Innovative preparation of catalysts by aerosol route for ... · Usually Fischer – Tropsch (FT)...

Innovative preparation of catalysts by aerosol route for the

Fischer – Tropsch synthesis

Maria Joana Figueiredo Rodrigues

Thesis to obtain the Master of Science Degree in

Chemical Engineering

Supervisors: Dr. Alexandra Chaumonnot (IFPEN)

Dr. Antoine Fécant (IFPEN)

Prof. Carlos Henriques (IST)

Examination Committee

Chairperson: Prof. José Madeira Lopes (IST)

Supervisor: Prof. Carlos Manuel Faria de Barros Henriques (IST)

Members of the committee: Prof. Maria Filipa Gomes Ribeiro (IST)

October 2015

This page was intentionally left blank

“If we knew what we were doing it would not be called research, would it?”

Albert Einstein

v

Resumo

Em termos gerais, os catalisadores usados no processo de Fischer – Tropsch (FT) são obtidos

através da deposição do cobalto (fase ativa) no suporte. Este óxido é habitualmente obtido pelo spray

– drying de uma solução que contém precursores inorgânicos. O principal objetivo deste trabalho é não

só a síntese de catalisadores para o processo de FT através da técnica de spray – drying, mas também

a sua caracterização.

Foram sintetizados sólidos suportados quer em sílica quer em alumina, onde se verificou uma

grande dispersão do cobalto, o que originou uma forte interação molecular entre o precursor de cobalto

e o precursor inorgânico molecular, confirmando-se assim a existência quer de silicatos de cobalto ou

aluminatos de cobalto. De modo a diminuir esta interação, foram modificados os vários parâmetros da

preparação, como por exemplo, a alteração do pH das soluções de atomização, de modo a induzir

atração ou repulsão electroestática entre o precursor de cobalto e o precursor de sílica ou alumina na

solução inicial. Foram obtidos melhores resultados quanto à acessibilidade e redutibilidade do cobalto

no caso da sílica para um pH mais ácido, apresentando, ainda assim, a formação de silicatos de cobalto.

Deverá ser realizado um estudo mais detalhado quer à sílica quer à alumina relativamente aos

parâmetros de síntese. Deverá ser igualmente testada a incorporação de um metal promotor de modo

a aumentar a redutibilidade do cobalto.

Palavras – chave: Catalisadores de fischer – Tropsch, spray – drying, aerossol, cobalto, sílica,

alumina.

vii

Abstract

Usually Fischer – Tropsch (FT) catalysts are obtained through the deposition of cobalt active phase

onto the support. This oxide support is normally obtained by a spray – drying synthetic pathway. The

present work aims the synthesis and characterization of FT catalyst through spray-drying pathway.

Silica and alumina based solids were investigated. It was found a very high dispersion of cobalt

leading to a high molecular interaction between the cobalt precursor and inorganic molecular precursor.

Therefore, it was confirmed the existence of cobalt silicates or aluminates. In order to weaken Co and

carrier atoms interactions, several preparation parameters were modified, like modifying pH of the

atomizing solution to induce electrostatic attraction or repulsion of Co and Si or Al precursors in the initial

solution. Better results regarding the cobalt accessibility and reducibility were achieved for a more acidic

media. Nevertheless, it was still noticed the presence of cobalt silicates.

A more exhaustive study should be done concerning synthesis parameters for both alumina and

silica carriers. Furthermore, the loading of a promoter metal should be tried in order to improve the cobalt

reducibility.

Keywords: Fischer – Tropsch catalysts, spray – drying, aerosol, cobalt, silica, alumina.

ix

Acknowledgements

First of all I would like to express my special thanks of gratitude to Prof. Filipa Ribeiro for making

possible, every year, the internships between IST and IFPEN. This opportunity allow us, students, the

chance of working in this well - known institution. Also, Victor Costa and Joana Fernandes thank you for

all your help and advices during this six months.

Secondly, I would like to thank my supervisors at IFPEN, Alexandra Chaumonnot, Antoine Fécant

and Souad Rafik – Clément for all the support since my arrival. Also, I am very grateful for your endless

patience answering to my questions, as well as, for all your efforts that helped me improving myself.

Surely, this was a successful work due to your contribution. Furthermore, I would like to thank to Prof.

Carlos Henriques for all you interest and availability as well as your suggestions to this work.

I would like to thank everyone in my department that made possible my integration, and how patient

they were when I could not speak a word of French. Also I would like to thank to Eugènie Rabeyrin for

your kindness and patience and effort to teach me French and to Adrien Berliet for helping me everytime

that I needed.

To all the phD students in Lyon: Leonor, Rúben, Sónia, Svetan, Fabien, Matthieu , Pedro, Leonel

and Dina thank you for receiving us so kindly.

To my family in Lyon: Catarina, Solange, David, Ana, Diogo, Tiago, João and Pedro thank you for

making me feel not so far from home. Also, Frederico thank you for visit, it was really important for me.

To Sofia, Gonçalo, Constança, Tomás and Tiago thank you for your support and comprehension

during all my course. Without your true friendship would not have been the same.

Last but not least, I would like to thank my mother, father, sister and brother for all you support

during my course. For certain I would not have done it without your presence in my life. Thank you for

all your encouragements and advices that made who I am today. Also, João and Carolina, thank you for

all your support and interest in my progress, is was truly meaningful for me.

xi

Table of Contents

Resumo ....................................................................................................................................................v

Abstract ................................................................................................................................................... vii

Acknowledgements ................................................................................................................................. ix

Table of Contents .................................................................................................................................... xi

Table List ............................................................................................................................................... xiii

Figure List ............................................................................................................................................... xv

Glossary ................................................................................................................................................ xvii

1. Introduction ...................................................................................................................................... 1

2. Bibliographic Study .......................................................................................................................... 3

2.1. Fischer – Tropsch Process ...................................................................................................... 3

2.1.1. Reactors .......................................................................................................................... 4

2.1.2. Reactions ......................................................................................................................... 5

2.1.3. Reaction mechanism ....................................................................................................... 5

2.1.4. Product distribution .......................................................................................................... 6

2.2. Fischer – Tropsch catalysts ..................................................................................................... 7

2.2.1. Active phase .................................................................................................................... 7

2.2.2. Supports .......................................................................................................................... 8

2.2.3. Metals and oxide promoters ............................................................................................ 8

2.2.4. Usual catalyst preparation methods ................................................................................ 9

2.2.5. Activity and selectivity .................................................................................................... 11

2.2.6. Catalyst deactivation...................................................................................................... 12

2.3. One – Pot synthesis: Incorporation of the metallic phase on a mesoporous oxide matrix.... 13

2.3.1. Sol-gel chemistry ........................................................................................................... 13

2.3.2. Mesostructured materials: definition .............................................................................. 14

2.3.3. Mesostructured materials: mechanisms ........................................................................ 15

2.3.4. Mesostructured materials: synthesis techniques ........................................................... 15

3. Experimental Work ........................................................................................................................ 21

3.1. Preparation Methods ............................................................................................................. 21

3.1.1. Supports ........................................................................................................................ 21

3.1.2. Catalysts ........................................................................................................................ 23

3.2. Spray – Drying ....................................................................................................................... 26

3.2.1. Spray Drying working principle ...................................................................................... 26

3.2.2. Spray – Dryer Parameters ............................................................................................. 28

3.3. Characterization Methods ...................................................................................................... 29

3.3.1. N2 adsorption-desorption ............................................................................................... 30

3.3.2. Temperature Programmed Reduction (TPR) ................................................................ 31

xii

3.3.3. X-Ray Diffraction (XRD)................................................................................................. 31

3.3.4. Low angles X-Ray Diffraction ........................................................................................ 32

3.3.5. X-ray Photoelectron Spectroscopy (XPS) ..................................................................... 32

3.3.6. Transmission Electronic Microscopy (TEM) .................................................................. 32

3.3.7. Scanning Electronic Microscopy (SEM) ........................................................................ 32

4. Results and Discussion ................................................................................................................. 35

4.1. Silica ...................................................................................................................................... 36

4.1.1. Supports ........................................................................................................................ 36

4.1.2. Cobalt catalysts ............................................................................................................. 40

4.2. Alumina .................................................................................................................................. 55

4.2.1. Supports ........................................................................................................................ 55

4.2.2. Cobalt catalysts ............................................................................................................. 57

5. Conclusions and future perspectives............................................................................................. 63

6. References .................................................................................................................................... 65

Appendix A: t- plot curves ........................................................................................................................ A

xiii

Table List

Table 1 – Reference molar composition for all the supports. ................................................................ 22

Table 3 – Synthesized supports. ........................................................................................................... 22

Table 4 – Reference molar composition for the catalysts on a silica matrix. ........................................ 23

Table 5 – Reference molar composition for the catalysts on an alumina matrix. .................................. 23

Table 6 – Synthesized catalysts. ........................................................................................................... 24

Table 7 – Spray – drier parameters for all the samples. ....................................................................... 29

Table 8 – Relation between analysis and the expected characterized properties. ............................... 29

Table 9 – BET surface, porous volume and pore diameter for JFR003, JFR005, JFR006 and JFR007.

............................................................................................................................................................... 37

Table 10 – Results of the elementary particle size determinate by SEM. ............................................. 39

Table 11 – BET surface, porous volume and pore diameter for JFR005, JFR014 and JFR016. ......... 41


............................................................................................................................................................... 45


............................................................................................................................................................... 48

Table 14 – Results from XPS analysis for JFR019 and JFR021. ......................................................... 52

Table 15 – BET surface, porous volume and pore diameter for JFR009 and JFR010. ........................ 56


............................................................................................................................................................... 58

Table 17 – BET surface, porous volume and pore diameter for JFR024 (reference) and JFR037. ..... 60

xv

Figure List

Figure 1 - Fuel consumption since 1965 up to 2035. [1] ......................................................................... 1

Figure 2 –FT process-based production scheme. Adapted from [4] ....................................................... 3

Figure 3 - FT stepwise growth process, where α stands for probability of chain growth and d stands for

desorbed species. [9] .............................................................................................................................. 6

Figure 4 - Product selectivity as function of ASF chain growth probability α. [10] .................................. 7

Figure 5 - Hydrolysis (H), Condensation (C) and Dissolution (D) kinetics for a system of TEOS. [20] . 14

Figure 6 – Micelle structure A: Sphere, B: Cylindric, C: Lamellar, D: Inverse micelle, E: Bicontinuous

phase, F: Liposomes. [20] ..................................................................................................................... 15

Figure 7- Main synthesis pathways to mesostructured materials. [21] ................................................. 15

Figure 8 - Spray - Dryer (Büchi B 290) working principle and material's structuration mechanism.

Adapted from [20] .................................................................................................................................. 18

Figure 9 – Calcination profile for the synthesized supports. ................................................................. 23

Figure 10 - Calcination profile for JFR014 and JFR016. ....................................................................... 25

Figure 11 - Calcination profile for JFR019, JFR021, JFR022, JFR023, JFR024 and JFR037. ............ 25

Figure 12 - Spray drying apparatus. [27] ............................................................................................... 26

Figure 13 – Overview over the spray-dryer parameters and their influence in the final product. [28] .. 28

Figure 14 – Typical isotherm for a mesostructured solid. [29] .............................................................. 30

Figure 15 – Nitrogen adsorption – desorption as a function of relative pressure and pore size

distribution determined by BJH adsorption for JFR003, JFR005, JFR006 and JFR007. ...................... 36

Figure 16 – SEM micrographs for (A) JFR005, (B) JFR006 and (C) JFR007. ...................................... 38

Figure 17 – Particles size distribution for JFR005, JFR006 and JFR007. ............................................ 38

Figure 18 – Low angles XRD patterns for JFR005, JFR006 and JFR007. ........................................... 40

Figure 19 - Nitrogen adsorption – desorption as a function of relative pressure and pore size

distribution determined by BJH adsorption for JFR005 (support reference), JFR014 and JFR016. .... 41

Figure 20 - Scheme representing the solid surfaces calcinated at 350°C and 550°C. Adapted from [20]

............................................................................................................................................................... 42

Figure 21 – Low angles XRD patterns for JFR014 and JFR016. .......................................................... 43


distribution determined by BJH adsorption for JFR005 (reference), JFR016 (reference), JFR028 and

JFR021. ................................................................................................................................................. 44

Figure 23 – TPR profile for JFR016 and JFR021. ................................................................................. 46


distribution determined by BJH adsorption for JFR016 (reference), JFR019, JFR021 and JFR022. ... 48

Figure 25 - Low angles XRD patterns for JFR019, JFR021 and JFR022. ............................................ 49

Figure 26 - TPR profile for JFR019, JFR021 and JFR022. ................................................................... 50

Figure 27 - XRD diagrams: A) JFR019 and B) JFR021 and JFR022. .................................................. 51

Figure 28 - TEM micrographs for the support of JFR019. ..................................................................... 52

xvi

Figure 29 - TEM micrographs for metallic phase of JFR019. ................................................................ 52

Figure 30 – Cobalt oxide particles size distribution in volume for JFR019. .......................................... 53

Figure 31 - TEM micrographs for the supports of: (A) JFR021 and (B) JFR022. ................................. 53


distribution determined by BJH adsorption for JFR009 and JFR010. ................................................... 55

Figure 33 - SEM micrographs for JFR009. ............................................................................................ 56

Figure 34 - Low angles XRD patterns for JFR009. ............................................................................... 57


distribution determined by BJH adsorption for JFR010 (reference), JFR023, JFR025 and JFR024. ... 58

Figure 36 - TPR profile for JFR023 and JFR024................................................................................... 59


distribution determined by BJH adsorption: JFR024 and JFR037. ....................................................... 60

Figure 38 - TPR profile for JFR024 and JFR037................................................................................... 61

xvii

Glossary

α– Probability of chain growth

β – Peak Breadth

λ – X – Ray wavelength (m)

Θ – Bragg angle (°)

BET – Brunauer – Emment – Teller

BJH – Barret – Joyner - Halenda

BTL – Biomass – to – liquid

C – Bragg constant

CMC – Critical Micelle Concentration

CTL – Coal – to – liquid

EDS – Energy Dispersive Spectroscopy

EISA – Evaporation Induced Self - Assembly

FT – Fischer – Tropsch

FWHM – Full – width half – maximum

GTL – Gas – to - liquid

HPA – Phospomolybdic heteropolyacid

HTFT – High Temperature Fischer –

Tropsch

L – Volume – averaged size of crystallites

LTFT – Low Temperature Fischer - Tropsch

𝑛 – Carbon number

PO – Saturation pressure

PPO – Polypropylene oxide

PS – Vapor pressure at the droplet surface

PZC – Point of zero charge

ROR – Reduction – Oxidation - Reduction

SEM – Scanning Electron Microscopy

TCD – Thermal Conductivity Detector

TEM – Transmission Electron Microscopy

TEOS – Tetraethyl Ortosilicate

TOF – Turnover frequency (s-1)

TON – Turnover number

TPAOH – Tretaprolylammonium hydroxide

TPR – Temperature Programmed Reduction

XPS – X – ray Photoelectron Spectroscopy

XRD – X – Ray Diffraction

WGS – Water Gas Shift reaction

𝑊𝑛 – Mass fraction of species with carbon

number, 𝑛

1

1. Introduction

According to the BP energy outlook for 2035, the primary energy demand will increase by 41%

between 2012 and 2035. [1] As one can see in Figure 1 more than 50% of the global energy consumption

is provided by oil, gas and coal. Among fossil fuels, gas presents the fastest growth, accounting to 1.9%

per year. Nowadays, coal represents the largest source of volume growth. However between 2025 and

2035, it is expected that coal adds less volume than oil, which is justified by China’s shift away from

coal-intensive industrialization.

Figure 1 - Fuel consumption since 1965 up to 2035. [1]

Therefore, with the development of the emerging economies (China, India, Brazil, etc.), it is

expected an increase in the number of vehicles. Thus, to keep up with the transports energy

requirements, it will be necessary to increase the diesel fuel production. In addition, it is crucial to have

an alternative fuel synthesis. Indeed, crude oil is becoming more and more heavier, shale gas only

provides very light cuts and on the other hand it is expected that environmental requirements become

more exigent.

The Fischer - Tropsch process (FT) transforms a mixture of H2 and CO, often called syngas, into

liquid fuels. FT plays an important role in the search for an alternative way to produce liquid fuels with

several plants all over the world. For instance, Sasol operates a coal-based plant in South-Africa which

has a capacity of 160 000 bpd (barrels per day), where the main products are waxes, naphtha, high

quality diesel and kerosene. [2] Furthermore, in order to produce middle distillates (diesel and kerosene)

and waxes, low temperature FT process (LTFT) is used. In this process, cobalt catalysts supported on

alumina matrix are preferred. These catalysts are normally obtained through the incipient wetness

impregnation method. During this synthesis, the metallic phase solution is deposited onto the oxide

2

support and this process could take numerous impregnations. After impregnating the desired amount of

cobalt onto the support, the solid should be dried, calcinated and reduced to form the catalyst active

phase.

Thereby, it is crucial to develop an alternative synthesis pathway that reduces the number of

preparation steps.

