Synthesis And Characterization Of Ag, Au and Cu

Dendrimer-Encapsulated Nanoparticles As Well As

Their Application In Catalysis.

by

Mulisa. S Nemanashi

(201042530)

A Dissertation submitted in partial fulfilment of the requirements for the Degree of

Masters in Chemistry in the department of Chemistry,

University of Johannesburg

Supervisor: Prof R. M. Meijboom

Co-supervisors: Dr F. A. Muller

: Dr A. Deshmukh

February 2012

i

Abstract

In this dissertation the synthesis, characterization and the application of Ag,

Au and Cu dendrimer encapsulated nanoparticles (DENs) in catalysis are described.

Ag, Au and Cu-DENs were synthesized using G4-G6 PAMAM-OH and G4-G6

PAMAM-NH2 dendrimers as templates as well as stabilizers. NaBH4 was used as a

reducing agent for the synthesis of DENs. Binding studies were carried out in order

to determine the maximum capacity of the dendrimer to which the metal ions can be

added. These binding studies were performed using UV-vis spectroscopy. The

synthesis of these nanoparticles (NPs) was carried out at room temperature. For the

synthesis of Ag and Au-DENs with PAMAM-NH2 dendrimers, the pH of the aqueous

dendrimer solution was first adjusted to acidic condition (~pH 2) using HCl before the

addition of the respective metal ion precursor to the dendrimer. This is done to avoid

coordination of the metal ions to the primary amine groups on the periphery of the

dendrimer, which might lead to particle agglomeration. These prepared DENs were

characterized by UV-vis spectroscopy and high resolution transmission (HRTEM)

microscopy. The synthesized DENs were evaluated as catalysts in the reduction of

4-nitrophenol to 4-aminophenol by NaBH4. This reaction was monitored by UV-vis

spectroscopy by following the absorbance at λ 400 nm These DENs were all found

to be active catalysts for the afore-mentioned process. The rate constant for the

reduction process was observed to decrease as the concentration of 4-nitrophenol

increased. As the concentration of NaBH4 is increased, the rate constant was also

found to increase, however this increase was only observed to a maximum

concentration of NaBH4.

The Au-DENs prepared using G4 PAMAM-NH2 dendrimers were

subsequently immobilized onto a titania support via the sol-gel (Ti-Au-s) and wetness

impregnation (Ti-Au-w) methods. The titania supported Au NPs were characterized

using HRTEM, powder X-ray diffraction (PXRD), thermal gravimetric analysis (TGA),

inductive coupled plasma-optical emission spectroscopy (ICP-OES) and Brunauer

Emmett Teller (BET) surface area analysis. The dendrimer template was removed by

calcining at 500 oC. The catalytic activity of these supported Au NPs was

investigated in the oxidation of styrene using tert-butyl hydroperoxide (TBHP) as an

ii

oxidant. Benzaldehyde and styrene oxide were observed as the major products. The

catalyst prepared by wetness impregnation method was found to give the highest

styrene conversion as compared to the one prepared via sol-gel method. At 60 oC,

the catalyst prepared by sol-gel method was found to selectively produce

benzaldehyde while on the other hand, the catalyst prepared by wetness

impregnation selectively produce styrene oxide. The highest conversion of styrene

was observed at 70 oC for both catalysts. Ti-Au-w catalyst was generally found to

give the highest styrene conversion.

iii

Acknowledgements

I would like to express my sincere appreciation to the following people, whose

contributions have helped me to complete my studies.

My supervisor, Prof Reinout Meijboom for believing in me as well his countless

assistance and the advice he gave me throughout the period of this study. I really

thank you very much for mentoring me and making my study a successful one.

I would also like to express my sincere appreciation to my co-supervisor, Dr Fanie

Muller, who equally played a crucial role in moulding my work to be the way it is now.

Thanks very much for all the suggestions and advice you gave me.

Dr Rehana Malgas-Enus for her advice and motivation, things would surely have not

been the same without your intervention. Thank you very much. Hope you’ll continue

carrying on with your kindness to other fellows in the lab.

I thank the Meta-catalysis group for their support. You guys have contributed a lot in

making this a reality more than just a dream. Hope we’ll keep on chaining up

together even much stronger.

I would also like to wholeheartedly send my gratitude to my family, especially my

mother as well as my brothers. Thank you all for your prayers and amazing support

you gave me throughout my studying career. Without your contribution, I would not

have made it this far. I really thank God for giving me you as a family.

The Council for Scientific and Industrial Research (CSIR) for allowing me to use their

HRTEM instrument.

The University of Johannesburg as well as the National Research Foundation (NRF)

for their financial support.

And above everything, I want to thank God for making everything possible. He is

indeed a living God. To God be the glory!!!

iv

Conference Presentations.

1. Poster titled: “Synthesis and catalytic evaluation of Au and Cu DENs on the

reduction of p-nitrophenols”, M. Nemanashi, R. Meijboom and F. Muller,

presented at CATSA 2010, Bloemfontein, South Africa, November, 2010

2. Poster titled: “Synthesis, characterization and application of titania-supported

Au nanoparticles using dendrimers as a template”, M. Nemanashi, R.Meijboom

and F. Muller, presented at CATSA 2011, Johannesburg, South Africa, November,

2011

v

Table of contents.

Chapter 1: The relevance of dendrimer dendrimer encapsulated nanoparticles

in homogeneous and heterogeneous catalysis 1

1.1 Introduction 1

1.2 Synthesis of metal nanoparticles 2

1.2.1 Co-precipitation method 3

1.2.2 Microemulsion 4

1.2.3 Template synthesis method 6

1.3 Dendrimers 6

1.3.1 Synthesis of dendrimers 7

1.3.1.1 Divergent dendrimer synthesis 9

1.3.1.2 Convergent dendrimer synthesis 13

1.4 Applications of dendrimers 17

1.5 Synthesis of dendrimer encapsulated nanoparticles 19

1.6 Supported dendrimer encapsulate nanoparticles 23

1.6.1 Synthesis of supported dendrimer encapsulated nanoparticles 24

1.7 Extraction of metal nanoparticles from the dendrimer template 25

1.8. Application of DENs and supported DENs in catalysis 27

1.8.1 Application of DENs as homogeneous catalysts 27

1.8.2 Application of supported DENs to heterogeneous catalysis 29

1.9 Aims and objectives of the project 31

1.10 References 32

vi

Chapter 2: Synthesis and characterization of Cu, Ag and Au dendrimer

encapsulated nanoparticles 38

2.1 Introduction 38

2.2 Experimental 42

2.2.1 Binding studies, synthesis and characterization of dendrimer encapsulated

Cu nanoparticles 44

2.2.1.1 Binding studies 44

2.2.1.1.1 Spectrophotometric titration of dendrimer with Cu2+ ions 44

2.2.1.2 Synthesis and characterization of Cu-DENs 48

2.2.2 Binding studies, synthesis and characterization of Ag-DENs 51

2.2.2.1 Binding studies 51

2.2.2.2 Synthesis and characterization of Ag-DENs 51

2.2.3 Binding studies, synthesis and characterization of Au-DENs 55

2.2.3.1 Binding studies 55

2.2.3.2 Synthesis and characterization of Au-DENs 55

2.3 Conclusions 57

2.4 References 58

Chapter 3: Ag, Au and Cu dendrimer encapsulated nanoparticles in the

reduction of 4-nitrophenol 60

3.1 Introduction 60

3.2 Application of Ag, Au and Cu DENs in the reduction of 4-NP 65

3.2.1 Experimental 65

3.2.2 Reaction conditions used for kinetic study 66

3.3 Results and discussion 66

vii

3.3.1 Cu-DENs as catalysts in the reduction of 4-nitrophenol 66

3.3.2 Ag-DENs as catalysts in the reduction of 4-nitrophenol 71

3.3.2.1 The effect of 4-NP concentration on the reaction rate constant 74

3.3.3 Au-DENs as catalysts in the reduction of 4-nitrophenol 75

3.3.3.1 The effect of temperature on the rate constant 76

3.3.3.2 The dependence of rate constant on the concentration of NaBH4 77

3.4 Conclusions 79

3.5 Reference 80

Chapter 4: Immobilization and characterization of dendrimer encapsulated Au

nanoparticles onto inorganic supports 82

4.1 Introduction 82

4.1.1 Inorganic oxide as supports for metal NPs 82

4.1.2 Methods for immobilization of NPs 84

4.2. Experimental 91

4.2.1 Synthesis of Au-DENs for immobilization via both wetness and sol-gel

methods 92

4.2.2 Synthesis of 2 µM Au-DENs and their immobilization onto commercial TO2

by wetness impregnation 92

4.2.3 Synthesis of 0.9 µM Au-DENs and their immobilization via sol-gel synthesis 93

4.3. Results and discussion 93

4.3.1 Immobilization and characterization of Au-DENs onto titania 94

4.3.1.1. Deposition of Au-DENs onto a commercial support by wetness

impregnation method 95

4.3.1.2 Immobilization of Au-DENs onto titania by sol-gel method 95

viii

4.4 Thermogravimetic and PXRD analyses of the synthesized G4-PAMAM-

NH2(Au55) and G4-Q32(Au55) 97

4.5 Physisorption analysis and metal loading of the titania-G4-PAMAM-NH2(Au55)

and G4-Q32(Au55) 99

4.6 Conclusions 101

4.7 Reference 103

Chapter: 5 Application of titania supported Au nanoparticles as catalysts in the

oxidation of styrene 105

5.1 Introduction 105

5.2 Experimental 112

5.3 Results and discussion 112

5.3.1 Effect of temperature on the oxidation of styrene 113

5.3.2 Effect of solvent on the oxidation of styrene 113

5.3.3 The effect of catalyst amount and metal loading on the oxidation of styrene 114

5.3.4 The effect of an oxidant on the formation of products during styrene

oxidation 115

5.3.5 Effect of reaction time on the conversion of styrene 116

5.4 Conclusions 118

5.5 References 119

Chapter 6: Dissertation summary 121

ix

List of Figures

Chapter 1 Figures

Figure 1.1: Size distribution of Au NPs capped with mercaptopropionate (MPA) at

various [MPA]/[Au] 4

Figure 1.2: Phase diagram for CTAB/hexanol/water system 5

Figure 1.3: Architectural components of a typical poly(propylene imine) (PPI)

dendrimer molecule 7

Figure 1.4: Schematic representation of dendrimer synthesis by a) divergent and b)

convergent methods 8

Figure 1.5: Synthesis of different generations of PAMAM dendrimers 10

Figure 1.6: Synthesis of EDA-core PAMAM dendrimers 12

Figure 1.7: Synthesis of polyether dendrimer using a convergent method 14

Figure 1.8: Half-wedge W2.5-TMS 14

Figure 1.9: Dumbbshell-shaped generation 3 synthesized by Washio et al 15

Figure 1.10: Star-shaped generation 2 synthesized by Washio et al 16

Figure 1.11: Structure of amino-terminated generation 3 dendrimers with carboxylic

groups at the core 16

Figure 1.12: Types of metallodendrimers 17

Figure 1.13: Synthesis of dendrimer encapsulated metal nanoparticles 20

Figure 1.14 Synthesis of bimetallic DENs by two different methods 22

Figure 1.15: Metal DENs synthesis by multiple, in situ displacement 22

Figure 1.16: Immobilization of DENs onto solid supports 24

Figure 1.17: Extraction of NPs from the dendrimer interior to the organic phase 26

x

Figure 1.18: HRTEM micrograms and particle-size distribution for MPC-12(Au55)

extracted from G4-PAMAM-OH dendrimer using the following HSC-12/Au mole

ratios: (a) 1, (b) 3 27

Chapter 2 Figures

Figure 2.1: Particle size distribution of (a) G4-NH2(Au55) and (b) G4-Q32(Au55) 42

Figure 2.2: Chemical structure of generation 4 PAMAM-NH2 dendrimer 43

Figure 2.3: Absorption spectra of G4-PAMAM-OH dendrimer (0.25 mM) solution

titrated with Cu2+ (250 mM) at λ 605 nm 46

Figure 2.4: Titration curve of the G4-PAMAM-OH dendrimer and Cu2+ ions at λ 604

nm and pH 5.7 46

Figure 2.5: Titration curve of the G5-PAMAM-OH dendrimer and Cu2+ ions at λ 608

nm and pH 5.7 47

Figure 2.6: Titration curve for the G6-PAMAM-OH dendrimers and Cu2+ ions at λ

604 nm and pH 5.7 47

Figure 2.7: End-points obtained (with error bars) for the titration of G4-G6 PAMAM-

OH dendrimers with Cu2+ ions at pH 5.7 and λ 605 nm 48

Figure 2.8: Colour change during the synthesis of Cu-DENs (a) aqueous dendrimer

solution, (b) Cu2+ loaded dendrimer solution, and (c) Cu-DENs solution 50

Figure 2.9: UV-vis spectra for the formation of Cu-DENs within PAMAM-OH

dendrimer template 50

Figure 2.10: Colour changes during the synthesis of Ag-DENs (a) aqueous

dendrimer solution, (b) Ag+ loaded dendrimer solution (c) Ag-DENs 52

Figure 2.11: UV-vis spectra for the preparation of Ag-DENs using PAMAM-NH2

dendrimers as a template 53

xi

Figure 2.12: a) HRTEM image and b) particle size histogram of G4-PAMAM-

NH2(Ag12) 53

Figure 2.13: a) HRTEM image and b) particle size histogram of G5-PAMAM-

NH2(Ag16) 54

Figure 2.14: a) HRTEM image and b) particle size histogram of G6-PAMAM-

NH2(Ag32) 54

Figure 2.15: Synthesis of Au-DENs a) aqueous dendrimer solution, b) Au3+ loaded

dendrimer solution c) Au-DENs 55

Figure 2.16: UV-vis spectra for the formation of Au-DENs within the PAMAM-NH2

dendrimer 56

Figure 2.17: a) HRTEM image and b) particle size histogram of G4-PAMAM-

NH2(Au55) 56

Chapter 3 Figures

Figure 3.1: Time base UV-vis spectra monitoring 4-NP reduction catalyzed by metal

NPs 61

Figure 3.2: Langmuir- Hinshelwood mechanism of the reduction of 4-NP by NaBH4

in the presence of metallic NPs 61

Figure 3.3: Colour change in the catalyzed reduction of 4-NP to AMP by NaBH4 67

Figure 3.4: First-order kinetic plot for the reduction of 4-NP when NaBH4 is added

last in the reaction mixture 68

Figure 3.5: First-order kinetic plot for the reduction of 4-NP when NaBH4 is added

first in the reaction mixture 68

Figure 3.6: The dependence of rate constant on temperature for Cu-DENs catalyzed

4-NP reduction by NaBH4 70

Figure 3.7: Arrhenius plot for Cu-DENs catalyzed reduction of 4-NP by NaBH4 71

xii

Figure 3.8: Typical time based UV-vis spectra monitoring the reduction of 4-NP by

NaBH4 catalyzed by Ag-DENs 71

Figure 3.9: First order plot for the reduction of 4-NP by NaBH4 in the presence of

Ag-DENs catalysts at 298 K 73

Figure 3.10: The effect of temperature on the rate constant for the Ag-DENs

catalyzed reduction of 4-NP by NaBH4 73

Figure 3.11: Arrhenius plot for the reduction of 4-NP by NaBH4 in the presence of

Ag-DENs catalysts 74

Figure 3.12: The effect of 4-NP concentration on the rate constant during 4-NP

reduction by NaBH4 at 298 K 75

Figure 3.13: First-order plot for the Au-DENs catalyzed 4-NP reduction by NaBH4 at

298 K 76

Figure 3.14: The effect of temperature on the reaction rate constant for Au-DENs

catalyzed 4-NP reduction by NaBH4 77

Figure 3.15: Arrhenius plot for the Au-DENs catalyzed reduction of 4-NP at different

temperatures 77

Figure 3.16: The dependence of rate constant on the concentration of NaBH4 during

4-NP reduction at 298 K 78

Chapter 4 Figures

Figure 4.1: Preparation of titania supported Au NPs from Au DENs precursor 87

Figure 4.2: Immobilization of dendrimer-encapsulated NPs into SBA-15 pores via

wetness impregnation 89

Figure 4.3: a) HRTEM image and b) particle size histogram of 0.92 µM Au-DENs

synthesized in 99 % MeOH 94

xiii

Figure 4.4: a) HRTEM image and b) particle size histogram of 2 µM Au-DENs

synthesized in 99 % MeOH 94

Figure 4.5: a) HRTEM image and b) particle size histogram of G4-Q32(Au55) after

calcination at 500 oC in air for 3 hours 95

Figure 4.6: Addition of TiO2 precursor (titanium isopropoxide) to the preformed G4-

PAMAM-NH2(Au55) solution 96

Figure 4.7: a) HRTEM image and b) particle size distribution histogram of G4-

PAMAM-NH2(Au55) after calcination at 500 oC in air 96

Figure 4.8: Thermogravimetric analysis plot of titania supported G4-NH2(Au55)

before calcination 97

Figure 4.9: Powder X-ray diffraction plot of the a) titania-G4-PAMAM-NH2(Au55)

before calcination; b) titania- G4-PAMAM-NH2(Au55) and c) titania support only, after

both materials were calcined at 500 oC in air for 3 hours 98

Figure 4.10: SEM images of titania supported G4-PAMAM-NH2(Au55) after

calcination at 500 oC in air for 3 hours 99

Figure 4.11: Nitrogen adsorption/desorptions isotherm for titania supported G4-

PAMAM-NH2(Au55) 100

Figure 4.12: BJH pore-size distribution of titania-G4-PAMAM-NH2(Au55) after

calcination at 500 oC in air for 3 hours 100

Chapter 5 Figures

Figure 5.1: Primary reaction products formed during the oxidation of styrene 106

Figure 5.2: The conversion of styrene at various reaction time using TBHP and

toluene at 70oC 117

xiv

List of Schemes

Chapter 1 Schemes

Scheme 1.1: The protection-deprotection method used by Tomalia et al 11

Scheme 1.2: Pathway for the formation of dendrimer-templated mesoporous

materials 19

Chapter 2 Schemes

Scheme 2.1: Indirect method for the preparation of Ag-DENs within PAMA-OH

dendrimers from Cu-DENs precursor 40

Chapter 3 Schemes

Scheme 3.1: Reduction of 4-NP to 4-AMP by NaBH4 in the presence of Cu-DENs 67

List of Tables

Chapter 2 Tables

Table 2.1: Concentrations used for the titration of G4, G5 and G6 PAMAM-OH

aqueous dendrimer solution with Cu2+ ions 45

Table 2.2: Number of amine groups on the PAMAM-OH dendrimer available for

binding with Cu2+ ions during Cu-DENs 45

Table 2.3: Endpoints for the titration of different generations of PAMAM-OH with

Aqueous Cu2+ 48

Table 2.4: Reaction quantities and conditions used for the synthesis of Cu-DENs

using G4, G5 and G6 PAMAM-OH dendrimers 49

Table 2.5: Quantities used for the synthesis of Ag-DENs using G4, G5 and G6

PAMAM-NH2 dendrimers 52

Table 2.6: The average particle sizes for Ag-DENs synthesized with G4, G5 and G6

PAMAM-NH2 dendrimers 54

xv

Chapter 3 Tables

Table 3.1: The rate constants obtained for the Cu-DENs catalyzed 4-NP

reduction.by NaBH4 69

Table 3.2: The effect of dendrimer generation on the rate constant for 4-NP

reduction 72

Table 3.3: The effect of 4-NP concentration on the rate constant during Ag-DENs

catalyzed 4- NP reduction by NaBH4 at 298 K 74

Table 3.4: The effect of 4-NP concentration on the rate constant during Cu-DENs

and Au-DENs catalyzed 4-NP reduction by NaBH4 at 298K 75

Table 3.5: The effect of NaBH4 concentration on the rate constant during Cu-DENs

and Ag-DENs catalyzed 4-NP reduction by NaBH4 at 298K 79

Chapter 4 Tables

Table 4.1: Summary of BET surface area, total pore volume and pore size of titania-

G4-PAMAM-NH2(Au55) and G4-Q32(Au55) before and after calcination 101

Chapter 5 Tables

Table 5.1: Results for the catalytic oxidation of styrene at different temperatures

using Ti-Au-s and Ti-Au-w catalysts 113

Table 5.2: Results for the oxidation of styrene using different solvents as well as Ti-

Au-s and Ti-Au-w catalysts 114

Table 5.3: The effect of metal loading on the formation of products for the oxidation

of styrene 115

Table 5.4: Results for the oxidation of styrene using TBHP and H2O2 oxidants with

Ti-Au-s and Ti-Au-w catalysts 116

Table 5.5: The effect of reaction time on the formation of products during styrene

oxidation catalyzed by Ti-Au-s and Ti-Au-w 117

xvi

List of Abbreviations

oC/min degree Celsius per minute

µmol micromole

µM micromolar

µL microliters

Å Ångstrom

4-NP 4-nitrophenol

4-AMP 4-aminophenol

∆S++ enthalpy change

∆H++ enthropy change

BET Brunauer Emmett Teller

BJH Barrett Joyner Halenda

BzA benzalydehyde

cm2 square centimetre

cm3/g centimeter cubed per gram

DMF dimethyl formamide

Ea activation energy

FT-IR Fourier Transform infrared spectroscopy

g grams

GC gas chromatography

G4 generation 4

G5 generation 5

G6 generation 6

hrs hours

xvii

HRTEM high resolution transmission electron microscope

ICP-OES inductive coupled plasma-optical emission spectroscopy

kobs observed rate constant

K Kelvin

kJ/mol kilojoule per mole

kV kilovolt

M molar

mA milliampere

MLCT metal-to-ligand charge transfer

mL milliliter

ML multilayer

mmol millimole

mM millimolar

mol/L mole per liter

MPC mono-protected cluster

NPs nanoparticles

nm nanometer

NMR nuclear magnetic resonance

PAMAM poly(amido amide)

PAMAM-OH hydoxyl-terminated PAMAM

PMMA poly(methyl metacrylate)

PPI poly(propylene imine)

rpm rotation per minute

s-1 per second

SEM scanning electron microscope

xviii

SO styrene oxide

TEM transmission electron microscope

TGA thermogravimetric analysis

TOF turn-over frequency

TPRS temperature programmed reaction spectroscopy

Ti(Oipr)4 titanium isopropoxide

UV-vis ultraviolet/visible

WGC world gold council

1

Chapter 1: Literature review.

The relevance of dendrimer encapsulated nanoparticles (DENs) in

homogeneous and heterogeneous catalysis.

1.1. Introduction:

Environmentally friendly (e.g. phosphine-free) catalysts should be designed

as required by green/catalysis chemistry. These specifically designed catalysts must

be able to be easily removed from the reaction mixture and subsequently also be

recycled as many times as possible with very high efficiency. All these demanding

conditions are driving forces for the design of catalysts that has both homogeneous

and heterogeneous properties. This could be ideal to fulfil all other challenges that

were far from being taken into consideration by the pioneers and even specialists in

each catalytic field in the past decades. Heterogeneous catalysts gain an advantage

over homogeneous catalysts in that the catalyst can be easily removed from the

reaction media as well as its possible use at high temperatures. Heterogeneous

catalysts, however, suffered for a long time from lack of selectivity and

understanding of mechanistic aspects that are absolutely necessary for parameter

improvements. Homogeneous catalysts on the other hand are very selective,

efficient and are used in a few industrial processes. It is limited by its difficulty to

remove the catalyst from the reaction mixture as well as its lower thermal stability.

The establishment of the desired optimized catalytic systems should possibly evolve

from the knowledge gained from the past research in homogeneous, heterogeneous,

supported and biphasic catalysis. These also include studies from non-classical

conditions (solvent-free, aqueous, use of ionic liquids, fluorine chemistry,

microemulsion, micelles, reverse micelles, vesicles, surfactants, aerogels, polymers,

or dendrimers). The use of supported transition metal nanoparticles (NPs) in

catalysis1 is very important as they mimic metal surface activation and catalysis at

the nanoscale, which in turn brings selectivity and efficiency to heterogeneous

catalysts.

A nanoparticle (or nanopowder or nanocrystal) is a collection of clusters

stabilized by ligands,2 surfactants,3 polymers4 or dendrimers5 protecting its surface.

They are also defined as a microscopic particle if at least one dimension is less than

100 nm. Due to a wide range of applications in biomedical, optical, electronics, and

2

catalysis, nanoparticle research is currently an area of intense scientific research.

They effectively bridge the gap between bulk material and atomic or molecular

structures. The latter application (catalysis) has proved to be the key for the

development of starting chemicals, fine chemicals and drugs from the raw materials.

Researchers believe that NPs have the potential to be more active and selective

catalysts than the previously preferred homogeneous complexes because a large

percentage of a nanoparticle’s metal atoms lie on the surface, and the surface atoms

do not necessarily order themselves in the same way that those in the bulk catalysts

do.6 The unique properties of these materials have also led to the realization that the

bottom-up approach should now replace the classic top-down approach, a strategic

move which is now common to several areas of nanoscience. Bottom-up approaches

involve formulation of the problem and solve it by introducing simple operations.

While on the other hand, top-down approach is more complicated because it is not

easy to formulate simple problems and end up with a global problem.

(i) Heterogeneous catalysts.

In heterogeneous catalysis, the catalyst provides a surface on which the

reactants (or substrates) temporarily become adsorbed. The components of the

system are in different phases in heterogeneous catalysis. Bonds in the substrate

become weakened sufficiently for new bonds to be formed in order to obtain the

product. Since the bonds between the products and the catalyst are weaker, the

products are released and allow the catalyst to be recovered.

(ii) Homogeneous catalysts.

These catalysts are in the same phase as the reactants. In homogeneous

catalysis, the catalyst is a molecule which facilitates the reaction by initiating the

reaction with one or more reactants to form intermediate(s) and in some cases one

or more products. Subsequent steps lead to the formation of the product and to the

regeneration of the catalysts.

1.2. Synthesis of metal nanoparticles.

Many transition metal NPs have been synthesized using several different

methods. These include but are not limited to: coprecipitation,7 microemulsion8-10

and template11 methods.

3

1.2.1. Co-precipitation method .

Many of the early preparations of NPs were achieved by the coprecipitation of

sparingly soluble products from aqueous solutions, followed by thermal

decomposition of those products to oxides. The precipitation of metals by chemical

reduction of metal ions can be performed both from aqueous and non-aqueous

solutions. This method generally involves the simultaneous occurrence of nucleation,

growth, coarsening and/or agglomeration. However the fundamental mechanisms of

coprecipitation are still not thoroughly understood. A number of research groups

have used this method to synthesize NPs.12-14

Tan et al have recently demonstrated that stable Au, Pd and Ag NPs can be

synthesized by reduction with potassium bitartrate in the presence of a suitable

stabilizing agent.7 Organic capping agents are used to prevent agglomerization and

can also serve as a reducing agent, as is the case in the Turkevish process for the

prepararion of gold colloids.15 It has previously been believed that whenever thiols

are used as stabilizing agents for the formation of aqueous Au NPs, they must be

synthesized using a borohydride or similar reducing agents. This is because

complexes formed between [AuCI4]– and thiols are too stable to be reduced with

citrate or other weak reducing agents. However, Yonezawa et al have shown that

[AuCl4]– can possibly be reduced by citrate in the presence of thiols if both the citrate

and the thiol are added simultaneously to the gold solution.16 Using this method, gold

NPs with 2-10 nm dimensions were achieved and narrow size distributions was

possible at high [thiol]/[Au] ratios (see Figure 1.1).

Han et al showed that gold can also be reduced in a non-aqueous solution. In

this case the solvent used, formamide (HCONH2), also served as a reducing agent.17

Although the particles produced showed a narrow size distribution, it was not clear

as to whether the particle size can be controlled. The molar ratio between the

stabilizing agent, polyvinylpyrrolidone (PVP) and Au, [PVP]/[Au], did not show any

influence on the particle size.

4

Figure 1.1: Size distribution of Au NPs capped with mercaptopropionate (MPA) at various ratios of

[MPA]/[Au].16

1.2.2. Microemulsion .

A microemulsion is a thermodynamically stable dispersion of two immiscible

fluids. It is one of the predominant techniques being used to prepare metal NPs due

its advantages of producing narrow size distributed NPs and the simple sample

preparation technique. The microemulsion system is stabilized by addition of a

surfactant. There are different types of microemulsions such as water-in-oil (w/o), oil-

in-water (o/w), water-in-supercritical-CO2 (w/sc-CO2) e.t.c. A water-in-oil

microemulsion is formed when water is dispersed in a hydrocarbon based

continuous phase, and is normally located towards the oil apex of water/oil/surfactant

triangular phase diagram as shown in Figure 1.2. It is in this region where

thermodynamically surfactant self-assembly generates aggregates known as reverse

or inverted micelles (Om phase on Figure 1.2). Spherical reverse micelles, which

minimize surface energy, are the most common form. These micelles in the system

are described as “nanoreactors”, which provide a suitable environment for controlled

growth and nucleation. Steric stabilization, provided by the surfactant layer, prevents

aggregation of NPs in the later stages of growth.

