Catalytic Partial Oxidation of Renewable Feedstocks A dissertation submitted to the faculty of the Graduate School of the University of Minnesota by Reetam Chakrabarti In partial fulfillment of the requirements for the degree of Doctor of Philosophy. Advisor: Lanny D. Schmidt July, 2012 c Reetam Chakrabarti 2012 All Rights Reserved ACKNOWLEDGMENTS I owe a lot of gratitude to my advisor, Professor Lanny Schmidt for giving me the opportunity to join his group, and the freedom to pursue my ideas. His breadth of knowledge and unending curiosity about various things in the world have always amazed me. I thank him for always having an open door to discuss problems. I would like to thank Professor Aditya Bhan for his help, both academic and non-academic over the past four years, ever since I applied to the University of Minnesota. I would also like to thank Professor Alon McCormick and Professor Ulrike Tschirner for agreeing to be a part of my defense committee and reviewing my thesis. Thanks to the staff at the Characterization Facility at the University of Minnesota for answering questions regarding characterization techniques presented in this thesis. Thank you to all the teachers through school and college in India. I have enjoyed working with the graduate students and undergraduate students in the lab, all of whom have helped me in my experiments. In particular, I would like to thank Dr. Dave Rennard, Dr, Brian Michael, Dr. Josh Colby, Dr. Jake Kruger, and Sam Blass for their help during various stages of my research. I would like to thank Dr. Dave Rennard, Dr. Brian Michael and Dr. Josh Colby for helping me get started in my first summer with experiments starting with an empty hood. I thank Dr. Joshua Colby and Dr. Brian Michael for the great conversations in the lab, on our way down to the coffee room on the first floor, and in their house, the `Green Monster'. Both of them have been an inspiration with their engineering creativity and approach to doing experiments. Outside of his immense help in my research, I am grateful to Dr. Jake Kruger for a number of things which include the great conference trips, organizing a trip from the sky to the ground, and the trip to Sheboygan, WI for his wedding. I would like to thank Sam Blass for his comments on writing, and the research and non-research related conversations in the ping-pong room next to the lab, and for tennis and ice skating. I would like to thank Dr. Christine Balonek for her wonderful desserts and Jeremy W. Bedard for tickets for baseball games and i ii rugby. Thanks to Richard `Walter' Hermann for help with lot of the experiments in this thesis. Thanks to Michael Skinner for introducing me to number of things while initially in the US, including darts and Easter gifts. Thank you to Hui Sun, Alex Marvin, David Nare for their help in research and insightful comments during group meetings. Thanks to the undergraduate students, Nils Persson, for his great commedy shows and piano skills, Tyler Josephson, Eric Hansen, Ed Michor, and Sheila Hunter for their help. I would like to thank my friends, Shameek Bose, Sujit Jogwar, Srinivas Ran- garajan, Romain Le Picard, Aloysius Gunawan, Andrew Yongky, and my roommate Parthiv Daggolu for great moments in Minneapolis. In addition, thanks to my friends from undergrad, Umang Desai, Karan Kadakia, and Mansi Seth for great conversa- tions that persisted through the last four years. I would like to thank my family in India, especially my parents and my sister Ishita for their support throughout my life. ABSTRACT The current world energy and economic infrastructure is heavily reliant on fossil fu- els such as coal, oil, and natural gas. The limited availability of fossil fuels along with environmental effects and economic uncertainties associated with their use has motivated the need to explore and develop other alternative sources of energy. Lig- nocellulosic biomass like fossil fuels is carbon-based and has the potential to partly supplant the energy supplied by fossil fuels. Lignocellulosic biomass is a complex mixture of polymers such as cellulose, hemi- cellulose, lignin along with small concentrations of inorganics and extractives. Recent research has shown that lignocellulosic biomass and biomass model compounds can be processed autothermally by catalytic partial oxidation in millisecond residence times over noble metal catalysts at high temperatures (600-1000 ◦C) to syngas (a mixture of carbon monoxide and hydrogen).1{3 The syngas stream can then be upgraded to fuels and chemicals. In Chapter 2, spatially resolved concentration and temperature profiles of methane and dimethyl ether, a model compound for biomass, are compared. Dimethyl ether can be produced renewably through syngas upgrading. Maximum temperature and concentration gradients were found within the oxidation zone. Most of the oxygen (∼ 95 %) was converted within the first 2.2 mm and syngas formation was observed despite the presence of oxygen. The catalytic partial oxidation process has been demonstrated using compounds which, unlike most biomass sources, contain