Dehydration of methanol and ethanol in the gas phase over heteropoly acid catalysts Thesis submitted in accordance with the requirements of the University of Liverpool for the degree of Doctor in Philosophy by Walaa Alharbi 2017 Abstract Dehydration of methanol and ethanol in the gas phase over heteropoly acid catalysts PhD thesis by Walaa Alharbi The aim of this thesis is to investigate heterogeneous catalysis for the dehydration of methanol and ethanol at a gas-solid interface over a wide range of solid Brønsted acid catalysts based on Keggin-type heteropoly acids (HPAs), focussing on the formation of dimethyl ether (DME) and diethyl ether (DEE), respectively. The dehydration of methanol to dimethyl ether (DME) was studied over a wide range of bulk and supported HPAs and was compared with the reaction over HZSM-5 zeolites (Si/Al = 10−120). Turnover rates for these catalysts were measured under zero-order reaction conditions. The HPA catalysts were demonstrated to have much higher catalytic activities than the HZSM-5 zeolites. A good correlation between the turnover rates and catalyst acid strengths, represented by the initial enthalpies of ammonia adsorption, was established. This correlation holds for the HPA and HZSM-5 catalysts studied, which indicates that the methanol-to-DME dehydration occurs via the same (or a similar) mechanism with both HPA and HZSM-5 catalysts, and that the turnover rate of methanol dehydration for both catalysts is primarily determined by the strength of catalyst acid sites, regardless of the catalyst pore geometry. I Dehydration of ethanol was also studied over a wide range of solid Brønsted acid catalysts based on Keggin-type HPAs in a continuous flow fixed-bed reactor in the temperature o range of 90-220 C. The catalysts included H3PW12O40 (HPW) and H4SiW12O40 (HSiW) supported on SiO2, TiO2, Nb2O5 and ZrO2 with sub-monolayer HPA coverage, as well as bulk acidic Cs salts of HPW (Cs2.5H0.5PW12O40 and Cs2.25H0.75PW12O40) and the corresponding core-shell materials with the same total composition (15%HPW/Cs3PW12O40 and 25%HPW/Cs3PW12O40, respectively) comprising HPW supported on the neutral salt Cs3PW12O40. The ethanol-to-DEE reaction was found to be zero order in ethanol in the range of 1.5-10 kPa ethanol partial pressure. The acid strength of the catalysts was characterised by ammonia adsorption microcalorimetry. A fairly good correlation between the catalyst activity (turnover frequency) and the catalyst acid strength (initial enthalpy of ammonia adsorption) was established, which demonstrates that Brønsted acid sites play an important role in ethanol-to-DEE dehydration over HPA catalysts. The acid strength and the catalytic activity of core-shell catalysts HPW/Cs3PW12O40 did not exceed those of the corresponding bulk Cs salts of HPW with the same total composition, which contradicts the claims in the literature of the superiority of the core-shell HPA catalysts. II List of publications The main results obtained in this thesis are disseminated in the following publications and conference presentations: 1. W. Alharbi, E. Brown, E. F. Kozhevnikova, I. V. Kozhevnikov, “Dehydration of ethanol over heteropoly acid catalysts in the gas phase”, J. Catal., 319 (2014) 174– 181. 2. W. Alharbi, E. F. Kozhevnikova, I.V. Kozhevnikov, “Dehydration of methanol to dimethyl ether over heteropoly acid catalysts: the relationship between reaction rate and catalyst acid strength”, ACS Catal., 5 (2015) 7186−7193. 3. W. N. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Poster day, University of Liverpool, Liverpool, United Kingdom, April, 2014 (poster). 4. W. N. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, 4th NORSC PG Symposium, Huddersfield University, Huddersfield, United Kingdom, October, 2014. 5. W. N. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, The 8th Saudi Students Conference, Imperial College London, London, United Kingdom, January, 2015. 6. W. N. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, Catalysis Fundamentals and Practice, University of Liverpool, Liverpool, United Kingdom, July, 2015. 7. W. N. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, The 9th Saudi Students Conference, University of Birmingham, Birmingham, United Kingdom, February, 2016. 