5

Figure 1.2: Phase diagram for CTAB/hexanol/water sytem.18

Chen et al used the microemulsion method to synthesize Au NPs with a

narrow size distribution of 7 nm.19 In another related study, Chen et al showed that

bimetallic core-shell NPs can also be prepared using the microemulsion method.20

Monodispersed Pd NPs of 6, 8 and 13 nm in diameter were prepared in a reverse

microemulsion of water/Aerosol-OT/isooctane and then used as catalysts for the

study of structure sensitivity of solvent-free selective hydrogenation of 2-methyl-3-

butyn-2-ol to 2-methyl-3-buten-2-ol.21 They found that, the catalytic activity, when

expressed as turnover frequency (TOF), increases as the Pd particle size increases,

but remains constant when expressed in terms of the number of specific atoms

(active site) on Pd nanoparticle facets. However, the particle size did not influence

the selectivity to the formation of the product.

6

1.2.3. Templated synthesis method.

One of the modern approaches for the formation of well-defined NPs is the

use of hyperbranched polymers as a template. This method has been given an

increasing amount of attention in the past few years and has proved to be much

better than the previously used methods. In this approach, a polymer or sub-class of

the hyperbranched polymers called dendrimers can be used as the templating agent.

Dendrimers, like polymers, are macromolecules but differ from polymers in that they

(dendrimers) are perfectly defined on a molecular level with a polydispersity of 1.0.22,

23

Platonova et al have reported the preparation of 1 nm Co in a polystyrene

(PS)-PVP copolymer.24 The synthesis of 1 nm Pd and Pt NPs within the dendrimer

template has been reported recently by Zhao and Crooks.25 The use of dendrimers

as a template for the formation of metal NPs offers a great advantage over other

techniques previously discussed, and is the main focus of this study.

Dendrimers are specifically well suited for hosting metal NPs (particularly

those to be utilized as catalysts) for a number of reasons: (1) the dendrimer

templates themselves are of uniform composition and structure, and therefore

should yield well defined NPs;26-28 (2) the NPs are stabilized by encapsulation within

the dendrimer and therefore do not agglomerate;27 (3) the encapsulated NPs are

confined primarily by steric effects, and therefore a substantial fraction of their

surface is unpassivated and available to participate in catalytic reactions;26-30 (4) the

dendrimer branches can be used as selective gates to control access of small

molecules or substrates to the encapsulated catalytic NPs;25, 30 (5) the terminal

groups on the dendrimer periphery can be tailored to control solubility of the hybrid

nanocomposite and used as a handle to help linking to surfaces and other

polymers.26, 31

1.3. Dendrimers.

Dendrimers are well-defined, three dimensional polymers which can be

synthesized by either convergent or divergent methods and are characterized by a

high density of peripheral groups.32 They are composed of three main distinguishing

architectural components, i.e. (a) an interior core, (b) interior layers (generations),

7

which are made up of repeating branching units attached to the initiator core and (c)

the exterior (periphery) attached to the outermost interior generation (Figure 1.3).33

NN

N

N N

N NH2

NH2

NH2

NH2

NH2

NH2

NH2

NH2

Figure 1.3: Architectural components of a typical of poly(propylene imine) (PPI) dendrimers .

dendrimer molecule 11

Dendrimers are produced through an iterative sequence of reaction steps, in which

additional iterations lead to higher generation dendrimers.

The first example of the synthetic procedure towards the well-defined

branched structure has been reported by Vögtle and co-workers in 1978 who

referred to this procedure as “cascade synthesis”.34 Based on this report, in 1985,

Tomalia et al managed to modify this procedure by developing an independent

divergent, macromolecular synthesis of “true dendrimers” in the form of

poly(aminoamide) (PAMAM) dendrimers.35 In the same year, Newkome et al

reported the preparation of “arborols” (a synonym for dendrimers).36

1.3.1. Synthesis of dendrimers.

The convergent and divergent pathways are the two synthetic methods for

dendrimer synthesis as discussed in the previous section.

b) branching

point

c) periphery

a) Core

8

The divergent method, introduced by Tomalia et al, involves the synthesis of

the dendrimer from the functional core molecule to the branching units and then

constructed to the periphery (Figure 1.4.a).35 Higher generations of dendrimers can

be synthesized by utilizing the reactive sites on the periphery of the previous

generation.

The divergent method has been used successfully to synthesize a large

diversity of dendrimers, because the molar mass of the dendrimer is almost doubled

in each generation-forming step. Although very large dendrimers have been

prepared using this approach, isolation of slightly impure samples is often

experienced due to incomplete growth steps and side reactions. With the convergent

method however, only two simultaneous reactions are needed for any generation

adding step, hence making the purification of the final dendrimer simpler. However,

the convergent method suffers from low yields if the synthesis of large dendritic

structures is attempted.

4 8 16. . .

2 x (a)

(a)

(b)

3 x (b)

Figure 1.4: Schematic representation of dendrimer synthesis by a) divergent and b) convergent

methods.

The convergent method, pioneered by Fréchet in the early 90’s, involves the

construction of the dendrimer from what will ultimately become its “periphery”, and at

each step, growth is designed to occur via reaction of only a very limited number of

reactive sites.37 In this method, the synthesis is started with what will eventually

become the surface functionality as well as a reactive functional group of the

b

a

9

dendritic macromolecule (Figure 1.4.b). When using the convergent dendrimer

synthesis method, the basic building blocks of dendrimer (dendrons or dendritic

wedges) are synthesized and then attached to the functionalized core.

1.3.1.1. Divergent dendrimer Synthesis .

A large number of dendrimer syntheses have been reported in the literature

since the first reports of the divergent synthetic method by Tomalia and Vögtle.

Using the divergent method, Tomalia et al managed to synthesize the different

generations of starburst oligomeric dendrimers poly(amidoamine) (PAMAM) using

ethylenediamine (EDA) and ammonia as initiator cores via a two step process

involving; (a) exhaustive Michael addition of the suitable amine initiator core with

methyl acrylate and (b) exhaustive amidation of the resulting esters with large

excesses of ethylenediamine as shown in Figure 1.5.35

Both ammonia (three binding sites) and ethylenediamine (with four possible

binding sites) were used as initiator cores in this case. Ethylenediamine was used as

the core for the synthesis of higher generation (generation 7) dendrimers and was

also found to be particularly suitable for the amidation step, since its boiling point

(110 oC) allowed the removal of the large excesses of EDA under conditions which

did not alter the dendrimer. The synthesized dendrimers were found to be soluble in

most normal organic solvents such as chloroform (CHCI3), methanol (MeOH),

dimethylformamide (DMF), as well as water.35

Contrary to what was proposed by Vögtle and co-workers34, Mülhaupt et al

showed that high yield divergent synthesis of dendrimers, which does not require the

use of large excess reagents and purification, can be achieved.38 With the use of

ammonia as an initiator core, different generations (G1-G5) were synthesized via

hydrogenation of nitriles or amine functional groups in the presence of Raney-nickel

catalyst at room temperature

10

Figure 1.5: Synthesis of different generations of PAMAM dendrimers.35

Padias et al also made use of the protection-deprotection method (Scheme

1.1) to synthesize two types of highly branched polyether dendrimers.39 In this study,

bicyclic ortho ester was used to temporarily mask three or two hydroxyl groups of the

used pentaerythritol core. Hydroxymethyl bicyclic orthoformate was used as the

dendrimer synthon for type I dendrimer synthesis. For the preparation of the type II

dendrimer, 1,1,1-tris(hydroxymethly)ethane and a (hydroxymethyl)dioxane were

used as core and synthon respectively. They also found that no spacer groups or

excess reagent were required in this synthesis.

11

Core OHn

Core

CyclicnY

Xn

Core o Cyclicn

Core o R OH m n

to generation 2

conversion to reactive function

reaction with masked synthon

deprotection

unmasked first generation

Scheme 1.1: The protection-deprotection method used by Tomalia et al 39

Peterson et al synthesized different generations of amino-terminated PAMAM

dendrimer (PAMAM-NH2) (Figure 1.6) using ethylenediamine (EDA) as core

initiator.40 They found that there were at least three main types of primary side-

reactions in PAMAM dendrimer synthesis resulting from 1) incomplete Michael

addition which causes the appearance of unsymmetrical dendritic structures; 2)

intramolecular cyclization, which can only occur during the synthesis of a full-

generation of the dendrimer; 3) retro-Michael addition. In an attempt to suppress

these side reactions, significant excess of EDA is used. Separation of different

dendrimer generations were performed by the application of capillary zone

electrophoresis (CZE).

Didier Astruc’s group reported the synthesis of a giant dendrimer41 which is far

beyond the De Gennes dense-packing limit42 using iteration of sequence reactions.

The Williamson reaction used for the synthesis of this dendrimer required heating for

2 days at 80 oC in DMF solvent. A one pot synthesis of the dendrimer series was not

achieved under these conditions, but three years later, the very same group reported

one-pot construction of 243-allyl dendrimer via Williamson reaction under ambient

conditions following the same procedure as in the previous study.43 This reaction

12

was carried out at room temperature and only took 5 minutes to complete. Sodium

chloride (NaCI) was observed as the only byproduct.

NH 2 N H 2

N N

H 3CO

H 3CO

O

O

OCH 3

OCH 3

O

O

OCH 3

O

NH 2 N H 2

NN

N H

N H

O

ONH 2

NH 2N H

N H

O

ON H 2

N H 2

OCH 3

O

NN

N H

N H

O

ON

NN H

N H

O

ON

N

OCH 3

OCH 3

O

O

OCH 3

OCH 3

O

O

H 3 CO

H 3 CO

O

O

H 3 CO

H 3 CO

O

O

NH 2 N H 2

NN

N H

N H

O

ON

N

N H

N H

O

O

N H 2

N H 2

N H

N H

O

O

N H 2

N H 2

N H

N H

O

ON

N

N H

N H

O

NH 2

NH 2

N H

N H

O

O

NH 2

NH 2

G -0 .5

G 0

G 0 .5

G 1

Figure 1.6: Synthesis of EDA-core PAMAM dendrimers.40

13

1.3.1.2. Convergent dendrimer synthesis.

Since the first report of the synthesis of dendrimers by the convergent

method, a large number of dendrimers have been prepared using this method. A few

examples of convergently synthesized dendrimers are discussed here.

In the one of the first published studies by Hawker and Fréchet in 1990,44 the

synthesis of a dendritic polyether macromolecule (dendrimers) with 64 surface

functional groups was based on 3,5-dihydroxybenzyl alcohol (structure M) as the

monomer unit. As shown in Figure 1.7, the synthesis of a polyether macromolecule

involves a repetitive two-step process. Benzylic bromide [G1]-Br (first generation) is

reacted with a monomer unit (M) in the presence of potassium carbonate and 18-

crown-6 to give the next generation alcohol [G2]-OH. This second alcohol generation

can be converted to the corresponding bromide [G2]-Br by reacting with carbon

tetrabromide or triphenylphosphine. Repetition of the two-step pocess leads to

successive generation up to sixth generation bromide [G6]-Br. This method is mild

enough to be used with organometallic functionalities on the surface, e.g. Moss and

co-workers have published several reports on the synthesis of organometallic

dendrimers using the convergent method.45-48

Moore et al used the modified convergent approach to synthesize rigid

dendritic macromolecules.49 They found that when synthesis of the rigid dendritic

wedges is based on 1-ethyl-3,5-disubstituted benzene, it was impossible to

synthesize the third generation wedge of the parent system. A half-wedge W2.5-

TMS (Figure 1.8) was observed rather. They attributed these results to two factors,

namely, poor solubility and steric inhibition. The addition of methoxy or tert-butyl

groups made it possible to obtain third generation wedges, although at low yield. In

an attempt to overcome the steric inhibition, a reaction for dendrimer growth in which

a monomer size was enlarged as a function of generation was considered. This

resulted in very high yields of generation 3 wedges. Moore’s group have also

reported other classes of convergently produced phenylacetylene dendrimers.50, 51

14

O

Br

OHO

OH

HO

+

(M)

[G1]-Br

K2CO3

OH

O

O

O

O

O

O

[G2]-OH

Br

O

O

O

O

O

O

[G2]-Br

Ph3PCBr4

18-C-6

[G6]-Br

Figure 1.7: Synthesis of polyether dendrimer using the convergent method.37

Si(CH3)3

BrBr

Si(CH3)3

W2.5-TMS

Figure 1.8: Half-wedge W2.5-TMS.49

15

Ueda and co-workers published the synthesis of perfectly branched third

generation polyamide dendrimers via the convergent method without repetitive

protection-deprotection procedures.52 This synthesis involved the direct

condensation of a carboxylic acid and unprotected AB2 building monomer, 3,5-bis(4-

aminophenoxy)benzoic acid, with the use of diphenyl (2,3-dihydro-2-thioxo-3-

benzoxazolyl)phosphonate (DBOP) as a condensing agent since it (DBOP) is very

useful for producing amides and esters from carboxylic acids and imines or phenols.

However DBOP is an expensive reagent to use for industrial large-scale production

and hence the discovery of inexpensive, commercially available reagent which can

substitute DBOP expanded the scope of dendrimer synthesis. Since these authors

have previously reported that thionyl chloride is an effective reagent for polyamide53

and ester54 synthesis in amide solvents, Ueda and co-workers extended this

knowledge to synthesize dumbbell-shaped dendrimers, star-shaped dendrimers and

dendrimers with a carboxylic acid at the core from unprotected AB2 building blocks.55

The three types of dendrimers structures produced are shown in Figure 1.9 – 1.11.

Figure 1.9: Dumbbell-shaped generation 3 dendrimer synthesized by Washio et al. 55

16

Figure 1.10: Star-shaped generation 2 dendrimer synthesized by Washio et al. 55

Figure 1.11: Structure of amine-terminated G3 dendrimer with carboxylic groups at the core.55

17

1.4. Applications of dendrimers.

Due to their unique structural topology and chemical versatility, dendrimers

have found a wide range of applications including drug delivery,56, 57 energy

transfer,58, 59 liquid crystal,60 molecular recognition,61, 62 and catalysis.11, 63, 64

Among all these applications, catalysis may be one of the most promising, because it

is easy to tune the structure, size and location of the catalytically active sites,63 and

most importantly, dendrimers have the potential to combine both heterogeneous and

homogeneous catalysis.64 A number of catalytic reactions involving composite

metallodendrimers have been reported thus far, and these include, but are not

limited to: Suzuki coupling,65 oxidation,66 hydroformylation,67 hydrogenations,68

alkene metathesis,69 and the Heck reaction.70 There are four common

metallodendrimer configurations known and are shown in Figure 1.12

Figure 1.12: Types of metallodendrimers:63 (a) dendrimer encapsulated metal NPs (DENs), (b)

dendrimer modified on the periphery with metal ions or complexes, (c) core metallodendrimers, (d)

focal-point metallodendrimer.

a b

c d

18

Dendrimers have recently been used as a templating agent that serves to

stabilize and control growth during the synthesis of metal NPs. Here, however, the

focus is exclusively on the synthesis of dendrimer encapsulated nanoparticles

(DENs) catalysts (Fig 1.12.a) as well as their application in catalysis. This method,

pioneered by Zhao and co-workers,5 offers a great advantage over other

metallodendrimer catalysts shown in Figure 1.12 in that; (1) it provides stability on

the formed NPs; (2) it is easy to have full control over the particle size and size

distribution of the NPs by tuning the dendrimer to metal molar ratio; (3) it eliminates

the possibility of agglomerization of clusters without reducing their catalytic

efficiency; (4) it is possible to prepare particles with sizes as small as 1 nm which are

nearly monodispersed; (5) the commonly used poly(amidoamine) (PAMAM) and

poly(propylene imine) (PPI) dendrimers are commercially available; (6) the

dendrimer template can be recycled, which in turn makes the whole process cost

effective and environmentally friendly. Other advantages of using dendrimers as

hosts for catalytically active metal NPs have been discussed under section 1.2.3.

However, it is worth mentioning that dendrimers have not only been used as a

templating agent for the synthesis of metal NPs. Some research groups have also

successfully used dendrimers as a templating agent for the synthesis of mesoporous

and microporous materials. Only a few examples regarding the use of dendrimers as

templating agents for the synthesis of mesoporous and microporous materials will be

briefly discussed here.

Kriesel and Tilley were the first to report the use carbosilane dendrimers to

synthesize silica xerogels which exhibit both meso- and microporous structures via a

sol-gel method in 1999.71 A year later, Larsen et al also reported the synthesis of

mesoporous silicas.72 In this study, polypropyleneimine tetrahexacontaamine (DAB-

Am-64) dendrimer was used as templating agent to control the size of the formed

silica particles as well as their pore sizes. The synthesis was simply carried out by

mixing tetra-ethyl orthosilicate (TEOS) with 1-propanol/DAB-Am-64 mixture. The gel

formation was allowed to form for 12 hours at 343 K in a closed container. In the

same year, Larsen and Lotero reported the use of G4 PAMAM-NH2 dendrimers as a

templating agent for the synthesis of silica gels from a TEOS precursor.73

19

Rogers et al used G4 and G5 PPI dendrimers as templating agents for the

synthesis of mesoporous titanosilicate and vanadosilicate materials via sol-gel

synthesis.74 The mesoporous silica materials were first synthesized as shown in

Scheme 1.2, then incorporated with the transition metals (Ti or V). Knecht et al

have also conducted a number of extensive studies on the use of dendrimers as a

template to control the growth of the silica nanospheres.75-77

Scheme 1.2: Pathway for the formation of dendrimer-templated mesoporous materials.74

1.5 Synthesis of dendrimer encapsulated nanoparticles.

Dendrimer encapsulated metal nanoparticles (DENs) can be synthesized by

the addition of metal ions to a dendrimer solution. Under favourable conditions, the

metal ions partition into the dendrimer interior where they coordinate strongly to the

tertiary amine groups of the dendrimer. Reduction of the dendrimer/metal-ion

composite with an excess of a chemical reducing agent such as sodium borohydride

(NaBH4) results in the formation of nearly monodispersed, zero-valent intra-

dendrimer metal NPs (Figure 1.13). The colour change in the solution after the

addition of the reducing agent confirms the reduction of metal ions to NPs and is also

accompanied by a formation of a new absorbance band in the UV-vis spectrum.

Although other types of dendrimer templates such as polyphenylene and

polyethylenegylcol can be used, two families of dendrimers are commonly used for

this purpose, namely: poly(amidoamine) (PAMAM) and poly(propylene imine) (PPI)

dendrimers. An important difference between these dendrimers is that the PPI

dendrimers are stable at high temperature (~ 400 oC) whereas the PAMAM

dendrimers undergo retro-Michael addition at temperature above ~ 100 oC.27 In the

case where the PAMAM-NH2 dendrimers are used, the metal ion can also coordinate

Mesoporous silicate

560 o C

Calcination

HCI, 70 o C

“Golf ball-like particle” Dendrimer

TEOS, MeOH

20

to the primary amine on the periphery of the dendrimer thereby forming inter-

dendrimer complexes with the dendrimer. The possibility of particle agglomeration is

very high when trying to form the inter-dendrimer NPs using amine-terminated

dendrimers. However this can be prevented by selective protonation of the primary

amine groups under mild acidic conditions or by functionalization of the terminal

amine groups.

Mn+

BH4-

DENs

Product Reactant

Dendrimer

Figure 1.13: Synthesis of dendrimer encapsulated metal nanoparticles (DENs) as well their

application to catalysis.11

Since the first published work on the synthesis of DENs by Crooks and co-

workers in 1998,5 there has been an increase in the number of reports on the

synthesis and application of dendrimer encapsulated monometallic,78 bimetallic79

and semiconductor80 NPs.

In their study, Crooks and co-workers used the hydroxyl-terminated PAMAM

(G4 PAMAM-OH) dendrimer to synthesize Cu-DENs with a diameter of less than 1.8

nm. The pH of the dendrimer solution was adjusted to weakly acidic before the

addition of the Cu2+ ions for complexation to occur. The amount of Cu2+ ions to be

extracted into the dendrimer was determined by spectrophotometric titrations, by

21

monitoring the absorbance at λ 605 nm. They found that each G4-PAMAM-OH

dendrimer absorbs 16 Cu2+ ions. These intra-dendrimer Cu NPs were extremely

stable despite their small size. However NPs formed in the presence of G4 PAMAM-

OH or G6 PAMAM-OH and with a Cu2+ loading less than the maximum threshold

values were found to be stable for at least a week. Balogh et al also reported a

related study using PAMAM dendrimers with different terminal groups.81 The Cu-

DENs synthesized were found to be stable after 90 days of being stored at room

temperature in the absence of oxygen. It must, however, be mentioned that metal

ions partition into the dendrimer because of strong ionic or covalent interactions with

the interior amines. In cases where the metal ions do not have high affinity for amine

coordination, an alternative strategy can be used as demonstrated by Zhao et al.27

Using this approach, the anticipated metal NPs can be prepared by a displacement

reaction. For example, Cu55 NPs within G6 PAMAM-OH dendrimer can be prepared

by direct reduction of the corresponding Cu2+ containing dendrimer to give Cu NPs

(G6-OH (Cu55)). If this prepared G6-OH (Cu55) is exposed to a solution containing

ions of the metal with higher standard electrode potential than Cu, the Cu is

displaced and the ions of the metal with higher standard electrode potential get

reduced (Figure 1.15).27 This reaction is fast and goes to completion, and the

resulting particles are stable.

This method has been used to successfully prepare metal NPs such as Ag

and Au from their respective precursor ions, Ag+ and Au3+ respectively, which do not

form strong complexes with the interior amine groups of the PAMAM-OH dendrimer

to form a metal-dendrimer complex. Bimetallic DENs have also been synthesized

using the co-complexation, sequential loading (Figure 1.14), as well as the

displacement reaction method (Figure 1.15). A number of studies on the preparation

of different monometallic and bi-metallic NPs within the dendrimer template via direct

reduction, partial displacement reaction, co-complexation and sequential methods

have been published thus far and a few examples are discussed here.

22

MAn+ + MB

m+Reduction

MAn+ Reduction

Bimetallic DENs

MBm+

ReductionGn-OH

Gn-OH

Figure 1.14: Synthesis of bimetallic DENs by two different methods.11

G 6 -O H G 6 -O H (C u 2 + )n

G 6 -O H (C u n )

G 6 -O H (A g ) 2 n G 6 -O H (A u 0 .6 7 n ,P tn ,P d n )

G 6 -O H (A u 0 .6 7 n ,P tn ,P d n )

C u 2 +

R e d u c in g A g e n t

A u 3 + ,P t2 + o r P d 2 +A g +

A u 3 + ,P t 2 + o r P d 2 +

C u + 2 A g + = 2 A g + C u 2 +

Reaction E0, V vs. NHE

Cu2+ + 2e = Cu 0.340

Ag+ + e = Ag 0.780

Pd2+ + 2e = Pd 0.830

Pt2+ + 2e = Pt

Au3+ + 3e = Au

1.2

1.5

Figure 1.15: Metal DENs synthesis by multiple, in situ displacement.27

23

Manna et al reported the intra-dendrimer synthesis of Ag and Au DENs via

direct reduction in a polar medium using G4 PAMAM-NH2 dendrimer and using

methanol as a solvent.82 After characterization of the synthesized NPs using

transmission electron microscopy (TEM), they found that the average particle sizes

of Ag (6.2 ± 1.7 – 12.2 ± 2.9) was larger than those of Au (3.2 ± 0.7 – 7.3 ± 1.5). The

average particle sizes were dependent on the metal ion-to-dendrimer ratio (M:D).

The particle size tends to decrease at low M:D ratio for both Ag and Au and a narrow

size distribution was also achieved at low M:D ratio. Stable metallic NPs of metals

such as Pt83 and Pd84 within the dendrimer have also been prepared using this

procedure.

Bimetallic NPs have been received greater interest than their monometallic

counterparts. This is because bimetallic systems have the ability to be fine-tuned to

give enhanced catalytic activity not achieved by monometallic systems.85 Chung et al

reported the synthesis of stable Pt-Pd bimetallic DENs using G4 PAMAM-OH

dendrimers by co-complexation of these two different metal ions.86 These bimetallic

DENs had an average size of ~2.3 ± 0.22 nm. Similarly, Scott et al also reported the

preparation of even smaller Pd-Pt dendrimer encapsulated catalysts with an average

diameter of 1.9 ± 0.4 nm.79 Both the former and the latter NPs were shown to be

catalytically active in hydrogenation reactions. Synthesis of other bimetallic DENs

catalysts such as Pd-Au,87 Pt-Au,88, 89 and Au-Ni90 have also been reported.

1.6. Supported dendrimer encapsulated nanoparticles.

Immobilization of NPs on solid supports is an important step in the preparation

of practical heterogeneous catalysts. Supported NPs resemble more closely the

catalysts used in real-world applications, and they often show catalytic properties

that are normally not observed for unsupported NPs. The use of these

heterogeneous catalysts in the chemical industry is certainly increasing as

environmental and economic pressures drive the movement towards clean, cost-

effective and selective chemical processes. This could be vital to companies

involved in the development of the hydrogen economy.91 The preparation of

supported dendrimer encapsulated NPs is briefly discussed.

24

1.6.1. Synthesis of supported dendrimer encapsulated nanoparticles .

Dendrimers used as template for the formation of NPs have surface groups

that provide a reactive handle for linking NPs to surfaces and other polymers. It is

therefore possible to use a dendrimer as a “vehicle” to transport NPs to solid

supports such as TiO2, SiO2, etc. For example, Crooks and Sun reported two

methods in which DENs can be immobilized onto solid supports.92 In the first method

(marked A in Figure 1.16), DENs are prepared in solution, and then chemisorbed

onto the surface of a solid support. In the second method (marked B in Figure 1.16),

dendrimers are first chemisorbed onto the surface of the solid support, and then the

DENs are prepared directly on the surface.

A B

substrate

DENs

Oxidative calcination

Surface Diffusion

substrate

Metal ion

Chemical Reduction

Figure 1.16: Immobilization of DENs onto solid oxide supports.92

These methods have potential advantages in that (a) the metal particles are

inherently small and monodispersed because the size of the particles is controlled by

the number of metal precursor ions coordinated to the dendrimer molecule and (b)

the location of immobilization may be controlled through specific interactions

between dendrimer and functional groups on the solid support. Incorporation of NPs

into the pre-existing solid support is often carried out via the wetness impregnation

25

method. Scott et aI. found that this method, however, could lead to a significant

increase in nanoparticle size after calcination of the dendrimer template.93 This is

contrary to the findings by Chandler and co-workers who showed that Pt could be

delivered to commercial silica support using Pt-DENs with better particle size

retention.94

Immobilization of DENs within a sol-gel matrix is another method for the

preparation of supported catalysts as described by Carpenter and co-workers.95

Scott et al used this method to synthesize titania supported Pd and Au catalysts.93

In this method, aqueous solutions of Pd and Au-DENs were added to titanium

isopropoxide to coprecipitate the DENs supported on TiO2. With this approach, both

the inside and the outside of the dendrimer act as a template. The exterior of the

dendrimer acts as a template for the pores within the sol-gel matrix, whereas the

inside of the dendrimer acts as a template for the metal NPs. The supported NPs

can be activated at high temperatures and thereby removing the dendrimer template

to leave the free metal NPs. However, in some cases, this calcination process tends

to poison the catalytic activity of the catalyst, presumably because at high

temperature, the organic dendrimer collapses onto the NPs, poisoning the metal

surface.94, 96 Therefore, the use of ideal activation conditions, enough to remove the

organic materials without causing particle agglomeration, is important for supported

DENs.

1.7. Extraction of metal nanoparticles from the dendrimer template.

A well-known and effective approach for the preparation of metal NPs

involves the reduction of metal ions in the presence of alkanethiols or other ligands

that complex to the growing metal NPs. Materials formed using these procedures are

referred to as monolayer-protected clusters (MPCs). These materials have many

attributes in that they can be repeatedly isolated from and redissolved in organic

solvents without irreversible aggregation or decomposition,97 their surface can be

functionalized with a vast range of modifiers98, 99 and they can be linked to polymers,

bio-molecules and monolith surfaces.100-102 Since some supported DENs are inactive

catalysts, these MPCs materials can offer a better alternative for the preparation of

active supported catalysts. The disadvantage of using this approach to synthesize

26

MPCs is that it produces NPs with a polydispersed size distribution.100, 103-105 In trying

to solve this problem, Garcia-Martinez and Crooks came up with a strategy to

convert prepared nearly monodispersed DENs to MPCs.106 In this method, aqueous

DENs are first synthesized, and then the NPs are extracted from within the

dendrimer into organic solution using long chain alkyl thiols. One of the advantages

of this method is that the dendrimer can be recycled and used again as template for

the synthesis of other metal NPs as shown in Figure 1.17.

