The increasing global consumption of petroleum-derived fuels and chemicals has resulted in rapid generation of atmospheric CO2, the accumulation of which has adverse effects on the global climate. One strategy for lowering the overall emission of CO2 from the combustion of petroleum-derived fuels and lubricants is to replace them with similar products derived from renewable sources. This approach has the potential to be both environmentally responsible and economical, particularly if policy changes incentivize the use of non-fossil energy resources in the future.
Biomass, such as agricultural waste, is a readily available source of renewable carbon for producing fuels and chemicals that does not compete with the demand for food. Efforts in biological fermentation of biomass-derived sugars via ABE fermentation have enabled the attainment of renewable butanol, acetone, and ethanol with the molar ratio of 6:3:1. These so-called platform molecules can be upgraded to produce higher carbon number fuels and chemicals using heterogeneous catalysts. This thesis is focused on developing an understanding of several heterogeneous catalytic pathways towards producing fuels and specialty chemicals from renewable platform molecules; specifically, acetone, ethanol, and other biomass-derived alcohols.
The first three chapters of this thesis are centered around understanding the etherification of biomass-derived alcohols and other platform molecules to produce ethers for use as fuels and lubricants. Ethers have attracted recent interest as diesel additives and specialty chemicals due to their high cetane numbers and excellent lubricant properties, and they can be produced via direct etherification of biomass-derived alcohols in the liquid phase. The competing reaction for alcohol dehydration over an acid catalyst is unimolecular dehydration to form alkenes, which is thermodynamically favored above approximately 350 K. To improve the activity and selectivity towards direct etherification of long chain alcohols in the liquid phase, it is necessary to develop an understanding of the mechanism and kinetics of etherification and dehydration reactions. In the first study of this dissertation, tungstated zirconia was identified as a selective solid acid catalyst for the liquid phase etherification of 1-dodecanol. Through kinetic modeling and mechanistic probing, this study suggested that a cooperative effect between Brønsted and Lewis acid sites on tungstated zirconia enhances the selectivity to ether by increasing the surface concentration of adsorbed alcohol molecules, promoting bi-molecular etherification over unimolecular dehydration. Kinetic isotope effects for linear alcohol dehydration were measured to elucidate the rate limiting steps in the mechanism. Effects of alcohol concentration and product inhibition were measured and fit to kinetic models consistent with the proposed mechanisms. In addition, acid site characterization and selective poisoning experiments were used to probe the role of the acid sites and support the proposed mechanism.
In the second study, the scope of alcohols for the synthesis of ethers via direct etherification over tungstated zirconia was expanded to include a variety of biomass-derived alcohols ranging from C6-C24 with varying degrees of carbon chain branches and substitution. The effects of alcohol length, position of carbon chain branches, and length of carbon chain branches were studied for etherification and dehydration reactions. Trends in the effects of alcohol structure on selectivity were consistent with the proposed mechanisms for etherification and dehydration and elucidated possible pathways to selectively form ethers from biomass-derived alcohols. In the third chapter of this thesis, these studies of direct etherification were explored within in the greater context of the etherification literature. Tradeoffs between catalyst selectivity, activity, stability, and reaction conditions required to achieve the most economically and environmentally favorable routes to biomass-derived ethers were discussed with the goal of identifying the combination of catalyst properties required to achieve high ether selectivity for a specified class of synthons.
Continuing the investigation of the valorization of alcohols via heterogeneous catalysis, the next study focused on the valorization of ethanol through oxidation reactions over Ag, Au, and Cu nanoparticles on Li2O/Al2O3. The interest in studying the oxidation of ethanol over arose from striking reports in the literature that identified Ag and Ag, Au, and Cu nanoparticles on Li2O/Al2O3 as highly selective catalysts for the single-step conversion of ethanol to ethylene oxide, a valuable precursor for the synthesis of many polymers and specialty chemicals. Motivated by these reports, a systematic study of the effects of catalyst support, Li2O loading, and various nanoparticle synthesis procedures was performed to provide an understanding of the unexpected selectivity reported in the literature. Systematic kinetic measurements and product characterization revealed that the primary product of this reaction is acetaldehyde, not ethylene oxide, and that errors in previous reports in the literature could be attributed to mis-identification of products with gas chromatography and mass spectrometry.
The last study in this thesis is centered around the valorization of bio-ethanol and acetone via conversion to isobutene. Isobutene is a valuable specialty chemical used in the production of fuel additives, polymers, and other high-value products. Isobutene is normally produced via steam cracking of petroleum naptha, thus the use of renewable platform molecules such as ethanol and acetone to synthesize isobutene has received increasing interest. Recent work in the literature has shown that zinc-zirconia mixed oxides selectively catalyze the production of isobutene from ethanol and acetone in the presence of water at 723 K. While this reaction is stable and selective, little is known about the mechanism, kinetics, and reaction pathway. In this study, a thorough investigation into the mechanism and kinetics of the acetone and ethanol conversion to isobutene was performed with the aim of elucidating the reaction pathway, the roles of active acidic and basic sites, and the role of water in promoting stability and selectivity. A reaction sequence for the conversion of ethanol to isobutene was proposed and supporting using a combination of catalyst synthesis, characterization, and kinetic measurements. The 5-step sequence starts with the dehydrogenation of ethanol to acetaldehyde, followed by oxidation to acetic acid, ketonization to acetone, and then dimerization to diacetone alcohol which then either undergoes decomposition or dehydration to mesityl oxide and subsequent hydrolysis to produce isobutene and acetic acid, which undergoes further ketonization to acetone. The dispersion of zinc oxide on zirconia was found to produce a balance between Lewis acidic and basic sites that promotes the cascade reactions of ethanol and acetone to isobutene.
Overall, this thesis brings together fundamental studies of the mechanisms and kinetics of these heterogeneous catalytic reactions to create a broader picture of the catalyst properties and reaction conditions necessary to enable the selective conversion of biomass-derived platform molecules to fuels and specialty chemicals. Bifunctional catalysts with cooperative Brønsted and Lewis acid sites, as well as Lewis acid and base sites, have emerged as versatile systems to study C-O and C-C bond forming reactions. This thesis also demonstrates the versatility of the zirconia support and various promoters in producing catalytic materials with tunable acid and base properties, and thus tunable catalytic activity and selectivity. By providing extensive catalyst characterization as well as kinetic and mechanistic probing, this thesis elucidates the catalytic properties and reaction conditions that favor the effective production of fuels and specialty chemicals from renewable platform molecules, specifically concentrating on the synthesis of ethers and isobutene. While this thesis focuses on the fundamental understanding of these reactions, the insights gained are part of a larger effort by the scientific community to develop creative solutions to the growing global energy demands in the dawn of an era when continuing to burn fossil fuels is triggering devastating effects on the health of the planet and its inhabitants.