Mesoporous carbon silica encapsulated molybdenum and ruthenium nanocatalysts for green chemistry applications

MESOPOROUS CARBON/SILICA ENCAPSULATED MOLYBDENUM AND RUTHENIUM NANOCATALYSTS FOR GREEN CHEMISTRY APPLICATIONS DOU JIAN NATIONAL UNIVERSITY OF SINGAPORE 2013 MESOPOROUS CARBON/SILICA ENCAPSULATED MOLYBDENUM AND RUTHENIUM NANOCATALYSTS FOR GREEN CHEMISTRY APPLICATIONS DOU JIAN (M ENG., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR OF PHILOSOPHY DEPARTMENT OF CHEMICAL AND BIOMOLECULAR ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE 2013 DECLARATION I hereby declare that the thesis is my original work and it has been written by me in its entirety I have duly acknowledged all the sources of information which have been used in the thesis This thesis has also not been submitted for any degree in any university previously Dou Jian 12 August 2013 Acknowledgements The journey to accomplishment of my PhD degree has been one of the most important steps in my life It would not have been possible without the help, support, and encouragement of the following people First and foremost, I would like to express my sincere appreciation to my advisor, Prof Zeng Hua Chun, for his generous support of my research in his group Through all the one-to-one discussions with him, he brought me into the world of nanocatalyst research He always inspires me with his broad knowledge and precious insights His integrity and dedication in research has been of great value for me The knowledge, experience and confidence gained from working with him will benefit me for all my life I would also like to thank my group members for many helps and fruitful discussions Dr Xiong Sheng Lin and Dr Li Cheng Chao have many research experiences and have taught me a lot Special thanks to Dr Xi Bao Juan for teaching me draw crystal structures and Mr Sheng Yuan for setting up the gas reactor I have learnt many tips of TEM operation from Dr Zhang Yu Xing, Dr Wang Dan Ping, Mr Yec Christopher Cheung and Ms Liu Min Hui I shall also thank Dr Zhang Sheng Mao, Dr Yao Ke Xin, Dr Pang Mao Lin, Dr Li Xuan Qi, Mr Li Zheng, Ms Wentalia Widjajanti, Ms Chng Tin Tin, Mr Zhan Guo Wu and Ms Zhou Yao for helping me in this or other ways i I would like to extend my gratitude to Ms Sandy Khoh and Ms Li Feng Mei for their laboratory technical support Ms Yasotha Kathiraser from Prof Kawi’s group has helped me with TPD analysis I shall thank Mr Chia Phai Ann and Mr Mao Ning for assisting in TEM analysis, Mr Morgan and Mr Liu Zhi Cheng for help with SEM and XRD analysis, and Dr Yuan Ze Liang for XPS analysis Sincere thanks also extend to Mr Bin Dolmanan Surani from institute of materials research and engineering (IMRE) for Raman characterization and Ms Han Yan Hui from Chemistry Department for NMR analysis I shall also acknowledge the research scholarship from Chemical and Biomolecular Engineering, NUS Finally, I would like to thank my family members for their enormous support and love all the way They make this accomplishment more meaningful ii Table of Contents Acknowledgements .i Table of Contents iii Summary…………………………………………………………………………….viii Symbols and Abbreviations x List of Tables……………………………………………………………………… xiii List of Figures……………………………………………………………………….xiv Publications Related to the Thesis xxiv Chapter Introduction 1.1 Overview………………………………………………………………………… 1.2 Objectives and Scope 1.3 Organization of the Thesis 1.4 References Chapter Literature Review 2.1 Overview of Nanocatalysts 2.2 Molybdenum Oxides and