Figure 1.17: Extraction of NPs from the interior of the dendrimer to the organic phase.107

Typically, a toluene solution containing an n-alkanethiol is added to the

aqueous Au-DENs solution and the two-phase system is shaken. The n-alkanethiol

presumably self-assembles onto the surface of the DENs, thereby extracting the NPs

from the dendrimers, and transports them to the toluene phase. After extraction,

characterization of the two phase system was done using UV-vis, FT-IR, and NMR

spectroscopy. It was determined that the dendrimer remains in the aqueous phase

after extraction and could therefore be recycled and used as a template to

synthesize additional metal particles. The high resolution transmission microscope

(HRTEM) results also showed that metal nanoparticle extraction does not affect the

size distribution of the particles or their optical properties (see Figure 1.18). This

result could be contrasted to the earlier reported work, which showed that it was

27

possible to extract both the dendrimer and the encapsulated NPs into toluene phase

upon the addition of an appropriate surfactant.29

Figure 1.18: HRTEM images and particle-size distribution for MPC-12(Au55) extracted from G4-

PAMAM-OH dendrimer using the following HSC12/Au mole ratios: (a) 1, (b) 3.106

Although many reports published so far have focused on Au,106, 108-110 the

extraction of other metal NPs such as Pd109, 111 and Ag110 have also been reported in

the literature. The extraction of Au-Ag DEN bimetallic NPs have also been published

by the same group in 2005 and similar results to the extraction of monometallic

DENs were obtained.112

1.8. Application of DENs and supported DENs in catalysis.

The application of catalysis has a significant role in that: (a) It is of paramount

importance in the chemical industry since the production of most industrially

important chemicals involves the use of catalysts. (b) Catalytic, rather than

stoichiometric reactions are preferred in environmentally friendly reactions due to the

reduced amount of waste generated.

1.8.1. Application of DENs as homogeneous catalysts.

In the section to follow, few examples on the application of DENs and

supported DENs in both homogeneous and heterogeneous catalysis will be

discussed.

As mentioned earlier, a dendrimer does not fully passivate the metal

nanoparticle surface and hence create reaction pathways for the substrate to the

encapsulated catalytic NPs.30 This has made it possible for DENs to be used as

28

active catalysts. DENs are soluble in many organic solvents and they have been

used as homogeneous catalysts in different reaction media such as water,25 organic

solvents,29 fluorous biphasic solvents,113 and supercritical CO2.107 A few examples of

the application of DENs in homogeneous catalysis will be summarized here.

Pd-DENs have been given more attention for the use in homogeneous

catalysis as compared to other metal DENs. Suzuki coupling,114 Heck coupling115

and hydrogenation116 reactions are the most extensively studied using Pd-DENs as

catalysts. Crooks and co-workers reported the preparation of Pd-DENs within

different generations (G4, G6 and G8) of PAMAM-OH dendrimers.117 The catalytic

activity of these Pd-DENs was evaluated for the hydrogenation of alkenes in a

methanol-water mixed solvent system. In this study they found that for the same

substrate the turnover frequency decreases as the dendrimer generation increases.

They also observed a decrease in turnover frequency for a particular dendrimer

generation as the substrates increase in size. These findings were attributed to the

fact that as the dendrimer generation increases, the surface becomes more crowded

and this in turn could limit the access of the substrate to the encapsulated NPs.

Ooe et al also used Pd-DENs prepared within different generations (G3-G5)

of triethoxybenzamide-terminated PPI dendrimers functionalized on the surface with

triethoxybenzoic acid chloride (TEBA).116 The catalytic activity of these Pd-DENs

was examined for the hydrogenation of different alkenes. The catalytic results

revealed that, as the dendrimer generation increases, the hydrogenation rate tends

to decrease for all ten different olefin substrates used in the study. These findings

are consistent with the work reported by Crooks et al.117However, in this study, the

decrease in the reaction rate as the ring size of the substrate increases was only

clearly observed when using Pd-DENs, prepared within G5 dendrimers. These

catalysts showed high selectivity towards cyclooctadiene amongst other alkenes

studied, giving a hydrogenation product of cyclooctene in 99 % yield. This was not

the case when carbon supported Pd catalyst (Pd/C) is used for the same reaction. In

case of Pd/C, cyclooctene is formed with 19 % of cylooctane as a side product and

hence low selectivity.

Esumi et al investigated the catalytic activity of Ag, Pt, and Pd DENs

prepared using different generations of aqueous solutions of both PAMAM and PPI

29

dendrimers.118 G3, G4, and G5 PAMAM dendrimers with amino surface groups were

used as well as G2, G3, and G4 PPI dendrimers with amino surface groups. The

catalytic activity of these dendrimer DENs were evaluated in the reduction of 4-NP. It

was found that the rate of reaction tends to decrease as the dendrimer concentration

increases for both the PPI and PAMAM dendrimers. Unlike in some other previously

reported studies, there was no distinct correlation observed between the rate

constant and the generation of both the PAMAM and the PPI dendrimers. However,

the rate constants for Pd-DENs prepared within PPI (G3 and G4) dendrimers were

found to be greater than those of Pt NPs prepared within the corresponding PAMAM

(G3, G4, and G5) at the same dendrimer concentration. For both PAMAM and PPI

dendrimers, Pd was shown to be more catalytically active than Ag and Pt. Ag-DENs

were less catalytically active than all three metals studied. This was because Ag-

DENs are often easily oxidized and their particle sizes were larger than other metal

NPs.

Chung et al reported the synthesis of Pt-Pd bimetallic DENs within the cavities

of G4 PAMAM-OH dendrimer using the co-complexation method.119 The catalytic

activity of these catalysts was also evaluated in the hydrogenation of cyclooctadiene.

These catalysts were found to have both higher activity as well as selectivity, than

what was observed when monometallic Pd-DENs were used as catalysts for the

same reaction.116

1.8.2. Application of supported DENs to heterogeneous catalysis .

DENs can be immobilized onto high surface area oxide supports and then

used as heterogeneous catalysts. The use of supported NPs as heterogeneous

catalysts has proved to be promising towards achieving clean, selective chemical

processes.120 As discussed in the previous sections, these catalysts can be prepared

via different routes. Firstly, DENs are prepared, and then immobilized onto the

support, followed by the activation and the removal of the dendrimer at high

temperature. Another route that is also used involves the preparation of DENs,

followed by the extraction of the NPs from the dendrimer using suitable thiols and

depositing them on the support before removing the thiols by calcining at high

temperature. In this section, the application of both monometallic and bimetallic

supported NPs in heterogeneous catalysis will be discussed.

30

Lang et al prepared silica supported Pt catalysts (Pt50/SiO2 and Pt100/SiO2)

from Pt-DENs precursor.94 Activation and dendrimer removal from these catalysts

yielded inactive catalysts. These catalysts were made active by removing the

dendrimer template in two steps which involved oxidation with O2 at 300 oC for 4

hours, followed by reduction with H2 at 300 oC for 2 hours. The activity of these

catalysts was investigated for the oxidation of CO as well as the hydrogenation of

toluene. These catalysts were found to be more catalytically active than the

traditionally prepared supported Pt catalyst (Pt/SiO2) in both reactions studied.

However, Pt50/SiO2 was more catalytically active than both Pt100/SiO2 and Pt/SiO2

catalysts.

Huang et al reported the preparation of Rh and Pt NPs synthesized within

the G4 PAMAM-OH dendrimers, followed by their immobilization onto a high surface

area mesoporous silica support.121 The catalytic behaviour of these catalysts

(Rh30/SBA-15 and Pt20/SBA-15) was studied in the hydrogenation of ethylene and

pyrrole. The dendrimer template was not removed here, as in some other related

studies. Both these catalysts were found to be active for all reactions studied. The

reaction rates on the hydrogenation of ethylene at low temperature for both

Rh30/SBA-15 and Pt20/SBA-15 were determined to be 0.6 s-1 and 0.5 s-1 respectively.

When these catalysts are treated at 423 K, the increase in reaction rate was

observed for both catalysts. Pt20/SBA-15 showed the greatest increase of 3.7 s-1 as

compared to 0.8 s-1 of Rh30/SBA-15. It was assumed that, at 423 K, the dendrimer

partially decomposes and this partial dendrimer decomposition resulted in the

increase of the NPs active sites and hence the increase in catalyst activity. However,

as the temperature was increased further, both catalysts showed a decrease in

activity until a temperature of 673 K. This could be due to the fact that, at high

temperature, the supported catalyst undergoes sintering which in turn decreases the

number of active sites. Based on these results, the maximum temperature of 423 K

was maintained for the hydrogenation of pyrrole. In this reaction, Rh catalysts

displayed higher catalytic activity than the Pt catalysts.

Lang et al also reported the synthesis and the catalytic activity of dendrimer

templated bimetallic Pt-Au NPs supported onto a silica surface.89 These catalysts

were thermally activated at 300 oC to remove the dendrimer before their catalytic

activity could be studied. Although Au32/SiO2 has previously been reported to be an

31

inactive catalyst, these bimetallic catalysts proved to be catalytically active for CO

oxidation within the temperature range of 30 oC to 160 oC. The catalytic activity on

CO oxidation of this bimetallic catalyst between 30 oC to 80 oC is attributed to either

Au or Pt-Au bimetallic active sites on the catalyst. The behaviour of this bimetallic Pt-

Au catalyst at temperatures of 120 oC or higher was the same as monometallic Pt

catalysts. It is assumed that Pt atoms are very active at these high temperatures.

Therefore the behaviour of this bimetallic Pt-Au catalyst can be described as one in

which Au sites are active at low temperatures and Pt sites are active at high

temperatures, when CO is less strongly bound to the surface. Recently, Chandler

and co-workers undertook a related study with the focus on the effect of the support

on catalysis.88

1.9. Aim and objectives of the project.

The objectives of this study include the following:

The synthesis of dendrimer-encapsulated metal NPs (Cu-DENs, Ag-DENs

and Au-DENs). The catalytic activity of these synthesized DENs will be evaluated in

the reduction of 4-NP. The effect of dendrimer generation on the activity and

selectivity of these NPs will be investigated. The synthesized Au-DENs will then be

subsequently immobilized onto a titania support via wetness impregnation and sol-

gel methods. These NPs will be characterized using UV-vis spectroscopy,

transmission electron microscopy (TEM), powder X-ray diffraction (PXRD), thermal

gravimetrical analysis (TGA), and physisorption studies. The extent of Au loading

onto the titania will be determined by inductively coupled plasma-optical emission

spectroscopy (ICP-OES). Supported Au NPs are known to be good catalysts for

oxidation reactions (especially CO oxidation).122 Therefore, the catalytic activity of

the titania-supported Au NPs will be further investigated in the oxidation of styrene.

The main aim of this project was therefore to synthesize metal (Cu, Ag, and

Au) NPs using PAMAM-OH and PAMAM-NH2 dendrimers as templates to provide

stability as well as avoiding particles agglomeration, and then investigate their

application in catalysis.

32

1.10. References.

1. H. D. Gesser and P. C. Goswami, Chem. Rev., 1989, 89, 765.

2. M. G. Warner, S. M. Reed and J. E. Hutchison, Chem. Mater., 2000, 12,

3316.

3. F. Mafune, J. Kohno, Y. Takeda, T. Kondow and H. Sawabe, J. Phys. Chem.

B, 2000, 104, 9111.

4. Y. Cheng, L. Yin, S. Lin, M. Wiesner, E. Bernhardt and J. Liu, J. Phys. Chem.

C, 2011, 115, 4425.

5. M. Zhao, L. Sun and R. M. Crooks, J. Am. Chem. Soc., 1998, 120, 4877.

6. R. Pool, Science, 1990, 248, 1186.

7. Y. Tan, X. Dai, Y. Li and D. Zhu, J. Mater. Chem., 2003, 13, 1069.

8. M. Boutonnet, J. Kizling, P. Stenius and G. Maire, Colloids sur., 1982, 5, 209.

9. P. Barnickel, A. Wokaun, W. Sager and H.-F. Eicke, J. Colloid Interface Sci.,

1992, 148, 80.

10. E. E. Foos, R. M. Stroud, A. D. Berry, A. W. Snow and J. P. Armistead, J. Am.

Chem. Soc., 2000, 122, 7114.

11. R. M. Crooks, M. Zhao, L. Sun, V. Chechik and L. K. Yeung, Acc. Chem.

Res., 2001, 34, 181.

12. T. Yonezawa, S. Onoue and N. Kimizuka, Langmuir, 2000, 16, 5218.

13. I. Pastoriza-Santos and L. M. Liz-Marzán, Langmuir, 1999, 15, 948.

14. M. T. Reetz and W. Helbig, J. Am. Chem. Soc., 1994, 116, 7401.

15. B. V. Enustun and J. Turkevish, J. Am. Chem. Soc., 1963, 85, 3317.

16. T. Yonezawa, M. Sutoh and T. Kunitake, Chem. Lett., 1997, 619.

17. M. Han, C. H. Quek, W. Huang and C. Chew, Chem. Mater., 1999, 11, 1144.

18. P. Ekwall, L. Mandell and P. Solyom, J. Colloid Interface Sci., 1970, 35, 266.

19. F. Chen, G. Xu and T. S. A. Hor, Mater. Lett., 2003, 57, 3282.

20. D. Chen, S. Liu, J. Li, N. Zhao, C. Shi, X. Du and J. Sheng, J. Alloys Compd.,

2009, 475, 494.

21. N. Semagina, A. Renken, D. Laub and L. Kiwi-Minsker, J. Catal., 2007, 246,

308.

22. D. A. Tomalia, A. M. Naylor and W. A. Goddard. III, Chem. Int. Ed. Engl.,

1990, 29, 138.

33

23. J. F. G. A. Jansen, E. M. M. d. B.-v. d. Berg and E. W. Meijer, Science, 1994,

266, 1226.

24. O. A. Platonova, L. M. Bronstein, S. P. Solodovnikov, I. M. Yanovskaya, E. S.

Obolonkova, P. M. Valestky, E. Wenz and M. Antonietti, Colloid Polym. Sci.,

1997, 275, 426.

25. M. Zhao and R. M. Crooks, Angew. Chem. Int. Ed., 1999, 38, 364.

26. M. Zhao and R. M. Crooks, Adv. Mater., 1999, 11, 217.

27. M. Zhao, Y. Liu and R. M. Crooks, J. Am. Chem. Soc., 1999, 121, 923.

28. L. K. Yeung and R. M. Crooks, Nano. Lett., 2001, 1, 14.

29. V. Chechik, M. Zhao and R. M. Crooks, J. Am. Chem. Soc., 1999, 121, 4910.

30. M. Zhao, H. Tokuhisa and R. M. Crooks, Angew. Chem. Int. Engl., 1997, 36,

2596.

31. M. Wells and R. M. Crooks, J. Am. Chem. Soc., 1996, 118, 3988.

32. G. R. Newkome, C. N. Moorefield and F. Vögtle, Dendritic molecules:

Concepts, Synthesis, Perspectives, Wiley-VCH, Weinheim,Germany, 1996.

33. D. A. Tomalia, H. Baker, J. R. Dewald, M. Halls, G. Kallos, S. Martin, J.

Roeck, J. Ryder and P. Smith, Macromolecules, 1986, 19, 2466.

34. E. W. Buhleier, W. Wehner and F. Vögtle, Synthesis, 1978, 155.

35. D. A. Tomalia, H. Baker, J. R. Dewald, M. Halls, G. Kallos, S. Martin, J.

Roeck, J. Ryder and P. Smith, Polym. J. , 1985, 17, 117.

36. G. R. Newkome, Z.-Q. Yao, G. R. Baker and K. Gupta, J. Org. Chem., 1985,

50, 2003.

37. C. J. Hawker and J. M. J. Fréchet, J. Am. Chem. Soc., 1990, 112, 7638

38. R. Mülhaupt and C. Wörner, Angew. Chem. Int. Ed. Engl., 1993, 32, 1306.

39. A. B. Padias, J. H. K. Halls, D. A. Tomalia and J. R. McConnell, J. Org.

Chem., 1987, 52, 5305.

40. J. Peterson, A. Ebber, V. Allikmaa and M. Lopp, Proc. Estonian Acad. Sci.

Chem., 2001, 50.

41. J. Ruiz, G. Lafuente, S. Marcen, C. Ornelas, S. Lazare, E. Cloutet, J.-C. Blais

and D. Astruc, J. Am. Chem. Soc., 2003, 125, 7250.

42. P.-G. d. Gennes and H. Hervet, J. Phys. Lett., 1983, 44, L-351.

43. C. Ornelas, J. R. Aranzaes, E. Cloutet and D. Astruc, Org. Lett., 2006, 8,

2751.

34

44. C. J. Hawker and J. M. J. Fréchet, J. Am. Chem. Soc. , Chem. Commun. ,

1990, 1010.

45. Y.-H. Liao and J. R. Moss, Organometallics, 1995, 14, 2130.

46. Y.-H. Liao and J. R. Moss, Chem. Commun., 1993, 23, 1774.

47. Y.-H. Liao and J. R. Moss, Organometallics, 1996, 15, 4307.

48. M. A. Hearshaw, A. T. Hutton, J. R. Moss and K. J. Naidoo, Adv. Dendri.

Macromol., 1999, 4, 1.

49. J. S. Moore and Z. Xu, Macromolecules, 1991, 24, 5893.

50. T. Kawaguchi, K. L. Walker, C. L. Wilkins and J. S. Moore, J. Am. Chem.

Soc., 1995, 117, 2159.

51. J. S. Moore, Acc. Chem. Res., 1997, 30, 402.

52. M. Okazaki, I. Washio, Y. Shibasaki and M. Ueda, J. Am. Chem. Soc., 2003,

125, 8120.

53. M. Ueda, S. Aoyama, M. Konno and Y. Imai, Makromol. Chem., 1978, 179,

2089.

54. Y. Imai, S. Aoyama, T. Q. Nguyen and M. Ueda, Makromol. Chem. Rapid

Commun., 1980, 1, 655.

55. I. Washio, Y. Shibasaki and M. Ueda, Macromolecules, 2005, 38, 2237.

56. A. K. Patri, I. J. Majoros and J. R. B. Jr, Curr. Opin. Chem. Biol., 2002, 6, 466.

57. C. Kojima, K. Kono, K. Maruyama and T. Takagishi, Bioconjugate Chem,

2000, 11, 910.

58. A. Adrovon and J. M. J. Fréchet, Chem. Commun., 2000, 18, 1701.

59. S. F. Swallen, Z. G. Zhu, J. S. Moore and R. Kopelman, J. Phys. Chem. B,

2000, 104, 3988.

60. P. Busson, H. Ihre and A. Hult, J. Am. Chem. Soc., 1998, 120, 9070.

61. M. W. P. L. Baars and E. W. Meijer, Top. Curr. Chem., 2000, 210, 131.

62. J. M. J. Fréchet, Proc. Natl. Acad. Sci. USA, 2002, 99, 4782.

63. D. Astruc and F. Chardac, Chem. Rev., 2001, 101, 2991.

64. R. v. Heerbeek, P. C. J. Kamer, P. W. N. M. v. Leeuwen and J. N. H. Reek,

Chem. Rev., 2002, 102, 3717.

65. R. B. Bedford, U. G. Singh, R. I. Walton, R. T. Williams and S. A. Davis,

Chem. Mater., 2005, 17, 701.

66. Z. Yang, Q. Kang, H. M. C. Li and Z. Lei, J. Mol. Cat. A: Chem., 2004, 213,

169.

35

67. L. Ropartz, R. E. Morris, G. P. Schwarz, D. F. Foster and D. J. Cole-Hamilton,

Inorg. Chem. Commun., 2000, 3, 714.

68. C. Köllner and A. Togni, Can. J. Chem., 2002, 79, 1762.

69. Z. Zheng, J. Chen and Y. Li, J. Organomet. Chem., 2004, 689, 3040.

70. T. R. Krishna and N. Jayaraman, Tetrahedron, 2004, 60, 10325.

71. J. W. Kriesel and T. D. Tilley, Chem. Mater., 1999, 11, 1190.

72. G. Larsen, E. Lotero and M. Marquez, Chem. Mater., 2000, 12, 1513.

73. G. Larsen and E. Lotero, J. Phys. Chem. B, 2000, 104, 4840.

74. M. C. Rogers, B. Adisa and D. A. Bruce, Catal. Lett., 2004, 98, 29.

75. M. R. Knecht and D. W. Wright, Chem. Mater., 2004, 16.

76. M. R. Knecht and D. W. wright, Langmuir, 2004, 20, 4728.

77. M. R. Knecht, S. L. Sewell and D. W. Wright, Langmuir, 2005, 21, 2058.

78. F. Gröhn, B. J. Bauer, Y. A. Akpalu, C. L. Jackson and E. J. Amis,

Macromolecules, 2000, 33, 6042.

79. R. W. J. Scott, A. K. Datye and R. M. Crooks, J. Am. Chem. Soc., 2003, 125,

3708.

80. B. I. L. III and R. M. Crooks, J. Am. Chem. Soc., 2000, 114, 12886.

81. L. Balogh and D. A. Tomalia, J. Am. Chem. Soc., 1998, 120, 7355.

82. A. Manna, T. Imae, K. Aoi, M. Okada and T. Yogo, Chem. Mater., 2001, 13,

1674.

83. D. Yamamoto, S. Watanabe and M. T. Miyahara, Langmuir, 2010, 26, 2339.

84. R. W. J. Scott, H. Heechang, R. R. Henriquez and R. M. Crooks, Chem.

Mater., 2003, 15, 3873.

85. J. H. Sinfelt, Acc. Chem. Res., 1987, 20, 134.

86. Y.-M. Chung and H.-K. Rhee, Catal. Lett., 2003, 85, 159.

87. R. W. J. Scott, O. M. Wilson, S.-k. Oh, E. A. Kenik and R. M. Crooks, J. Am.

Chem. Soc., 2004, 126, 15583.

88. B. J. Auten, H. Lang and B. D. Chandler, Appl. Catal. B: Environ., 2008, 81,

225.

89. H. Lang, S. Maldonado, K. J. Stevenson and B. D. Chandler, J. Am. Chem.

Soc., 2004, 126, 12949.

90. B. D. Chandler, C. G. Long, J. D. Gilbertson, C. J. Pursell and G.

Vijayaraghavan, J. Phys. Chem. C, 2010, 114, 11498.

91. C. S. Song, Catal. Today, 2002, 77, 17.

36

92. L. Sun and R. M. Crooks, Langmuir, 2002, 18, 8231.

93. R. W. J. Scott, O. M. Wilson and R. M. Crooks, Chem. Mater., 2004, 16, 5682.

94. H. Lang, R. A. May, B. L. Iversen and B. D. Chandler, J. Am. Chem. Soc.,

2003, 125, 14832.

95. J. P. Carpenter, C. M. Lukehart, S. R. Stock and J. E. Wittig, Chem. Mater.,

1995, 7, 201.

96. D. X. Liu, J. X. Gao, C. J. Murphy and C. T. Williams, J. Phys. Chem. B, 2003,

108, 12911.

97. M. J. Hostetler, J. E. Wingate, C.-Z. Zhong, J. E. Harris, R. W. Vachet, M. R.

Clark, J. D. Londono, S. J. Green, J. J. Stokes, G. D.Wignall, G. L. Glish, M.

D. Porter, N. D. Evans and R. W. Murray, Langmuir, 1998, 14, 17.

98. A. C. Templeton, M. J. Hostetler, C. T. Craft and R. W. Murray, J. Am. Chem.

Soc., 1998, 120, 1906.

99. A. C. Templeton, D. E. Cliffel and R. W. Murray, J. Am. Chem. Soc., 1999,

121, 7081.

100. R. L. Whetten, M. N. Shafigullin, J. T. Khoury, T. G. Schaaff, I. Vezmar, M. M.

Alvarez and A. Wilkinson, Acc. Chem. Res., 1999, 32, 397.

101. T. Pradeep and N. Sandhyarani, Pure Appl. Chem., 2002, 74, 1593.

102. M. Brust and C. J. Kiely, Colloids Surf., A, 2002, 202, 175.

103. A. C. Templeton, W. P. Wuelfing and R. W. Murray, Acc. Chem. Res., 2000,

33, 27.

104. R. L. Whetten, J. T. Khoury, M. M. Alvarez, S. Murthy, I. Vezmar, Z. L. Wang,

P. W. Stephen, C. L. Cleveland, W. D. Luedtke and U. Landman, Adv. Mater.,

1996, 8, 428.

105. S. Chen and R. W. Murray, Langmuir, 1999, 15, 682.

106. J. C. Garcia-Martinez and R. M. Crooks, J. Am. Chem. Soc., 2004, 126,

16170.

107. R. W. J. Scott, O. M. Wilson and R. M. Crooks, J. Phys. Chem. B, 2005, 109,

692.

108. R. J. Korkosz, J. D. Gilbertson, K. S. Prasika and B. D. Chandler, Catal.

Today, 2007, 122, 370.

109. Y.-G. Kim, J. C. Garcia-Martinez and R. M. Crooks, Langmuir, 2005, 21,

5485.

37

110. O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez and R. M. Crooks, Chem.

Mater., 2004, 16, 4202.

111. J. C. Garcia-Martinez, R. W. J. Scott and R. M. Crooks, J. Am. Chem. Soc.,

2003, 125, 11190

112. O. M. Wilson, R. W. J. Scott, J. C. Garcia-Martinez and R. M. Crooks, J. Am.

Chem. Soc., 2005, 127, 1015.

113. V. Chechik and R. M. Crooks, J. Am. Chem. Soc., 2000, 122, 1243.

114. J. Lemo, K. Hauzé and D. Astruc, Inorg. Chem. Acta, 2006, 359, 4909.

115. E. H. Rahim, F. S. Kamounah, J. Frederiksen and J. B. Christensen, Nano.

Lett., 2001, 1, 499.

116. M. Ooe, M. Murata, T. Mizugaki, K. Ebitani and K. Kaneda, Nano. Lett., 2002,

2, 999.

117. Y. Niu, L. K. Yeung and R. M. Crooks, J. Am. Chem. Soc., 2001, 123, 6840.

118. K. Esumi, R. Isono and T. Yoshimura, Langmuir, 2004, 20, 237.

119. Y.-M. Chung and H.-K. Rhee, Catal. Lett, 2003, 85, 159.

120. R. J. Chimentão, I. Kirm, F. Medina, X. Rodríguez, Y. Cesteros, P. Salagre

and J. E. Sueiras, Chem. Commun., 2004, 846.

121. W. Huang, J. N. Kuhn, C.-K. Tsung, Y. Zhang, S. E. Habas, P. Yang and G.

A. Somorjai, Nano. lett., 2008, 8, 2027.

122. R. M. T. Sanchez, A. Ueda, K. Tanaka and M. Haruta, J. Catal., 1997, 168,

125.

38

Chapter 2: Synthesis and characterization of Cu, Ag and Au-DENs.

2.1. Introduction.

In this chapter, the synthesis and characterization of Cu, Ag, and Au-DENs

will be described. Characterization of these aqueous DENs was performed by UV-vis

spectroscopy and high resolution transmission electron microscopy (HRTEM).

Since the first report on the template synthesis approach for the preparation of

metal nanoparticles (NPs) within dendrimers by Crooks and co-workers in 1998,1

there has been an increase of interest in the application of this strategy. In their

study, Crooks and collaborators used the G4 PAMAM-OH dendrimer as templating

agent for the preparation of Cu NPs. The maximum metal loading capacity of the

dendrimer was determined by spectrophotometric titration and the formation of Cu

NPs was confirmed by UV-vis spectroscopy and TEM. The Cu2+ ions show a broad,

weak absorption band at λ 810 nm in the UV-vis spectrum. In the presence of

dendrimers, this band shifts to λ 605 nm, with the formation of an additional strong

band around λ 300 nm. The formation of this new absorption band at λ 300 nm is

attributed to ligand-to-metal-charge-transfer (LMCT). Reduction of Cu2+ ions within

the dendrimer, using an excess of NaBH4 results in the disappearance of this strong

absorption at λ 300 nm. The disappearance of this band around λ 300 nm can be

used as an indication for the formation of Cu-NPs. TEM results indicated the

presence of Cu-NPs having a diameter of less than 1.8 nm, smaller than the

diameter of the G4 PAMAM-OH dendrimers, which is calculated as 4.5 nm.2 The

above observation in size differences between Cu-NPs and the dendrimer, along

with the fact that these NPs are stable in an oxygen-free solution, provides evidence

that the NPs i.e. Cu-DENs are formed inside the dendrimer.

In the same year, Balogh and Tomalia also reported the preparation of both

aqueous and methanol solutions of Cu-DENs.3 Their work entailed the use of

surface modified G4 PAMAM-OH, G4 PAMAM-NH2, and G5 PAMAM-CH3

dendrimers as templating agents. In the absence of oxygen, these Cu-DENs were

found to be stable for more than 90 days. Recently, the Crooks’s research group

also reported the synthesis of Cu-DENs using G4 PAMAM-OH.4 Although no TEM

39

results were obtained for these Cu-DENs, there was a lack of a plasmon peak at 570

nm after the formation of Cu-DENs, suggesting that Cu-DENs of diameter less than

5 nm are formed.5 These Cu-DENs were found to be catalytically active in the

reduction of 4-nitrophenol.