As one can find in the bibliographic study, a review [3] reported the synthesis in one step of

mesostructured materials (well-organized structure with an uniform and periodic porosity (2-50 nm)

leading to a narrow pore size distribution) containing heteroelements (Ca, Fe, Zr and Al, Pd) on a silica

matrix. This type of synthesis was accomplished due to the combination of sol – gel chemistry and the

aerosol process.

The main objective of the present work is the synthesis of active FT catalysts, in one step, by spray

– drying. More precisely, this technique accomplishes the direct incorporation of the metallic phase onto

the inorganic matrix synthesis.

Concerning the report outline, in the bibliographic study (chapter 2), it is described the FT process,

the usual FT catalysts and its normal synthesis procedure. After, it is given a brief explanation in the sol

– gel chemistry and synthesis techniques to obtain mesostructured solids, where it is included the spray

– drying technique.

In chapter 3, it is described the different preparations methods as well as a description on the spray

– dryer work and the characterization methods (experimental work).

In chapter 4, it is possible to find the results and discussion in the physic – chemical properties of

the synthetized materials.

Finally, in the last chapter, chapter 5, are presented conclusions and future work on this theme.

Study objectives

The main goal of this work is the synthesis of FT catalyst by the spray – drying process in one step.

This synthesis pathway accomplishes a reduction in the number of steps required to synthetize a usual

FT catalyst.

To do that, two solutions should be prepared: the first one contains an inorganic molecular

precursor, acidified water and a cobalt precursor; the second one contains a surfactant, ethanol and

acidified water. These solutions should be mixed and the resulting solution is spray – dried. The obtained

powder should be dried, calcinated and reduced in order to form the active phase.

Moreover, as referred in the bibliographic study, the aerosol process can form mesostructured

materials, however it is not the aim of this work, as the FT process does not requires mesostructured

catalysts. The priority is to have a mesoporous catalyst.

3

2. Bibliographic Study

2.1. Fischer – Tropsch Process

The FT process was proposed by Hans Fischer and Franz Tropsch in 1925. This process converts

a mixture of hydrogen and carbon monoxide, usually called synthesis gas (or syngas), into a

hydrocarbon mixture. As one can note in the following equations, this mixture is mainly composed by

paraffins, olefins and oxygenated compounds (mainly alcohols) with water, respectively.

(2𝑛 + 1)𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛+2 + 𝑛𝐻2𝑂 (Eq. 1)

2𝑛𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛 + 𝑛𝐻2𝑂 𝐶𝐻4 + 𝐻2𝑂 → 𝐶𝑂 + 3𝐻2 (Eq. 2)

2𝑛𝐻2 + 𝑛𝐶𝑂 → 𝐶𝑛𝐻2𝑛+2𝑂 + (𝑛 − 1)𝐻2𝑂 (Eq. 3)

Depending on the syngas source (non-petroleum feedstocks), the technology is often referred as

coal-to-liquids (CTL), when using coal to obtain syngas, or gas-to-liquid (GTL) when is used gas for

syngas production, and finally it is also possible to produce syngas from biomass, which is often referred

as biomass-to-liquid (BTL). The most used feedstock is methane due to its availability, and because it

is cheaper to build a methane-based plant than a coal-based plant (coal-based plants can cost up to

50% more than a methane-based plant). [3]

This process allows the production of methane (C1), petroleum gas (C2-C4), gasoline (C5-C11),

diesel and jet fuel (C12-C20) and wax (C21+), the latter being subsequently valorized into smaller

molecules (diesel and gasoline) through hydrocracking.

However, methane or petroleum gas production is undesirable due to less carbon conversion and

also since they are possible feedstocks for syngas synthesis.

The following figure exemplifies a FT synthesis based production scheme (Figure 2).

Figure 2 –FT process-based production scheme. Adapted from [4]

4

2.1.1. Reactors

The FT process can occur at high temperature (HTFT) (300-350°C) or at low temperature (LTFT)

(150-240°C). The first one is preferentially applied in circulating bed and fixed fluidized bed reactors with

iron catalysts, where it is only present the catalyst and gases. Regarding the second process (LTFT), it

occurs in a multibular fixed bed reactor where several tubes contain the catalyst and water is surrounding

them. Or it can happen in a slurry reactor where the catalyst is suspended in a liquid wax with syngas

bubbling through, with either an iron or cobalt catalyst.

Concerning the circulating bed and fixed fluidized bed reactors, they are normally used to generate

hydrocarbons between C1-C15, olefins. Concerning the generated oxygenated compounds, they are

separated and purified to produce alcohols, acetic acid and ketones. [5] Waxes cannot be produced in

these reactors because these compounds are liquid under normal FT conditions, thus the catalyst could

agglomerate and de-fluidize the reactor. Therefore, for wax production, it is preferred the LTFT process,

hence a multibular fixed bed or a slurry reactor.

Comparing all the above described reactors, for short-life catalysts, it is ideal to use a multibular or

a slurry reactor, in order to have longer runs, hence not having to stop the reactor to feed more active

catalyst (as required in fixed bed reactors). However, a drawback in using a slurry reactor is the need of

an additional device in order to remove the generated wax. This device is not needed in fixed bed

reactors because, as the wax runs through the bed, it is relatively easy to separate the catalyst from the

wax.

Khodakov et al. [6] stated several drawbacks from each reactor type. On one hand, in fixed bed

reactors there are diffusion limitations, a considerable pressure drop and an insufficient heat removal

that leads to substantial temperature gradients. Hence high levels of methane are expected due to

excessive cracking. On the other hand, in slurry reactors the major disadvantages are the catalyst

deactivation and attrition which leads to formation of fine powders.

An important benefit from the use of slurry reactors is the easier temperature control due to a well-

mixed slurry, which is crucial since FT reactions are exothermic. Hence, the operation in this reactor is

practically isothermal. Also, with slurry reactors, higher temperatures can be used without coke

formation, which prevents catalyst deactivation, and the pressure drop across the bed is much smaller

than in fixed bed reactors. [7]

The last FT developed technologies were about LTFT process and involved syngas with high H2/CO

ratio, which is originated through steam reforming (Eq. 4) of methane, where the latter reacts with steam

under pressure, and the syngas obtained has a H2/CO of 3.

Autothermal reforming (Eq. 5), presents the required energy for the endothermic steam reforming

which is provided by the exothermic oxidation reaction. Moreover, the obtained syngas will have a H2/CO

ratio of 1 or 2.5, whether the reagent used is CO2 or O2, respectively.

Syngas can also be obtained through partial oxidation of methane (Eq. 6), where the natural gas

reacts with less than the stoichiometric quantity of oxygen, generating a mixture with a H2/CO ratio of 2.

𝐶𝐻4 + 𝐻2𝑂 → 𝐶𝑂 + 3𝐻2 (Eq. 4)

𝐶𝐻4 + 𝐶𝑂2 → 2𝐶𝑂 + 2𝐻2 (Eq. 5)

5

𝐶𝐻4 +1

2𝑂2 ↔ 𝐶𝑂 + 2𝐻2

(Eq. 6)

2.1.2. Reactions

The main FT process reactions were already present in (Eq. 1), (Eq. 2) and (Eq. 3). However in

each process there are side reactions, and the FT process is not an exception. As one can see in the

previous equations, water is a FT product, and it slows the reaction rate. The generated water can react

with carbon monoxide (CO) to form carbon dioxide (CO2) and hydrogen (H2), which is called the water-

gas shift reaction (WGS) (Eq. 7) and is one of the most important side reactions.

𝐶𝑂 + 𝐻2𝑂 ↔ 𝐶𝑂2 + 𝐻2

(Eq. 7)

Also, α-olefins can participate in side reactions (α-olefins hydrogenation), as well as paraffins

(paraffins isomerization). [8] It can also occur the transformation of CO into solid carbon and carbon

dioxide, which is the so-called Boudouard equilibrium (Eq. 8). Nevertheless, CO can react with H2 to

produce methane which is the inverse reaction of (Eq. 6).

2𝐶𝑂 ↔ 𝐶 + 𝐶𝑂2

(Eq. 8)

All FT products are free of sulfur and nitrogen compounds, which make FT products very attractive.

With the LTFT process, better cetane numbers are achieved: diesel after hydrotreatment presents a

cetane number of 75, and the market requires cetane numbers of 45 up to 50.

Also, in the LTFT technology, waxes (mainly linear) represent 50% of the total products. One can

note that the FT straight-run naphtha, as well as the one produced in waxes hydrotreatment, contains

no aromatics, consisting in mainly linear hydrocarbons, hence an extensive isomerization and Pt-

reforming is needed to generate a high octane gasoline.

2.1.3. Reaction mechanism

One of the proposed FT mechanisms is the hydrogenation of the adsorbed CO generating CHx

monomers which are polymerized in order to produce hydrocarbons with a wide range of chain length.

Chain growth occurs by addition of surface methylene (CH2) groups either by β-hydrogen abstraction to

produce linear α-olefins or by hydrogen addition to form n-paraffins. The first one is a reversible

termination at FT normal conditions. Nevertheless, chain termination can also occur by CO insertion into

the surface which will lead to alcohols formation. [8]

The initial step involves the dissociation of CO in order to form chemisorbed carbon, which is

hydrogenated to surface methyl and methylene groups, as it is a catalytic reaction. [8]

Therefore, at the catalyst surface, each specie has the option to desorb to generate an olefin, or to

be hydrogenated and desorb afterwards to produce a paraffin, or it can simply continue the chain growth

by adding another methylene group (Figure 3).

6

.

Figure 3 - FT stepwise growth process, where α stands for probability of chain growth and d stands for desorbed species. [9]

2.1.4. Product distribution

The product distribution of FT synthesis can be described by the equation of Anderson-Schulz-

Flory (ASF). The ASF equation (Eq. 9) refers to the chain length distribution, which is related with the

probability of chain growth, given by α.

𝑙𝑜𝑔 (𝑊𝑛

𝑛) = 𝑛 𝑙𝑜𝑔𝛼 + 𝑙𝑜𝑔

(1 − 𝛼2)

𝛼

(Eq. 9)

In the above equation, 𝑊𝑛 is the mass fraction of species with carbon number 𝑛. This equation

assumes that the chain is formed by adding C1 monomers, and the probability of chain growth is given

by α, which is independent from the chain length.

There are several factors that influence the α-value, for instance, the operating temperature, the

catalysts promoters, as well as the partial pressures in contact with the catalysts. When the temperature

increases, the α-value decreases. Thus it produces a higher amount of light hydrocarbons, which is

undesirable. Besides, increasing the pressure will lead to an increase in the α-value.

Therefore, representing 𝑙𝑜𝑔 (𝑊𝑛

𝑛) as function of 𝑛, it is possible to obtain the value of α. Figure 4

shows the product selectivity towards α, and, as one can see, the selectivity of gasoline and diesel is

about 40%, for a α-value of 0.73 and 0.84, respectively. As these selectivities are too low for industrial

applications, FT synthesis has to be operated with a α-value higher than 0.9, which will lead to a

production of a reasonable amount of liquid fuels and large amounts of waxes that will be cracked

afterwards. Cobalt catalysts have a high α-value, more precisely in the range of 0.90. [5]

7

Figure 4 - Product selectivity as function of ASF chain growth probability α. [10]

Some deviations from the ASF distribution were reported concerning higher methane selectivities,

than expected, and lower C2 (mainly ethylene) yields. Besides these deviations, products with higher

carbon numbers do not always follow the ASF distribution. There are several explanations for these

phenomena, for instance, different termination mechanisms or side reactions are causing the deviation

for products with longer chains. The latter topic is due to olefins readsorption on the catalyst that can

lead to secondary reactions, such as hydrogenation and isomerization, and even olefins condensation

leading to longer chains.

2.2. Fischer – Tropsch catalysts

The FT catalysts are composed by an oxide support, a metallic active phase and sometimes metal

and/or oxide promoters. The following paragraphs will described these topics.

2.2.1. Active phase

For FT applications only Fe-, Co-, Ni- and Ru- based catalysts have a satisfactory activity. Due to

the low availability of Ru, and consequently its high price, Ru-based catalysts are not used.

Ni catalysts have a high hydrogenolysis capability and hence, produce large amounts of methane.

Moreover, at the temperature and pressure at which FT plants operate, they present a low selectivity for

long chain hydrocarbons, which is not the FT purpose. Also, Ni based catalysts generate volatile

carbonyls, which result in continuous loss of metal.

Concerning Fe- and Co-based catalysts, despite the higher cost of cobalt catalysts, the latter

presents a higher activity and longer life than Fe-based catalysts. Indeed, cobalt is the most active metal

for long chain hydrocarbons. However, cobalt based catalysts are very sensitive to temperature

changes, thus a small increase in temperature could lead to a high production of methane. [9] [11]

More in detail, cobalt catalysts have a higher resistance to deactivation than iron based catalysts.

In addition, the WGS reaction has higher activity on Fe-based than on Co-based catalysts, thus it slows

the reaction rate on Fe-catalysts. Besides, for Fe-based catalysts, the syngas should not contain more

than 0.2 ppm of sulfur, and for Co-based catalysts, the sulfur content must be less than 0.1 ppm. [12]

8

Finally, cobalt catalysts are more suitable for higher H2/CO ratios, approximately 2 and on the other

hand, iron catalysts are preferred for lower H2/CO ratios.

This study will be focused on Co-based catalysts, due to their stability, higher activity and higher

hydrocarbon productivity. These catalysts are considered an optimal choice for long-chain hydrocarbons

production in the LTFT process. [5]

2.2.2. Supports

Concerning the support material, it is crucial to assure the stabilization of the Co particles in order

to provide a good catalyst activity. Changing the support’s surface, structure and pore size could lead

to an improved metal dispersion, reducibility and the diffusion coefficients of reactants and products. As

previously mentioned, Al2O3 and SiO2, are the most used catalysts supports due to their high surface,

and strong mechanical strength. [13] The following paragraphs will describe the most often supports

used in the FT process.

Silica – Supported catalysts

A better cobalt reducibility is achieved in this type of support due to a relatively weak interaction

between the support and cobalt. However, the cobalt dispersion is much lower in silica-supported

catalysts than the one achieved with alumina-supported catalysts. Khodakov et al. [5] reported that

catalysts with a pore size of 6 – 10 nm displayed higher FT activity and higher C5+ activity.

It was found that periodic mesoporous silicas including MCM-41, SBA-15 and SHS have expanded

their utilization in cobalt based catalysts, due to their desirable properties such as large surface area,

controllable pore size and narrow pore size distributions. [13]

Moreover, Jung et al. [13] reported successful catalytic tests with cobalt dispersed into periodic

mesoporous silicas by incipient wetness impregnation method (method described in the following

paragraphs). Also, the support’s periodic mesoporosity enhances the reactants access to active sites

as well as the transportation of higher hydrocarbon products. Industrially, silica is not the most used

support.

Alumina – Supported catalysts

Alumina has been one of the mostly used supports for cobalt FT catalysts. There are formed

small cobalt crystallites due to strong interactions between cobalt oxide with this support. Cobalt

reducibility is one of the most important problems of alumina-supported cobalt FT catalysts. However,

promotion with noble metals can improve cobalt reducibility.

2.2.3. Metals and oxide promoters

According to the literature [11], cobalt-based catalysts are composed by an oxide support, metal

and oxide promoters. Khodakov et al. [5] reported that the metal promoters are normally Pt, Ru, Ir and

Re, which are described as “reduction promoters” that will result in an ease of cobalt reduction and an

enhancement in the cobalt dispersion. Iglesia et al. [14] revealed that the promotion with ruthenium

delayed irreversible catalyst deactivation.

9

Khodakov and co-workers [5] described that the most often oxide promoters are ZrO2, La2O3, MnO

and CeO2 and the implementation of these oxides could lead to the catalyst texture and porosity

modification, as well as reduction of the formation of cobalt mixed oxides, increase in the cobalt

dispersion, and enhance mechanical and attrition resistance of the catalysts. In this study it was found

that promoting the catalyst with zirconia would lead to higher FT reaction rates, along with an increase

in C5+ selectivity.

Finally, the shape of the catalyst depends on the type of the reactor.

2.2.4. Usual catalyst preparation methods

The choice of the deposition method of the active phase will strongly influence the catalytic activity

of the final catalyst. The following paragraphs will address the most usual preparation methods to

prepare cobalt-based catalysts for FT synthesis: impregnation and deposition-precipitation methods. [5]

The preparation of cobalt-based FT catalysts includes the following steps [5]:

1) Synthesis of the catalyst support

Normally the supports used in the FT process are obtained through the spray-drying of a solution

which contains inorganic precursors of the oxide support.

2) Preparation of cobalt precursors, and possibly promoters

3) Cobalt precursors dispersion onto the catalyst support

The purpose of dispersing the active phase, cobalt in this study, is to spread it along the porous

support and to create metal clusters. [5] Moreover, it is necessary to generate a significant concentration

of stable cobalt metal surface sites, and this depends on the size of cobalt particles as well as their

reducibility.

Incipient wetness impregnation is the most common method for preparing cobalt-based catalysts

for the FT synthesis. This method consists on preparing a solution of a cobalt salt which is contacted to

a dry porous support.

After being contacted, the solution is aspired by capillary forces within the support pores. The

incipient occurs when all pores of the support are filled with the liquid and there is no excess of moisture.

