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The Pennsylvania State University

The Graduate School

Department of Chemical Engineering

THE DESIGN OF A SHAPE SELECTIVE PLATINUM-CARBON CATALYST

WITH HIGH EFFECTIVENESS

A Dissertation in

Chemical Engineering

Maryam Peer Lachegurabi

 2014 Maryam Peer Lachegurabi

Submitted in Partial Fulfillment of the Requirements for the Degree of

Doctor of Philosophy

May 2014

The dissertation of Maryam Peer Lachegurabi was reviewed and approved* by the following:

Henry C. Foley Professor of Chemical Engineering Dissertation Advisor Chair of Committee Executive Vice President for Academic Affairs, University of Missouri System

Andrew Zydney Professor of Chemical Engineering Chemical Engineering Department Head

Darrell Velegol Professor of Chemical Engineering

Chunshan Song Professor of Fuel Science and Chemical Engineering Director of EMS Energy Institute

*Signatures are on file in the Graduate School

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ABSTRACT

Polyfurfuryl alcohol forms an amorphous non-graphitizing carbon upon pyrolysis at temperatures higher than 600 °C. This carbon is inherently microporous with the average pore size of 0.5 nm. The mean pore size and total porosity of PFA-derived carbon can be shifted and enhanced using different techniques either during polymerization (adding pore forming agents) or after carbonization (oxidation using CO2). The narrow pore size distribution which is a unique characteristic of this carbon, provides selective adsorptive properties and hence size and shape selectivity. Mass transport of bulky and non-planar molecules is significantly hindered within the slit-like ultra micropores of PFA-derived carbon. Because of having large surface area and high thermal and chemical stability, PFA-derived carbon can be utilized in a wide variety of applications such as gas separation, catalysis and energy storage.

Heterogeneous catalysis is one of the most promising areas in which carbon offers several advantages. High hydrothermal stability and inert surface chemistry of PFA-derived carbon grant its long active life under harsh reaction conditions such as extremely high or low pH. On the other hand, shape selective catalysts with much higher sintering resistance compared to conventional impregnated catalysts, could be synthesized by embedding active metal nanoparticles within the carbon microstructure. These catalysts selectively convert those molecules that are able to diffuse into the pores, or promote the reaction in specific pathways that lead to the formation of molecules which are able to diffuse out. Due to the clean surface of the carbon pores, undesired side reactions, which are the main reason for decreased selectivity on zeolite-based catalysts, are avoided. However, engineering the textural and morphological properties of PFA-derived carbon (catalyst support) is the main challenge on the way to commercialization of these types of shape selective catalysts. In this study, based on the well-

iv known fundamentals of heterogeneous catalysis, practical approaches are developed and utilized in order to design the optimum catalyst for liquid phase hydrogenation reactions.

An emulsion polymerization approach using pluronic F-127 as the structure-directing agent and furfuryl alcohol as the monomer was utilized to prepare poly(furfuryl alcohol) spheres.

Upon pyrolysis at 800 °C, the spheres transform to microporous carbon. By controlling the polymerization conditions, the diameter of the spheres, hence the diffusion length could be varied. Carbon spheres with the average size of a few microns all the way down to 50 nm were synthesized. In next step, platinum nanoparticles were embedded within the microstructure of the carbon spheres through emulsion polymerization of furfuryl alcohol in the presence of pre-formed

Pt nanoparticles. The synthesized embedded catalyst was evaluated in liquid phase hydrogenation of linear and branched alkenes and showed high selectivity towards hydrogenation of smaller linear alkenes compared to larger and more branched ones. To improve the embedded catalyst activity, diffusion length, total porosity and average pore size were varied by varying the emulsion polymerization parameters, using pore forming agents and selective oxidation of carbon in CO2 stream. Catalyst activity was improved by more than one order of magnitude by optimizing the morphological and textural properties of the carbon support.

The experimental catalysis data were analyzed and modeled with a transient reaction- diffusion equation within the ultramicropores, in order to obtain kinetic, adsorption and diffusion parameters. The results showed that those reactions that are happening within the confined spaces of the pores are significantly different from the reactions occurring on the outer surface of a supported conventional catalyst. The force field of the pore walls and the steric hindrance imposed by micropores, stabilize the molecules and increase the adsorption equilibrium constants and reaction rate constants. As the molecule size and bulkiness increases, the effect of confinement becomes more pronounced. It was shown that intrinsic forward reaction rate

v constant for 2-methyl-1-pentene hydrogenation within the micropores is almost one order of magnitude larger than it is for the same reaction over the supported catalyst.

In order to tailor carbon-based catalysts for reactions that involve bulkier molecules, such as poly-aromatic hydrocarbons or biomass-derived chemicals, the presence of mesoporosity is essential to lower the mass transfer limitation imposed by the microporous nature of the carbons.

In this study, a simple one step method that can be used to synthesize a carbon with a bimodal pore size distribution is presented. Simultaneous polymerization of furfuryl alcohol and phloroglucinol-formaldehyde in the presence of a structure directing agent (Pluronic F-127) was carried out and the resultant polymer was pyrolyzed to yield the bimodal carbon. The mean micro and mesopore size in this carbon could be varied by varying the surfactant and acid concentrations and monomer composition.

TABLE OF CONTENTS

List of Figures ...... ix

List of Tables ...... xiii

Acknowledgements ...... xiv

Chapter 1 Polymer-derived porous carbons: An overview ...... 1

1.1. Introduction to polymer-derived carbons ...... 1 1.2. Polyfurfuryl alcohol-derived carbon ...... 3 1.3. Pyrolysis procedure and evolution of porosity in PFA-derived carbon ...... 5 1.4. Control of porosity ...... 8 1.4.1. Activation ...... 8 1.4.2. Pore forming agents ...... 10 1.4.3. Hard and soft templating ...... 12 1.5. Application of porous carbon ...... 15 1.5.1. Gas adsorption and storage ...... 15 1.5.2. Membrane separation ...... 17 1.5.3. Heterogeneous catalysis ...... 18 1.6. Organization of Thesis ...... 24 1.7. References ...... 26

Chapter 2 Experimental Procedures ...... 39

2.1. Synthesis ...... 39 2.1.1. Synthesis of PFA-derived microporous carbon spheres ...... 39 2.1.2. Synthesis of Platinum nanoparticles with controlled size ...... 40 2.1.3. Synthesis of supported and embedded carbon-based catalyst ...... 41 2.1.4. CO2 Activation ...... 42 2.1.5. Synthesis of mesoporous carbon using soft-templating approach ...... 42 2.1.6. Synthesis of carbon with bimodal porosity ...... 43 2.2. Characterization ...... 44 2.2.1. Nitrogen adsorption ...... 44 2.2.2. Methyl chloride adsorption ...... 45 2.2.3. X-ray diffraction (XRD) ...... 45 2.2.4. Scanning Electron Microscopy (SEM) ...... 46 2.2.5. Pulse Chemisorption ...... 47 2.2.6. Transmission Electron Microscopy (TEM) ...... 47 2.2.7. Fourier Transform Infrared Red (FTIR) Spectroscopy ...... 48 2.2.8. Thermal Gravimetric Analysis (TGA) ...... 48 2.2.9. Gas chromatography ...... 48 2.3. Catalytic tests ...... 49 2.4. References ...... 50

Chapter 3 Surfactant-assisted polymerization of furfuryl alcohol ...... 51

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3.1. Introduction ...... 51 3.2. Experimental ...... 53 3.2.1. Synthesis of PFA-derived carbon spheres ...... 53 3.2.2. Characterization of synthesized carbons ...... 54 3.3. Results ...... 54 3.3.1. Pseudo-ternary phase diagram and carbon microstructure ...... 54 3.3.2. Effect of monomer (FA) concentration ...... 57 3.3.3. Effect of surfactant concentration ...... 58 3.3.4. Effect of initiator (acid) concentration ...... 59 3.3.5. Effect of solvent composition...... 62 3.4. Discussion ...... 65 3.5. Conclusions ...... 69 3.6. References ...... 70

Chapter 4 Shape selective carbon spheres-based catalyst: Synthesis and Application ...... 74

4.1. Introduction ...... 74 4.2. Experimental ...... 77 4.2.1. Materials ...... 77 4.2.2. Catalyst synthesis ...... 77 4.2.3. Catalyst Characterization ...... 79 4.2.4. Catalytic tests ...... 80 4.3. Results ...... 81 4.3.1. Supported catalyst properties and performance ...... 81 4.3.2. As-synthesized embedded catalyst properties and performance ...... 84 4.3.3. Enhancing the activity of embedded catalyst ...... 90 4.4. Discussion ...... 97 4.5. Conclusion ...... 103 4.6. References ...... 104

Chapter 5 Effect of confinement in nanopores on reaction kinetics and adsorption parameters ...... 108

5.1. Introduction ...... 108 5.2. Analysis of hydrogenation reaction data ...... 110 5.2.1. Alkene hydrogenation kinetics over supported catalyst ...... 110 5.2.2. Transient diffusion-reaction in embedded catalyst ...... 111 5.3. Results and discussion ...... 113 5.3.1. Supported catalyst ...... 113 5.3.2. Embedded catalyst ...... 115 5.4. Conclusion ...... 124 5.5. Nomenclature ...... 125 5.6. References ...... 126

Chapter 6 Synthesis of carbon with bimodal porosity ...... 129

6.1. An overview on hierarchical/bimodal carbon synthesis and its application ...... 129 6.2. Experimental ...... 131 6.2.1. Synthesis of homo-polymers, polymer blends and polymer mixtures ...... 131

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6.2.2.Characterization of synthesized polymers and carbon ...... 132 6.3. Results ...... 133 6.3.1. Characterization of homopolymer derived carbons ...... 133 6.3.2. Characterization of bimodal porous carbons ...... 137 6.4. Discussion ...... 141 6.5. Conclusion ...... 147 6.6. References ...... 149

Chapter 7 Conclusions and future directions ...... 153

7.1. Conclusions ...... 153 7.2. Future directions ...... 155 7.3. References ...... 158

LIST OF FIGURES

Figure 1-1. Schematic of folded graphite-like layers in an amorphous carbon [35]...... 2

Figure 1-2. Different molecular structures formed during polymerization of FA...... 4

Figure 1-3. Evolution of carbon compounds during pyrolysis. (A) methylene bridge, (B) backbone conjugation, (C) 2,5 carbons, (D) 3,4 carbons, (E) terminal methyl group, (F) cross-links, (G) polyaromatic domain, (H) carbonyl group [2]...... 5

Figure 1-4. Schematic of proposed structures for a) graphitizing and b) non-graphitizing carbons [36]...... 7

Figure 1-5. Schematic models of graphene layers with non-hexagonal rings [33]...... 8

Figure 1-6. Effect of activation time on pore size distribution and total pore volume of the PFA-derived NPC (15, 42, 69 and 84 wt% burn-off) [3]...... 10

Figure 1-7. Pore size distribution of PFA-derived carbon templated with 25 wt% PEG having different molecular weights [44]...... 11

Figure 1-8. Different types of shape selectivity in heterogeneous catalysis [119]...... 20

Figure 1-9. a) Platinum nanoparticles kinetically frozen within PFA-derived carbon, b) embedded catalyst activity in hydrogenation of alkenes [98]...... 23

Figure 3-1. Pseudo-ternary phase diagram of solvent/surfactant/furfuryl alcohol system showing morphology evolution of carbon, A: spherical particles, B: interconnected structure, C: flaky structure, D: interconnected structure and E: carbon chunks, Black points are showing the samples synthesized...... 55

Figure 3-2. Characterization of carbon spheres synthesized by emulsion polymerization, as synthesized and 45% activated, a) methyl chloride pore size distribution, b) cumulative pore volume, c and d) TEM images of carbon spheres...... 57

Figure 3-3. Effect of monomer (furfuryl alcohol) amount on average carbon particles size, All samples synthesize with 20 wt.% pluronic F-127...... 58

Figure 3-4. Effect of surfactant (Pluronic F-127) on average carbon particles size, all samples made with 9 wt.% furfuryl alcohol...... 59

Figure 3-5. Effect of initiator (HCl) molarity on average carbon particles size and morphology, red: 10.5 wt.% FA, 10.5 wt.% pluronic F-127, blue: 40 wt.% pluronic F-127, 12 wt.% FA...... 60

Figure 3-6. SEM images of samples synthesized at 40 wt.% pluronic F-127, 12 wt.% FA, 48 wt.% solvent and at different HCl molarities: a) 2 M, b) 3 M, c) 5.2 M...... 61

Figure 3-7. a) SEM and b) TEM images of carbon sample synthesized at HCl molarity of 7, 40 wt.% pluronic F-127, 12 wt.% FA, 48 wt.% solvent...... 62

Figure 3-8. Effect of ethanol and water composition in the solvent on average particles size, samples synthesized with 10.5 wt.% furfuryl alcohol and 10.5 wt.% pluronic F- 127...... 63

Figure 3-9. Effect of ethanol and water composition on the size distribution of carbon particles, samples synthesized with 10.5 wt.% furfuryl alcohol and 10.5 wt.% pluronic F-127, a and c: ethanol fraction of 0.7, b and d: ethanol fraction of 0.9...... 64

Figure 3-10. Acid catalyzed polymerization scheme of furfuryl alcohol...... 66

Figure 4-1. Comparison of XRD patterns of the supported catalysts made using alcohol reduction method at 120°C and 160°C respectively...... 82

Figure 4-2. Reactant conversion versus time on 5 wt.% Pt supported catalyst used for alkene hydrogenation reactions...... 83

Figure 4-3. a) Pore size distribution and b) cumulative pore volume of the embedded catalyst, measured by gravimetric methyl chloride gas adsorption...... 84

Figure 4-4. Comparison of the XRD patterns for embedded catalyst before and after heat treatment...... 85

Figure 4-5. a) FESEM image of carbon spheres, b) TEM, c and d) STEM images of embedded platinum nanoparticles in carbon spheres...... 86

Figure 4-6. Elemental mapping of carbon and platinum, a&b) supported catalyst, c&d) embedded catalyst...... 87

Figure 4-7. FESEM images of a) supported and b) embedded catalyst...... 88

Figure 4-8. Conversion versus time on embedded catalyst for a) different alkenes with pre-soaking, b) 1-hexene without pre-soaking...... 89

Figure 4-9. a) Pore size distribution and b) cumulative pore volume of the embedded catalyst synthesized with and without PEG addition, measured by methyl chloride gas adsorption...... 91

Figure 4-10. Conversion versus time for different alkenes on the embedded catalyst synthesized using a) PEG600 and b) PEG8000 as the pore forming agent...... 92

Figure 4-11. Effect of diffusion length (carbon sphere size) on embedded catalyst activity for a) 1-hexene, b) 2-methyl-1-pentene...... 94

Figure 4-12. Conversion versus time of different alkenes on the as-synthesized embedded catalyst and catalysts oxidized at a) 650°C and b) 700°C...... 96

Figure 4-13. Conversion versus time of 1-hexene and 2-methyl-1-pentene on the embedded catalyst synthesized with all three approaches combined...... 97

Figure 4-14. Comparison between TOF changes with conversion for 1-hexene hydrogenation over supported and embedded catalyst...... 99

103

Figure 4-15. Comparison of the effectiveness factor of the embedded catalysts synthesized using different approaches...... 103

Figure 5-1. hexene hydrogenation reaction, conversion versus time on 5 wt.% Pt supported on activated carbon spheres at different hydrogen pressures...... 115

Figure 5-2. 1-hexene hydrogenation conversion on three different embedded catalysts (markers: experimental, dashed lines: modeling results...... 117

Figure 5-3. a) Platinum nanoparticle embedded in microporous carbon, b) 1-hexene and 2-methyl-1-pentene diffusing inside carbon micropore, c)1-hexene and d) 2-methyl- 1-pentene molecule at active site inside the pore (double bond is interacting with platinum surface (dashed red arrows)...... 119

Figure 5-4. Reaction rate constants for different molecules on different catalysts...... 122

Figure 5-5. a) 1-hexene conversion-time data on supported catalyst at different temperatures, b) ln(k1) versus reciprocal of temperature...... 123

Figure 5-6. a) 1-hexene conversion-time data on embedded catalyst at different temperatures, b) ln(k1) versus reciprocal of temperature...... 124

Figure 6-1. a) Pore size distribution, b) cumulative pore volume, c and d) FESEM images of PFA-derived carbon synthesized without and with surfactant, respectively (insets: zoomed in images) ...... 134

Figure 6-2. a) Pore size distribution and b) cumulative pore volume of the Ph-C synthesized at different HCl molarities, c) FESEM and d) dark field STEM image of homo polymer of phloroglucinol (inset: bright field image) ...... 136

Figure 6-3. a) Pore size distribution and b) cumulative pore volume of PFA-Ph-C and FA-Ph-C samples, c) FESEM and STEM images of FA-Ph-C and d) FESEM image of PFA-Ph-C...... 139

Figure 6-4. a) Pore size distribution and b) pore volume of FA-Ph-C synthesized at different surfactant concentrations...... 140

Figure 6-5. a) Pore size distribution and b) cumulative pore volume of bimodal and activated bimodal carbon...... 141

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Figure 6-6. Polymerization of FA in the presence of pluronic F-127 and synthesis of FA- Plu-C (the thin white lines in the resultant carbon structure, are the micropores) ...... 143

Figure 6-7. Schematic of polymerization of phloroglucinol in the presence of pluronic F- 127 and synthesis of Ph-C...... 144

Figure 6-8. Schematic of simultaneous polymerization of phloroglucinol and FA in the presence of pluronic F-127 and synthesis of bimodal carbon (FA-Ph-C)...... 146

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LIST OF TABLES

Table 4-1. Effect of PEG addition on overall reaction rates (r) and ratio of rates (shape selectivity) of the embedded catalyst...... 93

Table 4-2. Variation of carbon spheres diameter (diffusion length) by surfactant concentration...... 93

Table 4-3. Overall reaction rates (r) and ratio of rates (shape selectivity) for different diffusion lengths ...... 94

Table 4-4. Overall reaction rates (r) and ratio of rates (shape selectivity) after 10 min oxidation at two different temperatures...... 95

Table 4-5. TOF for 1-hexene hydrogenation...... 98

Table 5-1. Adsorption equilibrium constants and reaction rate constant on supported catalyst...... 114

Table 5-2. Average relative error between model and experimental data for 1-hexene (two different assumptions: constant and variable kinetic parameters)...... 116

Table 5-3. Diffusion coefficients and adsorption equilibrium constants derived from simulation for 1-hexene hydrogenation on supported and embedded catalysts...... 118

Table 5-4. Diffusion coefficients and adsorption equilibrium constants derived from simulation for 1-decene hydrogenation on supported and embedded catalysts...... 120

Table 5-5. Diffusion coefficients and adsorption equilibrium constants derived from simulation for 2-methyl-1-pentene hydrogenation on supported and embedded catalysts...... 120

Table 6-1. Effect of surfactant on micro and mesoporosity of PFA-derived carbon...... 135

Table 6-2. Effect of acid concentration on micro and mesoporosity of FA-Plu-C...... 135

Table 6-3. Textural properties of bimodal carbon synthesized with different monomer compositions (2 mmoles of HCl)...... 137

xiv

ACKNOWLEDGEMENTS

I first wish to thank God who has always empowered me with faith and courage, guided my curiosity, provided strength to never give up and blessed me with priceless family, friends and great mentors.

I have many people to thank, who without them the completion of this thesis was not possible and I would not have reached where I am today without their support. First and foremost, I would like to thank my adviser, Professor Henry C. Foley, who guided me during the last five years and helped me grow as a research scientist. It was a great honor and privilege for me to work under his guidance. One of the greatest things that always kept me motivated was his enthusiasm and passion for the research. Every time I had a scientific conversation with him, I was amazed and affected by his creativity, intelligence and his problem solving approach. While guiding and supporting me with his extensive scientific knowledge, he allowed me to work and think independently throughout my study. Professor Foley was always supportive, caring, and ready to take risks and try new ideas. He taught me the true success is achieved when you enjoy what you are doing and be part of it. I hope that I can apply what I have learned from his unequivocal mentorship in the future to help and inspire others.

I would also like to thank Dr. Ramakrishnan Rajagopalan for all his help and support during my PhD. He was always available when I wanted to discuss my results, talk about next steps and to help me find answers to the questions I had. He helped me to stay focused and motivated, as well as approach problems systematically and learn from my mistakes. Dr.

Rajagopalan devoted a significant portion of his time to help me and always persuaded me to set the standards higher in my research. I also would like to express my appreciation to his wife,

Prashanti, for her patience and support when I was keeping Ram busy with my questions and requests. I wish both of them a long happy and healthy life full of joy and success.

I would like to thank Professor Andrew Zydney, Professor Chunshan Song and Professor

Darrell Velegol for serving as my committee members. Their brilliant comments and suggestions made my defense, candidacy, and comprehensive exams dynamic and joyful. I wish to thank my master’s thesis adviser, Professor Toraj Mohammadi who gave me the opportunity to work in his lab as a young researcher. I am always indebted to him for all his continuous support especially when I decide to continue my studies in Penn State.

I would also like to thank all my friends and group members during my PhD work. I am fortunate to have had such hardworking and enthusiastic group members. Particularly, I would like to thank past and previous group members, Dr. Billy-Paul Mathew Holbrook and Dr. Ali

Qajar, who I had the opportunity to work closely and collaborate with. Thank you to all my friends outside the academic environment, who made my life more exciting by their presence.

I wish to thank my family for their unconditional love and support through all the stages of my personal and academic life. My parents encouraged me to always follow my dreams and try to be the best at whatever challenge I accepted. My greatest and most wonderful teacher, my mother, taught me to read and write my first words and since then she has always been encouraging and teaching me not to be afraid of taking risks and setting higher goals. My father who believed in me and supported me all the way through; my sister and brother who are two of the biggest treasures I have and make my life joyful and invaluable with their presence and smile.