Ruthenium Based Nanocomposit Catalysts 17 2.2.1 Molybdenum Oxides 18 2.2.1.1 Molybdenum trioxide and molybdenum dioxide 18 iii 2.2.1.2 Molybdenum based heteropoly acids 21 2.2.2 Ruthenium Nanoparticles 23 2.2.3 Integrated Nanocatalysts 25 2.3 Green Chemistry 30 2.3.1 Friedel-Crafts Alkylation 30 2.3.2 Oxidative Desulfurization 33 2.2.3 CO2 Hydrogenation 36 2.5 References ……………………………………………………………………… 41 Chapter Characterization Methods……………………………………………… 49 3.1 Powder X-ray Diffraction (XRD) and Small-angle X-ray Diffraction (SAXRD)… 49 3.2 Field Emission Scanning Electron Microscopy (FESEM) and Energy Dispersive X-ray Spectroscopy (EDX) 50 3.3 Transmission Electron Microscopy (TEM) 50 3.4 Nitrogen Adsorption-Desorption Analysis 51 3.5 X-ray Photoelectron Spectroscopy (XPS) 52 3.6 Thermogravimetric Analysis (TGA) 52 3.7 Fourier Transform Infrared Spectroscopy (FTIR) 53 3.8 Gas Chromatography (GC) and Mass Spectroscopy (MS) 53 3.9 References… 55 iv Chapter Preparation of Mo-Embedded Mesoporous Carbon Microspheres for Friedel-Crafts Alkylation 56 4.1 Introduction 56 4.2 Experimental Section 59 4.2.1 Materials Preparation 59 4.2.2 Materials Characterization 60 4.2.3 Benzylation of Toluene 61 4.3 Results and Discussion 62 4.4 Conclusions 81 4.5 References 83 Chapter Targeted Synthesis of Silicomolybdic Acid (Keggin acid) inside Mesoporous Silica Hollow Spheres for Friedel-Crafts Alkylation 86 5.1 Introduction 86 5.2 Experimental Section 91 5.2.1 Preparation of MoO2 Nanoparticles 91 5.2.2 Preparation of MoO2@SiO2 Core-Shell Spheres 92 5.2.3 Preparation of MoVI@mSiO2 Hollow Spheres 93 5.2.4 Preparation of H4SiMo12O40@mSiO2 Hollow Spheres 93 5.2.5 Friedel-Crafts Benzylation of Toluene 94 5.2.6 Materials Characterization 95 5.3 Results and Discussion 96 v 5.4 Conclusions 127 5.5 References 129 Chapter Integrated Catalyst-Adsorbent of Mo/SiO Nanowires with Highly Accessible Mesopores for Oxidative Desulfurization of Model Diesel………… 134 6.1 Introduction 134 6.2 Experimental Section 139 6.2.1 Preparation of Mesoporous Silica (mSiO2) Nanowire-Networks 139 6.2.2 Preparation of Mo/mSiO2 Network Catalysts 139 6.2.3 Oxidative Desulfurization (ODS) of Model Diesel 140 6.2.4 Regeneration of Mo/mSiO2 Catalyst-Adsorbent 140 6.2.5 Materials Characterization 141 6.3 Results and Discussion 142 6.4 Conclusions 169 6.5 References ………………………………………………………………………171 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO2 to CO 174 7.1 Introduction 174 7.2 Experimental Section 177 7.2.1 Preparation of Ruthenium Nanoparticles 177 7.2.2 Preparation of Ru@mSiO2 Nanowires 177 7.2.3 Hydrogenation of CO2 178 vi 7.2.4 Materials Characterization 179 7.3 Results and Discussion 180 7.4 Conclusions 199 7.5 References 201 Chapter Conclusions and Recommendations 204 8.1 Conclusions 204 8.2 Recommendations 207 8.3 Rererences ………………………………………………………………………209 vii Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO 0.8 1.2 1.6 2.0 2.4 2.0 2.4 Diameter (nm) 0.8 1.2 1.6 Diameter (nm) 0.8 1.2 1.6 2.0 2.4 Diameter (nm) Figure 7.12 TEM images of (a, b) Ru@mSiO2-1-N after reaction, (c, d) Ru@mSiO2-3-N after reaction, and (e, f) Ru@mSiO2-5-N after reaction 195 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO 10 15 20 Diameter (nm) 12 16 Diameter (nm) 12 Diameter (nm) Figure 7.13 TEM images of (a, b) Ru@mSiO2-1-A after reaction, (c, d) Ru@mSiO2-3-A after reaction, and (e, f) Ru@mSiO2-5-A after reaction 196 002 101 Intensity (a u.) 100 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO f e d c b a 30 40 50 60 