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Catalytic Hydrodeoxygenation of Guaiacol overNoble Metal CatalystsDanni GaoPurdue University

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Thesis/Dissertation Acceptance

This is to certify that the thesis/dissertation prepared

By

Entitled

For the degree of

Is approved by the final examining committee:

Approved by Major Professor(s): ____________________________________

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Head of the Department Graduate Program Date

Danni Gao

CATALYTIC HYDRODEOXYGENATION OF GUAIACOL OVER NOBLE METAL CATALYSTS

Doctor of Philosophy

Arvind Varma

Fabio H. Ribeiro

Doraiswami Ramkrishna

Mahdi Abu-Omar

Arvind Varma

John Morgan 09/22/2014

To the best of my knowledge and as understood by the student in the Thesis/Dissertation Agreement, Publication Delay, and Certification/Disclaimer (Graduate School Form 32), this thesis/dissertation adheres to the provisions of Purdue University’s “Policy on Integrity in Research” and the use of copyrighted material.

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CATALYTIC HYDRODEOXYGENATION OF GUAIACOL OVER NOBLE METAL

CATALYSTS

A Dissertation

Submitted to the Faculty

of

Purdue University

by

Danni Gao

In Partial Fulfillment of the

Requirements for the Degree

of

Doctor of Philosophy

December 2014

Purdue University

West Lafayette, Indiana

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To my dearest family

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ACKNOWLEDGMENTS

The five years I spent at Purdue have been a great experience. I have grown

tremendously as a researcher, an independent thinker, a risk taker, and most

importantly, a person. I will not be where I am without my mentors, friends and

family.

First and foremost, I would like to thank my advisor, Professor Arvind Varma, for

accepting me as his graduate student. He is a great mentor who has not only

shared with me his academic wisdom but also pushed me to grow as a person.

He was always ready to make time from his exceedingly busy schedule for

meetings and discussions whenever I needed them. I also greatly appreciate his

support and conviction throughout my graduate research.

I would also like to acknowledge my committee members, Professor Fabio

Ribeiro, Mahdi Abu-omar and Doraiswami Ramkrishna. Their input and

encouragement has been of great value to my study. I want to especially thank

Prof. Fabio Ribeiro for allowing me to use a number of essential instruments in

his lab.

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I also express my gratitude to my group members who I worked with on a daily

basis. They are Hyun-Tae Hwang, Ranjita Ghose, Gregory Honda, Yang Xiao,

Wenbin Hu, Shinbeom Lee and Ahmad Al-Kukun. Hyun-Tae and Ahmad were

the first people in the group whom I worked with and they helped to initiate my

lab experiences. Hyun-Tae was very helpful during my transition between

research projects. His considerable research experience and expertise helped

accelerate the process. Greg and Yang were always available when I needed a

quick discussion on my thoughts, while Ranjita was a great companion and friend

through good and bad times. I look forward to reuniting with her in Houston. I am

also thankful to Christopher Schweitzer, Timothy Lehnert and Yucheng Wang for

contributing to the research project as undergraduate researchers.

I am grateful to all the faculty and staff in the School of Chemical Engineering.

Your work ensured that I can focus on my studies and research work. It has been

a great pleasure working with all of you. Special thanks to Dr. Enrico Martinez for

his input to my research, and Dr. Yury Zvinevich for his encouragement and help

in the laboratory. You both have always believed in me and made me feel

confident of my own ability.

I also thank all of my friends from my batch, including Lei Ling, Silei Xiong, Hung-

Wei Tsui, Renay Tsu, Vinod Kumar, Dhairya Mehta, Harsh Choudhari, Gautum

Yadav and Andy Koswara for accompanying me through the first semester. I still

remember the surprise birthday party all of you planned for me and the nights we

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spent studying together in Forney Hall. The friendship made the first semester

bearable and my transition into the new culture smooth.

In addition, I thank my friends Rong Zhang, Ye Cheng, Che-Chi Chu, Wen-

Sheng Lee, Jiannan Dong, Haijing Gao, Shuang Chen, Jianfeng Li, Yanran Cui,

Vicky Hsu, Xiaohui Liu, Qing Zhu, Haiyu Fang, Haoran Yang, Xin Zhao, Yang

Yang, Betty Yang, Si Chen, Zhijian Zhao, Zhenglong Li and Jun Wang for

treating me as one of your own. I will always cherish the time we spent together.

I am grateful to my fiancee Andy Koswara for always being there for me through

all the ups and downs. Your love, understanding and support made me strong.

Every day is just that much better with your presence.

Finally, I would like to thank my parents who always stand by me and let me

pursue my passions and dreams. Your unconditional love teaches me how to be

a caring person; your understanding and support make me strong and

determined, and your encouragement made me believe in myself to continue to

grow and be a better person.

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TABLE OF CONTENTS

Page

LIST OF TABLES ................................................................................................. x

LIST OF FIGURES ...............................................................................................xi

NOMENCLATURE ............................................................................................. xiv

ABSTRACT ..................................................................................................xv

CHAPTER 1. INTRODUCTION ........................................................................ 1

1.1 Biomass for fuels and chemicals production .................................. 1

1.1.1 Background ............................................................................. 1

1.1.2 Methods................................................................................... 4

1.2 Upgrading of pyrolysis bio-oils ....................................................... 6

1.2.1 Characteristics of bio-oils ........................................................ 6

1.2.2 Hydrodeoxygenation (HDO) .................................................. 10

1.3 Guaiacol hydrodeoxygenation ..................................................... 12

1.4 Thesis objectives ......................................................................... 15

CHAPTER 2. CATALYST ACTIVE METAL SCREENING .............................. 16

2.1 Introduction .................................................................................. 16

2.2 Experimental methods ................................................................. 17

2.2.1 Materials ................................................................................ 17

2.2.2 Catalyst characterization ....................................................... 17

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Page

2.2.3 Catalyst performance measurements .................................... 18

2.2.4 Product analysis .................................................................... 21

2.3 Results and discussions .............................................................. 22

2.4 Conclusions ................................................................................. 28

CHAPTER 3. OPTIMIZATION OF OPERATING CONDITIONS .................... 31

3.1 Introduction .................................................................................. 31

3.2 Experimental ................................................................................ 31


3.4 Conclusions ................................................................................. 39

CHAPTER 4. EFFECTS OF SUPPORT ......................................................... 40

4.1 Introduction .................................................................................. 40

4.2 Experimental ................................................................................ 41

4.2.1 Material.................................................................................. 41

4.2.2 Catalyst characterization ....................................................... 41

4.2.3 Reaction apparatus ............................................................... 42

4.2.4 Reaction product analysis ..................................................... 42


4.4 Conclusions ................................................................................. 47

CHAPTER 5. REACTION PATHWAYS .......................................................... 49

5.1 Introduction .................................................................................. 49

5.2 Methods ....................................................................................... 50

5.2.1 Experimental ......................................................................... 50

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Page

5.2.2 Computational ....................................................................... 50


5.3.1 Reaction pathways ................................................................ 52

5.3.2 DFT calculations .................................................................... 56

5.4 Conclusions ................................................................................. 59

CHAPTER 6. REACTION KINETICS ............................................................. 61

6.1 Introduction .................................................................................. 61

6.2 Methods ....................................................................................... 62

6.3 Results and discussion ................................................................ 63

6.3.1 Absence of heat and mass transfer limitations ...................... 63

6.3.2 Model selections .................................................................... 63

6.3.3 Results and discussions ........................................................ 65

6.4 Conclusions ................................................................................. 71

CHAPTER 7. DEACTIVATION STUDIES ...................................................... 72

7.1 Introduction .................................................................................. 72

7.2 Characterization methods ............................................................ 72


7.3.1 Thermal degradation ............................................................. 73

7.3.2 Coking ................................................................................... 76

7.4 Conclusions ................................................................................. 81

CHAPTER 8. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE

WORK ................................................................................................. 82

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Page

8.1 Summary ..................................................................................... 82

8.1.1 Catalyst screening and optimization of reaction conditions ... 82

8.1.2 Catalyst deactivation study .................................................... 83

8.1.3 Reaction pathways and kinetics study ................................... 84

8.2 Recommendations for future work ............................................... 85

8.2.1 DFT calculations for guaiacol HDO ....................................... 85

8.2.2 Phenol production from bimetallic catalysts........................... 86

8.2.3 Other bio-oils model compounds ........................................... 87

REFERENCES ................................................................................................. 89

APPENDIX ............................................................................................... 106

VITA ............................................................................................... 107

COPYRIGHT PERMISSIONS .......................................................................... 109

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LIST OF TABLES

Table .............................................................................................................. Page

Table 1.1 Bio-oils composition in wt % on the basis of different biomass sources

and production methods (taken from reference [19]). ........................................... 7

Table 1.2 Comparison between characteristics of bio-oil and crude oil (adapted

from [14, 19]) ........................................................................................................ 8

Table 2.1 Characterization results for fresh catalysts. ........................................ 18

Table 4.1 Characterization of supported Pt catalysts. ........................................ 41

Table 6.1 The reaction rate constants. ............................................................... 67

Table 6.2 Activation energy values. .................................................................... 69

Table 8.1 Metal candidates for bimetallic catalyst research. .............................. 87

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LIST OF FIGURES

Figure ............................................................................................................. Page

Figure 1.1 Energy availability for diesel, gasoline and cellulosic waste biomass

(adapted from reference [4]). ................................................................................ 3

Figure 1.2 Minimum fuel product selling price for diesel, gasoline and biofuel

(adapted from reference [5]). ................................................................................ 4

Figure 1.3 Examples of guaiacol-like species in lignin derived pyrolysis

bio-oils. ............................................................................................................... 14

Figure 2.1 Schematic diagram of the experimental setup. .................................. 20

Figure 2.2 Van Krevelen diagram for liquid products for the four carbon

supported metal catalysts. .................................................................................. 24

Figure 2.3 Guaiacol conversion versus reaction time for the four carbon

supported metal catalysts. .................................................................................. 26

Figure 2.4 Distribution of major products for the four carbon supported metal

catalysts. ............................................................................................................ 30

Figure 3.1 Van Krevelen diagram for liquid products at different temperatures. . 34

Figure 3.2 Guaiacol conversion versus reaction time at different temperatures. 36

Figure 3.3 Carbon recovery in liquid and gaseous products at different

temperatures. ..................................................................................................... 37

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Figure ............................................................................................................. Page

Figure 3.4 Selectivity of major products at different temperatures. ..................... 38

Figure 4.1 Guaiacol conversion versus reaction time for supported Pt

catalysts. ............................................................................................................ 44

Figure 4.2 Selectivity of major liquid products for supported Pt catalysts. .......... 45

Figure 4.3 Possible reaction mechanisms on supported Pt catalysts ................. 48

Figure 5.1 The distribution of major liquid products versus inverse space velocity

for Pt/C catalyst. ................................................................................................. 53

Figure 5.2 Proposed guaiacol HDO reaction pathways for Pt/C catalyst. ........... 54

Figure 5.3 13C NMR spectra for reaction products (solvent CDCl3).................... 55

Figure 5.4 Energy level for reaction coordinate .................................................. 57

Figure 5.5 Schematic illustrations of (a) the most stable guaiacol configurations

adsorbed on platinum slabs (b - d) other guaiacol configurations adsorbed on

platinum slabs; (e) the most stable partially hydrogenated intermediate adsorbed

on platinum slabs; (f) the most stable cyclopentanone configuration adsorbed on

platinum slabs. .................................................................................................... 60

Figure 6.1 Fitting results of guaiacol conversion based on second-order

kinetics. .............................................................................................................. 64

Figure 6.2 Fit of kinetic data at 300oC. ............................................................... 67

Figure 6.3 Parity plot for major compounds at 275, 300 and 325 oC. ................. 68

Figure 6.4 Arrhenius plots for the rate constants. ............................................... 69

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Figure ............................................................................................................. Page

Figure 7.1 TEM images for (a) fresh Pt/C, (b) used Pt/C, (c) fresh Ru/C, and (d)

used Ru/C catalysts, and the corresponding particle size distributions for (a‟)

fresh Pt/C, (b‟) used Pt/C, (c‟) fresh Ru/C, and (d‟) used Ru/C catalysts. ........... 76

Figure 7.2 Examples of compounds observed. .................................................. 78

Figure 7.3 TGA patterns for Pt/C and Ru/C catalysts. ........................................ 80

Figure A.1 SEM images for (a) fresh Pt/C, (b) used Pt/C, (c) fresh Ru/C, and (d)

used Ru/C catalysts. ......................................................................................... 106

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NOMENCLATURE

GUA guaiacol

CAT catechol

PHE phenol

CYC cyclopentanone

MET methane

WAT water

MEO methanol

CDO carbon dioxide

ki reaction rate constant,

W catalyst weight, g

F flow rate, cc/min

Pi partial pressure for compound i, atm

E activation energy, kJ/mol

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ABSTRACT

Gao, Danni. Ph.D., Purdue University, December 2014. Catalytic Hydrodeoxygenation of Guaiacol over Noble Metal Catalysts. Major Professor: Arvind Varma. Pyrolysis of biomass is a promising technology to convert solid biomass into

liquid bio-oils. However, bio-oils have high water and oxygen content which

subsequently lowers their energy density relative to conventional hydrocarbons.