The fundamental phenomena underlying impregnation and drying are extremely complex. Thus, it is

needed a careful control of temperature, time of support drying, rate of addition of impregnating solution,

etc.

At the moment right after impregnation, the interactions between the metal precursor and the

support are relatively weak, which allows the redistribution of the active phase over the support during

drying and calcination steps.

The distribution of Co2+ cations depends on the support charge, whether it is silica, alumina or

titania. For each material, there are different points of zero charge (PZC), which correspond to the pH

at which the positive and negative charges on the surface are equal and cancel. Therefore, at a pH

below the PZC, the oxides surface is positively charged, and at a pH higher than PZC the oxides surface

is negatively charged. Hence, if the impregnation solution has a pH below PZC, repulsion between the

support surface and Co2+ ions will lead to a nonhomogeneous distribution of Co2+ ions. On the other

10

hand, if the impregnation solution has pH above the PZC, Co2+ ions will be homogeneously distributed.

[5] In a first approximation this last statement is correct. However, at a pH of 12-13, Co in water is not

in the usual form (Co2+), it is in the form of Co(OH)42-, thus repulsion would occur.

4) Post-treatment

After impregnation, the catalyst does not present the active phase in the final form, hence it is

required additional treatments. In order to eliminate the solvent, it is needed a drying step followed by

thermal treatments, as calcination and activation. The following paragraphs will be focused on the post-

treatments in cobalt-based catalysts.

Drying

As previously mentioned, the drying step wishes the removal of the solvent present in the catalyst

pores. Coulter and Sault [15] have stated that the surface properties of Co/SiO2 catalyst, prepared by

incipient wetness impregnation with a cobalt nitrate precursor, are extremely influenced by the drying

and calcination conditions. They showed that, after drying the samples at 110°C and calcinating it in air

at 400°C, a surface phase of Co3O4 is created and it is easily reduced. However, drying under vacuum

conditions formed an irreducible Co3O4 phase due to the migration of Co2+ cations into the silica matrice.

Therefore, vacuum dried samples present a higher nitrate concentration on the surface, which leads to

the formation of a silicate surface. On the other hand, the presence of a gas phase NOx formed during

the decomposition of the cobalt nitrate precursor promotes the oxidation of the intermediate Co2+ to form

large Co3O4 particles.

Calcination

During calcination, several chemical reactions can take place. These reactions are responsible for

the precursor’s thermal decomposition, as well as the relief of volatile compounds.

According to Krylova et al. [16], for Co/SiO2 and Co/Al2O3 catalysts, the calcination temperature

influences the metal reducibility and the active phase dispersion. On one hand, the calcination at higher

temperatures of Co/Al2O3 catalysts causes the formation of new surface compounds which increases

the hydrocarbon yield (however, the selectivity on C5+ decreases leading to a higher methane

production). On the other hand, the calcination of Co/SiO2 catalysts at higher temperatures leads to a

higher selectivity on C11-C18, thus an increase in the average carbon number.

5) Reducing treatments

The activation is the last step on the catalyst preparation. This operation involves a thermal

treatment at high temperatures and it occurs usually under a hydrogen flow.

The reduction mechanism is generally assumed to occur in two steps, which are showed in the

following equations. The first step, (Eq. 10), usually takes place at low temperatures, 100-350°C, and

(Eq. 11) normally takes place at 400-600°C. [5]

11

According to Petru and co-worker [17], the metal particle size is a determinant factor in the reduction

process of oxidized cobalt catalysts. In this study, it was showed that the degree of reduction of Co3O4

strongly depends on the conditions of its preparation. For instance, the particles prepared at 600°C were

easily reduced to Co at 300°C, while those prepared at 900°C were not reduced even at 500°C. Two

effects were proposed to explain this: diffusion limitations occurring with increasing particle size, and

differences in the microstructure of the particles. Moreover, high reduction temperatures (>400°C) can

lead to sintering cobalt particles. [5]

Therefore, as above mentioned, cobalt-supported FT catalysts are normally loaded with Pt, Ru, Ir

and Re which will result in an ease of cobalt reduction and an enhancement in the cobalt dispersion.

However, this can increase the catalyst cost and affect the economic efficiency of the overall FT

technology. [5]

2.2.5. Activity and selectivity

The activity of the catalyst will dictate the process economics, thus, regarding the FT process the

size of the particles that create the active phase will interfere in the catalyst activity. Hence, there is a

necessity to find out the influence of the cobalt particles size and the most effective method to disperse

the active phase.

One of the most important catalyst characteristics is the quantity of reactant transformed per time

unit, per unit of mass, or area unit of active phase. The catalyst activity can be quantified by the turnover

number or frequency number, TON or TOF, respectively. The turnover frequency describes the number

of moles transformed per active site per time unit.

Bezemer et al. [18] studied the influence of cobalt particle size on the catalyst activity, turnover

frequency (TOF), methane selectivity and C5+ selectivity, at 1 and 35 bar, and at 210 and 250°C

regarding C5+ selectivity. Referring to the catalytic performance at 1 bar, it was found that in particles

ranging from 27 to 6 nm the activity increases from 0,64 × 10-5 to 3,51× 10-5 molCOgCO-1s-1, respectively.

This increase in activity is a result of a higher specific cobalt surface area. However, for particles smaller

than 6 nm the activity quickly decreases. [18]

Concerning the TOF, it has been concluded that, for particles ranging from 6 to 27 nm, TOF is

relatively constant, but for smaller particles it rapidly decreases. Therefore, for small cobalt particles

(smaller than 5 nm) there is a higher selectivity for methane than in particles larger than 5 nm.

Furthermore, the methane selectivity remains constant for particles ranging from 5 to 27 nm. This high

selectivity in small cobalt particles might point out a lower quantity of active sites for chain growth,

leading to a higher methane production. [18]

The C5+ selectivity depends on the cobalt particle size, and varies from 76 to 84 wt.% at 210°C and

from 51 to 74 wt.% at 250°C. [18]

Regarding the catalytic performance at 35 bar, Bezemer and co-workers [18] showed that TOF is

constant for samples with cobalt particles larger than 8 nm, while for smaller particles it starts to

𝐶𝑜3𝑂4 + 𝐻2 → 3𝐶𝑜𝑂 + 𝐻2𝑂 (Eq. 10)

3𝐶𝑜𝑂 + 3𝐻2 → 3𝐶𝑜 + 3𝐻2𝑂 (Eq. 11)

12

decrease. Thus, it can be concluded that the catalytic performance at 1 and 35 bar shows the same

trend, with constant TOF-values for larger Co particles and size dependency for small Co particles.

According to Khodakov et al. [5], catalysts containing small cobalt particles do not exhibit adequate

catalytic activity, as they are very hard to reduce and as these particles are not stable at normal FT

conditions. As they are not stable, they could lead to catalyst deactivation (sinter, carburize, reactions

with the support) or the use of small particles can radically change the adsorption properties.

Therefore, Bezemer and co-workers [18] concluded that one should aim for catalysts with cobalt

particles size close to 6 – 8 nm. As larger particles show a lower activity and smaller particles show a

lower selectivity and activity.

2.2.6. Catalyst deactivation

About the catalyst deactivation, there are several mechanisms that are responsible, such as,

poisoning by sulfur, chlorine and nitrogen compounds, sintering of cobalt crystallites, carbon effects, re-

oxidation, attrition, etc. The following paragraphs will briefly describe each one of these topics.

Sulfur is potentially present in the feed when the feedstock is coal or biomass, and Tsakoumis et

al. [19] stated that one sulfur atom adsorbed on a Co/Al2O3 catalyst poisons more than two cobalt atoms.

Also, it was found that small amounts of N-compounds have an immediate effect on the catalyst activity.

Nevertheless, the deactivation seems to be reversible with an in situ hydrogen treatment, which can

recover 100% of the catalyst activity.

Sintering of cobalt crystallites leads to a reduction of the active surface area, and it is

thermodynamically driven by the surface energy minimization of the crystallites. Tsakoumis and co-

workers [19] reported that high temperatures and water accelerate the sintering process, and this is

recognized as an irreversible process. However, by a reduction-oxidation-reduction (ROR) sequence it

is possible to have a re-dispersed cobalt phase. As FT synthesis accomplish an exothermic reaction,

the potential for sintering is relatively high, thus, as above mentioned, special attention should be given

to the reactor choice, since it is extremely important to have isothermal conditions.

Side reactions, as the Boudouard reaction, generate carbon that could interact with the metal and

form either inactive species or species that act like reaction inhibitors (amorphous, graphitic or carbon

species (coke)). As water is an oxidizing agent, and as it is the most abundant byproduct of FT synthesis,

it could lead to surface oxidation of the cobalt particles. Thus, as inactive cobalt oxides are generated,

the catalyst activity decreases.

Attrition is the breakdown of solid particles, which could be due to abrasion/erosion or fracture. This

fracture can result in the production of fine powders which means a low catalytic performance. Some

studies [19] reported that the addition of cobalt to alumina or silica supports increase the resistance

towards attrition. Catalyst deactivation due to attrition effects is particularly valid for moving bed reactors.

Lastly, Sadeqzadeh et al. [12] reported that higher H2/CO ratios promote higher initial catalytic

performance, as well as higher initial deactivation rate. This could be related to a higher water

production, which is directly associated with catalyst deactivation (sintering).

13

2.3. One – Pot synthesis: Incorporation of the metallic phase on a mesoporous oxide

matrix

The supports used in the FT process, as previously mentioned, are normally silica or alumina,

which are obtained through the spray – drying of an inorganic solution. In this procedure, sol – gel

reactions take place.

The following paragraphs will briefly describe the sol-gel chemistry and a particular case of the

application of sol – gel chemistry: mesoporous materials.

2.3.1. Sol-gel chemistry

The chemistry involved in the sol-gel process is based on inorganic polymerization reactions. Thus,

it allows the formation of inorganic polymers through polycondensation of precursors, such as, metallic

salts or alkoxides. This method is easy to apply, therefore it is widely applied in the manufacture of

catalyst supports. [3] The polymerization reactions involve two steps: hydrolysis and condensation of

the precursors.

In order to produce silica or alumina it is needed a precursor, such as Tetraethyl Orthosilicate

(TEOS) (Si(OCH2CH3)4), or aluminum chloride hexahydrate AlCl3∙6H2O, respectively. One can note that

metal alkoxides are popular precursors as they will lead to more pure solids than if a salt was used, and

also because it is easier to control the kinetics of hydrolysis and condensation reactions. In the following

equations is described the mechanism to synthesize silica. The reaction proceeds first through

hydrolysis (Eq. 12) which is the hydrolysis of alkoxy groups. Once reactive hydroxy groups are formed,

the generation of oligomers and polymers occurs via polycondensation reactions that can occur via (Eq.

13) or (Eq. 14).

One can note that the structure and morphology of the resulting network is strongly dependent on

the nature of the precursors, as well as the water content, the pH, temperature, and the relative

contribution of each one of these reactions. As mentioned before, the surface of an oxide is differently

charged depending on the pH. In the case of silica synthesized using TEOS as precursor, the

condensation reactions are catalyzed through acid-basic reactions, where the groups Si-OH2+ and Si-

O- interfere, respectively, in an acid or basic media. As one can note in Figure 5, the hydrolysis kinetics

presents a minimum at a pH of 7, and the condensation kinetics presents a minimum at a pH of 2 which

corresponds to the PZC of silica. [20]

𝑆𝑖(𝑂𝑅)4 + 𝑛 𝐻2𝑂 → 𝑆𝑖(𝑂𝑅)4−𝑛(𝑂𝐻) + 𝑛 𝑅𝑂𝐻 (Eq. 12)

𝑆𝑖(𝑂𝑅)3 − 𝑂𝐻 + 𝑆𝑖(𝑂𝑅)3 − 𝑂𝐻 → (𝑂𝑅)3𝑆𝑖 − 𝑂 − 𝑆𝑖(𝑂𝑅)3 + 𝐻2𝑂 (Eq. 13)

𝑆𝑖(𝑂𝑅)3 − 𝑂𝑅 + 𝑆𝑖(𝑂𝑅)3 − 𝑂𝐻 → (𝑂𝑅)3𝑆𝑖 − 𝑂 − 𝑆𝑖(𝑂𝑅)3 + 𝑅𝑂𝐻 (Eq. 14)

14

Figure 5 - Hydrolysis (H), Condensation (C) and Dissolution (D) kinetics for a system of TEOS. [20]

The sol-gel chemistry allows the synthesis of a large variety of amorphous or crystalline solids, with

very different porosities, and in particular it can form mesoporous materials. As the condensation

happens at ambient temperature, it is possible to add organic molecules in the reaction media, which

will create a mesostructure. [20]

The following paragraphs describe this type of materials.

2.3.2. Mesostructured materials: definition

A mesostructured material presents pores that according to IUPAC classification, consist in a well-

organized structure with an uniform and periodic porosity at the mesoporous scale and the pore diameter

is between 2 and 50 nm. The principle of synthesis is derived from the sol-gel synthesis, previously

described, except that a (supra)molecular (surfactant) is used to generate periodic mesoporosity. [21]

The surfactants can be ionic or non-ionic. In the first type, one should take in account the charge

of the oxide. On one hand, taking the example of silica, above the PZC (pH>2) the surface is negatively

charged, thus a cationic surfactant would be the better choice. On the other hand, bellow the PZC

(pH<2), the surface is positively charged, thus an anionic surfactant would be more suitable.

Therefore, using non-ionic surfactants allows the synthesis of mesostructured materials in a large

range of pH. These type of surfactants include the Brij family (CH3(CH2)x-[EO]y-OH) and the Pluronic®

family ([PEO]x-[PPO]y-[PEO]x), with PEO: polyethylene oxide and PPO: polypropylene oxide.

It is well known that amphiphilic molecules can arrange themselves in micelles (aggregate of

surfactant molecules). A typical micelle in an aqueous solution forms an aggregate with the hydrophilic

part that is in contact with the surrounding solvent, whereas the hydrophobic part forms an aggregate in

the micelle center. Figure 6 describes several forms of micelles arrangement.

One can note that the solid final geometry depends on the geometry and arrangement of the

micelles.

In order to have a mesostructured solid, it is necessary the hydrolysis and condensation reactions

of the inorganic phase (sol-gel chemistry), the micellization of organic phase and finally a good

interaction between these two phases, as it is explained in 2.3.3.

15

Figure 6 – Micelle structure A: Sphere, B: Cylindric, C: Lamellar, D: Inverse micelle, E: Bicontinuous phase, F:

Liposomes. [20]

2.3.3. Mesostructured materials: mechanisms

To explain what occurs during the synthesis of mesostructured materials, two mechanisms were

proposed, depending on the surfactant concentration (c): True Liquid Crystal templating (TLC) and

cooperative self-assembly mechanism. The first one refers to the situation where c>>CMC (Critical

Micelle Concentration (CMC): above this surfactant concentration micelles are spontaneously formed),

hence micelles soon begin to form. After, hydrolysis and condensation reactions of inorganic precursors

take place around this template. This pathway can be observed in Figure 7a).

If c≈CMC, the cooperative self-assembly mechanism takes place, where several processes occur

simultaneously, such as hydrolysis and condensation reactions of the inorganic species, the self-

assembly of the surfactant and the interaction between these two phase. In Figure 7b) it is illustrated

the pathway of this mechanism.

Figure 7- Main synthesis pathways to mesostructured materials. [21]

2.3.4. Mesostructured materials: synthesis techniques

The following paragraphs describe usual synthesis techniques for the production of mesostructured

materials.

1) Precipitation

In mildly acidic to basic conditions, it is known that silica polymerization starts with the condensation

of monosilicic acid into cyclic oligomers, which grow to three-dimensional polymer particles. These

16

particles may grow through monomer addition and if they grow large enough, they will eventually

precipitate. [22]

The formation of mesoporous solids through precipitation is rapid. In only 3 – 5 minutes in a cationic

surfactant solution, it is possible to detect well-ordered mesostructures. However, if a nonionic surfactant

is used, the formation of mesostructures is slower (normally 30 minutes). One can note that the pH value

affects the mesostructure formation rate. [22]

This technique accomplish the following steps to produce mesostructured materials: dissolution of

the reactants (inorganic precursor and surfactant) followed by the formation of a suspension. Afterwards,

this suspension is put inside an autoclave, and the autoclave goes to a pre - heated oven at 110°C and

it remains there for 24 – 48 hours. Later, the precipitate requires a wash/filtration step. Finally, a post-

treatment (drying and calcination) step is required.

2) Evaporation

The so called Evaporation Induced Self-Assembly (EISA) presents a mechanism that can be

explained by the cooperative self-assembly mechanism.

This synthesis method, EISA, consists in the progressive evaporation of a solution containing

inorganic precursors and organic surfactants. Initially, this solution is diluted, thus, the surfactant

concentration is lower than the CMC, which inhibits micelles formation, and also the lower concentration

in inorganic precursor allows a low hydrolysis and condensation kinetics. During the evaporation, the

solution will become more concentrated, which induces the mesostructuration process. Hence, this

method depends on the solvent evaporation kinetics, template structuration and inorganic species

condensation.