70 80 2-Theta angle (degree) Figure 7.14 XRD patterns of (a) Ru@mSiO2-1-N-used, (b) Ru@mSiO2-3-N-used, (c) Ru@mSiO2-5-N-used, (d) Ru@mSiO2-1-A-used, (e) Ru@mSiO2-3-A-used, and (f) Ru@mSiO2-5A-used a) b) Si 2p 103.3 Ru 3p Intensity (a u.) Intensity (a u.) 462.1 106 104 102 466 Binding energy (eV) c) 462 460 458 Binding energy (eV) O 1s Intensity (a u.) 532.5 464 533.7 530.5 536 534 532 530 Binding energy (eV) Figure 7.15 XPS spectra of Ru@mSiO2-3-N-used: (a) Si 2p, (b) Ru 3p5/2, and (c) O 1s 197 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO a) 103.3 b) Si 2p Ru 3p Intensity (a u.) Intensity (a u.) 461.6 106 104 102 464 Binding energy (eV) c) 532.6 462 460 458 Binding energy (eV) Intensity (a u.) O 1s 533.8 530.6 536 534 532 530 Binding energy (eV) Figure 7.16 XPS spectra of Ru@mSiO2-3-A-used: (a) Si 2p, (b) Ru 3p5/2, and (c) O 1s catalysts were reduced to Ru metal by hydrogen after reaction The oxidation states of Ru in the used catalysts were further evidenced by XPS analysis (Figures 7.15 and 7.16) Referenced to Si 2p at 103.3 eV, the Ru 3p3/2 value of Ru@mSiO2-3-A-used catalyst was measured as 461.6 eV corresponding to reduced Ru(0) For Ru@mSiO23-N-used catalyst, the Ru 3p3/2 value at 462.1 eV is slightly higher than that of fresh catalyst (Ru 3p3/2 = 461.8 eV), which can still be assigned to Ru(0) state The stability of prepared Ru@mSiO2-3-N catalyst was verified by extended reaction test at 400 oC for 50 h Shown in Figure 7.17a, the average conversion of CO2 at initial h reaction was 24.2%, which gradually increased to 25.7% at end of 50 h reaction The CO selectivity was measured as 79.7% at initial run and dropped slightly to 77.6% at end of 50 h run 198 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO 40 100 b) 80 30 Selectivity of CO (%) Converstion of CO2 (%) a) 20 10 60 40 20 0 10 20 30 Time-on-stream (h) 40 50 10 20 30 40 50 Time-on-stream (h) Figure 7.17 Reverse water gas shift reaction with Ru@mSiO2-3-N catalsyts: (a) conversion of CO2 and (b) selectivity of CO over CH4 Reaction conditions: 100 mg of catalyst, 25 mL/min of CO2/H2 (1:4) gas mixture, weight hours space velocity (WHSV) = 250 mL· -1· -1, temperature g o (T) = 400 C 7.4 Conclusions In summary, mesoporous silica encapsulated Ru nanoparticles have been successfully prepared by in-situ hydrolysis and condensation of TEOS in the presence of presynthesized Ru nanoparticles Through thermal treatment in nitrogen, organic template (CTACl) was burnt off to generate mesopores within integrated silica nanowires The size of encapsulated Ru nanoparticles remained similar as those before thermal treatment On the contrary, oxidation and sintering of encapsulated Ru nanoparticles occurred when the fresh samples heated in air The calcined catalysts were investigated for hydrogenation of CO2 It was observed that high selectivity of CO was achieved with catalysts heated in nitrogen, while high selectivity of CH was favored with catalysts heated in air The high selectivity of CO over CH4 is possibly due to the small size of Ru nanoparticles in samples heated in nitrogen With more unsaturated surface Ru sites, small Ru nanoparticles bind with CO molecules more strongly, leading to higher activation barrier for further methanation reaction Instead 199 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO of hydrogenation to CH4, the surface carbonyls desorb from Ru metal surface to form CO products The resulted Ru@mSiO2-N catalyst has been demonstrated to be very stable within extended reaction