For these reasons, an upgrading process is required. Catalytic

hydrodeoxygenation (HDO) is a rapidly developing technology for oxygen

removal from pyrolysis bio-oils and noble metal catalysts have shown promising

activities, especially as compared to the traditional hydrodesulphurization

catalysts (e.g. CoMo/Al2O3 and NiMo/Al2O3). However, further understanding and

development of the catalysts through improving robustness, increasing the oil

yield and reducing the hydrogen consumption are still required. In this work,

guaiacol, a phenol derived compound produced by the thermal degradation of

lignin, was selected as a model compound to study the HDO process. Guaiacol

is selected because it is among the major components of pyrolysis bio-oils, but it

is thermally unstable and leads to catalyst deactivation.

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In this study, four noble metals (Pt, Pd, Rh and Ru) and three catalyst supports

(activated carbon, alumina and silica) were selected to investigate the activity of

different metals and the effects of catalyst support. The screening criteria were

as follows: (1) High degree of deoxygenation, (2) Low hydrogen consumption, (3)

High carbon recovery in liquid phase, and (4) Long catalyst lifetime. The

screening was performed systematically in a fixed-bed reactor at atmospheric

pressure. The results show that among all the tested catalysts, Pt/C catalyst has

the highest activity and stability. Additionally, the operating temperature for the

Pt/C catalyst was optimized and 300 oC was found to be optimum.

For Pt/C catalyzed guaiacol HDO reaction, three major liquid products were

observed (i.e. phenol, catechol and cyclopentanone). Based on the experiments

performed under various space velocities and feed compositions, a reaction

network including 5 sub-reactions was proposed. Furthermore, kinetic studies

were conducted under integral conditions. The power-law model was found to

describe the system well and the corresponding rate constants and activation

energies for the 5 sub-reactions were obtained. In addition, the formation of

cyclopentanone from guaiacol was investigated via density functional theory

(DFT) calculations and a thermodynamically feasible pathway was proposed

based on the results.

Finally, since Pt/C showed negligible deactivation during the 5 h testing period

while Ru/C had significant deactivation, the catalyst deactivation mechanisms

were investigated using Pt/C and Ru/C catalysts. Two possible causes for

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deactivation (thermal degradation and coking) were investigated. The results

from catalyst characterization (SEM and TEM images, BET surface area

measurements, TGA experiments and dichloromethane dissolution) showed that

polyaromatic deposits, especially the condensed ring compounds, were the most

likely cause for catalyst deactivation.

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CHAPTER 1. INTRODUCTION

1.1 Biomass for fuels and chemicals production

1.1.1 Background

The increasing worldwide energy demand accompanied by the rising cost for

fossil fuels production has led to diversification of the global energy portfolio. A

recent report from BP suggested that, based on the estimated rate of future

worldwide energy consumption, the current fossil fuel reserves would last for only

about 50 years [1]. Although this forecast is likely to improve due to the

availability of newly developing sources, such as shale gas and tar sands, for the

longer term there is a need to develop renewable resources for fuels and

chemicals production. These candidates would need to meet many performance

criteria, some of which are determined in relation to the properties of fossil fuels

and others by the existing energy infrastructure tailored towards fossil fuel

processing. These factors include competitive pricing, comparable if not better

carbon efficiency, high expansion capacity and flexible implementation to the

existing infrastructure. In light of all these factors, biomass has been shown to be

an important renewable energy source [2].

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There are currently two types of biofuels which can be derived from biomass.

These include the “first-generation” and “second-generation” biofuels. Specifically

the former refers to those produced from edible feedstock, such as corn. While

these have yielded positive results, this is not a sustainable option since it

directly competes with the food supply. In contrast, second-generation biofuels,

which are derived from non-edible lignocellulosic materials (composed of lignin,

cellulose and hemicellulose), such as cornstove and wood, have attracted

considerable interest as alternative energy sources [3].

The mass availability of cellulosic wastes, one of the sources for second-

generation biofuels, in the US has been reported by Holmgren et al [4]. Based

on the reported data, analysis in terms of energy availability (EJ/year) is

presented in relation to those of diesel and gasoline in Figure 1.1. The results

show that cellulosic waste alone, which represents only 35-50% of lignocellulosic

biomass feedstock [5] has the potential to produce more than half of the energy

currently being generated by gasoline. Figure 1.2 shows the minimum fuel selling

price (yellow bars) for gasoline, diesel and fuel produced from biomass based on

a case study performed by the Pacific Northwestern National Laboratory [5]. The

report concluded that the price of fuel derived from biomass could be even lower

than that of diesel and gasoline, which further suggests that the second

generation biomass is a promising renewable resource for fuel production. In

addition, there is tremendous potential for biomass conversion to chemicals as

well [6].

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Figure 1.1 Energy availability for diesel, gasoline and cellulosic waste biomass (adapted from reference [4]).

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Figure 1.2 Minimum fuel product selling price for diesel, gasoline and biofuel (adapted from reference [5]).

1.1.2 Methods

It took Nature millions of years to form fossil fuels from biomass and other dead

organisms through anaerobic decomposition. It is therefore not surprising that

there are significant challenges for humans to engineer a similar process which

would work in a much shorter period of time (hours to days). Currently, the two

major approaches for conversion of lignocelluloses into fuels are biological and

thermochemical. Within the thermochemical route, combustion, gasification and

fast pyrolysis are the major processes [7].

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The biological methods are based on fermentation technologies. In Brazil,

ethanol produced from biomass fermentation is already being used to power

vehicles [8]. However, there are still challenges in the pretreatment of

lignocelluloses – to effectively break down the lignocelluloses into enzymatic

degradable compounds (e.g. simple sugars) [9, 10]. Also, this process is

generally more time-consuming than thermochemical-based processes.

Combustion of biomass is a traditional route for heat and power production and

has existed since the beginning of civilization. However, the energy efficiency for

this process is very low and simply cofiring biomass in existing combustors may

lead to clogging of the feed systems [7].

Gasification is another biofuel generation process which converts biomass under

a controlled level of oxygen into syngas (carbon monoxide, carbon dioxide and

hydrogen) at high temperatures. While syngas can be burnt directly for energy

production, its energy density is much lower and thus requires further treatment

(e.g. Fischer-Tropsch) [11].

In comparison, pyrolysis of biomass converts solid biomass into liquid bio-oils in

the absence of oxygen [12]. It normally takes only seconds for the large biomass

molecules to break down into smaller compounds in vapor and then condense

into a mixture of fuel-like liquid, which is referred to as pyrolysis bio-oils. The

short process time, relatively mild conditions and high liquid yields of fast

pyrolysis technology are advantageous as compared to other approaches.

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Recently, there is a significant expansion of research in this area all around the

world [13].

However, pyrolysis bio-oils are chemically corrosive and unstable, and have both

high water and oxygen content which in turn lower their energy density relative to

conventional hydrocarbon fuels [14-16]. Therefore, before the bio-oils can be

commercially used as transportation fuels or converted to chemicals, an

upgrading process is required [17].

1.2 Upgrading of pyrolysis bio-oils

1.2.1 Characteristics of bio-oils

In general, any form of biomass may be used as a starting material for fast

pyrolysis [13] and the acquired pyrolysis bio-oils are typically a dark brown liquid.

Depending on the feedstock and the processing conditions, as many as 400

different compounds may be present in the bio-oils [14, 18, 19]. Mortensen et

al.[19] have collected relevant information and provided a summary of the

compositions of bio-oils derived from different biomass sources and pyrolysis

reactors, shown in Table 1.1. It can be seen that the water content of these

products is high, and the oil may contain carbonhydrates, alcohols, ketones,

furans, and phenolics, with their compositions highly dependent on the feedstock

and reactor type [14, 18-21].

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Table 1.1 Bio-oils composition in wt % on the basis of different biomass sources and production methods (taken from reference [19]).

Corn cobs Corn

stover Pine Softwood hardwood

Ref. [22] [22] [23],[24] [25] [25]

T[◦C] 500 500 500 520 500

Reactor Fluidized

bed Fluidized Transport Rotating Transport

Water 25 9 24 29–32 20–21

Aldehydes 1 4 7 1–17 0–5

Acids 6 6 4 3–10 5–7

Carbohydrates 5 12 34 3–7 3–4

Phenolics 4 2 15 2–3 2–3

Furan etc. 2 1 3 0–2 0–1

Alcohols 0 0 2 0–1 0–4

Ketones 11 7 4 2–4 7–8

Unclassified 46 57 5 24–57 47–58

Table 1.2 shows a comparison between bio-oils and heavy petroleum fuel. One

major difference is the elemental composition; Bio-oils contain 28 – 52 wt%

oxygen, while heavy petroleum fuel has only around 1 wt%. This high oxygen

content of bio-oils results in many differences in terms of its physical and

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chemical properties from those of petroleum fuel. These include low energy

density, low stability and immiscibility with hydrocarbon fuels [14].

Table 1.2 Comparison between characteristics of bio-oil and crude oil (adapted from [14, 19])

Characteristic Fast pyrolysis Bio-oil Heavy petroleum fuel

Water content, wt% 15 - 30 0.1

Insoluble solids 0.5 - 0.8% 0.01%

pH 2.5 - 3.8 --

Carbon 39 - 65 85.2

Hydrogen, % 5 - 8 11.1

Oxygen, % 28 - 53 1.0

Nitrogen, % < 0.4 0.3

Sulfur, % < 0.05 2.3

Ash < 0.3 --

HHV, MJ/kg 16 - 19 40

Density, g/ml 1.23 0.94

Viscosity(@ 50oC), cp 10 - 150 180

Distillation residue, wt% 50 1

The water in bio-oils is either from the moisture which are originally presented in

the feedstock, or formed through the dehydration reactions during pyrolysis [26].

Moreover, the water content of bio-oils covers a wide range (15-30%) depending

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on the feedstock and the pyrolysis conditions. The presence of water in this

concentration range gives bio-oils a polar nature, as well as the immiscibility [3,

14, 27].

The pH of bio-oils ranges from 2 to 4, primarily because of the presence of

organic acids [14, 28]. This high acidity makes bio-oils highly corrosive to regular

construction materials, such as carbon steel, aluminum and even sealing

materials. At elevated temperatures, the corrosiveness is even more severe [14,

29].

Instability and aging issues during storage are also pronounced problems

associated with bio-oils. Specifically, the presence of highly reactive organic

compounds in bio-oils adversely affects their viscosity, heating value, and density.

For example, olefins, under the presence of air, could repolymerize changing bio-

oils‟ viscosity. Consequently, the quality of bio-oils usually decreases with

increasing storage time [19].