It should be noted that the most important step in the synthesis of metal DENs

is the coordination between the metal ions and the tertiary amine groups of the

dendrimer. For some metal ions, e.g. Ag+, this coordination to dendrimer such as

PAMAM-OH proved to be difficult. However, Ag+ and other metal ions such as Au3+

have been reported to coordinate well with PAMAM-NH2 dendrimers.6 However, this

coordination with the PAMAM-NH2 dendrimers, does not occur in a stoichiometric

ratio. Another challenge is that most of the metal ions might coordinate to the

primary amine groups rather than with the tertiary amine groups within the

dendrimer, which lead to the formation of dendrimer stabilized NPs rather than

DENs. Manna et aI reported the use of G4-PAMAM-NH2 dendrimers as a templating

agent for the synthesis of Ag NPs.7 The synthesis was carried out in a polar medium

of MeOH and water mixture. These Ag NPs were synthesized using different

concentration ranges of Ag+ ions (0.075 - 100 mM) as well as dendrimer/Ag+ ratios

(1:0.01 - 1:1). The average particle size range of (6.2 ± 1.7) – (12.2 ± 2.9) nm was

revealed by TEM and suggests that these NPs might not have formed inside the

dendrimer as the diameter of G4-PAMAM-NH2 is only 4.5 nm. This was attributed to

the fact that more of the Ag+ ions might have coordinated to the primary amines on

the pheriphey of the dendrimer, and this resulted in the formation of a inter-

dendrimer composite, which resulted in the formation of Ag NPs stabilized mostly by

the primary amino groups of the dendrimers. However, the possibility of initial NPs

formation inside the dendrimer, followed by diffusion and conglomeration should not

be ruled out. The very same research group also reported preparation of dendrimer

stabilized Ag NPs using different generations of PAMAM-NH2 and amine-terminated

PPI dendrimers.8

Chechik et al showed that adjusting the pH of the dendrimer solution to acidic

will protonate the primary amine at the periphery of the dendrimer, and thus promote

coordination of the metal ions to the internal tertiary amine groups.9

40

Crooks et al also developed a strategy to form DENs of weakly coordinating

metal ions within some dendrimers such as G4 PAMAM-OH.5 This indirect method of

DEN preparation basically entails a metal ion exchange with a preformed DEN

consisting of a different metal ion. Thus for example, to prepare Ag-DENs within

PAMAM-OH dendrimers, Cu-DENs are prepared first, followed by the addition of Ag+

ions. Because Ag+ is a stronger oxidizing agent than Cu2+, intradendrimer exchange

of the Cu atoms by Ag+ will take place. Cu-DENs will be oxidised to Cu2+ upon

exposure to Ag+ ions, and Ag+ ions get reduced to zero-valent Ag-DENs (see

Scheme 2.1). The result is the formation of a strong plasmon peak at λ 390 nm,

which is associated with the formation of Ag-DENs. Depending on the pH of the

solution in which the reaction is carried out, the Cu2+ ions resulting from the

displacement reaction may remain trapped within the dendrimer. The Ag-DENs

prepared this way were found to be stable for more than 60 days, even when not

stored under inert conditions.

C u D E N s + 2 A g + C u 2 + + 2 A g D E N s

Scheme 2.1: Indirect method for the preparation of Ag-DENs within PAMAM-OH dendrimers from Cu-

DENs precursor

Not all dendrimers show weak coordination with Ag+ ions, e.g. Esumi et aI.

showed that stable Ag-DENs can be prepared via direct reduction of the Ag+ ions if

the carboxyl-terminated G3.5 - G5.5 PAMAM dendrimers are used.6 It is assumed

that Ag+ ions are strongly adsorbed on the carboxyl-terminated dendrimers through

electrostatic attractive forces. The prepared Ag-DENs were found to have a high-

wavelength absorption, due to the dielectric properties of very small NPs (< 5 nm).

Although no size distribution or TEM images were shown in this report, only Ag-

DENs prepared at [dendrimer]/[Ag] molar ratio of 5:1 and less were detected by

TEM. Particles formed at [dendrimer]/[Ag+] molar ratio of 5:1 and above were too

small to be observed by TEM.

Dendrimers can also be used as a template for the preparation of stable Au

DENs as reported by Gröhn et al.,10 where they have used PAMAM-NH2 (G2-G4)

dendrimers. They found that stable Au-DENs can be formed if the concentration of

the dendrimer is low (dendrimer mass fraction < 1%). However, the resulting Au-

DENs show a wide size distribution. When a dendrimer mass fraction of 0.12 % or

lower is used, Au-DENs with a narrow size distribution were observed. The formation

41

of these uniform Au-DENs is also dependent on the rate of the addition of the

NaBH4.

Esumi et al also prepared Au-DENs stabilized by different generations (G3,

G4, and G5) of PAMAM-NH2 dendrimers. The synthesis of Au-DENs was monitored

by UV-vis spectroscopy. Before reduction, an aqueous gold ion solution shows a

strong absorption band at λ 220 nm in the UV-vis spectrum as well as a shoulder at λ

290 nm. The addition of a reducing agent (NaBH4) to the mixture results in the

reduction of Au3+ ions to zero-valent atoms. Upon the formation of Au-DENs the

absorbance peak of the aqueous Au ions at λ 220 nm disappears completely and

new bands at λ 280 nm as well as a broad band at λ 520 nm (attributed to the

plasmon band of Au NPs) are observed. The broad band at λ 520 nm is attributed to

a plasmon band of Au NPs. For all dendrimer generations used, the average NPs

diameter decreased as the [dendrimer]/[Au3+] ratio increases. The average diameter

of the NPs formed with G3 dendrimers was greater than those for G4 and G5 at the

same [dendrimer]/[Au3+] ratio but are still smaller than the diameter of the dendrimer

used. Contrary to this and other reports,6, 11 Crooks and co-workers showed that the

size of Au NPs does not depend on the generation of the dendrimers but rather on

the [dendrimer]/[Au3+] ratio.12 In this study, Crooks and co-workers used G4 and G6

PAMAM-NH2 dendrimers to synthesize narrowly distributed Au-DENs of 1-2 nm.

These dendrimers were first partially functionalized on their periphery by quaternary

ammonium groups. This method has two main advantages: firstly, the

unfunctionalized primary amine groups of the partially quaternized dendrimer can be

used as a handle to covalently attach Au-DENs to surfaces of other molecules, and

secondly, the quaternized amines provide a large positive charge on the dendrimer

periphery, which in turn reduces the possibility of dendrimer agglomeration.10 To

illustrate the effect of partially quaternized dendrimers on the preparation of Au-

DENs, Au-DENs synthesized with PAMAM-NH2 dendrimers were also prepared for

comparison. A [dendrimer]/[Au3+] ratio of 1:55 was used in both cases. HRTEM

revealed that the particle size and size distribution of the NPs prepared within

PAMAM-NH2 have a comparable size distribution to those prepared within partially

quaternized dendrimers, as can be seen in Figures 2.1.a and 2.1.b respectively.

42

Figure 2.1: Particle size distribution of (a) G4-NH2(Au55) and (b) G4-Q32(Au55).12

It was also found that, the use of magic numbers for the [[dendrimer]/[Au3+] ratios

(1:13, 1:55 and 1:140) lead to the formation narrowly distributed NPs while when

other ratios (e.g., 1:100) resulted in formation of polydispersed Au-DENs.

Based on the above literature findings, the synthesis of Cu, Ag, and Au-DENs using

G4-G6 PAMAM-OH dendrimers, G4-G6 PAMAM-NH2 dendrimers as well as different

dendrimer concentrations was considered for this study. Described below in detail

are the synthesis and characterization for these systems.

2.2. Experimental.

Generation 4 (10% wt in methanol), 5 (5% wt in methanol) and 6 (5% wt in

methanol) PAMAM dendrimers with hydroxyl terminal groups (G4-G6 PAMAM-OH)

and generation 4 (10% wt in methanol), 5 (5% wt in methanol) and 6 (5% wt in

methanol) PAMAM dendrimers with amine terminal groups (G4-G6 PAMAM-NH2)

and HAuCI4 were all purchased from Sigma Aldrich. Both NaOH and CuSO4.5H2O

were purchased from Merck and hydrochloric acid (32%) was purchased from

Associated Chemical Enterprise (PTY) LTD). Sodium borohydride (NaBH4) was

purchased from Fluka. All chemicals were of analytical grade and used as received.

All experiments were performed using de-ionized water.

Spectrophotometric titrations as well as absorption spectra were obtained

using a 10 mm quartz cuvette with a VARIAN Cary 100 Conc UV-visible

spectrophotometer equipped with Cary WIN UV software. Particle images were

obtained using a JEOL-Jem 2100 HRTEM. Ag and Au-DENs particle sizes were

analyzed from HRTEM images taken at an accelerating voltage of 197 kV on a

Philips JEOL-Jem 2100 HRTEM using a Gatan GIF and an Oxford INCA energy

dispersive X-ray analysis system (EDX). Samples for TEM analysis were prepared

a b

43

by placing one drop of a Au or Ag-DENs aqueous solution on a holey-carbon coated

Cu grid (200 mesh). Particle size distributions were calculated using ImageJ

software.13

pH Measurements were performed using an ORION model 520A, SCHOTT

pH electrode blueline 25. Calibration of the pH meter was done using pH 4.01 and

pH 10.01 standard solutions (Scientific ADWA ). All pH adjustments were done using

NaOH (0.1 M) and HCI (0.1 M)

N

NHO

NH

O

N

N

NH

O

N

NH

O

N

NH

O

N

NH

O

N

NH

ON

NH

O

N

NH

O

N

NH

O

N

NH

O

N

NHO

N

NHO

N

NH

O

N

NH

O

NH2

NH

O

NH2

NH

O

NH2NHO

NH2

NHO

NH2

NH

O

NH2

NHO

NH2

NH

O

NH2

NHO

NH2

NH

O

NH2

NHO

NH2

NH

O

NH2NH

O

NH2NH

O

NH2

NH

O

NH2

NHO

NH2

N

NH O

NH

O

N

NNH

ON

NH

O

N

NH

O

N

NHO

N

NH O

N

NH

O

N

NH

O

N

NH O

N

NH O

N

NH

O

N

NH

O

N

NH

O

N

NH

O

NH2

NH

O

NH2

NH

O

NH2

NH

ONH2

NH

O

NH2

NHO

NH2NH

O

NH2

NH

ONH2

NH

O

NH2

NH

ONH2

NH

O

NH2

NH

ONH2

NH

ONH2

NH

O

NH2

NH

O

NH2

NH

O

NH2

Figure 2.2: Chemical structure of generation 4 PAMAM-NH2 dendrimer with ethylenediamine core.

Shown in Figure 2.2 is the typical chemical structure of generation 4 PAMAM-NH2.

This structure is the same for PAMAM-OH dendrimers except that the primary NH2

groups are replaced by the OH groups in the case of PAMAM-OH dendrimers. Each

branching unit represents a generation.

44

2.2.1. Binding studies, synthesis and characterization of dendrimer-

encapsulated copper NPs (Cu-DENs).

2.2.1.1. Binding studies.

There are many metal binding sites on a dendrimer (Table 2.1). This made it

necessary to investigate the optimum metal loaded onto the dendrimer in a metal

nanoparticle synthesis. In order to determine the maximum capacity of each

dendrimer for which the metal ion can be coordinated to the tertiary amine groups,

spectrophotometric titration experiments for all dendrimer generations were

performed. Any excess addition of metal ions to the dendrimer may lead to particle

agglomeration.1 Details of this procedure are given in the next section.

2.2.1.1.1 Spectrophotometric titration of dendrimers with Cu2+ ions.

The protocol used for the synthesis of Cu-DENs here was adapted from the

literature.4 G4-G6 of PAMAM-OH dendrimers were used for this study. Before the

aqueous solution of G4-G6 PAMAM-OH dendrimer was titrated with aqueous CuSO4

solution, the pH of the aqueous dendrimer solution was adjusted to 5.7.

Generally, the titration was carried out by adding 3 mL of the aqueous

dendrimer solution into a cuvette followed by the addition of 5 µL aliquots of CuSO4

solution to the cuvette each time with vigorous stirring (15-20 seconds) while

monitoring the absorbance at λ 605 nm by UV-vis spectroscopy. The concentration

of the dendrimer and the CuSO4 used are summarized in Table 2.1. The addition of

5 µL aliquots of CuSO4 results in the absorbance increasing around λ 605 nm (see

Figure 2.3). This is the wavelength (λ 605 nm) associated with the complexation of

Cu2+ within a dendrimer. When the excess of Cu2+ ions have been added to the

dendrimer, the increase in absorbance levelled off, signalling the maximum capacity

of the dendrimer for Cu2+ being reached (see Figures 2.3 and 2.4 respectively). Any

additional Cu2+ added after this point will only slightly change the absorbance at this

wavelength due to dilution effects.

45

Table 2.1: Concentrations used for the titration of G4, G5 and G6 PAMAM-OH aqueous dendrimer

solution with Cu2+ ions.

Dendrimer generation Dendrimer concentration (mM) Concentration of CuSO4

(mM)

G4 0.25 250

G5 0.058 80

G6 0.085 204

Based on the reported work by Crooks and co-workers which showed that

one Cu2+ ion will coordinate with the maximum number of four tertiary amine

groups,4 the calculated titration end points for G4, G5 and G6 PAMAM-OH were

expected to be 16, 32 and 64 respectively. The maximum number of tertiary amine

groups for G4, G5 and G6 PAMAM-OH are 62, 126, and 254 respectively as shown

in Table 2.2.

Table 2.2: Number of amine groups on the PAMAM-OH dendrimer available for binding with Cu2+ ions during Cu-DENs.

Dendrimer

generation

Number OH groups

on the dendrimer

periphery

Number of internal

tert-amine groups

in the dendrimer

Number of

outermost internal

tert-amine groups

in the dendrimer

4 64 62 32

5 128 126 64

6 256 254 128

46

Figure 2.3: Absorption spectra of G4 PAMAM-OH dendrimer (0.25 mM) solution titrated with Cu2+

(250 mM) at λ 605 nm.

The spectrophotometric titration curves for the titration of G4, G5, and G6

PAMAM-OH dendrimers with Cu2+ are shown in Figure 2.4, 2.5, and 2.6 respectively.

Figure 2.4: Titration curve of the G4 PAMAM-OH dendrimer and Cu2+ ions at λ 604 nm and pH 5.7.

y = 0.015x + 0.012 y = 0.001x + 0.236

-0.1

0

0.1

0.2

0.3

0.4

0.5

0 5 10 15 20 25 30 35 40 45

Ab

sorb

an

ce a

t λ

60

4 n

m

Cu2+/G4-PAMAM-OH molar ratio

Cu2+/G4 PAMAM-OH titration curve

16:1 Molar excess

16.0

47

Figure 2.5: Titration curve of the G5 PAMAM-OH dendrimer and Cu2+ ions at λ 608 nm and pH 5.7.

Figure 2.6: Titration curve for the G6 PAMAM-OH dendrimer and Cu2+ ions at λ 604 nm and pH 5.7.

The maximum molar excess of Cu2+ ions that can be loaded within the G4, G5

and G6 PAMAM-OH dendrimer were found to be 16.0, 31.8, and 63.8 close to the

expected value. Results for titration of G4, G5, and G6 of PAMAM-OH with Cu2+ ions

are also summarized in Table 2.3. These values are consistent with the calculated

y = 0.003x + 0.005y = 0.000x + 0.101

-0.04

-0.02

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0

Ab

sorb

an

ce a

t λ

60

8 n

m

Cu 2+/G5-PAMAM-OH molar ratio

Cu2+/G5 PAMAM-OH titration curve

y = 0.005x + 0.036y = 0.000x + 0.355

0

0.1

0.2

0.3

0.4

0.5

0.6

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0

ab

sorb

an

ce a

t λ

60

4 n

m

Cu2+/G6-PAMAM-OH molar ratio

Cu2+/G6 PAMAM-OH titration curve

63.8:1 molar ratio excess

31.8:1 molar excess

31.8

63.8

48

values published in the literature.4 Each end-point value has been calculated as an

average of three titration experimental runs (see Figure 2.7)

Table 2.3: Endpoints for the titration of different generations of PAMAM-OH with Aqueous Cu2+

PAMAM-OH dendrimer

generation

Calculated endpoint Experimentally obtained

endpoint

4 16 16.0

5 32 31.8

6 64 63.8

Figure 2.7: End-points obtained (with error bars) for the titration of G4-G6 PAMAM-OH dendrimers

with Cu2+ ions at pH 5.7 and λ 605 nm.

2.2.1.2. Synthesis and Characterization of Cu-DENs .

Given the results obtained from the binding studies, the synthesis of Cu-DENs

with G4, G5 and G6 of PAMAM-OH dendrimers are discussed in this section. Similar

synthetic procedures were followed for all three dendrimer generations which

involves the removal of the volatiles from the dendrimer solution in vacuo, followed

by the addition of de-ionized water (15 mL) which results in an aqueous dendrimer

solution with a concentration of 10 µM. This concentration was kept constant for all

dendrimer generations. The pH of the aqueous dendrimer solution was adjusted to

R2= 0.981

49

5.7 before the addition of the CuSO4. The correct amount of aqueous CuSO4 (as

calculated from the binding studies) was added to each generation of prepared

aqueous dendrimer solution. Molar ratios of Cu2+ ions used in the case of G4, G5,

and G6 PAMAM-OH were 16, 32, and 64 respectively. The solution was then stirred

for 20 minutes to allow the metal ions to complex to the tertiary amine groups of the

dendrimer. To this solution, excess (20 molar excess) aqueous NaBH4 was added

dropwise. The Cu2+ ions are reduced to zero-valent Cu atoms, which results in the

formation of the desired Cu-DENs in each dendrimer. The formation of Cu-DENs is

accompanied by the change in the colour of the solution from light blue to a light

amber colour (see Figure 2.8). Only the concentration of 0.1 M was used for both

CuSO4 and NaBH4 for all dendrimer generations. The quantities and reaction

conditions used for each dendrimer generation are listed in Table 2.4.

Table 2.4: Reaction quantities and conditions used for the synthesis of Cu-DENs using G4, G5 and

G6 PAMAM-OH dendrimers.

Dendrimer

generation

Mass of dendrimer

dissolved (g)

Volume of CuSO4

(0.1 M) added (µL)

Volume of NaBH4

(0.1 M) added (µL)

G4 0.02 (10% wt in

MeOH)

24 480

G5 0.086 (5% wt in

MeOH)

48 960

G6 0.18 (5% wt in

MeOH)

96 1920

The formation of Cu-DENs was also monitored by UV-vis spectroscopy as

shown in Figure 2.9. This spectrum shows the aqueous dendrimer solution giving a

weak absorption band at around λ 280 nm (labelled 1). Upon the addition of Cu2+,

the increase in absorption at λ 300 nm is observed. This increase in absorbance is

due to the coordination of Cu2+ to the dendrimer. The peak for the presence of Cu2+

ions around λ 600 nm could not be clearly observed since the concentration of

CuSO4 was too small. A peak at λ 300 nm disappears immediately after the addition

of excess reducing agent (NaBH4), signalling the formation of Cu-DENs (labelled

3).This UV-vis spectra were in good agreement with the previously reported for the

50

formation of Cu-DENs.1 Similar UV-vis spectra were also observed for the other

dendrimer generations. The pH of the prepared Cu-DENs was then adjusted to

pH=8.1 where they are more stable.

Figure 2.8: Colour changes during the synthesis of Cu-DENs (a) aqueous dendrimer solution, (b)

Cu2+ loaded dendrimer solution, and (c) Cu-DENs solution.

Figure 2.9: UV-vis spectra for the formation of Cu-DENs within PAMAM-OH dendrimers template.

Since Cu is a first row transition element, it is difficult to determine Cu particle

sizes of the nanoparticles by using TEM.5 Although no HRTEM were obtained for

Cu-DENs for this study, the lack of an absorption peak around 570 nm from the

spectrum attributed to the formation of Cu-DENs (spectrum 3 in Figure 2.9) indicates

Cu2+ NaBH4

Excess

Dendrimer solution Cu2+/dendrimer complex Cu-DENs

a b c

1

2

3

51

that the average particle size of these Cu-DENs is less than 5 nm as already

explained in section 2.1.3, 4

2.2.2. Binding study, synthesis and characterization of Ag-DENs.

2.2.2.1 Binding studies .

Lesniak et al reported the binding of Ag to PAMAM dendrimers by applying

potentiometric titrations to monitor the Ag ion binding to PAMAM-NH2 using a molar

ratio (Ag/dendrimer) of 30 and 45.14 In their study, complexation of Ag+ ions to the

PAMAM-NH2 dendrimer induced deprotonation of the nitrogen binding sites and the

titrations were performed with Ag+ ions. They found that at pH range of 2.8 to 7

binding of Ag+ ions will occur with the PAMAM-NH2 dendrimer. At the dendrimer to

Ag ratio of 30, complete binding is observed. However, precipitation of AgOH

occurred at the ratio of 45. At pH levels above 7.5, hydrolosis of Ag ions occurs. In

our study the synthesis of stable Ag-DENs proved to be difficult, since Ag+ ions are

only weakly coordinating inside the dendrimer.15 Since it is believed that Ag+ ions do

not coordinate very well with PAMAM-OH dendrimers16 G4, G5 and G6 of PAMAM-

NH2 dendrimers were used for the preparation of Ag-DENs in this study. Any binding

study using UV-vis spectrophotometry proved inconclusive in our case. Nonetheless

we have used the synthetic method reported in the literature.

2.2.2.2 Synthesis and characterization of Ag-DENs.

The synthetic method used was adapted from the reported literature.8 A

similar synthetic procedure was used for G4, G5 and G6 PAMAM-NH2 dendrimer

solutions. For G4, G5 and G6 PAMAM-NH2 a molar excess of 12, 16 and 32 Ag+

ions to dendrimer were used respectively to give a theoretical complexation of all

amines inside each dendrimer. This synthesis involves the in vacuo removal of

volatiles from the dendrimer solution. The dendrimer is then dissolved in de-ionized

water (20 mL) to give a concentration of 10 µM aqueous dendrimer solution. The pH

of this aqueous dendrimer solution was adjusted to approximately 2 in order for Ag+

ions to coordinate to the primary amine groups inside the dendrimer. Aqueous

AgNO3 solution (0.1 M) was added to the aqueous dendrimer solution. Addition of

Ag+ ions to the acidified dendrimer solution did not show any colour change. (Figure

2.10.b). The mixture was stirred for 1 hour to allow for coordination of Ag+ ions with

52

the tertiary amine groups of the dendrimer. This was followed by addition of 10 molar

excess aqueous NaBH4 (0.1 M) added drop-wise. The solution immediately changes

colour from colourless to yellow, signalling the formation of Ag-DENs (Figure 2.10.c).

The concentration of 0.1 M for both AgNO3 and NaBH4 was used for all dendrimer

generations. The reaction quantities and conditions used are given in Table 2.5.

Table 2.5: Quantities used for the synthesis of Ag-DENs using G4, G5 and G6 PAMAM-NH2

dendrimers.

Dendrimer

generation

Mass of dendrimer

used (g)

Volume of AgNO3

(0.1 M) added (µL)

Volume of NaBH4

(0.1 M) added (µL)

G4 0.2 (10% wt in

MeOH)

24 240

G5 0.058 (5% wt in

MeOH)

32 320

G6 0.0232 (5% wt in

MeOH)

40 400

The addition of Ag+ ions to the dendrimer gives rise to the spectrum (marked

2) as can be seen in Figure 2.11. Reduction of the metal ions coordinated to the

dendrimer with an excess of a NaBH4 resulted in the formation of a strong absorption

band at λ 400 nm, which is attributed to the formation of Ag NPs (spectrum 1). These

observed spectra were also consistent with the reported literature for the Ag-DENs.6,

7 To simplify our discussion, we refer to the particles formed here as DENs although

their true nature cannot be easily determined. The formation of dendrimer stabilized

NPs cannot be ruled out as well.

Figure 2.10: Colour changes during the synthesis of Ag-DENs: a) aqueous dendrimer solution, b) Ag+

loaded dendrimer solution c) Ag-DENs.

Ag+ NaBH4

Excess

Aqueous dendrimer solution Ag+/dendrimer complex Ag-DENs

a b c

53

Figure 2.11: UV-vis spectra for the preparation of Ag-DENs using PAMAM-NH2 dendrimer as a template. From the HRTEM analysis, the average size distribution of the Ag-DENs

synthesized using G4 PAMAM-NH2 dendrimers was determined to be 3.1 ± 0.6 nm

(200 particles counted). The HRTEM image and the size distribution histogram for

G4-PAMAM-NH2(Ag12) are shown in Figure 2.12. The molar ratio of dendrimer to Ag+

ions of 1:12 was used for the synthesis of G4-PAMAM-NH2(Ag12).

Figure 2.12: a) HRTEM image and b) particle size histogram of G4-PAMAM-NH2(Ag12).

Dendrimer/Ag+ molar ratio of 1:16 was used to synthesis G5-PAMAM-

NH2(Ag16). The average particle size of 2.9 ± 0.5 nm (230 particles counted) was

obtained for Ag-DENs synthesized within G5 PAMAM-NH2 dendrimers. Figure 2.13

shows the HRTEM image as well as particle size histogram of G5-PAMAM-

NH2(Ag16).

2

1

a b

3

54

Figure 2.13: a) HRTEM image and b) particle size histogram of G5-PAMAM-NH2(Ag16).

For the synthesis of Ag-DENs using G6 PAMAM-NH2 dendrimer as a template, the

molar ratio of 1:32 was used. The particle size distribution of G6-PAMAM-NH2(Ag32)

was determined to be 2.7 ± 0.5 nm (210 particles counted). Figure 2.14 shows the

HRTEM image as well as the particle size histogram of G6-PAMAM-NH2(Ag32).

Figure 2.14: a) HRTEM image and b) particle size histogram of G6-PAMAM-NH2(Ag32).

The average particle sizes obtained when different dendrimer generations are

used for the synthesis of Ag-DENs are summarized in Table 2.6.

Table 2.6: The average particles sizes for Ag-DENs synthesized with G4, G5 and G6 PAMAM-NH2 dendrimer. PAMAM-NH2 dendrimer generations

Dendrimer:Ag+ molar ratio Average particle size (nm)

G4 1:12 3.1 ± 0.5

G5 1:16 2.9 ± 0.5

G6 1:32 2.7 ± 0.5

a b

a b

55

2.2.3. Binding studies, Synthesis and Characterization of Au-DENs. 2.2.3.1. Binding studies .

As with the preparation of Ag-DENs, the use of Au ions also proved to be

difficult,16 and the stoichiometric ratios between dendrimers and metal atom (as with

PtCl42+, PdCl4

2- and Cu2+) could not be obtained.

2.2.3.2 Synthesis and Characterization of Au-DENs .

The synthesis of Au-DENs was done following the procedure from literature.12,

17 In the present study we have used only the G4 PAMAM-NH2 dendrimer. For the

preparation of Au-DENs, G4 PAMAM-NH2 dendrimer solution (0.029 g, 2.05 µmol,

10% wt in MeOH) was added into a small round bottom flask. The MeOH was

removed in vacuo. The dendrimer solution was then dissolved in de-ionized water

(20.5 mL). The pH of the aqueous dendrimer solution was adjusted to approximately

2. This was done to prevent coordination of Au3+ ions on the periphery groups of the

dendrimer which may consequently lead to nanoparticle agglomeration. Aqueous

Au3+ ions (112.75 µL of a 0.1 M solution) were added at 55 molar excess to the

aqueous dendrimer solution while stirring. The solution turned yellow (at λ 310 nm)

and a shoulder band is formed at ~ λ 300 nm in the UV-vis spectrum (see Figure

2.16.b). The reaction mixture was stirred for 5 minutes. NaBH4 (222.5 µL of 0.5 M

solution prepared in 0.3 M aqueous NaOH) was added dropwise under vigorous

stirring to reduce Au3+ to Au-DENs. The solution changed from yellow to a

maroon/brownish colour, indicating the formation of Au-DENs as shown in Figure

2.15.c.

Figure 2.15: Synthesis of Au-DENs a) aqueous dendrimer solution, b) Au3+ loaded dendrimer solution

c) Au-DENs.

Aqueous dendrimer solution

Au3+/dendrimer complex Au-DENs

AuCI4— NaBH4

Excess

a b c

56

The addition of Au3+ ions to the aqueous dendrimer solution resulted in the

formation of an absorption bands at approximately λ 300 nm and 220 nm

respectively (spectrum 2 of Figure 2.16). These bands are assigned to ligand-to-

metal-charge-transfer (LMCT) transitions.6 Reduction of metal ions by NaBH4 results

in the formation of a new spectrum with a small absorption band at approximately λ

550 nm (Spectrum 1 of Figure 2.16) attributed to the formation of Au-DENs.12 The

formation of a plasmon band around λ 550 nm indicates that the size of Au-DENs is

larger than 2 nm.18 This was confirmed by HRTEM images shown in Figure 2.17.

The TEM images revealed that the Au-DENs formed have an average diameter of

3.1 ± 0.5 nm (210 particles counted). A particle-size histogram showed that these

Au-DENs have a fairly narrow size distribution as compared to other metal NPs

synthesized without the use of dendrimer template.12, 19

Figure 2.16: UV-vis spectra for the formation of Au-DENs within the PAMAM-NH2 dendrimers.

Figure 2.17: a) HRTEM image and b) particle size histogram of G4-PAMAM-NH2(Au55).

3.1 ± 0.5 nm a b

1 2 3

57

2.3. Conclusions.

PAMAM dendrimers were successfully used as template as well as stabilizer

during encapsulated metal nanoparticle preparation. Binding studies were conducted

to determine the maximum molar ratio between the dendrimer and the metal ions.