In order to achieve a better control on the mesostructuration process, it is necessary to understand

the influence of certain parameters, for instance pH, temperature and surfactant/silica ratio. The

parameter surfactant/silica volume ratio, studied by Sanchez et al. [23], can have a significant impact

on the level of organization. The type of obtained micelles will vary on this ratio, and more curved

micelles will form worm – like phases, that will lead to a low level of organization in the final solid.

Moreover, the surfactant will dictate the pore diameter.

Moreover, the EISA has great advantages over the precipitation method. Firstly, the

surfactant/silica stoichiometry is retained after evaporation; secondly the shaping into spheres takes

place with control of the diameter, and finally, large objects with controlled dimensions (slow

evaporation) are possible.

2.1) Spray - drying

The spray-drying method is a sub-process of the evaporation technique, where a limpid solution (in

fact it is a colloidal solution, as hydrolysis and condensation reactions have already begun some

oligomers will be in solution) is transformed into a dry powder.

The aerosol technique presents many benefits when compared to the precipitation methods. It is

a continuous process (the time between a droplet and a dry solid is between 1 to 4 seconds), it can be

17

easily scaled-up to industrial scale, and, lastly, it allows a perfect control of the chemical composition of

the final solid due to the existence of non volatile components that once were present in the atomization

solution.

Therefore, this process permits the formation of materials that were not possible to be generated

through the usual preparation methods, due to the fast evaporation that the precursors are obliged to

co-arrange inside the matrix under a metastable state. Another benefit in using this method is the small

amount of waste that is generated, in terms of energy, it is only needed to treat the gas that is released.

There are several spray-drying techniques, all related with the mechanical destabilization of the

solution/atmosphere interface. The liquid feed can be atomized by several nozzles types depending on

the required droplet size and this colloidal dispersion of liquid droplets in a gas is usually called “aerosol”.

[24]

As one can see in Figure 8, firstly a very diluted solution is pumped to the nozzle. Furthermore, the

aerosol is generated where liquid droplets are carried by the vector gas. Concerning the drying step, the

solvent evaporation allows the material’s structuration through a mechanism of auto assembly which is

induced by the solvent evaporation. The evaporation driving force is the difference between (P0 – PS),

where PS is the vapor pressure at the droplet surface and P0 is the saturation pressure. Therefore, when

P0 = PS the evaporation process stops. After solids that are smaller than what is required are sent to an

air filter, and the solids that meet the specifications are recovered in a collector.

Moreover, the non volatile species are responsible for the polarity fluctuation and the viscosity

change in the droplet depth profile. Hence, the diffusion of volatile species inside the particles through

the solid/gas interface, has an important role in the particles’ structure. Therefore, parameters related

with this diffusion process, such as droplet size, residence time in the chamber, concentration, the carrier

gas relative pressures in volatile species, temperature and flux, must be controlled.

18

Figure 8 - Spray - Dryer (Büchi B 290) working principle and material's structuration mechanism. Adapted from [20]

Sanchez et al. [25] reported a direct aerosol synthesis of aluminosilicates in a basic medium. The

microstructure-directing agent used was the tretaprolylammonium hydroxide (TPAOH) in a Pluronic®

F127 template. The porosity showed to be homogeneous throughout the sample, and the pores size

was between 6 to 17 nm. Therefore, it is possible to conclude that the micillization and self-assembly

can occur between the aluminosilicate and the F127 copolymer until a pH of 11. Also, it has been

reported in this study that the pore size varies with the amount of TPAOH added to the solution, due to

the fact that this last compound slows the inorganic condensation reactions.

Heteroelements can be loaded onto the oxide matrix synthesis. It is important to understand and

control this incorporation.

If mesostructured solids are manufactured by spray-drying, the localization, structure and

properties of the final particles will depend on the precursor solution chemistry and on the fast drying

conditions. The uniform dispersion of the heteroelements onto the mesostructured matrice depends

mainly on two parameters: the homogeneity of the nanoparticles dispersion onto the precursor

silica/surfactant solution, and the nanoparticles surface/silica/surfactant interactions.

It was reported in a review [3] that spray-drying is a very convenient way to synthesize powders

with a high loading of heteroelements. It was also reported the incorporation of the following

heteroelements in acidic conditions: aluminum, zirconium, calcium, iron, phosphate ions and palladium.

At low concentrations, heteroelements are found homogeneous distributed within the final particles.

Normally the mesostructures obtained with pure silica are not affected by the incorporation of small

19

amounts of heteroelements. One should note, that the trapping limit of heteroelements is conditioned

by its attractive interactions with silica oligomers or with the structuring agent.

Furthermore, Sanchez and co-workers [26] reported a synthesis procedure that allows the

formation of mesoporous silica with cobalt (Co) and molybdenum (Mo) already loaded. In this study,

the co-location of molybdenum HPA precursor close to the micelles upon the evaporation of aerosol

droplets, allowed the complete accessibility of the active phase. It is interesting to draw attention to the

fact that the simultaneous existence of Co and Mo did not affect the mesostructuration. However, this

study reveals that the presence or absence of Co may interfere with the localization.

21

3. Experimental Work

The catalysts prepared in this study are cobalt based catalysts either on silica or alumina matrix

and they were obtained through spray-drying performed with an ultrasonic nozzle.

Several supports and cobalt catalysts were produced, and concerning the latter, all of them have

15 wt.% of cobalt regarding the total mass of catalyst.

All the prepared supports and catalysts were characterized through N2 adsorption – desorption and

the latter was also characterized by TPR. Besides this, in the most promising catalysts or supports other

analysis were performed, such as XRD, low angles XRD, SEM and TEM.

This chapter will focus on describing the preparation methods (typical solution molar composition

and the differences between each sample), the spray-drying working principle as well as its parameters.

Finally, it will describe the characterization techniques.

3.1. Preparation Methods

The materials were obtained through a solution referred as “atomization solution”. The preparation

of this solution requires several steps, the first one being the hydrolysis of the matrix precursors. After

that, a surfactant solution (which will induce the mesostructuration), and a solution of the metallic

precursors are prepared. Finally, these three solutions should be mixed and afterwards atomized in a

Büchi B-290 spray – dryer. One should note that the atomization solution should be limpid to the eye

during all the atomization period (without any precipitation or gelification phenomenon).

Solutions nature

The atomization solution results from the mixture of two solutions: an inorganic solution and an

organic solution.

Inorganic solution: is a mixture of a silica or alumina precursor and acidified water, and if

a catalyst is synthesized, it will also contain a solution of the metallic precursor.

Organic solution: is a mixture of surfactant (Pluronic®P123 (PEO20-PPO70-PEO20), where

PEO stands for polyethylene oxide and PPO stands for polypropylene oxide), ethanol and

acidified water.

One should note that the water in both solutions can be acidified with a solution of HCl or HNO3.

3.1.1. Supports

Silica and alumina matrices were produced. To obtain a silica matrix, tetraethyl orthosilicate (TEOS

(Si(OEt)4), 98% Sigma – Aldrich) was used as precursor, and, regarding alumina, aluminum chloride

hexahydrate (AlCl3·6H2O, 99% Sigma – Aldrich) was used as precursor. The precursor’s solution

contains the inorganic precursor and acidified water at a pH of 2. The organic solution contains Pluronic®

P123, ethanol and acidified water at a pH of 2. In Table 1 it is described the molar composition for all

the synthesized supports.

22

Table 1 – Reference molar composition for all the supports.

For preparing a typical silica support, a solution with 15.97 g of TEOS (98% Sigma – Aldrich) and

24.15 g of acidified water at a pH = 2, was left stirring during one night. The organic solution contained

4.45 g of Pluronic® P123, 10.69 g of ethanol and finally 44.84 g of water at a pH = 2. In this procedure

the water in both solutions was acidified with a HCl solution (HCl, Sigma – Aldrich 37 wt.% or with a

HNO3 solution (HNO3, Sigma – Aldrich 68 wt.%).

Regarding the alumina supports, 18.05 g of AlCl3·6H2O were mixed with 23.55 g of water at a pH

of 2. This inorganic solution was left hydrolyzing during 30 minutes. Finally, the organic solution,

contained 4.34 g of Pluronic® P123, 10.33 g of ethanol and finally 43.73 g of water at a pH = 2. In this

procedure, the water in both solutions was acidified with a HCl solution (HCl, Sigma – Aldrich 37 wt.%)

or with a HNO3 solution (HNO3, Sigma – Aldrich 68 wt.%)..

In Table 2 is a full description of all synthesized supports, and for all the samples the molar ratio

between (EtOH:H2O) was 0.06, and the surfactant was always Pluronic®P123.

Table 2 - Synthesized supports.

After spray-drying, all the solids were dried at 100°C during one night, in a ventilated oven, and

calcinated at 550°C. The calcination profile for all the supports can be seen in Figure 9.

TEOS or AlCl3·6H2O H2O HCl or HNO3 EtOH Surfactant (P123)

1 50 0.009 (pH = 2) 3 0.01

Reference Matrix/Inorganic precursor nature pH;

(HCl/HNO3) Post - Treatment

JFR003

Silica/TEOS 2;HCl

Calcination at

550°C

JFR005

JFR006

JFR007

JFR028 2;HNO3

JFR009

Alumina/AlCl3·6H2O 2;HCl

JFR010

JFR025 2;HNO3

23

Figure 9 – Calcination profile for the synthesized supports.

3.1.2. Catalysts

In order to synthesize a catalyst, a metallic precursor solution has been added to the atomization

solution. The metallic precursor solution used in this study was cobalt nitrate solution in water (Co(NO3)2

with 13.4% of Co, with a solution density of 1.48). Usually, the cobalt nitrate solution is added to the

inorganic solution just before atomization starts. Furthermore, the organic solution remains equal as

described in the supports. The only difference is in the inorganic solution that accomplishes the addition

of the cobalt nitrate solution. Catalysts with 15 wt.% of cobalt, concerning the final catalyst mass, were

produced.

In Table 3 and Table 4 it is described the reference molar composition for catalysts on a silica

matrix and alumina matrix, respectively.

Table 3 – Reference molar composition for the catalysts on a silica matrix.

Table 4 – Reference molar composition for the catalysts on an alumina matrix.

For preparing a typical cobalt catalyst on a silica matrix, a solution with 13.53 g of TEOS (98%

Sigma – Aldrich) and 24.22 g of acidified water at a pH =2, was left stirring during one night. Just before

atomization, 5.03 g of cobalt nitrate solution in water (13.4% of Co with a solution density of 1.48) were

added to the latter inorganic solution. The organic solution contained 4.46 g of Pluronic®P123, 10.63 g

of ethanol and finally 44.98 g of water at a pH = 2. In this procedure, the water in both solutions was

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20 22 24

Te

mp

era

ture

(°C

)

Time (h)

JFR003; JFR005; JFR006;JFR007; JFR028; JFR009;JFR010; JFR025

TEOS Co H2O HCl or HNO3 EtOH Surfactant (P123)

0.85 0.15 50

0.09 (pH = 1)

0.009 (pH = 2)

0.0009 (pH = 3)

3 0.01

AlCl3·6H2O Co H2O HCl or HNO3 EtOH Surfactant (P123)

0.87 0.13 50 0.009 (pH=2) 3 0.01

12h

2°C/min

24

acidified with a HCl solution (HCl, Sigma – Aldrich 37 wt.%) or with a HNO3 solution (HNO3, Sigma –

Aldrich 68 wt.%).

Regarding the preparation of a cobalt catalyst on an alumina matrix, 15.77 g of AlCl3·6H2O were

mixed with 23.69 g of water at a pH of 2. This inorganic solution was left hydrolyzing during 30 minutes.

Just before atomization, 4.27 g of cobalt nitrate solution in water (13.4% of Co with a solution density of

1.48) were added to the latter inorganic solution. Finally, the organic solution, contained 4.36 g of

Pluronic®P123, 10.40 g of ethanol and finally 43.99 g of water at a pH = 2. In this procedure, the water

in both solutions, was acidified with (HCl, Sigma – Aldrich 37 wt.%) or with a HNO3 solution (HNO3,

Sigma – Aldrich 68 wt.%).

Moreover, besides aluminum chloride other alumina precursor was tested: aluminum nitrate

(Al(NO3)3, Sigma – Aldrich 99.997%).

In Table 5 it is a full description of all synthesized catalysts. All of them were produced through an

ultrasonic nozzle and for all the samples the molar ratio between (EtOH:H2O) was 0.06.

After spray-drying, all the solids were dried at 100°C during one night, in a ventilated oven. Three

calcination temperatures were performed in the muffle at 350, 400 and 550°C.

The calcination profile for all samples can be seen in Figure 10 and Figure 11.

Table 5 - Synthesized catalysts.

Reference Matrix/Inorganic precursor nature

pH; (HCl/HNO3)

Surfactant Post - Treatment

JFR014

Silica/TEOS

2; HCl

Pluronic®

P123

Calcination at 550°C

JFR016 Calcination at 350°C

JFR019 1; HNO3

Calcination at 400°C

JFR021 2; HNO3

JFR022 3; HNO3

JFR023 Alumina/AlCl3·6H2O

2; HCl

JFR024 2; HNO3

JFR037 Alumina/Al(NO3)3

25

Figure 10 - Calcination profile for JFR014 and JFR016.

Figure 11 - Calcination profile for JFR019, JFR021, JFR022, JFR023, JFR024 and JFR037.

0

100

200

300

400

500

600

0 2 4 6 8 10 12 14 16 18 20 22 24

Te

mp

era

ture

(°C

)

Time (h)

JFR014

JFR016

0

50

100

150

200

250

300

350

400

450

0 2 4 6 8 10 12 14 16 18 20 22 24

Te

mp

era

ture

(°C

)

Time (h)

JFR019

JFR021; JFR022; JFR023;JFR024; JFR037

2°C/min

12h

1°C/min

12h

2°C/min

26

3.2. Spray – Drying

3.2.1. Spray Drying working principle

The main objective is the transformation of a solution, which contains reagents of the desired

product, into solid spherical elementary particles that were dried through an atomization process

(evaporation). The following paragraphs describe the main steps of the spray drying process.

1) Solution pumping: The equipment responsible for

the solution pumping is a peristaltic pump. This pump

is responsible for feeding the solution to the nozzle.

2) Aerosol generation: The aerosol is generated

through a nozzle where the solution is mixed with a

vector gas. The diameter reduction in the outlet of the

nozzle combined with the vector gas flux allows the

formation of small droplets that are carried by the

gas. This vector gas is chosen according the desired

atmosphere, for instance, it can be an inert gas if an

inert atmosphere is required. There are different

kinds of energy used to disperse the liquid feed into

fine droplets.

Two fluid nozzles: The energy required for

atomization is provided by a rapid ejection of

the spray gas, which was previously mixed

with the liquid feed within the nozzle. It is

more suitable for a laboratory scale, due to

its low pressure consumption, low particle

velocity and thus shorter required length in the spray chamber. The droplets produced

by this kind of nozzle range from 5 - 30 µm.

Ultrasonic nozzles: The droplet size is controlled by the frequency at which the nozzle

vibrates, and by the surface tension and density of the liquid being atomized. One can

note, that the higher the frequency, the smaller the median droplet size. Moreover, there

is no need to use air pressure, the liquid is pumped to the nozzle vibrating surface. The

produced droplets range from 2 - 100 µm.

There are other type of nozzles, such as rotary disks and pressure nozzles, however they are more

suitable for an industrial scale, and because of that they are not mentioned in this report. In this study,

an ultrasonic nozzle was chosen instead of a two fluid nozzle (the nozzle often used in the laboratory

scale), because the size of the produced elementary particles is more appropriate for the final application

of these catalysts (FT synthesis).

Figure 12 - Spray drying apparatus. [27]

27

3) Aerosol evaporation: The liquid droplets have to be in contact with a hot gas, which allows the

evaporation of the solvents and the droplets transportation. Therefore, the inlet temperature

corresponds to the temperature of this heated vector gas. This temperature needs to be

controlled which is detailed described in the spray – drier parameters.

The droplets can shrink, agglomerate or lose sphericity as moisture and the solvent are

evaporated. At the end, the dried particle surface temperature approximates the temperature of

the surrounding gas, as it is described later (spray – drier parameters).

The manner in which the sprayed liquid droplets contact with the drying gas has a major impact

on the droplet’s behavior and in the dried product properties. The usual flow configurations are:

Co-current flow: The solution is sprayed in the same downwards direction as the flow of

the drying gas. The gas temperature in the outlet is the lowest because the gas comes

in contact with the droplets at the top. Therefore, this configuration is appropriated for

heat sensitive products.

Counter-current flow: The solution is sprayed in the opposite direction of the drying gas.

The drying gas enters at the bottom and flows upwards. The product becomes hotter at

the end in comparison with the co-current mode. This approach is suitable for thermally

stable products.

In this study it is used the co-current flow, because it is required that the liquid droplets

experiment a higher temperature at the chamber inlet, in order to start the solvent evaporation.

4) Powder collection: is done through a cyclone that confers the particles separation by size.

Based on inertial forces, the particles flow to the cyclone wall and are separated from the gas

as a downwards strain. The smaller particles are sent to an air filter while the particles with a

larger diameter are collected in the vessel under the cyclone.