From the above review, we conclude that for pyrolysis bio-oils to be used as fuel,

the main challenge is to reduce its oxygen content, while retaining its carbon

content and minimizing hydrogen consumption [4, 30]. Furthermore, the cost of

bio-oils based on the current technologies is still much higher (10 to 100%) than

fossil fuels [14]. Therefore, the improvement of pyrolysis technology needs to

also focus on reducing the cost to make it economically feasible [3]. According to

Bridgwater [20], 62kg hydrogen is required for the hydrodeoxygenation of 1 ton

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of wood-derived bio-oil. From this perspective, decreasing the amount of

hydrogen consumed is also essential for process economics.

In order to improve the quality of bio-oils and produce fuels and valuable

chemicals, an upgrading process is required.

1.2.2 Hydrodeoxygenation (HDO)

One of the most promising technologies to upgrade the pyrolysis bio-oils is

catalytic hydrodeoxygenation (HDO), which is analogous to the more well-known

hydrodesulphurization (HDS) and hydrodenitrogenation (HDN) processes for

sulphur and nitrogen removal elimination from crude petroleum oil in the refinery

industry. As indicated by its name, the purpose of HDO is to remove oxygen with

the assistance of hydrogen or other hydrogen-donating compounds in the

presence of a suitable catalyst [19, 31, 32]. The HDO reactions typically occur at

high pressure (75 – 300 bar) and at temperature between 250 oC and 450 oC [33].

Based on the composition of bio-oils, a generalized equation for the HDO

processes has been proposed as follows: [19]

1.4 0.4 2 2 20.7 1" " 0.4CH O H CH H O (1.1)

According to this description, “CH2” represents any unspecified hydrocarbon

product. Generally, the reaction is exothermic and on average, the overall heat of

reaction is around 2.4 MJ/kg [24]. As indicated above, water may be formed

during HDO. It has also been observed that distinct phases of the reaction

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products are generated: two organic phases separated by one aqueous phase. It

is likely that this phase separation is associated with the degree of

deoxygenation [19, 24].

In summary, the key challenges of HDO process arise from several aspects. The

first is the complex composition of bio-oils [8]. Currently, the use of model

compounds is a common approach in studying the HDO process. Moreover,

developing durable catalysts which could be applied to pyrolysis oil from different

feedstocks is crucial in maintaining a year-round production. Another challenge,

which is especially important during lignin-derived pyrolysis oil upgrading process,

is to develop catalysts that selectively cleave C-O bonds but not C=C bonds [34].

In addition, since bio-oils tend to form coke during the upgrading process, which

leads to catalyst deactivation, developing catalysts which are coke-resistant is

also critical.

Generally, two categories of catalysts have been investigated for the bio-oils

HDO process. Conventional sulfide catalysts (e.g. CoMo, NiMo on Al2O3 support)

are clear candidates as they have been studied thoroughly for the HDS process

in the petroleum industry. However, co-feeding of H2S is required for catalyst

activation, which may generate trace amounts of sulfur-containing compounds

during the HDO process [35]. This is considered as a major drawback because

bio-oils are nearly sulfur-free [36]. Recently, there is more research focused on

noble metal catalyzed bio-oils HDO. Wildschut et al. [16, 17] studied the activity

of Ru/C, Pd/C and Pt/C for beech bio-oil HDO in a batch reactor at 200 bar and

12

12

350 oC, and concluded that Ru/C and Pd/C were good candidates for the

reaction as they showed higher degree of deoxygenation with sufficiently high

yields. Despite the higher cost relative to conventional sulfide catalysts, noble

metals have great potential for facilitating the HDO reaction by having higher

activity at moderate operating conditions and a more flexible catalyst design [37].

In addition, sulfur is not required for catalyst activation [38, 39]. Nevertheless,

further development of the noble metal catalysts through improving robustness,

increasing the yield, and reducing the amount of hydrogen consumed are still

required [40]. This may be achieved by improved understanding of the reaction

kinetics [15, 38, 39, 41-45] and the catalyst deactivation mechanisms [44].

Therefore, one key to the success of bio-oils upgrading technologies is to

develop effective noble metal catalysts with high selectivity and durability in the

bio-oils HDO process [3, 4].

1.3 Guaiacol hydrodeoxygenation

Although a number of bio-oils HDO processes have been studied, bio-oils

derived from different feedstocks typically consist of more than 400 different

organic compounds, which significantly complicate the study of catalytic activities

and reaction pathways of the HDO process [8]. In this context, it is important to

select model compounds which represent the raw bio-oils for providing

fundamental insight into the HDO process. In the present study, guaiacol (2-

methoxyphenol) was selected as the model compound. During bio-oils

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13

production, guaiacol-like species are typically formed by thermal degradation of

lignin [46].

More specifically, guaiacol is chosen for the study due to the following reasons:

- Guaiacol represents a large group of substituted phenolic compounds in bio-

oils from lignin pyrolysis process [38, 45]. These include guaiacol, vanillin,

and eugenol and their structures are shown in Figure 1.3 [37]. The overall

concentration of guaiacols in the oil phase can be as high at 34 wt%

depending on the pyrolysis conditions [34, 47].

- Guaiacol has two different oxygenated functions: a phenolic (Ar-OH) and a

methoxy (Ar-OCH3) group. Among the two functions, the phenolic group is

thermodynamically more stable which indicates that the cleavage of the

corresponding C-O bond is more difficult and requires severe conditions [41,

48],[49].

- Guaiacol has a low thermal stability and can transform to coke during the

HDO process [44, 48]. It is observed that among one and two-oxygen-

containing-benzenic structures, guaiacol has the highest tendency for coke

formation and therefore the highest coke content in its HDO product [44]. It

has been reported that the benzenic rings with two or more oxygenated

substituents like veratrole, catechol, dimethoxyphenol form char more easily

than those containing only the oxygenated substitutes without the benzene

rings [44, 50]. In comparison to guaiacol, anisole has less tendency to

produce coke during the HDO process [48]. Utilization of guaiacol as a model

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14

compound therefore provides the opportunity to investigate deactivation

mechanisms while also serving as a test for catalyst robustness.

-

Figure 1.3 Examples of guaiacol-like species in lignin derived pyrolysis bio-oils.

The HDO of guaiacol has recently been summarized by Zakzeski and

collaborators [51], where activities of noble metal catalysts were compared with

traditional sulfide and base metal catalysts. Gutierrez et al. [39] tested four

catalysts (Rh, Pd and Pt supported on ZrO2 and CoMo/Al2O3) and reported that

Rh/ZrO2 was the most active one. Zhao and co-workers [43] compared the

activity of several phosphide catalysts (Ni2P, Co2P, Fe2P, WP and MoP

supported on SiO2) with a noble metal catalyst (Pd/Al2O3) in a fixed-bed reactor.

They found that Pd/Al2O3 provided higher guaiacol conversion than all base

metal catalysts tested, indicating the superior performance of the noble metal

catalyst. To assist guaiacol HDO reactions, two functions are required for the

catalyst: to activate the oxygen containing groups and to facilitate hydrogen

donation [19]. It should be noted that although noble metal catalysts have shown

promising performance for guaiacol HDO reaction, their activities and lifetime

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15

have not been systematically compared, and their deactivation mechanisms are

also not well understood [19].

1.4 Thesis objectives

The goals of this research are to investigate the activity of noble metal catalysts

in guaiacol HDO reactions to achieve fundamental understanding of guaiacol

HDO mechanisms, and to provide a basis for investigations of other compounds

present in pyrolysis bio-oils. The thesis has the following objectives:

- Systematically compare the activity of four monometallic noble metal catalysts

(Pt, Pd, Rh and Ru) and three commonly used supports (carbon, alumina and

silica) and identify the superior supported catalyst;

- Optimize the reaction conditions and study their effects on catalyst

performance;

- Provide insights into reaction networks and mechanisms through both

experimental and computational studies;

- Investigate the mechanism for catalyst deactivation using various

characterization techniques;

- Study the kinetics of the reaction network for the most promising catalyst.

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CHAPTER 2. CATALYST ACTIVE METAL SCREENING

2.1 Introduction

As discussed in CHAPTER 1, for pyrolysis bio0oils upgrading, noble metal

catalysts have shown promising performance as compared to the conventional

sulfide catalysts. However, more investigation is still required, especially for their

deoxygenation and hydrogenation activities, lifetime and deactivation

mechanisms, etc. [19]. The goal of the work described in this chapter is to

systematically compare the activities of the four selected noble metal catalysts

(Pt/C, Pd/C, Rh/C and Ru/C) for the guaiacol HDO reaction and identify the most

promising catalyst. The screening criteria were: (1) high degree of deoxygenation,

(2) low hydrogen consumption, (3) high carbon recovery in the liquid phase, and

(4) long catalyst lifetime

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2.2 Experimental methods

2.2.1 Materials

Catalysts used in this study were purchased from Alfa Aesar as powders: Pt, Pd,

Rh and Ru, all supported on activated carbon. The metal loadings for all catalysts

were 5 wt%. The catalysts were sieved using a digital sieve shaker (Octagon

D200), and particles of size 100 ± 25 μm were used for the study. Guaiacol

(>98.0%) and all other calibration compounds (methanol, hexane, cyclohexene,

cyclohexanone, benzene, phenol, anisole, guaiacol, cresol, dimethoxybenzene

and dihydroxybenzene) were purchased from Sigma Aldrich. Ultra high purity

(99.999%) hydrogen and nitrogen gases were purchased from Indiana Oxygen.

2.2.2 Catalyst characterization

The BET surface area and pore diameter were measured for the samples using

surface area and porosimetry analyzer (ASAP 2000, Micromeritics). Scanning

electron microscopy (SEM, FEI Philips XL-40) and Transmission electron

microscopy (TEM, FEI Titan 80-300) were used to investigate the morphology

and metal particle sizes of catalysts, and the results are summarized in Table 2.1.

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18

Table 2.1 Characterization results for fresh catalysts.

Pt/C Pd/C Rh/C Ru/C

BET surface area (m2/g) 715.9 885.7 772.1 708.0

Pore diameter (Å) 33.7 33.3 35.3 32.0

Metal particle size (nm) 2.40±0.54 2.92±0.46 3.19±0.74 2.56±0.47

2.2.3 Catalyst performance measurements

A schematic diagram of the experimental setup is shown in Figure 2.1. The

experiments were conducted in a fixed-bed reactor made of 316 stainless steel

tubing (OD = 12.7 mm, ID = 10.2 mm). Stainless steel meshes and quartz wool

plugs were inserted in both ends of the reactor to hold the catalyst bed in place.

Prior to the catalytic reaction, the packed catalyst was activated at 400 oC, 1 atm

for 4 h under a gas mixture flow (H2:N2=1:2). The reactor was heated by a tubular

furnace (Lindberg/Blue M) and the reactor temperature was monitored by a set of

K-type thermocouples. Gas feed mixtures were prepared from gas cylinders

using mass flow controllers (Millipore Tylan 2900) while a precisely controlled

flow of guaiacol was introduced to the reactor in an up-flow configuration by a

syringe pump (KDScientific 410). To ensure the evaporation of guaiacol, the

injection line was pre-heated at 150 oC prior to reaching the reactor. The product

stream passed through a double-wall condenser, under ice water circulation

controlled by a water circulator (Thermo Haake C10). The liquid products were

collected periodically and the compositions were analyzed. The gas products

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19

were analyzed on-stream every 3 min. The standard reactor operating conditions

were: 300 oC, 1 atm, 0.5 g catalyst, total gas (H2:N2=1:1) flow rate 100 mL/min

and guaiacol feed rate 0.025 mL/min (liquid, at room temperature). For the case

of Pt/C, the tests were conducted between 275 and 325 oC.

Blank test of carbon support with no metal loading was conducted under the

standard reaction conditions and guaiacol conversion was less than 1%. All

experiments have mass balance of 85 ± 3%, and is due to the incomplete

collection of condensed material in condenser and experimental set-up and the

material deposited on used catalysts. These mass balance values are similar to

those reported in the literature [52, 53]. Possible factors affecting mass balance

include liquid hold-up in various locations in the system, particularly the

condenser, and coke deposit on the catalyst.

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20

Figure 2.1 Schematic diagram of the experimental setup.

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21

2.2.4 Product analysis

The collected liquid samples were analyzed by a GC/MS (LECO Pegasus 4D

GCxGC-TOF) to identify the composition. The GC/MS was coupled with an auto

sampler (CTC, GC-xt) and equipped with a DB-WAX column (30 m x 0.32 mm).

