(677b) Solvation of Water and Alkanols Confined within Brønsted Acid Zeolites: Kinetic Effects of Protonated Clusters and Extended Networks

During aqueous-phase catalysis, Brønsted acid zeolites stabilize clusters of water-solvated hydronium ions ((H₃O⁺)(H₂O)_x), protonated reactants (e.g., alcohol n-mers), mixed solvent-reactant clusters, and extended solvent networks, yet their consequences for acid catalysis remain incompletely understood. Here, we use gas-phase bimolecular alkanol dehydration kinetics to interrogate the structure of water and alkanol clusters and extended solvent networks under conditions approaching intrapore condensation of water and alkanols within Brønsted acid zeolites.

Turnover rates (373 K, per H⁺) of gas-phase ethanol dehydration to diethyl ether (DEE), adsorption isotherms, and infrared (IR) spectra, are measured on a suite of H-Al-Beta-F zeolites of varying Al content and silanol defect density. The structures of most-abundant (C₂H₅OH)(H⁺)(H₂O)_n clusters are corroborated by ab initio molecular dynamics simulations, and IR spectra show that extended hydrogen-bonded networks form as capillary condensation occurs (10–75 kPa H₂O). Kinetic data and metadynamics simulations show that solvation of (C₂H₅OH)(H⁺)(H₂O)_n clusters and transition states by extended solvent networks causes strong inhibition (P_H2O^-3), which is described by non-ideal thermodynamic formalisms that account for the more extensive disruption of hydrogen bonds by DEE transition states relative to the relevant precursors. We also show how different micropore geometries influence the reorganization of such water networks.

Turnover rates (373 K, per H⁺) of gas-phase bimolecular dehydration of C₂H₅OH-CH₃OH mixtures to form methyl ethyl ether (MEE) and dimethyl ether (DME) products on a CHA zeolite are measured under conditions where H⁺ are covered by (H⁺)(CH₃OH)_n clusters (n = 1–4). The dependence of DME and MEE formation rates on CH₃OH pressure is consistent with kinetically relevant formation of DME through trimolecular pathways, whereas larger MEE transition states prefer bimolecular pathways. The speciation of protonated clusters that influence kinetics within alkanol-filled pores contrasts the preeminence of extended hydrogen-bonded networks and their reorganization within H₂O-filled micropores.