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January 2014
Catalytic Hydrodeoxygenation of Guaiacol overNoble Metal CatalystsDanni GaoPurdue University
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Recommended CitationGao, Danni, "Catalytic Hydrodeoxygenation of Guaiacol over Noble Metal Catalysts" (2014). Open Access Dissertations. 1498.https://docs.lib.purdue.edu/open_access_dissertations/1498
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Thesis/Dissertation Acceptance
This is to certify that the thesis/dissertation prepared
By
Entitled
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Is approved by the final examining committee:
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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
3.3 Results and discussions .............................................................. 32
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
4.3 Results and discussions .............................................................. 43
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 Results and discussions .............................................................. 52
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 Results and discussions .............................................................. 73
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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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
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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
13
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
14
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
15
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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16
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
17
17
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
19
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.
20
20
Figure 2.1 Schematic diagram of the experimental setup.
21
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.
22
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
23
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
24
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%)
25
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.
26
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
27
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.
28
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
29
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.
30
30
Figure 2.4 Distribution of major products for the four carbon supported metal catalysts.
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31
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).
32
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)
3.3 Results and discussions
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
33
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
34
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
35
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.
36
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
37
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.
38
38
Figure 3.4 Selectivity of major products at different temperatures.
39
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.
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.
40
40
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.
41
41
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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42
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.
43
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.
4.3 Results and discussions
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].
44
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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48
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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51
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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52
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 Results and discussions
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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58
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 Results and discussions
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.
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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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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85
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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86
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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89
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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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COPYRIGHT PERMISSIONS
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