time of 50 h 200 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO 7.5 References Zeng, H C Acc Chem Res 2013, 46, 226-235 Somorjai, G A.; Frei, H.; Park, J Y J Am Chem Soc 2009, 131, 16589-16605 Astruc, D.; Lu, F.; Aranzaes, J A Angew Chem Int Ed 2005, 44, 7852-7872 An, K.; Somorjai, G A ChemCatChem 2012, 4, 1512-1524 Pushkarev, V V.; Musselwhite, N.; An, K.; Alayoglu, S.; Somorjai, G A Nano Lett 2012, 12, 5196-5201 Kliewer, C J.; Aliaga, C.; Bieri, M.; Huang, W.; Tsung, C K.; Wood, J B.; Komvopoulos, K Somorjai, G A J Am Chem Soc 2010, 132, 13088-13095 Kuhn, J N.; Huang, W.; Tsung, C K.; Zhang, Y.; Somorjai, G A J Am Chem Soc 2008, 130, 14026-14027 Lei, Y.; Mehmood, F.; Lee, S.; Greeley, J.; Lee, B.; Seifert, S.; Winans, R E.; Elam, J W.; Meyer, R J.; Redfern, P C.; Teschner, D.; Schlogl, R.; Pellin, M J.; Curtiss, L A.; Vajda, S Science 2010, 328, 224-228 Wang, W.; Wang, S.; Ma, X.; Gong, J Chem Soc Rev 2011, 40, 3703-3727 10 Song, C Catal Today 2006, 115, 2-32 11 Arakawa, H.; Aresta, M.; Armor, 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S Catal Lett 1998, 51, 41-45 27 Yao, K X.; Zeng, H C Chem Mater 2012, 24, 140-148 28 Bock, C.; Paquet, C.; Couillard, M.; Botton, G A.; MacDougall, B R J Am Chem Soc 2004, 126, 8028-8037 29 Wang, Y.; Ren, J.; Deng, K.; Gui, L.; Tang, Y Chem Mater 2000, 12, 1622-1627 30 Dou, J.; Zeng, H C J Am Chem Soc 2012, 134, 16235-16246 31 Moller, K.; Kobler, J.; Bein, T Adv Funct Mater 2007, 17, 605-612 32 Dou, J.; Zeng, H C submitted 2013 33 Ouyang, R.; Liu, J X.; Li, W X J Am Chem Soc 2013, 135, 1760-1771 34 Simonsen, S B.; Chorkendorff, I.; Dahl, S.; Skoglundh, M.; Sehested, J.; Helveg, S J Am Chem Soc 2010, 132, 7968-7975 35 Ravikovitch, P I.; Domhnaill, S C O.; Neimark, A V.; Schuth, F.; Unger, K K Langmuir 1995, 11, 4765-4772 36 Sharma, S.; Hu, Z.; Zhang, P.; McFarland, E W.; Metiu, H J Catal 2011, 278, 202 Chapter Mesoporous Silica Nanowires Encapsulated Ru Nanoparticles for Selective Hydrogenation of CO to CO 297-309 37 Shen, J Y.; Adnot, A.; Kaliaguine, S Appl Surf Sci 1991, 51, 47-60 38 Karelovic, A.; Ruiz, P J Catal 2013, 301, 141-153 39 Eckle, S.; Anfang, H G.; Behm, R J J Phys Chem C 2011, 115, 1361-1367 40 Fisher, I A.; Bell, A T J Catal 1996, 162, 54-65 203 Chapter Conclusions and Recommendations Chapter Conclusions and Recommendations 8.1 Conclusions In this work, we have prepared mesoporous carbon and silica supported molybdenum and ruthenium nanocatalysts via three different assembly strategies The synthesized nanocatalysts have been investigated for green chemistry applications such as benzylation of toluene, oxidative desulfurization and hydrogenation of carbon dioxide The main conclusions drawn from this work are summarized as following: A) Hydrothermal treatment of glucose and ammonium heptamolybdate aqueous solution has been demonstrated to be an effective method for in-situ formation and encapsulation of molybdenum dioxide (MoO2) nanoparticles within carbon spheres In this synthetic route, glucose functions as a reducing agent for reduction of ammonium heptamolybdate to molybdenum dioxide Simultaneously, glucose molecules react with each other via condensation and polymerize to form carbonaceous spheres The mesoporosity was generated by thermal treatment in 204 Chapter Conclusions and Recommendations nitrogen and air consecutively Oxidation of encapsulated MoO2 to MoO3 was achieved by thermal treatment without destructing carbon spheres Mesoporous carbon embedded MoO3 nanocatalyst has been demonstrated