Maximum amount of Cu2+ that coordinate with the tertiary amino groups of the G4,

G5 and G6 PAMAM-OH dendrimers were determined to be 16, 32 and 64

respectively. It was also confirmed, based on this binding study, that one Cu2+ is

coordinated to at-least four tertiary amine groups of the PAMAM-OH dendrimers

since G4, G5 and G6 PAMAM-OH dendrimers contain 62, 126 and 254 tertiary

amine groups respectively. Some metals coordinate stronger to the tertiary amine

groups within a certain dendrimer than others, for example, metal ions such as Ag+

and Au3+ do not bind well with the binding sites on the PAMAM-OH dendrimers, but

bind strongly with other dendrimers such as PAMAM-NH2 and carboxyl-terminated

PAMAM dendrimers. Some metal ions such as Cu2+ coordinate with the tertiary

amines of the dendrimer in a stoichiometric ratio, while some (e.g., Ag and Au) do

not. The use of dendrimers also prevents nanoparticle agglomerization during their

synthesis. When using NH2 terminated PAMAM dendrimers, coordination of the

metal ions to the primary amine groups on the periphery of the dendrimer can occur,

giving rise to interdendrimer NPs. This may subsequently lead to nanoparticle

agglomerization. However, functionalization of the periphery of the dendrimer, or

adjusting the pH of the dendrimer solution to acidic conditions can help in avoiding

the coordination of the metal ions to the terminal amine groups of the dendrimer, and

hence suppress the nanoparticle agglomeration when PAMAM-NH2 dendrimers are

used.

During nanoparticle synthesis, the change of the solution’s colour can be

used as an indicator for the formation of NPs. The formation of Cu-DENs is

accompanied by the light amber colour while Au-DENs and Ag-DENs show maroon

and yellow colours respectively. Absorbance spectroscopy can also be used to study

NPs because the peak position and shape are sensitive to particle size.5, 18 For

example, during the synthesis of Cu-DENs reported by Feng et al,4 the absence of a

plasmon peak around λ 570 nm was used as an indication that the size of formed

Cu-DENs were below 5 nm. The UV-vis analysis was also used in our study to

estimate the particle average diameters in cases where TEM images cannot be

58

obtained. UV-vis spectroscopy showed that the synthesized Cu-DENs in our study

also lack a Plasmon peak at approximately λ 570 nm. It was therefore reasonable to

conclude that the average sizes of these particles were below 5 nm and therefore

were formed inside the dendrimer. Similar UV-vis spectra were obtained irrespective

of the generation of dendrimer (G4-G6 PAMAM-OH and G4-G6 PAMAM-NH2) used

for the synthesis of Cu, Ag and Au-DENs. Although these DENs were prepared

according to the procedures adapted from the literature, different reaction conditions

(e.g dendrimer: metal ions molar ratio, dendrimer concentration e.t.c) were used for

this study. Therefore, their average particle sizes could not be easily compared with

those reported in the literature. The HRTEM image showed that both Au and Ag-

DENs synthesized have an average particle size of less than 4 nm. The NPs size

can be controlled by tuning the dendrimer/metal ion molar ratio. The average

particles size decreases when the dendrimer generation and dendrimer/Ag+ molar

ratio increase during the synthesis of Ag-DENs.

2.4. References.

1. M. Zhao, L. Sun and R. M. Crooks, J. Am. Chem. Soc., 1998, 120, 4877.

2. C. L. Jackson, H. D. Chanzy, F. P. Booy, D. A. Tomalia and E. J. Amis, Proc.

Am. Chem. Soc., 1997, 77, 222.

3. L. Balogh and D. A. Tomalia, J. Am. Chem. Soc., 1998, 120, 7355.

4. Z. V. Feng, J. L. Lyon, J. S. Croley, R. M. Crooks, D. A. V. Bout and K. J.

Stevenson, J. Chem. Educ., 2009, 86, 368.

5. M. Zhao and R. M. Crooks, Chem. Mater., 1999, 11, 3379.

6. K. Esumi, A. Suzuki, A. Yamahira and K. Torigoe, Langmuir, 2000, 16, 2604.

7. A. Manna, T. Imae, K. Aoi, M. Okada and T. Yogo, Chem. Mater., 2001, 13,

1674.

8. K. Esumi, R. Isono and T. Yoshimura, Langmuir, 2004, 20, 237.

9. V. Chechik, M. Zhao and R. M. Crooks, J. Am. Chem. Soc., 1999, 121, 4910.

10. F. Gröhn, B. J. Bauer, Y. A. Akpalu, C. L. Jackson and E. J. Amis,

Macromolecules, 2000, 33, 6042.

11. M. E. Garcia, L. A. Baker and R. M. Crooks, Anal. Chem., 1999, 71, 256.

12. Y.-G. Kim, S.-K. Oh and R. M. Crooks, Chem. Mater., 2004, 16, 167.

13. W. Rasband, National Institute of health, USA, http://rsb.info.nih.gov/ij.

59

14. L. Lesniak, A. U. Bielinska, K. Sun, K. W. Janczak, X. Shi, J. R. B. Jr and L. P.

Balogh, Nano. lett., 2005, 5, 2123.

15. A. Castonguay and A. K. Kakkar, Advances in Colloid and Interface Science,

2010, 160, 76.

16. R. M. Crooks, M. Zhao, L. Sun, V. Chechik and L. K. Yeung, Acc. Chem.

Res., 2001, 34, 181.

17. J. C. Garcia-Martinez and R. M. Crooks, J. Am. Chem. Soc., 2004, 126,

16170.

18. C. N. R. Rao, G. U. Kulkarni, P. J. Thomas and P. P. Edwards, Chem. Eur. J.,

2000, 8, 29.

19. R. W. J. Scott, O. M. Wilson and R. M. Crooks, Chem. Mater., 2004, 16, 5682.

60

Chapter 3:

Ag, Au and Cu dendrimer encapsulated NPs (Ag, Au and Cu-DENs) for the

reduction of 4-nitrophenol.

3.1. Introduction.

4-Nitrophenols (4-NP) are considered to be amongst the most prevalent

organic pollutants in waste-waters generated from industrial and agricultural sources.

This also includes companies that manufacture explosives, dyestuffs, insecticides

and other products.1-3 They are considered as hazardous wastes and priority toxic

pollutants by the United States Environmental Protection Agency (USEPA).4

Because nitrophenols are not environmentally friendly compounds, their reduction to

4-aminophenol (4-AMP) is important. On the other hand, 4-AMP is a commercially

important intermediate for the manufacture of analgesic and antipyretic drugs such

as paracetamol.5

In this chapter, the catalytic activity of the dendrimer-encapsulated NPs

(DENs) synthesized in Chapter 2 is evaluated. Ag, Au, and Cu-DENs were evaluated

as catalysts for the reduction of 4-NP to 4-aminophenol. Several factors that may

influence the reaction rate were evaluated. These include effects of temperature, and

substrate concentration. The effect of dendrimer generation on the rate of reaction

was also studied. These results are discussed and compared to some of the

reduction of 4-NP studies reported in the literature. Relatively few published literature

studies on the reduction of 4-NP are available where dendrimers where used as

templates/stabilizers for NPs synthesis.

The shape and size of the nanoparticles (NPs) can be correlated to their

catalytic properties through a precise kinetic analysis. However, this type of analysis

can only be done for model reactions that proceed in a well-defined manner from a

single reactant to a single product. Pal and co-workers6 and Esumi et aI7 were the

first to identify that the reduction of 4-NP to 4-AMP by borohydride ions (BH4—) is

suitable as such a model reaction. This reaction is considered the best model for

studying catalytic behaviour of NPs, because there are no side reactions, and the

reaction can be easily followed using UV-vis spectroscopy. 4-NP is yellow in colour,

61

and therefore shows a strong absorbance at λ 400 nm. The addition of excess BH4—

in the presence of the catalyst results in the decrease in absorption at λ 400 nm and

the formation of a new absorption band around λ 300 nm which is attributed to the

formation of 4-AMP (Figure 3.1). The isosbestic point at approximately λ 325 nm

shown in Figure 3.1 indicates a clean conversion of 4-NP to 4-AMP without side

reactions.6, 8

Figure 3.1: Time based UV-vis spectra monitoring 4-NP reduction catalyzed by metal NPs.

The reaction is treated as a pseudo-first order reaction and it is assumed to follow

the Langmuir-Hinshelwood mechanism (Figure 3.2.).9

4-AMPMetal NP

BH4-

H H

H

4-NP

H

HH4-NP

H

Figure 3.2: Langmuir- Hinshelwood mechanism of the reduction of 4-NP by BH4- in the presence of

metallic NPs.9

In the Langmuir-Hinshelwood mechanism, both reactants must first be

adsorbed to the nanoparticle surface prior to reaction. After the adsorbed species

have reacted, the product dissociates from the surface. Although a number of

Isosbestic point

62

studies in the reduction of 4-NP are based on this mechanism, Khalavka et aI

recently reported a study in which they concluded that only hydrogen needs to be

adsorbed onto the surface of the NPs (Eley-Rideal mechanism).10 In the Eley-Rideal

mechanism, one substrate is absorbed onto the surface first and the other one

passes by in order to interact with the substrate adsorbed on the surface, resulting in

the formation of a product. In this study however, the Langmuir-Hinshelwood model

will be considered as it has previously been used to study the catalytic behaviour of

NPs.8, 9, 11, 12 In the rate law of this mechanism, the apparent kinetic rate constant

kapp is strictly proportional to the total surface, S, of all metal NPs. Therefore, the

kinetic rate constant, k1, and the apparent kinetic rate constant, kapp, can be defined

through Equation 1, Where cNip is the concentration of 4-NP.

�����

��=−���. ���� =−�. �. ���� (1)

Pradhan et al reported the reduction of 4-NP with borohydride in the presence

of Ag NPs.6 Here, the Ag NPs were prepared as either a still growing microelectrode

(GME) or a full grown microelectrode (FGME). FGME Ag NPs were synthesized

using different reducing agents, such as NaBH4, N2H4 and ascorbic acid, whereas

only NaBH4 was used for the preparation of GME. It was found that there is always a

delay (or a “lag”) for an amount of time before any visible change in the absorbance

can be observed. This inactive period is referred to as induction time/period. Some

authors have interpreted this induction period as the time needed for the reactants to

diffuse to the surface of the particles.13 The rate of reduction in the presence of GME

was found to be faster than that of the reduction in the presence of FGME under

similar conditions. However, amongst the results obtained for FGME prepared using

different reducing agents, similar catalytic activity and kinetic results were obtained.

The increase in the concentration on NaBH4 was found to decrease the induction

period as well as forcing the reaction to follow a pseudo-first order reaction kinetics.

The linear Arrhenius plot for the reaction was achieved when the NaBH4

concentration is higher, showing that the reaction was following a pseudo-first order

kinetics.

Esumi and co-workers reported the preparation of Au-DENs by laser

irradiation using G3 and G5 PAMAM as well as G3 and G4 poly(propyleneimine)

63

(PPI) dendrimers with surface amino groups as templates.14 The Au-DENs were

prepared by irradiation of the dendrimer/Au3+ complex in a quartz cell with a second

harmonic (532 nm) output of the laser focused on a spot (0.196 cm2) on the solution

surface by a lens. The catalytic activity of these Au-DENs were evaluated in the

reduction of 4-NP. The rate constant was found to be independent on the molar ratio

of [dendrimer]/[Au3+]. Also the rate constants decreased with increase in dendrimer

concentration in the case of PAMAM dendrimers, whereas the opposite was

observed in case of PPI dendrimers. However, these observations in the rate

constants were not substantial enough to draw any valid conclusions. The sizes of

Au NPs show a decrease as [dendrimer]/[Au3+] ratio increases. However, the Au

NPs prepared within the PPI dendrimers were shown to be highly catalytically active

as compared to the ones prepared within the PAMAM dendrimers. Because the

diameters of PPI dendrimers are generally smaller than those of PAMAM of the

same generation, they assumed that the rate constant is affected by the size of the

DENs.

In another related study, Esumi et al reported the reduction of 4-NP with Ag,

Pt and Pd-DENs prepared using different generations as well as concentrations of

PPI and PAMAM dendrimers.15 In this case, no correlation between the rate constant

and the generations for both PAMAM and PPI dendrimers was observed. This could

be attributed to the fact that these NPs were only stabilized by the dendrimers rather

than being encapsulated within the dendrimer cavities. However, the rate constants

for all the systems were observed to decrease with an increase in dendrimer

concentration. This latter observation is also consistent with the findings of the

previous study reported by Esumi et al.7 However, the rate constants for Pt and Pd-

DENs obtained from the PPI dendrimer were found to be significantly faster than

those of the corresponding PAMAM dendrimer template of the same generation.

This was also observed in the work reported by the same research group where Au-

DENs prepared in different generations of both PAMAM and PPI dendrimers were

used as catalysts.14 For the Ag-DENs, the change in rate constants were found

neither to be influenced by the type of dendrimer nor the generation. No induction

period was observed in all these studies.

Feng et al also reported the catalytic activity of Cu-DENs in the reduction of 4-

NP.4 G4 PAMAM-OH dendrimers were used to synthesize Cu-DENs. The Cu-DENs

64

were found to be catalytically active for this reaction. When 4-NP is added first to the

reaction mixture, the induction period was observed, whereas the induction period

disappeared if NaBH4 is added first. The sequence of addition of 4-NP and NaBH4

does not affect the rate constant, but only the induction period. When NaBH4 is

added last, the induction period was observed whereas the induction period was

completely reduced or even not observed when 4-NP is added last. These

observations were also evident in our study and would be discussed later in this

Chapter.

Kuroda et al reported the application of polymer supported Au NPs as catalyst

for the reduction of 4-NP.11 These Au NPs were supported on poly(methyl

methacrylate) (PMMA). A rate constant of 0.0079 s-1 was obtained in this study.

Contrary to the work reported by Pal and co-workers, the rate constant obtained here

was found to be significantly higher as compared to other polymer supported Au NPs

with smaller average sizes.12, 16, 17 This could be due to the fact that different

polymers as well as synthetic procedures were used. The same trend was observed

in the work reported by Liu et al,18 where the polymer supported Au NPs with an

average diameter of 10 nm showed high catalytic activity as compared to other

polymer supported Au NPs with smaller average particle sizes for the same reaction.

Thus, some polymer supported Au NPs with a certain size showed higher catalytic

activity than other polymer supported NPs.

Liu et al reported the preparation of Ag NPs within multilayer polymer films.19

These multilayer polymer thin films were synthesized by a sequential electrostatic

deposition of a positively charged G3 PAMAM dendrimer and negatively charged

poly(styrenesulfonate) (PSS) and poly(acrylic acid) (PAA). The catalytic activity of

these silver NPs was investigated for the reduction of 4-NP. They monitored the

reaction using UV-vis spectroscopy and found that these Ag NPs proved to be active

for this reaction. However, no kinetic results were reported in this case. Recently,

Balamurugan et al also reported the synthesis and application of polymer stabilized

Ag NPs.20 Here, polymer stabilized Ag NPs were prepared using 3,4-

ethylenedioxythiophene (EDOT) as a reductant and polystyrene sulfonate (PSS—) as

a dopant as well as stabilizer. The synthesized Ag NPs had an average diameter of

10-15 nm. UV-vis spectroscopy also showed that these Ag NPs were catalytically

65

active in the reduction of 4-NP. In this instance also, no kinetic results in terms of

reaction rates were reported.

Tang et al reported the use of Ag and bimetallic Ag-Au NPs that are

supported on carbon spheres (CSs) as heterogeneous catalysts in the reduction of

4-NP.21 The activity for the reduction catalyzed by Ag-C was found to be higher as

compared to others reported in the literature for supported monometallic Ag NPs for

the same reaction.22-24 However, the bimetallic Ag-Au-C NPs were highly active as

compared to a monometallic Ag-C composite. It is clear from literature that the

catalytic conversion of 4-NP to 4-AMP has been extensively studied.25-32 and very

few of these include studies where DENs were used as catalyst for this 4-NP

reduction to form 4-AMP and this motivated the current study.4

3.2. Application of Ag, Au, and Cu DENs in the reduction of 4-NP.

In this section, the catalytic activity of the synthesized Ag, Au and Cu-DENs

discussed in Chapter 2 was evaluated for the reduction of 4-NP. The effect of factors

such as dendrimer generation, temperature and substrate concentration on the

reaction rate will be studied. G4, G5 and G6 PAMAM-OH dendrimers were all used

to prepare Cu-DENs respectively in this study. G4, G5 and G6 PAMAM-NH2

dendrimers were also used to synthesize Ag-DENs. Only the G4 PAMAM-NH2

dendrimers were used to prepare Au-DENs.

3.2.1. Experimental.

G4 (10% wt in methanol), G5 (5% wt in methanol) and G6 (5% wt in

methanol) PAMAM dendrimers with hydroxyl terminal groups (G4-G6 PAMAM-OH)

and G4 (10% wt in methanol), G5 (5% wt in methanol) G6 (5% wt in methanol),

PAMAM dendrimers with amine terminal groups (G4-G6 PAMAM-NH2) and HAuCI4

were all purchased from Sigma Aldrich. NaOH and CuSO4.5H2O, AgNO3 were

purchased from Merck. Sodium borohydride (NaBH4) was purchased from Fluka. All

chemicals were of analytical grade. All experiments were performed using de-ionized

water. All graphs were plotted using Microsoft Excel 2007 and Origin software.33

All kinetic data were obtained using a 10 mm glass path cuvette using either

VARIAN Cary 100 Conc or Shimadzu UV-visible 1800 spectrophotometers

66

(equipped with Cary WIN UV software). Reaction rate constants were calculated

using Kinetic Studio software.34 For all reactions, each reaction rate is calculated an

average of at least three reaction runs.

pH measurements were performed using an ORION model 520A, SCHOTT

pH electrode blueline 25. Calibration of the pH meter was performed using pH 4.01

and pH 10.01 standard solutions (Scientific ADWA).

3.2.2. Reaction conditions used for kinetics study.

3.2.2.(i) Kinetic study using Cu-DENs as catalysts. A typical catalysis experiment was carried as follows: Cu-DENs (600 µL, 5

µM), NaBH4 (1.5 mL, 0.1 M), de-ionized water (1.6 mL) and 4-NP (250 µL, 600 µM)

were mixed together in a 10 mm path length glass cuvette. The change in the

absorbance with time of 4-NP was monitored by a UV-vis spectrophotometer at λ

400 nm. This wavelength was used for all kinetic study. The reaction time varies

depending on the activity of the catalyst

3.2.2.(ii) Kinetic study using Ag-DENs as catalysts.

Ag-DENs (300 µL, 5 µM), NaBH4 (1 mL, 0.1 M), de-ionized water (1.6 mL)

and 4-NP (600 µM, 250 µL) were mixed together in a cuvette.

3.2.2.(iii) Kinetic study using Au-DENs as catalysts.

Au-DENs (100 µL, 5 µM), NaBH4 (1 mL, 0.5 M), de-ionized water (2 mL) and

4-NP (250 µL, 600 µM) were all mixed together and the same procedure was

followed.

3.3. Results and discussion.

3.3.1 Cu-DENs as catalysts in the reduction of 4-NP

As mentioned in the previous section, 4-NP exhibits a bright yellow colour with

an absorbance peak at λ 400 nm in basic aqueous solution. The catalytic activity of

the synthesized Cu-DEN (discussed in Chapter 2) were evaluated in the reduction of

4-NP to 4-aminophenol reaction (Scheme 3.1). Although no TEM analysis was done

for Cu-DENs, the average particle size of the synthesized Cu-DENs was assumed to

be less than 5 nm (explanation given in Chapter 2). The reaction is set up to follow

67

pseudo-first-order kinetics when NaBH4 is added in excess to the Cu-DENs catalyst

and 4-NP. In the absence of Cu-DENs, the concentration of this 4-NP peak does not

change over an extended period of time after the addition of a reducing agent

(NaBH4). However the addition of Cu-DENs in the reaction mixture results in the

decrease of the absorbance peak at λ 400 nm and the appearance of a new

absorbance peak around λ 310 nm (Figure 3.1), which means that the reduction of 4-

NP only takes place in the presence of a reducing agent and a catalyst. The change

in absorbance is also visually observed as a colour change from yellow (4-NP) to a

colourless solution, indicating the formation of 4-aminophenol (see Figure 3.3).

NO2

OH

NH2

OH

NaBH4(Excess)

Cu-DENs

4-nitrophenol 4- aminophenol

Scheme 3.1: Reduction of 4-NP to 4-aminophenol by NaBH4 in the presence of Cu-DENs. It seems the Cu-DENs is oxidized to copper oxides with time and this results

in an induction period (as mentioned earlier in this chapter) when 4-NP is added first.

This induction period is presumed to be the time needed by NaBH4 to eliminate the

surface oxides on the Cu-DEN. Again, if 4-NP is added first, time lag is needed for

NaBH4 to also get adsorbed on the particle surface. However, if NaBH4 is added first,

this surface oxide elimination happens undetected, and upon the addition of 4-NP

the reaction appears to proceed without the induction period previously observed.

This is illustrated in Figure 3.4 and 3.5.

4-NP (yellow) 4-AMP (colourless)

Figure 3.3: Colour change in the catalyzed reduction of 4-NP to AMP by NaBH4.

Cu-DENs

NaBH4 (Excess)

68

Figure 3.4: First-order kinetic plot for the reduction of 4-NP (600 µM) when NaBH4 (0.1 M) is added

last in the reaction mixture.

Figure 3.5: First-order kinetic plot for the reduction of 4-NP (600 µM) when NaBH4 (0.1 M) is added

first in the reaction mixture.

The observed rate constant kobs, was obtained by plotting the natural log of

corrected absorbance against time t at λ 400 nm. It should be noted that the

absorption values were corrected by subtracting the final absorbance value from

each absorbance value obtained. This was done to account for the increase in the

spectral intensity due to the presence of NaBH4. As shown in Table 3.1, the rate

constants obtained when NaBH4 is added first for the reduction of 4-NP catalyzed by

Cu-DENs synthesized within G4, G5 and G6 PAMAM-OH were 2.61 x 10-2 s-1, 1.84 x

y = -0.023x + 1.829

R² = 0.987

-6

-5

-4

-3

-2

-1

0

0 50 100 150 200 250 300

In[A

40

0(t

) -

A4

00

(tfi

na

l)]

t/s

y = -0.026x - 0.763

R² = 0.978

-8

-7

-6

-5

-4

-3

-2

-1

0

0 50 100 150 200 250 300

ln[A

40

0(t

) -

A4

00

(tfi

na

l)]

t/s

69

10-2 s-1 and 1.66 x 10-2 s-1 respectively. However, when NaBH4 is added last, the rate

constants for the reduction of 4-NP were 2.43 x 10-2 s-1, 1.78 x 10-2 s-1 and 1.59 x 10-

2 s-1respectively. This afore-mentioned observation could have been due to the fact

that as the dendrimer generation increases, the dendrimer branches also increases

making it difficult for the reactants to access the active site of the encapsulated NPs

and hence the decrease in rate constant. These rate constant values were both

lower than the reported values (0.035 s-1 and 0.037 s-1 when 4-NP is added first and

last respectively) for Cu-DENs prepared using G4-PAMAM-OH dendrimers in this

reaction (4-NP reduction).4 We suspect that these observed lower rate constants

were due to the fact that the instrument (UV-vis spectrophotometer) used for this

study, which unlike the one used in the literature, did not have a magnetic stirrer had

an effect or due to a difference in particle sizes. For each dendrimer generation, the

addition of NaBH4, first or last in the reaction, does not seem to have a significant

effect on the reaction rate but rather only the induction period. This is in good

agreement with the reported studies which suggested that NaBH4 must first get

adsorbed on the catalyst surface before the reaction starts and that Cu-DENs

oxidises to Cu oxide with time, and also that time is required for NaBH4 to re-reduce

surface oxides on the Cu-DENs, reactivating the Cu-DENs to catalyze the reduction

of 4-NP.4, 9

Table 3.1: Effect of dendrimer generation on the rate constant for Cu-DENs (5 µM) catalyzed 4-NP

(600 µM) reduction by NaBH4 (0.1 M) at 298 ± 0.1 K.

PAMAM-OH generations

used to prepare Cu-DENs

Rate constants (10-2 s-1 )

when NaBH4 is added first

Rate constants (10-2 s-1)

when NaBH4 is added last

G4-(Cu16) 2.61 ± 0.03 2.43 ± 0.03

G5-(Cu32) 1.84 ± 0.09 1.78 ± 0.09

G6-(Cu64) 1.66 ± 0.07 1.59 ± 0.08

The rate constant obtained when using Cu-DENs prepared from G4 PAMAM-

OH was significantly higher compared to those prepared from G5 and G6 PAMAM-

OH dendrimers. There was also a decrease in rate constant from G5 PAMAM-OH to

G6 PAMAM-OH dendrimer catalysts.

In order to calculate the activation energy for the Cu-DENs catalyzed 4-NP

reduction, kinetic runs were carried out at different temperatures.

70

ln = −��

�.�

�� ln� (2)

Equation 2 is a simplified form of the Arrhenius equation which can be used to

calculate the activation energy, where k is the rate constant, R is the molar gas

constant, T is the temperature and EA is the activation energy. If this equation 2 is

expressed in terms of equation 3, then, the slope, m = ��

�

y = mx + c (3)

Figure 3.6 shows a plot for the influence of temperature on the rate constant

for Cu-DENs catalyzed 4-NP reduction. The Cu-DENs synthesized using G4-

PAMAM-OH dendrimers were used for this study. Kinetic runs were carried out at

various temperatures (278 ± 0.1 K, 288 ± 0.1 K, 293 ± 0.1 K, 298 ± 0.1 K and 308 ±

0.1K).

Figure 3.6: The dependence of rate constant on temperature for Cu-DENs (5 µM) catalyzed 4-NP

(600 µM) reduction by NaBH4 (0.1 M)

The rate constant was found to increase with an increase in temperature. The

data obtained from Figure 3.6 were used to construct an Arrhenius plot for the same

reaction. The Arrhenius plot and the equation from which the activation energy was

calculated are shown in Figure 3.7. The EA for this reaction was calculated to be 65.5

kJ/mol. The enthalpy change (∆H++) and the entropy change (∆S++) were calculated

to be 56.0 ± 3.7 kJ/mol and -89.2 ± 8.1 J/K.mol respectively.

R2= 0.996

71

Figure 3.7: Arrhenius plot for Cu-DENs (5 µM) catalyzed reduction of 4-NP (600 µM) by NaBH4 (0.1

M).

3.3.2. Ag-DENs as catalysts in the reduction of 4-NP.

Presented in this section are the kinetic evaluations of 4-NP reduction with

Ag-DENs prepared from G4-G6 PAMAM-NH2 dendrimers. Figure 3.8 shows the

spectra taken over time when prepared Ag-DENs were added to the reaction mixture

followed by the addition of NaBH4 and 4-NP respectively.

Figure 3.8: Typical time based UV-vis spectra monitoring the reduction of 4-NP (600 µM) by NaBH4

(0.1 M) catalyzed by Ag-DENs (5 µM).

R2= 0.994

Y= -7876x + 22.83

72

The average particle size of the G4, G5 and G5 PAMAM-NH2 Ag-DENs were

calculated to be 3.1 ± 0.6 nm, 2.9 ± 0.5 nm and 2.7 ± 0.5 nm respectively (HRTEM

images shown in Chapter 2). The rate constants for Ag-DENs prepared using G4,

G5 and G6 PAMAM-NH2 dendrimers are summarized in Table 3.2. Although

different reaction conditions were followed, these values were higher than the

reported ones when dendrimer stabilized Ag NPs were used as catalysts for the

same reaction.15

Table 3.2: The effect of dendrimer generation on the rate constant for Ag-DENs (5 µM) catalyzed

reduction of 4-NP (600 µM) by NaBH4 (0.1 M) at 298 ± 0.1 K.

Generations of PAMAM-NH2 used to

prepare Ag-DENs.

Rate constant (10-3 s-1)

G4-(Ag12) 7.1 ± 0.5

G5-(Ag16) 6.1 ± 0.3

G6-(Ag32) 2.0 ± 0.3

The linear plot (often referred to as “first-order”) for the reduction of 4-NP in

the presence of Ag-DENs catalysts is shown in Figure 3.9. This plot is only linear if

the reaction followied first-order kinetics. Since NaBH4 was added last into the

reaction mixture, the induction period was observed in the first 100 s and is not

shown in the figure. This induction period is not observed when NaBH4 is added first.

The kinetic runs at various temperatures were also carried out in order to get the

Arrhenius plot. The dependence of rate constant on the temperature for the Ag-

DENs catalyzed reduction of 4-NP is shown in Figure 3.10. All other variables were

kept constant and only the temperature was varied.

73

Figure 3.9: First order plot for the reduction of 4-NP (600 µM) by NaBH4 (0.1 M) in the presence of

Ag-DENs (5 µM) catalyst at 298 ± 0.1 K.

Figure 3.10: The effect of temperature on the rate constant for the Ag-DENs (5 µM) catalyzed

reduction of 4-NP (600 µM) by NaBH4 (0.1 M).

The Arrhenius plot for the reduction of 4-NP in the presence of Ag-DENs

catalysts is shown in Figure 3.11. The activation energy for this reaction was

calculated to be 45.7 kJ/mol while the enthalpy change and the entropy change were

determined to be 35.0 ± 2.7 kJ/mol and -89.2 ± 11.3 J/K.mol respectively.