A GC (Agilent GC6890) with flame ionization detector (FID), equipped with a DB-

1701 column (30 m x 0.25 mm) was used for quantitative analysis of the liquid

products. Based on the GC/MS analysis results, 11 liquid compounds (methanol,

hexane, cyclohexene, cyclohexanone, benzene, phenol, anisole, guaiacol, cresol,

dimethoxybenzene and catechol) were selected to generate standard calibration

curves. In order to reduce the error introduced by instrument operation, acetone

was used as the internal standard.

The gaseous effluent was analyzed using a Micro GC (Agilent 3000A Micro GC)

equipped with two columns (Column A: MolSieve 5A, 10 m x 0.32 mm; Column B:

Plot U, 8 m x 0.32 mm) and two thermal conductivity detectors (TCD). Using

argon as carrier gas, hydrogen, nitrogen, oxygen, methane, and carbon

monoxide were analyzed by column A. Meanwhile, carbon dioxide, ethylene,

ethane, propane, and propylene were measured by column B using helium as

carrier gas. Standard calibration curves were prepared for hydrogen, nitrogen,

oxygen, methane, carbon monoxide, carbon dioxide, ethane, ethylene, propane,

and propylene.

Good reproducibility was achieved for all quantitative analyses; the relative

standard deviation of 3 injections is less than 5% for each calibrated compound.

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22

Various parameters including guaiacol conversion (XGUA), product selectivity of

compound A (SA), oxygen to carbon molar ratio (O/C) and hydrogen to carbon

molar ratio (H/C) are defined as follows, where n represents moles:

, ,

,

GUA in GUA out

GUA

GUA in

n nX

n

(2.1)

carbon in AA

carbon in iproduct i

nS

n

(2.2)

/O in liquid product

C in liquid product

nO C

n (2.3)

/H in liquid product

C in liquid product

nH C

n (2.4)

The liquid product was collected over different intervals, so the conversion over

any particular time period is based on the guaiacol injected and the product

collected over that period (Equation 1.1). Thus, the conversion is an average

over a given time interval and is reported at the mean of the interval in

subsequent figures.

2.3 Results and discussions

The four catalysts (Pt/C, Pd/C, Rh/C and Ru/C) were tested for HDO

performance under standard operating conditions described in section 2.2.3. In

this study, the Van Krevelen diagram (Figure 2.2) was used to assess the

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23

performance of the catalysts by analyzing the elemental composition of the liquid

product. The Van Krevelen diagram was originally developed to study the various

reaction processes with coal using a graphical-statistical method [54]. Here, the

diagram allows for comparison of hydrogenation and deoxygenation

performances. Due to the deactivation of catalysts with time, the results reported

in the diagram are based on the elemental composition of liquid products

collected between 60 and 80 min. In the calculation of O/C and H/C, unreacted

guaiacol was not included because its high O/C value would otherwise obscure

the discrimination of the results. The corresponding conversions are reported in

Figure 2.3 (between 60-80 min). As shown in Figure 2.2, both Pt/C and Ru/C

catalysts provide lower oxygen content in the liquid product, indicating higher

deoxygenation ability as compared to other catalyst candidates.

The results in this work are in contrast to those reported by Gutierrez at al.,

where Rh catalyst exhibited improved deoxygenation as compared with Pt and

Pd [39]. In their calculation of H/C and O/C values, unreacted guaiacol was

included. While it was reported by the authors that the reaction was complete at

300 °C with benzene and cyclohexanol as main products, back calculation shows

significantly lower conversion for Pt/ZrO2. This is in agreement with their results

at 100 °C, where Pt/ZrO2 achieved 10% the conversion of the Rh catalyst.

Conversely, in the present work, the highest guaiacol conversion was observed

for Pt/C catalyst. This is not surprising because different catalyst supports are

used, as it is widely accepted that supports participate in the hydrodeoxygenation

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24

reaction and thus lead to different product distribution and conversion [55, 56].

Using activated carbon as support, the same order of deoxygenation and

hydrogenation activity was observed for Ru/C and Pd/C catalysts by Chang et al.,

with the differences in the results from the current work being due to differences

in operating conditions [52].

Figure 2.2 Van Krevelen diagram for liquid products for the four carbon supported metal catalysts.

Figure 2.3 presents guaiacol conversion profiles with reaction time for all the

catalysts. To examine the stability of each catalyst, the reaction time was

extended to 5 h. Pt/C catalyst provided the highest guaiacol conversion (~87%)

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25

along with the highest stability for the tested period of time, while all other

catalysts (Pd/C, Rh/C and Ru/C) showed significant deactivation. To confirm that

this result is not due to excess Pt catalyst, experiments were also conducted at

lower guaiacol conversions (~30%) under the same operating conditions, and no

deactivation was observed. Although high deoxygenation ability was observed for

Ru/C (shown in Figure 2.2), significant deactivation occurred under tested

conditions: for a reaction time of 5 h, guaiacol conversion decreased from 75 to

46%. While improved catalyst stability can often be achieved by increasing

operating pressure (hydrogen partial pressure), Chang et al. still observed

significant deactivation for Ru/C catalyst even when conducting the guaiacol

HDO reaction at 4.0 MPa [52]. Furthermore, as hydrogen pressure increases,

hydrogenation of the aromatic ring may occur before deoxygenation, [37] thus

leading to higher hydrogen consumption. It is likely that the higher stability of

Pt/C observed in this work results from a different reaction mechanism as

compared to the other tested catalysts, which prevents deactivation of the Pt/C

catalyst throughout the entire 5 h test period at low hydrogen pressure; this

aspect is further discussed below.

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26

Figure 2.3 Guaiacol conversion versus reaction time for the four carbon supported metal catalysts.

The selectivity values for major reaction products (defined as compounds with

selectivities of above 1%) for different catalyst systems are shown in Figure 2.4

As described earlier, analysis results of products collected between 60 and 80

min were reported. Similar product compounds were observed for all the tested

catalysts, although their compositions varied. Phenol is the most abundant liquid

product with selectivity between 45 to 85%. Phenol is likely to be produced by

two pathways: (1) through the direct removal of the methoxy group from the

aromatic ring, as it has been confirmed through thermodynamic calculations that

the aromatic-methoxy functional group has a lower bonding dissociation energy

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27

as compared to the phenolic group, which makes it easier to be detached from

the aromatic ring [32], and (2) through the removal of a water molecule from

catechol (1,2-dihydroxybenzene) [57]. For the Pt/C, Pd/C and Rh/C catalysts,

another major liquid product is cyclopentanone. Particularly for the Pt/C catalyst,

cyclopentanone is the second most abundant liquid product with selectivity above

20%. Reactions which produce cylcopentanone are likely to involve

hydrogenation, ring-opening, ring-closing, and decarbonylation reaction [58].

Formation of this compound in large quantities (17% selectivity) has been

observed by other researchers for Pt/MgO catalyst but not for Pt/Al2O3 [58]. This

indicates that the catalyst support also participates in the reaction and affects

reaction pathways, as has been observed for supported Ru catalysts [56].

However, in contrast to the results of this work for Pt/C, cyclopentanone has

previously not been observed for the guaiacol HDO reaction [57]. Differences

between results are likely due to the diverse structures which may occur in

activated carbon, its complexity as a support with acid and base functionalities,

[56, 59] and changes in catalyst behavior with different operating conditions. In

addition, cyclopentanone was observed by Chen et al. during dehydro-

aromatization of 1,2-cyclohexanediol to catechol over Na- and Ni- modified

HZSM-5 catalysts [60]. They proposed that the pathway which produced

cyclopentanone involves decarbonylation of 2,3-dihydroxyl-1,3-cyclohexadiene

and/or 1,2-cyclohexanone, which agrees with our hypothesis. Further

investigation is ongoing to understand the formation of cyclopentanone.

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28

Figure 2.4 also shows the distribution of gaseous products, where the primary

ones include carbon monoxide, methane, and carbon dioxide. For all the

catalysts, carbon monoxide was the most abundant gas product, followed by

methane and carbon dioxide. In addition to decarbonylation reaction, carbon

monoxide could also be produced through methanol decomposition, since both

Pt and Ru metals are well known to catalyze this reaction. It has also been

reported that Pt catalyst has higher methanol decomposition activity as

compared to Ru, [61] which agrees with our result that Pt/C catalyst provides

lower methanol selectivity as compared to Ru/C catalyst. Carbon dioxide could

be produced through the water gas shift reaction, especially for Pt/C catalyst [62].

Rh and Ru catalysts, however, were found to also promote methanation reaction

which consumes the carbon dioxide produced from water gas shift reaction [63].

This may explain the higher presence of carbon dioxide in the Pt/C catalyzed

reaction, but not others. Moreover, yields to gas products for the tested catalysts

follow the trend Pd/C < Ru/C < Rh/C < Pt/C.

2.4 Conclusions

The performance of four noble metal catalysts (Pt, Pd, Rh and Ru) supported on

activated carbon was tested systematically for guaiacol HDO in a fixed-bed

reactor at atmospheric pressure. The catalysts were evaluated based on their

deoxygenation, hydrogenation, liquid recovery activities as well as stability. The

results show that, among the tested catalysts, Pt/C is the most promising catalyst

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29

because of its higher deoxygenation activity and stability under the standard

operating conditions. Therefore, it is selected for further optimizations and

studies, as described in later chapters.

Note: Adapted with permission from Industrial & Engineering Chemistry

Research (“Conversion of Guaiacol on Noble Metal Catalysts: Reaction

Performance and Deactivation Studies”, DOI: 10.1021/ie500495z, Authors: D.

Gao, C. Schweitzer, HT. Huang and A. Varma). Copyright (2014) American

Chemical Society.

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30

Figure 2.4 Distribution of major products for the four carbon supported metal catalysts.

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CHAPTER 3. OPTIMIZATION OF OPERATING CONDITIONS

3.1 Introduction

The results presented in CHAPTER 2 have shown that, among the four tested

carbon supported noble metal catalysts, Pt/C provides the best performance for

guaiacol HDO reaction under standard operating conditions. To examine the

effect of temperature on the performance of this catalyst, three operating

temperatures were selected: 275, 300 and 325 °C. The optimum operating

temperature was identified based on the screening criteria described in section

2.1.

3.2 Experimental

The Pt/C catalyst was purchased from Alfa Aesar. The reaction experiments

were performed in the fixed-bed reactor described in Figure 2.1. and the

experimental procedure has been described in Section 2.2. Experiments were

conducted at three different operating temperatures (275, 300 and 325 °C) under

the following operating conditions: 1 atm, 0.5 g catalyst, total gas (H2:N2=1:1)

flow rate 100 mL/min and guaiacol feed rate 0.025 mL/min (liquid, at room

temperature).

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32

The liquid products were collected periodically and analyzed using an Agilent GC

6890, and the gaseous products were analyzed on-stream every 5 min using an

Agilent Micro GC.

The following equations were applied in the calculation guaiacol conversion

(XGUA), product selectivity of compound A (SA), oxygen to carbon molar ratio (O/C)

and hydrogen to carbon molar ratio (H/C) are defined as follows, where n

represents moles:

, ,

,

GUA in GUA out

GUA

GUA in

n nX

n

(3.1)

carbon in AA

carbon in iproduct i

nS

n

(3.2)

/O in liquid product

C in liquid product

nO C

n (3.3)

/H in liquid product

C in liquid product

nH C

n (3.4)


The results of the screening study indicate that Pt/C provides the best

performance for guaiacol HDO reaction under standard operating conditions. To

examine the performance of this catalyst further, three operating temperatures

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33

were selected: 275, 300 and 325 °C. The screening criteria described previously

were applied again to identify the optimum operating temperature.

The Van Krevelen diagram was used to analyze the elemental composition of

the reaction products in the liquid phase (as described in Chapter 2, the product

was collected between 60 and 80 min). As shown in Figure 3.1, the O/C molar

ratio decreases from 0.22 to 0.19 when the operating temperature increases from

275 to 300 oC. Further increase in operating temperature from 300 to 325 oC

does not alter the O/C ratio significantly. The H/C molar ratio is expected to

increase with temperature due to an increase in hydrogenation of aromatic rings.