negligible quantities of inorganics. Some of these inorganics have catalytic properties themselves and some may act as poisons for the Rh-based catalyst. The effects of common biomass-inorganics (silicon, calcium, magnesium, sodium, potassium, phosphorus, sulfur) on rhodium-based catalysts in autothermal reactors have been studied. To understand the effects of biomass inorganics on Rh catalysts, two sets of ex- iii iv periments surveying common inorganics were performed - in the first set, inorganics were directly deposited on the rhodium catalyst and tested using steam methane reforming as a model reaction (Chapter 3); whereas in the second set, inorganics were introduced to a clean catalyst in an ethanol feed to simulate actual inorganic- containing biomass (Chapter 4). In both sets of experiments, performance testing, catalyst characterization and regeneration were carried out to probe the mechanism of inorganic interaction with the rhodium-based catalyst. Large decreases in reform- ing activity were observed on phosphorus- and sulfur-doped catalysts. Deactivation due to calcium and magnesium was primarily due to blocking of active sites. Potas- sium and silicon were volatile at the high temperatures within the reactor. Potassium introduced alkaline chemistry promoting acetaldehyde formation from ethanol while phosphorus introduced acid chemistry promoting formation of ethylene from ethanol. The effects of potassium and phosphorus on catalytic partial oxidation of methane and ethanol at different concentrations and temperatures have been studied in Chap- ter 5. The synergistic effects of potassium and phosphorus were studied by distribut- ing the inorganics together on the catalyst as monobasic potassium phosphate. The effects of both potassium and phosphorus were observed in the catalytic partial oxi- dation of methane on a potassium phosphate-doped catalyst at low temperatures. At high temperatures, only effects due to phosphorus were observed because of potassium volatilization. The results show that biomass-sources containing low concentrations of inorganics can be processed autothermally to a high selectivity syngas stream. The distribution and interactions of the inorganics within the catalyst can be used to design better pre- treatment, processing, and regeneration strategies to minimize catalyst deactivation during biomass processing. Alcohols represent an important intermediate in different biomass upgrading routes. Chapter 6 discusses the behavior of butanol isomers, 1-butanol, isobutanol, 2-butanol, and tert-butanol over four different catalysts; Rh, Pt, RhCe, and PtCe at different fuel to oxygen (C/O) ratios. At low C/O ratios, equilibrium species such as CO, CO2,H2 and H2O were obtained while non-equilibrium species such as carbonyls and olefins were dominant at high C/O ratios. Low reforming activity was observed on Pt and PtCe catalysts. All isomers decompose primarily by dehydrogenation through a carbonyl intermediate except tert-butanol which decomposes by dehydration to isobutene; however, the reactivity of tert-butanol was unaffected. v In Chapter 7, isobutanol autothermal reforming is integrated with a water gas shift stage downstream to produce hydrogen containing low concentrations of carbon monoxide for portable fuel cell applications. A RhCe-based catalyst was selected to carry out autothermal reforming of isobutanol while a PtCe catalyst was selected for the water gas shift stage. This staged reactor produced high yields of hydrogen (> 120 % selectivity) containing low concentrations of CO (< 2 mol %) in less than 100 ms making the effluent ideal for portable high temperature PEM fuel cell applications. The water gas shift stage also reduced the concentration of non-equilibrium products formed in the autothermal reforming stage by over 50 %. Thermodynamic analysis of the system showed that staged autothermal reforming of isobutanol integrated with a fuel cell can potentially lead to 2.5 times more efficient energy usage when compared to burning isobutanol in a conventional combustion engine. The results in this thesis give an insight into the mechanisms and processing challenges involved in converting renewable feedstocks to syngas by catalytic partial oxidation. Further experiments based on the conclusions of this thesis are discussed in Chapter 8. Spatial profile experiments to determine roles of mass transfer, steam reforming, and dry reforming during catalytic partial oxidation of oxygenates are proposed. Spatial profile studies for catalytic partial oxidation over inorganic-doped catalysts and feed are proposed to determine their concentrations and nature on the catalyst surface during reactor operation. CONTENTS Acknowledgments i Abstract iii Table of Contents vi List of Tables x List of Figures xi 1 Introduction 1 1.1 Current World Energy Scenario . .1 1.1.1 Limited Fossil Fuel Reserves . .2 1.1.2 Developing countries . .2 1.1.3 Climate Impact . .3 1.1.4 Energy Security . .3 1.2 Biomass as an Energy Source . .3 1.2.1 Structure of Biomass . .4 1.2.2 Converting Biomass to Fuels and Chemicals .