R2= 0.994

Y= -0.007x + 1.984

R2= 0.997

74

Figure 3.11: Arrhenius plot for the reduction of 4-NP (600 µM) by NaBH4 (0.1 M) in the presence of

Ag-DENs (5 µM) catalyst.

3.3.2.1 The effect of 4-NP concentration on the reaction rate constant .

Ag-DENs prepared employing G4 PAMAM-NH2 dendrimers were used to

investigate the effect of the 4-NP concentration on the reaction rate constant. Only 4-

NP concentration was varied while other parameters were kept constant. It was

found that, as the 4-NP concentration is increased, the rate constant decreases as

shown in Table 3.3 and Figure 3.12 respectively. This trend was also observed for

Cu-DENs and Au-DENs synthesized in G4 PAMAM-OH and G4 PAMAM-NH2

dendrimers respectively. Table 3.4 shows the results obtained for Cu-DENs and Au-

DENs.

Table 3.3: The effect of 4-NP concentration on the rate constant during Ag-DENs (5 µM) catalyzed 4-

NP reduction by NaBH4 (0.1 M) at 298 ± 0.1 K.

4-NP concentration (µM) Reaction rate constant (10-2 s-1)

100 1.13 ± 0.07

200 0.71 ± 0.05

300 0.53 ± 0.09

400 0.34 ± 0.08

Y= -5492x + 13.82

R2=0.995

75

Table 3.4: The effect of 4-NP concentration on the rate constant during Cu-DENs (5 µM) and Au-

DENs (5 µM) catalyzed 4-NP reduction by NaBH4 (0.1 M) at 298 ± 0.1 K.

4-NP concentration

(µM)

Reaction rate constant for Cu-

DENs (10-2 s-1)

Reaction rate constant for

Au-DENs (10-2 s-1)

100 5.7 ± 0.3 4.9 ± 0.1

200 4.1 ± 0.3 3.7 ± 0.2

300 3.6 ± 0.2 3.0 ± 0.1

400 2.9 ± 0.2 2.7 ± 0.1

Figure 3.12: The effect of 4-NP concentration on the rate constant during Ag-DENs (5 µM) catalyzed

4-NP reduction by NaBH4 (0.1 M) at 298 ± 0.1 K.

3.3.3. Au-DENs as catalysts in the reduction of 4-NP .

Presented in this section are the kinetic evaluations of the 4-NP reduction with

Au-DENs prepared with G4-PAMAM-NH2 dendrimers. The detailed synthetic

procedure of these Au-DENs is discussed in Chapter 2. The average particle size of

these Au-DENs was found to be 3.1 ± 0.5 nm (HRTEM images shown in chapter 2).

Figure 3.13 shows the first-order plot for the Au-DENs catalyzed reduction of 4-NP

with NaBH4. The reducing agent (NaBH4) was added before 4-NP and hence no

76

induction period was observed in this case. The rate constant for the Au-DENs

catalyzed reduction of 4-NP was determined to be 0.025 ± 0.00033 s-1. The effects of

both temperature as well as the reducing agent (NaBH4) concentration on the rate

constant were also investigated using Au-DENs as a catalyst.

Figure 3.13: First-order plot for the Au-DENs (5 µM) catalyzed 4-NP (600 µM) reduction by NaBH4 at

298 ± 0.1 K.

3.3.3.1. The effect of temperature on the rate constant .

The effect of temperature on the reaction rate constant was investigated by

carrying out kinetic runs keeping the concentration of all substrates constant ( while

varying the temperature only (278 ± 0.1 K, 288 ± 0.1 K, 293 K, 298 ± 0.1 K, and 308

± 0.1 K). The rate constant increases as the temperature increases as shown if

Figure 3.14. From the data obtained in Figure 3.14, the Arrhenius plot can be

constructed in order to calculate the activation energy for the Au-DENs catalyzed 4-

NP reduction reaction. Figure 3.15 shows the Arrhenius plot for this reaction. The

calculated EA for this reaction was found to be 40.3 kJ/mol. ∆H++ and ∆S++ were

calculated to be 35.0 ± 3.1 kJ/mol and -156.3 ± 9.4 J/K.mol respectively

R2=0.992

Y=-0.018x – 0.039

77

Figure 3.14: The effect of temperature on the reaction rate constant for Au-DENs (5 µM) catalyzed 4-

NP (600 µM) reduction by NaBH4 (0.1 M).

Figure 3.15: Arrhenius plot for the Au-DENs (5 µM) catalyzed reduction of 4-NP (600 µM) by NaBH4 (0.1 M) at different temperatures.

3.3.3.2. The dependence of rate constant on the concentration of NaBH 4.

In this section, the influence of NaBH4 concentration on the rate constant

during 4-NP reduction is investigated. For this study, kinetic runs were carried out

varying the concentration of NaBH4 while the other substrate concentrations were

R2=0.992 Y= -4842x +12.64

R2= 0.991

78

kept constant. As shown in Figure 3.16, the rate constant increases non linearly as

the concentration of NaBH4 increases.

Figure 3.16: The dependence of rate constant on the concentration of NaBH4 during Au-DENs (5 µM)

4-NP (600 µM) reduction at 298 K.

However, no increase in reaction rate constant was observed after the

concentration of NaBH4 reaches 0.1 mol/L, i.e the observed rate constant was found

to remain almost the same with increasing concentration of NaBH4 above 0.1 mol/L.

This indicates that both reactants must compete for a reactive site on the particle

surface. As a result, there must be an optimal concentration where the reaction rate

has a maximum and hence the Langmuir-Hinshelwood mechanism. The same trend

was also observed for all metals (Ag and Cu) studied. Table 3.5 shows the results

obtained when Ag and Cu-DENs are used as catalyst respectively. The rate constant

was also observed to increase with an increase in NaBH4 concentration. However,

this increase in rate constant was only observed to a certain NaBH4 concentration.

79

Table 3.5: The effect of NaBH4 concentration on the rate constant during Cu-DENs (5 µM) and Ag-

DENs (5 µM) catalyzed 4-NP reduction by NaBH4 at 298 ± 0.1 K

NaBH4 concentration

(Mol/L)

Reaction rate constant for

Cu-DENs (10-2 s-1)

Reaction rate constant for

Ag-DENs (10-3 s-1)

0.2 2.7 ± 0.2 7.2 ± 0.8

0.15 2.6 ± 0.2 7.2 ± 0.9

0.1 2.7 ± 0.1 7.1 ±0.9

0.05 1.8 ± 0.2 5.1 ± 0.7

0.025 1.5 ± 0.2 4.6 ± 0.6

0.0125 0.9 ± 0.2 3.1 ± 0.5

3.4. Conclusions.

Dendrimer encapsulated Ag, Au and Cu NPs (Ag, Au and Cu DENs) can be

used to catalyze the reduction of 4-NP to 4-AMP. The rate constant was found to be

dependent on the dendrimer generation for both Ag-DENs and Cu-DENs. The rate

constant was found to decrease as the dendrimer generation increased in the case

of Cu-DENs. The same trend was observed in case of Ag-DENs prepared within G4-

G6 PAMAM-NH2 dendrimers. This afore-mentioned observation could have been

due to the fact that as the dendrimer generation increases, the dendrimer branches

also increases making it difficult for the reactants to access the active site of the

encapsulated NPs and hence the decrease in rate constant. It is evident from the

literature that the rate constant for this reaction (4-NP reduction) does not only

depend on the size of the NPs but rather on the morphology of the NPs as well as

the type of catalyst support used.18 The rate constant was found to increase with an

increase in temperature for all catalysts (Ag-DENs, Cu-DENs and Au-DENs) used.

This observation can be explained in terms of collision theory. Particles not at 0 K

tend to move around and therefore collide. During this collision particles may react.

The higher the temperature, the faster the collision of these particles and hence the

reaction will occur faster. The rate constant was found to decrease as the

concentration of 4-NP increases. This is because a high concentration of 4-NP

molecules leads to nearly full coverage of the NPs, which consequently results in

slowing down the reaction. On the other hand, the increase in the concentration of

NaBH4 resulted in the increase in rate constant up to a certain concentration of

NaBH4. This indicates that both reactants must compete for a reactive site on the

80

particle surface. As a result, there must be an optimal concentration where the

reaction rate has a maximum.9

The sequence for the addition of NaBH4 and 4-NP does not significantly affect

the rate constant, but rather the induction period only. Addition of NaBH4 first into the

reaction mixture eliminates the induction period almost completely. There is always a

time needed for NaBH4 to reactivate the nanoparticles surface before the reaction

can start. Both Ag-DENs and Au-DENs were found to have comparable activation

energies of 40.2 kJ/mol and 45.6 kJ/mol respectively. The calculated activation

energy for both Au-DENs and Ag-DENs were comparable with those previously

reported when NPs were used as catalysts for the same reaction i.e 41 kJ/mol and

43 kJ/mol for Ag (FGME) and Au (stabilized by spherical polyelectrolyte brushes)

NPs respectively.6, 26, 27 ∆S++ was found to be negative for all reaction systems.

∆H++ for all reaction systems was calculated to be positive, hence all reactions were

endothermic.

3.5. References.

1. Y. Iinuma, E. Bruggemann, T. Gnauk, K. Müller, M. O. Andreae, G. Helas, R.

Parmar and H. Herrmann, J. Geophys. Res, 2007, 112, D08209.

2. T. Mori, T. Watanuki and T. Kashiwagura, Environ. Toxicol, 2007, 22, 58.

3. C. M. Li, S. Taneda, A. K. Suzuki, C. Furuta, G. Watanabe and K. Taya,

Toxicol. Appl. Pharmacol, 2006, 217, 1.

4. Z. V. Feng, J. L. Lyon, J. S. Croley, R. M. Crooks, D. A. V. Bout and K. J.

Stevenson, J. Chem. Educ., 2009, 86, 368.

5. C. V. Rode, M. J. Vaidya, R. Jaganathan and R. V. Chaudhari, Chem. Eng.

Sci., 2001, 56, 1299.

6. N. Pradhan, A. Pal and T. Pal, Colloids and Surfuces A: Physicochem. Eng.

Aspects, 2002, 196, 247.

7. K. Esumi, K. Miyamoto and T. Yoshimura, J. Colloid Interface Sci., 2002, 254,

402.

8. Y. Mei, Y. Lu, F. Polzer and M. Ballauff, Chem. Mater., 2007, 19, 1062.

9. S. Wunder, F. Polzer, Y. Lu, Y. Mei and M. Ballauff, J. Phys. Chem. C, 2010,

114, 8814.

10. Y. Khalavka, J. Becker and C. J. Sönnichsen, J. Am. Chem. Soc., 2009, 131,

1871.

81

11. K. Kuroda, T. Ishida and M. Haruta, J. Mol. Cat. A: Chem., 2009, 298, 7.

12. H. Wu, Z. Liu, X. Wang, B. Zhao, J. Zhang and C. Li, J. Colloid interface Sci.,

2006, 302, 142.

13. Y. Gao, X. Ding, Z. Zheng, X. Cheng and Y. Peng, Chem. Commun., 2007,

36, 3720.

14. K. Hayakawa, T. Yoshimura and K. Esumi, Langmuir, 2003, 19, 5517.

15. K. Esumi, R. Isono and T. Yoshimura, Langmuir, 2004, 20, 237.

16. Y. Wang, G. Wei, W. Zhang, X. Jiang, P. Zheng, L. Shi and A. Dong, J. Mol.

Cat. A: Chem., 2007, 266, 233.

17. M. Zhang, L. Liu, C. Wu, G. Fu, H. Zhao and B. He, Polymer, 2007, 48, 1989.

18. W. Liu, X. Yang and W. Huang, J. Colloid interface Sci., 2006, 304, 160.

19. Z. Liu, X. Wang, H. Wu and C. Li, J. Colloid interface Sci., 2005, 287, 604.

20. A. Balamurugan, K.-C. Hong and S.-M. Chen, Synth. Met., 2009, 159, 2544.

21. S. Tang, S. Vongerh and X. Meng, J. Mater. Chem., 2010, 20, 5436.

22. H. MdRashid and T. K. Mandal, J. Phys. Chem. C, 2007, 111, 16750.

23. P. Liu and M. F. Zhao, Appl. Surf. Sci., 2009, 255, 3989.

24. L. Xie, M. Chen and L. M. Wu, J. Polym. Sci. Part A: Polym. Chem., 2009, 47,

4919.

25. K. Y. Lee, J. Hwang, Y. K. Lee, J. Kim and S. W. Han, J. Colloid interface

Sci., 2007, 316, 476.

26. M. Schrinner, F. Polzer, Y. Mei, Y. Lu, B. Haupt, M. Ballauff, A. Gölden, M.

Drechsler, J. Preussner and U. Glatzel, Macromol. Chem. Phys., 2007, 208,

1542.

27. Y. Mei, G. Sharma, Y. Lu and M. Ballauff, Langmuir, 2005, 21, 12229.

28. R. Narayanan and M. A. El-Sayed, J. Phys. Chem. B, 2005, 109.

29. D. Wei, Y. Ye, X. Jia, C. Yuan and W. Qian, Carbohydr. Res., 2010, 345, 74.

30. J. Huang, S. Vongehr, S. Tang, H. Lu, J. Shen and X. Meng, Langmuir, 2009,

25, 11890.

31. S. K. Ghosh, M. Mandal, S. Kundu, S. Nath and T. Pal, Appl. Catal., A, 2004,

268, 61.

32. T. Tamai, M. Watanabe, T. Teremura and N. Nishioka, Macromol. Symp.,

2009, 282, 199.

33. Microcal (TM) origin® version 6.0.

34. Kinetic studio version 2.0.8.14953.

82

Chapter 4:

Immobilization and characterization of dendrimer encapsulated Au

nanoparticles onto inorganic oxide supports.

4.1. Introduction.

Gold nanoparticles (NPs) have been used for many different purposes.1

Amongst these, their catalytic properties have traditionally been considered weak.

This perception changed when Haruta2 and Hutchings3 discovered, simultaneously

and independently, that small gold particles could be very active for the

heterogeneous low-temperature oxidation of carbon monoxide (CO). Building on this

finding, we explore in this chapter the immobilization and characterization of titania

supported Au NPs. These synthesized titania supported Au NPs were characterized

by UV-vis spectroscopy, high resolution transmission microscopy (HRTEM), Powder

X-ray diffraction (PXRD), thermal gravimetric analysis (TGA), inductive coupled

plasma-optical emission spectroscopy (ICP-OES) and Brunauer-Emmett-Teller

(BET) surface area analysis. The catalytic activity of these titania supported NPs will

be evaluated in the oxidation of styrene in the next chapter.

The different types of oxide supports, which are ideal for immobilization of Au

and other metal NPs, are also discussed. The different methods for immobilization of

metal NPs will be outlined, a few examples of the synthesis of supported Au NPs

shall be discussed, and the use of supported Au and other metals as heterogeneous

catalysts is also summarized.

4.1.1 Inorganic oxides as supports for metal NPs .

Immobilization of NPs onto a solid support is an important step in the

preparation of practical heterogeneous catalysts. Solid materials that are used as

support during the preparation of heterogeneous catalysts can be classified into

three categories: (i) inorganic oxides such as alumina, titania, silica and zeolites, (ii)

carbonaceous materials, and (iii) polymers.4 The latter two types of supports differ

from inorganic supports in terms of surface properties. In this study, only the

inorganic oxides are briefly discussed. Inorganic oxides have often been used as

supports for both bulk metals or metal NPs.5, 6 The advantages of using inorganic

83

oxides over other solid supports is mainly due to their high surface area as well as

their thermal stability.

In the case of amorphous silica, alumina and titania, the unconnected oxygen

atoms at the surface are usually protonated to give hydroxyl groups, e.g., silanols in

the case of silica.7 A disadvantage of amorphous silica is its irregular pore structure,

which results in mass transport limitations and this can consequently lower the

catalyst activity. In the early 90’s, researchers at Mobil Oil Company reported the

synthesis of meso-structured materials with a narrow pore size distribution of 3 nm in

a highly regular hexagonal arrangement.8 They referred to these materials as

mesoporous M41S silicate materials. MCM-41, MCM-48 and SBA-15 are the most

widely studied support materials from the family of M41S. MCM-41, first reported by

the Mobil researchers,8 consists of the hexagonally ordered structure and

unidirectional pore structure. MCM-48 on the other hand, consists of a cubic array

with a 3-dimensional pore network.9

SBA-15, first reported by Zhao et al. consists of hexagonally ordered

mesoporous channels, interconnected by secondary micropores in the wall,

providing it with a three dimensional pore structure.10 Tetraethoxysilane,

tetramethoxysilane and tetramethyl orthosilicate can all be used as silica precursors

for the synthesis of these mesoporous silica materials. Tetraethyl orthosilicate was

found to be the best precursor for the synthesis of MCM-48.10

Porous titania,11 zeolites12 and alumina12 are also some of the common

supports used for nanoparticle immobilization. They also have sufficient thermal

stability. Silica has been reported as not ideal for the immobilization of Au NPs.13

This is because Au NPs form very unreactive (+50 nm) particles on the silica surface

when treated at high temperature.14 Titania on the other hand has been the most

common support used for immobilization of Au NPs.15, 16 Several methods are used

to immobilize metal NPs onto solid supports and they will be discussed here. It is

worth mentioning that NPs are generally insoluble. However in this study, metal

DENs are rendered soluble by the dendrimer template.

84

4.1.2 Methods for immobilization of NPs .

(i) Co-precipitation.

This relatively easy, one step process involves the addition of sodium

carbonate to an aqueous solution of the metal precursor and the addition of a nitrate

salt of the metal oxide that will consequently leads to the production of the support.17

The precipitates are then washed, dried, and calcined in air, resulting in the

formation of supported NPs. This support has a high surface area and usually leads

to highly dispersed metal NPs.

(ii) Deposition-precipitation

This method involves precipitation of the metal hydroxide on the oxide

support.18 It is generally used to prepare small sized oxide-supported metal NPs.

This method can be carried out by adding the support to an aqueous solution of

metal precursor, after which the pH of the suspension is raised to usually 7 or 8,

followed by addition of sodium hydroxide (NaOH). This subsequently results in the

reduction of metal precursor to form NPs. The suspension is then heated between

70 oC to 80 oC with stirring for 1 hour. The resulting composite is washed with water

at 50 oC to remove NaOH. The product is subsequently dried in vacuo at 100 oC and

calcined in air at higher temperatures.

(iii) Wetness impregnation.

This is also a simple method to carry out, and can be used with any support.

In this method, the soluble NPs are formed first. These are then immobilized into the

pores of the support. This can be carried out by simply mixing the soluble catalyst

and the insoluble support together using a solvent, after which the solvent is

removed during the activation of the catalyst at high temperature.

(iv) Sol-gel process.

This involves a single step for preparing oxide-supported metal catalysts. The

soluble NPs are formed first using any method, followed by hydrolysis of a metal

alkoxide percursor in a water-alcohol solution. The solvents were removed from the

85

resulting slurry/gel by drying as well as during the activation step at higher

temperature. The oxide support produced here is normally amorphous. The use of

alcohol in the sol-gel process can lead to an increased size of metal NPs as

compared to when water is used alone.16 The difference in dielectric constant

between water and MeOH was assumed to be the reason for this observation.19

Sun and Crooks reported two methods for the deposition of dendrimer-

encapsulated Pd nanoparticles (Pd-DENs) onto flat mica or highly oriented pyrolytic

graphite (HOPG) surfaces via a surface chemical reaction.20 PAMAM-OH and

PAMAM-NH2 dendrimers were used in the first and second method respectively. In

the first method, Pd-DENs were first prepared21 and then chemisorbed onto the

support surface. Electrostatic attraction between the positively charged dendrimers

(at neutral pH) and the negatively charged mica surface is expected to occur.22, 23

Again, the surface functional groups on the dendrimer may also interact with the

support through hydrogen bonding.24 The dendrimer template was subsequently

removed at 630 oC. Unfortunately, this calcination step led to an increased size in

the Pd NPs, due to surface diffusion of the NPs. This is due to the fact that the

support is not interacting strongly enough with the NPs. In the second method, Pd-

DENs were prepared directly on the surface of the support. The aqueous dendrimer

solution was first added to the support surface, followed by the addition of metal ion

precursor to the immobilized dendrimer. Addition of a suitable reducing agent results

in the formation of supported Pd-DENs. As in the first method, the dendrimer

template was removed by calcination at 630 oC. The NPs prepared by this method

were found to be catalytically active for the electroless deposition of Cu.

Chandler and co-workers reported a method for immobilization of Pt-DENs in

a sol-gel silica matrix.5 In this study, Pt-DENs were added at two different times to

prepare two catalysts: (i) before the condensation reactions were initialized (catalyst

A) and (ii) 24 hours after the condensation reactions and the formation of the sol but

before the solution had gelled (catalyst B). The prepared Pt-DENs from both

methods were added to tetramethoxysilane (TMOS) and the solution was then

combined with a mixture of glacial acetic acid (HOAc) and water. The resulting

solution was stirred slowly at room temperature for 3 days. The formation of a gel

was observed after 2 days of stirring. The formed gel was then dried in a furnace for

48 hours at 80 oC. After the dried product was ground, it was calcined at 300 oC for

86

16 hours. The catalytic activity of these two catalysts was investigated in the

oxidation of CO as well as hydrogenation of toluene. It was found that, both catalysts

A and B showed similar activities for CO oxidation. However, catalyst A appeared to

be catalytically more active than catalyst B in the hydrogenation of toluene. This was

attributed to the fact that catalyst B has a lower Pt surface area as compared to

catalyst A. Lower Pt surface area in catalyst B was attributed to the fact that the

decomposition of the dendrimer might have poisoned the catalyst during the

activation step.

In another study, Korkosz et al reported the immobilization of Au NPs

extracted from Au-DENs precursors.25 Here, both aqueous and organic dendrimer

solutions were used to prepare Au-DENs separately. The aqueous Au NPs were

subsequently extracted from the dendrimer into an organic phase using decanethiol

and toluene while the organic Au NPs were extracted into an aqueous phase using

N-(2-mercaptopropionyl)glycine (tiopronin) dissolved in HCl. Both decanethiol and N-

(2-mercaptopropionyl)glycine serve as stabilizers for the extracted Au NPs. These

thiol protected Au NPs are referred to as monolayer-protected clusters (MPCs).

These MPCs were then deposited onto commercial titania via wetness impregnation

as shown in Figure 4.1. The organic and aqueous MPCs were immobilized via the

wetness impregnation method in this study. This involves stirring of these soluble

extracted MPCs with titania for 4-6 hours. Both these supported catalysts were then

heated at 300 oC for 2 hours to remove the protecting thiols. The catalytic activity of

these titania supported Au catalysts were evaluated on the oxidation of CO. These

two catalysts showed comparable to slightly higher activity for CO oxidation

compared to a similarly treated World Gold Council Au/P25 titania catalyst (WGC) at

moderate temperatures (35–95 oC). The WGC was also subjected to the same

treatment as other prepared catalysts for a better comparison.

87

G o ld M P C s in so lu t io n

T ita n ia S u p p o rt

S u p p o rted M P C s

A c tiv a t io n

(H ea t, O 2 , H 2 )

M P C s D er ived C a ta lys t

P ro tec t in g th io ls

N P s

D E N s N P s e x tra c tio n

Figure 4.1: Preparation of titania supported Au NPs from Au DENs precursor.25

Shortly after this report, Chandler and co-workers reported a related study on

the preparation of titania supported Au NPs from the MPCs precursors.26 In this

study they extended their previous study25 by applying Michaelis-Menten (M-M)

kinetics. This mechanism is usually applied to biological systems (enzyme kinetics)

where there is a reversible equilibrium between enzyme and substrate. Thus the two

can either decompose back to enzyme and substrate, or the substrate can be

converted into a product and the enzyme left unchanged. The Michaelis-Menten

kinetics was also applied by Augustine et al for the investigation of catalyst active

sites during partial oxidation of 2-propanol over a supported Pt catalyst.27 This model

allowed Chandler and co-workers to quantitatively compare the similarities and the

differences of their catalysts with some other catalysts prepared by different

methods. This was done by measuring the O2 binding constant for the Au catalysts,

which in turn allows for comparison of the relative number of active sites between

different catalysts. The catalysts in this study were pre-treated for a longer time (16

hours) as compared to the previously discussed catalysts reported by the same

88

research group (2 hours). This longer pre-treatment time allowed a full

decomposition of the protecting thiols and complete removal of sulphur, which

resulted in more active sites available for catalysis. It was found that all the catalyst

pre-treatment steps used here did not significantly change the NPs size or affect the

activity of the catalyst. The synthesized Au catalyst and the reference WGC catalyst

were found to have essentially identical rate laws as well as activation energies for

CO oxidation. However, the Au catalyst synthesized here was found to be 50 %

more active than the standard WGC catalyst under the experimental conditions used

in this study for the oxidation of CO. This can be attributed to the findings from the

Michaelis-Menten treatment which revealed that the Au catalyst prepared contain

roughly 50 % more active sites than the traditionally prepared WGC catalyst.

Chandler and co-workers also reported the preparation of titania supported Au

nanoparticle catalysts from Au-DENs precursors.28

In the same year, Huang et al reported the synthesis of SBA-15 supported Rh

and Pt NPs from Rh and Pt-DENs precursors.29 The DENs were synthesized first as

reported in the literature.30 Both these Pt and Rh-DENs were then immobilized onto

the SBA-15 support via wetness impregnation method (see Figure 4.2). To achieve

this, the acidity of the dendrimer solution was adjusted to pH 5. This was done to

deprotonate the dendrimer which will allow better electrostatic interaction with the

SBA-15 support, which is negatively charged under these conditions.20 This

electrostatic interaction, together with hydrogen bonding between the terminal

hydroxyl groups on both the dendrimer and the SBA-15 silica surface, creates a

stable supramolecular entity.20 The immobilization is usually carried out by mixing

the aqueous DENs solution with the SBA-15 and the resulting slurry is sonicated for

3 hours at room temperature. The SBA-15 supported NPs were separated from the

solution using a centrifuge and subsequently dried under ambient conditions for 2

days and then at 100 oC for 4 days. The dendrimer template of the DENs remained

intact during this heat treatment. The catalytic activities of these prepared catalysts

were evaluated for the hydrogenation of pyrrole and ethylene. In this study, Rh

catalysts showed both higher activity and selectivity for hydrogenation of pyrrole as

compared to the Pt catalysts.

Scott et al also reported the synthesis of titania supported Au and Pd NPs via

both wetness impregnation and sol-gel methods.16 Here, Au and Pd-DENs were

89

used as the precursors for the preparation of titania supported Au and Pd NPs

respectively. Typically Au and Pd-DENs were first synthesized separately and

subsequently immobilized onto the high surface area titania. In the sol-gel method,

the dendrimer was used to perform a dual templating role. The inside of the

dendrimer templated the metal NPs while the outside of the dendrimer templated the

pores within the sol-gel titania matrix. The dendrimer was then removed by calcining

at 500 oC in air. They found that the wetness impregnation method leads to a 4-fold

increase in the average sizes of Au NPs. The BET surface area of titania supported

Pd-DENs prepared via the sol-gel method was determined to be 34.0 m2/g before

calcination and 50.2 m2/g after calcining. This increase in the surface was attributed

to the mesoporosity introduced by the removal of dendrimer template. However, the

pore volume was found to decrease from 0.18 to 0.10 cm3/g before and after

calcination respectively. This decrease in pore volume was assumed to be due to

loss of macroporosity arising from crystallization of the titania matrix upon

calcination.

Loading

SBA-15

DENs

DENs

Pore

Figure 4.2: Immobilization of dendrimer-encapsulated NPs into SBA-15 pores via wetness

impregnation.4

Chandler et al recently reported the synthesis of titania supported bimetallic

Au-Ni NPs.31 These titania supported bimetallic Au-Ni NPs were prepared from the

dendrimer-encapsulated Au-Ni nanoparticle precursor. In their study, the amine-

terminated PAMAM dendrimer was first anchored onto functionalized silica. The

functionalization of silica was carried out using 3-(triethoxysilyl)propylsuccinic

90

anhydride. The anchored dendrimers were subsequently alkylated with 1,2-

epoxydodecane to yield a hydrophobic dendrimer surface. This anchored alkylated

amine-terminated dendrimer was then used as a template for the preparation of Au-

Ni bimetallic NPs. The bimetallic DENs were prepared by simultaneously adding

aqueous Ni2+ and Au3+ ions to the anchored dendrimer.The metal:dendrimer ratio of

147:1 was used while Au:Ni ratios of 2, 1, 0.5, 0.3 were used. The mixture was

stirred for 1 hour at 60 rpm. Addition of NaBH4 resulted in the formation of Ni-Au

DENs, which is accompanied by the formation of a brown colour. The synthesized

bimetallic Au-Ni DENs were then extracted from the dendrimer and subsequently

immobilized onto the titania support. Extraction of the bimetallic Ni-Au NPs was

carried out by adding a deoxygenated solution of decanethiol (3 mL) in toluene (5

mL) to the bimetallic Ni-Au DENs mixture. The solution was stirred for 30 minutes,

yielding a brown extracted Ni-Au MPCs. Centrifugation resulted in the separation of

MPCs from excess n-alkanethiol and other impurities. The catalytic activity of these

supported NPs was evaluated using CO-oxidation. It was found that the presence of

Ni has a dramatic effect on the CO-oxidation as compared to standard Au NPs

catalyzed CO-oxidation. Although no clear conclusion could be drawn from this, it

was suspected that the presence of Ni resulted in increased O2 chermisorption onto

the Au-Ni catalyst. Chandler and co-workers also reported the synthesis of

supported bimetallic Pt-Au NPs.13, 32 These bimetallic Pt-Au NPs were immobilized

on high surface area supports such as silica, titania and alumina and were all found

to have a higher catalytic activity in the CO-oxidation as compared to supported Au

NPs.