28

3.2.2. Spray – Dryer Parameters

The final product characteristics (particle size, final humidity, yield and outlet temperature of the

drying gas) depend on several parameters that are set in the beginning of the atomization. There are

several parameters that influence the final product characteristics, however only the following

parameters will be described, as they are the only parameters that were modified.

1) Inlet temperature: The heated vector gas is sucked by the aspirator and it is heated up before

it enters in the spray chamber. This measurement takes place at the inlet of the drying chamber.

The heating is done though a resistance, and it can heat up until 220°C.

2) Aspirator rate: the aspiration motor sucks the drying gas creating vacuum. A higher or lower

drying gas flow rate will have a great impact on the drying performance. Also, a higher

aspiration rate provides a better separation in the cyclone. However, it will lead to a larger

moisture amount, due to a shorter residence time in the drying chamber.

3) Feed pump rate: a higher feed flow rate means a higher energy to evaporate the droplets into

solid particles, resulting in a lower outlet temperature. Thus, the feed pump rate will influence

the inlet and outlet temperature difference.

Moreover, the parameters optimization is done by using the trial and error procedure. In Figure

13 is given a summary over the spray dryer parameters and their influence in the final product

characteristics.

Figure 13 – Overview over the spray-dryer parameters and their influence in the final product. [28]

29

In Table 6 is possible to find the used parameters in the synthesis of all samples.

Table 6 – Spray – drier parameters for all the samples.

3.3. Characterization Methods

The characterization methods that were performed and will be described in the following

paragraphs are: N2 adsorption – desorption, Temperature Programed Reduction (TPR), X-Ray

Diffraction (XRD), low angles XRD, X-Ray Photoelectron Spectroscopy (XPS), Transmission Electron

Microscopy (TEM) and Scanning Electron Microscopy (SEM). In Table 7 it is described the properties

that are expected to get from the analysis and characterize each samples.

Table 7 – Relation between analysis and the expected characterized properties.

Reference Nozzle Type

Inlet temperature

(°C)

Aspiration rate (m3/h)

Feed pump rate (mL/min)

Ultrasonic nozzle power

(kW)

JFR003 Two – fluid

nozzle

220

35 9.0 -

JFR005 –

JFR009 Ultrasonic

nozzle 17,5

1.8

2.0 JFR010

2.1 JFR014 –

JFR037

Analysis Characterized properties

N2 adsorption -

desorption Textural properties (BET area, porous volume, pore diameter)

TPR Cobalt reducibility and accessibility

XRD Cobalt crystalline phases and crystal’s size

Low angles XRD Organization of the porosity of the support (mesostructuration)

XPS Degree of oxidation of various elements

TEM

Information of the support: organization of the porosity of the support

(mesostructuration).

Information on the metallic phase: form in which the cobalt is, size of the

cobalt particles

SEM Information of the support: average size and morphology of the aerosol

elementary particles

30

3.3.1. N2 adsorption-desorption

This method is widely used for studying textural properties of catalysts. Adsorption equilibrium is

represented by isothermal plots, which describe the adsorbed quantity as function of the equilibrium

pressure P of the gas in contact with the solid. Actually, relative pressure P/P0 is used instead of P,

where P0 is the saturated vapor pressure of the adsorbate at the measurement temperature (≈ 77 K in

case of nitrogen). Therefore, an adsorption – desorption isotherm consists in measuring the quantity of

gas that is adsorbed on (or desorbed from) the surface of the solid, at a given temperature. [29]

Nitrogen is the most common molecule used in this technique. However, other molecules can be

used for specific purposes: for instance, argon, a very small monoatomic gas, is more appropriated for

the study of microporous samples, or krypton, where the low saturated vapor pressure (≈ 2 torr) can be

used to measure small adsorbed quantities precisely (for specific surface areas below 1 m2/g). [29]

Before measuring the adsorbed quantity, a degassing (or pre-treatment) stage is carried out in order

to eliminate the compounds adsorbed on the surface of the sample (H2O, CO2, etc.). The isotherms are

obtained by gradually increasing the pressure, where the small pores are filled first. The gas condenses

in successively larger pores until a saturated vapor pressure level is reached at which the entire porous

volume is saturated with liquid. Furthermore, the adsorption – desorption phenomena is highly suitable

for the study of samples where the pore size is in the mesoporous domain. [29]

The desorption isotherm is not often superimposed over the adsorption isotherm, which shows up

as a hysteresis phenomenon. This fact happens due to capillary condensation, which corresponds to a

phase transition effect caused by the interactions with the surface of the solid where the gas phase

abruptly condenses in the pore, accompanied by the formation of a meniscus at the liquid – gas

interface.

In Figure 14 it is described a typical isotherm of a mesostructured solid with a very good level of

organization. Moreover, the pore size distribution of a mesostructured solid is very narrow, as the pore

diameter is very uniform.

Figure 14 – Typical isotherm for a mesostructured solid. [29]

The main parameters obtained through this technique are the specific surface area, pore size

distribution, specific porous volume and information on the structure (pore shape, interconnection, etc.).

These information can be given by several models, where the BET model is used to determine the

specific surface area, the BJH method is used in the pore size distribution, while the t-plot method

consists in comparing the adsorption isotherm of a given solid in terms of the adsorbed thickness, what

makes possible to characterize the micro- and mesoporosity.

31

In this study, these analyses were performed in a Micromeretics ASAP 2420 equipment.

3.3.2. Temperature Programmed Reduction (TPR)

This method is used to evaluate the catalyst reducibility and the oxidation degree of the active

phase. The hydrogen consumption is followed as a function of temperature. TPR is extremely attractive

due to its high sensitivity to chemical changes induced by a catalyst promoter or by the support. [29]

This technique consists in measuring the consumption of hydrogen while heating a catalyst with a

linear temperature rate under continuous gas flow. TPR profiles do not provide direct information about

the modification of the catalyst structure, because hydrogen consumption could be attributed to different

reduction processes. In this study, the TPR analysis were performed in a Mircrometrics Auto Chem II

2920. The catalyst, during this analysis, was under a mixture of 5% H2 in air, at 58 cm3/min, and the

temperature was raised up to 1000°C, at a rate of 5°C/min. The hydrogen consumption is followed

through a thermal conductivity detector (TCD), and, it can be seen that, during the reduction process,

several products such as H2O, CO2 or CO are formed.

3.3.3. X-Ray Diffraction (XRD)

XRD is often used for identification of cobalt crystalline phases and evaluation of the crystal size

using the Debye - Scherrer equation (Eq. 15), where, β is the angular breadth of a diffraction line, C is

a constant, λ is the X-ray wavelength, L is volume – averaged size of crystallites, and finally, θ is the

Bragg angle.

The peak breadth, β, can be either calculated through the full-width half-maximum (FWHM) or by

the “integral width”, which corresponds to the area under diffraction peak divided by peak maximum.

Therefore, is often some uncertainty in measuring the size of cobalt crystallites with the Debye - Scherrer

equation, because the β definition will determine the value of the Bragg constant (C).

For very small and very large crystallites, it has been noticed a low accuracy in measuring the

crystallite sizes. The XRD technique is not very sensitive to the presence of small crystallites of cobalt

oxides (< 2 – 3 nm), since the peaks get too broad to be identified and measured. Broadening of the

XRD lines is caused by structural imperfections of the sample. [5]

The values measured during an analysis are the relative intensity levels of the diffraction lines,

corresponding to beam intensities. Moreover, the measurement of the scattering angle is done in lattice

planes which are identified using indices in three dimensions: h, k and l. These last three are related to

the directions of axes defining the crystalline system. [29]

Therefore, in order to follow the Bragg condition (Eq. 16), there is no possibility of scattering planes

with a spacing less than half the wavelength. Where n is an integer, λ is the wavelength, d is the plane

spacing, and finally, θ is the Bragg angle.

𝛽 =𝐶𝜆

𝐿 cos 𝜃

(Eq. 15)

𝑛𝜆 = 2𝑑 sin 𝜃 (Eq. 16)

32

These analyses, ranging from 5 to 72° (2 𝜃), were performed in a PANalytical X’Pert Pro equipment

with the reflection configuration.

3.3.4. Low angles X-Ray Diffraction

Low angles X-Ray diffraction analysis is often used to obtain parameters such as size, morphology

and distribution of particles. Normally, X-ray diffraction (XRD) covers angles in the range of 10° to 100°,

corresponding to a typical wavelength of 0.1 nm, to interatomic distances. Low angles XRD concerns

angles under 5°, thus, distances are greater than a nanometer. [29]

Therefore, low angles XRD processes at a higher resolution and scattering strength while

producing the scattering peaks, making it a suitable method for characterize mesostructured materials.

These analysis were performed in a PANalytical X’Pert Pro equipment in the transmission configuration,

and the analysis range was from 0.2 to 10° (2 𝜃). Moreover, the analysis were done without beam-stop,

in order to prevent the detector saturation. Also, it was used a Cu0.2 attenuator.

3.3.5. X-ray Photoelectron Spectroscopy (XPS)

XPS allows the characterization of the external surface layers of the catalyst (5 to 10 nm) [35] and

the degree of oxidation or electronic state of various elements (chemical neighborhood). The sample

that will be characterized is bombarded by an X-ray photon beam, thus, electrons of different elements

are emitted in terms of number and energy by an appropriate detector. The measured kinetic energy is

directly connected to the electron biding energy by the photoelectric effect. [29]

The characterization in this study was carried out in an ESCA KRATOS Axis Ultra spectrometer

with a monochromatic source of Al. The obtained spectrum are compared with references in order to

identify the signals.

3.3.6. Transmission Electronic Microscopy (TEM)

TEM provides detailed information concerning the composition and structure of heterogeneous

catalysts with real-space resolution down to the atomic level.

Once a sample is crossed by an electron beam, this may be partially adsorbed or deflected. A

certain fraction of these electrons, and those that have not been deflected, are combined to form an

image. Therefore, the use of transmission electron microscopy is based in controlling the electrons

involved in image formation. Thus, the image of the sample depends on the electron – matter

interactions. One can note that atoms with high atomic number scatter more electrons and at larger

angles than those with lower atomic number. Therefore, a particle of heavy metal on a light oxide

support, for instance silica or alumina, will appear dark. [29]

This characterization was carried out in a JEM 2100F equipment. To improve the contrast between

the support and the metallic phase, it is easiest to analyze it in a dark – field image, by selecting the

plan (220) for metallic cubic cobalt and the plan (440) for cobalt oxide (Co3O4).

3.3.7. Scanning Electronic Microscopy (SEM)

This characterization technique allows, along with TEM, to a local chemical and textural

characterization of catalysts. The principle of operation is very close to the one used in TEM. Therefore,

the image is obtained through the interaction of material’s and the electron beam, also known as electron

33

probe, with energies between 0.5 and 35 kV. However, when using SEM microscopes, the image

resolution is limited by the size of the electron probes. [29]

SEM provides information about the size, shape and 3D arrangement of the catalyst particles.

Nevertheless, SEM is not able to fully identify the intrinsic structure of the catalyst, thus, TEM should be

performed to fully characterize the pore structure.

In the present study, this type of analyses was performed with a Supra 40 equipment, with no pre-

treatment of the sample.

35

4. Results and Discussion

Generic remarks

As the final application of these catalysts is the FT synthesis, it is more suitable the use of an

ultrasonic nozzle than a two fluid nozzle (normally used at the laboratory scale). Indeed, it produces

larger elementary particles. Therefore, the produced elementary particles would be closer to the ones

of the industrial catalysts that have elementary particles ranging from 80 to 100 µm.

In the following paragraphs, first are described the silica results and after alumina results. One

should take into account, that the main priority of this study is not the synthesis of a mesostructured

solid, but the synthesis of a mesoporous solid, since the FT process does not require mesostructured

catalysts.

On one hand, alumina matrix is the most used support in the FT synthesis. On the other hand,

there is more information in the literature concerning silica sol – gel chemistry and also IFPEN has a

vast experience in synthetizing silica based materials through the aerosol process. Therefore, a more

detailed study was performed on silica matrixes.

Supports were firstly tried to produce instead of trying to produce directly a catalyst, in order to

know if it was possible to synthesize it and also to characterize the elementary particles obtained through

the ultrasonic nozzle, which had never been used at IFPEN.

All the synthesized supports and catalysts were characterized through N2 adsorption – desorption

isotherm and the latter was also characterize through TPR. Besides these analysis, other

characterization techniques were applied when needed.

36

4.1. Silica

The following paragraphs tackle the synthesis of supports and the introduction of cobalt onto a

silica matrix, using as inorganic precursor TEOS.

4.1.1. Supports

Comparison with the two-fluid nozzle, generic characteristics and trials reproducibility

First of all, a trial was done with the two-fluid nozzle (JFR003) as it was the usual nozzle used in

the laboratory at IFPEN, as previously mentioned. This trial allowed a comparison between the two –

fluid and the ultrasonic nozzle, making it possible to evaluate the major differences between these two

equipments.

Concerning trials with the ultrasonic nozzle, two trials (JFR005 and JFR006) with the same spray

– drier parameters were performed in order to evaluate the trials reproducibility. Moreover, two samples

(JFR006 and JFR007) were collected along one trial, in order to evaluate if there were any differences

in the samples collected in the begging and the end of one trial.

Textural properties

Concerning the supports textural properties (BET surface and pore size distribution) nitrogen

adsorption – desorption isotherms were performed, which are described in Figure 15.

Figure 15 – Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR003, JFR005, JFR006 and JFR007.

In Table 8 it is described the surface area, the porous volume and the pore diameter for all the

previous samples.

0

50

100

150

200

250

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d

Vo

lum

e (

mL

/g)

Relative Pressure

JFR003

JFR005

JFR006

JFR007

0

0,02

0,04

0,06

0,08

0,1

0,12

0 10 20

dV

/dD

vo

lum

e (

mL

/g)

Average pore diameter (nm)

37

Table 8 - BET surface, porous volume and pore diameter for JFR003, JFR005, JFR006 and JFR007.

Comparing the isotherms of JFR003 and JFR005, one can see that the solid obtained with the two

– fluid nozzle presents a hysteresis loop with more vertical lines than the solids obtained with the

ultrasonic nozzle. Thus, JFR003 seems more organized than JFR005. However, low angles XRD and

TEM analysis should be performed to fully verify this statement.

Furthermore, the isotherms shape of JFR005 and JFR006 are very similar, hence, it is possible to

conclude that the trials are reproducible. Likely, the isotherms shape of JFR006 and JFR007, are very

similar too, so it is possible to conclude that there is no change along time in solids belonging to the

same trial.

Higher surface areas are achieved with the two – fluid nozzle which is due to an increase in the

mesoporous volume.

Silica particles size distribution

In order to characterize the elementary particles morphology produced through the ultrasonic

nozzle, SEM analysis were performed in JFR005, JFR006 and JFR007.

As is possible to conclude through Figure 16 the synthesized silica particles are spherical, and as

one can see, the particles present the same shape throughout the trials, showing once more the trials

reproducibility (Figure 16 (A) and (B)). Furthermore, looking at the SEM micrographs (B) and (C) one

can conclude once more that there are no significant changes along the same trial.

Reference Porous type BET surface (m2) Porous volume (mL/g) Pore diameter (nm)

JFR003

Mesoporous

183 0.37 7.8

JFR005 114 0.20 7.8

JFR006 101 0.18 7.9

JFR007 107 0.18 7.9

(A) (B)

(C)

38

Figure 16 – SEM micrographs for (A) JFR005, (B) JFR006 and (C) JFR007.

Moreover, it was not possible to precise if the elementary particles are hollow as there are no

damage elementary particles.

For each sample it was done a particle’s size distribution, which are shown in Figure 17.

Figure 17 – Particles size distribution for JFR005, JFR006 and JFR007.

0

0,05

0,1

0,15

0,2

0 10 20 30 40 50

Fre

qu

en

cy (

vo

lum

e)

Elementary particles diameter (µm)

JFR005

0

0,05

0,1

0,15

0,2

0,25

0 10 20 30 40 50 60

Fre

qu

en

cy (

vo

lum

e)


JFR006

0

0,05

0,1

0,15

0,2

0,25

0 10 20 30 40 50 60

Fre

qu

en

cy (

vo

lum

e)


JFR007

(A) (B)

(C)

39

Regarding these histograms, it is possible to find the same results as before concerning the trials

reproducibility.

Table 9 - Results of the elementary particle size determinate by SEM.

The particle size distribution presents for the three supports a wide range of particles size, as it can

be seen in the histograms, since the particles size range from 2 to 60 µm. However, the maximum of

population is found for 30 and 35 µm.

As previously mentioned, the ultrasonic nozzle was used instead of the two – fluid nozzle due to

the final application of these catalysts in the FT process. It was expected to have the majority of

elementary particles ranging from 80 – 100 µm, and as one can see the medium elementary particle

diameter is around 30 µm, which value is acceptable for testing.

Organization of the porosity of the support

In order to confirm the existence of a mesostructured solid, low angles XRD was performed which

is shown in Figure 18. The presence of a peak at 0.75° (2θ) for the three samples confirms the existence

of an organized structure.