In this study, however, the highest H/C ratio was achieved at 275 oC, and it is

only slightly higher than at 300 or 325 oC. Nevertheless, there are differences in

major product compounds obtained at each operating temperature, and a more

detailed discussion is presented later in the section.

Figure 3.2 shows the effect of temperature on guaiacol conversion for up to 5 h.

The conversion increases with operating temperature and the highest value was

achieved at 325 oC (~97% for product collected between 60 and 80 min). At this

temperature, however, there is a slight decrease of guaiacol conversion with

reaction time, indicating catalyst deactivation. This observation can be explained

by the competition between aromatic hydrogenation and condensation.

Hydrogenation of the aromatic ring is thermodynamically favorable at high

pressure and low temperature, while aromatic condensation reactions are

favored at low pressure and high temperature [64]. The condensed aromatic

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34

compounds (e.g. naphthalene) have high propensity for catalyst deactivation [65].

A more detailed investigation on catalyst deactivation is presented in CHAPTER

7.

Figure 3.1 Van Krevelen diagram for liquid products at different temperatures.

As described previously, carbon recovery in the liquid phase is another important

factor to assess the performance of the catalyst. For this reason, carbon

distributions in liquid and gas phases were calculated for each operating

temperature. Figure 3.3 shows that the highest carbon recovery in liquid product

(~70%) is achieved at 300 oC. There are two major factors affecting carbon

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35

recovery in the liquid phase: guaiacol conversion and further ring-opening

reactions to produce C1 gaseous products, which are inevitable on the carbon-

supported noble metal catalysts [57]. Figure 3.2 and Figure 3.3 clearly show that

both guaiacol conversion and gas production increase with reaction temperature

[66, 67], indicating that owing to trade-off between them, there exists an optimum

temperature to achieve maximum carbon recovery in the liquid phase. Existence

of gaseous product formation from ring opening of 5 or 6 carbon ring compounds

(especially at higher temperature) is also indicated by the ratio of carbon in gas

and in liquid products [57]. In fact, carbon yields in gas products exceed the

theoretical amount from direct removal of single carbon compounds from the

aromatics ring, especially at higher temperature.

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36

Figure 3.2 Guaiacol conversion versus reaction time at different temperatures.

Figure 3.4 shows the product selectivity at different operating temperatures.

Cyclohexanol, a six-carbon ring hydrogenation product, was observed only at the

lower temperature (275 oC), while cyclopentanone, a five-carbon ring compound,

was observed at all temperatures but with significantly higher selectivity at higher

temperatures (300 and 325 oC). It is likely that hydroxycycloalkene is produced

first and followed by the ring-opening reaction, which is more favorable at higher

temperature and leads to the production of cyclopentanone [58, 68]. More

investigation will be presented in CHAPTER 5. Selectivity of methanol decreased

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37

with increasing temperature, which may be due to increase in the methanol

decomposition rate with increasing operating temperature [69].

Figure 3.3 Carbon recovery in liquid and gaseous products at different temperatures.

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38

Figure 3.4 Selectivity of major products at different temperatures.

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39

3.4 Conclusions

The effect of operating temperature on the performance of Pt/C catalyst for

guaiacol HDO reaction was studied. Although the highest guaiacol conversion

was achieved at 325 oC, noticeable deactivation was also observed. At 300 oC,

however, despite a slightly lower guaiacol conversion, a comparable level of

oxygen removal and maximum liquid phase carbon recovery were obtained. In

addition, most importantly, the highest stability was observed. Based on these

results, the operating temperature of 300 oC was determined to be optimal for the

Pt/C catalyzed guaiacol HDO reaction.





Chemical Society.

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CHAPTER 4. EFFECTS OF SUPPORT

4.1 Introduction

In CHAPTER 2, the activities of the four active metal (Pt, Pd, Rh and Ru)

catalysts supported on activated carbon were compared, and it was found that

Pt/C catalyst offers the best activity. Another important aspect of catalyst

formulation for HDO is the selection of carrier material, not only because the

interactions between catalyst metal and support may play an important role in the

mechanism, but also due to its potential contribution to carbon formation which

may lead to the catalyst deactivation. In this chapter, Pt supported on three

different materials (activated carbon, alumina and silica) were tested to

investigate the effects of support.

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4.2 Experimental

4.2.1 Material

Catalysts used in the study were purchased as powders. Pt/C was purchased

from Alfa Aesar, while Pt/Al2O3 and Pt/SiO2 were purchased from Strem

Chemicals. The platinum loadings for all catalysts were 5 wt%. The catalysts

were sieved using a digital sieve shaker (Octagon D200), and particles of size

100 ± 25 μm were used for the study. Guaiacol (>98.0%) and all other chemicals

(methanol, phenol, anisole, cresol and catechol) were purchased from Sigma-

Aldrich. Ultra high purity (99.999%) hydrogen and nitrogen gases were

purchased from Indiana Oxygen.

4.2.2 Catalyst characterization

The BET surface area, pore size and metal dispersion were measured for the

fresh catalyst samples using surface area and porosimetry analyzer (ASAP 2020,

Micromeritics). The results are summarized in Table 4.1

Table 4.1 Characterization of supported Pt catalysts.

Pt/C Pt/Al2O3 Pt/SiO2

BET surface area (m2/g) 716 135.4 215

Pore diameter (Å) 33.7 28.5 94.6

Metal dispersion (%) 36.6 9.3 5.0

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4.2.3 Reaction apparatus

The experiments were conducted in a continuous flow system similar to that

described in CHAPTER 2. The catalyst powder was packed in a stainless steel

reactor (OD = 12.7 mm, ID = 10.2 mm), with quartz wool plugs placed on both

ends. Stainless steel meshes were used on both ends of the quartz wool plugs in

order to hold the catalyst in place and guide the thermocouple into the catalyst

bed to measure the bed temperature during the reaction. The reactor was heated

by a tubular furnace. Gas feed flows were controlled by a set of mass flow

controllers (Millipore Tylan 2900) while guaiacol feed was controlled using a

syringe pump (KDScientific 410). The reaction was carried out in an up-flow

configuration, with the guaiacol feed line preheated to 180 oC. A customized

double-wall condenser under ice water circulation was used to collect the liquid

reaction product, while the gas products were analyzed on-stream every 10 min.

4.2.4 Reaction product analysis

Liquid product samples were analyzed by a gas chromatograph (Agilent GC6890)

equipped with flame ionization detector and a DB-WAX (30 m x 0.32 mm) column.

Standard calibration curves were generated for 6 liquid compounds (methanol,

phenol, anisole, guaiacol, cresol and catechol), with acetone being the internal

standard. 13C NMR analysis of the liquid product was also conducted for some

cases, using the Bruker ARX400.

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43

The gaseous product passing through the condenser was analyzed by a Micro

GC (Agilent 3000A Micro GC) equipped with a MolSieve 5A column and a Plot U

column. Calibration was performed for hydrogen, nitrogen, methane, carbon

monoxide and carbon dioxide.


Experiments were performed under the standard operation conditions using three

supported platinum catalysts: Pt/Al2O3, Pt/C and Pt/SiO2. Figure 4.1 shows the

guaiacol conversion versus reaction time for the three tested catalysts. Among

these three, Pt/C provided the highest guaiacol conversion (90%), which was

stable for the 5 h testing period. The Pt/ Al2O3 catalyst offered high guaiacol

conversion, but noticeable deactivation occurred (66% to 59% in 5 h). The lowest

guaiacol conversion was observed with Pt/SiO2 (10%). Similar sequence and

comparable values have been reported by Boonyasuwat et al. for supported Ru

catalysts [56]. Wu et al. compared the activity of Ni2P supported on Al2O3, ZrO2

and SiO2, and also observed a much lower guaiacol conversion with SiO2

supported catalyst as compared to Al2O3 [70].

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44

Figure 4.1 Guaiacol conversion versus reaction time for supported Pt catalysts.

Deactivation associated with Pt/Al2O3 catalyst is likely to be caused by coking,

particularly condensed-ring compounds (more details will be presented in

CHAPTER 7). An additional evidence is that when alumina was used either alone

or with Ru loading, heavy oxygenated hydrocarbons (above 12 carbon atoms)

were observed in the products [56, 71]. It has also been reported that when using

Ru, Ni2P or CoMo supported on alumina, the carbon deposit is more than on

corresponding silica supported catalyst [72]. Popov et al. investigated the

adsorption of phenol and guaiacol on Al2O3 and SiO2 [73, 74]. It was found that

on Al2O3, guaiacol interacts with the Lewis acid sites and forms the doubly

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45

anchored phenates at room temperature, which subsequently transform into coke

as temperature increases. On SiO2, however, guaiacol adsorbs mostly through

the –OH and yield methoxy phenates, which are less likely to form coke. The

adsorption of guaiacol on activated carbon is more complex since it is

determined by both its pore structure and surface chemical composition [75].

Figure 4.2 Selectivity of major liquid products for supported Pt catalysts.

Figure 4.2 shows the selectivity of major liquid products (catechol, phenol and

cyclopentanone) for the three supported platinum catalysts. The presented data

is based on the product collected between 60 and 80 min. The selectivity of

catechol is comparable for Pt/Al2O3 and Pt/SiO2, but significantly lower for Pt/C

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46

catalyst. Pt/Al2O3 provides the highest phenol selectivity while Pt/SiO2 offers the

lowest. Formation of cyclopentanone is minimal with Pt/Al2O3, but relatively high

(selectivity of 22%) with Pt/C.

Hydrogenation and deoxygenation of guaiacol are competing routes. It is of

importance to investigate how the two types of reactions are affected by

metal/support selection. It has been reported that when alumina is used as

catalyst in the absence of active metal, even under higher hydrogen pressure (~

4 MPa), no hydrogenation of the aromatic ring was observed, indicating minimal

hydrogenation activity [71]. With the addition of Pt, high selectivity (30 – 40%) of

catechol was observed under low partial pressure of hydrogen, [45] and when

hydrogen pressure was increased (> 3 MPa), the reaction produced mostly

saturated-rings compounds [76, 77]. Experiments with activated carbon along as

catalyst was performed in this work under the standard operating condition,

during which guaiacol conversion was ~ 5% and catechol and cresol were the

major products. These results suggest that catalyst support alone does not lead

to ring saturation even at elevated hydrogen pressure but may assist the

hydrogenation process in the presence of active metal due to hydrogen spillover.

Based on the discussion above, one possible route of reaction initiated from

adsorption on the support is proposed (Figure 4.3): first the phenolic group

interacts with the support followed by the dealkylation of the methoxy group and

yields catechol [56]. With the addition of active metals, the adsorbates may also

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47

interact with the dissociated hydrogen on support due to spillover,[78] and

produce catechol and phenol.

Due to the adsorption characteristics of silica support, [74] it is less active for

step 2, which is critical for the following steps to proceed. This could lead to the

low activity for silica supported catalysts. Alumina support, as noted above, is

more active but leads to coke formation. When activated carbon is the support,

the high surface area and complex chemical groups on its surface lead to more

adsorption sites and increase its activity.

4.4 Conclusions

For the guaiacol HDO reaction, Pt supported on three different supports

(activated carbon, alumina and silica) were compared under atmospheric

pressure using a fixed-bed reactor. The carbon supported catalyst was found to

provide superior activity and stability for the 5 h tested period. A possible reaction

mechanism starting from the adsorption of guaiacol on catalyst support was

proposed and used to explain the difference in activities observed for the three

tested catalysts.

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Figure 4.3 Possible reaction mechanisms on supported Pt catalysts

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CHAPTER 5. REACTION PATHWAYS

5.1 Introduction

The general pathways of guaiacol HDO over carbon supported noble metal

catalyst have been proposed under various operating conditions [52, 56, 57].

Typical reactions include hydrogenation, demethoxylation, hydrogenolysis and

demethylation. However, since reaction pathways and products depend

significantly on the active metal, support structure and the operating conditions, it

is of great importance to understand the individual steps for the tested Pt/C

catalyst.