Based on these literature reports, the immobilization of Au-DENs onto titania

support via both wetness impregnation and sol-gel method is outlined. The catalytic

activity of these prepared titania-Au NPs will be evaluated and discussed in the next

chapter.

91

4.2. Experimental.

G4-PAMAM-NH2 dendrimer (10% wt in methanol) having a ethylenediamine

core, Degussa P25 TiO2, titanium isopropoxide (Ti(OiPr)4) (99.99%), sodium

borohydride (NaBH4), glycidyltrimethylammonium chloride (90%), HAuCI4 and MeOH

(99.5 %) were all purchased from Sigma Aldrich. NaOH, HCl and a Au ICP-EOS

standards were purchased from Merck. All chemicals were of analytical grade. All

experiments were performed using de-ionized water.

pH Adjustment were performed using an ORION model 520A, SCHOTT pH

electrode blueline 25. Calibration of the pH meter was performed using pH 4.01 and

10.01 standard buffer solutions (Scientific ADWA (M) Sdn.Bhd.). MeOH was dried

by distillation over magnesium and iodine.33

All UV-vis spectra were obtained using a 10 mm path length cuvette with

either a VARIAN Cary 100 Conc or a Shimadzu 1800 UV-visible spectrophotometers

equipped with Cary WIN UV and kinetic studio software respectively.

All TEM images were obtained using a JEOL-Jem 2100 HRTEM. Au DENs

particle sizes and images were analyzed by HRTEM at an accelerating voltage of

197 kV on a Philips JEOL-Jem 2100 HRTEM using a Gatan GIF Tridiem and an

Oxford INCA energy dispersive X-ray (EDX) analysis system. Samples for TEM were

prepared by placing one drop of aqueous Au DENs or a methanolic slurry of titania

supported Au NPs solution on a holey carbon-coated Cu grid (200 mesh) and

allowing the solvent to evaporate in air. Particle size distributions were calculated

using ImageJ software.34

SEM images were obtained using a JEOL-JSM-5600 SEM. Porosity and

surface area measurements were carried out using physisorption (Micrometrics

Tristar 3000 v6.05 A). Pore size distributions were calculated using the Barrett-

Joyner-Halenda (BJH) method. Thermogravimetric analyses were performed using a

TGA Perkin-Elmer STA6000 with a heating rate of 10 oC/min. The X-ray diffraction

powder patterns were collected by a Phillips diffractometer (Cu-Kα, 40 kV, 40 mA)

using a collection range of 2Theta= 4-80 degrees at 25 oC. Au loading was

determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES).

The calibration curve was obtained using different concentrations of Au standard

92

solutions diluted in aqua-regia solution (1:3 v/v of HCl:HNO3). The sample was

prepared by digestion of 0.2 g of titania supported Au NPs with aqua-regia solution.

4.2.1. Synthesis of Au-DENs for immobilization via both wetness impregnation

and sol-gel methods .

Au-DENs precursors for both sol-gel and wetness impregnation

immobilization methods were prepared according to the reported literature.35 The

general synthetic route involves preparing an aqueous/ organic dendrimer solution.

The Au3+ ions (55 molar excess) are then added to the dendrimer solution. The

metal ions underwent coordination with the tertiary amine groups within the

dendrimer. This was followed by the addition of excess reducing agent such as

sodium borohydride (NaBH4) which resulted in the formation of the Au-DENs.

4.2.2. Synthesis of 2 µM Au-DENs and their immobilization onto commercial

TiO2 by wetness impregnation .

For this synthesis, the surface of the dendrimer was first functionalized with

glycidyltrimethylammonium chloride to give partially quaternized G4-PAMAM

dendrimers (G4-Qp), where p is the number of quaternary amines per dendrimer.

The concentration of 2 µM of G4-PAMAM-NH2 dendrimer solution was used for this

synthesis.

The following procedure was adopted from the literature to synthesize 2 µM

G4-Q32(Au55).16, 36 G4-PAMAM-NH2 dendrimer (0.142 g, 10 wt % in MeOH, 1.00

µmol) was dissolved in de-ionised water (100 mL). Glycidyltrimethylammonium

chloride (9.5 µL, 64 µmol) was added dropwise to the aqueous dendrimer solution

under stirring. The solution was stirred for 2 days at 40 oC. The functionalized

dendrimer was then added to MeOH (400 mL). Aqueous HAuCl4 (550 µL, 0.1 M) was

added to the functionalized dendrimer solution. After the mixture was stirred for 5

minutes, NaBH4 (550 µL, 1.0 M) dissolved in aqueous NaOH (0.3 M) was added.

The solution changed colour from yellow to brown-red signalling the formation of the

Au-DENs. The MeOH solvent was removed from the prepared G4-Q32(Au55) DEN

composite by rotary evaporation. The remaining water was removed by placing the

solution over phosphorus pentoxide (P2O5) in a desiccator overnight. The resulting

93

Au-DENs powder was mixed together with Degussa P25 titania (0.99 g) and MeOH

(2 mL). The slurry was dried in a desiccator overnight then calcined at 500 oC in air

for 3 hours. Since chandler and co-workers have reported the removal of the

dendrimer template at 300 oC,5 it was assumed that all the dendrimer residual atoms

would be removed at this annealing temperature used ( 500 oC).

4.2.3. Synthesis of 0.92 µM Au-DENs and their immobilization onto titania via

sol-gel synthesis.

The procedure described in the literature was followed to synthesize a 0.92

µM G4-NH2(Au55) DEN solution which wa immobized on the titania.16, 36 G4-PAMAM-

NH2 (0.131 g, 10 wt % in MeOH) was added to MeOH (1L, 99%). Aqueous HAuCl4

solution (510 µL, 0.1 M) was added to the dendrimer solution. After the mixture was

stirred for 5 minutes, 10 molar excess of NaBH4 (510 µL, 1.0 M) dissolved in

aqueous NaOH (0.3 M) was added. The solution changed colour from yellow to

brown-red, indicating the formation of the DENs. After the solution was stirred for

several hours under N2, HCl (20 µL, 0.3 M) was added to neutralize excess NaBH4

and NaOH. This was to avoid the reduction of Ti(OiPr)4. After the mixture was stirred

3 hours, Ti(OiPr)4 (3.52 g, 99.99%) dissolved in dried MeOH (5 mL) was added

dropwise to the solution. The solution became turbid after 1 minute as the alkoxide

precursor hydrolyzed. After 5 minutes, de-ionized water (100 mL) was added. The

resulting precipitate was left at room temperature for 24 hours and filtered. The

precipitate was dried at 100 oC. The powder was then calcined at 500 oC in air for 3

hours.

4.3. Results and Discussion.

Figure 4.3 shows the representative HRTEM image and size distribution of

the Au-DENs prepared for the immobilization via the sol-gel method. The average

particle sizes were calculated using the ImageJ software. The average size

distribution of the Au-DENs was found to be 2.3 ± 0.4 nm. This size distribution is

slightly higher than the one reported in the literature (1.9 ± 0.5 nm) using the same

synthetic method.16 The average particle size of the Au-DENs prepared within the 2

µM dendrimer solution for the immobilization via the wetness impregnation method

was measured to be 1.9 ± 0.4 nm. Figure 4.4 shows the representative HRTEM

image as well as the particle size histogram.

94

Figure 4.3: a) HRTEM image and b) particle size histogram of 0.92 µM Au-DENs synthesized in 99 %

MeOH.

Figure 4.4: a) HRTEM image and b) particle size histogram of 2 µM Au-DENs synthesized in 99 %

MeOH.

The size distribution here was comparable to the one reported in the literature (1.7 ±

0.4 nm).16

4.3.1 Immobilization and characterization of Au-DENs onto titania.

The Au-DENs synthesized using amine-terminated as well as partially

quaternized surface G4-PAMAM dendrimer concentrations (0.92 µM and 2 µM) were

deposited onto the titania using two different methods. Firstly, Au-DENs synthesized

using a partially quaternized dendrimer (G4-Q32(Au55)) were immobilized onto a

commercial titania via the wetness impregnation method. Secondly, the Au DENs

synthesized in 0.92 µM dendrimer (G4-PAMAM-NH2(Au55)) solution were

incorporated into the amorphous titania network prepared using sol-gel chemistry.

10 nm

a b

b a

95

4.3.1.1 Deposition of Au-DENs onto a commercial titania support by wetness

impregnation .

The dried titania-Au-DENs powder was then calcined at 500 oC in air for 3

hours to remove the dendrimer template, leaving only Au NPs on the titania support

(the optimum calcination temperature was determined by TGA). This resulted in

significant agglomeration of Au NPs as can be seen in Figure 4.5. The average

particle size increased to 7.9 ± 1.9 nm after calcination. This particles agglomeration

could have been due to the removal of the organic stabilizers during calcination, i.e

dendrimers

Figure 4.5: a) HRTEM image and b) particle size histogram of G4-Q32(Au55) after calcination at 500 oC in air for 3 hours.

This observation was not in agreement with the work reported by Chandler and co-

workers which showed that metal DENs can be deposited onto a commercial silica

support to yield supported metal NPs and still retain their initial size distribution even

after calcination at 300 oC.37 However Sun et al found that NPs aggregate at the step

edges of a graphite support when the dendrimer was removed at high temperature.20

4.3.1.2. Immobilization of Au-DENs onto titania by sol-gel method .

After Ti(OiPr)4 dissolved in dried MeOH was added to the solution, the

solution became turbid after one minute as the alkoxide precursor hydrolyzed (see

Figure 4.6).

b a

96

Figure 4.6: Addition of TiO2 precursor (titanium isopropoxide) to the preformed G4-PAMAM-

NH2(Au55) solution.

HRTEM revealed that the average particle size of these titania supported Au

NPs did not change significantly when the sol-gel method is used to support Au

DENs onto titania (see Figure 4.7.).

Figure 4.7: a) HRTEM image and b) particle size distribution histogram of G4-PAMAM-NH2(Au55)

after calcination at 500 oC in air.

The average particle size of the Au NPs increases from 2.3 ± 0.4 nm (before

supported) to 3.2 ± 0.7 nm after being supported (after calcination at 500 oC for 3

hours). This improved retention of the average NPs size when the sol-gel method is

used has been attributed to the fact that Au NPs are well dispersed within the titania

matrix as opposed to the Au NPs made by wetness impregnation method.

Ti(OiPr)4

Au-DENs After addition of Ti(OiPr)4

a b

97

4.4. Thermogravimetric and PXRD analyses of the synthesized titania

supported G4-PAMAM-NH4(Au55) and G4-Q32(Au55).

As mentioned earlier, the maximum calcination temperature of these titania

supported G4-PAMAM-NH2(Au55) and G4-Q32(Au55) was determined by

thermogravimetric analysis. As can be observed in Figure 4.8, TGA showed that the

weight loss reached its maximum value in the range 500-550 oC. For this analysis, a

higher synthetic dendrimer-to-titania mass ratio (0.13 as compared to 0.013 used for

earlier preparations) was used. This was done as the detection limit of the TGA is

not adequate to give a proper profile. Figure 4.8.a shows the percentage derivative

weight loss of the G4-PAMAM-NH2(Au55) as the temperature increases. The plot for

the percentage weight loss of the same material is represented by Figure 4.8.b.

There is a 15 % weight loss between 100 oC and 300 oC. This weight loss is

attributed to the evaporation of moisture from the sample. Another weight loss is

observed between 300-400 oC which is attributed to decomposition of the dendrimer

template. Final loss of weight was also observed between 500-550 oC. This is the

loss due to the decomposition of the Ti-Au NPs sample. A similar TGA plot was

obtained for the G4-Q32(Au55) material.

Figure 4.8: Thermogravimetric analysis plot of titania supported G4-PAMAM-NH2(Au55) before

calcination.

a

b

98

Powder X-ray diffraction pattern revealed that the synthesized titania

supported G4-PAMAM-NH2(Au55) material is amorphous before calcination (see

Figure 4.9.a). It should be noted that the peaks appearing in Figure 4.9.a are due to

the presence of Au only. However, after calcination, the Powder X-ray diffraction plot

showed that the titania is partially converted to the crystalline anatase phase (see

Figure 4.9.b).38 No rutile phase was observed after calcination. SEM images of the

synthesized titania supported G4-PAMAM-NH2(Au55) after calcination are shown in

Figure 4.10. It is clear from Figure4.10.a that the titania was converted to a

crystalline phase after being calcined. Figure 4.9.c shows the powder X-ray

diffraction of the synthesized titania support without any metal loading.

Figure 4.9: Powder X-ray diffraction plot of the a) titania-G4-PAMAM-NH2(Au55) before calcination; b)

titania- G4-PAMAM-NH2(Au55) and c) titania support only, after both materials were calcined at 500 oC

in air for 3 hours.

The appearance of some new peaks (2θ= 38 o, 46 o, 66 o and 78 o) in Figure 4.9.b

was compared to the PXRD patterns obtained for the titania support only without

metal loading( Figure 4.9.c). The new peaks are due to the presence of Au NPs

immobilized onto titania as shown in Figure 4.9.a. Similar PXRD profiles were also

obtained for G4-Q32(Au55) material.

TiO2 TiO2, Au

Au

TiO2

c

b

TiO2 TiO2

Au Au TiO2

Au

Au a Au Au

TiO2

99

Figure 4.10: SEM images of titania supported G4-PAMAM-NH2(Au55) after calcination at 500 oC in air

for 3 hours.

4.5. Physisorption analysis and metal loading of the titania-supported G4-

PAMAM-NH2(Au55) and G4-Q32(Au55).

As summarized in Table 4.1, the surface area determined by the BET analysis

for the synthesized titania supported G4-PAMAM-NH2(Au55) was found to be 9.2

m2/g before calcination. After calcining at 500 oC, this surface area increased

significantly to 56.9 m2/g. The BET surface area for G4-Q32(Au55) was determined to

be 11.3 m2/g and 61.6 m2/g prior and after cacination respectively. This increase in

surface area could be due to mesoporosity introduced by the dendrimer. Before

calcination, the synthesized titania supported G4-PAMAM-NH2(Au55) and G4-

Q32(Au55) showed a type lll isotherm, which is typical of macroporous materials.39

The total pore volumes were determined to be 0.04 cm3/g and 0.12 cm3/g for G4-

PAMAM-NH2(Au55) prior to and after calcination respectively. This trend in the

change of total pore volume is not in agreement with what was observed for the

titania supported Pd NPs prepared from Pd-DENs precursor reported in the

literature.16 This decrease in pore volume in the case of titania supported Pd NPs

was attributed to the loss of macroporosity arising from crystallization of the titania

matrix upon calcination. The average pore size was measured to be 16.5 nm before

and 8.0 nm after calcination respectively. Figure 4.12 shows the BJH pore-size

distribution of titania/G4-PAMAM-NH2(Au55) after calcination at 500 oC. Both these

pore sizes are typical characteristics of mesoporous materials. Figure 4.11 shows

the nitrogen adsorption-desorption isotherms for a synthesized G4-PAMAM-

NH2(Au55) after calcination at 500 oC in air for 3 hours.

a b

100

Figure 4.11: Nitrogen adsorption/desorptions isotherm for titania supported G4-PAMAM-NH2(Au55).

After calcination, these titania supported Au NPs showed hysteresis loops which are

closed in the pressure region (see Figure 4.11). This type of isotherm is

characteristic of a mesoporous material, i.e it is a type IV isotherm.39

Figure 4.12: BJH pore-size distribution of titania-G4-PAMAM-NH2(Au55) after calcinations at 500 oC in

air for 3 hours.

101

Table 4.1: Summary of BET surface area, total pore volume and pore size of titania supported G4-

PAMAM-NH2(Au55) and G4-Q32(Au55) before and after calcination.

The extent of Au loading onto the titania support for both G4-Q32(Au55) and

G4-PAMAM-NH2(Au55) was determined using ICP-OES elemental analysis. The Au

loading was found to be 1.8 and 1.3 wt % for G4-Q32(Au55) and G4-PAMAM-

NH2(Au55) respectively

4.6. Conclusions.

DENs can successfully be immobilized onto oxide supports via different

methods such as wetness impregnation, sol-gel and precipitation deposition. Of

these methods used in this study, sol-gel was found be a good method for NPs

immobilization as it does not lead to severe particles agglomerization as compared

to the wetness impregnation method. Silicaand titania are considered to be ideal for

use as NPs support in heterogeneous catalysis. The primary reason for this is that

these supports are mildly acidic, inert and they have desirable mechanical

properties. Tetraethyl orthosilicate (TEOS) has been reported as the better silica

precursor. Although silica is considered amongst some of the best supports for NPs,

the preparation of silica supported Au NPs is difficult. This is because Au NPs are

extremely mobile on silica surfaces and therefore form very large (+50 nm)

catalytically inactive particles when treated at higher temperatures.14 Additionally, it

was found that silica does not effectively stabilize Au NPs against agglomerization.

Titania-G4-PAMAM-NH2(Au55) before calcination.

Titania-G4-PAMAM-NH2(Au55) after calcination.

G4-Q32(Au55) before calcination.

G4-Q32(Au55) after calcination.

BET surface

area (m2/g)

9.2

56.9

11.3

61.6

Total pore

volume (cm3/g)

0.04

0.12

0.06

0.14

Pore size (nm) 16.5

8.0

13.4

7.2

102

The precipitation-deposition method is not ideal for the immobilization of Au NPs

onto silica.18 This is because the silica surface is negatively charged at a pH required

to precipitate [Au(OH)x] on a support. Titania is the most widely used solid support

for Au NPs as compared to other oxide supports. Titania can be used successfully

as a stable support for Au NPs.

PAMAM-NH2 dendrimers with NPs encapsulated within their cavities can be

utilized successfully to deliver these NPs onto a solid support. This is possible

through a covalent linkage between the NH2 groups on the dendrimer periphery and

the support surface. Again, PAMAM-NH2 dendrimers can be used to template and

stabilize both the NPs as well as the porosity of the support using sol-gel chemistry.

Wetness impregnation yields particles with much larger sizes as compared to the

sol-gel method. Calcination usually leads to agglomeration of NPs. The synthesized

titania Au NPs composite material was found to be stable to a maximum temperature

of 500 oC. Calcination of the amorphous titania at 500 oC led to a partial conversion

of titania to the crystalline anatase phase. The powder X-ray diffraction method can

be used to confirm the immobilization of metal NPs onto the solid support. The

nitrogen adsorption-desorption isotherm revealed that the synthesized material has a

characteristic of a mesoporous materials after calcination at 500 oC. Calcination of

the synthesized G4-PAMAM-NH2(Au55 ) and G4-Q32(Au55) composite materials

resulted in a significant increase in surface area as well as decrease in average pore

size respectively.

103

4.6. References

1. R. Sardar, A. M. Funston, P. Mulvaney and R. W. Murray, Langmuir, 2009,

29, 13840.

2. R. M. T. Sanchez, A. Ueda, K. Tanaka and M. Haruta, J. Catal., 1997, 168,

125.

3. A. S. K. Hashmi and G. J. Hutchings, Angew. Chem., Int. Ed., 2006, 45, 7896.

4. K. P. d. Jong, Synthesis of solid catalysts, WILEY-VCH Verlag GmbH & Co.,

Weinheim, 2009.

5. L. W. Beakley, S. E. Yost, R. Cheng and B. D. Chandler, Appl. Cat., A, 2005,

292, 124.

6. S. Todorova, V. Pârvulescu, G. Kadinov, K. Tenchev, S. Somacescu and B.-

L. Su, Microporous Microporous Mater., 2008, 113, 22.

7. L. T. Zhuravlev, Colloids Surf. A: Physicochem. Eng. Aspects, 2000, 173, 1.

8. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. C. Beck,

Nature, 1992, 359, 710.

9. J. Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem., Int. Ed., 1999, 38,

56.

10. D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredericks, B. F. Chmelka

and G. D. Stucky, Science, 1998, 279, 548.

11. J. J. Pietron, R. M. Stroud and D. R. Rolison, Nano. Lett., 2002, 2, 545.

12. N. Zheng and G. D. Stucky, J. Am. Chem. Soc., 2006, 128, 14278.

13. H. Lang, S. Maldonado, K. J. Stevenson and B. D. Chandler, J. Am. Chem.

Soc., 2004, 126, 12949.

14. A. Wolf and F. Schuth, Appl. Cat., A, 2002, 226, 1.

15. R. W. J. Scott, C. Sivadinarayana, O. L. Wilson, Z. Yan, D. W. Goodman and

R. M. Crooks, J. Am. Chem. Soc., 2005, 127, 1380.

16. R. W. J. Scott, O. M. Wilson and R. M. Crooks, Chem. Mater., 2004, 16, 5682.

17. L. D. Socaciu, J. Hagen, T. M. Brenhardt, L. Wöste, U. Heiz, H. Häkkinen and

U. Landmann, J. Am. Chem. Soc., 2003, 125, 10437.

18. M. Haruta, Catal. Today, 1997, 36, 153.

19. Y. Niu and R. M. Crooks, Chem. Mater., 2003, 15, 3463.

20. L. Sun and R. M. Crooks, Langmuir, 2002, 18, 8231.

104

21. R. W. J. Scott, H. Ye, R. R. Henriquez and R. M. Crooks, Chem. Mater., 2003,

15, 3873.

22. H. G. Hansma and D. E. Laney, Biophys. J., 1996, 70.

23. J. Y. Ye, K. Umemura, M. Ishikawa and R. Kuroda, Anal. Biochem., 2000,

281, 21.

24. H. Tokuhisa, M. Zhao, L. A. Baker, V. T. Phan, D. L. Dermody, M. E. Garcia,

R. F. Peez, R. M. Crooks and T. M. Mayer, J. Am. Chem. Soc., 1998, 120,

4492.

25. R. J. Korkosz, J. D. Gilbertson, K. S. Prasifka and B. D. Chandler, Catal.

Today, 2007, 122, 370.

26. C. G. Long, J. D. Gilbertson, G. Vijayaraghavan, K. J. Stevenson, C. J. Pursell

and B. D. Chandler, J. Am. Chem. Soc., 2008, 130, 10103.

27. R. L. Augustine and L. K. Doyle, J. Catal., 1993, 141, 58.

28. H. Hartshorn, C. J. Pursell and B. D. Chandler, J. Phys. Chem. C 2009,

10718.

29. W. Huang, J. K. Kuhn, C.-K. Tsung, Y. Zhang, S. E. Habas, P. Yang and G.

A. Samorjai, Nano. Lett., 2008, 8, 2027.

30. R. M. Crooks, M. Q. Zhao, L. Sun, V. Chechiv and L. K. Yeung, Acc. Chem.

Res., 2001, 34, 181.

31. B. D. Chandler, C. G. Long, J. D. Gilbertson, C. J. Pursell, G. Vijayaraghavan

and K. J. Stevenson, J. Phys. Chem. C, 2010, 114, 11498.

32. B. J. Auten, H. Lang and B. D. Chandler, Appl. Cat., B, 2008, 81, 225.

33. R. Malgas-Enus, The Preparation And Characterization Of Multinuclear

Catalysts Based On Novel Dendrimers: Application In The Oligomerization

And Polymerization Of UnsaturatedHydrocarbonsUniversity of Stellenbosch,

2011.

34. W. Rasband, ImageJ 1.42q, National Institute of health, USA.

35. M. Zhao and R. M. Crooks, Chem. Mater., 1999, 11, 3379.

36. S.-K. Oh, Y.-G. Kim, H. Ye and R. M. Crooks, Langmuir, 2003, 19, 10420.

37. H. Lang, R. A. May, B. L. Iversen and B. D. Chandler, J. Am. Chem. Soc.,

2003, 125, 14832.

38. V. Chhabra, V. Pillai, B. K. Mishra, A. Morrone and D. O. Shah, Langmuir,

1995, 11, 3307.

105

39. P. A. Webb and C. Orr, Analytical methods in fine particle technology,

Micrometrics Instrument Corp., Norcros, U.S.A, 1977.

Chapter 5:

Application of titania-supported Au nanoparticles as catalyst in the oxidation

of styrene.

In the past, gold was not considered to be catalytically active in both oxidation

and hydrogenation reactions.1 However, since the discovery by Haruta et al that

oxide supported nano-sized gold can be used as highly active catalyst for CO

oxidation,2, 3 supported nano-sized gold catalysts have received considerable

attention from many researchers. Transition metal nanoparticles (NPs) supported on

different oxide supports have successfully been used as catalysts in styrene

oxidation reaction. A significant amount of literature has been reported for styrene

oxidation and Au has been one of the most studied metals so far. In this chapter, a

few relevant examples from the reported literature on the application of supported

NPs as catalysts in the oxidation of styrene are discussed. Based on this literature

survey, we envisioned that titania-supported Au NPs prepared from DENs

precursors can also be used effectively as catalysts in the styrene oxidation reaction.

The application of the titania supported Au NPs (synthesized via sol-gel and wetness

impregnation methods discussed in chapter 4) as catalysts in the oxidation of

styrene is the focus in this chapter. Titania-supported Au NPs prepared via sol-gel

and wetness impregnation methods will be referred to as Ti-Au-s and Ti-Au-w

respectively in this chapter. The effect of factors such as the oxidizing agent, solvent,

reaction temperature and reaction time on the conversion and selectivity of styrene

was also given attention. The reactions were monitored using gas chromatography

(GC).

5.1. Introduction.

Epoxidation of alkene to their corresponding epoxides is an important step in

the manufacturing of a large number of fine, bulk and pharmaceutical chemicals.4

Currently epoxides are prepared from alkenes on an industrial scale from oxygen,

peroxides and peracids.5 However, the use of peracids (e.g., peracetic acid and

106

percarboxylic acid) is unsafe and the acids are corrosive and generate large

amounts of waste. Therefore, the synthesis of epoxides by means of a safer and

more environmentally friendly oxidizing agent is an active field of research in both

industry and academia.5 Additionally there is also a drive to find more active

/selective, easily separable and reusable catalysts for this reaction. Some of the

driving forces for the industry and academic research can be identified as: (i) the

formulation of alternative or new catalysts, (ii) reduction of the number of process

steps, (iii) elimination of waste by-products and (iv) the development of new

processes.

As compared to other chemical processes, the epoxidation of terminal

alkenes such as styrene, is difficult and requires long reaction times.6 The oxidation

of styrene yields benzaldehyde (BzA), styrene oxide (SO) and acetophenone (AP) as

main products (see Figure 5.1).

CH2Au-NPs catalystOxidant

+O+ CH3

OO

Styrene Styrene oxideBenzaldehyde Acetophenone

Figure 5.1: Primary reaction products formed during the oxidation of styrene.

Patil et al reported the use of alkaline earth metal oxide supported Au NPs as

catalysts in the liquid phase oxidation of styrene.7 This was the first report on the use

of supported Au NPs catalysts in the liquid phase epoxidation of olefins. In their

study, Au NPs were immobilized onto MgO, CaO, SrO and BaO by homogeneous

deposition-precipitation (HDP) as well as deposition precipitation (DP) methods.

Anhydrous tert-butyl hydroperoxide (TBHP) was used as the oxidant in this case.

Both styrene conversion and selectivity towards styrene oxide were found to be high

and did not show any dependence on the type of support used when the HDP

immobilization method is used. However, best results in terms of styrene conversion

as well as styrene oxide selectivity was observed when Au NPs supported onto MgO

by the HDP method, when a metal loading (7.5% wt) was used. When MgO-

supported Au NPs (DP method with low metal loading (4.1% wt)) were used as

catalysts, much lower activity and styrene oxide selectivity were observed. This was

107

attributed to low metal loading as well as larger Au particle sizes (17. 9 ± 3.4 nm).

Using the MgO support by itself also showed a significant styrene conversion activity

(15.9 %) with benzoic acid observed as the major product and with very poor

selectivity towards styrene oxide (16.1 %). The MgO-supported Au NPs prepared by

HDP with high metal loading (7.5% wt) also showed good recyclability as improved

activity and selectivity was obtained when this catalyst was reused as compared to

the initial run.

In the same year, Patil et al also reported use of Au NPs supported over a

number of transition metals support (TiO2, Cr2O3, MnO2, Fe2O3, Co3O4, NiO, CuO,

ZnO, Y2O3, ZrO2, La2O3 and U3O8) for the oxidation of styrene.8 The HDP method

was used in this case for immobilization of Au onto various metal oxide supports. It

was found that all these supported Au catalysts have good activity/selectivity towards

styrene oxide except for MnO2-supported Au and U3O8-supported Au catalysts which

favoured the formation of benzaldehyde and other products such as benzoic acid.