8. W. N. Alharbi, E. F. Kozhevnikova, I. V. Kozhevnikov, The Royal Society of Chemistry, Burlington House, London, United Kingdom, April, 2016. III Acknowledgements First and foremost I would like to express my appreciation to Prof. Ivan V. Kozhevnikov for his guidance during my PhD. journey. Without his valuable assistance, this work would not have been completed. I would also like to give a heartfelt, special thanks to Dr. Elena F. Kozhevnikova. She was not the only assistant in solving multiple technical problems during my PhD, but my mentor and friend. Her patience, flexibility, and faith in me during PhD journey enabled me to attend to life while also earning my PhD. She is motivating, encouraging, and enlightening. For this, I cannot thank her enough. I am forever grateful. Thank you Dr. Elena! Special thanks should also be given to all members of the technical support team in the Chemistry Department and Kozhevenikov's group at the University of Liverpool. I gratefully acknowledge the funding sources that made my PhD work possible. I was funded by the King Abdullah scholarship program for three years. My project was also supported by King Khalid University for last year of my PhD. Last, but certainly not least, I must acknowledge with great and deep thanks to my mum, my husband, a special friend (Khadijah), my kids, and all my family. Through their love, patience, support and unwavering belief in me, I have been able to complete this long PhD journey. At the same time, they have also given me so many happy and beautiful memories throughout this journey. Thank you from all my heart and soul for your devotion, unconditional love, support, sense of humour, patience, optimism and advice; it was more valuable than you could ever imagine. IV List of Abbreviations HPA Heteropoly acid HPW H3PW12O40 HSiW H4SiW12O40 CsPW Cesium salt (Cs2.5) of phosphotungstic acid (H3PW12O40) MeOH Methanol DME Dimethyl ether EtOH Ethanol DEE Diethyl ether BET Brunauer-Emett-Teller method TGA Thermogravimetric analysis IR Infrared spectroscopy XRD X-ray diffraction DSC Differential scanning calorimetry ICP Inductively coupled plasma GC Gas chromatography FTIR Fourier transform infrared spectroscopy TOF Turnover frequency V Contents Abstract ............................................................................................................................... List of publications ..........................................................................................................iii Acknowledgements ......................................................................................................... iv List of Abbreviations ....................................................................................................... v Contents ........................................................................................................................... vi Chapter 1. Introduction .................................................................................................. 1 1.1 Catalysis: the scope and definition ........................................................................... 1 1.2 Heterogeneous catalysis ........................................................................................... 3 1.2.1 Basic features of heterogeneous catalysis ......................................................... 3 1.2.2 Effect of mass transfer in solid porous catalysts ............................................. 12 1.3 Catalysis by heteropoly acids ................................................................................. 13 1.3.1 Introduction to heteropoly acids (HPA) .......................................................... 13 1.3.2 Structural hierarchy of solid heteropoly acids ................................................. 15 1.3.3 Properties of heteropoly acids ......................................................................... 19 1.3.4 Types of heterogeneous catalysis by heteropoly acids .................................... 26 1.3.5 Application of HPAs in heterogeneous acid catalysis ..................................... 28 1.4 Zeolites ................................................................................................................... 29 1.4.1 Properties of Zeolites ....................................................................................... 29 1.4.2 Application of zeolites in heterogeneous catalysis .......................................... 32 1.5 Alcohol dehydration: scope and mechanism .......................................................... 34 1.6 Transformation of biomass into biodiesel .............................................................. 37 1.7 Production of ethers through dehydration of alcohols ................................................. 39 1.7.1 Liquid phase dehydration of alcohols .............................................................. 39 1.7.2 Dimethyl ether production through dehydration of methanol in gas phase ..... 40 1.7.3 Diethyl ether production through dehydration of ethanol in gas phase ........... 43 VI 1.8 The scope and objectives of thesis ......................................................................... 46 1.9 Organisation of the thesis ....................................................................................... 47 1.10