One can see that there are not three peaks as it is characteristic in mesostructured solids with a

very good level of organization. There are two hypotheses that can justify this absence: it can be

characteristic of samples where parts of them are very well organized and other parts do not present

organization at all. It can also be characteristic of samples where the entire sample is organized but not

with a good level of organization. Moreover, one can suppose that the three samples have a worm-like

structure (second option) and it will be confirmed thanks to a TEM analysis.

Moreover, the above assumption seems to be true, as the TEM analysis of a catalyst (later

described) reveals an organization throughout the sample, however with a low level of organization,

which is characteristic of worm – like structures.

Reference

Elementary particles medium

diameter (µm)

Elementary particles minimum

diameter (µm)

Elementary particles maximum

diameter (µm)

Standard deviation (µm)

JFR005 34.18 2.15 52.68 14.28

JFR006 34.51 2.34 59.38 17.40

JFR007 29.99 2.83 56.55 16.51

40

Figure 18 – Low angles XRD patterns for JFR005, JFR006 and JFR007.

4.1.2. Cobalt catalysts

It was synthetized a silica matrix containing 15 wt.% of cobalt concerning the total mass of solid

(JFR016). This solid was obtained by atomizing a solution which contained HCl and a pH, approximately,

of 2. This synthesis consisted mainly in introducing the cobalt precursor solution into the solution that

was used to produce the support. This solid was tested in a FT catalytic unit, as it will be later described.

The following paragraphs describe a preliminary study on the calcination temperature in order to

determine the best conditions to favor the formation of active catalytic species.

One should note, that at this point it is not possible to know if the solids with cobalt loaded are

catalysts or not, as they were not catalytic tested. However, as a simplification they will be referred as

“catalysts”.

Calcination temperature influence

JFR014 and JFR016 were calcinated at different temperatures, 550°C and 350°C, respectively.

The first one experimented the same calcination temperature as the support, and JFR016 was

calcinated with a lower temperature in order to prevent the formation of cobalt silicates, which are hardly

reducible species. [5]

Textural properties

Hereby are presented several analysis results to evaluate the influence of the calcination

temperature on the textural properties of the support.

JFR005 (silica matrix) is only represented in Figure 19 so it can be seen the differences in

introducing cobalt on the textural properties.

Blanc - File: C15N2584.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 - Di

140187*0.33 - File: C15K2589.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3:

140090 - File: C15K2588.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 -



Lin

(C

ou

nts

)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

25000

2-Theta - Scale

0.22 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

0.75° (118Å)

JFR005

JFR007

JFR006

41

Figure 19 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR005 (support reference), JFR014 and JFR016.

In Table 10 is summarized the surface area (BET surface), the porous volume and the pore

diameter.

Table 10 - BET surface, porous volume and pore diameter for JFR005, JFR014 and JFR016.

The support isotherm shape is very close to the one of the catalysts. Comparing the t-plot of the

catalyst (JFR014) and the support (JFR005), which have the same calcination temperature, a higher

microporous volume is achieved for the catalyst (0.054 cm3/g), while the support (JFR005) presents a

microporous volume of 0.015 cm3/g. In terms of microporous surface area, JFR014 has 131 m2/g and

JFR005 has 42 m2/g. Therefore, the introduction of cobalt nanoparticles induces a higher microporosity,

and also a higher mesoporosity, as there was an increase in the porous volume along with a decrease

in the pore diameter. (Find the t-plot in appendix A)

Moreover, the isotherms shape of both catalysts is very similar. It was confirmed by the t-plot that

a higher microporosity is found in JFR014, which was calcinated at 550 °C. This is expected because

0

20

40

60

80

100

120

140

160

180

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d

Vo

lum

e (

mL

/g)

Relative Pressure

JFR005 (Support)

JFR014 (Catalyst, calcination at 550°C)

JFR016 (Catalyst, calcination at 350°C)


JFR005

Mesoporous

114 0.20 7.8

JFR014 226 0.26 6.6

JFR016 164 0.26 6.5

0

0,01

0,02

0,03

0,04

0 5 10 15

dV

/dD

vo

lum

e (

mL

/g)


42

with a higher calcination temperature small voids in the microporosity domain are created in the

surfactant hydrophilic part. Figure 20 illustrates this phenomenon.

Figure 20 - Scheme representing the solid surfaces calcinated at 350°C and 550°C. Adapted from [20]

Both catalysts present a very similar porous size distribution. For JFR014 the maximum population

is found at 6.6 nm while for JFR016 is found at 6.5 nm, therefore both solid have mesopores.

The support (JFR005) presents higher pore diameters than the catalysts (JFR014 and JFR016).

This smaller pore diameter could be due to a different micelle – inorganic precursor interaction.

Organization of the porosity of the silica matrix

To attest the existence of a mesostructured solid, low angles XRD was performed which is shown

in Figure 21. For JFR014 and JFR016 it was not found a long distance organization because no peak

at small angles was observed.

It is also possible to conclude that the introduction of cobalt onto the support has made solids with

no long distance organization, due to the impact of cobalt species on the chemical reactions inducing

the mesostructuration process.

Besides that, the calcination temperature does not have influence on the mesostructuration.

Vµ pore: 0.054 cm3/g

Mesoporous volume: 0.11 cm3/g

Vµ pore: 0.015 cm3/g

Mesoporous volume: 0.12 cm3/g

43

Figure 21 – Low angles XRD patterns for JFR014 and JFR016.

Finally, the calcination temperature was set up as 400°C, an intermediate value that accomplishes

a higher surfactant removal and trying to avoid the formation of cobalt silicates. Also, there is no interest

in creating micropores for the final application of these catalysts, because the long produced

hydrocarbons chains would have difficult leaving the catalyst.

Catalytic test

Moreover, before having all the analysis results, a part of JFR016 (before being dried or calcinated)

was reduced under hydrogen flow, and tested in a FT catalytic test unit (slurry reactor). After 16 hours,

there was no activity, in another words, there were no products formation. As there were no other

catalytic tests in this work, no setup or reaction condition are presented here.

Therefore, two main hypotheses were proposed for the catalyst lack of activity: first, the presence

of chlorine could affect the catalyst activity, and second, it could exist a strong interaction between the

silica precursor and the cobalt precursor, consequently affecting the cobalt dispersion. In the fowling

paragraphs is described in detail each hypothesis.

1) Presence of chlorine in the catalyst

The presence of chlorine in FT catalysts has showed a significant decrease in activity. This lack of

activity could be due to the poising of several surface sites by Cl atoms. [30] This could be related with

the chlorine strong electronegativity, which prevents the CO dissociation on the catalyst. Moreover, it

was found that the effect of chlorine atoms on the catalyst was slowly reversible.

1 4 2 59 4 i*0.7 - F ile : C 1 5 K 2 58 3 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 20 0 . s - T em p. : 2 5 °C (R o om ) - T im e S ta rte d : 0 s - 2 -Th e ta : 0.21 2 ° - Th e ta : 0.10 5 ° - A u x1: 0 .0 - A ux2 : 0.0 - A u x3 :

1 4 2 38 0 - F i le : C 15 K 2 5 8 0. raw - Typ e: P S D fa s t-sca n - S ta rt: 0. 21 2 ° - E n d: 4 .9 9 2 ° - S te p: 0 .0 1 7 ° - S tep t im e: 2 0 0. s - Te m p .: 2 5 °C (Ro o m ) - T im e S ta rte d : 0 s - 2-T he ta: 0 .2 1 2 ° - T he ta: 0 .1 0 5 ° - A u x1 : 0 .0 - A u x2: 0 .0 - A u x3 : 0 .0 -


1)



B la n c - F i le : C1 5 N 25 8 4 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 2 0 0 . s - T em p . : 2 5 °C (R o om ) - T im e S ta rte d: 0 s - 2 -Th e ta : 0 .21 2 ° - Th e ta : 0 .10 5 ° - A u x1 : 0 .0 - A ux2 : 0.0 - A u x3 : 0 .0 - D i

1 4 2 1 39 - L ef t A n g le: 0 .5 8 0 - R ig h t A n gle : 1 .1 9 8 - L e ft In t. : 5 5 9 8.1 00 C o u nts - Rig h t I nt .: 12 1 1 .4 2 9 Co u n ts - P e aks : 0 - P a ram s : 0 - W e ig ht : -1 .0 0 0 - kA 2 Ra tio : 0.5 - R e l. : 1 .8 0 8 % - T h .R .: 1 .5 46 % - R .In t. : 0 . 00 0 %

Lin

(C

ou

nts

)

0

1 000

2 000

3 000

4 000

5 000

6 000

7 000

8 000

9 000

1 000 0

1 100 0

1 200 0

1 300 0

1 400 0

1 500 0

1 600 0

1 700 0

1 800 0

1 900 0

2 000 0

2 100 0

2 200 0

2-Theta - Scale

0.24 0.3 0.4 0.5 0.6 0.7 0.8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4

0.90° (d=98Å)

0.80° (d=110Å)

JFR014 JFR016

44

In order to prevent this effect HCl was replaced for HNO3 in the initial solution. The following

paragraphs describe the effect of HNO3, either on the supports and catalysts.

Textural properties

In Figure 22, one can see the nitrogen adsorption – desorption isotherms for the supports (JFR005

(HCl) and JFR028 (HNO3)) and for the catalysts (JFR016 (HCl) and JFR021 (HNO3)). One can note that

JFR005 and JFR016 are just represented as references, since they were synthetized with HCl.

Moreover, JFR016 was calcinated at 350°C and JFR021 at 400°C. A different calcination

temperature induces changes in the textural properties, but it was made the choice to compare them.

Figure 22 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR005 (reference), JFR016 (reference), JFR028 and JFR021.

Concerning the isotherms shape (supports and catalysts isotherms), one can conclude that, there

are no significant changes, except that JFR021 presents a hysteresis loop with more straight lines,

leading to a more organized structure, which is further confirmed.

In Table 11 it is summarized the surface area (BET surface), the porous volume and the pore

diameter for all the previous samples.

0

50

100

150

200

250

300

350

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d

Vo

lum

e (

mL

/g)

Relative Pressure

JFR005 (Support, HCl)

JFR016 (Catalyst, HCl)

JFR028 (Support, HNO3)

JFR021 (Catalyst, HNO3)

0

0,02

0,04

0,06

0,08

0,1

0 5 10

dV

/dD

vo

lum

e (

mL

/g)


45


Both supports either with HCl (JFR005) or HNO3 (JFR028) present a different porous size

distribution.

Concerning the supports, one can conclude that the introduction of HNO3, increases (variation of

13%) the surface area along with an increase in the porous volume and in the pore diameter, possibly

due to a different micelle – inorganic precursor interaction.

Regarding the catalysts, using HNO3 instead of HCl increases significantly (variation of 103%) the

surface area as well as the porous volume, and reduces the pore diameter. This change in the pore

diameter could be due to the presence of cobalt species on the chemical reactions inducing the

mesostructuration process.


Concerning the support (JFR005) and catalyst (JFR016) synthetized with HCl, the organization of

the porosity of the support has changed with the introduction of cobalt, since JFR005 was an organized

structure and JFR016 did not present any organization.

Regarding the samples that were synthetized with HNO3, the organization of the porosity of JFR028

(support) should be confirmed thanks to low angles XRD analysis, and the organization of JFR021

(catalyst) was confirmed thanks to the same analysis (Page 49, Figure 25).

Therefore, one can conclude that the presence of HCl or HNO3 has influence in the

mesostructuration of the catalysts, probably due to a different interaction of the cobalt species on the

chemical reactions inducing the mesostructuration process.

Reference

Porous type

BET surface (m2)

Porous volume (mL/g)

Pore diameter

(nm)

Supports JFR005 (HCl)

Mesoporous

114 0.20 7.8

JFR028 (HNO3) 129 0.24 8.4

Catalysts JFR016 (HCl) 164 0.26 6.5

JFR021 (HNO3) 345 0.46 7.1

46

Cobalt reducibility

The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 23, it is the

TPR profile for both samples (JFR016 and JFR021) after calcination. After calcination both samples

were violet (which normally corresponds to the Co nitrate precursor coloration).

As previously mentioned, in chapter 2, the cobalt oxide reduction to metallic cobalt happens in two

reactions. The first one (Eq.10) corresponds to the reduction of Co3O4→CoO, which normally happens

between 100 - 350°C and the second reaction (Eq.11) corresponds to the reduction of CoO→Co°, which

normally happens between 400 - 600°C.

As one can see in Figure 23, for JFR016 there is a major peak at 796°C and for JFR021 there are

two main peaks, the first at 842°C and the second at 899°C.

Both samples present peaks at high temperatures which does not correspond to the reduction of

CoO→Co°, since there is no peak corresponding to the reduction of Co3O4→CoO (between 100-350°C)

and as it is at a much higher temperature than usual (400-600°C). Actually, these peaks may suggest

the existence of cobalt silicates which are due to a strong interaction between the support and cobalt

oxide. These species are hardly reducible, thus, it can explain the existence of peaks at elevated

temperatures.

At this point, we can conclude that the hypothesis aforementioned of strong Co-O-Si interactions

may be responsible for the lack of catalytic activity due to difficulties in the Co reduction.

Furthermore, using HCl (JFR016) or HNO3 (JFR021) does not have influence in the cobalt

reducibility. However, as previously stated, the presence of Cl atoms can affect negatively the activity

of the catalysts in the FT synthesis. Thus, from now on, all the catalysts were synthesized with HNO3 in

the initial solution.

Figure 23 – TPR profile for JFR016 and JFR021.

0

0,2

0,4

0,6

0,8

1

1,2

0 200 400 600 800 1000

Hyd

rog

en

co

ns

um

pti

on

(mL

/g c

ata

lys

t)

Temperature (°C)

JFR016 (HCl)

JFR021 (HNO3)

47

2) Strong interaction between the silica precursor and the cobalt precursor

As it can be seen in Figure 23, hardly reducible cobalt species were being formed, which supports

the hypothesis of a strong interaction between silica precursor and the cobalt precursor. These oxides

(cobalt silicates) are often amorphous, which makes it harder to characterize those using conventional

techniques, such as XRD. A low cobalt content and a high surface area favor the formation of hardly

reducible oxides. Mixed cobalt – silicium or aluminum oxides can be formed during the catalysts

preparation, oxidative and reductive pretreatments, and in the course of FT reaction. [5]

Therefore, in this case if there is a strong interaction between cobalt and TEOS in the solution,

cobalt in the final powder could be ultra – dispersed which leads to cobalt silicate and/or very small

cobalt particles that are very hard to reduce, as seen in chapter 2.

In order to change the interaction between silica precursor (TEOS) and the cobalt precursor (cobalt

nitrate), were made three solutions at different pH. The formed oligomers are mainly linear and easily

condensable in an acid media (0<pH<2) and ramified in a less acid media (pH>2). The silica PZC is

approximately at pH=2, hence a solution was made approximately at a pH=PZC (as in the usual

procedure), and the other two at a pH above and below the PZC.

Consequently, changing the pH of the solution would affect the size and the dispersion of the cobalt

particles. Atomizing a solution with a pH below the PZC provides a silica surface positively charged,

hence repulsion will occur between Co2+ ions and the surface leading to a “non – homogenous

distribution” (creating perhaps cobalt domains and hence larger particles of cobalt species). On the other

hand, atomizing a solution with a pH above the PZC provides a silica surface negatively charged, and

the Co2+ ions will be “homogenous distributed”.

Consequently, the following samples were prepared: pH=1 (JFR019), pH=2 (JFR021) and pH=3

(JFR022).

Textural properties

Hereby is described the pH influence on the textural properties. In Figure 24 it is described the

nitrogen adsorption – desorption isotherms for the three samples (JFR019, JFR021 and JFR022), and

JFR016 is only represented as a reference (this solid was synthesized through an atomization solution

which contained HCl and present a pH approximately of 2).

48

Figure 24 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR016 (reference), JFR019, JFR021 and JFR022.

In Table 12 it is summarized the surface area (BET surface), porous volume and pore diameter for

the previous samples.


Concerning the isotherms shape, one can see that JFR019 (pH = 1) is slightly different from the

others. JFR019 presents a hysteresis loop with more vertical lines, indicating the existence of a more

organized solid, which is confirmed by TEM analysis, further presented. The shape of JFR021 (pH = 2)

presents a hysteresis loop with more straight lines than JFR022 (pH = 3), but less pronounced than

JFR019.

All the three samples (JFR019, JFR021 and JFR022) present different porous size distributions.

This is due to different interactions between the surfactant and the inorganic species induced by the

different pH.

0

50

100

150

200

250

300

350

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d V

olu

me

(m

L/g

)

Relative Pressure

JFR016

JFR019 (pH = 1)

JFR021 (pH = 2)

JFR022 (pH = 3)

Reference Porous type BET surface

(m2) Porous volume

(mL/g) Pore diameter

(nm)

JFR016

Mesoporous

164 0.26 6.5

JFR019 (pH = 1) 338 0.38 5.2

JFR021 (pH = 2) 345 0.46 7.1

JFR022 (pH = 3) 258 0.35 6.3

0

0,03

0,06

0,09

0,12

0 5 10 15

dV

/dD

vo

lum

e (

mL

/g)


49

JFR019 and JFR021 present higher surfaces areas than JFR022 due to a higher mesoporosity, as

JFR021 presents a higher porous volume, and, JFR022 presents a slightly higher porous volume, but a

much smaller pore diameter.