No matter how far I have been from them during last few years, they have always been in my heart. Last but not least, I must express my profound gratitude to Mehdi, my husband, my love, and my friend. He has always been there through all the ups and downs, happiness and sadness, laughs and tears, all the late night experiments, scientific discussions, helping me in the lab, the exams, through it all, with all his love and continuous support and encouragement. He made me believe I can do anything and I share all my success and accomplishments with him. Life is such a wonderful journey with him.

Chapter 1

Polymer-derived porous carbons: An overview

1.1. Introduction to polymer-derived carbons

The precursor for making porous carbon materials can be either natural or synthetic.

Synthetic sources for producing carbon are polymers and resins that are processed in a controlled heat treatment step to form carbonaceous materials. Both, the precursor used in the synthesis and the heat treatment procedure, determine the final textural properties of the carbon. Porous carbon materials derived from synthetic polymers are divided into two distinct categories: graphitizing and non-graphitizing. The degree of graphitization in a porous carbon depends on the nature of the polymer precursor. It has been shown that some polymers, such as polyvinyl chloride (PVC) form graphitizing carbon upon heating at temperatures as low as 1000°C, while other polymers like poly(furfuryl alcohol) don’t [1]. This phenomenon was first shown by Franklin in 1951 by carbonizing polyvinyl chloride and the mixture of polyvinylidene chloride (PVDC) and PVC.

Carbon derived from the polymer mixture did not graphitize, even after heat treatment at 2000°C

[1]. It was found that the presence of cross-linking in the polymer network inhibits graphitization during heat treatment. While the most thermodynamically favorable state of the carbon is graphite, the crosslinks in the polymer lead to bonds that resist graphitization and formation of any well-ordered carbon. Non-graphitizing carbons are inherently porous but the pores have dimensions less than one nanometer and they are thermally stable. The origin of porosity in this type of carbon is the misalignment of graphene layers [2, 3]. Figure 1.1 shows one suggested structure of an amorphous carbon.

Figure 0-1. Schematic of folded graphite-like layers in an amorphous carbon [35].

Polymer-derived carbons usually have less surface functionalization when compared to biomass (naturally)-derived carbon; although the surface of a polymer-derived carbon is subject to functionalization by secondary treatment. Thus, they also contain many fewer heteroatoms and they have very little in the way of impurities [4-6]. One of the important characteristics of these carbons is that their textural and morphological properties can be varied by the appropriate choice of the polymer precursor (monomer), polymerization conditions and heat treatment (pyrolysis) procedure.

In order to fine-tune carbon-based materials to meet specific requirements for different applications, all the steps involved in the synthesis need to be carefully controlled. The important structural properties that significantly affect the performance include pore size and distribution of pores, morphology and surface area.

3 1.2. Polyfurfuryl alcohol-derived carbon

Furfuryl alcohol (FA) is a biomass-derived alcohol that can be polymerized through cationic polymerization in the presence of an acid initiator [7, 8]. Polymerization of FA proceeds through head-to-tail and head-to-head condensation of FA molecules and, hence, the growth of a linear chain ensues [9]. In the linear chains of polyfurfuryl alcohol (PFA), the furan rings are connected by methylene or ether bridges. If the poly condensation of furan rings and linear chain growth were the only mechanisms in the polymerization of FA, then the resultant polymer would be a colorless polymer of high molecular weight. However, PFA is a dark brown polymer, indicating other processes have taken place in addition to linear polymerization that lead to larger rings and lower energy absorption bands in the visible spectrum.

Many researchers have studied the polymerization mechanism of FA and have used different spectroscopic techniques to clarify the polymerization pathway [10-14]. Choura et al., have used a combination of IR, UV-vis and NMR data on PFA and cured PFA, from which they have confirmed cross-linking and branching arising from Diels-Alder reactions and methylene bridge formation between linear chains of PFA [10, 11, 14]. It has been shown that by increasing temperature and acid (initiator) concentration, the rate of cross-linking increases [15-20]. Figure

1.2 shows the schematic of main pathways suggested for FA polymerization.

Figure 0-2. Different molecular structures formed during polymerization of FA.

Lamond and coworkers for the first time reported the use of PFA for nanoporous carbon synthesis in early 1960 [21, 22]. They studied physical properties of carbon (such as surface area) by measuring adsorption of different gases including carbon dioxide and nitrogen. After conducting a series of experiments these researchers found that carbon derived from PFA heat treated at 800°C had the highest adsorption and hence the largest surface area. Later on, Walker studied the use of PFA-derived carbon as a molecular sieving material [23]. Along with Walker’s studies of PFA-derived carbon, Fitzer and Schafer examined the formation of carbon from pyrolysis of PFA using thermal gravimetric measurements [24, 25]. Figure 1.3 shows the evolution of carbon compounds during heat treatment (pyrolysis) of PFA.

Figure 0-3. Evolution of carbon compounds during pyrolysis. (A) methylene bridge, (B) backbone conjugation, (C) 2,5 carbons, (D) 3,4 carbons, (E) terminal methyl group, (F) cross- links, (G) polyaromatic domain, (H) carbonyl group [2].

1.3. Pyrolysis procedure and evolution of porosity in PFA-derived carbon

The decomposition of PFA during pyrolysis and the formation of carbon have been studied in detail using TEM, infrared and Raman spectroscopy along with NMR, x-ray and temperature programmed oxidation [26-31]. It has been shown through differential thermal analysis that during pyrolysis of PFA two maximums are observed in weight loss spectra [24, 25].

The first one, which happens at around 340°C, is attributed to carbon dioxide and water losses and the second weight loss, happening at 440°C, corresponds to carbon monoxide and water losses. InfraRed (IR) spectroscopy has shown that furan rings are stable up to 275°C. Increasing the temperature to higher than 300 °C results in the rupture of furan ring and evolution of carbon monoxide, carbon dioxide and water. At temperatures around 400°C, aromatic benzene-like rings start to form and larger aromatic domain emerge. When temperatures go higher than 450°C,

6 hydrogen gas is evolved from the reaction of water with methylene bridges and leading to the formation of carbonyl groups in the structure. The other gas evolved is methane, which is also formed at the higher temperatures, and is due to elimination of methylene bridges. It is shown that during pyrolysis and in the intermediate states between polymer and carbon, some mesopores

(with sizes around 10 nm and smaller) are formed [2, 32]. The presence of these large pores in the structure facilitates the transports of gaseous products during pyrolysis of FA. With time, at temperatures higher than 400°C, the mesoporosity drops due to loss of carbonyl groups (see

Figure 1.3.) At even higher temperatures (600°C), more heteroatoms are removed and the mesoporosity decreases toward zero.

It has been shown that by increasing the pyrolysis temperature to 800°C, the maximum inherent pore volume of PFA-derived carbon (0.2 cc/g) is reached. The mean micropore size in this carbon is around 0.5 nm and this is the result of cross-linking in the polymer structure, which subsequently leads to the formation of odd-membered rings (5 or 7 membered) and, thusly, to curvature within the carbon structure. The very small amount of remaining mesoporosity in the

PFA-derived carbon structure is attributed to the presence of oxygen moieties [33-35].

Several rudimentary models have been suggested for the structure of graphitizing and non-graphitizing carbon. Rosalind Franklin suggested a model containing sp2 –hybridized domains for both types of carbons as shown in Figure 1.4 [1]. In graphitizing carbon, these domains are well-ordered and with a small driving force (heat treatment at low temperature) it is possible to form graphite. While, in the case of non-graphitizing amorphous carbon, the sp2 hybridized domains are mis-oriented and are connected by curved and disordered carbon sheets which is the origin of porosity.

Figure 1-4. Schematic of proposed structures for a) graphitizing and b) non-graphitizing carbons [36].

The effect of curvature on the shape of graphitic layers has also been shown in a study by

Smith et al [33] as can be seen in Figure 1.5. It has been suggested that high temperature heat treatment can rearrange the rings, change the odd-membered ring to six member rings and align the graphitic layers. However in the case of PFA-derived nanoporous carbon, high temperature heat treatment (even higher than 2000°C) does not result in to ordered graphitic layers. The effect of heat treatment (annealing) on nanoporous carbon structure was studied by Burket et al [3]. It was shown that although the as-synthesized nanoporous carbon does not graphitize even after annealing at 2000°C, activation of the carbon using CO2 (selective oxidizing in CO2 stream) increases its tendency for graphitization, significantly [3]. Burket showed that CO2 activation etches the carbon layers in the pore walls and decreases the number of graphitic layers in the carbon walls as confirmed also by XRD. After activation, the thinner layers of carbon are more flexible and easier to align by annealing. It has also been shown by McNamara that the presence of tetrahedral carbons between the graphitic layers can be the main barrier to graphitization in

8 amorphous carbon. These tetrahedral carbons can be partly removed from the structure by activation of carbon [37]. It was shown that after partly or completely removal of tetrahedral carbons by activation, it is possible to partly or completely graphitize the amorphous carbon. The decrease in total porosity and the increase in density (approaching graphite density) that can accompany very high temperature treatment are also indicative of “graphitization” taking place and this is confirmed by XRD and TEM studies [3].

Figure 0-5. Schematic models of graphene layers with non-hexagonal rings [33].

1.4. Control of porosity

1.4.1. Activation

The micro and mesoporosity in the PFA-derived carbon can be controlled using different approaches. Oxidation of carbon using different oxidizing agents has been the main method to create more porosity or to shift the mean pore size; these are generally called activation processes

[38-40]. In so-called physical activation, usually steam or other oxidizing atmospheres are used,

9 while in chemical activation, oxidizing chemical compounds such as KOH, NaOH or ZnCl2 are used to etch the carbon atoms in order to change the textural properties. In the activation process, the carbon atoms at the edges and steps are more susceptible to oxidation so these are removed progressively by the oxidizing agent. Many of the activation methods also change the surface chemistry of the carbon walls and, hence, change the material’s adsorption characteristics based on the types of surface functional groups. For example, treating with steam at high temperature puts a large amount of hydroxyl group on the carbon surface. Treating with KOH and NaOH on the other hand, adds hydroxyl groups and impurities in the form of metal ion left behind in the carbon structure. Among the oxidizing agents used for carbon activation, CO2 was shown to have the least effect on the surface chemistry. It was shown by XPS that after activating carbon using

CO2, the total oxygen-containing functional groups decreased to less than 2 at.% compared to more than 10 at.% before activation [41]. At the same time the CO2 molecule is small in size (3.3

Å) and so it can diffuse into the micropores of carbon and etch the carbon atoms at the pore walls.

Burket et al., studied the activation process of carbon using CO2. By systematic changes in activation temperature and time he was able to correlate the extent of activation with the textural properties of the carbon. It was shown that the micropore size and total pore volume could be shifted from 5 Å to around 10 Å and 0.16 to 1.3 cc/g, respectively, by increasing the activation time [3]. The effect of activation time on mean pore size and pore size distribution is shown in

Figure 1.6. It can be observed that in addition to shifting the mean pore size, activation also broadens the pore distribution.

Figure 0-6. Effect of activation time on pore size distribution and total pore volume of the PFA- derived NPC (15, 42, 69 and 84 wt% burn-off) [3].

1.4.2. Pore forming agents

In order to increase the total porosity of NPC, to shift the mean micropore size or to induce mesoprosity in the structure, the addition of pore forming agents to the polymer resin has also been utilized [42-44]. The presence of both micro and mesoporosity improves transport properties of the carbon-based adsorbents, catalysts and electrodes [45-51]. Lafyatis et al, has studied the effect of poly(ethylene glycol) (PEG) as an additive on the pore size of carbon and its total nitrogen adsorption. They showed that mixing PEG having molecular weights higher than

2000 amu with PFA prior to pyrolysis, the total nitrogen uptake could be significantly enhanced

[43]. The effect of different molecular weights of PEG molecule and the composition of the

PEG/PFA mixture on the resultant carbon porosity were also studied thoroughly by Strano et al.

11 [44]. Adsorption of dextran with different molecular weights was used to determine the effect of

PEG on textural properties of NPC. It was shown that mixing PEG, which has negligible carbon yield, with PFA, forms a miscible polymeric blend which phase separates later on during pyrolysis. The PEG-rich domains then decompose and leave behind some mesoporosity. The composition of PEG/PFA mixture and the molecular weight of PEG were optimized to synthesize a carbon with well-connected porosity and enhanced pore volume. Figure 1.7 shows pore size distribution plots of carbons derived from PFA mixed with 25 wt% PEG having different molecular weights [44].

Figure 1-7. Pore size distribution of PFA-derived carbon templated with 25 wt% PEG having different molecular weights [44].

12 1.4.3. Hard and soft templating

Carbon with high surface area and hierarchical porosity is of great interest in different applications including energy storage, catalysis, separation, chromatography and Li ion batteries

[52]. Presence of well-developed interconnected porosity facilitates mass transport within the carbon structure and, hence, improves the diffusion process. To synthesize high surface area carbon having both micro- and mesoporosity, researchers have used two distinctly different approaches, soft templating and hard templating. In hard templating approaches, an inorganic template, such as silica, zeolite, etc… is used and the synthesis process generally involves three main steps. First, the carbon precursor-inorganic composite is prepared by impregnation or infiltration of the precursor on the template. In the second step, the carbon precursor is carbonized at high temperature and finally the template is removed [53]. On the other hand, in soft templating approaches, an organic molecule, such as a block copolymer is used as the soft template. A well-ordered porous network is formed as the result of supra-molecular arrangement of the surfactant and the carbon precursor. The soft template skeleton is removed during the carbonization process at high temperature, leaving behind the hierarchical carbon network [52].

Knox et al., used hard templating approach for the first time in 1986, to synthesize porous carbon, using spherical silica particles as the hard template and phenolic resin as the carbon precursor [54]. Since then, different types of hard templates have been used to synthesize carbon- based materials with controlled micro, meso and macropore size [55-63]. Ryoo et al. and Hyeon et al., both, used mesoporous silica as the hard template to synthesize ordered mesoporous carbon

[55, 56]. Ryoo and coworkers used MCM48 mesoporous silica and filled the mesopores with the mixture of sucrose and sulfuric acid, as the carbon precursor. After pyrolysis and removal of the silica template, they obtained mesoporous carbon (CMK-1) which was the replica of the silica template [55].

13 A wide variety of hard templates such as SBA-15, MCM-41, HAM, SBA-7 and etc., have been used by researchers [64-66]. Carbons with high surface area (1000-2500 m2/g) and well- ordered porosity could be synthesized by choosing the appropriate template, carbon precursor and polymerization condition.

Bimodal porous carbon materials were also synthesized, using combined nanocasting and imprinting (dual templating approach). In this method, mesoporous silica was used as the first template to form one pore system, while colloidal silica nanoparticles were added into the solution as the second template, to form another pore system [67, 69].

Polymer microspheres have also been used, as the hard templates for synthesizing macroporous carbon. Baumann et al. used colloidal polystyrene microspheres, and infiltrated the carbon precursor into the interstitial spaces between the spheres. The template was then removed by toluene washing. Carbon with large cavities (~ 100 nm) and smaller mesopores (6 nm) that connect the larger holes was prepared by heat treating the polymeric structure [70].

Soft templating technique has recently attracted great attention due to its simplicity and higher cost effectiveness [51]. This method involves nanocasting of the carbon precursor, using micelles of an amphiphilic block copolymer as the soft template. Pluronics (PEO-PPO-PEO) are the most common organic molecules used as the soft templates, in this approach [71-73]. In order for the templating to occur, the carbon precursor should have enough interaction with the copolymer molecule. In a typical synthesis the block copolymer is mixed with the solvent and self-assembly in the form of micelles occurs. The carbon precursor that is usually resorcinol- fromaldehyde (RF resin) or pgloroglucinol-formaldehyde (PF resin), then, is added to the micellar solution. The micellar structure is stabilized by cross-linking between the carbon precursor and the surfactant molecules [71-77]. The removal of soft template happens simultaneously with the carbonization process, during high temperature heat treatment.

14 Zhao et al., showed that mesopore size and pore morphology, could be controlled by varying the fraction of the hydrophobic block of the copolymer or the ratio of the carbon precursor to the template [72, 75-77]. They used pluronics having different molecular weights, to show the effect of the copolymer composition on the resultant carbon structure.

Dai and coworkers, used pluronic F-127 and phlorolglucinol-fromaldehyde mixture to synthesize carbon films, monoliths and fibers. They were able to obtain carbons with variable mesopore sizes in the range of 5 to 10 nm by varying the polymerization condition and utilizing different casting approaches [60].

Both, hard and soft templating approaches, have been used to control the morphology of the carbon particles, as well. The morphology of porous carbon materials is a very important feature from the practical point of view. For example, in gas sensors and membrane separation processes, thin films are the desired morphology, while, in chromatography, mono-dispersed spheres are needed. In soft templating approach, mesopore size and morphology of the particles, both, could be simultaneously controlled, by the appropriate choice of surfactant, carbon precursor and their relative composition. Carbons with different morphologies including thin films, rods, spheres and discs were synthesized, using the different micellar morphologies formed by surfactant molecules, as the template [78, 79]. In order to prepare carbon fibers, copolymers having shear-aligning feature were used. Spin coating and extrusion techniques, were used to impose shear forces in specific directions and synthesize carbon films and fibers [70]. In hard templating approach, the final carbon morphology represents the exact replica of the hard template. Using the hard templating approach, different morphologies including spheres, monoliths, hollow spheres and rods have also been synthesized. As an example, hollow carbon spheres with porous walls that could be used in catalysis and controlled drug delivery were synthesized by Li et al. They used alumina-silicate hollow spheres as the hard template and infiltrated the carbon precursor within the porous structure, using incipient –wetness

15 impregnation method [80]. Silica spheres with solid core and mesoporous shell were also used, as the hard template for the same purpose [81].

Controlling the porosity and the morphology of the carbon-based materials, has equipped researchers with the necessary tools, for synthesizing carbons with improved performance in different applications. Varying morphology and porosity, directly affect the other physical properties including density, mechanical stability, etc., which are very important factors when looking at the potential industrial applications of the carbon-based materials.

1.5. Application of porous carbon

Porous carbon materials possess high surface area, good thermal and mechanical stability and tunable textural and morphological properties. Polymer-derived carbons usually have surfaces which are chemically inert, but prone to functionalization [4-6]. Because of having the above mentioned desired physical and chemical properties, carbon-based materials have been used in a wide variety of applications, including gas adsorption and storage, separation, heterogeneous catalysis, energy storage and Li ion batteries [82-98]. In this section, few of these applications are explained along with the examples in each category.

1.5.1. Gas adsorption and storage

Porous carbon derived from PFA, is predominantly microporous with the average pore size of 0.5 nm [36]. Theoretical and experimental adsorption studies have shown that, the pores in the PFA-derived carbon have slit-like shape. So, pore geometry is different from what is observed in other types of porous materials such as circular, cylindrical or polygonal pores [33, 36]. It has

16 been shown that, carbon with the average pore size of 0.4 nm, can adsorb planar molecules like benzene, due to its slit-like pores; while a zeolite structure with the similar pore size but different pore geometry doesn’t show any tendency for benzene adsorption [82]. Due to its unique pore geometry, carbon can selectively adsorb benzene in a mixture containing cyclohexane or isobutene [83].

Measuring gas adsorption on a carbon adsorbent is of interest from two different perspectives. First of all, gas adsorption measurement is the standard technique, which is being widely used for characterization and evaluation of the textural properties of porous materials.

Carbon dioxide, nitrogen and argon are the standard gases that are used to determine pore size distribution, in carbon and other porous materials [99]. On the other hand, in energy and environmental-based applications, in addition to characterization, measuring adsorption capacity of carbon for specific gases such as H2, CO2, CH4, NH3 and H2S, is critical. Hydrogen as an alternative fuel has attracted great attention and, hence, department of energy (DOE) has set 9 wt% as the target for adsorption capacity by 2015. Although despite the significant amount of research in the field of porous carbon materials, the highest numbers obtained are still in the range of 1-2 wt% [100-103]. It has been shown that, the adsorption affinity and capacity could be controlled by varying the textural properties of the PFA-derived carbon. Saha et al., found that, both heat of adsorption and adsorption capacity, could be enhanced by doping carbon with rare earth metals such as K ions [104].

Adsorption of carbon dioxide and ammonia on carbon adsorbents, have also been extensively studied [84, 85, 105]. Carbon dioxide adsorption is a very important process in refining natural gas, in order to meet the specific criteria for natural gas transport and usage. It has been shown by Rodriguez et al. that the adsorption capacity of 4.8 mmol/g could be reached using a nanoporous carbon synthesized by pyrolysis of a vacuum residue pitch. The high

17 adsorption capacity, was achieved at atmospheric pressure and room temperature and the selectivity compared to methane, was significantly high [84].

Methane is widely used for heat and energy production. The DOE target for methane adsorption, which was set to be 185 cc/g, was reached by Pfeiffer et al [106]. They used coconut char as the carbon precursor and obtained a porous carbon material with the methane adsorption capacity of 192 cc/g.

1.5.2. Membrane separation

PFA-derived nanoporous carbon was used by Shifflett et al. as the molecular sieve for air separation. They made a tubular membrane module using ultrasonic deposition of PFA on a stainless steel support. The coated module was heat treated to form a thin layer of porous carbon on the support. Synthesized membrane, showed significantly high selectivity for oxygen compared to nitrogen (separation factor ~ 30) [86]. Application of PFA-derived carbon as the membrane for O2/N2 separation was further explored by Merritt et al [87-91]. In their studies, porous stainless steel discs were used as the supports. To improve permeability and retain high selectivity, the crack-free thickness of the deposited PFA layer was decreased, by pre-filling the pores of the support, using carbon black spheres.

Carbon molecular sieves (CMS) derived from other polymer precursors, including polyimide, polyacrylonitrile, polyetherimides and phenolic resins, have been synthesized and evaluated in different applications, including natural gas sweetening, olefin/paraffin and oxygen/nitrogen separation [107-109]. In many of these applications, the CMS materials have shown superior performance, compared to the polymeric membranes. In addition to higher permeability and selectivity, CMS membranes also possess high thermal and chemical stability.

18 The preparation procedure, polymerization condition, choice of precursor and the heat treatment procedure, were studied by many researchers [110-112].

