(211c) Mechanism and Kinetics of 1-Dodecanol Etherification and Dehydration over Tungstated Zirconia

As atmospheric carbon dioxide levels continue to rise as a result of burning fossil fuels, there is a growing interest in developing renewable fuels derived from biomass to meet the world’s increasing energy demands. Ethers have attracted recent interest as diesel additives and specialty chemicals due to their high cetane numbers and excellent lubricant properties, and 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. In order to optimize the 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 this study, 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 suggests 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 in order 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.