No correlation in the performance of these catalysts was established between Au

loading or/and particle size, except for TiO2-supported Au and CuO-supported Au

catalysts. These catalysts, with comparable Au loading, yielded comparable

performance in the epoxidation reaction, although their particle sizes were very

different (2.8 ± 0.8 and 11.7 ± 2.6 nm, respectively). These two catalysts also

showed good recyclability and showed nearly the same epoxidation activity when

reused for six times. No Au leaching was observed when using these catalysts.

In another study, Patil et al reported the immobilization of Au onto group IIIa

metal oxides (TI2O3, Ga2O3, In2O3 and Al2O3) via the DP and HDP method.9 Of all

these, Au/TI2O3 was found to have both the highest Au loading and average particle

size when the HDP method is used. This was because TI2O3 has the greatest

surface basicity and thus the highest reduction potential as compared to the other

three metal oxide supports used. The catalytic activity of these supported Au NPs

was evaluated in the styrene oxidation by TBHP. All four supported Au NPs proved

to be catalytically active for oxidation of styrene with the Au/TI2O3 catalyst prepared

via the HDP method showing the highest activity as well as the best selectivity for

the oxidation of styrene.

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Chimentão et al reported the effect of morphology for both Al2O3 and MgO

supported Ag NPs when used as catalysts for the styrene oxidation in the gas

phase.10 Ag NPs with different morphology (nanowire and nanopolyhedra) were

synthesized via the polyol process using poly(vinylpyrolidone) (PVP) as a template.

The morphologies of these Ag NPs were found to depend on the experimental

conditions such as temperature and the molar ratio between PVP and AgNO3. Two

methods of immobilizing these Ag NPs were used i.e. immobilization by either

dispersion of preformed Ag NPs on the support or by adding nanoaprticle metal ion

precursors to the support first, followed by reduction of metal ions to form supported

NPs. In their study oxygen was used as the oxidant. For both these catalysts,

phenylacetaldehyde and styrene oxide were observed as the main products. Al2O3-

supported Ag nanowires and nanopolyhedra with similar metal loadings both showed

comparable results on the styrene conversion. The increase in the O2 : styrene molar

ratio resulted in an increase in the styrene conversion, but decreased selectivity

towards styrene oxide. The addition of potassium hydroxide (KOH) to the Ag

nanowires resulted in the increase in the catalytic activity. No explanation was given

of the afore-mentioned observations.

Purcar et al recently reported the catalytic activity of three types of Ag NPs

supported on hybrid film materials,11 one containing methyltrimethoxysilane (Ag-

MeTMS), one comprised of both MeTMS and 3-mercaptopropyltrimethoxysilane

(MPTMS) as a modifier (Ag-MeTMS/MPTMS) and in the third one a starch was

added to Ag-MeTMS as a modifier/ reducing agent (Ag-MeTMS/starch). All these

catalysts showed activity, but different selectivity, for the oxidation of styrene.

Although Ag-MeTMS/MPTMS catalysts did not show the highest selectivity for

styrene oxide, they proved to be more active and showed good reusability as

compared to Ag-MeTMS and Ag-MeTMS/starch catalysts.

Yin et al reported the immobilization of Au onto mesoporous alumina with

different surface basicities.12 The difference in surface basicity was achieved by

using different structure-directing agents during the synthesis of these mesoporous

alumina oxides. All these alumina supported Au NPs were found to have more basic

surfaces than the commercial alumina. Triblock poly(ethylene glycol)-poly(propylene

glycol)-poly(ethylene glycol) (P123), cetyltrimethyl ammonium bromide (CTAB) and

chitosan were used as a structure-directing agent for the synthesis of alumina oxides

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which were denoted as meso-Al2O3-a, meso-Al2O3-b and meso-Al2O3-c respectively.

The meso-Al2O3-a and Al2O3-b were found to have higher specific surface area,

larger pore volume and wider pore diameter than meso-Al2O3-c. As expected, the

specific surface area, pore volume and mean pore decreases after deposition of Au

particles on the meso-Al2O3-a as well as on reference commercial Al2O3 supports.

This was in agreement with the previously reported literature when Au particles were

deposited on a SBA-15 support.13 However, a different trend was observed with

respect to the other two supports (meso-Al2O3-b and meso-Al2O3-c). The highest

number of surface basic sites was found to follow the order of meso-Al2O3-c > Al2O3-

b > meso-Al2O3-a > commercial Al2O3. The catalytic activity of these alumina

supported Au NPs catalysts was evaluated in the oxidation of styrene. No significant

difference was observed in terms of selectivity to styrene oxide for these three

catalysts including the reference Au/Al2O3 catalyst. However, Au/meso-AI2O3-b

showed to have the highest activity of these catalysts. This activity was attributed to

the fact that Au/meso-Al2O3 has smaller Au particle size as well as more surface

basic sites on the support.

Friend and co-workers reported the oxidation of styrene catalyzed by oxygen

covered Au NPs.14 This deposition of oxygen atoms on Au was done by electron

bombardment of condensed NO2 that allows control of the extent of oxygen

coverage to 0.4 ML (monolayer). The oxide support used to immobilize Au NPs was

not explained in this communication. These oxygen-covered Au NPs were used as

catalysts in the styrene oxidation and showed high selectivity (~53%) for styrene

oxide during styrene oxidation reaction. Other important products such as benzoic

acid and benzeneacetic acid were also formed. The reason for improved selectivity

in styrene oxide when Au-NPs were covered by O2 was not explained. The same

research group later reported the use of chlorine as a promoter for enhancing

selectivity in the oxygen-covered Au catalyzed styrene oxidation.15 The chlorine was

introduced to the Au surface by exposing the Au particles to CI2 gas at 200 K. The

presence of chlorine slightly reduces the oxygen coverage in this case to 0.3 ML. In

the absence of chlorine, almost the same amount of styrene oxide is formed as

reported in the first study. In the presence of chlorine, a small amount of CO2 is

formed, and neither benzoic acid nor benzeneacetic acid were detected. The

formation of the styrene oxide was detected by temperature programmed reaction

110

spectroscopy (TPRS) and a styrene oxide peak was observed at 100 K lower than

when the chlorine is not present and hence the decrease in the activation energy in

the epoxidation of styrene. When styrene reacts with Au covered with only 0.06 ML

of O2 in the absence of CI2, CO2, benzoic acid and benzeneacetic acid were still

formed. This was therefore evidence that this substantial increase in selectivity and

suppression of combustion was not due to a decrease in oxygen coverage but rather

solely due to the presence of CI2. The presence of CI2 in this experiment disperses

the Au-O complex so as to promote the addition of O2 to the styrene, which may

result in the increase in epoxidation rate and decrease in temperature for the

formation of styrene oxide.

Recently, Zhang et al reported the catalytic evaluation of Au NPs supported

on a hexagonal layered double hydroxide (LDH) in the conversion of styrene to

styrene oxide with TBHP via epoxidation.16 LDH is represented by a general formula:

[ Mll (1-x)Mlll x(H2O)2] A

n- x\n.mH2O, where Mll include Mg2+, Co2+ e.t.c.; Mlll may be Al3+,

Cr3+ e.t.c. and An- might be any organic and/or inorganic anion. LDH supported Au-

NPs were prepared by a HDP technique using urea as a precipitating agent

precursor.17 Styrene oxide, benzaldehyde and phenyl acetayldehyde were observed

as the main products in this case. During the catalytic investigation, it was found that

the pure LDH (without Au) has a selectivity of 73.3% for styrene oxide but with a very

low reaction activity. As Au loading (from 0.10 % to 0.66 %) on the LDH support

increased, an increase in styrene conversion (from 41.8% to 67.4) was observed.

This conversion however decreases as the Au loading was increased to 1.93 %. As

the Au loading was increased to 5.48 %, an increase in styrene conversion was

observed again. No correlation was observed between the metal loading and

selectivity for styrene oxide. The reusability of these catalysts was tested using

Au/LDH with a Au loading of 0.66 % and it was found that the catalyst retained its

original activity after being reused three times.

Turner at al reported the application of Au-NPs supported on boron nitrate

(BN), silicon dioxide (SiO2) and carbon as catalysts in the oxidation of styrene with

dioxygen. 18 The NPs synthesized in their study varied in diameter from 1.4 nm to

more than 30 nm. Au-NPs supported on BN and SiO2 (0.6% wt) with diameters less

than 5 nm were found to have comparable turnover frequencies with respect to

styrene conversion and epoxide production. This was in agreement with previous

111

reports which assumed that extremely small supported Au-NPs can absorb and

activate O2 for selective oxidation.14, 19 This occurs through dissociation of O2 to yield

O adatoms that initiate the oxidation reaction. It was found that SiO2 supported Au-

NPs showed higher styrene conversion but lower selectivity for styrene oxide as

compared to BN supported Au-NPs with a comparable Au loading and diameter. The

reusability of the Au/SiO2 catalyst was also studied and it was found that the initial

re-use resulted in the decrease in styrene conversion and increased selectivity

towards styrene oxide formation. A further drop in activity was observed when the

catalyst was reused for the second time. The influence of phosphorus and chlorine

introduced during catalyst preparation was also investigated and it was found that

their presence did not have any influence on the observed catalytic activity. Au-NPs

supported on carbon with diameter of 17 nm were found to be catalytically inactive in

the oxidation of styrene.

Liu at al recently reported the synthesis of Au-NPs supported on

hydroxyapatite (HAP) via the wetness impregnation method from a ligand stabilized

Au precursor. 20 The resulting particles were determined to have an average

diameter of 1.4 ± 0.6 nm and a Au loading of 0.5% wt. Other catalysts with the same

Au content were also prepared for comparison, using the impregnation (IP),

deposition precipitation (DP) and adsorption (Ad) methods. The catalytic activity of

these supported Au-NPs was also evaluated in the oxidation of styrene with TBHP.

All these catalysts showed good activity in styrene conversion as well as selectivity

towards styrene oxide. Although styrene oxide was observed as the major product,

benzayldehyde, benzyl alcohol and benzoic acid were also produced. After 12 hours,

a styrene conversion was found to be above 90% for all catalysts. The HAP

supported Au-NPs showed high selectivity for styrene oxide (90-95%) as compared

to other catalysts prepared via IP and Ad methods (50-60%). As a result, this

catalyst gave the highest yield of styrene oxide (92%). To determine the reason for

this high selectivity, the rate constants for all of these three catalysts were compared

on the decomposition of styrene oxide in methylcyclohexane. Although the prepared

catalyst had the smallest average particle diameter compared to the other catalysts

prepared via DP, IP and Ad methods, it showed the poorest conversion (<15%) as

compared to the catalysts with larger average particle sizes. Based on this size

112

effect, it was suspected that the high selectivity of this catalyst was due to an

extremely slow decomposition of styrene oxide by Au-NPs.

5.2 Experimental.

Styrene (99%), tert-butyl hydroperoxide (TBHP) (70 % in water), hydrogen

peroxide (H2O2) (30 % v/v), acetonitrile, toluene and cyclohexane were all purchased

from Sigma Aldrich. GC standards (styrene oxide, benzalydehyde and phenyl

acetaldehyde) were all purchased from Sigma Aldrich. The reaction products were

analyzed and identified by known standards with a Shimadzu GC-2010 plus

equipped with a flame ionization detector (DB-35 capillary column, 30 mm x 0.25

mm). Helium was used as a carrier gas and an injection temperature of 250 oC was

used.

The following method was adapted from the literature to conduct the catalytic

reaction.21 The catalytic oxidation of styrene with either TBHP or H2O2 as oxidant

was carried out in a round-bottomed flask (50 mL) using a magnetic stirrer and a

reflux condenser. Ti-Au-s (Ti-supported Au NPs prepared via sol-gel method) and Ti-

Au-w (Ti-supported Au NPs prepared via wetness impregnation method) whose

preparations are discussed in Chapter 4 were used as catalysts. In order to control

the temperature, the batch reactor was immersed in a paraffin oil bath. A typical

oxidation reaction was carried out by mixing a catalyst (0.1 g), styrene (299 µL, 2.6

mmol), TBHP (0.334 g, 2.6 mmol) and acetonitrile (10 mL, 99. 9 %) in a round

bottom flask. The reaction temperature was varied from 50 oC to 70 oC. The reaction

time was varied from 6 to 15 hours. After the reaction was cooled to room

temperature, the catalyst was separated from the reaction mixture by centrifugation.

Each value for styrene as well as product selectivity is reported as an average of a

minimum of three reaction runs.

5.3 Results and Discussion.

In this study, the effect of catalyst amount, temperature, solvent, oxidant and

reaction time on the styrene conversion and product selectivity was investigated. For

all the reaction systems studied, styrene oxide (SO) and benzaldehyde (BzA) were

detected as known products.

113

5.3.1 Effect of temperature on the oxidation of styrene .

Table 5.1 shows the results obtained for the oxidation of styrene at different

temperatures with other parameters kept constant. Reaction conditions are shown at

the bottom of the Table 5.1. For both catalysts, the highest styrene conversion was

achieved at 70 oC. Styrene conversions of 16 and 21 % were achieved for Ti-Au-s

and Ti-Au-w catalysts respectively. Both these catalysts generally showed high

selectivity towards the benzaldehyde. At 60 oC, Ti-Au-s was found to selectively form

benzaldehyde while Ti-Au-w was selective towards the formation of styrene oxide.

Table 5.1: Results for the catalytic oxidation of styrene by TBHP at different temperatures using Ti-

Au-s and Ti-Au-w catalysts.

Catalyst Temperature (oC) Styrene conversion Product selectivity (%)

(%) BzA SO Others

Ti-Au-s 50 0.1 100 — —

Ti-Au-w 50 7 81 19 —

Ti-Au-s 60 2 100 — —

Ti-Au-w 60 9 61 39 —

Ti-Au-s 65 8 69 31 —

Ti-Au-w 65 16 54 46 —

Ti-Au-s 70 16 50 47 3

Ti-Au-w 70 21 33 64 3

— : no product detected, catalyst amount: 0.1 g, reaction time: 12 hours, solvent: acetonitrile. oxidant: TBHP.

5.3.2. Effect of solvent on the oxidation of styrene.

The choice of solvent also proved to have an effect on the product formation

during the oxidation of styrene as can be seen in Table 5.2. Reaction conditions are

outlined at the bottom of Table 5.2. The highest styrene conversion was observed

when toluene was used with a Ti-Au-s catalyst. Both Ti-Au-s and Ti-Au-w catalysts

114

showed to have high selectivity for benzaldehyde when toluene is used as a solvent.

When acetonitrile is used as a solvent, the conversion of styrene is higher for Ti-Au-

w. The Ti-Au-s catalyst was found to have the same selectivity towards both

benzaldehyde and styrene oxide as when acetonitrile was used as a solvent. The

lowest styrene conversion was observed when Ti-Au-w catalyst is used with a

cyclohexane as solvent. Ti-Au-s showed high selectivity towards benzaldehyde when

cylohexane is used while Ti-Au-w showed high selectivity towards other products

that could not be identified. These observations could be attributed to the difference

in the polarity of the solvents used here. As the temperature is increased, the styrene

conversion was found to increase. The results for the oxidation of styrene using

different solvents are summarized in Table 5.2.

Table 5.2: Results for the oxidation of styrene by TBHP using different solvents as well as Ti-Au-s

and Ti-Au-w catalysts.

Catalyst Solvent Styrene conversion product selectivity (%)

(%) BzA SO Others

Ti-Au-s acetonitrile 18 50 50 —

Ti-Au-w acetonitrile 25 46 54 —

Ti-Au-s toluene 41 61 39 —

Ti-Au-w toluene 23 67 33 —

Ti-Au-s cyclohexane 37 98 1.0 1

Ti-Au-w cyclohexane 4 6 3 91

—: no product detected, catalyst amount: 0.1 g, temperature: 70 oC, reaction time 15 hours, oxidant: TBHP

5.3.3. The effect of catalyst amount and metal loading on the oxidation of styrene

The metal loadings for Ti-Au-s and Ti-Au-w catalysts were determined to be

1.31 and 1.8 wt % respectively (see chapter Chapter 4). Reaction conditions used

here are given at the bottom of Table 5.3. Ti-Au-w catalyst was found to give high

115

styrene conversions for all reaction systems studied. Although both catalysts showed

high selectivity towards the formation of benzaldehyde, Ti-Au-s catalyst was found to

have the highest selectivity for benzaldehyde. No other unknown products were

detected in this case. The results for the effect of metal loading on the oxidation of

styrene are summarized in Table 5.3

Table 5.3: The effect of metal loading on the formation of products for the oxidation of styrene by TBHP.

Catalyst catalysts metal styrene conversion product selectivity

amount (g) loading (wt %) (%) BzA SO

Ti-Au-s 0.05 1.3 3 100 —

Ti-Au-w 0.05 1.8 7 81 19

Ti-Au-s 0.1 1.3 7 68 32

Ti-Au-w 0.1 1.8 12 58 42

—: no product detected, reaction temperature: 70 oC, solvent: acetonitrile, reaction time: 12 hours. Oxidant TBHP.

5.3.4. The effect of an oxidant on the formation of products during styrene

oxidation .

Reaction conditions used are given at the bottom of Table 5.4. The styrene

conversion was found to decrease when H2O2 was used as an oxidant as compared

to when TBHP was used for under the same reaction conditions. This could have

been due to weak interaction between H2O2 and the catalysts which consequently

led to non-homogeneous system with respect to liquids. High selectivity for

benzaldehyde was also observed for both catalysts even when H2O2 was used.

Table 5.4 compares the results for the oxidation of styrene when H2O2 and TBHP are

used as oxidants.

116

Table 5.4: Results for the oxidation of styrene using TBHP and H2O2 oxidants with Ti-Au-s and Ti-Au-

w catalysts.

Catalyst Oxidant Styrene conversion products selectivity (%)

(%) BzA SO other

Ti-Au-s TBHP 18 50 50 —

Ti-Au-w TBHP 25 44 56 —

Ti-Au-s H2O2 10 54 39 7

Ti-Au-w H2O2 13 59 37 4

—: no product detected, reaction temperature: 70 oC, solvent: acetonitrile, reaction time 15 hours.

No conversion of styrene was observed at reaction temperature below 70 oC

when H2O2 is used as an oxidant. The same was observed when the reaction time

was less than 12 hours. Additionally, no conversion of styrene was observed in the

absence of a solvent (12 hours).

5.3.5. Effect of reaction time on the conversion of styrene .

Presented in Table 5.5 are the results obtained for the styrene oxidation

products when only the reaction time is varied. The styrene conversion was found to

increase as the reaction time increased. The styrene conversion was found to

increase almost two fold when the reaction time increased from 6 to 12 hours. As the

reaction time was increased, the increase in the conversion of styrene was observed

for both these catalysts. This can be attributed to the fact that styrene oxidation

reaction requires long time and as a result, the increase in time would results in the

improved styrene conversion irrespective of the catalyst used. Ti-Au-w catalyst gave

the highest styrene conversion as compared to Ti-Au-s for a reaction time of 6 and

12 hours respectively. However, when the reaction time is increased to 15 hours, Ti-

Au-s showed a significant increase in the styrene conversion as opposed to Ti-Au-w

catalyst. The results for the conversion of styrene at various reaction times are

summarized graphically in Figure 5.2.

117

Table 5.5: The effect of reaction time on the conversion of styrene using Ti-Au-s and Ti-Au-w catalysts.

Catalyst Reaction time Styrene conversion Product selectivity

(hrs) (%) BzA SO Others

Ti-Au-s 6 4 78 22 —

Ti-Au-w 6 9 81 18 1

Ti-Au-s 12 7 68 32 —

Ti-Au-w 12 12 58 42 —

Ti-Au-s 15 18 50 49 1

Ti-Au-w 15 25 46 54 —

—: no product detected, catalyst amount: 0.1 g, temperature: 70 oC, solvent: acetonitrile,

oxidant: TBHP.

.

Figure 5.2: The conversion of styrene at various reaction times using TBHP and toluene at 70 oC

118

5.4. Conclusions.

Titania supported Au NPs can be used as catalyst for the oxidation of styrene.

Both catalysts showed high selectivity towards the formation of benzaldehyde. The

titania supported Au catalyst (Ti-Au-w) prepared via wetness impregnation showed a

highest styrene conversion as compared to the sol-gel method for all reaction

systems studied. The increase in styrene conversion was observed as the

temperature is increased for Ti-Au-s and Ti-Au-w catalysts. At low temperature (~50-

60 oC), only benzaldehyde was detected as a product when Ti-Au-s is used while

only styrene oxide was detected when Ti-Au-w is used. The formation of unknown

products was only observed at high temperature (˃ 70 oC) for both catalysts.

Ti-Au-s catalyst gave the highest styrene conversion (41%) when toluene

solvent is used as compared to acetonitrile and cyclohexane, while Ti-Au-w catalyst

gave the highest styrene conversion when acetonitrile was used. Ti-Au-w showed

the lowest styrene conversion as well as high selectivity for other unknown products

when cyclohexane was used as solvent. Irrespective of the solvent used, both

catalysts showed high selectivity towards the formation of benzaldehyde.

The use of H2O2 as an oxidant resulted in a decrease in the styrene

conversion for both catalysts as compared to when TBHP was used. Ti-Au-w gave

the maximum styrene conversion of 13% observed when H2O2 was used. As the

amount of catalyst increased, an increase in styrene conversion was observed for

both catalysts. The same trend was observed when the reaction time is increased for

both catalysts. Since Ti-Au-w has a higher Au loading than Ti-Au-s catalyst, it can

therefore be concluded that a higher metal loading results in higher conversion of

styrene. The increase in temperature also resulted in the increase in conversion. The

Ti-Au-w generally gave higher styrene conversion than Ti-Au-s catalyst in all system

studied. . The study on the re-usability as well as catalyst sintering can be

considered for future work.

119

5.6. References.

1. B. Hammer and J. K. Nørskov, Nature, 1995, 376, 238.

2. M. Haruta, T. Kobayashi, H. Sano and N. Yamada, Chem. Lett., 1987, 405.

3. M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Cat., 1989, 115, 301.

4. H. Nakajima, S. Takase, H. Terano and H. Tanaka, J. Antibiot., 1997, 50, 96.

5. L. Espinal, S. L. Suib and J. F. Rusling, J. Am. Chem. Soc., 2004, 126, 7676.

6. C. Coperet, H. Adolfson and K. B. Sharpless, Chem. Commun., 1997, 1565.

7. N. S. Patil, B. S. Uphade, P. Jana, S. K. Bharagava and V. R. Choudhary, J.

Catal., 2004, 223, 236.

8. N. S. Patil, B. S. Uphade, D. G. McCulloh, S. K. Bhargava and V. R.

Choudhary, Catal. Commun., 2004, 5, 681.

9. N. S. Patil, R. Jha, B. S. Uphade, S. K. Bhargava and V. R. Choudhary, App.

Catal. A Gen., 2004, 275, 87.

10. R. J. Chimentão, I. Kirm, F. Medina, X. Rodríguez, Y. Cesteros, P. Salagre

and J. E. Sueiras, Chem. Commun., 2004, 846.

11. V. Purcar, D. Donescu, C. Petcu, R. Luque and D. J. Macquarrie, App. Catal.

A Gen., 2009, 363, 122.

12. D. Yin, L. Qin, J. Liu, C. Li and Y. Jin, J. Mol. Catal. A., 2005, 240, 40.

13. C. M. Yang, M. Kalwei, F. Schüth and K. J. Chao, App. Catal. A Gen., 2003,

254, 289.

14. X. Deng and C. M. Friend, J. Am. Chem. Soc., 2005, 127, 17178.

15. D. S. Pinnaduwage, L. Zhou, W. Gao and C. M. Friend, J. Am. Chem. Soc.,

2007, 129, 1872.

16. F. Zhang, X. Zhao, C. Feng, B. Li, T. Chen, W. Lu, X. Lei and S. Xu, ACS

Catal., 2010, 1, 232.

17. M. Adachi-Pagano, C. Forano and J.-P. Besse, J. Mater. Chem., 2003, 13,

1988.

18. M. Turner, V. B. Golovko, O. P. H. Vaughan, P. Abdulkin, A. Berenguer-

Murcia, M. S. Tikhov, B. F. G. Johnson and R. M. Lambert, Nature, 2008, 454,

981.

19. R. M. Lambert, F. J. Williams, R. L. Cropley and A. Palermo, J. Mol. Catal., A,

2005, 228, 27.

120

20. Y. Liu, H. Tsunoyama, T. Akita and T. Tsukuda, Chem. Commun., 2010, 46,

550.

21. S. Gómez, J. Villegas, R. Ghosh, O. Giraldo and S. L. Suib, J. Cat., 2005,

233, 60.

121

Chapter 6:

Dissertation Summary

The main objectives of this study were to synthesize Ag, Au and Cu

dendrimer encapsulated nanoparticles (DENs) as well as to evaluate their catalytic

behaviour. The detailed synthetic procedure and characterization of these DENs are

explained in Chapter 2. G4, G5 and G6 PAMAM-OH were used as a template for

the synthesis of Cu-DENs, while G4, G5 and G6 PAMAM-NH2 dendrimers were used

for the preparation of Ag- DENs. Only G4 PAMAM-NH2 dendrimers were used for the

synthesis of Au-DENs. From binding studies, it was found that Cu2+ ions coordinate

in a stoichiometric ratio with the tertiary amine groups of the PAMAM-OH

dendrimers, and this is shown from the binding studies in which UV-vis was used.

However, no stoichiometric coordination of the Ag+ and Au3+ ions with the tertiary

amine groups of the PAMAM-NH2 dendrimers were established. From the binding

studies it was decided to use the dendrimer/Cu2+ molar ratio of 1:16, 1:32, and 1:64

for G4, G5 and G6 PAMAM-OH dendrimer were used respectively. In case of Ag

DENs, dendrimer/Ag+ molar ratio of 1:12, 1:16 and 1:32 were used for the synthesis

of Ag-DENs in G4, G5 and G6 PAMAM-NH2 dendrimers respectively. Only G4

PAMAM-NH2 was used for the preparation of Au-DENs. When PAMAM-NH2

dendrimers are used as templating agent, the pH of the aqueous dendrimer solution

was first adjusted to give an acidic condition (~pH 2) before the addition of metal

ions. This was done to avoid the coordination of the metal ions with the primary

amine groups on the periphery of the dendrimers, which might subsequently lead to

the agglomeration of NPs. These synthesized Au, Ag and Cu-DENs were

characterized by UV-vis spectroscopy as well as by HRTEM. These techniques were

helpful in determining the average size of the NPs formed as well as predicting

whether the NPs were formed inside the dendrimer or not.

The catalytic activity of these prepared Ag, Au and Cu-DENs was initially

evaluated on the reduction of 4-nitrophenol to 4-aminophenol by NaBH4. All these

particles showed activity for this reaction. For all systems studied, the reaction was

set up to follow pseudo-first order kinetics by adding molar excess of NaBH4 to 4-

nitrophenol. It was found that the sequence of addition of 4-nitrophenol and NaBH4

to the reaction mixture does not affect the rate of reaction but rather the induction

122

period. The rate constant was found to decrease as the concentration of 4-

nitrophenol increased. As the concentration of NaBH4 increased, an increase in the

reaction rate constant was observed which reached a maximum after a certain

NaBH4 concentration. This trend is also observed for all metals (Ag, Au and Cu) in

the study. Based on the ∆H++ (>0) and ∆S++ (<0) values obtained for all these

metals, the reaction was endothermic. Extraction of these NPs from the dendrimer

into aqueous/organic phase as well as their catalytic evaluation for this reaction can

be considered for future work

The pre-formed Au-DENs were further immobilized onto a titania support via

the sol-gel and wetness impregnation methods. The dendrimer template was

removed by calcination at 500 oC for 3 hours in air. These titania supported Au NPs

were characterized by UV-vis spectroscopy, HRTEM, SEM, PXRD, TGA and ICP-

OES. The physisorption analysis of these titania supported Au NPs was performed

using the BET method. HRTEM revealed that the wetness impregnation method was

found to yield large NPs as compared to the sol-gel method. The PXRD spectra

showed that these supported Au NPs are amorphous before calcination. However,

after calcination, the titania is partially converted to the anatase phase. No rutile

phase was observed after calcination. The TGA revealed that the synthesized

titania-Au-NPs are stable up to 500 oC. ICP-OES showed a metal loading of 1.3 wt

% and 1.8 wt % for titania-Au-NPs prepared via sol-gel and wetness impregnation

method respectively. The BET surface of the titania-Au-NPs showed a significant

increase in surface area after calcination, but it is also accompanied by a decrease

in pore size. After calcination, these titania-Au-NPs showed a hysteresis nitrogen

adsorption/desorption isotherm at intermediate partial pressure, which is a typical of

mesoporous materials.

The catalytic evaluation of these titania-Au-NPs on the oxidation of styrene by

TBHP was carried out. Both these catalysts were found to be active, and the catalyst

prepared via wetness impregnation showed high styrene conversion. Benzaldehyde

and styrene oxide were detected as the main products with both catalysts showing

higher selectivity for benzalydehyde. The reaction conditions played a major role in

the reaction. It was found that, an increase in temperatures, as well as reaction time

favours the styrene conversion. The use of different solvents also played a major

role, i.e. using toluene as a solvent showed an increase in conversion as compared

123

to acetonitrile. It was also found that the use of H2O2 as an oxidant resulted in a

decrease in styrene conversion as compared to when TBHP is used under similar

conditions. The reusability as well as the possibility of catalyst sintering can be

considered for future work.