1.5.3. Heterogeneous catalysis

In many industrial processes for chemicals and petrochemicals production, harsh reaction condition such as high pressure and temperature and extremely high or low pH, can adversely affect the catalytic materials being used in the process and shorten the active life time of the catalyst [113]. One of the promising alternates in such processes is using carbon as the catalyst support that is not usually affected by the harsh operational condition. In addition to having high surface area and unique textural properties, carbon surface chemistry can be easily varied using different functional groups. Functionalization of the carbon surface is used to enhance the dispersion of the precious metal nanoparticles on the surface, to induce some sort of catalytic functionality and/or to control the interaction of reactants, products and transition states with the surface, during the reaction [114-116]. Porous carbon materials have been largely used as the support for synthesizing conventional impregnated catalyst or embedded catalyst with active catalytic sites dispersed within the porous structure.

1.5.3.1. Shape selective catalysis: definition and application

One of the important goals in 21st century chemical industry is to develop new and useful heterogeneous catalysts for carrying out multipath reactions with high selectivity which, in turn, leads to major gains in energy efficiency [117, 118]. Directing the catalyst performance towards higher selectivity while maintaining high activity, will lead to lower overall production costs. For

19 example, fewer separation units will be needed to obtain a high purity of the targeted product and fewer by products will be formed, thus less waste to deal with.

A shape selective catalyst support possesses the catalytic active sites within a pore system penetrable only by molecules smaller than the pore size. Thus only molecules small enough to enter the pores will react, and only product molecules of a similarly small size will appear outside the catalyst support pellet or particle [92]. There are three designated types of shape selective catalyzed reactions that have been studied; the three types are reactant shape selectivity, product shape selectivity and transition state shape selectivity. Differences between reactant or product molecules’ diffusion coefficients or size of the potential intermediates that can be formed based on the pore size of the support are the reasons for observing shape selectivity. In reactant and product shape selectivity the molecules are discriminated based on the difference between their diffusion coefficients in the pores and changing the catalyst particle size affects the selectivity.

However, in transition state shape selectivity, changing the catalyst particle size (diffusion length) does not affect the product composition [119]. Figure 1.8 shows a schematic representation of different types of selectivity. So far, zeolites are the only commercial shape selective catalyst supports that have been used in industry.

Figure 1-8. Different types of shape selectivity in heterogeneous catalysis [119].

Alkylation of aromatics is a widely used reaction in the large-scale synthesis of petrochemicals, fine chemicals and intermediates [120]. To synthesize advanced polymers such as polyethylene naphtalate and polybutylene naphtalate, 2,6-dialkyl naphtalenes is required for preparing the monomers [121]. Alkylation of benzene and toluene has been commercialized using zeolite (ZSm-5) as the catalyst supports [119]. However, mass transfer limitations in the small pores of zeolites limit their performance, especially in the case of reactions that involve bi-cyclic aromatics. Approaches such as synthesis of ultra large-pore zeolites and ordered mesoporous materials (MCM-41, SBA-15, etc.) are utilized to overcome this problem. Although application of these kinds of materials is limited, because of the complexity of the synthesis approach and the relatively low thermal and hydrothermal stability of the highly-ordered mesoporous materials

[122]. Cheng et al. have shown the selective production of ortho-diethyl benzene at short contact times in the benzene ethylation over MCM-22 [119]. They concluded that, the reaction is taking

21 place on the external surface sites. After poisoning the acidic sites on the external surface, they were able to increase the selectivity towards the para-isomer.

Selective hydrogenation of naphthalene to either cis- or trans-decalin and regio-selective hydrogenation of heteroatom-containing aromatic compounds are two other reactions that are of practical interest. The products in such reactions are specialty chemicals that are used as the monomers for synthesizing advanced polymers [119]. Song et al. have studied the activity and selectivity of different zeolite supports with Pt or Pd as the active phase, in hydrogenation of naphthalene [123].

1.5.3.2. Carbon as catalyst support

Although zeolites have shown high conversion and selectivity in different reactions, many synthesis pathways, especially liquid phase reactions, need chemically inert supported precious metal catalysts. By using an inert support, acid catalyzed reactions that decrease selectivity via isomerization, cracking or alkylation, are avoided. Since carbon is inert, many of these unwanted isomerization reactions and also coking could be hindered. Walker et al., for the first time, used carbon as the shape selective catalyst support. They mixed the Pt/PFA-derived carbon composite with activated carbon and evaluated the resultant catalyst in gas phase hydrogenation reactions. It was shown that, the synthesized catalyst, selectively hydrogenates 3- methyl-1-butene compared to the other reactants examined in their study, such as 1-butene and cyclopentene [92, 93].

Trimm and Cooper used platinum sequestered in microporous carbon catalyst, in the hydrogenation of different linear and branched alkenes. They found that, the synthesized catalyst has higher selectivity towards the linear alkane production and higher conversion for the linear alkenes compared to branched and nonlinear ones [124]. Application of carbon as the catalyst

22 support was further explored by Foley et al., who designed a composite material of silica- alumina-supported carbon molecular sieve. Product shape selectivity of this catalyst was investigated in the reaction of methanol and ammonia for the production of methylamines [125].

Evidence for the transition state shape selectivity, was found for the carbon molecular sieve- supported nickel catalyst, synthesized by Miura et al., in the methanol decomposition reaction.

The synthesized hybrid catalyst, displayed high selectivity toward syn-gas production, due to the steric constraints imposed by the carbon pore walls [126].

Platinum-loaded nanoporous carbon membranes were also studied by Strano et al. [96].

They investigated the gas phase hydrogenation of short chain alkenes. Differences in the diffusivities due to the differences in alkenes’ chain lengths, was found to be the reason for observing different reaction rates. Dandekar et. al, have shown selective performance of the carbon- supported copper catalyst in the hydrogenation of crotonaldehyde, even when the catalyst was synthesized using activated carbon and conventional wet impregnation method [127]. In another study, Rioux et al., have synthesized activated carbon-supported copper catalyst, and evaluated the activity and selectivity of the catalyst in isopropyl alcohol dehydrogenation to either acetone or propylene. Taking advantage of both, product shape selectivity and surface functional groups’ effect, they could reach the selectivity of 100% toward the production of acetone compared to propylene [128, 129]. Rajagopalan et al. pre-formed Pt nanoparticles in the carbon precursor, FA, in the presence of a surfactant (Triton X-100). Then the mixture was polymerized to form Ptx/PFA [98]. The resultant polymer was pyrolyzed at 800 °C to form carbon. This catalyst was heat treated at 800 ˚C under hydrogen flow for multiple times, and the Pt nanoparticles were observed to resist sintering. The high stability against sintering, confirmed that the Pt nanoparticles were kinetically frozen within the carbon microporous structure. This catalyst also showed high selectivity towards the hydrogenation of linear alkenes in gas phase reactions.

Figure 1.9a shows a platinum particle embedded within the amorphous carbon structure. In

23 Figure 1.9b, shape selective catalyst activity towards hydrogenation of smaller alkenes compared to bulkier ones, is illustrated.

a b

Figure 1-9. a) Platinum nanoparticles kinetically frozen within PFA-derived carbon, b) embedded catalyst activity in hydrogenation of alkenes [98].

In a recent study, Holbrook extended the use of embedded catalyst to liquid phase hydrogenation reactions of alkenes. The catalyst, showed very low activity in liquid phase hydrogenation reaction while selectivity was significantly high. By using Triton X-100 as the solvent during polymerization, a slight amount of mesoporosity was created in the carbon structure and, hence, activity was improved. However, the improvement in activity was accompanied by a decrease in selectivity [130, 131]. Although, embedding the platinum nanoparticles within the carbon structure results in the desired features such as high stability against sintering and shape selective performance, the catalysts of this type have shown low activity in liquid phase reactions, due to the pronounced mass transport limitations exerted by the ultra micropores. To prepare carbon-supported catalysts that are industrially interesting, it is

24 important to develop different approaches to improve their activity while retaining the high shape selectivity. These approaches generally include controlling the morphology and porosity of the carbon materials (catalyst support).

1.6. Organization of Thesis

The goal of this work is to explore the use of PFA-derived carbon as the shape selective catalyst support for liquid phase hydrogenation reactions. To reach this goal, morphological and textural properties of the carbon need to be modified. These properties, directly affect the catalyst activity and selectivity. The results of this research will provide applicable and simple techniques that can be utilized to tune physical properties of a carbon-based catalyst, in order to obtain desired catalytic performance.

Chapter 2 summarizes the different synthesis and preparation methods used in this study along with the characterization techniques implemented to evaluate the synthesized materials. In

Chapter 3, the use of an emulsion polymerization approach, for controlling carbon morphology is presented. A systematic study on the synthesis of carbon spheres with controlled size, along with the effect of different polymerization parameters is covered. Chapter 4 explains the use of carbon spheres as a media for dispersing platinum nanoparticles. Shape selective catalytic properties of the synthesized catalyst are illustrated. In this chapter, a structure-performance relationship, for the synthesized catalyst in liquid phase hydrogenation reactions, is developed. Different approaches are applied to control pore size and carbon particles size, and finally an optimum catalyst is designed. In Chapter 5, the reaction mechanism and kinetics inside the ultra micropores of the carbon-based catalyst, in the presence of significant confinement and steric hindrance, is studied. It is shown in this chapter that, how reaction rate constants and adsorption equilibrium constants are affected by the pore size. In chapter 6, a new approach is used to synthesize carbon

25 materials with bimodal porosity. This is achieved by simultaneous polymerization of furfuryl alcohol and phloroglucinol. The method presented in this chapter, is a simple route to control the textural properties of the carbon, and is highly applicable to the synthesis of catalytic and adsorptive materials with improved transport properties. Finally, Chapter 7, summarizes the main results and achievements of this study and suggests future promising research areas for carbon based materials.

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Chapter 2

Experimental Procedures

2.1. Synthesis

2.1.1. Synthesis of PFA-derived microporous carbon spheres

In the previous studies conducted in our group, bulk irregular PFA-derived carbon, had been synthesized using para- Toluene sulphonic acid (p-TSA) as the initiator, FA as monomer and tetrahydrofuran (THF) or Triton X-100 as the solvent. The carbon derived from this approach was a predominantly microporous carbon with an average pore size of 0.5 nm and irregular morphology.

In this study an emulsion polymerization approach was adapted to synthesize carbon particles with regular morphology and controlled size [1, 2]. This surfactant-assisted polymerization of furfuryl alcohol (FA) involved two major steps. In the first step, 20 ml ethanol was mixed with 6 cc water and this mixture was used as the solvent. To this solution, 1.4 g HCl

(36.5 wt%) was added. Then, 3 g pluronic F-127 which is a tri-block copolymer (surfactant) was dissolved in the solvent. At this step micelles are formed in the solution due to self-assembly of surfactant molecules. After complete dissolution of the solid in the solvent, 3 g FA was added drop-wise to the micellar solution with the rate of 10 ml/min using a syringe pump. The polymerization of FA in the presence of HCl was continued for 12 hours under stirring. When FA starts to polymerize, the resulting oligomer and polymeric chains are hydrophobic and have higher tendency for the hydrophobic core of the micelles formed by pluronic f-127 molecules.

40 Hence, polymerization proceeds within the core of the micelles. After the first step slow polymerization, 15 ml of 5M sulfuric acid was added to the mixture and the temperature was increased to 90 ºC. In this step, the acid and heat help to create a lot of crosslinking in the polymer network. The spheres which are formed during the first step grow in the second step and the surface of the spheres hardens due to high degree of cross-linking. This is a critical step, to remove excess surface functionality and avoid sticking of individual spheres together later during pyrolysis. The polymerization along with solvent evaporation was continued until all the solvents were evaporated and a brown residue was left. To remove the unreacted monomer (FA) and acid, the polymeric paste was washed and centrifuged at least three times using copious amount of distilled water. The washing was stopped when a clean, transparent and neutral in pH supernatant was obtained. The washed polymer then was dried overnight in an oven at 90-100 ºC. To form microporous carbon, the dried polymer was pyrolyzed in a quartz tubular furnace at 800 ºC.

Temperature ramp was 1 ºC/min and the soaking time was 5 hours. The resultant carbon has spherical morphology with almost mono-dispersed distribution of spheres’ size. Carbon spheres synthesized using this approach are microporous with a narrow pore size distribution centered at

0.5 nm and BET surface area of 380 m2/g.

2.1.2. Synthesis of Platinum nanoparticles with controlled size

To synthesize platinum nanoparticles, platinum (II) acetylacetonate (Pt(acac)2) was used as the precursor. Alcohol reduction method which has been used widely for preparing platinum and other transition metal nanoparticles was adapted [3, 4]. Alcohols can reduce metal salts at high temperatures. The optimum temperature to obtain the desired size of the nanoparticles depends on the alcohol used and the metal precursor. In a typical synthesis, 6 g of FA was mixed with given amount of Pt(acac)2 (based on the final desired loading and considering the carbon

41 yield), in a round bottom flask and was heated in a reflux set-up. Heating was carried out for 16-

17 hours and at different temperatures (80, 120 and 170 ºC) to examine the effect of temperature on the resultant platinum nanoparticles’ size. Upon heating, the initial solution color which is transparent yellow becomes dark brown which is an indicative of formation of nanoparticles. In this study it is shown that 80 ºC is not high enough and FA cannot reduce platinum precursor to

Pt0 at this temperature. XRD patterns were used to determine the size of the platinum nanoparticles synthesized at different temperatures. It was shown that 120 ºC is the optimum temperature to obtain smallest platinum nanoparticles and highest reduction extent. Using this approach platinum nanoparticles with the average size of 4-5 nm were prepared. Both XRD and

HRTEM of the catalysts synthesized using these particles were used to confirm the size of the nanoparticles.

2.1.3. Synthesis of supported and embedded carbon-based catalyst

Two different types of catalysts were synthesized in this study, supported and embedded.

In order to synthesize the supported catalyst, carbon spheres were activated in CO2 stream at 900

ºC to reach to 45% weight loss. This process increases the available surface area to around 1100 m2/g and helps to increase the dispersion of platinum nanoparticles on the surface. The platinum nanoparticles prepared in FA then were used to impregnate these spheres. 1 g of activated carbon spheres was mixed with the proper amount of the FA/Ptx sol (based on the final desired loading of the platinum) and kept under stirring for 4 hours. The mixture then was washed and centrifuged several times using acetone to remove the excess FA. The synthesis procedure was followed by drying the catalyst in an oven at 100 ºC.

In order to embed platinum nanoparticles within the microstructure of carbon spheres, the pre-made FA/Ptx sol was added drop-wise to the micellar solution as both the monomer and the

42 source of platinum nanoparticles. All other steps were conducted in the same way as described for making carbon spheres. In some of the samples, poly (ethylene glycol) (PEG) with different molecular weights was added to the monomer (FA) prior to addition to the micellar solution. PEG is a non-carbonizing polymer which is miscible with PFA. During carbonization, PEG domains decompose and leave behind some mesoporosity in the carbon structure.

2.1.4. CO2 Activation

In a typical activation process, 1 g sieved carbon was placed in a quartz boat and treated in a tubular quartz furnace under flowing CO2. The temperature was increased to 900 ºC in one hour under argon and was kept at that temperature for an additional hour. Then the gas was switched to CO2 and flow rate was adjusted at 900 sccm. The oxidizing reaction was continued for a certain time length based on the final desired weight loss (activation). Then the gas was switched back to argon and the sample was cooled down to room temperature. The activation process and evolution of porosity during CO2 activation was thoroughly studied by Burket et al., previously [5, 6]. Using CO2 activation the mean pore size can be shifted toward bigger pores and surface area and total pore volume can be increased. This will provide a much more facile transport of reactants and products inside the pores during catalysis.

2.1.5. Synthesis of mesoporous carbon using soft-templating approach

In this study, a soft-templating approach was adapted from Dai work [7]. In this approach, pluronic F-127 is used as the structure directing agent and phlorglucinol/formaldehyde system is used as the monomer. The enhanced hydrogen bonding between the hydroxyl groups of the phloroglucinol and oxygen of the poly (ethylene oxide) blocks of the surfactant molecule is

43 the main reason for directing the polymerization within the corona of the micelles. This procedure forms a polymeric network around the micelles’ cores which are the origin of the mesopores in the resultant carbon. Formaldehyde acts as a linker between to phloroglucinol molecules.

In a typical synthesis, 9.4 g ethanol and 8.5 g water were mixed and used as the solvent.

Then, 2.5 g pluronic F-127 and 2.5 g phloroglucinol were added and the mixture was stirred until complete dissolution of the solids. 0.2 g HCl was added afterwards as the polymerization catalyst.

After 30 min of stirring, 2.6 g formaldehyde was added to the solution and stirring was continued for an additional hour. After one hour, gellation occurred resulting in a phase separation with water-rich phase at the top and a polymer-rich phase at the bottom. Water-rich phase was removed and polymerization was continued overnight at room temperature. The rubbery non- stick polymer film was then placed in oven at 90 ºC for around 12 hours to dry. Pyrolysis was conducted at 850 ºC for 5 hours which was reached with a 5 ºC/min rate. The resultant carbon is predominantly mesoporous with average pore size of 8.3 nm and total pore volume of 0.6 cc/g.

2.1.6. Synthesis of carbon with bimodal porosity

Simultaneous polymerization of FA and phloroglucinol was used in this study as a novel approach to synthesize carbon with bimodal porosity having narrow pore distribution in both the micro- and mesopore region. These kinds of porous carbon possess superior transport properties due to presence of mesopores in addition to the high surface area and molecular sieving feature because of the micropores existence.

In a typical synthesis, 8.5 g water and 9.4 g ethanol were mixed and to this solution 2.5 g pluronic F-127 was added. Then a certain amount of phloroglucinol was added and upon complete dissolution, 0.2 HCl and a specific amount of FA were added to the solution. The amount of FA and phloroglucinol was changed to see the effect of monomer composition on the

44 final carbon textural properties. The rest of the process was conducted exactly following the same steps as for synthesizing phloroglucinol-derived mesoporous carbon. To study the effect of HCl and pluronic, the concentration of these compounds were changed in the polymerization step. It was shown that by carefully adjusting the monomers ratio and surfactant concentration, it is possible to obtain carbon with bimodal porosity, controlled mesopore size and total pore volume.

2.2. Characterization

In order to study and examine the physical and chemical properties of the synthesized carbon-based materials and catalysts in this study, a series of characterization techniques were used. It is important to characterize different materials thoroughly to be able to develop a structure-performance relationship and design the optimum material. Some of the characteristics which were examined using characterization tools are BET surface area and pore size distribution

(textural properties), metal nanoparticles size and dispersion, microcrystalline structure of carbon, morphology and size of the carbon particles, etc. The reaction products also were analyzed using gas chromatography technique.

2.2.1. Nitrogen adsorption

Nitrogen adsorption measurements were conducted on a Micromeritics ASAP2020 adsorption instrument. The carbonaceous samples were first outgassed at temperatures higher than 100 °C under vacuum. BET surface area was measured using multipoint BET analyzer program and the adsorption equilibrium data. Using nitrogen physisoption data at low relative pressures (P/P0 < 0.14), the micropore volume of the samples were determined. To estimate total pore volume of the samples, total amount of nitrogen uptake (at P/P0 >0.98) was used. The

45 mesopore volume was calculated by subtracting micropore volume from the total pore volume.

Nitrogen adsorption isotherm along with density functional theory (DFT) and Barrett-Joyner-

Halenda (BJH) models was used to determine micropore and mesopore size distribution, respectively.

2.2.2. Methyl chloride adsorption

It has been previously shown that methyl chloride gas adsorption can be used effectively to determine micropore size distribution of carbon-based materials [8]. To collect the adsorption data, a custom-made instrument which works based on gravimetric measurements, was used. The measurements were conducted at temperatures between 249 K and 313 K and pressures lower than atmospheric pressure. One of the advantages of using methyl chloride for microporus materials is that, the saturation pressure can be reached at higher temperatures compared to 77 K for nitrogen. At higher temperatures the diffusion of molecules inside the pores is faster and hence equilibrium is reached faster (faster adsorption dynamic). This will decrease the total analysis time for a microproous material [9]. To conduct the measurements, sample was loaded on a quartz pan and degassed at 350 °C under vacuum for at least three hours. After cooling down to room temperatures, methyl chloride was dosed at different relative pressures and the equilibrium data points were recorded. Horvath-Kavazoe and BJH models were used to calculate micro and mesopore size distribution, respectively.

2.2.3. X-ray diffraction (XRD)

X-ray diffraction was used to determine microcrystalline structure of the carbon and the average size of the platinum nanopartciles. It is a simple and relatively fast method. Using Schrrer

46 equation and the diffraction pattern obtained from the measurement, the size of the nanoparticles or crystalline domains can be calculated as follows:

K d  Equation 2-1  cos

Where K is the shape factor (typically 0.9), λ is the x-ray wavelength (1.5412 Å), β is the line broadening at half the maximum intensity of the peak and θ is the Bragg angle (the peak location in the X-ray scattering pattern). X-Ray Diffraction peaks will be used in this study to see the effect of different methods for synthesizing nanoparticle on their size and the effect of heat treatment on sintering the platinum nanoparticles. A PANalytical Empryean X-Ray

Diffractometer was used to conduct the measurements.

2.2.4. Scanning Electron Microscopy (SEM)

Scanning electron microscopy (SEM) is a type of electron microscope that images the sample surface by scanning it with a high-energy beam of electrons. The structure of the catalyst made can be investigated using SEM images. In addition, using SEM, the average size of the carbon nanospheres can be determined by the counting method. This instrument will be used to see the effect of different parameters like: surfactant concentration and temperature on the size of the carbon catalyst particles. The distribution of bigger pores (mesopores) can also be visualized using FESEM. Using energy dispersive spectroscopy (EDS) detector, it is also possible to perform elemental mapping for the synthesized catalysts. In this study, scanning electron micrographs were collected either on a Hitachi S-3000H SEM instrument or a FEI NanoSEM 630

FESEM.

