Catalysis in condensed phases causes Brønsted acid zeolites to stabilize various protonated molecular clusters that include reactants, solvents, and their mixtures. Such molecular clusters can be further solvated by extended networks of hydrogen-bonded solvent molecules that are also contained within microporous voids. Yet, the influence of confining environments on the structures of these clusters and extended networks, and in turn their consequences for acid catalysis, are incompletely understood. Here, we measure turnover rates (per H⁺, 373 K) of bimolecular alkanol dehydration, in which ethanol reactants present at low coverages serve as a kinetic reporter of solvent structure under widely varying conditions where molecular clustering and solvation by extended networks dominate.

Chabazite (CHA) zeolites were used to quantify turnover rates of gas-phase bimolecular dehydration of C₂H₅OH-CH₃OH mixtures to form methyl ethyl ether (MEE) and dimethyl ether (DME) products, in regimes where (H⁺)(CH₃OH)_n clusters (n = 1–4) are stabilized but those containing C₂H₅OH are not (C₂H₅OH/CH₃OH < 0.02). Turnover rates to form MEE and DME depend similarly on the C₂H₅OH/CH₃OH ratio when ether products form through transition states (TS) that include two clustered alkanol molecules (0.1–1 kPa CH₃OH), but depart from this dependence at higher methanol pressures (> 1 kPa CH₃OH) because a trimolecular TS to form DME becomes kinetically relevant while the bimolecular TS remains the preferred cluster size to form MEE.

Zeolites of varying topology were used to measure ethanol dehydration turnover rates in kinetic regimes (C₂H₅OH/H₂O < 0.2) corresponding to (C₂H₅OH)(H⁺)(H₂O)_n clusters that become solvated by extended H₂O networks. Ethanol dehydration rates are −1 order in H₂O pressure once protonated clusters form (2–10 kPa H₂O), because one water coordinated in (C₂H₅OH)(H⁺)(H₂O)_n clusters must be displaced by ethanol to form the S_N2 TS that eliminates DEE. The solvation of (C₂H₅OH)(H⁺)(H₂O)_4–5 clusters and TS by extended solvent networks (10–75 kPa H₂O) causes strong inhibition (~P_H2O⁻³), which is described by non-ideal thermodynamic formalisms that account for the more extensive disruption of hydrogen bonds within extended networks by DEE-formation TS than by their relevant precursors. Micropore geometries that constrict the reorganization of such networks through cage-window motifs (e.g., CHA, AEI) impose further energetic penalties to cause more severe inhibition.

Jason Bates

University of Virginia

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