Therefore, is possible to conclude that regarding the textural properties, atomizing solutions with

different pH leads to solids with different properties.


In order to study the influence of the pH on the mesoporosity, low angles XRD analysis was

performed.

As one can see in Figure 25, for the sample JFR022 (pH = 3) there is no long distance organization.

While for JFR019 (pH = 1) and for JFR021 (pH = 2), it was confirmed the existence of a mesostructured

organization.

This can be explained, since a higher condensation rate is expected at pH = 1 and pH = 3. However,

JFR019 (pH = 1) is more organized than JFR022 (pH = 3) due to a better interaction between the

surfactant and the inorganic molecular precursor, as for each pH, the silica surface is differently charged.

The mesoporosity was also confirmed by TEM analyses, which are described later on.

Furthermore, one can conclude that the pH of the atomized solution has influenced in the

mesostructuration process, as it was expected.

Figure 25 - Low angles XRD patterns for JFR019, JFR021 and JFR022.

1 4 2 59 4 i*0.7 - F ile : C 1 5 K 2 58 3 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 20 0 . s - T em p. : 2 5 °C (R o om ) - T im e S ta rte d : 0 s - 2 -Th e ta : 0.21 2 ° - Th e ta : 0.10 5 ° - A u x1: 0 .0 - A ux2 : 0.0 - A u x3 :



1)



B la n c - F i le : C1 5 N 25 8 4 .ra w - T yp e : P S D fa s t-s can - S ta rt: 0 .2 1 2 ° - E n d : 4.99 2 ° - S te p : 0 .01 7 ° - S te p tim e : 2 0 0 . s - T em p . : 2 5 °C (R o om ) - T im e S ta rte d: 0 s - 2 -Th e ta : 0 .21 2 ° - Th e ta : 0 .10 5 ° - A u x1 : 0 .0 - A ux2 : 0.0 - A u x3 : 0 .0 - D i

1 4 2 1 39 - L ef t A n g le: 0 .5 8 0 - R ig h t A n gle : 1 .1 9 8 - L e ft In t. : 5 5 9 8.1 00 C o u nts - Rig h t I nt .: 12 1 1 .4 2 9 Co u n ts - P e aks : 0 - P a ram s : 0 - W e ig ht : -1 .0 0 0 - kA 2 Ra tio : 0.5 - R e l. : 1 .8 0 8 % - T h .R .: 1 .5 46 % - R .In t. : 0 . 00 0 %

Lin

(C

ou

nts

)

0

1 000

2 000

3 000

4 000

5 000

6 000

7 000

8 000

9 000

1 000 0

1 100 0

1 200 0

1 300 0

1 400 0

1 500 0

1 600 0

1 700 0

1 800 0

1 900 0

2 000 0

2 100 0

2 200 0

2-Theta - Scale

0.24 0.3 0.4 0.5 0.6 0.7 0.8 0 .9 1 .0 1 .1 1 .2 1 .3 1 .4 1 .5 1 .6 1.7 1.8 1.9 2.0 2.1 2.2 2.3 2.4

0.90° (d=98Å)

0.80° (d=110Å)

JFR019

JFR021

JFR022

50

Cobalt reducibility

The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 26, it is the

TPR profile for JFR019, JFR021 and JFR022 after calcination.

It is important to refer that JFR019 after calcination was black, which is the characteristic color of

cobalt oxide (Co3O4), whereas JFR021 and JFR022 were violet after the same post – treatment.

Figure 26 - TPR profile for JFR019, JFR021 and JFR022.

As one can see in Figure 26, for JFR019 there is a first peak at 375°C and a second peak at 816°C.

As the first peak is at a higher temperature than usual, it could correspond to the reduction of

Co3O4→CoO and/or to the decomposition of residual NOx groups (exothermic reaction). These NOX

groups are due to the cobalt precursor solution (cobalt nitrate) and the HNO3 used to acidify the solution,

because JFR019 is the more acidic solution so the more NO3- concentrated. [5] If one decomposes the

second peak, a part of it might correspond to the CoO→Co° reduction. And the majority of this peak, as

it happens at an elevated temperature may suggest, once more, the existence of cobalt silicates which

are due to a strong interaction between the support and cobalt oxide.

Regarding JFR021 it shows two peaks at 842°C and 899°C, and, JFR022 presents one peak at

825°C. These peaks may only suggest again the existence of cobalt silicates due to a strong interaction

between silica and cobalt oxide.

A XRD analysis was performed in order to characterize the crystalline cobalt oxide phases present

in the three catalysts.

Oxide phases

In order to know in which phase is the cobalt XRD analysis were performed. The results are

described in the following XRD diagrams. In Figure 27 A) the green line is refered to another sample

not mentioned in this part.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

0 200 400 600 800 1000

Hyd

rog

en

co

ns

um

pti

on

(mL

/g c

ata

lys

t)

Temperature (°C)

JFR019 (pH = 1)

JFR021 (pH = 2)

JFR022 (pH = 3)

51

Figure 27 - XRD diagrams: A) JFR019 and B) JFR021 and JFR022.

As one can see in Figure 27, for JFR019 it was found a small peak of cobalt oxide (Co3O4) which

is in agreement with the TPR analysis. These particles were measured at 36.8° (2θ) and they have a

size of 22 nm.

Moreover, for all the three samples it was found a peak of metallic cobalt and this was not expected

since the samples were only calcinated, and not reduced.

It was expected after the conclusions taken by the TPR, which revealed the possible existence of

cobalt silicates, to found a peak of cobalt silicates in the XRD diagrams. The absence of this peak may

be justified by the cobalt silicates being amorphous, thus does not appear on the XRD diagrams.

Therefore, a XPS analysis was performed in JFR019 (which presented a first peak in TPR and a peak

of Co3O4 in XRD), and in JFR021. This analysis was not done in JFR022 since it present very similar

results to the ones of JFR021. The results from XPS are summarized in Table 13, which describe in

which form is the cobalt.

140971 - 142139

0 4-00 5 -96 5 6 (A) - C ob alt - C o - Y : 8 .4 7 % - d x b y: 1 . - W L: 1 .5 4 06 - C u bic - a 3.54 4 30 - b 3 .5 44 3 0 - c 3.54 4 30 - a lp ha 9 0 .0 00 - b eta 90 .0 0 0 - g a m m a 9 0.00 0 - Fa ce -ce ntered - Fm -3m (2 25 ) - 4 - 44 .5 2 37 - I/ Ic PD F 7 .3 - F7 =10 0 0(0 .0

0 0-00 9 -04 1 8 (A) - C ob alt O x id e - C o 3 O 4 - Y : 9.05 % - d x b y : 1. - W L : 1 .54 06 - C u b ic - a 8 .0 84 0 0 - b 8 .0 8 40 0 - c 8 .0 84 0 0 - a lp h a 90 .0 0 0 - b e ta 9 0.00 0 - ga m m a 9 0.00 0 - Fa ce-ce n te re d - F d-3 m (22 7 ) - 8 - 5 2 8.29 8 - F1 9= 3 4(0 .0 2 30 ,

O p e rat io n s: S li ts F ixed | Im po rt

1 42 1 39 - Fi le : F1 5K 15 6 85 .raw - Typ e: PS D fas t -scan - S tar t: 5 .0 17 ° - End : 7 1.99 5 ° - S te p: 0 .0 3 3 ° - S tep t im e : 49 9 .7 s - Te m p .: 2 5 °C (R oo m ) - T im e S ta rte d : 0 s - 2-T he ta : 5 .0 17 ° - Th eta: 2 .5 08 ° - Au x1 : 0.0 - Au x2 : 0.0 - Aux3 : 0.



Lin

(C

ps)

0

1

2

3

4

5

6

7

8

2-Theta - Scale

6 10 20 30 40 50 60 70

142380 -142594

0 4-00 5 -96 5 6 (A) - C ob alt - C o - Y : 8 .4 7 % - d x b y: 1 . - W L: 1 .5 4 06 - C u bic - a 3.54 4 30 - b 3 .5 44 3 0 - c 3.54 4 30 - a lp ha 9 0 .0 00 - b eta 90 .0 0 0 - g a m m a 9 0.00 0 - Fa ce -ce ntered - Fm -3m (2 25 ) - 4 - 44 .5 2 37 - I/ Ic PD F 7 .3 - F7 =10 0 0(0 .0

0 0-00 9 -04 1 8 (A) - C ob alt O x id e - C o 3 O 4 - Y : 9.05 % - d x b y : 1. - W L : 1 .54 06 - C u b ic - a 8 .0 84 0 0 - b 8 .0 8 40 0 - c 8 .0 84 0 0 - a lp h a 90 .0 0 0 - b e ta 9 0.00 0 - ga m m a 9 0.00 0 - Fa ce-ce n te re d - F d-3 m (22 7 ) - 8 - 5 2 8.29 8 - F1 9= 3 4(0 .0 2 30 ,





Lin

(C

ps)

0

1

2

3

4

5

6

7

8

2-Theta - Scale

6 10 20 30 40 50 60 70

JFR019

(A)

(B)

Co°

Co3O4

JFR021

JFR022

Co°

52

Table 13 - Results from XPS analysis for JFR019 and JFR021.

The results from XPS analysis, confirmed the existence of cobalt oxide in JFR019, and it reveals

the existence of Co2+ in both samples. XPS analysis is only able to specify that there is a specie in the

catalyst that presents an oxidation state of Co2+, but it could most likely be cobalt silicates.

Furthermore, it is interesting to note that no Co° was observed by XPS.

Organization of the porosity of the silica matrix and cobalt dispersion

In order to study with more detail the silica mesoporosity and the cobalt phase, TEM analysis were

performed. The following pictures show TEM micrographs for the previous three samples.

Figure 28 - TEM micrographs for the support of JFR019.

Regarding the silica matrix, as one can see in Figure 28, the porous have a worm-like shape. Also,

TEM showed that the pore size was approximately 5 nm, what is in agreement with the value taken from

N2 adsorption-desorption isotherm.

Figure 29 - TEM micrographs for metallic phase of JFR019.

Reference Co2+ Co3O4

JFR019 55,3% 44,7%

JFR021 100% -

Co3O4

Co°

53

The metallic phase (Figure 29) is present in the form of particles which are heterogeneously

distributed and ultra – dispersed onto the matrix.

Moreover, there are zones where cobalt particles were not visible. However EDS analysis showed

that Co is also present. Images in a dark – field mode did not show any diffraction, what makes possible

to conclude that cobalt, in these zones, is in an amorphous form. Besides that, cobalt was found in two

crystalline forms: cobalt oxide (Co3O4) and metallic cobalt (Co°).

For JFR019 was done a particle size distribution. The cobalt oxide particle size distribution in

volume is represented in Figure 30, and the size range is very broad going from about 10 to 50 nm.

Figure 30 – Cobalt oxide particles size distribution in volume for JFR019.

Concerning the samples JFR021 and JFR022 (Figure 31), globally they present the same

morphology. The oxide matrix presents a mesoporous domain as expected, and the structuration was

still happening, leading to a worm – like structure. The metallic phase was present in an ultra-dispersed

form, and, as seen in JFR019, it was found an amorphous phase rich in cobalt.

Figure 31 - TEM micrographs for the supports of: (A) JFR021 and (B) JFR022.

Conclusions

To conclude, it was found that JFR019 presented the most organized structure since it was

synthesized under the most acidic conditions, as previously stated.

0

0,05

0,1

0,15

0,2

0,25

0,3

0,35

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54

Fre

qu

en

cy (

Vo

lum

e)

Co3O4 particles diameter (nm)

JFR019

(B) (A)

54

Furthermore, these results are in agreement with the initial hypothesis: there is a peak in TPR at

elevated temperatures which is due to existence of cobalt silicates, however this species are not

detectable on XRD due to their non – crystallinity, and finally TEM analysis show that the cobalt in its

majority is in an amorphous form.

To sum up, there is a strong interaction between silica and cobalt, and changing the pH was not

enough to change the Si-O-Co interaction. However, with the solution at pH = 1 (JFR019), better results

were achieved, therefore more acidic conditions can be tested, to improve the solids obtained with TEOS

as inorganic precursor.

Silica conclusions

It was possible to load cobalt on a silica oxide matrix by spray – drying. Therefore, a solid with

cobalt already loaded (15 wt.% concerning the final catalyst mass) was synthetized by atomizing a

solution that had TEOS as inorganic precursor and acidified water with HCl at pH = 2. The resulting solid

was tested under a catalytic unit and there was no product formation. In order to modify the lack of

catalytic activity, the pH of the initial solution was modified.

First of all, the water was acidified with HNO3 instead of HCl to avoid chlorine atoms that can poison

active sites of the catalyst. It was supposed that the lack of activity was not mainly due to the presence

of Cl but to cobalt silicate. Also, the presence of HCl or HNO3 influences the mesostructuration process

of the solids. However, as chlorine atoms can affect the activity of the FT catalysts, all the solids started

to be synthetized with acidified water with HNO3.

Moreover, it was notice by TPR analysis the existence of peaks at high temperatures. This could

indicate the presence of cobalt silicates, which are hardly reducible species. [5]

In order to change the Si-O-Co interaction solutions with different pH were atomized. Three

solutions were made at pH = 1, pH = 2 and pH = 3. For all the samples different textural properties were

achieved (surface area, porous volume and pore diameter). Also, it was concluded that the pH has

influence on the mesostructuration process, as only the samples at pH = 1 and pH = 2 were

mesostructured. Moreover, it was concluded that the Si-O-Co interaction was only modified at pH =1,

however not enough to prevent the formation of cobalt silicates.

55

4.2. Alumina

Study of Aluminum chloride as inorganic precursor

The following paragraphs tackle the synthesis of alumina and cobalt alumina solids, using as

inorganic precursor AlCl3·6H2O. As previously mentioned, this study is not so detailed as the one of

silica, due to a larger knowledge and experience at IFPEN in synthetizing silica based materials by spray

- drying.

4.2.1. Supports

Generic characteristics and spray – drier parameters influence

JFR009 was synthesized in order to characterize the alumina supports produced by the ultrasonic

nozzle. Moreover, JFR010 was synthesized with a faster feed pump rate (2.1 mL/min) in order to know

the influence of this parameter in the support textural properties and to know if further atomizations could

proceed with this feed pump rate. Finally, the feed pump rate was set up at 2.1 mL/min.

Textural properties

Concerning the supports textural properties nitrogen adsorption – desorption isotherms were

performed, which are described Figure 32.

Figure 32 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR009 and JFR010.



0

50

100

150

200

250

300

350

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d V

olu

me

(m

L/g

)

Relative Pressure

JFR009

JFR010

0

0,02

0,04

0,06

0,08

0 5 10 15 20dV

/dD

vo

lum

e (

mL

/g)


56

Table 14 - BET surface, porous volume and pore diameter for JFR009 and JFR010.

In Figure 32, one can see that the isotherm shape of JFR009 and JFR010 is far different from the

typical isotherm for a mesostructured solid, and it is possible to state that this solid is probably not

mesostructured. Furthermore, this statement is confirmed by low angles XRD analysis, which is

described in the following paragraphs.

Concerning the difference in the isotherm shape of JFR009 and JFR010 and the differences in the

pores diameter, a more detailed study on alumina oxide matrix should be performed in order to justify

these differences. But it seems that the organic molecule (surfactant) does not play the role of templating

agent as with silica matrix, because of the much lower pore size.

Only JFR009 was characterized with more detail, because it was synthesized in the same

conditions as silica supports. Thus, it is possible to make comparisons between them.

Morphology of the aerosol elementary particles

In order to characterize the aluminum elementary particles morphology produced through the

ultrasonic nozzle, SEM analysis was performed in JFR009.

Figure 33 - SEM micrographs for JFR009.

Comparing Figure 33 with Figure 16 (page 37), one can see that alumina elementary particles have

a more fragile aspect than silica elementary particles. This can be due to different hydrolysis,

condensation and micellization reactions, and also, because alumina is not mesostructured (described

in Figure 34). Also, it is possible to see that alumina elementary particles are less spherical than the

ones of silica, and, that alumina elementary particles are hollow inside, as they are crushed, which did

not happen with silica elementary particles.


JFR009 Mesoporous

248 0.41 1.8

JFR010 258 0.47 3.2

57

Organization of the porosity of the support

In order to confirm the existence of a mesostructured solid, low angles XRD was performed which

is shown in Figure 34. It is possible to conclude that alumina is not mesostructured as there is no peak

in the low angles XRD analysis.

Figure 34 - Low angles XRD patterns for JFR009.

As previously mentioned, this lack of organization is not restricting in the final application of these

catalysts. The priority is to have a porous solid, which is the case but with a lower pore diameter.

4.2.2. Cobalt catalysts

Cobalt catalysts on an alumina matrix were synthesized containing 15 wt.% of cobalt, concerning

the final mass of catalyst. Following the same strategy as in silica, the presence of chloride in the catalyst

should be avoided. However, to understand the influence on having HNO3 instead of HCl, a trial was

done where the atomization solution contained with HCl (JFR023) and another where it contained HNO3

(JFR024). Also, some trials were made with aluminum nitrate (Al(NO3)3) as inorganic precursor, to avoid

the presence of chlorine atoms in the final solid.