47 2.2.5. Pulse Chemisorption

The goal of this work was to increase the active metal surface area while retaining shape selectivity for the synthesized catalysts. Pulse chemisorptions technique was used to study the dispersion of metal in the catalyst. This was achieved by measuring the uptake of a gas that is chemisorbed on the metal but negligibly so on the support. The measurement was done under conditions that allow the coverage corresponding to a monolayer [10]. Pulse Chemisorption tests were done using CO pulse chemisorption in a Micromeritics AutoChem II 2920. The effect of different procedures and precursors used for synthesizing the metal particles on dispersion was studied using this technique.

2.2.6. Transmission Electron Microscopy (TEM)

Transmission Electron Microscopy is a microscopy technique with very high resolution whereby a beam of electrons is transmitted through an ultra thin specimen, interacting with the specimen as it passes through. An image is formed from the interaction of the electrons transmitted through the specimen. In this study, TEM was used for verification of the Pt nanoparticle sizes obtained by XRD and chemisorptions as well as to indicate how well the platinum is dispersed throughout the carbon network.Transmission electron micrographs were captured on a Gatan LaB6 Joel 2010 and Field-emission 2010F transmission electron microscopes.

48 2.2.7. Fourier Transform Infrared Red (FTIR) Spectroscopy

In this technique, the adsorption or transmission of light within a wide range of wavelength is measured. The ability of sample to adsorb or transmit light at specific wavelengths, gives information about the different bonds and functional groups which exist in the sample. This technique was used for both polymers and carbons, synthesized in this study. The data were collected using a Bruker Vertex V70 instrument.

2.2.8. Thermal Gravimetric Analysis (TGA)

TGA is typically used to determine the composition or thermal and oxidative stability of different materials. In this study, TGA was used to obtain the actually total platinum content of the synthesized catalysts. A temperature ramp of 10-20 °C/min was used and the maximum temperature was 1000 °C. The experiment was conducted under flowing air to burn the carbon support and determine the weight of the remained compound which was platinum oxide. TGA profiles were collected on a TGA 2050 instrument.

2.2.9. Gas chromatography

Gas chromatography was used to analyze the reaction products and determine the composition of different compounds. An Agilent (model#: 7890A) GC with a HP-5 capillary column was used for this purpose. Calibration of the GC was conducted using standard mixtures prepared with reactant, product and solvent at different compositions. The relative response factor

(RF) of the column for different components were determined and used for calculating the unknown composition in the product mixture.

49 2.3. Catalytic tests

A low pressure Parr reactor was used to run the catalytic hydrogenation reactions. The reactor includes a glass vessel equipped with a stirrer and a jacket for heating and cooling.

Adjusting the temperature is achieved by circulating heavy oil in the reactor jacket. Reactor pressure was adjusted by controlling the outlet pressure of the gas cylinder through a gas regulator and it was measured by a pressure guage installed on the reactor. The catalyst was loaded into the reactor and reduced in situ by flowing hydrogen and heating to 130 °C. After cooling down to room temperature under argon, reactant/solvent mixture was added to the reactor through a deep feeding tube installed on the vessel head. N-undecane was used as solvent in all the reactions. Hydrogen was then introduced to the reactor at the desired pressure and temperature was adjusted at the reaction temperature. The reaction started by turning on the stirrer. Samples were collected during reaction and analyzed using GC.

50 2.4. References

[1] Zubizarreta L, Arenillas A, Pis JJ, Preparation of Ni-doped carbon nanospheres with different surface chemistry and controlled pore structure, Applied Surface Science 2008; 254:3993-4000.

[2] Yao J, Wang HT, Liu J, Chang KY, Zhang LX, Xu NP, Preparation of colloidal microporous carbon spheres from furfuryl alcohol, Carbon 2005; 43:1709-1715.

[3] Toshima N, Hirakawa K, Polymer-protected bimetallic nanocluster catalysts having core/shell structure for accelerated electron transfer in visible-light-induced hydrogen generation. Polymer

Journal 1999; 31(11):1127-1132.

[4] Rajagopalan R, Ponnaiyan A, Mankidy PJ, Brooks AW, Yi B, Foley HC. Molecular sieving platinum nanoparticle catalysts kinetically frozen in nanoporous carbon. Chemical

Communications 2004; 21:249824-99

[5] Burket CL, Rajagopalan R, Foley HC. Overcoming the barrier to graphitization in a polymer- derived nanoporous carbon. Carbon 2008; 46(3):501-10.

[6] Burket CL, Rajagopalan R, Foley HC. Synthesis of nanoporous carbon with pre-graphitic domains. Carbon 2007; 45(11):2307-10.

[7] Liang CD, Dai S. Synthesis of mesoporous carbon materials via enhanced hydrogen-bonding interaction. Journal of the American Chemical Society 2006; 128(16):5316-5317.

[8] Mariwala RK, Foley HC, Calculation of Micropore Sizes in Carbogenic Materials from the

Methyl Chloride Adsorption Isotherm. Ind. Eng. Chem. Res. 1994; 33(10):2314-21.

[9] Qajar A, Peer M, Rajagopalan R, Foley HC, Characterization of Micro- and Mesoporous

Materials Using Accelerated Dynamics Adsorption. Langmuir 2013; 29(40):12400-12409.

[10] Satterfield CN, Heterogeneous Catalysis in Practice, 1980, McGraw-Hill Inc., New York

Chapter 3

Surfactant-assisted polymerization of furfuryl alcohol

(This chapter is published in Carbon journal. Maryam Peer, A. Qajar, R. Rajagopalan, H.C. Foley, “On the effects of emulsion polymerization of furfuryl alcohol on the formation of carbon spheres and other structures derived by pyrolysis of polyfurfuryl alcohol ”, Carbon, 51, 85-93 (2013))

3.1. Introduction

Porous carbon materials derived from the pyrolysis of polymer precursors may be used for a wide range of applications that include as catalyst supports, gas adsorbents, membranes for gas separation, electrodes for fuel cells, electrochemical capacitors and metal/air batteries [1-14].

Control of porosity, surface area and pore size distribution is a key feature that makes these carbons useful for most of these applications [15]. Manipulation of the internal structures of these carbons, as measured in terms of their surface area, pore size and porosity is crucial to their performance. However, the control of their external properties, that is their primary particle size and its geometry, can affect the rate and efficiency of process as central to their performance as the mass transport of gases or liquids to and from their surfac-es. Mass transport is also critical to improve the power density of electrodes and to provide high interfa-cial surface area for heterogeneous catalysis and electrochemistry.

Emulsion polymerization is commonly used to produce polymer particles with sizes in the range from a few nanometers to several microns [16-20]. In this technique, micelles act as the template to assist the formation of spherical particles during polymerization. However, under certain conditions, other shapes such as rods, bicontinuous layers and sponge-like structures can

52 also be formed [21-22]. Thermodynamic equilibrium, as captured by the ternary phase diagram of the surfactant, solvent and monomer, dictates the formation of these interesting structures in solution. The size and shape of the polymeric particle is also influenced by the polymerization kinetics. A question then arises as to whether and to what extent the polymer structures formed in the emulsion can be preserved in the carbon solid after their pyrolysis at high temperatures.

Recently, the emulsion polymerization technique was used to form carbon spheres from furfuryl alcohol (FA) precursor using a two-step approach. In the first step, furfuryl alcohol was acid-polymerized in the presence of pluronic F-127 used as a structure directing surfactant. This step was followed by crosslinking the synthesized polyfurfuryl alcohol (PFA) spheres using a strong acid such as sulfuric acid. The authors reported that carbon spheres in the range of 200 nm

- 1µm can be prepared by this method [23] and a similar approach was used to synthesize nickel doped carbon spheres [24].

Previously, the evolution of porosity in PFA-derived carbon during pyrolysis has been thoroughly studied. In addition, we have developed methods to overcome the barrier to graphitization in the carbons by selective oxidation and annealing treatments. These carbons have been demonstrated viability for a variety of applications that include supported membranes, adsorbents and catalysts. Control of porosity, pore size distribution and surface area of these carbons is achievable by well described synthesis methods [2-7, 25-29]. However, the control of morphology using emulsion polymerization of FA, pro-vides a way to manipulate the external structures of these materials while preserving the unique internal properties of these carbons.

In this chapter, we report a systematic study that determines the effect of various solutions variables that influence the size, shape and nanoscale geometry of the carbon derived from emulsion polymerization of FA. In particular, the goal of this chapter is to develop a deeper understanding of the interplay between micellar growth, polymerization kinetics and crosslinking of PFA formed using this method. A pseudo-ternary phase diagram is developed to map the

53 different emulsion conditions to the resultant structural and morphological features of synthesized carbons.

3.2. Experimental

3.2.1. Synthesis of PFA-derived carbon spheres

An amphiphilic triblock copolymer (EO106PO70EO106) Pluronic F-127 was used as the surfactant and FA was used as monomer. Both compounds were purchased from Sigma-Aldrich and were used as received. Distilled water and ethanol were used as solvents. The synthesis procedure was adapted from Yao et. al. [23]. In a typical synthesis, a known amount of surfactant

(pluronic F-127) was added to an ethanol/water mixture. After letting the surfactant mix completely to form a uniform solution, HCl was added as the polymerization initiator. To this mixture the monomer, FA, was added drop-wise using a syringe pump. The solution was stirred at room temperature for 12 hours to allow the polymerization to proceed. After this step, 5 M sulfuric acid, equal to one-half the volume of the mixture, was added to the polymerized mixture and heated to 90°C until the solvent had evaporated to leave behind a viscous residue. The viscous black polymer was washed using copious amount of distilled water and centrifuged several times before the solid was collected. The collected solid was then dried in an oven at

90°C in air. The product of this step was solid polymer spheres that were then carbonized under a flow of argon at 650°C in a simple tube furnace.

Experiments were designed around this basic synthesis methodology in order to study the effects of monomer concentration, surfactant concentration, initiator concentration and solvent

54 composition during the first step of polymerization. In each of these studies, all the other variables were kept constant while changing only the parameter of interest.

3.2.2. Characterization of synthesized carbons

A Hitachi S-3000H Scanning Electron Microscope and a field-emission scanning electron micro-scope (FEI Nova NanoSEM 630) were used to examine the morphology and the size of the carbon par-ticles. Standard deviation and average particle size for these spherical particles were determined by im-aging approximately 200 particles. Field Emission Transmission Electron

Microscopy (JEOL 2010F) was used to observe the microstructure of the carbon samples. Methyl chloride gas adsorption measurement combined with Horvath-Kawazoe method was used to determine pore size distribution and cumulative pore volumes of the sample [30].

3.3. Results

3.3.1. Pseudo-ternary phase diagram and carbon microstructure

A series of samples were synthesized at different compositions in order to explore the regions of the pseudo-ternary phase diagram for the solvent/monomer/surfactant mixture. Five different regions la-beled A - E were identified as shown in Figure 3.1.

Polymers synthesized in Region A were spherical particles and they retained their morphology even after pyrolysis. In this region of the composition space, a uniform and transparent micellar solution was formed with the surfactant/acid/solvent mixtures to which FA was then added drop-wise. Depending upon the composition of surfactant, monomer and solvent, carbon spheres with different diameters ranging from 50 nm to few micrometers were synthesized. Scanning electron micrographs of the carbon spheres produced in region A showed smooth and uniform spheres which are distinct and well-separated. Methyl chloride gas

56 adsorption indicated the microporous nature of the samples with a very narrow pore size distribution and a mean pore size of 0.5 nm; nitrogen BET measurements indicated a surface area of 480 m2/g. Physical activation of these spheres under CO2 at 900°C led to a 45% decrease in the initial mass and this resulted in an increase in surface area to 1200 m2/g with a mean micropore size of 0.63 nm and narrow pore size distribution as shown in Figure 3.2 (a and b).

Images of carbon spheres taken by TEM showed typical glassy carbon morphology with highly disordered polyaromatic domains (Figure 3.2c and d). Carbon materials derived from synthesis with mixtures having composi-tions within regions B and D, showed interconnected porous reticulated morphology. The mixture with composition in Region B is FA deficient (< 10 wt.%) while that in region D was solvent deficient. Syn-thesis from mixtures in both these regions led to very viscous polymers mixtures that significantly hin-dered the formation of homogeneous morphological features. Synthesis from a mixture with a composi-tion in Region C led to a surfactant-rich phase and this resulted in a highly viscous gel. The final carbon structures made in this region were observed to be dense, flaky and highly irregular. Similarly, solids formed from mixtures with compositions in the FA rich region E showed dense, flaky morphology.

Figure 3-1. Characterization of carbon spheres synthesized by emulsion polymerization, as synthesized and 45% activated, a) methyl chloride pore size distribution, b) cumulative pore volume, c and d) TEM images of carbon spheres.

3.3.2. Effect of monomer (FA) concentration

In order to understand the variation in size and shape of the carbon structures, a systematic study was done varying parameters such as FA concentration, acid concentration, surfactant concentration and solvent composition. Figure 3.3 shows that there is a strong linear dependence between the size of the final carbon particles after pyrolysis and the FA concentration during polymer synthesis. By increasing the FA concentration from 0.3 wt.% to 10 wt.%, the

58 average size of the carbon spheres increased from 150 nm to 540 nm, respectively. The spheres were mostly but not perfectly mono-dispersed.

Figure 3-3. Effect of monomer (furfuryl alcohol) amount on average carbon particles size, All samples synthesize with 20 wt.% pluronic F-127.

3.3.3. Effect of surfactant concentration

Unlike the FA composition, any change in surfactant concentration had a profound effect on the morphology of the resultant carbon. At a very low concentration of pluronic F-127 (<2 wt.%), a large and highly distorted spherical morphology with huge dispersion as shown by the error bars (Figure 3.4a) was observed. On the other hand, when the surfactant concentration was

59 very high (>35 wt.%), indis-tinct, agglomerated and highly irregular small spherical particles

(Figure 3.4b) were observed. When the surfactant concentration ranged between 5 and 35 wt.%, we saw regular spherical morphology and a monotonic decrease in size with each increase in surfactant concentration (Figure 3.4c).

Figure 3-4. Effect of surfactant (Pluronic F-127) on average carbon particles size, all samples made with 9 wt.% furfuryl alcohol.

3.3.4. Effect of initiator (acid) concentration

The effect of acid concentration on morphology was yet more complex. The size of the carbon parti-cles increased with an increase in acid concentration during polymerization, but only

60 up to a certain lim-it, beyond which, the final particle size decreased significantly as shown in

Figure 3.5.

Figure 3-5. Effect of initiator (HCl) molarity on average carbon particles size and morphology, red: 10.5 wt.% FA, 10.5 wt.% pluronic F-127, blue: 40 wt.% pluronic F-127, 12 wt.% FA.

The limiting acid concentration at which this reversal occurred was influenced by the surfactant concentration, as it shifted to lower molarities at lower surfactant concentration. By using a 40 wt.% surfactant mixture, we synthesized monodispersed carbon spheres with average particle sizes of 50 nm at both a low and high acid concentration, as shown in Figure 3.6a and c.

However, the polydispersity increased in the vicinity of the maximum and we observe a bimodal distribution as shown in Figure 3.6b. Under these conditions, particles in the range of 200 to 300

61 nm and 500 to 600 nm, respectively, were observed. At very high acid concentration (> 6 M), we saw interconnected, possibly hollow, rod-like features as can be seen in Figure 3.7a and b.

a b

Figure 3-6. SEM images of samples synthesized at 40 wt.% pluronic F-127, 12 wt.% FA, 48 wt.% solvent and at different HCl molarities: a) 2 M, b) 3 M, c) 5.2 M.

a b

Figure 3-7. a) SEM and b) TEM images of carbon sample synthesized at HCl molarity of 7, 40 wt.% pluronic F-127, 12 wt.% FA, 48 wt.% solvent.

3.3.5. Effect of solvent composition

The effect of solvent on the morphology was also explored by varying the composition of etha-nol/water mixture in the solution. In general, the average size of carbon particles increased with ethanol fraction in the solvent as shown in Figure 3.8. When the ethanol fraction was less than 0.5, we observed an irregular dense morphology similar to Figure 3.4b. As the ethanol fraction increased, spherical carbon particles were formed. When the ethanol fraction was between 0.5 – 0.8, monodisperse carbon spheres with diameters in the range of 150 nm to 1µm were noted as shown in Figure 3.9a and c. At very high ethanol fractions, a bimodal dispersion was formed with particles ranging in diameter from 150 – 300 nm and from 1.2 – 1.4 µm as can be seen in Figure 3.9b and d.

Figure 3-8. Effect of ethanol and water composition in the solvent on average particles size, samples synthesized with 10.5 wt.% furfuryl alcohol and 10.5 wt.% pluronic F-127.

Distorted

irregular morphology

a a EtOH/(EtOH+Water)= 0.7 EtOH/(EtOH+Water)= 0.9 b b

c c d d

65 3.4. Discussion

In order to understand the morphological evolution of PFA, it is important to recall the key steps in-volved in the acid polymerization of FA. Figure 3.10 shows the three main steps involved in the cationic polymerization of FA as presented in the work of Choura et al [31]. It is initiated by the formation of a cationic species due to the dehydration reaction with the acid initiator (HCl). This is then followed by repetitive addition of the carbocation to the monomer, leading to growth of linear polymeric chains. This step usually controls the observed rate of polymerization. However, in addition to the linear polymeriza-tion of FA, both NMR and

UV/VIS studies have shown that there can be an hydride abstraction from the linear polymer leading to formation of conjugated species. This reaction can further induce chain cross- linking leading to either Diels-Alder condensation reactions or to methylene bridging [31]. Both hydride ion abstraction and the crosslinking reactions can act as chain termination reactions.

In the case of emulsion polymerization of FA, the composition of pluronic F-127, HCl, water and ethanol forms and controls the micellar growth. This is followed by the diffusion of monomeric furfuryl alcohol into these micelles triggering the polymerization and growth of the spheres. After polymerization, as reported by Yao et al., a second step that involves addition of highly concentrated sulfuric acid to solidify or crosslink the synthesized spheres was done before pyrolysis [23]. In all experiments, the conditions for the second step was fixed, but varied the parameters during the first step, the step that controls polymerization.

Figure 3-10. Acid catalyzed polymerization scheme of furfuryl alcohol.

67 The rate of polymerization for this process is given as [32]:

k p nN p [M ] p Rp  Equation 3-1 N A

Where kp is the polymerization rate constant, is the number of cationic species per micelle, Np is the number of micelles and [M]p is the concentration of monomer in micelles and

NA is Avogadro’s number.

As seen from the equation 3.1 the rate of polymerization is directly proportional to the monomer concentration in each micelle. Our studies show that the carbon spheres grow linearly with an increase in the FA concentration (as shown in Figure 3.3) as we would expect from the rate expression. An increase in surfactant concentration reduces the interfacial surface tension and this leads to an increase in the number of micelles, thereby increasing the available surface area for polymerization. We see evidence for such an effect under conditions wherein the surfactant concentration is between 5 and 25wt.%. At very low concentration of surfactant, the number of micelles formed is less and the polymerization occurs both within the micelles and in the solvent- rich phase simultaneously, leading to the formation of irregular and larger particles. At high concentrations, a large number of very small micelles are formed and this leads to an increased tendency to aggregate and to produce less orderly morphological features (Figure 3.4).

Increasing the acid concentration should increase the rate of initiation leading to a larger number of available carbocations in each micelle. This, in turn, should enhance the rate of polymerization and lead to larger carbon particles. However, as shown in the second step, higher acid concentration can also induce a significant increase in the rate of crosslinking leading to an increased rate of termination.

68 Hence, beyond a certain critical concentration of acid, due to competitive termination reactions, we observe a reversal and a significant decrease in the particle size. The rate of termination versus polymerization can also be dependent on the amount of surfactant and monomer species that control the viscosity of the solution.

Micellar growth can also be controlled by varying the composition of the solvent.

Addition of ethanol to water can help to swell both the hydrophobic (polypropylene oxide) and hydrophilic end (poly-ethylene oxide) of the pluronic F-127. It has been shown that addition of ethanol to water reduces the interfacial tension between the hydrophobic segment of the surfactant and the solvent. Hence the aggregation number, the micelles’ core size and hard sphere interaction diameter decrease and number density of micelles increases [33, 34]. At high concentrations of ethanol in solvent (VEtOH/V(H2O+EtOH)> 0.8), the small size and the high number density of the micelles lead to two different mechanisms for their growth. Nucleation and growth in each individual micelle form the smaller spheres, while coagulation of some micelles during growth (because of small hard sphere interaction diameter) results in larger spheres. However, as the ethanol fraction is reduced, the micellar growth rate dominates and determines the particle size. At very low ethanol fraction in solvent (VEtOH/V(H2O+EtOH)< 0.5), the critical micellar concentration is less leading to increased aggregation of micelles resulting in irregular carbon morphology. It is the interplay of these factors that determines the morphological features resulting from the emulsion polymerization of FA done in different regions of the pseudo-ternary phase diagram.

3.5. Conclusions

The pseudo-ternary phase diagram of solvent/furfuryl alcohol/pluronic F-127 was studied by exam-ining the effect of different parameters on the size and morphology of the carbon particles that result from polymerization done with mixtures having different compositions within its regions. The region in the ternary phase diagram with homogeneous micellar structure was determined by synthesizing samples at different surfactant and monomer concentrations.

Monomer-rich and surfactant-rich regions, however, resulted in irregular flaky carbon structures upon pyrolysis. Based on the emulsion polymerization rate theory, how different parameters affect the size and morphology of the polyfurfuryl alcohol carbon could be rationalized. While in this study only small laboratory sized samples were produced, it is feasible that this emulsion polymerization process could be sufficiently scaled to produce larger quantities of these carbon spheres with sizes tuned for specific technological applications.

Carbon spheres synthesized using this approach has a narrow micropore size distribution centered at 0.5 nm. The narrow micropore distribution along with controlled morphology and size make this material suitable for different applications including catalysis, gas adsorption and energy storage.

70 3.6. References

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[2] Strano MS, Wyre J, Foley HC, Novel heteropolyacid nanoporous carbon reactive barriers for supra-equilibrium conversion and in situ component separation, Industrial & Engineering

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[4] Rajagopalan R, Ponnaiyan A, Mankidy P.J, Brooks AW, Yi B, Foley HC, Molecular sieving platinum nanoparticle catalysts kinetically frozen in nanoporous carbon Chemical

Communications 2004; 21:2498-2499.