to be an efficient solid acid for benzylation of toluene B) Mesoporous silica based strong solid acids have been prepared by an inside-out preinstallation-infusion-hydration method Molybdenum dioxide nanoparticles were prepared via hydrothermal synthesis using polyvinylpyrrolidone as capping agent The prepared MoO2 nanoparticles were used as seeds to grow mesoporous silica shells to generate MoO2@SiO2 core-shell nanospheres After thermal treatment, encapsulated MoO2 nanoparticles were oxidized to Mo6+ and infused into mesoporous silica shells, forming heptamolybdate species (Mo7O246-) Due to evacuation of encapsulated MoO2 nanoparticles, the silica spheres were hollowed With further hydration treatment, encapsulated heptamolybdate species were converted to silicomolybdic acid by reaction with silica spcies and water The resulted mesoporous silica encapsulated silicomolybdic acid is a strong solid acid catalyst for benzylation of toluene, with its activity 2.6 times as high as commercial Amberlyst-15 catalysts Furthermore, the silicomolybdic acid nanocatalyst can be reused after reaction with regeneration treatment C) Silica nanowires with diameter of 14 nm were prepared by an emulsion templated method With thermal treatment, the as-synthesized silica nanowires assembled together to form hierarchical porous structure originated from organic template and inter nanowires spacing The resulted hierarchical mesoporous silica nanowires were used as high surface area support for immobilization of 205 Chapter Conclusions and Recommendations molybdenum trioxide nanoparticles via impregnation The prepared molybdenum trioxide nanocatalysts were used as efficient catalysts for oxidative desulfurization of model diesel The oxidized dibenzothiophene sulfone adsorbed on the surface of mesoporous silica Thus the oxidation and adsorption of sulfur compound was achieved within one stop reaction using the bifunctional catalystadsorbent system In addition, the adsorbed sulfone can be removed by thermal treatment or washing with organic solvent D) Mesoporous silica supported ruthenium nanocatalysts were assembled by in-situ growth silica nanowires in the presence of ruthenium nanoparticles Thermal treatment of as-prepared supported ruthenium nanocatalysts in nitrogen resulted in ruthenium nanoparticles with size of 1.3 ± 0.5 nm For samples heated in air, big RuO2 nanocrystals with sie of to 30 nm appeared in addition to small nanoparticles, due to sintering of RuO2 nanoparticles When the ruthenium nanocatalysts were used for hydrogenation of carbon dioxide, selectivity of carbon monoxide over methane varied with particle size of encapsulated ruthenium nanoparticles Nanocatalyst with smaller ruthenium nanoparticles favors formation of carbon monoxide, possibly due to adsorption of surface carbon monoxide strongly and leaing to higher activation barrier for methanation reaction The mesoporous silica encapsulated ruthenium nanocatalyst was very stable for 50 h of reaction, which has the potential for large scale conversion of carbon dioxide to useful carbon monoxide feedstock 206 Chapter Conclusions and Recommendations 8.2 Recommendations As summarized above, a number of mesoporous silica/carbon supported molybdenum and ruthenium nanocatalysts have been successfully assembled and investigated for green chemistry applications The synthetic methods may be extended for preparation of other nanocatalysts, while the nanocatalysts prepared can be used for other reactions as well Some of the