It has been noted in the previous chapters that for Pt/C catalyzed guaiacol HDO

reaction, the main liquid phase reaction products were phenol, catechol and

cyclopentanone. In this chapter, a network for this reaction system is proposed

and supported through various experimental studies.

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50

In addition, cyclopentanone, a five-membered ring compound has only been

observed a few times in the literature, and its formation from guaiacol, a six-

membered ring compound, is still unclear [49, 58]. In this work, additional

experiments were conducted to confirm its presence. Moreover, computational

studies were also performed using density functional theory (DFT) in order to

better understand the interactions between chemical species and catalyst

surface, and to propose thermodynamically feasible steps which lead to the

formation of cyclopentanone.

5.2 Methods

5.2.1 Experimental

The experiments were conducted in the continuous flow system under the

standard operating conditions described in CHAPTER 2. During the study, the

space velocity was varied by adjusting both guaiacol feed rate and catalyst. The

same product analysis and calculations methods as described in CHAPTER 2

were followed. In addition, 13C NMR analysis of the liquid product was conducted

for some cases, using the Bruker ARX400.

5.2.2 Computational

DFT calculations were performed using the periodic plane-wave-based code

Vienna Ab-initio Simulation Package (VASP) [79, 80] via the projector

augmented wave (PAW)[81, 82] approach for ionic cores and PW91[83] form of

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exchange-correlation functional at generalized-gradient approximation level. A

cutoff energy of 400 eV for plane-wave basis set was used and for the Brillouin

zone, a 5 × 5 × 1 k points Monkhorst-Pack mesh was sampled [84]. In all cases,

a first-order Methfessel-Paxton model with a swearing width of 0.15 eV was

applied and the total energies were calculated by extrapolating to zero

broadening [85].

An ideal Pt (111) surface was represented by a five-layer periodic (3 × 3) unit cell

slab model in a super-cell geometry with 1.4 nm vacuum spacing between them.

During the geometry optimization, the two upper Pt layers with the adsorbents

were allowed to relax while the bottom three Pt layers were fixed according to

bulk-terminated geometry. A converging criterion of 1 × 10–4 eV was used for the

self-consistent iterations, and 0.02 eV/Å for the ionic steps. Dipole corrections

were included only in the direction perpendicular to slab surface. To account for

the possible presence of unpaired electrons, spin polarization was applied for

gas phase radicals.

The binding energy (BE) is defined as:

/=ads ad slab ad slabE E E E (5.1)

A negative value of BE implies an exothermic process or a favorable interaction,

while a positive value means an endothermic process or an unfavorable

interaction.

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In order to calculate the free energy, zero-point energy corrections were applied

to each structure. The entropy correction was performed at 300 oC, and based on

the assumption that each adsorbate is a localized oscillator with only vibrational

modes.


5.3.1 Reaction pathways

To understand the reaction pathways, experiments were conducted using Pt/C

by varying the space velocity under the standard operating conditions and the

selectivity of the three major liquid compounds (phenol, catechol and

cyclopentanone) were reported in Figure 5.1. When W/F increased from 0.1 to

0.3 g cat•h/(g GUA), the selectivity of phenol increased from 30% to 42.5%, while

the selectivity of catechol decreased from 14% to 5%. The cyclopentanone

selectivity remained constant at about 19%. These results indicate that catechol

and/or guaiacol is an intermediate which may lead to the formation of phenol and

cyclopentanone. Qualitative experiments were then conducted by feeding

catechol as the reactant, where both phenol and cyclopentanone were observed

in large quantities in the liquid product. When phenol was the feed, no

cyclopentanone was observed.

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Figure 5.1 The distribution of major liquid products versus inverse space velocity for Pt/C catalyst.

Based on analysis of the results, a reaction network is proposed (Figure 5.2).

Specifically, two pathways exist for phenol production: direct demethoxylation of

guaiacol and dehydrolysis of catechol. Cyclopentanone, may be generated

directly from guaiacol, and from catechol, possibly through partial hydrogenation

of the aromatic ring; isomerization;[86, 87] Keto–enol tautomerism; [88] α-

diketones decarbonylation [89, 90] and ring-closing [91]. It is likely that some

intermediates, which are undetectable under current operating conditions, exist.

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The thermodynamic feasibilities of the proposed steps are investigated through

DFT calculations and presented in the following section (section 5.3.2).

Figure 5.2 Proposed guaiacol HDO reaction pathways for Pt/C catalyst.

Remarkably, during guaiacol hydrodeoxygenation study, cyclopentanone has

only been observed a few times by other authors during guaiacol

hydrodeoxygenation study. Its formation depends highly on the catalyst active

metal and the support [49, 58]. To confirm that cyclopentanone forms during the

reaction, an additional analytical method, 13C NMR analysis was performed for

the reaction product obtained under standard condition with Pt/C as catalyst

(Figure 5.3). By comparing the acquired spectrum with NIST database, the three

peaks at 224.9 ppm, 38.1 ppm and 22.6 ppm confirmed the existence of

cyclopentanone. The presence of phenol and catechol, as the other two major

reaction products, was also verified in this manner.

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Figure 5.3 13C NMR spectra for reaction products (solvent CDCl3).

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5.3.2 DFT calculations

Since cyclopentanone has not been observed often in guaiacol

hydrodeoxygenation studies, and it is also unclear how a six-membered ring

opens to form a five-membered ring compound, calculations based on density

functional theory were performed in an attempt to gain insight into its

thermodynamic feasibility.

First, structure of the Pt bulk was optimized and the lattice constant was found to

be 3.99 Å, which is consistent with values reported in the literature and compares

well with the experimental value (3.92 Å) [92]. Based on the steps proposed in

the previous section, the relative free energy levels of each absorbate on Pt at

300 oC were calculated and are presented in Figure 5.4. Although

cyclopentanone can be produced from either guaiacol or catechol (see Figure

5.2), their reaction mechanisms were similar. Thus, only the results with guaiacol

as reactant are presented.

It is noted that after partial hydrogenation, the free energy levels for all following

steps decrease, indicating increasing molecule stability. Thus, after the first step

the formation of cyclopentanone is thermodynamically feasible. The effect of

support is not accounted for because the structure of activated carbon is complex

and the calculation is computationally expensive, beyond the scope of this work.

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Figure 5.4 Energy level for reaction coordinate

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To verify the feasibility of the first step, adsorption of guaiacol on Pt cluster was

calculated based on DFT theory to provide information on how the compound

interacts with the metal cluster. Several plausible adsorption configurations of

guaiacol were constructed and optimized, among which four converged to the

desired level (Figure 5.5 a-d). The calculations show that the most stable

configuration of adsorbed guaiacol is in a tilted position via the para-carbon

(Figure 5.5 a), with the binding energy -16.48 kcal/mol. The other three

optimized adsorption positions (Figure 5.5 b-d) are meso-stable with binding

energies -4.94 kcal/mol, -3.70 kcal/mol, and -3.04 kcal/mol, respectively.

For the most stable structure, the Pt-C bond length is 2.15 Å, which indicates

chemisorptions [93]. The result illustrates that the proposed first step for

cyclopentanone formation, i.e. partial hydrogenation, is configurationally possible.

It is well-known that hydrogen dissociates rapidly on Pt [94]. Thus, after the

guaiacol adsorption, the dissociated hydrogen on Pt could diffuse on the metal

surface and hydrogenate the C-C bond. The structure for this partially

hydrogenated compound was also optimized (Figure 5.5 e). In this case, the

oxygen-containing groups, instead of the ring, are adsorbed on metal surface,

which could further activate the binding intermediate to react through the steps

proposed in Figure 5.4. The other two main reaction products, catechol and

phenol, could be produced from all four adsorbed guaiacol structures on Pt, or

via guaiacol adsorption on the support as described in CHAPTER 4.

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Several adsorption configurations of cyclopentanone were also optimized and the

most stable one is shown in Figure 5.5 f. The cyclopentanone molecule is

adsorbed on the Pt surface through the oxygen, with a binding energy of -5.79

kcal/mol. This implies that the adsorption of cyclopentanone on Pt cluster is weak.

Thus, cyclopentanone may desorb rapidly after its formation and has lower

probability to react further to other compounds.

5.4 Conclusions

In this chapter, the reaction pathways for Pt/C catalyzed guaiacol HDO reaction

were investigated through experiments. Plausible reaction steps for

cyclopentanone generation from guaiacol were proposed via DFT calculations.

This experimental and theoretical study provides insight into the mechanisms of

Pt-catalyzed guaiacol hydrodeoxygenation reactions and offers a basis for

investigations of other phenolic compounds present in pyrolysis bio-oils.

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(a)

(b)

(c)

BE = -16.5 kcal/mol BE= - 4.9 kcal/mol BE= - 3.7 kcal/mol

(d)

(e)

(f)

BE = - 3.0 kcal/mol BE= - 5.4 kcal/mol BE = - 5.8 kcal/mol

Figure 5.5 Schematic illustrations of (a) the most stable guaiacol configurations adsorbed on platinum slabs (b - d) other guaiacol configurations adsorbed on platinum slabs; (e) the most stable partially hydrogenated intermediate adsorbed on

platinum slabs; (f) the most stable cyclopentanone configuration adsorbed on platinum slabs.

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CHAPTER 6. REACTION KINETICS

6.1 Introduction

The work presented so far has shown that Pt/C catalyst provides better

performance than other noble metals and supports and exhibits no deactivation

for the testing period of 5 h. Its reaction network has been reported in CHAPTER

5 (Figure 5.2), and includes the following 5 sub-reactions (Eq. 6.1 – 6.5). In this

chapter, kinetic study is performed for the proposed reaction pathways. The goal

of this work is to provide more insight into the individual reaction steps.

(6.1)

(6.2)

(6.3)

(6.4)

(6.5)

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6.2 Methods

Prior to reaction experiments, the catalysts were activated for 4 h under the

following conditions: 350 oC, 1 atm, total gas flow 100 mL/min (H2:N2=1:2). The

standard reaction conditions were: 300 oC, 1 atm, 0.5 g catalyst, total gas

(H2:N2=1:1) flow rate 100 mL/min and guaiacol feed rate 0.025 mL/min (liquid, at

room temperature). Both catalyst loading and guaiacol feed rate were varied in

order to acquire data at different residence times. When the gas feed rate was

varied, the reported hydrogen flow rate was adjusted to maintain a constant

molar feed ratio, H2/guaiacol=10.

The calculations for conversion and selectivity were performed based on

equations reported in CHAPTER 2. The mass balance for each run was above

90%. The accuracy of gas flow measurements was confirmed by evaluating

nitrogen balance, with difference between inlet and outlet being below 3%. Since

it is not possible to collect all condensed liquid (liquid drops are visible on the

condenser wall), it was reasonable to assume that the total carbon and mass

losses were caused by the incomplete liquid product collection. Thus, the product

distribution was corrected by assuming 0.1 g liquid product (equivalent to 2-3

drops) was held in the condenser. After applying this correction, mass balances

for all runs were above 96%.

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To acquire the experimental data, space velocity was varied by changing both

guaiacol feed rate and catalyst weight at three temperatures (275, 300 and 325

oC) under integral operating conditions

6.3 Results and discussion

6.3.1 Absence of heat and mass transfer limitations

Before conducting the kinetics study, it is important to ensure the absence of any

mass transfer limitation and heat transfer limitations. The absence of mass

transfer limitations was verified the criteria described by Weisz and Prater, [95]

where ψ<0.05 in all cases. To confirm the absence of heat transfer limitations,

criteria for fixed-bed reactors proposed by Mears [96] were applied and the

results confirmed that there were no intrareactor, interphase or intraparticle heat

transfer limitations under the tested conditions.

6.3.2 Model selections

Three common kinetic models (i.e. power-law, Langmuir–Hinshelwood and

Rideal – Eley mechanisms) were evaluated. For the two adsorption based

models, a large number of parameters exist which may significantly decrease the

reliability of the data fitting. Also, after applying several adsorption/dissociative

mechanisms in attempts to describe the experimental values, the results were

unsatisfactory. When the power-law kinetics model was applied, however, good

fitting results and reasonable reaction kinetics parameters were obtained.