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[16] Ovanda-Medina VM, Peralta RD, mendizabal E, Martinez—Guiterrez H, Lara-Ceniceros

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72 [20] Schreiber E, Zeiner U, Manzke A, Plettl A, Ziemann P, Landfester K, Preparation of

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2008;254:3993-4000.

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Carbon 2007;45:2307-2310.

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Chapter 4

Shape selective carbon spheres-based catalyst: Synthesis and Application

(This chapter is published in Carbon journal. Maryam Peer, A. Qajar, R. Rajagopalan, H.C. Foley, “Platinum embedded within carbon nanospheres for shape selective liquid phase hydrogenation”, Carbon, 57, 485-497 (2013))

4.1. Introduction

Porous carbon materials are used commercially as heterogeneous catalyst supports for fine chemicals synthesis because of their combined physical and chemical properties [1, 2]. They possess high surface area, chemically variable surfaces and high stability which make them suitable as a support for dispersing transition metal nanoparticles [3-6]. Carbon-supported catalysts figure prominently in electrocatalysis and fuel cells [7-10]. Synthetic, polymer-derived carbons can be prepared reproducibly with high purity, and they can be modified systematically to control their total porosity, their pore size distribution and their level of surface functionality and they have been known to be molecular sieves for some time [11-13]. These types of nanoporous carbon have been studied as catalyst support media for a number of different reaction types [14-18].

Shape selective catalysis has been mainly demonstrated in zeolites due to tunable and well-ordered pore structure [19]. Zeolite-based shape selective catalysts with controlled pore structure and acidity have been used in applications such as selective toluene methylation, naphthalene hydrogenation and isomerization reactions [20]. Herein, we examine the liquid phase hydrogenation of alkenes with platinum embedded within nanoporous, molecular sieving carbon.

Because the platinum particles are larger than the pores by an order of magnitude or more, by

75 embedding them in the carbon, they cannot move; they are immobilized and frozen in place [16,

21]. Thus they do not sinter or agglomerate. By embedding, we invert the usual catalyst structure.

Instead of the particles being supported on an activated carbon surface residing within a pore, the platinum particles reside within the polyfurfuryl alcohol derived nanoporus carbon (PFA-NPC).

The pores that are in the vicinity of the metals end at the metal particle surfaces. In this way the locus of catalytic activity is at the junction of a pore and a portion of the platinum particle surface. In principle, the reaction that takes place at this junction of pore and particle must be subject to the maximal level of steric factors that can be imposed by the walls of the pore.

To fully realize this potential for shape selective catalysis with the platinum embedded within PFA-NPC (Pt-Em-PFA-NPC) materials, their activity must approach that of platinum supported on activated carbon catalysts. It has been shown that Pt-Em-PFA-NPCs are active for the gas phase hydrogenation of ethylene, propene, 1-butene, but to be of interest for fine chemicals synthesis, these catalysts must be useful for reactions of larger molecules [15, 16]. In reaction engineering terms, the effectiveness factor of the Pt-Em-PFA-NPC catalysts must be sufficiently close to that of a more standard supported platinum catalyst. We know from first principles that to increase the effectiveness factor, we must decrease the Thiele Modulus. In the case of an isothermal reaction with first order kinetics in a spherical catalyst this means that the radius of the sphere must be small enough to reduce the diffusion length, and that the diffusivity must be large enough to significantly lower internal mass transfer resistance. Put another way the

magnitude of the characteristic time for diffusion within the catalyst, diffusion, must be small

relative to the magnitude of the characteristic time for chemical reaction reaction , according to

Equation 4-1,

2 R k 1 R k 1 diffusion     Equation 4-1 3 D 3 D 3 reaction

φ = Thiele Modulus

R= radius of the spherical particle (cm)

ρ = density of the particle (g/cm3)

k = first order rate constant (cm3/g.sec)

τ = tortuosity

D = diffusivity of reactant within the catalyst (cm2/sec)

ε = void fraction within the catalyst

In this chapter Thiele Modulus is used as a guide for improving the activity of the embedded platinum catalysts for the hydrogenation of longer chain and bulkier alkenes in the liquid phase. To assess the effectiveness of the changes that was made in the catalyst structure, hydrogenation reaction rates were used as measures of the extent of improvement in the effectiveness of embedded catalysts. By comparing the hydrogenation rates of linear alkenes with increasing chain lengths, or with a branching methyl group, it is possible to assess how the changes made in the catalyst structure affected the relative activities and reactant shape selectivities of the embedded catalysts. We then took the ratio of the rate of any given hydrogenation reaction over an embedded platinum catalyst to the rate of reaction over a supported platinum catalyst to compute an “effectiveness factor”. This allows us to place these effectiveness factors for the new catalysts on a general plot of effectiveness factor versus the

Thiele modulus in order to progressively solve the catalyst design problem.

77 4.2. Experimental

4.2.1. Materials

Pluronic F-127, which is a tri-block copolymer consisting poly(ethylene oxide) as the hydrophilic part and poly(propylene oxide) as the hydrophobic segment, was purchased from

Sigma-Aldrich and was used as received. Ethanol and distilled water were used as solvents and hydrochloric acid solution (HCl, 36.5 wt.%) was used as the polymerization initiator. Furfuryl alcohol (FA) (98% purity) was purchased from Sigma-Aldrich and used as received as the monomer. All the alkenes (and alkanes) that were used for the experiments were purchased from

Sigma-Aldrich. N-undecane which was used as the main solvent in catalysis was purchased from

Alfa-Aesar and was used as received. Platinum acetylacetonate (Pt(acac)2) was purchased from

Strem Chemicals and was used for synthesizing platinum nanoparticles.

4.2.2. Catalyst synthesis

4.2.2.1. Supported control catalyst

An emulsion polymerization approach adapted from Yao’s work [22] was used to synthesize carbon spheres with controlled size [23] and these were used as the support for the platinum; this is the supported catalyst that we take as the control in this study. For a typical synthesis, 6 g of surfactant (Pluronic F-127) were dissolved in a mixture of ethanol and water (40 cc ethanol and 12 cc water). The mixture was magnetically stirred to form a homogeneous solution. After complete dissolution of solid, 2.8 g HCl was added as the polymerization initiator.

This step was followed by adding FA to the solution, drop wise. The polymerization proceeded

78 for 12 hours. The first step, polymerization under mild conditions, was followed by a rapid polymerization and cross-linking under forcing conditions that were catalyzed by concentrated sulfuric acid. To do this, 5 M sulfuric acid was added to the polymerized solution and the mixture was heated to 90°C until all the solvent evaporated. The viscous brown residue of high polymer furfuryl alcohol was then washed and centrifuged several times with a copious excess of distilled water. The solid that resulted was dried in an oven overnight. To transform these polyfurfuryl alcohol (PFA) spheres to carbon, pyrolysis was done under argon atmosphere at 800°C. To synthesize the supported catalyst, these spheres were “activated” and then used as the support for platinum. We refer to the carbon spheres after activation as ACS for activated carbon sphere. The activation of the carbon spheres was carried out under flowing CO2 at 900°C. The activation process was stopped once the mass of the carbon was 55% of the original mass, which is to say after 45% of the original pyrolized mass was removed. This process increases the porosity of the carbon spheres and also increases the available surface area for dispersing the platinum nanoparticles. More details on the carbon spheres can be found elsewhere [22, 23].

Platinum nanoparticles were prepared by reactive reduction in solution with furfuryl alcohol [16]. Briefly, a known amount of Pt(acac)2 was massed based on the calculated desired final platinum loading in the carbon, and this was mixed with FA in a 25 ml round bottom flask.

The mixture was heated to 120°C and kept at this temperature for 16 hrs. The heating temperature was chosen to be 120°C after a series of experiments was done to determine how the platinum nanoparticles size varied with the temperature of this reduction step. The initial color of the solution was transparent yellow, but this turned to a black sol after reaction, indicating the reduction of Pt(acac)2 to metallic platinum nanoparticles. To compare the activity of the catalyst, the reduction step was done at three different temperatures (80, 120 and 160°C). These suspensions of platinum nanoparticles in FA were used to impregnate the pre-made carbon spheres in order to prepare the control catalyst. In a typical synthesis of this supported catalyst,

79 the suspension (containing specific amount of Pt based on the final desired loading) was mixed with 1 g of activated carbon sphere. Mixing was continued for 4 hours and then the carbon spheres were washed and filtered using acetone, several times. The catalyst was then dried in an oven at 100 °C overnight.

4.2.2.2. Embedded catalyst

To make the embedded catalyst, the platinum nanoparticle/FA sol, prepared as described, was added drop wise to the micellar solution used in the first polymerization step. All subsequent steps were conducted in exactly the same way as described for making the high polymer spheres.

Pyrolysis was performed at 800°C and resulted in platinum nanoparticles embedded within the nanoporous carbon spheres. To increase the porosity of these materials, poly (ethylene glycol),

PEG, was added as a pore forming agent during the synthesis step. Two different molecular weights, 600 and 8000 amu with the amounts of 10 and 2 g, respectively, were used following preparative methods described elsewhere [24, 25]. The ultimate diameters of the primary carbon particles were varied by adjusting surfactant concentration during polymerization [23].

4.2.3. Catalyst Characterization

Both supported and embedded catalysts were characterized by several different methods.

Field Emission Scanning Electron Microscopy (FESEM) was used to examine the morphology of the catalyst. X-ray Diffraction (XRD) and Scanning TEM (STEM) were used to determine the platinum particles’ sizes and their dispersion, either on the surface of, or embedded within, the

80 carbon spheres. Methyl chloride gas adsorption was used to measure the cumulative pore volume and pore size distribution of the catalysts. An BET surface area of the catalysts was obtained from the nitrogen physisorption isotherm. Thermal gravimetric analysis (TGA) was used to determine actual platinum content of the catalysts. Carbon and metal in the as-synthesized embedded platinum catalysts were combusted in flowing air and by heating to 1000°C. The residue is platinum oxide (PtO2) and from its weight the actual platinum content of each sample was back calculated. Carbon monoxide and hydrogen chemisorption measurements were conducted in a

Micromeritics ASAP 2020 instrument.

4.2.4. Catalytic tests

All the catalysts were pre-reduced before catalytic reaction tests under hydrogen/argon

(5% hydrogen) mixture. For the supported catalyst, pre-reduction was done at 400°C for 30 minutes to avoid sintering of the metal nanoparticles. In the case of the embedded catalyst, pre- reduction was carried out at higher temperature (800°C) and for longer time (4 hrs). After subsequent high temperature heat treatment, it was confirmed that the platinum nanoparticles in the embedded catalyst were immobilized and thus remained highly stabilized against sintering.

A low pressure batch reactor (Parr) equipped with temperature and pressure controllers, was used to conduct the catalytic reaction tests. For each reaction, 500 mg of the catalyst was loaded into the reactor after pretreatment. An in-situ reduction under pure hydrogen at 130°C was conducted in the reactor before catalysis. After cooling down to room temperature, a solution containing 15 cc of alkene and 60 cc of solvent (n-undecane) was added into the reactor. The reactor dead volume was purged with nitrogen several times and then the catalyst was stirred in the reactant/solvent mixture for 2 hours prior to start of the reaction. After this mixing step, the

81 reactor was pressurized with hydrogen to 4 bars and the reaction started. During the reaction, at specific time intervals, small volumes of reaction mixture were sampled through a dip tube and valve connected to the reactor. Analysis of these samples was done using a Varian gas chromatography with an HP-5 column connected to a FID detector.

4.3. Results

4.3.1. Supported catalyst properties and performance

The supported Pt/ACS catalyst was synthesized by the alcohol reduction method [26, 27].

In order to form the smallest platinum nanoparticles and to improve their dispersion, reduction temperature and time were varied. Particle size characterization by x-ray diffraction showed that the reduction time did not affect the platinum particle size. However, by decreasing the reduction step temperature from 160 to 120°C, the platinum particle size was cut in half, from 4 to 2 nm, respectively. When this reduction step was attempted at 80°C, it did not result in formation of platinum nanoparticles, the temperature was too low.

In Figure 4-1 we see a comparison of the x-ray diffraction patterns resulting from the supported catalysts with particles synthesized at 160°C and 120°C, respectively. The Pt (111) and

(200) lines are less pronounced for particles produced at 120°C, indicating they have smaller sizes. Hence 120°C was chosen as the temperature for the reduction of Pt(acac)2 and the formation of the platinum nanoparticles.

Pt (111)

Pt (200)

Figure 4-1. Comparison of XRD patterns of the supported catalysts made using alcohol reduction method at 120°C and 160°C respectively.

Conversion versus time data for three different alkenes on the supported catalyst can be seen in Figure 4-2. This catalyst showed nearly equal activity toward each of the alkenes. The reaction reached complete conversion in a short time, 20-25 min, but no significant differences in the rates of reaction were observed. That the reaction rate is almost the same for 1-hexene, 1- octene and 2-methyl-1-pentene (~ 480 mmol/g.hr), clearly shows that there was no molecular sieving effect in operation for this catalyst. Further the rate of hexene hydrogenation did not vary with catalyst particle size or with mixing of the solution thus showing the rate was neither internally nor externally mass transfer controlled. On this basis it was concluded that this supported catalyst could be used as a proper control catalyst since it had an effectiveness factor of

83 unity (no variation in rate with variation in particle size) and that its behavior could be compared to the behaviors of the embedded catalysts. Hydrogenation reactions on this control catalyst were also conducted without premixing of the catalyst and reactants, and no significant difference was observed between the rates with or without the presoaking adsorption step as is consistent for a catalyst that is not limited by internal mass transfer.

100 90

30 1-hexene Reactant conversion (mol.%) Reactant 20 1-octene 10 1-decene 0 0 5 10 15 20 25 30

Time (min)

Figure 4-2. Reactant conversion versus time on 5 wt.% Pt supported catalyst used for alkene hydrogenation reactions.

84 4.3.2. As-synthesized embedded catalyst properties and performance

To make the new molecular sieving catalysts, platinum particles were embedded in the microporous structure of the carbon spheres. The average pore size of the platinum particles within the spheres was 0.5 nm, as was measured using methyl chloride gas adsorption. Figure 4-3 shows the pore size distribution and cumulative pore volume of typical as-synthesized, embedded catalyst.

0.5 0.2 ( a ( b

0.4 0.16 a) b)

0.3 0.12

0.2 0.08 dV/dlog(l)

0.1 0.04

Cumulative pore volume (cc/g) volume pore Cumulative

0 0 1 10 100 1000 10000 1 10 100 1000 10000 l (A) l (A)

Figure 4-3. a) Pore size distribution and b) cumulative pore volume of the embedded catalyst, measured by gravimetric methyl chloride gas adsorption.

Platinum nanoparticles embedded in the microporous structure of the carbon were highly stable towards sintering, even after heating at 800°C as shown in Figure 4-4. In contrast, the supported catalyst showed a significant increase in particle size even after a short period of heating at 800°C.

Figure 4-4. Comparison of the XRD patterns for embedded catalyst before and after heat treatment.

Figure 4-5 shows SEM, TEM and STEM images of the as-synthesized, embedded catalyst. As is clear, smooth and fairly uniformly sized carbon spheres were formed. Within these carbon spheres the platinum nanoparticles were moderately well dispersed. Note that the carbon catalyst in this image had an average carbon particle size of 270 nm and a platinum nanoparticle size of 4 – 5 nm as synthesized. The platinum nanoparticles appear to be mostly uniform in size but there are a few large particles always present. The nitrogen BET surface area was measured to

2 be 400 m /g. The extents of chemisorption of either CO or H2 on both the embedded platinum catalysts and on the supported platinum catalyst were quite low, and these low values resulted in very low values for the apparent platinum dispersion. Based on the measured values for CO and

H2 chemisorption, the platinum dispersions for both the supported and the embedded catalysts were between 3 and 5 %. Different pretreatment conditions for these catalysts prior to the chemisorption were used, but the results were always the same – the extents of chemisorption corresponded to an apparent dispersions of 3-5%.

a b

c d

0 nm Figure 4-5. a) FESEM image of carbon spheres, b) TEM, c and d) STEM images of embedded platinum nanoparticles in carbon spheres.

87 FESEM-EDS elemental mapping for both supported and embedded catalysts along with high magnification images are shown in Figures 4-6 and 4-7. Comparing Figures 4-6 b and d it can be concluded that in the embedded catalyst most of the Pt particles are indeed located within the carbon spheres at different depths, as the Pt map for this catalyst looks less sharp compared to the Pt map for the supported catalyst. FESEM high magnification images of these two catalysts

(Figure 4-7) clearly reveal well-dispersed small platinum nanoparticles on the surface of supported catalyst compared to few particles on the surface for the embedded catalyst.

a b

c d

Figure 4-6. Elemental mapping of carbon and platinum, a&b) supported catalyst, c&d) embedded catalyst.

a b

Figure 4-7. FESEM images of a) supported and b) embedded catalyst.

Figure 4-8 shows the performance of the embedded catalyst for the liquid phase hydrogenation of a series of alkenes. As can be seen in this figure the shortest and straightest alkene tested in this study, 1-hexene, showed the highest conversion. It is worth noting that after admission of hydrogen there was an instantaneous jump in the conversion for all the reactions with the embedded catalyst. The jump results from presoaking the catalyst with the alkene prior to the introduction of hydrogen. During the presoaking step, adsorption of the alkene takes place in the micropores of the carbon. The instantaneous conversion jump does not happen if the catalyst is not allowed to equilibrate adsorptively prior to introduction of hydrogen (Figure 4.8b).

However, in order to avoid transient internal mass transfer effects associated with adsorption that would mask the rate early in the experiment, a preadsorption time of 2 hours was implemented for each of the alkenes. As the reaction progresses with time, the rate drops and moves to a lower value that arises due to the transport limitation of the rate.

89 To evaluate the effect of steric hindrance on the diffusion of reactant molecules and reactivity of the catalyst, the hydrogenation reaction was carried out with increasingly longer chain alkenes and, then, these results were compared to those for a shorter, but branched chain alkene, 2-methyl-1-pentene (a branched reactant). The embedded catalyst showed higher activity towards the linear alkenes as compared to the shorter, but branched alkene, as shown in Figure

8a; the activity ratio was about 10.

a b

Figure 4-8. Conversion versus time on embedded catalyst for a) different alkenes with presoaking, b) 1-hexene without pre-soaking.

90 4.3.3. Enhancing the activity of embedded catalyst

Although the embedded catalyst showed significant differences in alkene hydrogenation rates with differences in chain length or branching, we also saw that the overall activity was much lower when compared to the supported control catalyst. The global reaction rate for the hydrogenation of 1-hexene using the supported catalyst was 480 mmol/g.h, and that was 20 times higher than the corresponding reaction rate for the embedded catalyst (24 mmol/g.h).

Three distinct approaches were taken to enhance the catalytic activity of the embedded catalyst while also seeking to preserve the reactant shape selectivity. In the first approach, polyethylene glycol was blended with the polymer precursor and pyrolyzed to yield a predominantly microporous carbon with a very small amount of mesoporosity. In other instances, it has been shown that even a small additional amount of mesoporosity created in this way disproportionately facilitates the ease of transport of reactant and product molecules. [24-25, 28-

29]. By adding extra porosity, we sought to increase the effective diffusivity of molecules transported within the carbon. In the second approach, we sought to synthesize substantially smaller catalyst particles. Smaller particle sizes lead to decreased micropore diffusion lengths and this can also lead to increased effectiveness. To achieve this we had to modify the polymerization methodology. The third approach involved mild oxidation of carbon with carbon dioxide to add additional porosity. This results in increased internal volume and may also lead to a higher surface area of platinum being exposed.

91 4.3.3.1. Approach 1: pore forming agents

By adding PEG with two different molecular weights (600 g/mol and 8000 g/mol) we examined how the small effect on the porosity of carbon that this produces, might also affect the embedded catalyst’s activity and shape selectivity. It is important to point out that the addition of

PEG to the synthesis mixture did not have any effect on the morphology of the carbon particles as they retained their spherical geometry. Both TEM and STEM images showed highly dispersed platinum nanoparticles with average particles sizes of 4-5 nm, i.e. as we observed for the as- synthesized embedded catalyst. Figure 4-9 compares the pore size distribution and pore volume as a function of PEG additive. Addition of PEG induced a small amount of extra mesoporosity while preserving similar micropore volumes as shown in Figure 4-9b.

a b

Figure 4-9. a) Pore size distribution and b) cumulative pore volume of the embedded catalyst synthesized with and without PEG addition, measured by methyl chloride gas adsorption.

92 However, though the change in overall porosity was small, even this low level of additional mesoporosity had a profound effect on the activity of the catalyst as shown in Figures

4-10a and b. Table 4-1 summarizes the overall reaction rates for the three different catalysts. The addition of either PEG600 or PEG8000 caused a 50% to 100% increase in catalyst activity

(depending on the reactant molecule size) with very little change in the reactant shape selectivity of the catalyst (~7-8).

a b

Figure 4-10. Conversion versus time for different alkenes on the embedded catalyst synthesized using a) PEG600 and b) PEG8000 as the pore forming agent.

93 Table 4-1. Effect of PEG addition on overall reaction rates (r) and ratio of rates (shape selectivity) of the embedded catalyst.

Pore r2-methyl-1- Catalyst forming r1-hexene r1-octene r1-decene pentene Shape Selectivity number agent (mmol/g.h) (mmol/g.h) (mmol/g.h) (mmol/g.h) (r1-hexene/r2-methyl-1-pentene) 1 None 24.3 10.4 5.3 2.36 10.3 2 PEG600 35.3 16.8 12.6 4.3 8.2 3 PEG8000 36.3 18 13.1 5.3 6.8

4.3.3.2. Approach 2: Reducing the size of the catalyst support (carbon spheres)

Control of the size of the carbon particles was achieved by changing the surfactant concentration during the polymerization step. How the average sphere size changed with the surfactant (Pluronic F-127) to FA mass ratio is shown in Table 4-2.

