Kinetic and Theoretical Insights into the Mechanism of Alkanol Dehydration on Solid Bronsted Acid Catalysts

Elementary steps that mediate ethanol dehydration to alkenes and ethers are determined here from rate and selectivity data on solid acids of diverse acid strength and known structure and free energies derived from density functional theory (DFT). Measured ethene and ether formation rates that differed from those expected from accepted monomolecular and bimolecular routes led to our systematic enumeration of plausible dehydration routes and to a rigorous assessment of their contributions to the products formed. H-bonded monomers, protonated alkanol dimers, and alkoxides are the prevalent bound intermediates at conditions relevant to the practice of dehydration catalysis. We conclude that direct and sequential (alkoxide-mediated) routes contribute to ether formation via S(N)2-type reactions; alkenes form preferentially from sequential routes via monomolecular and bimolecular syn-E2-type eliminations; and alkoxides form via bimolecular S(N)2-type substitutions. The prevalence of these elementary steps and their kinetic relevance are consistent with measured kinetic and thermodynamic parameters, which agree with values from DFT-derived free energies and with the effects of acid strength on rates, selectivities, and rate constants; such effects reflect the relative charges in transition states and their relevant precursors. Dehydration turnover rates, but not selectivities, depend on acid strength because transition states are more highly charged than their relevant precursors, but similar in charge for transition states that mediate the competing pathways responsible for selectivity.