recommendations are discussed as following: A) The hydrothermal treatment of glucose and ammonium heptamolybdate solution has been demonstrated to be an effective method for preparation of carbon supported molybdenum oxide nanocatalyst Other metal oxides such as iron oxide and cobalt oxide have been immobilized on carbon support using this method, which are very active for production of hydrocarbons from syngas.1 This method may be further extended for encapsulation of noble metals, which can be used for hydrogenation reactions such as biomass conversion.2 Furthermore, the porous carbon supported metal oxide/metal materials may be further used as a hard template for preparation of other supported catalysts (i.e., from Mo@C to Mo@SiO2, Mo@TiO2 and Mo@CeO2 etc.) The porosity can be generated by burning off the carbon matrix in air B) Mesoporous silica supported silicomolybdic acid has been prepared by the preinstallation-infusion-hydration method By this method, mesoporous silica supported other heteropoly acids such as silicotungstic acid could be synthesized as well The supported silicomolybdic acid has been demonstrated to be a recyclable strong solid acid for benzylation of toluene This catalyst can be 207 Chapter Conclusions and Recommendations applied for other acid catalyzed reactions as well such as hydrolysis of saccharides.3 C) Hierarchical mesoporous silica supported molybdenum oxide is an excellent catalyst-adsorbent system for removing dibenzothiophene from model diesel, with the sulfur concentration reduced from 400 to less than 20 ppm in one step This nanocatalyst is worth further investigation for removing sulfur compounds from real diesel by batch or continuous reactions D) Mesoporous silica supported ruthenium nanoparticles with size of 1.3 ± 0.5 nm have been synthesized via in-situ encapsulation and thermal treatment in nitrogen This method would be posssible for preparing other noble metals or alloy nanocatalysts (i.e., Pt, Pd, Rh, Au, and Ag etc.) as well In particular, Pt based catalysts are active for reverse water gas shift reaction It is interesting to study the particle size effect on the catalytic performance of supported Pt nanocatalysts Cerium oxide supported PtRu alloy catalyst has been investigated for water gas shift reaction It was reported that less methane was produced over PtRu/CeO2 catalyst comparing with Ru/CeO2 It is worth to study the catalytic activity and selectivity of mesoporous silica supported ruthenium alloy nanocatalysts for hydrogenation of carbon dioxide 208 Chapter Conclusions and Recommendations 8.3 References Yu, G.; Sun, B.; Pei, Y.; Xie, S.; Yan, S.; Qiao, M.; Fan, K.; Zhang, X.; Zong, B J Am Chem Soc 2010, 132, 935-937 Fukuoka, A.; Dhepe, P L Angew Chem Int Ed 2006, 45, 5161-5163 Tagusagawa, C.; Takagaki, A.; Iguchi, A.; Takanabe, K.; Kondo, J N.; Ebitani, K.; Hayashi, S.; Tatsumi, T.; Domen, K Angew Chem Int Ed 2010, 49, 1128-1132 209 .. .MESOPOROUS CARBON/ SILICA ENCAPSULATED MOLYBDENUM AND RUTHENIUM NANOCATALYSTS FOR GREEN CHEMISTRY APPLICATIONS DOU JIAN (M ENG., NUS) A THESIS SUBMITTED FOR THE DEGREE OF DOCTOR... within mesoporous supports for green chemistry applications In this work, molybdenum oxide and ruthenium nanoparticles are selected for assembly within mesoporous carbon and silica supports Three... selective and stable nanocatalysts for green chemistry applications In this thesis, molybdenum oxide and ruthenium nanoparticles were selected as model metal oxide and metal compounds to be encapsulated