Table 4-2. Variation of carbon spheres diameter (diffusion length) by surfactant concentration. Pluronic F-127/FA (mass ratio) 1 2 4 Avergae carbon spheres’ size 276 220 172

Three different particle sizes of embedded catalysts were tested to evaluate the effect of diffusion length on the activity and shape selectivity. The results are shown in Figure 4.11 and

Table 4.3.

a a b b

Figure 4-11. Effect of diffusion length (carbon sphere size) on embedded catalyst activity for a) 1-hexene, b) 2-methyl-1-pentene.

By decreasing the spheres size from 276 nm to 172 nm, the overall catalyst activity increased by a factor of 300% while maintaining similar reactant shape selectivity, approximately

8:1, for 1-hexene versus 2-methyl-1-pentene.

Table 4-3. Overall reaction rates (r) and ratio of rates (shape selectivity) for different diffusion lengths

Catalyst Spheres r1-hexene r2-methyl-1-pentene Shape Selectivity number size (nm) (mmol/g.h) (mmol/g.h) (r1-hexene/r2-methyl-1-pentene) 1 276 24.3 2.36 10.3 4 220 34.4 3.86 8.9 5 172 61.8 8.11 7.6

95 4.3.3.3. Approach 3: Mild oxidation of carbon

Mild oxidations of the catalyst were conducted with a flow of carbon dioxide at two different temperatures (650 and 700°C) for 10 minutes. Hydrogenation reaction rates for 1- hexene and 2-methyl-1-pentene over the mildly oxidized catalysts versus the as-synthesized embedded catalyst are shown in Figure 4.12 and Table 4.4. The mildly oxidized catalysts showed higher activity compared to the as-synthesized embedded catalyst and the activity improved even further at higher oxidation temperature. As summarized in Table 4.4, catalytic activity increased by 300% and 800% after selective oxidation at 650 and 700°C, while reactant shape selectivity remained high at 11:1 and 7:1, respectively.

Table 4-4. Overall reaction rates (r) and ratio of rates (shape selectivity) after 10 min oxidation at two different temperatures.

Catalyst Oxidation r1-hexene r2-methyl-1-pentene Shape Selectivity number temperature (°C) (mmol/g.h) (mmol/g.h) (r1-hexene/r2-methyl-1-pentene) 1 none 24.3 2.36 10.3 6 650 62.7 5.4 11.6 7 700 191.6 25.6 7.5

a b

Figure 4-12. Conversion versus time of different alkenes on the as-synthesized embedded catalyst and catalysts oxidized at a) 650°C and b) 700°C.

4.3.3.4. Combining three approaches in one catalyst

To combine all the three approaches and to improve the catalyst further, an embedded catalyst was synthesized with an average carbon sphere size of 172 nm, with the addition of

PEG600 as the pore forming agent and with 10 minutes carbon dioxide oxidation at 650°C.

Hydrogenation reaction results for 1-hexene and 2-methyl-1-pentene are shown in Figure 4.13.

As can be seen, this catalyst showed an even higher activity when compared to all other embedded catalysts, since it has all these features together. It should be noted that for these reactions, just 300 mg of the catalyst was loaded into the reactor. The overall reaction rates for 1- hexene and 2-methyl-1-pentene based on conversion and mass of the catalyst were 330 and 43 mmol/g.h, respectively and the reactant shape selectivity (ratio of rates) was 7.6.

Figure 4-13. Conversion versus time of 1-hexene and 2-methyl-1-pentene on the embedded catalyst synthesized with all three approaches combined.

4.4. Discussion

This paper demonstrates several different approaches to preparing highly active, shape selective carbon catalysts for liquid phase reactions. Platinum nanoparticles were embedded in the microporous structure of PFA-derived carbon spheres using an emulsion polymerization and pyrolysis. Because of the highly cross-linked network of the polyfurfuryl alcohol chains, the platinum nanoparticles were inhibited from aggregation in the polymer structure during pyrolysis.

To be able to compare the reaction rates and catalysts effectiveness factors, it was necessary to have the same Pt loading in all the catalysts. Although the nominal platinum content

98 for the synthesized catalysts was 5 wt.%, the final platinum content was somewhat lower due to

platinum loss in solution during catalyst preparation. TGA results for the different as-synthesized

catalysts were used to determine the Pt content and all the catalysts were shown to have actual Pt

contents of Suppressed chemisorption was observed for all the catalysts even though they were

synthesized using different approaches and this effect was independent of the reduction time and

temperature. The maximum dispersion value which was obtained for the catalysts synthesized in

this study was ~ 10%. Similarly, low apparent metal dispersions were reported for carbon-

supported Pt catalysts elsewhere and these were shown to arise from presence of carbon atoms

that had migrated onto the surface of transition metal nanoparticles from the support as the

catalysts were cooled to room temperature after reduction [30, 31]. Nonetheless, turnover

frequency (TOF) values were calculated for both the supported and the embedded catalysts and

the results are shown in Table 4.5 for 1-hexene. These values for TOF are comparable to those

reported by Ng et al., [32] for platinum nanoparticles encapsulated in microporous carbon (also

shown in Table 4.5). However, in that study the authors did not report the number of exposed Pt

atoms from chemisorption. The values reported in Table 4.5 are calculated based on the initial

reaction rates (conversion< 20%).

Table 4-5. TOF for 1-hexene hydrogenation.

Supported control catalyst Embedded as- Embedded catalyst as- Catalyst synthesized in this study synthesized in this study synthesized by Ng. et al.[30] TOF (sec-1) 20 26 25

99 Figure 4.14 shows plots of TOF versus conversion, for both the supported and embedded catalysts. As can be observed in this figure, TOF remains relatively constant for the supported catalyst over the whole range of conversion until the reactant concentration is nearly depleted. In the case of the embedded catalyst, the TOF was highest, as we expected, early in the experiment

(due to preadsorption) and it started to decrease immediately due to a pronounced internal mass transfer limitation effect.

TOF (1/sec) TOF 10 Supported catalyst Embedded catalyst 5

0 0 20 40 60 80 100 1-Hexene conversion (mol%)

Figure 4-14. Comparison between TOF changes with conversion for 1-hexene hydrogenation over supported and embedded catalyst.

As measured by methyl chloride gas adsorption, the average pore size of the as- synthesized catalyst was 0.5 nm, which is similar to the size of the reactant molecules. Because of the near parity of the pore dimensions and the molecular dimensions, diffusion of the reactants and products in and out of the pores is affected and can even be severely hindered by the steric

100 hindrance imposed by pore walls. This diffusion-controlled mechanism governs the rate of the catalytic reaction and this leads to shape selective effects displayed by the embedded catalyst.

While, on the one hand, it is necessary to have diffusion limitations to obtain shape selectivity, it is also important to have sufficient activity to make this kind of catalyst technologically viable.

This issue is a matter that is best described by the quantitative relationship of the effectiveness factor and the Thiele modulus.

The catalyst efficiency can be evaluated using two parameters defined for heterogeneous catalysts, namely, Thiele modulus (φ) and effectiveness factor (η) (Equations 4-1 and 4-2) [33]:

1  1 1       Equation 4-2   tanh  

A general plot of effectiveness factor versus Thiele modulus for a spherical catalyst particle and a pseudo-first order reaction at constant temperature is shown in Figure 15. To improve catalyst effectiveness, the Thiele modulus must be decreased. In this work, we have done this by decreasing the size of the spheres (using the emulsion polymerization method of synthesis), and by increasing the diffusion coefficient by adding extra meso and macro porosity

(with the addition of PEG or by mildly oxidizing with carbon dioxide).

To experimentally determine an effectiveness factor for these new catalysts, the reaction rate on each embedded catalyst was divided by that of the supported (control) catalyst according to Equation 3. The supported catalyst was shown to have an effectiveness factor of unity with no mass transfer limitation. This was confirmed by changing the catalyst particles size (carbon spheres’ size) and observing the same activity for the supported catalyst. The shape selectivity of the embedded catalysts for hydrogenation of two different reactants, either with different lengths or different structures (linear or branched), was defined as the ratio of the overall reaction rates.

101

Equation 4-3

As seen from Figure 4.15, the three approaches significantly enhanced catalyst effectiveness, moving it closer to the knee of the curve with each improvement while maintaining shape selectivity. In the first approach, by adding PEG, more mesoporosity was created in the carbon spheres microstructure. Addition of PEG leads to a miscible blend with PFA that then phase separates during pyrolysis and leaves behind a small amount of mesoporosity in addition to the inherent microporosity [24, 25]. The extent of mesoporosity induced, for the same mass ratio of PEG to FA depended on the molecular weight of the PEG that was used. This small addition of mesoporosity decreased the Thiele modulus and increased the catalyst effectiveness factor (2 and

3 in Figure 4.15). The second approach, decreased the size of the carbon spheres, and thus the

Thiele modulus also decreased, which also led to higher catalyst effectiveness due to shorter diffusion lengths (4 and 5 in Figure 4.15).

The porosity can also be selectively changed by mild oxidation with carbon dioxide. It was previously shown that upon oxidation under CO2, the oxidation-susceptible domains that are less graphite-like oxidize first and this process opens the porosity of the carbon and makes more of the pores accessible. However, treating for longer time, or at higher temperatures oxidizes the graphitic carbon layers of the pores, thins the pore walls and widens the pores [34, 35]. The average pore size of the catalyst did not change after carbon dioxide oxidation at 650°C, but the platinum particles may have catalyzed the oxidation of the less graphitic carbon domains in close vicinity.

It is also possible that some of the closed porosity present in the carbon structure is opened in an even more specific way. Within PFA- derived carbons, there are carbon atoms that are the vestiges of cross-links between polymer chains of the precursor. These carbon atoms that

102 are thought to be closer tetrahedral than to trigonal hybridization and while small in population, prop open the structure and they resist graphitization due to their thermal and kinetic stability.

Although they are kinetically stable when heated to high temperatures, they are also very susceptible to oxidation under much milder condition [34, 35]. We suspect that it is these same carbons that block access to pores within the structure. When they are removed, access improves and does so apparently disproportionately to the small mass of carbon removed. It is in this way that the mild oxidation by carbon dioxide improved the catalytic activity and effectiveness factor of the embedded platinum catalysts (see 6 in Figure 4.15) while also retaining similar high selectivity. In a similar study it has been shown that mild oxidation of carbon-based catalyst improves hydrogen uptake and catalyst activity [36].

When conducted at higher temperature (700 °C), the oxidation with carbon dioxide is less mild. At this higher temperature there is a measurable change in the pore size and the total porosity of the carbon. These changes are no doubt produced by increased rates of oxidation of bulk carbon as well as that which is in the cross-linking positions. This process decreased the magnitude of the Thiele modulus (due to increased porosity) further and also improved the effectiveness factor (7 in Figure 4.15). This catalyst has an effectiveness factor and Thiele modulus that places its relative reactivity very close to the so-called “knee” of the effectiveness factor – Thiele modulus curve. The knee region of the curve is the optimal region of effectiveness for a heterogeneous catalyst because in this region the catalyst is operating close to a maximal overall rate at a density that is optimal for mechanical strength and for filling of the reactor.

Finally, we combined all three of these methods into the preparation of one catalyst. We synthesized small particles, added PEG to the synthesis mix and then we oxidized the catalyst with carbon dioxide at 650°C. This catalyst, 8, on the effectiveness factor curve (Figure 4.15), showed the highest activity for both reactants, 1-hexene and 2-methyl-1-pentene, and a shape selectivity of about 7:1 was retained.

103

1: 276 nm, as-synthesized 2: 276 nm, with PEG600 3: 276 nm, with PEG8000 4: 220 nm, as-synthesized 5: 170 nm, as-synthesized 6: 276 nm, oxidized at 650°C 7: 276 nm, oxidized at 700°C 8: all three approaches combined

Figure 4-15. Comparison of the effectiveness factor of the embedded catalysts synthesized using different approaches.

4.5. Conclusion

Embedded catalysts comprised of platinum nanoparticles contained within microporous carbon spheres were shown to be quite active and reactant shape selective for liquid phase hydrogenation reactions involving longer chain molecules. We report three distinct approaches that can be combined to raise effectiveness to requisite levels by fine tuning porosity, lowering overall mass transfer resistance and by increasing the active surface of the platinum particles while maintaining shape selectivity. These results bring us a few steps closer to being able to design highly stable, shape selective, transition metals embedded within microporous carbon catalysts for selective liquid phase reactions of practical as well as fundamental interest.

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108

Chapter 5

Effect of confinement in nanopores on reaction kinetics and adsorption parameters

(This chapter is published in Carbon journal. Maryam Peer, A. Qajar, R. Rajagopalan, H.C. Foley, “On the effects of confinement within a catalyst consisting of platinum embedded within nanoporous carbon for the hydrogenation of alkenes”, Carbon, 66, 459-466 (2014))

5.1. Introduction

Reactions that take place within the confined spaces of nanopores can proceed differently from those that happen within larger pores or on the outer surface of a catalyst [1-4]. Both

Transition State Theory and Reactive Monte Carlo simulation have been used to analyze and simulate the effect of nanopore size on reaction and adsorption. It has been shown that when the molecules are confined inside pores with dimensions similar in size to those of the molecule, the reaction barrier decreases and the reaction rate constant increases [5]. This kind of enhancement in reaction rate due to pore confinement in carbon slit pores with different sizes, or in nanotubes with different diameters was shown both for formaldehyde decomposition and ammonia synthesis

[5, 6]. As the pore size decreases, the stabilization effect of the pore walls for a specific molecule increases and, hence, the reaction rate constant increases. Both pore size and pore geometry was found to have significant effects on molecule stabilization, the surface density of the molecules on the pore wall and on the equilibrium adsorption constants [5, 6].

Shape selective catalysis combines reaction with diffusion in a way that can be used to control the distribution of products, or to select one molecule for reaction from a mixture of

109 molecules. [7-11]. However, diffusion of molecules inside ultramicropores with dimensions commensurate with the sizes of molecules can be very slow (10-10 to 10-17 m2/s) [12-15]. Thus even though having pores close to the size of the reactant molecule can enhance the reaction rate constant [5, 6], these same pores decrease the rate of mass transfer into and out of the catalyst such that the global rate is severely diffusion limited. As is well known, these mass transfer limitations can diminish the reported, apparent reaction rate constant and apparent reaction order compared to the actual values when reaction and diffusion effects are handled properly [16-18].

In this chapter, platinum nanoparticles embedded in ultramicroporous carbon spheres were synthesized previously (as explained in previous chapters) were used as a model catalyst to understand the effect of pore confinement and steric hindrance on the reaction rate constant and diffusion coefficients for alkene hydrogenations. The carbons synthesized in this study are considered to possess slit-like pore geometry with an average width of 0.5 nm [19]. A control catalyst was also prepared using the same carbon and similar platinum nanoparticles by supporting these on the surface of the carbon rather than embedding them within the pores of the carbon. This catalyst is the reference case catalyst since it works in the absence of any internal mass transfer limitations. Hence, the reaction kinetics over this catalyst should be intrinsic since internal mass transfer effects do not mask them. Liquid phase hydrogenations of different alkene molecules were conducted as probe reactions. Details on catalyst synthesis (both supported and embedded) and characterization were reported in chapter 4 [11, 19-21]. As also shown in that chapter, the embedded catalyst’s activity was significantly improved by changing the ultimate carbon particle size and by increasing the average pore size and extent of the pore size distribution in order to overcome transport limitations [11]. In this chapter we have analyzed the kinetics of the catalytic reaction that takes place inside the carbon ultramicropores by solving the reaction-diffusion equations that describe the overall process. With this separation of diffusion

110 from reaction terms, it is possible to better assess the effect of steric hindrance imposed by pore confinement on both reaction kinetics and the diffusional process.

5.2. Analysis of hydrogenation reaction data

5.2.1. Alkene hydrogenation kinetics over supported catalyst

Hydrogenation of an alkene can be represented by the following elementary steps that proceed through dissociative hydrogen adsorption on active sites (*) and a stepwise hydrogen addition to the adsorbed alkene (Horiuti-Polanyi mechanism) where A, denotes reactant and B denotes product [22, 23].

Hydrogen is assumed to be adsorbed dissociatively on the platinum surface of the catalyst

(in our case, platinum nanoparticles) as shown in step 1. The chemisorbed atomic hydrogen then reacts with an alkene molecule adsorbed on an adjacent active site (step 3). The half- hydrogenated alkene, a chemisorbed alkyl intermediate, then reacts with the second adsorbed hydrogen to form the final hydrogenated product (alkane) adsorbed on the surface (step 4). The adsorbed alkane then desorbs from the surface and leaves behind a vacant adsorption site for subsequent reaction to happen. Here, it was assumed that hydrogen and alkene are both

111 competing for the same adsorption sites and that the platinum sites are uniformly dispersed within the catalyst particle. Based on these assumptions and considering the addition of the first hydrogen atom to the adsorbed alkene to be the rate determining step (RDS), which is observed at low hydrogen pressures, we obtain the well-known Langmuir-Hinshelwood (LH) kinetic equation for the reaction rate:

Equation 5-1

2 k in above equation is defined as k1*[L] in which k1 is the reaction rate constant of the rate determining step (g/mol.s) and [L] is the concentration of active site in the catalyst (mol/g) which can be estimated using chemisorptions data. As shown in reaction elementary steps, ,

and are adsorption equilibrium constants of Hydrogen, reactant and product, respectively.

5.2.2. Transient diffusion-reaction in embedded catalyst

To obtain better insight into what takes place within the ultramicropores and at the locus of reaction that is at the nexus of pores and a portion of the surface of the platinum nanoparticle, the differential equations for coupled diffusion and reaction for all the three components involved in the reaction (reactant, product and hydrogen) were solved numerically. For one dimensional diffusion and reaction in the slit-like pores of the carbon, the mass balance for a component inside a differential element of the spherical catalyst particle, obeys the following equation:

112

Equation 5-2

In which “i” denotes the corresponding component and is the production or consumption rate of that component due to chemical reaction inside the pores. Based on the above equation, the observed change in concentration is the collective result of diffusion and reaction inside the pores. This partial differential equation can be solved numerically for all the components inside the pores (reactants and products). The initial and boundary conditions for solving the equations are taken as follows:

I.C.1: ,

B.C.1: ,

B.C.2: and

At time zero (before reaction starts) concentration inside the pores is equal to bulk concentration, since the catalyst was pre-mixed with reactant/solvent mixture for two hours prior to reaction (I.C.1). Concentration on the outer surface of catalyst (carbon spheres) is equal to bulk concentration at any specific time, since there is no significant bulk mass transfer limitation. It is assumed that there are very few active sites on the surface of carbon spheres and consequently, the reaction at the outer surface of the carbon is negligible and the total concentration change on the surface is determined by the rate of diffusion into or out of the pores (B.C.2).

For hydrogen, the first two conditions are the same, but at the sphere surface, the hydrogen concentration is taken to remain constant since hydrogen was being provided to the reactor to keep the pressure constant during the reaction. The corresponding boundary condition for hydrogen can be written as follows:

113

is equal to hydrogen solubility in the hydrocarbon at the corresponding pressure and temperature. To solve the differential equations, the reaction term in Equation 2, was replaced by the kinetic equation derived for the supported catalyst.

5.3. Results and discussion

5.3.1. Supported catalyst

Previously, we had extensively characterized the synthesized catalysts using various characterization techniques [11]. X-ray diffraction (XRD) showed that the average size of platinum nanoparticles was 4-5 nm. Transmission Electron Microscopy (TEM) and Scanning

Electron Microscopy (SEM) were also used to evaluate platinum nanoparticles size, dispersion, morphology and size of the carbon support.

The supported catalyst showed high activity (1.3×10-3 mol/g.s, almost complete conversion after 25 minutes) towards hydrogenation of different alkenes used in this study. But, the catalyst also showed no significant difference in reactivity toward the different alkenes though they varied in molecules shape and size. The hydrogenation of 1-hexene on the supported catalyst was conducted at four different hydrogen pressures (2, 4, 7 and 9 bar). At each hydrogen pressure, the solubility of hydrogen in the reactant-solvent mixture was calculated using data in

-3 -3 literature [24, 25]. These concentrations were calculated to be 1.3×10 mol/gcat, 2.6×10 mol/gcat,

-3 -3 4.5×10 mol/gcat and 5.8×10 mol/gcat, respectively. Since the supported catalyst showed similar activities for all the alkenes used in this study, the rate equations and the calculated constants are the same for all the reactants. The overall 1-hexene hydrogenation reaction rates on the supported

114 catalyst increased by increasing pressure, as can be seen in Figure 5.1. In this figure, dashed lines show the results of fitting the LH model to the experimental data. The parameters obtained by fitting the experimental data to the kinetic model, are shown in Table 5.1.

Table 5-1. Adsorption equilibrium constants and reaction rate constant on supported catalyst. Kinetic Parameter Values

KH2 80

KA 104

KB 28 9 k1 3.6×10

As shown in Table 5.1, the alkene has a higher adsorption constant when compared to that of the saturated product (alkane), which shows that the product can desorb more readily from the surface after it is formed. As shown in literature [22, 23], it is also common to assume that surface concentration of the hydrogenated product is negligible compared to adsorbed reactants which is consistent with the adsorption constant being lower for the alkane.

115

Figure 5-1. hexene hydrogenation reaction, conversion versus time on 5 wt.% Pt supported on activated carbon spheres at different hydrogen pressures.

5.3.2. Embedded catalyst

In the case of the embedded catalyst, the major mechanism for diffusion is somewhat like surface diffusion and is often referred to as configurational diffusion [26]. Molecules that move in and out of the pores have their motions severely affected by the overlapping force fields exerted by the pore walls [14]. The effective configurational diffusion coefficients of different components are functions of the dimensions of the molecules, the pore size and the concentration of the components within the micropore. The embedded platinum catalyst showed real

116 differences in the global rates of alkene hydrogenation depending on their size and shape. The global rates of hydrogenation were higher for smaller alkenes because they have higher diffusion coefficients within these pores. Hence the conversions observed for the embedded catalyst are controlled by mass transfer limitations.