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Therefore, the power-law kinetic model was selected to describe the reaction

system.

Figure 6.1 Fitting results of guaiacol conversion based on second-order kinetics.

First, guaiacol conversion was evaluated to obtain the reaction order for Eq. 1, 4

and 5. The design equation for a plug-flow packed-bed reactor was integrated

based on the assumption that the reaction order was zero, one, two or three. The

results showed that good fitting was achieved when the reaction order was 2

(Figure 6.1). Thus, second-order model appears to be appropriate to describe

guaiacol conversion. Runnebaum et al. have proposed a first-order model for

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guaiacol conversion, where Pt/Al2O3 was tested under differential condition [97].

For the present work, however, the fitting results based on first-order kinetics

were unsatisfactory.

Therefore, reactions 1, 3 and 4 are assumed to be second order with respect to

guaiacol. Different reaction orders were also investigated for reaction 2 and 5,

and first order with respect to catechol provided the best fitting results.

6.3.3 Results and discussions

Based on the design equation for the packed-bed reactor and the reaction

network, the formation/consumption rates for each of the major components are

listed in Eq. 6 – 9. Since excess hydrogen is used, its partial pressure can be

considered constant during the entire reaction and lumped into the rate constants.

For given conditions, these differential equations were solved using the MATLAB

ode45s subroutine. Meanwhile, the difference between the calculated

concentration profiles as functions of the residence time and the experiment data

were minimized using the non-linear fitting subroutine fmincon, and the optimum

kinetic parameters were determined. To increase the accuracy of mathematical

fitting, during the process, partial pressures for all compounds were normalized

based on the initial guaiacol partial pressure.

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(6.6)

(6.7)

(6.8)

(6.9)

The method was applied to the three temperatures (275oC, 300oC and 325oC)

separately to acquire the reaction rate constants. Figure 6.2, which is typical,

shows the normalized partial pressure at 300 oC with respect to the inverse

space velocity. Good agreement was reached between the experimental data

(represented by points) and the calculated results (represented by curves) for all

cases.

Figure 6.3 summarizes the goodness of fit in a parity plot for each component at

all three temperatures. The values for all components are close to the diagonal

line and relatively evenly distributed on both sides, indicating good fit. The

obtained rate constants are listed in Table 6.1.

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Figure 6.2 Fit of kinetic data at 300oC.

Table 6.1 The reaction rate constants.

Temperature 275 oC 300 oC 325 oC

k1 x 10-4 ( gGUA/(gcat•h•atm)) 0.14 0.70 1.37

k2 x 10-2 (gGUA /( gcat •h)) 0.31 1.05 1.94

k3 x 10-4 ( gGUA/(gcat•h•atm)) 0.31 0.86 1.71

k4 x 10-4 ( gGUA/(gcat•h•atm)) 0.11 0.70 1.67

k5 x 106 (gGUA /( gcat •h)) 0.59 1.67 5.85

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Figure 6.3 Parity plot for major compounds at 275, 300 and 325 oC.

Based on the obtained rate constants at different temperatures, the effect of

temperature was evaluated to calculate the activation energies for the various

reactions. Using Arrhenius law and linear regression (Figure 6.4), activation

energies for the five sub-reactions were obtained and are listed in Table 6.2

along with the corresponding R2 values.

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Figure 6.4 Arrhenius plots for the rate constants.

Table 6.2 Activation energy values.

E1 E2 E3 E4 E5

Activation Energy (kJ/mol) 125.5 99.8 92.7 149.0 124.6

R2 value 0.98 0.98 0.99 0.98 0.99

Direct consumption of guaiacol occurs in reactions 1, 3 and 4 and forms catechol,

phenol and cyclopentanone, respectively. The rate constants obtained for the

three reactions are in the same order of magnitude. The results show while that

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k3 is almost twice as large as k1 and k4 at 275oC, it becomes similar to k1 and k4

at 325 oC because of its lower activation energy. Regarding catechol

consumption, it is noted that k2 is much larger than k5, indicating that the majority

of the reacted catechol forms phenol.

The apparent activation energy of guaiacol was also calculated for comparison

with literature values. Based on the values of k1, k3 and k4 obtained at the three

temperatures, the value was determined to be 116.8 kJ/mol. It is higher than the

values reported for Co-Mo, Ni-Mo and Ni-Cu catalysts, which are 71.2, 58.7 and

89.1 kJ/mol, respectively, [98, 99], and the values reported for a series of metal

phosphide catalysts, which are in the range of 40 – 65 kJ/mol [43]. The difference

is likely due to the catalyst nature (noble metal versus others) which leads to

different reaction pathways and deactivation profiles, since it has been reported

that the formation of condensed-ring compounds has lower activation energy as

compared to hydrogenation and oxygenation reactions [99]. Owing to lack of

literature data for Pt catalyst, the activation energy value reported in this work

cannot be compared directly.

Based on the data analysis, the consumption of guaiacol appears to be second-

order. A possible explanation is that under the operating conditions, adsorption of

guaiacol is the rate controlling step. Liu and Shou have derived the adsorption

rate equation based on Langmuir kinetics, and found that depending on the

relative values of initial concentration, maximum adsorption capacity, dosage of

adsorbent and equilibrium constant, the adsorption rate may appear to be

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second order with respect to the adsorbate [100]. This could explain the apparent

second-order reaction for guaiacol conversion observed in this study.

6.4 Conclusions

In this chapter, reaction kinetics study was conducted under integral conditions at

three temperatures (275, 300 and 325 oC). The power-law model was found to

describe the kinetics well, and the rate constants and activation energies were

obtained for all sub-reactions in the network. The apparent activation energy for

guaiacol conversion was also calculated and compared with the reported values.

This kinetic study provides insight into Pt-catalyzed guaiacol hydrodeoxygenation

mechanisms, and serves as a basis for investigations of other phenolic

compounds present in pyrolysis bio-oils.

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CHAPTER 7. DEACTIVATION STUDIES

7.1 Introduction

Catalyst stability is a key factor for its successful industrial application; however,

limited research has been conducted in this direction for the bio-oils upgrading

process during which catalyst deactivation is a prevalent issue [43, 44, 46, 48,

101, 102]. In the previous chapters, it has been reported that catalyst stability is

affected by many factors, e.g. catalyst metal, support and operating conditions.

Generally, there are two main possible causes of catalyst deactivation in guaiacol

HDO reaction: (1) thermal degradation (i.e. sintering), and (2) coking [103]. In this

chapter, the mechanisms of catalyst deactivation are investigated through

detailed characterization of selected catalysts.

7.2 Characterization methods

Both Ru/C and Pt/C catalysts were selected to understand the deactivation

mechanism, since Ru/C had significant deactivation while it was negligible for the

Pt/C catalyst (see Figure 2.3).

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Scanning electron microscopy (SEM, FEI Philips XL-40) and Transmission

electron microscopy Transmission electron microscopy (TEM, FEI Titan 80-300)

were used to investigate the morphology and metal particle sizes of catalysts.

Used catalysts were treated with dichloromethane to identify compounds

deposited on the catalyst surface. To prepare the samples, 600 mg of

dichloromethane was added to 100 mg of used catalyst. The solution was then

mixed and centrifuged to separate the solid and liquid phases. The liquid phase

containing the deposits was analyzed using a Gas Chromatograph/Mass

Spectrometry system (GC/MS, LECO Pegasus 4D GCxGC-TOF).

Thermalgravimetric analysis under the flow of nitrogen was conducted in a TGA

(TA Q500). During the analysis, temperature was increased from room

temperature to 100 oC, stabilized for 30 min to remove moisture, and then

increased to 600 oC at rate 10 oC /min.


7.3.1 Thermal degradation

As reaction temperature increases, sintering causes a decrease in the catalyst

surface area available for reaction through metal crystallite growth and the

disruption of the structure of the catalyst support material [103]. To investigate

possible changes in the catalyst support, BET analysis was conducted for fresh

catalysts tested in the present work. As seen in Table 2.1, Pt/C and Ru/C show

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similar values of surface area and pore diameter. The BET surface area for used

Ru/C (after reaction), which showed the most significant deactivation among the

tested catalysts, was also analyzed. It was found that there was no significant

change (< ± 7%) in surface area after the reaction for this catalyst. The images

from SEM (Figure A.1) showed that for both Pt/C and Ru/C, the support surface

is more rounded for the used samples as compared to the fresh ones which,

however, does not alter the surface area.

To analyze metal crystallite growth, particle sizes of the active metal in the

catalysts were measured based on TEM images for used Pt/C and Ru/C (Figure

7.1). It is known that metal catalyst on support sinters at elevated temperatures

and causes deactivation. The average sizes of metals for both fresh and used

Pt/C and Ru/C were determined. In order to ensure that the population variance

is not significantly different from that of the test, more than 100 metal particles in

each catalyst were measured. The average sizes of metal particles for both

catalysts (Pt/C and Ru/C) increased slightly after reaction, indicating that some

metal sintering occurred during the HDO reaction. For Pt/C and Ru/C, average

sizes increased from 2.40 ± 0.54 to 2.67 ± 0.62 nm and from 2.56 ± 0.47 to 2.87

± 0.63 nm, respectively. Although similar levels of sintering were determined for

both catalysts, significant deactivation was observed for Ru/C while little

deactivation for Pt/C (Figure 7.1). This result suggests that sintering is not the

primary cause of catalyst deactivation for Ru/C catalyst. This may be expected

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because the reaction temperature 300 oC is significantly lower than Tammann

temperature for both Pt (750 oC) and Ru (990 oC) [104].

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Figure 7.1 TEM images for (a) fresh Pt/C, (b) used Pt/C, (c) fresh Ru/C, and (d) used Ru/C catalysts, and the corresponding particle size distributions for (a‟)

fresh Pt/C, (b‟) used Pt/C, (c‟) fresh Ru/C, and (d‟) used Ru/C catalysts.

7.3.2 Coking

Another main cause of catalyst deactivation is coking, which refers to deposition

of polymerized heavy hydrocarbons [103]. In the present study, two

characterization techniques were utilized to identify and quantify the coke

formation.

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First, to identify the compounds absorbed/deposited on catalyst during reaction,

dichloromethane was used as solvent to treat the catalyst samples. This

approach was originally designed to identify hydrocarbons and oxygenated

compounds deposited on zeolite catalysts [105]. Dichloromethane dissolves the

deposits on the catalyst which enables the characterization of their chemical

composition. The method is suggested to be appropriate for analyzing coke

deposits formed below 350 °C [106]. In this study, used Ru/C and Pt/C catalysts

were treated with dichloromethane and the obtained solutions (SRu and SPt) were

analyzed using GC/MS. Although quantitative data could not be obtained from

this analysis, the relative quantities of major components in the deposits were

determined.

More than 20 different aromatic compounds were identified in both SRu and SPt.

Based on their tendency to form coke, the observed compounds can be

categorized into two groups: linked ring series (e.g. biphenyl; benzene, 1,1'-(1,4-

butanediyl)bis-) and condensed ring series (e.g. naphthalene; naphthalene, 1-

methyl-) (Figure 7.2). It has been reported that aromatic compounds in the

condensed ring series lead to more rapid coke formation as compared to the

linked ring types [65]. Quantitative data cannot be obtained from this analysis

because it is not feasible to obtain the GC response factors for all the

compounds detected. A comparison of relative peak areas for the soluble coke

deposits can, however, still provide some insights. The following discussion is

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based on the assumption that the GC response factors for all compounds are the

same.

Figure 7.2 Examples of compounds observed.

Biphenyl was the most abundant compound observed in both SRu and SPt. Since

it was not detected in the liquid products, it is likely that the compound is mostly

adsorbed on the catalyst surface. Also, the concentration of biphenyl in SPt was

more than twice that in SRu. This result indicated that biphenyl, a linked-ring

aromatic compound, is not primarily responsible for catalyst deactivation since

little deactivation was observed for Pt/C. Among the condensed ring compounds

detected, naphthalene was the most abundant in both SRu and SPt. Significantly

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higher (4-5) concentration of naphthalene was observed in SRu as compared to

SPt. Based on these results, it can be suggested that naphthalene and possibly

larger condensed ring compounds originated from it, are the main deposited

components which result in deactivation of the Ru/C catalyst.

