Substrate-Structure and Solvent Mediated Environment Around the Brønsted Acid Sites Governs the Dehydration Rate Enhancement of Alcohols inside Zeolite Confinements

Molecular-sized zeolite confinements enhance reaction rates for a wide range of organic transformations. These rate enhancements for active sites within these environments are accompanied by an increased complexity that limits understanding on a fundamental level. The dehydration of alcohols by Brønsted acid sites (BAS) is used in various organic transformations, including the deoxygenation of biomass-derived oxygenates. The nature of the BAS changes with the reacting environment. BAS are likely localized on the zeolite framework in an apolar non-aqueous environment, but form hydrated hydronium ions in the presence of a large concentration of water. Due to the structural complexity of biomass, alcohols with a diversity of molecular structure will need upgrading. The question arises, therefore, (1) how zeolites can be tailored for conversion of more complex feedstocks and (2) how different solvents influence the environments around the BAS and its influence on reaction rates. Herein, thermochemical and kinetic measurements along with isotope labeling were used to quantitatively investigate the reaction pathway and kinetics of the (1) dehydration of C₆-C₈ alkanols and (2) dehydration of cyclohexanol in apolar solvent, decalin to study the influence of alcohol structure and solvents on the organization of the substrates in the pore and the stabilization of the elimination transition state (TS) inside zeolites. Overall, the study elucidates the influence of the environment around the active on the reaction kinetics and mechanism for alcohol dehydration reactions inside zeolite confinements.