In the first pass at fitting the experimental conversion data for the embedded catalyst, the diffusion coefficients for the three components were allowed to vary, while the kinetic rate constants and adsorption equilibrium constants were fixed and set equal to the values that had been obtained for supported catalyst. In other words if the only difference between the global rates of hydrogenation were due to the imposition of a diffusion process, then the same intrinsic rate constants should be about equal for the embedded platinum catalyst as for the supported platinum catalyst. However, when this was done, it was observed that the model could not be properly fitted to experimental results. At best the fit had a large relative error confirming that the kinetic parameters are also changing. Table 5.2 summarizes average relative errors between experimental data and modeling results for 1-hexene hydrogenation and three different catalysts.

Table 5-2. Average relative error between model and experimental data for 1-hexene (two different assumptions: constant and variable kinetic parameters).

Catalyst Variable kinetic parameters Constant kinetic parameters Pt/C 6% 20% Pt/C PEGDiacid 6.6% 21% Pt/C activated 7.9% 24%

117 When the kinetic parameters were also allowed to vary along with the diffusion coefficients, a good fit with a much lower relative error (maximum relative error of 10%) was produced. The fit of the simulated results to the data for 1-hexene hydrogenation on the embedded catalyst are shown in Figure 5.2. It can be observed that model fits the experimental data with a good reasonable accuracy and the corresponding effective diffusion coefficients and adsorption parameters are given in Table 5.3.

Figure 5-1. 1-hexene hydrogenation conversion on three different embedded catalysts (markers: experimental, dashed lines: modeling results.

In Tables 5.2-5.5, Pt/C PEGDiacid denotes the embedded catalyst synthesized with

PEGDiacid additive as a pore forming agent and Pt/C activated denotes the embedded catalyst that was selectively oxidized for 10 minutes at 700ºC to etch the pores around the platinum

118 particles. Adsorption and diffusion parameters calculated for 1-decene and 2-methyl-1-peneten hydrogenation are also shown in Tables 5.4 and 5.5, respectively.

Table 5-3. Diffusion coefficients and adsorption equilibrium constants derived from simulation for 1-hexene hydrogenation on supported and embedded catalysts.

Dhexene Dhexane Dhydrogen Khexene Khexane Khydrogen Catalyst (m2/s) (m2/s) (m2/s) (g/mol) (g/mol) (g/mol) Supported - - - 104 28 80 Pt/C 3.8×10-15 1.1×10-15 2.7×10-13 160 190 0.4 Pt/C PEGDiacid 3.9×10-15 1.2×10-15 8.1×10-13 130 180 1 Pt/C activated 3.6×10-14 1.2×10-14 1.2×10-12 110 55 10

Within the embedded catalyst, the reaction occurs in spaces that are similar in size to the dimensions of the molecules within them. The steric hindrance that is imposed by the pore walls on the reactant and product molecules can affect the reaction pathways. As shown in Table 5.3, for 1-hexene hydrogenation reaction inside the ultramicropores of the embedded catalyst, adsorption equilibrium constants on platinum nanoparticles within the carbon increased for both the reactant (1-hexene) and the product (hexane) when compared to the same equilibrium constants for adsorption on the supported platinum catalyst. Since the effects of diffusion are already separated from the kinetics in the differential equation describing the system, the observed effect can be explained purely by the effect of the micropore on adsorption, desorption and reaction of the molecules on the platinum. The 1-hexene molecules that have diffused into the micropore interact with the pore walls and the exposed surface of the platinum nanoparticle. The adsorbed state of the alkene in this situation is stabilized when compared to adsorbed state of the alkene on platinum dispersed on supported catalyst. This arises as a synergistic effect that combines the adsorption on platinum with adsorption in the molecular-sized pore. Figure 5.3a, shows a schematic of a platinum nanoparticle embedded in a porous carbon structure and Figure

119 5.3b, illustrates 1-hexene and 2-methyl-1-pentene molecules diffusing inside the ultramicropores of carbon. In Figure 5.3c and d, molecules have reached the locus of reaction and each molecule experiences a different degree of confinement and loss of degrees of freedom, based on its size and shape. The confinement effect constrains the motion of the molecules and it is an effect that is imposed by the pore walls.

a c

b d

Figure 5-3. a) Platinum nanoparticle embedded in microporous carbon, b) 1-hexene and 2- methyl-1-pentene diffusing inside carbon micropore, c)1-hexene and d) 2-methyl-1-pentene molecule at active site inside the pore (double bond is interacting with platinum surface (dashed red arrows).

120 This kind of confinement effect has been considered in the context of transition state theory- [3, 5-6]. As can be observed in Table 5.3, after widening the pores by adding of

PEGDiacid to the precursor or by selective oxidation of the catalyst with carbon dioxide, the steric hindrance and the confinement effect was diminished. Consistent with our expectation, this led to a decrease in the adsorption equilibrium constants. For the oxidized version of the embedded catalyst the magnitudes of the equilibrium constants approach those of the supported catalyst as expected. The effect of the micropore on the adsorption equilibrium constant for the product molecule was even more pronounced, presumably because the product is bulkier than the reactant. Similar trends were observed for other reactant molecules as shown in Tables 5.4 and

5.5. As the reactant molecule increased in size and bulk, the increase in adsorption constants was much more pronounced.

Table 5-4. Diffusion coefficients and adsorption equilibrium constants derived from simulation for 1-decene hydrogenation on supported and embedded catalysts.

Ddecene Ddecane Dhydrogen Kdecene Kdecane Khydrogen Catalyst (m2/s) (m2/s) (m2/s) (g/mol) (g/mol) (g/mol) Supported - - - 104 28 80 Pt/C 1.4×10-15 5×10-16 1.2×10-13 270 1800 0.11 Pt/C PEGDiacid 1.8×10-15 8.7×10-16 1.9×10-13 200 660 0.7 Pt/C activated 1.2×10-14 8×10-15 6.1×10-13 150 100 5

Table 5-5. Diffusion coefficients and adsorption equilibrium constants derived from simulation for 2-methyl-1-pentene hydrogenation on supported and embedded catalysts.

Ddecene Ddecane Dhydrogen Kdecene Kdecane Khydrogen Catalyst (m2/s) (m2/s) (m2/s) (g/mol) (g/mol) (g/mol) Supported - - - 104 28 80 Pt/C 8.2×10-16 3.5×10-16 6.7×10-14 520 22800 0.028 Pt/C PEGDiacid 9×10-16 4.5×10-16 8.1×10-13 450 18000 0.2 Pt/C activated 8×10-15 2×10-15 1.3×10-13 210 1190 3

121 Interestingly, the hydrogen adsorption equilibrium constant in the embedded catalysts was smaller than the same constant in the supported catalyst. For the embedded catalyst, hydrogen must compete with alkene and alkane for adsorption on the exposed platinum surface within the carbon micropore. Because there is so little room in the vicinity of the active site, hydrogen is not able to compete with the alkenes as effectively as it can when there is more space in the vicinity of the platinum and when more of the platinum surface is exposed. These effects decrease the hydrogen adsorption equilibrium constant to lower values as observed in Tables 5.3-

5.5.

The values of the diffusivities calculated for three different embedded catalysts and three different reactant molecules (as presented in Tables 5.3, 5.4 and 5.5), show that effective diffusion coefficients of the molecules inside the pores were increased by catalyst modification.

As expected, the branched molecule used in this study (2-methyl-1-pentene) had the lowest diffusion coefficient in the carbon slit pores. This is due to the 2-methyl-1-pentene having the highest deviation from planarity (0.625) compared to other two reactant molecules (0.436 and

0.602 for 1-hexene and 1-decene, respectively. These deviations from planarity were estimated using MM2 energy minimization in ChemBio3D software.

The conversion versus time data for all the alkene molecules tested in this study were fitted to the reaction-diffusion model and the corresponding reaction rate constants were obtained and are shown in Figure 5.4. The results show that reaction rate constant (k1) increased by almost one order of magnitude when reaction happens inside the ultramicropores. The extent of this effect depends on molecule size.

122

Figure 5-4. Reaction rate constants for different molecules on different catalysts.

The reaction rate constant (k1) depends on the pre-exponential factor, the reaction barrier

(activation energy) and the temperature:

Equation 5.3

Based on Equation 5.3, the increase in reaction rate constant inside the pores can be attributed either to a decrease in activation energy and an increase in pre-exponential factor or both. The pre-exponential factor is comprised of the ratio of partition functions for the transition

123 state to those of the reactant molecules. To calculate the activation energy, 1-hexene hydrogenation was conducted at three different temperatures over both the supported and embedded catalysts. Conversion versus time data on the supported and embedded catalyst, and the plot of ln(k1) versus reciprocal of temperature, are shown in Figures 5.5 and 5.6.

The activation energy for 1-hexene hydrogenation on the embedded catalyst was almost 1 kJ/mol lower than that of supported catalyst. Although the change is small, this effect of confinement on activation energy was consistent with other results found in the literature [5, 6].

13 The pre-exponential factor ( ) increased from 5.2×10 g/mol.s for supported catalyst to

9.7×1013 g/mol.s for embedded catalyst. The increase in the pre-exponential factor and the decrease in the activation energy clearly should be even more pronounced for larger molecules.

a b

Ea=24.3 kJ/mol =5.2×1013

g/mol.s

Figure 5-5. a) 1-hexene conversion-time data on supported catalyst at different temperatures, b) ln(k1) versus reciprocal of temperature.

124

a b

Ea=23.2 kJ/mol 13 =9.7×10 g/mol.s

Figure 5-6. a) 1-hexene conversion-time data on embedded catalyst at different temperatures, b) ln(k1) versus reciprocal of temperature.

5.4. Conclusion

In this chapter, liquid phase hydrogenation of different alkenes over supported catalyst versus an embedded catalyst was analyzed. A kinetic rate form was first determined for the supported catalyst; then by coupling this rate form with the configurational diffusion within the micropores, we were able to accurately describe the behavior of the embedded catalyst, but only if we varied both the parameters for diffusion and those for reaction. The diffusion coefficients for different reactants and products inside micropores were found to fall in the range of 10-16 to

10-12 m2/s, depending on the size of the molecule. It was shown that reaction rate constant is also significantly affected by the pore confinement and changes in following order: 2-methyl-1- peneten > 1-decene > 1-hexene. The increase in the reaction rate constant by almost one order of magnitude for the largest reactant in this study (2-methyl-1-peneten), can be attributed to

125 increased confinement that is expressed kinetically as a simultaneous decrease in the activation energy and an increase in the pre-exponential factor.

5.5. Nomenclature

KA and KB: Adsorption equilibrium constants of reactant and product (g/mol) k1: reaction rate constant of the rate determining step (g/mol.s)

[L]: concentration of active catalytic sites (mol/g)

Ci: concentration of reactants and products (mol/g)

: radial distance inside carbon spheres (m) ri: reaction rate of component i (mol/g.s)

R: catalyst particle size (m)

Acat: catalyst outer surface area (m2) mcat: mass of loaded catalyst (g)

Di: diffusion coefficient of component i (m2/s)

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Chapter 6

Synthesis of carbon with bimodal porosity

6.1. An overview on hierarchical/bimodal carbon synthesis and its application

Porous carbon materials possess unique properties such as high surface area, large pore volume, and good thermal stability. These characteristics make them suitable candidates for different applications including catalysis, gas separation, adsorption and electrodes in electrochemical capacitors for energy storage purposes [1-4]. To have acceptable performance in many applications, an interconnected porous structure with both meso and microporosity is necessary. The presence of meso and macropores facilitate mass transfer processes that are the controlling steps in many applications. It is the micropores that provide high surface area and size selectivity at the molecular level in the adsorption process. Researchers have been using different methods to engineer the pore size and connectivity in carbon so as tailor its properties and performance for different applications [5-13]. Carbons derived from thermosetting resins intrinsically contain micropores in the range of 0.4 – 0.5 nm. The porosity in these carbons is the result of misalignment of the graphitic domains [2] and is created when the thermoset polymer is heat treated at temperatures higher than 300°C but not above 800°C. The micropore size of these carbons can be enlarged by either physical or chemical activation while maintaining a narrow pore size distribution [14-17]. Morphology and size of the carbon particles can also be controlled using structure-directing agents, for example during emulsion polymerization of the carbon precursor [18, 19]. It has been shown that by decreasing the diameter of carbon spheres synthesized by the emulsion polymerization of furfuryl alcohol, mass transport of reactant and

130 products inside the pores can be controlled. This in turn improves catalytic activity in liquid phase hydrogenation reactions, while maintaining high selectivity due to microporosity [20, 21].

To form pores in the mesopore range (2 – 50 nm) within polymer-derived carbons, it is necessary to use a template (either soft or hard) during the synthesis of the corresponding polymer [5, 7]. Using the hard templating method (nanocasting), mesoporous inorganic oxides

(zeolite, silica, etc.) are infiltrated with a monomer and then polymerization is initiated within the pores. Mesoporosity is formed within the carbon by removal of the hard template with strong acid or strong base after pyrolysis. The mesopore size distribution and pore volume are determined by the structural of the hard template [8-11]. Although the carbons so-produced have interesting structures, the post synthesis processing involves the use of harsh chemicals and the destruction of the rather expensive mesoporous inorganic oxide.

It is also possible to synthesize mesoporous carbons using soft-templating methods. In this technique, a structure-directing agent such as a tri-block copolymer is used as an organic template. In the presence of solvent, these polymers self-assemble and create micellar structures.

The polymer precursor to the carbon is then formed around these micellar structures during emulsion polymerization [7, 22]. Mesopore size in these types of carbon depends mostly on the size and molecular weight of the templating reagent. Recently, it has been shown that it is possible to make ordered mesoporous carbon using this approach with phloroglucinol- formaldehyde as the polymer precursor [23]. A carbon with well-ordered mesopores in the range of 8 – 20 nm can be formed in this way.

In the present study, we demonstrate that it is possible to use the soft-templating approach to develop carbons with well-defined bimodal pore distributions. This was made possible by simultaneous polymerization of two monomers (furfuryl alcohol and phloroglucinol) in the presence of a structure-directing agent (Pluronic F-127) molecule. The effects of

131 polymerization conditions on the formation of micro- and mesopores were studied in detail and the role played by the surfactant molecule was elucidated.

6.2. Experimental

6.2.1. Synthesis of homo-polymers, polymer blends and polymer mixtures

Furfuryl alcohol and phloroglucinol were purchased from Sigma-Aldrich and used as received. Deionized water and ethanol were used as solvents in the synthesis of the polymers. An amphiphilic triblock copolymer (EO106PO70EO106), Pluronic F-127, was used as the structure- directing agent (purchased from Sigma-Aldrich). Formaldehyde and HCl were used as linker and polymerization initiator, respectively.

To make mesoporous carbon, pholoroglucinol was polymerized as described in Dai’s work [23]. In a typical synthesis, 8.5 g water and 9.4 g of ethanol were mixed. 2.5 g of pluronic

F-127 and 2.5 g of phloroglucinol were then added to this solvent. After complete dissolution of the solid powder in the solvent, 0.2 g HCl was added as the polymerization initiator. After 30 minutes of stirring, 2.6 g formaldehyde was added and polymerization was continued for an additional hour. At this step, phase separation occurred and water rich phase was removed. The polymerization continued for an additional 12 hours at room temperature; then the polymer was placed in an oven at 100°C for 12 hours. Finally the resultant polymeric solid was pyrolyzed at

850°C under an argon atmosphere. The mesoporous carbon made using phloroglucinol is referred to as Ph-C. A sample was synthesized under exactly the same conditions with furfuryl alcohol.

Basically, all other parameters and amounts were kept constant except that 2.5 g of furfuryl alcohol was added instead of phlologlucinol. This sample is referred as FA-Plu-C. To study the

132 effect of surfactant, furfuryl alcohol alone was polymerized in a water/ethanol solution under the same conditions used for preparation of FA-Plu-C, but without any surfactant (FA-C).

Both monomers were also simultaneously polymerized as follows: 8.5 g of water was mixed with 9.4 g of ethanol. To this solution, 2.5 g pluronic F-127 and 2.5 g phloroglucinol were added. After complete dissolution of the solid powders in the solvent mixture, 0.2 g HCl was added and the solution stirred for few minutes; this was followed by addition of the second monomer: furfuryl alcohol. The solution was stirred for 30 more minutes, and until the color turned to light pink. At this point, 2.6 g formaldehyde was added to the solution and the polymerization was continued for an additional hour after which phase separation occurred with the polymer-rich phase in the bottom of the beaker and water-rich phase above. The water-rich phase was separated and polymerization in the polymer-rich phase was continued overnight

(almost 12 hours). The result of the polymerization was a rubbery polymer film that was placed in an oven at 90°C for 12 hours. The dried polymer was pyrolyzed at 850°C for 5 hours to obtain porous carbon (FA-Ph-C). In another experiment, in order to make a physical blend of the two homopolymers, 1 g of synthesized PFA was added to solution during the polymerization of phloroglucinol; this was done in the presence of pluronic F-127 (2.5 g) using 0.2 g HCl as initiator and 2.6 g of formaldehyde as the linker (PFA-Ph-C). The effects of polymerization conditions on porosity of the resultant carbons were studied by varying the concentration of the monomers, acid and surfactant.

6.2.2. Characterization of synthesized polymers and carbon

Field Emission Scanning Electron Microscopy (FEI Nova NanoSEM 630) was used to examine the morphological and textural properties of the synthesized polymers and the carbons.

Field Emission Transmission Electron Microscopy (JEOL 2010F) was used to image the carbon

133 structure and the pore arrangement. The pore size distribution and the total pore volume of the synthesized carbon samples were determined by methyl chloride gas adsorption combined with the Horvath-Kawazoe model for micropore region and the Kelvin equation for mesopore region.

The BET surface areas were measured using nitrogen gas adsorption.

6.3. Results

6.3.1. Characterization of homopolymer derived carbons

Our previous studies have shown that carbons derived from polyfurfuryl alcohol by acid- catalyzed polymerization results in a microporous material with a narrow pore size distribution centered around 4 – 5 Å [19, 24]. In this study, we have synthesized PFA in a water/ethanol solvent both with and without Pluronic F-127. As seen before, PFA made without surfactant present, was purely microporous with a pore size around 0.5 nm [25]. However, the PFA synthesized in the presence of surfactant (FA-Plu-C), had both micropores similar to PFA and a broad mesopore distribution ranging from 2 – 20 nm as shown in Figure 6.1. As can be seen in this figure, when PFA is synthesized without surfactant (6.1C), a rather smooth monolithic carbon structure without any mesopores is obtained upon pyrolysis. In the sample made with surfactant, the existence of mesopores is evident from FESEM image (6.1d) and it can be attributed to the gaps between small carbonaceous domains that are agglomerated during pyrolysis. The micropore volume of FA-Plu-C (0.12 cc/g) was slightly less than FA-C (0.18 cc/g) while significant mesopore volume ~ 0.064 cc/g (35% of total porosity) was generated.

134

a b

c d

Mesopore

Figure 6-1. a) Pore size distribution, b) cumulative pore volume, c and d) FESEM images of PFA-derived carbon synthesized without and with surfactant, respectively (insets: zoomed in images)

135 The effect of acid concentration on the pore size distribution was also studied. In general, the micropore volume increased from 0.09 cc/g to 0.15 cc/g when the acid concentration was increased from 0.04 M to 0.18 M, as shown in Table 6.2.

Table 6-1. Effect of surfactant on micro and mesoporosity of PFA-derived carbon.

Mean Mean Micropore Mesopore Total pore FA HCl Surfactant micropore mesopore volume volume volume (mmoles) (mmoles) (g) size (nm) size (nm) (cc/g) (cc/g) (cc/g)

25.5 2 2.5 0.52 Broad(2-20) 0.12 0.064 0.184 25.5 2 - 0.6 - 0.176 - 0.176

Table 6-2. Effect of acid concentration on micro and mesoporosity of FA-Plu-C.

HCl Mean Mean Micropore Mesopore Total pore FA Molarity micropore mesopore volume volume volume (mmoles) (M) size (nm) size (nm) (cc/g) (cc/g) (cc/g)

10 0.04 0.6 - 0.095 - 0.095 10 0.18 0.63 Broad(2-20) 0.15 0.02 0.17

Phloroglucinol-derived carbon was predominantly mesoporous with narrow pore size distribution centered on 8.2 nm and a small volume of micropores centered at about 0.6 nm. The mean mesopore size was a strong function of the acid concentration, while the micropore size and volume were invariant. By increasing the HCl concentration from 0.04 M to 0.18 M, the mean mesopore size can be increased from 4.7 to 11 nm. The total mesopore volume also scaled with

136 increase in acid concentration with the cumulative pore volume reaching 0.7 cc/g, as shown in

Figure 6.2a and b. The porous texture of the carbon was examined using FESEM and STEM imaging (Figure 6.2c and d). The interconnected mesoporosity in these carbons can be visualized as the gaps between dense carbon domains.

137 6.3.2. Characterization of bimodal porous carbons

Next, we systematically varied the relative amounts of furfuryl alcohol and phloroglucinol while keeping the concentrations of HCl, formaldehyde and Pluronic F-127 constant during the simultaneous polymerization of both the monomers.

Table 6-3. Textural properties of bimodal carbon synthesized with different monomer compositions (2 mmoles of HCl).