To further confirm coke formation and the different types of aromatic deposits on

used catalysts, TGA was performed under nitrogen flow for both fresh and used

samples for Pt/C and Ru/C catalysts (Figure 7.3). Both fresh catalyst samples

were reduced before the TGA analysis. The different weight loss for the two fresh

catalysts under increasing temperature are likely due to differences in textural

and chemical properties of the activated carbon support [59, 107]. Further, with

increasing temperature, coke deposited on used catalysts desorbs and leads to

more weight loss as compared to fresh catalysts. By comparing the weight loss

profiles between the fresh and used catalysts, information about the coke

deposits can be obtained.

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Figure 7.3 TGA patterns for Pt/C and Ru/C catalysts.

For Pt/C catalyst, noticeable desorption of coke initiated at 130 oC, but for Ru/C

catalyst, it started at 250 oC. This result indicates that more weakly-bonded coke

is present on Pt/C catalyst as compared to Ru/C. As temperature increases, coke

continues to desorb from both catalysts. At 400 oC, for example, the weight

losses for Pt/C and Ru/C catalysts were 6 wt% and 2.6 wt%, respectively. Due to

limitation of the instrument, experiments were only performed below 1000 oC and,

at this temperature, similar weight (~ 17%) of coke desorption is observed from

both catalysts, which is comparable to other reported values [55, 56]. Those

results indicate that more coke desorbed from used Ru/C catalyst at higher

temperatures as compared to the used Pt/C catalyst.

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This result is consistent with the observation from the dichloromethane

dissolution method. Being less strongly adsorbed, the linked-ring compounds are

likely to be responsible for the weight loss observed at lower temperature, while

the more strongly adsorbed condensed-ring products desorbed at higher

temperatures [103]. The heavier condensed-ring compounds which have low

solubility in dichloromethane exist more on used Ru/C catalysts than on used

Pt/C, which explains the different TGA profiles for the two catalysts.

7.4 Conclusions

The deactivation mechanism of catalyst in guaiacol HDO reaction was studied

using Ru/C and Pt/C catalysts. Based on the results from dichloromethane

dissolution and thermogravimetric analysis for both fresh and used samples, it

can be concluded that polyaromatic deposits, particularly condensed ring series

compounds, are the main cause of Ru/C catalyst deactivation.





Chemical Society.

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CHAPTER 8. CONCLUSIONS AND RECOMMENDATIONS FOR FUTURE WORK

8.1 Summary

Guaiacol, which contains two oxygen-containing groups and represents a large

fraction of pyrolysis bio-oils, was selected as a model compound to study the

catalytic hydrodeoxygenation of bio-oils. The main results obtained in this work

include the following aspects.

8.1.1 Catalyst screening and optimization of reaction conditions

Four noble metals (Pt, Ru, Pd and Rd) supported on activated carbon were

selected for the active metal screening. The experiments were conducted under

atmospheric pressure using a fixed-bed reactor. Four criteria were applied in

evaluating the catalysts‟ performance, namely (1) high deoxygenation activity, (2)

low hydrogenation activity, (3) high liquid carbon recovery, and (4) high catalyst

stability.

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The catalysts were compared under standard operating conditions (300 oC, H2

flow 50cc/min, guaiacol feed rate 0.025 mL/min at room temperature) for a period

of 5 h. The results showed that Pt/C catalyst offers the highest guaiacol

conversion and the highest stability, and the major reaction liquid products are

phenol, catechol and cyclopentanone.

Furthermore, the effect of catalyst support was investigated using Pt supported

on alumina, silica and activated carbon. The results showed that activated

carbon supported catalyst offers the highest activity and stability, while the silica

counterpart has the lowest activity and the alumina catalyst deactivates. Based

on the reaction results and literature studies, a plausible reaction mechanism

starting from the adsorption of guaiacol on catalyst surface was proposed to

interpret the variance in the catalyst activity on different supports.

The operating temperature of Pt/C, which is the superior catalyst, was further

optimized. Catalyst performance under three temperatures (275, 300 and 325 oC)

was investigated. It was found that guaiacol conversion increases with increasing

temperature; however, a slight deactivation and relatively lower liquid carbon

recovery were observed at 325 oC. Thus, the operating temperature of 300 oC

was determined to be optimal for Pt/C catalyzed guaiacol HDO reaction.

8.1.2 Catalyst deactivation study

Catalyst deactivation is an inevitable issue in pyrolysis bio-oils upgrading. In this

study, it was found that deactivation is affected by the properties of active metal

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and catalyst support, as well as the operating temperature. Two catalysts, Pt and

Ru supported on activated carbon, were selected to investigate deactivation

mechanism because Pt/C showed little deactivation in a testing period of 5 h,

whereas Ru/C had significant deactivation under the same operating conditions.

The fresh and used catalysts were characterized using various techniques, such

as physisorption/chemisorption, SEM, TEM, dichloromethane dissolution method

and thermal gravimetric analysis. Two types of common deactivation

mechanisms (thermal degradation and coking) were investigated. The analytical

results showed that coking, particularly via the deposition of condensed-ring

compounds, is responsible for the deactivation of Ru/C catalyst.

8.1.3 Reaction pathways and kinetics study

For Pt/C catalyst, there are three main reaction products (i.e. phenol, catechol

and cyclopentanone). Experiments were conducted with different feed

compounds and at various space velocities. Based on the results, the simplified

reaction network including 5 sub-reactions was proposed.

Among the three major products, cyclopentanone was typically not observed in

the prior literature. Thus, its existence was confirmed using 13C NMR technique

in addition to GC-MS. For its formation from guaiacol, plausible steps were

proposed and supported via density functional theory calculations.

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For the proposed overall reaction pathways, kinetic study was conducted under

integral conversion conditions. The power-law model was found to describe the

kinetics well and the reaction orders with respect to guaiacol and catechol were

determined based on the experimental data. Moreover, rate constants and

activation energies were obtained for all sub-reactions in the network. The

apparent activation energy of guaiacol conversion was also calculated and

compared with the values reported in the literature.

8.2 Recommendations for future work

8.2.1 DFT calculations for guaiacol HDO

Density functional theory is a powerful tool to understand the interactions

between reaction compounds and the catalyst surface. For reactions with smaller

molecules, such as ammonia synthesis, reasonable agreement was reached

between the quantum chemical computational results and the experimental data

[108]. In this work, the computational results support the proposed reaction steps

for cyclopentanone formation. With more investment in time and resources, the

computation could be extended to obtain a more comprehensive understanding

of the reaction mechanism of Pt catalyzed guaiacol HDO reaction.

A few possible future research directions include (1) determining the transition

states for each reaction step in cyclopentanone (and other products) formation,

(2) optimizing the adsorption configuration of each reaction compound on Pt

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surface and eventually (3) computing the reaction kinetic parameters to be

compared with experimental data.

In addition, the computational approach may also serve as a screening tool in

searching for more effective catalysts with desired activities.

8.2.2 Phenol production from bimetallic catalysts

Phenol is an important petrochemical in the plastics industry and is typically

manufactured from crude oil. The phenolic nature of lignin make it a potential

renewable resource for phenol production. As compared with the price of

gasoline, the bulk price of phenol is approximately 7 times higher. This means

that it may be more profitable to produce phenol rather than fuel from biomass. In

this work, it was found that phenol could be generated from guaiacol over noble

metal catalysts. The selectivity of phenol may be improved through the

application of bimetallic catalysts.

It is known that bimetallic catalysts offer improved activity and selectivity as

compared to monometallic catalysts. The addition of a second metal may alter

the activity of the monometallic catalysts in the following aspects: (1) electronic

effect by alloys, (2) geometric rearrangement, and (3) mixed-sites with dual

functionalities [109].

The addition of the second metal, along with optimization of reaction conditions,

may improve the phenol selectivity. Specifically, the metal candidate needs to

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increase the selectivity of Ar-OMe bond cleavage with minimal activities in Ar-OH

bond breaking and ring hydrogenation. Based on preliminary literature search,

the following metals are promising candidates:

Table 8.1 Metal candidates for bimetallic catalyst research.

Metal Potential advantages Examples References

Sn

Improve catalyst stability

Improve catalyst activity

Targeting C-O bond, not C=C bonds

RuSn

PtSn [48, 110]

Mo Improve the selectivity of C-O hydrogenolysis

over aromatic ring hydrogenation

RuMo

PdMo [111]

Fe Improve C-O cleavage PdFe

RhFe [112]

Co

Modify adsorption strength for chemical

groups

Improve reaction rate

PdCo

PtCo [109, 113]

8.2.3 Other bio-oils model compounds

The ultimate goal of studying catalytic HDO process with a model compound is to

apply the technology in the upgrading of pyrolysis bio-oils as a whole. Thus, it is

of great importance to investigate the activity of the promising catalyst(s) on other

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model compounds which have different chemical groups and are also abundant

in the bio-oils.

Two examples of the other model compound candidates are furan and ketone.

The fixed-bed continuous system used in this study could be of use in testing the

catalysts‟ activity on their hydrodeoxygenation reactions. Similarly, catalyst

activity, reaction kinetics and mechanism should be investigated and compared

with those from the guaiacol counterpart.

Once the reactivity of the model compounds containing different oxygen-

containing functional groups is evaluated individually, the next step is to study the

catalytic HDO reactions involving a mixture of all model compounds in order to

understand their interactions and competition. Eventually, the reaction kinetic

and mechanism study of catalytic upgrading of both individual and mixture of

model compounds would lead to the understanding and development of an

optimized and integrated reaction system for bio-oils upgrading.

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APPENDIX

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APPENDIX

Figure A.1 SEM images for (a) fresh Pt/C, (b) used Pt/C, (c) fresh Ru/C, and (d) used Ru/C catalysts.

(a) (b)

(d) (c)

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VITA

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VITA

Danni Gao received her Bachelor‟s Degree in Chemical Engineering and

Industrial Biological Engineering from Tsinghua University in China in 2009.

During her undergraduate studies, she conducted research on the catalyst

preparation and reaction kinetics of thiophene desulfurization under the

supervision of Prof. Yujun Wang. In addition to academic work, Danni also had

the opportunity to serve as the Main Desk Receptionist in the International

Broadcast Center during the Beijing 2008 Olympics.

Immediately after receiving her Bachelor‟s Degree, Danni joined the School of

Chemical Engineering at Purdue University to pursue her PhD degree. Under the

supervision of Prof. Arvind Varma, she first worked on regeneration of ammonia

borane from spent fuel for hydrogen fuel cells, and then on catalytic

hydrodeoxygenation of guaiacol over noble metal catalysts, which is the focus of

her thesis. During her studies, Danni has published one article in Industrial &

Engineering Chemistry Research, and submitted another manuscript for

publication. Danni has presented her work at several national conferences, such

as the AIChE Annual Meetings in Fall 2012 and 2013, ACS Annual Meeting in

Fall 2013, NASCRE Meeting in the Spring 2013 and has received several travel

grants. Her research work was recognized with awards at the 2013 AIChE

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and the 2013 and 2014 CACS (Chinese American Chemical Society) Annual

Meetings. Selected by GasUnie/KEMA among applicants from all over the world,

Danni participated in the „NRG Battle‟ during the 25th World Gas Conference in

Kuala Lumpur, Malaysia, and worked on an innovative project „Power to Gas‟.

During her time at Purdue, Danni was also involved in various activities and

services, including serving as an Energy Ambassador for the Purdue Energy

Center, the Publicity Chair of Chemical Engineering Graduate Student

Organization and the Safety Coordinator for the research group of Prof. Varma.

She also volunteered at the Lafayette Adult Resource Academy to tutor adult

learners preparing for their GED in Math and Science. Danni will graduate with a

Ph.D. degree in Chemical Engineering in Fall 2014, and will join Shell Oil

Company as a Research Engineer in Houston, TX.

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