(281a) Effect of Alcohol Structure on the Kinetics and Mechanism of Acid Catalyzed Etherification and Unimolecular Dehydration over Tungstated Zirconia

Branched and linear ether molecules have attracted recent interest as diesel additives and lubricants that can be built from various structures of biomass-derived alcohols. In this study, a comprehensive investigation of the effect of alcohol structure on the mechanism and kinetics of liquid phase etherification and unimolecular dehydration reactions was performed. In this investigation, 12 wt% WO_x/ZrO₂ was identified as an effective and green solid acid catalyst for selective etherification of primary alcohols in the liquid phase, achieving ether selectivities of >96% for C₆-C₁₂ linear alcohols at 120 °C, even though at this temperature unimolecular dehydration to alkenes is thermodynamically preferred. The proposed mechanisms for Brønsted acid-catalyzed unimolecular dehydration and etherification were supported by isotopic labeling and by measuring kinetic isotope effects. The catalyst was characterized with XRD, BET, and DRIFTS FTIR with pyridine adsorption. It is proposed that, while the desired etherification reaction proceeds over a small number of strong Brønsted acid sites, the surrounding Lewis acid sites help bring reactant molecules to the surface to facilitate the bimolecular reaction. The effects of alcohol structure including linear alcohol length, carbon chain branching positions, and carbon chain branching size, were probed by measuring the apparent kinetics for dehydration reactions over tungstated zirconia. Trends in apparent activation energies and selectivities were consistent with the proposed mechanism, and can elucidate feasible pathways to produce valuable ether products from biomass-derived alcohols.