Mean Mean Micropore Mesopore Total FA Ph FA/Ph micropore mesopore volume volume pore (mmoles) (mmoles) size (nm) size (nm) (cc/g) (cc/g) volume (cc/g) 25 0 - 0.5 Broad(2-20) 0.12 0.06 0.18 36 20 1.8 0.5 - 0.14 - 0.14 20.4 20 1.02 0.54 Broad(2-20) 0.155 0.03 0.185 15 20 0.75 0.53 3.2 0.13 0.08 0.21 10 20 0.5 0.51 3.5 0.23 0.15 0.36 7.5 20 0.375 0.68 5.7 0.12 0.25 0.37 5 20 0.25 Broad 10.2 0.18 0.33 0.51 0 20 0 Broad 8.3 0.17 0.41 0.58

Table 6.3 summarizes the effect of monomer composition on the effective pore size distribution of the derived carbons. When the mole ratio of FA to HCl was 12.5, it yielded a carbon with a micropore volume of 0.12 cc/g and mesopore volume of 0.06 cc/g. For our study, we intended to keep the moles of phloroglucinol constant while the furfuryl alcohol content was varied. When the [FA]/[Ph] ratio was 1.8, the mesopore volume significantly decreased and the carbon was essentially microporous in nature with a total pore volume of 0.14 cc/g. We saw that total pore volume steadily increased with an increase in the phloroglucinol content. Even with an equimolar ratio of [FA]/[Ph] ~ 1, the carbon was still mainly microporous (0.155 cc/g) with only a broad tail of mesopore volume (~ 0.03 cc/g). When the [FA]/[HCl] ratio was decreased to 7.5

138 and the [Ph]/[HCl] ratio was held at 10, and the [FA]/[Ph] ratio was 0.75, a carbon with a bimodal pore size distribution with narrow pore size distribution and slightly higher total pore volume

(0.21 cc/g) was obtained. The mean micropore size was 0.5 nm and the mean mesopore size was

3.2 nm. This new bimodality was retained when the [FA]/[Ph] ratio was decreased to 0.5 and even when it was further decreased to 0.37. The mesopore volume and the average mesopore size increased from 0.21 to 0.37 cc/g and 3.2 to 5.7, respectively, moving towards higher phloroglucinol concentration. Decreasing the [FA]/[Ph] ratio further to 0.25, yielded a carbon with a large mesopore volume, quite similar to that obtained with the phloroglucinol-derived carbon, but this new carbon had both broad micropore and mesopores size distributions.

In addition to polymerizing the monomers simultaneously, we also used blended pre- made PFA during the polymerization of phloroglucinol. This resulted again in a carbon with a bimodal pore size distribution with similar mean micropore size of 0.52 nm and a slightly larger mean mesopore size of 4.8 nm. It is interesting and important to note that the total pore volume of the carbon derived from simultaneous polymerization was significantly higher than that derived by simply blending the two polymers.

The textural properties and distribution of the pores through the carbon samples were also studied using FE-SEM. As shown in Figure 6.3c and d, in the case of simultaneous polymerization to produce FA-Ph-C, more uniform textural properties with distinct mesopores and interconnected porosity were obtained. On the other hand, by blending the two polymers the resultant carbon (PFA-Ph-C) showed a more dense morphology, with texture similar to Ph-C.

139

Figure 6-3. a) Pore size distribution and b) cumulative pore volume of PFA-Ph-C and FA-Ph-C samples, c) FESEM and STEM images of FA-Ph-C and d) FESEM image of PFA-Ph-C.

Since the surfactant plays the role of the templating agent and leads to the formation of the mesoporosity, the amount of surfactant was varied to study the corresponding effect on the pore size and porosity of the synthesized carbon. As can be seen in Figure 6.4a and b, an increase in the surfactant concentration resulted in a smaller mesopore size (shifting from 5 to 3.5 nm) and

140 a higher total porosity, while keeping the average micropore size almost constant. In all cases examined, the narrow distribution of the pores in both the micro and mesopore region was also maintained.

Figure 6-4. a) Pore size distribution and b) pore volume of FA-Ph-C synthesized at different surfactant concentrations.

Another approach to control both the micro- and mesopore size and the surface area is through selective oxidation of carbon in a CO2 stream at 900ºC. Figure 6.5 shows how the pore size changes when the co-polymer derived carbon (FA-Ph-C) was treated for 70% yield (30% burn off) and 33% yield (67% burn off). As can be seen in this figure, CO2 oxidation of the carbon resulted in an increase in the mean micropore size from 5.2 Å to 6.8 Å, while the mean mesopore size shifted from 3.5 nm to 5.5 nm for 30% activated sample and then up to 6 nm for

67% activated sample (Figure 6.5). The BET surface area for the as-synthesized bimodal carbon

141 was 440 m2/g and this increased to 1500 m2/g after 67% activation in CO2. The extent of activation can be furthered to obtain even larger pores in both regions.

a b

Figure 6-5. a) Pore size distribution and b) cumulative pore volume of bimodal and activated bimodal carbon.

6.4. Discussion

Carbons that contain only micropores have important, but limited application, such as the recovery of nitrogen from air by pressure swing adsorption. To be more generally useful in catalysis and possibly in other separation processes, new carbons having a hierarchical pore structure that includes meso- and macropores connected to the micropores are of real interest. For many applications, except those with only the very smallest molecules, mass transport to the micropores must be fast and efficient. Diffusion through micropores alone imposes limitations on

142 rates that are too severe to tolerate, yet it is in the micropores where molecular discrimination based on size and shape may take place and that can provide previously unseen effects.

Interestingly, we have seen in multiple cases that engineering into the carbon even a small volume of mesoporosity results in markedly faster kinetics and therefore more effective use of the micropores [20, 24-31]. In this study, we have synthesized much more mesoporosity in PFA- derived carbons, more than ever before, by simultaneously polymerizing furfuryl alcohol and phloroglucinol. The result is a carbon with distinctly bimodal pore size distribution.

Homo-polymerization of furfuryl alcohol in an emulsion with a surfactant molecule can result in formation of well-defined nanospheres as shown previously [18, 19]. In this paper, we adopt a similar approach in which furfuryl alcohol is polymerized using an acid initiator (HCl) in the presence of Pluronic F-127 (surfactant). Pluronic surfactants have been studied widely by other researchers and it has been shown that these surfactants form spherical core-shell structures at the low concentrations studied in this work [32, 33]. During polymerization, furfuryl alcohol diffuses into the hydrophobic core of the micelles and polymerizes cationically. This forms a dense polymer structure as shown in Figure 6.6. In order to form distinct spheres, it is important to crosslink the polymer through a secondary acid treatment followed by removal of unreacted excess monomer. For this study, we did not perform this process and hence we obtain irregular carbons particles. However, this carbon (FA-Plu-C) shows presence of both micropores and small amount of broad mesopores. The origin of microporosity is related to the intrinsic curvature in the polyaromatic structures of the carbon formed due to the presence of five or seven membered rings in the domains [34]. The mesopore formation is influenced by the presence of surfactant induced micellar structures as shown in Figure 6.6. The polymer fills the micellar core and during pyrolysis, these cores coalesce together resulting in a semi-continuous carbon structure with mesopores created due to the presence of gaps between the carbon particulates.

143

Pluronic F-127 micelle PFA growing within the micelle core

Figure 6-6. Polymerization of FA in the presence of pluronic F-127 and synthesis of FA-Plu-C (the thin white lines in the resultant carbon structure, are the micropores)

The mechanism of phloroglucinol polymerization is different from that of furfuryl alcohol. While HCl still acts as a catalyst, phloroglucinol polymerization proceeds through a step growth polymerization mechanism resulting in a polycondensation reaction between phloroglucinol and formaldehyde. Surfactant molecule plays a key role in determining the mesopore size. It has been shown that phloroglucinol forms strong hydrogen bonds with the outer hydrophilic domains of the micelles and then it begins to polymerize slowly along the outer edge of the micelles [23] as shown in Figure 6.7. After pyrolysis, the unreacted inner core of the

144 micelles forms the mesopores in these carbons [23]. Previously, it has also been shown that increasing the concentration of the acid catalyst accelerates the polymerization rate, limiting the transport of phloroglucinol monomer into the micelles thereby forming the polymer in both the solvent phase and around the micellar region, leading to the formation of bigger pores in the mesopore region [35].

Phloroglucinol polymerizing Pluronic F-127 micelle around the micelle corona

Figure 6-7. Schematic of polymerization of phloroglucinol in the presence of pluronic F-127 and synthesis of Ph-C.

Although phloroglucinol homo-polymer forms a carbon with both micro and mesoporosity upon pyrolysis, the micropores have a broad distribution and the micropore volume

145 is less than the mesopore volume. In order to prepare a carbon with narrow micro and mesopore distribution, large micropore volume and tunable mean mesopore size, both monomers were polymerized simultaneously. Simultaneous polymerization of both the monomers results in an interesting polymerization phenomenon. In order to understand the bimodal distribution, it is important to appreciate the two different mechanisms of polymerization. Furfuryl alcohol polymerization is cationic and hence has faster kinetics as compared to sol gel polymerization of phloroglucinol [36-38]. This effect can be seen clearly in the porosity data presented in Table 6.3.

When the ratio of monomers is bigger than one ([FA]/[Phloroglucinol] > 1), micropores alone are the key feature of the carbon and only a limited volume of mesopores with a very broad distribution are formed. The broad tail of mesopores in the overall pore size distribution results from the micellar templating. When the [FA]/[Ph] < 0.25, the effects of the phloroglucinol polymerization dominate and the result is a mesoporous carbon structure similar to that of pure phloroglucinol-derived carbon. However, when the [FA]/[Ph] falls in the range from 0.4 to 0.75, the conditions seem to be optimal for the formation of a distinctly bimodal distribution of pore sizes.

The origin of the distinct bimodality in the pore sizes can be visualized by considering the loci of polymerization of both the monomers. Assuming that the polymerization of both the monomers is favorable, furfuryl alcohol polymerization ensues at the center of the micelle, while phloroglucinol polymerization can occur both in the solvent and around the corona of the micelle as shown in Figure 6.8. After pyrolysis this results in the observed bimodal distribution and in a smaller mesopore size that results from pyrolysis of the phloroglucinol homopolymer.

146

Figure 6-8. Schematic of simultaneous polymerization of phloroglucinol and FA in the presence of pluronic F-127 and synthesis of bimodal carbon (FA-Ph-C).

We also demonstrated that the mesopore size can be altered by increasing the surfactant concentration. Increasing the surfactant concentration decreases the micelles’ sizes but increases the number of micelles, upon pyrolysis this translates into carbons with higher pore volume, but a smaller mesopore size. The effect of simultaneous polymerization can further be understood by comparing it with the results of PFA-Ph-C, which is synthesized by simply blending pre-formed

PFA into solution during the polymerization of phloroglucinol. In this case, we again form bimodal pore size distribution, but the overall pore volume is significantly lower. Since PFA is

147 hydrophobic it diffuses into the micellar core. However, as the molecular weight of the PFA grows this transport process becomes hindered due to slower diffusion of high mass PFA.

Therefore when we add FA directly to the polymerization solution, the low molecular weight material may diffuse into the micelle, but the high molecular weight PFA does not and so this material forms a dense low volume carbon upon pyrolysis. The measured cumulative pore volumes clearly confirm this effect. As shown in Figure 6.3, when the monomers were polymerized simultaneously, total pore volume of the resultant carbon was around 0.4 cc/g which is higher than the total pore volume of the carbon made with mixing the polymers by a factor of

1.5. It is not surprising that when we use the FA monomer we get the best results, because the monomer diffuses relatively rapidly into the micelles and is subsequently polymerized at that location. These results in the best combination of events to provide the highest pore volume that can be attained.

6.5. Conclusion

This chapter describes the designed synthesis of a carbon with distinctly bimodal porosity made up of micropores and mesopores. The designed synthesis was made possible by the surfactant-aided polymerization of furfuryl alcohol and phloroglucinol. The origin and control of mesoporosity was studied by varying the polymerization variables of acid concentration, monomer composition and surfactant concentration. We proposed a mechanism in which both cationic polymerization and sol gel polymerization occur simultaneously within the micelles. The micropore size of the derived carbon was ~ 0.6 nm, while the mesopore size and pore volume varied from 5 – 3.5 nm and from 0.2 – 0.4 cc/g with the changes in synthesis variables. This soft templating approach is a highly promising route to the development of carbon nanostructures

148 with unique textural properties that can make carbon molecular sieves more useful for wider range of new applications.

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29(11):3839-3850.

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153 Chapter 7

Conclusions and future directions

7.1. Conclusions

Poly(furfuryl alcohol)-derived carbon is an amorphous porous material with high thermal and chemical stability [1, 2]. A predominantly microporous structure with a narrow pore size distribution centered at ~0.5 nm makes this carbon useful for applications such as gas separation and adsorption, catalysis and energy storage. Among these applications, shape and size selective catalysis is one of the applications in which carbon offers great potential. A carbon-based shape selective catalyst, synthesized by embedding platinum nanoparticles within the cross-linked poly(furfuryl alcohol) network prior to pyrolysis, showed high activity and selectivity in gas phase hydrogenation reactions involving ethylene, propylene and butene [3]. However in reactions that involved bigger/bulkier molecules, the embedded catalyst suffered from low activity due to severe mass transfer limitations for bulky molecules in the ultra micropores of carbon. This low activity due to severe mass transfer resistance was addressed by incorporating mesopores in the carbon structure using pore forming agents [4, 5].

Firstly, to control carbon particle morphology and ultimate size, an emulsion polymerization of furfuryl alcohol was adapted. It was shown that carbon spheres with variable diameters from about 5 microns to 50 nm could be synthesized by adjusting surfactant (Pluronic

F-127), initiator (HCl) and monomer (furfuryl alcohol) concentrations as well as solvent composition (water/ethanol). By controlling carbon sphere diameters, we were able to control global diffusion lengths. Secondly, platinum nanoparticles were dispersed within the carbon

154 spheres as the active catalytic phase and the catalyst prepared in this way was evaluated in liquid phase hydrogenation reactions of different alkenes. The controlled addition of pore forming agents, such as poly(ethylene glycol) having different molecular weights, also shortened diffusion lengths. Thirdly, selective CO2 oxidation of the carbon after formation was used to widen the pores further and to increase the overall void fraction of the solid. These three approaches were used separately and in combination to optimize the catalyst’s activity and shape selectivity. Each of these methods provided much more facilitated mass transport processes while maintaining the narrow micropore size distribution of the carbon in the vicinity of the active platinum particles thus also preserving the size and shape selectivity. It was shown in Chapter 4 that the catalyst’s effectiveness factor could be improved by almost one order of magnitude without a significant loss in selectivity. The methodology presented in this chapter may be generalized to the design and synthesis of other catalysts with metallic particles embedded in nanoprous carbon for different reactions involving molecules with different sizes and shapes.

In chapter 5, the reaction kinetics and diffusion process dynamics inside the pores wherein the effect of pore walls’ confinement was imposed, were studied. By analyzing the transient diffusion-reaction processes, we were able to fit and understand the experimental catalysis data and we obtained the relevant diffusion, adsorption and kinetic parameters that quantify the process. When one reacting molecule was bulkier or was more non-planar than another, then mass transfer was comparatively more hindered (lower diffusion coefficients), but the reaction and adsorption constants were enhanced. Since the reactants and intermediates are stabilized and adsorbed on the surface more strongly, thus the reaction barrier decreases and, hence, the intrinsic reaction rate constant increases. The increase in the intrinsic reaction rate constant is also related to an increase in the pre-exponential factor. This is the case because the nanopores confinement causes a decrease in the partition functions (degrees of freedom) of reactant molecules within the ultra micropores. The analysis presented in this chapter, explains

155 how the catalyst support particle size and the mean pore size affect the observed activity and selectivity.

As stated earlier, control of textural properties in these carbon-based materials is an important step towards using them in industrial applications. Most of the methods used previously to produce carbons with ordered porous structure, employ complex synthesis routes and multiple steps, which are impractical and not cost effective. By contrast in chapter 6, a simple approach to synthesizing a carbon with bimodal porosity was developed using simultaneous polymerization of furfuryl alcohol and phlorolgucinol in the presence of pluronic F-127 as the soft-templating agent. This one-step approach, provides an hierarchical carbon with micropore size of ~ 0.5 nm and mesopore size ranging from 3 to ~ 20 nm. Even higher surface area bimodal carbon was obtained by CO2 oxidation of the as-synthesized carbon. This methodology produces a more open and apparently more well-connected bimodal porosity that provides facilitated mass transport, which is advantageous in applications such as catalysis, separation and energy storage.

7.2. Future directions

Carbon-based catalysts possess high hydrothermal stability in aqueous media making them suitable for reactions in water and at high temperatures and pressures. Specifically, the idea of embedding active catalytic sites within the carbon structure makes this type of catalyst highly stable and active for a much longer time than the conventional impregnated catalysts. Recently, biomass is being studied as a new source for fuels. Although homogeneous catalysts are the most common type of catalysts for biomass conversion, the separation of these catalysts from the product is a significant and unsolved challenge. Polymer-derived carbons with high selectivity, could be alternatives to homogeneus catalysts for these kinds of reactions. In order to control the catalyst’s activity and its selectivity, the morphology and porosity of the carbon can be controlled

156 using the approaches presented in this study. Further, different functionalization techniques can be applied to modify the carbon surface with specific functional groups in order to promote various reactions. As an example, sulfonated carbon catalysts have been studied for different reactions such as the hydrolysis of cellulose and the catalytic production of biodiesels [6].

The selective dehydrogenation of alkanes to alkenes is another reaction for which an engineered carbon catalyst of the kind produced in this work may offer advantages over other types of porous catalysts. Dehydrogenation is an endothermic reaction that occurs at high temperatures (500-700 °C). At such high temperatures, thermal and catalytic side reactions result in a decrease in selectivity. For example, solid acid catalysts such as zeolites, at high temperatures, promote the cracking reactions that ultimately lead to the formation of coke. The reactions pathway that leads to coke formation begins with skeletal isomerization reactions.

Nanoporous carbon catalysts as an alternative to zeolites, have been used successfully in oxidative dehydrogenation of ethylbenzene to styrene [7]. Recently, ordered and metal-free mesoporous carbon has been applied for steam-free dehydrogenation of propane to propylene [8].

This reaction is an industrially important reaction for the production of propylene. It was found that the presence of mesoporosity facilitates the mass transport of propylene, preventing further dehydrogenation to propyne.

The bimodal carbon synthesized in this study showed promising results in the dehydrogenation of propane when conducted at 500 °C, which is a relatively low temperature at which to conduct this transformation. It would be interesting to study the temperature dependence of this reaction when catalyzed over this new carbon. It is anticipated that there will be effects of both mesopore size and distribution on this catalyst’s activity and selectivity. Selective dehydrogenation of propane using metal free carbon catalyst, having oxygen containing surface functional groups as the active sites, could be a breakthrough in the catalytic science of oxidative dehydrogenation.

157 Porous polymer-derived carbon materials are also widely used in energy storage applications. Carbons with hierarchical pore structures have shown superior performance as electrodes in super capacitors due to the fast transport of ions through the pores that improves the kinetics (fast charge-discharge rate), even as the high surface area enhances the total storage capacity [9, 10]. Nano-structured carbon (spheres or rods) with regular morphology and hierarchical porosity can be synthesized by combining the methods used in chapters 3 and 6. It is possible to synthesize carbon spheres with a bimodal pore size distribution and small diameters

(50-100 nm), by using simultaneous emulsion polymerization of furfuryl alcohol and phloroglucinol. Interstices between the spheres would provide the macropores, while the spheres contain micro- and mesopores. The textural and morphological properties of this carbon could be tailored using the different approaches presented in this study, in order to optimize the electrochemical performance.

Other carbon morphologies, including honeycombs, rods and bi-continuous layers can be synthesized using different types of soft or hard templates with an appropriate polymerization condition. In a recent effort, we have been able to prepare just such a honeycomb carbon network by polymerizing FA in the presence of hexagonally packed polystyrene spheres having different sizes. Initial promising results, showed that it is possible to synthesize PFA-derived carbon structures with thin walls containing micropores along with large transport channels ( > 50 nm).

This structure would be ideal for applications such as catalytic production or transformation of bulky molecules (pharmaceuticals and their intermediates) as well as energy storage applications.

158 7.3. References

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Materials 1995; 4:407-433.

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[3] Rajagopalan R, Ponnaiyan A, Mankidy PJ, Brooks AW, Yi B, Foley HC. Molecular sieving platinum nanoparticle catalysts kinetically frozen in nanoporous carbon. Chemical

Communications 2004; 21:2498-2499.

[4] Holbrook BPM, Rajagopalan R, Dronvajjala K, Choudhary YK, Foley HC. Molecular sieving carbon catalysts for liquid phase reactions: Study of alkene hydrogenation using platinum embedded nanoporous carbon. Journal of Molecular Catalysis A-Chemical 2013; 367:61-68.

[5] Holbrook BPM. Nanoporous Carbon Mediated Catalysis and Hydrogen Adsorption, PhD thesis, The Pennsylvania State University, 2009.

[6] Kang S, Ye J, Chang J. Recent Advances in Carbon-Based Sulfonated Catalyst: Preparation and Application. International Review of Chemical Engineering 2013; 5(2):133-144.

[7] Delgadoo JJ, Su DS, Rebmann G, Keller N, Gajovic A, Schlogl R. Immobilized carbon nanofibers as industrial catalyst for ODH reactions. Journal of Catalysis 2006; 244(1):126-129.

[8] Liu L, Deng QF, Agula B, Zhao X, Ren TZ, Yuan ZY. Ordered mesoporous carbon catalyst for dehydrogenation of propane to propylene. Chemical Communications 2011; 47:8334-8336.

[9] Mun Y, Jo C, Hyeon T, Lee J, Ha KS, Jun KW, et al. Simple synthesis of hierarchically structured partially graphitized carbon by emulsion/blockcopolymer co-template method for high power supercapacitors. Carbon; 2013; 64:391-402.

159 [10] Suss M, Baumann TF, Worsley MA, Rose KA, Jaramillo TF, Stadermann M, et al.

Impedance-based study of capacitive porous carbon electrodes with hierarchical and bimodal porosity. Journal of Power Sources 2013; 241:266-273.

1 VITA

Maryam Peer Lachegurabi

Maryam Peer Lachegurabi was born in Rasht, Iran on January 9th, 1982. After graduating high school, she moved to Tehran to continue her studies in Sharif University of Science and

Technology where she got her B.S. in Chemical Engineering. In 2004 she entered Iran University of Science and Technology and continued her graduate study (M.S.) in Chemical Engineering

Department with focus on Membrane Separation Processes. She traveled to United States and started her PhD in The Pennsylvania State University, in January 2009, under the guidance of

Professor Henry C. Foley in the Department of Chemical Engineering. Synthesis and application of nanoporous carbon-based materials for catalysis and adsorption was the major focus of her doctorate study. Her future plan is to continue her studies at chemical engineering department at

MIT, as a post-doctoral student.