One should note, that at this point it is not possible to know if the solids with cobalt loaded are

catalysts or not, as they were not catalytic tested. However, as a simplification they will be referred as

“catalysts”.

Blanc - File: C15N2584.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3: 0.0 - Di

140187*0.33 - File: C15K2589.raw - Type: PSD fast-scan - Start: 0.212 ° - End: 4.992 ° - Step: 0.017 ° - Step time: 200. s - Temp.: 25 °C (Room) - Time Started: 0 s - 2-Theta: 0.212 ° - Theta: 0.105 ° - Aux1: 0.0 - Aux2: 0.0 - Aux3:




Lin

(C

ou

nts

)

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

11000

12000

13000

14000

15000

16000

17000

18000

19000

20000

21000

22000

23000

24000

25000

2-Theta - Scale

0.22 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2.0

0.75° (118Å)

JFR009

58

Textural properties

In Figure 35, one can see the nitrogen adsorption – desorption isotherms for the supports (JFR010

and JFR025) and for the catalysts (JFR023 and JFR024). One can note that JFR010 is just represented

as an oxide matrix reference, since it only differs in the presence of HCl or HNO3.

Figure 35 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by BJH adsorption for JFR010 (reference), JFR023, JFR025 and JFR024.




As one can see in Figure 35, the isotherms shape change significantly with the presence of HCl or

HNO3. As previously mentioned in the supports sub – chapter, the supports (JFR010) synthetized

through an atomization solution which contained HCl are not mesostructured, and likelihood, the

catalysts which contained HCl (JFR023) are not also. Moreover, the isotherm shape of the support which

contained HNO3 is very different from the typical isotherm for a mesostructured solid, thus is reasonable

0

50

100

150

200

250

300

350

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d V

olu

me

(m

L/g

)

Relative Pressure

JFR010 (Support, HCl)

JFR023 (Catalyst, HCl)

JFR025 (Support, HNO3)

JFR024 (Catalyst, HNO3)

Reference

Porous type

BET surface (m2)

Porous volume (mL/g)

Pore diameter (nm)

Supports JFR010 (HCl)

Mesoporous

258 0.41 3.2

JFR025 (HNO3) 260 0.45 3.1

Catalysts JFR023 (HCl) 65 0.15 2.9

JFR024 (HNO3) 121 0.19 1.8

0

0,02

0,04

0,06

0,08

0,1

0 5 10

dV

/dD

vo

lum

e (

mL

/g)


59

to affirm that both supports and catalysts containing HNO3 are not mesostructured. One should take into

account that to verify this statement a low angles XRD and TEM analysis should be performed.

Concerning the supports (JFR010 and JFR025), one can conclude that the introduction of HNO3,

almost does not modify the surface area, porous volume or pore diameter. Regarding the catalysts

(JFR023 and JFR024), using HNO3 instead of HCl increases significantly (variation of 86%) the surface

area. However there is a reduction in the pore diameter along with a reduction in the porous volume

with the introduction of HNO3 in the catalyst, instead of HCl.

The incorporation of cobalt on the alumina matrix changes significantly the textural properties,

reducing considerably the surface area, the porous volume and the pore diameter, whereas it is the

contrary with silica matrix. Therefore, the textural properties of the obtained solids are not satisfactory

for the application of these catalysts in the FT process. Hence, a further study should be performed in

order to improve these textural properties.

Cobalt reducibility

The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 36 it is the

TPR profile for JFR023 and JFR024.

Figure 36 - TPR profile for JFR023 and JFR024.

After spray drying both solutions, the collected powder was light blue, which is the typical color of

cobalt aluminates. In order to confirm this supposition, a TPR analysis was performed.

For both samples there is only one peak at high temperatures which does not correspond to the

reduction of CoO→Co°, since there is no peak corresponding to the reduction of Co3O4→CoO (between

100-350°C) and as it is at higher temperature than usual (400-600°C).

Therefore, these peaks may suggest the existence of cobalt aluminates, which is in agreement with

the powder color, due to a strong interaction between the oxide matrix and cobalt oxide.

To completely avoid the presence of chlorine atoms in the final catalyst aluminum nitrate (Al(NO3)3)

was tested as inorganic precursor.

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

1,8

0 200 400 600 800 1000

Hyd

rog

en

co

ns

um

pti

on

(mL

/g c

ata

lys

t)

Temperature (°C)

JFR023 (HCl)

JFR024 (HNO3)

60

Study of Aluminum nitrate as inorganic precursor

In the following paragraphs it is described the synthesis of cobalt catalysts on an alumina matrix,

with 15 wt%., concerning the final catalyst mass.

Textural properties

In Figure 37 is described the nitrogen adsorption – desorption isotherms for JFR037 and JFR024.

One should note that the latter is only represented as a reference, as the inorganic precursor is the only

difference between the both samples.

Figure 37 - Nitrogen adsorption – desorption as a function of relative pressure and pore size distribution determined by

BJH adsorption: JFR024 and JFR037.



Table 16 - BET surface, porous volume and pore diameter for JFR024 (reference) and JFR037.

Comparing the isotherms shape it is possible to affirm that JFR037 is not mesostructured, however

a low angles XRD and TEM analysis should be performed to confirm this.

0

20

40

60

80

100

120

140

0 0,2 0,4 0,6 0,8 1

Ad

so

rbe

d V

olu

me

(m

L/g

)

Relative Pressure

JFR024 (AlCl3)

JFR037 (Al(NO3)3)


JFR024 Mesoporous

121 0.19 1.8

JFR037 32 0.11 12

0

0,01

0,02

0,03

0 10 20 30

dV

/dD

vo

lum

e (

mL

/g)


61

Concerning the differences in the pore diameter and in the surface area, a more detailed study on

should be performed in order to justify these differences, but it seems that with Al(NO3)3 the organic

molecule (surfactant) plays a role in building the porosity as it is wider than with AlCl3 precursor.

The increase in the pore diameter is positive, as the produced hydrocarbons chains would be

desorbed from the catalyst more easily. However, the porous volume it is not satisfactory for the FT

process, hence a more detailed study should be done to improve the textural properties.

Cobalt reducibility

The cobalt reducibility as well as accessibility was studied by TPR analysis. In Figure 38 it is the

TPR profile for JFR037 and JFR024. It is important to refer that after calcination, both samples were

black, which is the characteristic color of cobalt oxide (Co3O4), instead of violet for JFR024.

Figure 38 - TPR profile for JFR024 and JFR037.

As one can see, in Figure 38, the TPR profile for the samples obtained with Al(NO3)3 as inorganic

precursor have changed significantly.

If one decomposes the peaks of JFR037 it is found a first peak around 415°C, a second peak at

615°C and finally a third peak at 895°C.

For JFR037 sample the first peak likelihood corresponds to Co3O4→CoO reduction, whereas the

second peak may correspond to CoO→Co° reduction. Concerning the first peak, one can note that it is

at a more elevated temperature than usual, which might be explained through the decomposition of

residual NOX groups (exothermic reaction). This group exists due to the presence of NO3 in the inorganic

precursor, cobalt precursor and acidified water.

Finally, JFR037 presents a third peak that probably correspond to the existence of cobalt

aluminates. To confirm this, a more detailed characterization should be performed.

Alumina conclusions

The loading of cobalt onto an alumina matrix was possible. Moreover, it is believed that none of the

synthetized alumina based solids are mesostructured (due to the low angles XRD for the support and

to the isotherms shape).

0

0,2

0,4

0,6

0,8

1

1,2

1,4

1,6

0 200 400 600 800 1000

Hyd

rog

en

co

ns

um

pti

on

(mL

/g c

ata

lys

t)

Temperature (°C)

JFR024 (AlCl3)

JFR037 (Al(NO3)3)

62

To follow the same strategy as in silica, two solids with cobalt already loaded (15 wt.%, concerning

the final catalyst mass) were synthetized through two solutions containing HCl and HNO3 and aluminum

chloride as inorganic precursor. The loading of cobalt in the alumina matrix decreases the surface area,

porous volume and the pore diameter. One should note, that the achieved textural properties are not

satisfactory, therefore a further study should be performed to improve these. Moreover, for both cases

it was noticed a strong interaction between cobalt and alumina precursor, as in TPR analysis, there were

peaks at elevated temperatures.

In order, to avoid the presence of chlorine atoms in the final solid, aluminum nitrate was tested as

inorganic precursor.

One should note that the change in the aluminum precursor allows a modification in the interaction

between alumina and cobalt, hence, the reduction of Co3O4 → Co° was achieved.

63

5. Conclusions and future perspectives

The aim of this work is the synthesis of active FT catalysts by spray – drying. At this point, is not

possible to conclude if the synthetized solids are catalysts, as they were not catalytic tested. It is only

possible to affirm that it was possible to synthetize, by spray – drying, solids with a cobalt loading of 15

wt.% onto a silica and alumina matrix.

Concerning the solids synthetized onto a silica matrix, it was noticed that all of them present very

good textural properties for final application in the FT process. Nevertheless, the existence of cobalt

silicates was observed, due to high cobalt dispersion that lead to a strong interaction between the

inorganic precursor (TEOS) and cobalt. Therefore in order to reduce this interaction the pH of the

atomization solution was changed.

Only at pH = 1 better results were achieved, as there was a reduction of Co3O4 →Co°. However, it

was noticed the formation of cobalt silicates. Nevertheless, a more detailed study should be done, to

really evaluate which parameter could change the interaction between silica and cobalt. Also, it should

be understood the presence of metallic cobalt in samples that were not reduce. Depending on the

catalytic tests results, it should be understood where the cobalt nanoparticles are located on the support

and it could be tried the loading of a promoter metal in order to see if the cobalt reducibility was improved.

Moreover, concerning the solids synthetized onto an alumina matrix, it was noticed that the textural

properties are not satisfactory for the application in FT process. In solids produced with aluminum

chloride, as inorganic precursor, it was notice the existence of cobalt aluminates. Another aluminum

precursor, aluminum nitrate, was tested: the cobalt reducibility was improved since there was a reduction

of Co3O4 →Co° but, there were still cobalt aluminates. Though, concerning alumina matrixes a more

detailed study should be performed in order to improve the textural properties and also to completely

understand several differences that were reported in the results chapter.

65

6. References

[1] BP Energy Outlook 2035, January 2014;

[2] B. Anantharaman, D. Chatterjee, S. Ariyapadi, R. Gualy, Hydrocarbon Processing (2012),

pp.47-53;

[3] C. Boissière, D. Grosso, A. Chaumonnot, L. Nicole, C. Sanchez, Advanced Materials, Vol.23

(2011), pp.599-623;

[4] http://www.netl.doe.gov/research/coal/energy-systems/gasification/gasifipedia/ftsynthesis,

visited on 13/03/2015;

[5] A. Khodakov, W. Chu, P. Fongarland, Chemical Reviews, Vol.107 (2007), pp. 1692-1744;

[6] S. Chambrey, P. Fongarland, H. Karaca, S. Piché, A. Griboval-Constant, D. Schweich, F. Luck,

S. Savin, A. Khodakov, Catalysis Today, Vol. 171 (2011), pp. 201-206;

[7] B. Jager, R. Kelfkens, A. Steynberg, Studies in Surface Science and Catalysis, Vol. 81 (1994),

pp. 419–425;

[8] E. Iglesia, S. Reyes, R. Madon, and S. Soled, Advances in Catalysis, Vol. 39 (1993), pp. 221-

302;

[9] M. Dry, Journal of Chemical Technology and Biotechnology, Vol.77 (2001), pp. 43-50;

[10] https://web.anl.gov/PCS/acsfuel/preprint%2520archive/Files/45_3_WASHINGTON%2520DC_

08-00_0592.pdf+&cd=1&hl=fr&ct=clnk&gl=fr, visited on 20/03/2015;

[11] J. Casci, C. Lok, M. Shannon, Catalysis Today, Vol.145 (2009), pp. 38-44;

[12] M. Sadeqzadeh, S. Chambret, S. Piché, P. Fongarland, F. Luck, D. Curulla-Ferré, D. Schweid,

J. Bousquet, A. Khodakov, Catalysis Today, Vol. 251 (2013), pp. 52-59;

[13] J. Jung, S. Kim, D. Moon, Catalysis Today, Vol. 185 (2012), pp. 168-174;

[14] E. Iglesia, S. Soled, R. Fiato, G. Via, Journal of Catalysis, Vol. 143 (1993), pp. 345;

[15] K. Coulter, A. Sault, Journal of Catalysis, Vol. 154 (1995), pp. 56-64;

[16] A. Lapidus, A. Krylova, J. Rathousky, A. Zuka, M. Jancalkova, Applied Catalysis A: General,

Vol. 80 (1992), pp. 1-11;

[17] D. Petru, L. Kepinski, Catalysis Letters, Vol. 73 (2001), pp. 41-46;

[18] G. Bezemer, J. Bitter, H. Kuipers, H. Oosterbeek, J. Holewijn, X. Xu, F. Kapreijn, A. Dillen and

K. Jong, J. Am. Chem. Soc., vol. 128 (2006), pp. 3956-3964;

[19] N. Tsakoumis, M. Rønning, Ø. Borg, E. Rytter, A. Holmen, Catalysis Today, Vol. 154 (2010),

pp. 162-182;

[20] F. Colbeau-Justin, Design de nouveaux catalyseurs par incorporation d’hétéropolyanion dans

une matrice mésostructurée, 2012 (pHD thesis);

[21] A. Chaumonnot, F. Tihay, A. Coupé, S. Pega, C. Boissière, D. Grosso, C. Sanchez, Oil and

Gas Science and Technology – Rev. IFP, Vol. 64 (2009), No.6, pp. 681-696;

[22] D. Zhao, Y. Wan, W. Zhou, Ordered Mesoporous Materials, Wiley-VCH, 2013;

[23] N. Baccile, D. Grosso, C. Sanchez, Journal of Materials Chemistry, Vol. 13 (2003), pp.3011-

3016;

[24] F. Iskandar, Advanced Podwer Technology, Vol.20 (2009), pp. 283-292;

66

[25] S. Pega, C. Boissière, D. Grosso, T. Azaïs, A. Chaumonnot, C. Sanchez, Angew. Chem. Int.

Ed., Vol.48 (2009), pp. 2784-2787;

[26] F. Colbeau-Justin, C. Boissière, A. Chaumonnot, A. Bonduelle, C. Sanchez, Advanced

Functional Materials, Vol.24 (2014), pp. 233-239;

[27] http://www.buchi.com/en/products/spray-drying-and-encapsulation/solution-spray-drying-large-

10-%E2%80%93-60-%CE%BCm, visited on 18/05/2015;

[28] S. Kleinhans, C. Arpagaus, G. Schönenberger, The Laboratory Assistant, Büchi labortechnik,

2007;

[29] J. Lynch, Physico-Chemical Analysis of Industrial Catalysts - A Pratical Guide to

Characterisation, Editions Technip, 2003;

[30] A. Paredes-Nunez, D. Lorito, N. Guilhaume, C. Mirodatos, Y. Schuurman, F. Meunier, Catalysis

Today, Vol. 246 (2014), pp.178-183;

[31] J. Figueiredo, F. Ribeiro, Catálise Heterogénea, 2 ed., Lisboa: Fundação Calouste Gulbenkian,

2007;

[32] P. Bruinsma, A. Kim, J. Liu, S. Baskaran, Chem. Mater., Vol.9 (1997), pp. 2507;

[33] C. Brinker, Y. Lu, H. Sellinger, H. Fan, Adv. Mater., Vol. 11 (1999), pp. 579;

[34] C. Sassoye, C. Laberty, H. Khanh, S. Cassaignon, C. Boissière, M. Antonietti, C. Sanchez,

Advanced Functional Materials, Vol. 19 (2009), pp. 1922-1929;

[35] Y. Lu, H. Fan, A. Stump, T. Ward, T. Rieker, C. Brinker, Nature, Vol. 398 (1999), pp. 223-226;

[36] V. Dufaud, F. Lefebvre, G. Niccolai, M. Aounine, J. Mater. Chem., Vol.19 (2009), pp. 1142;

[37] L. Catita, Search for new innovating catalyst for the Fischer-Tropsch synthesis, 2014 (master

thesis);

[38] E. Gorrepati, P. Wongthahan, S. Raha, H. Fogler, Langmuir, Vol. 13 (2010), pp.10467- 10474;

[39] E. Rytter, A. Holmen, Catalysts, Vol. 5 (2015), pp.478-499;

[40] R. Andreson, W. Hall, A. Krieg, B. Seligman, Reduced and Carbonized Cobalt Catalysts, Vol.

71 (1949), pp.183 – 187;

A

Appendix A: t- plot curves

Figure A1 - t – plot for samples JFR014 and JFR016.

0

20

40

60

80

100

120

140

160

180

0 0,5 1 1,5 2 2,5

Ad

sro

be

d V

olu

me

(m

L/g

)

Thickness statistiques (nm)

JFR014

JFR016

Innovative preparation of catalysts by aerosol route for ... · Usually Fischer – Tropsch (FT)...

Documents

Transcript of Innovative preparation of catalysts by aerosol route for ... · Usually Fischer – Tropsch (FT)...