(119d) Effects of Intrapore Hydroxyl Density on Confined Water Structures and Ethanol Dehydration Kinetics within Microporous Brønsted Acids

Water-solvated hydronium ions ((H₃O⁺)(H₂O)_x) confined within the micropores of zeolites catalyze a variety of aqueous-phase reactions involving polar molecules [1–3], yet their precise structures [4,5] in reactant-solvent mixtures and the distinct mechanistic pathways that they traverse [6] remain incompletely understood. Hydrophilic binding sites located within hydrophobic siliceous domains of zeolite frameworks, including both Si-O(H⁺)-Al and Si-OH groups, stabilize clustered solvent structures and co-adsorbed reactant-solvent complexes that may inhibit turnover rates. The scope of mechanistic investigations is limited in liquid phases because aqueous solvent structures cannot be controlled independent of reactant or co-solvent concentrations. Here, we circumvent these limitations using a gas-phase probe reaction, bimolecular ethanol dehydration to diethyl ether, under conditions approaching intrapore condensation of water within zeolite Beta to identify kinetic reporters of the structure of clustered reactant-solvent intermediates, supported by infrared spectra and density functional theory calculations.

Bimolecular dehydration turnover rates (373 K, per H⁺) measured on a suite of H-Al-Beta-F zeolites (Si/Al = 23–220; 0.1–2.0 H⁺ per unit cell) synthesized in fluoride media to minimize Si-OH defect densities give rise to apparent first-order rate constants in ethanol pressure when H⁺ active sites are saturated with clustered (C₂H₅OH)(H⁺)(H₂O)_n intermediates (P_C2H5OH/P_H2O < 0.2). Adsorption of a second ethanol during catalytic turnover disrupts clustered H₂O molecules to form (C₂H₅OH)₂(H⁺)(H₂O)_m precursors to bimolecular dehydration transition states, resulting in apparent H₂O reaction orders equivalent to m – n. Measured values of –3 under conditions approaching intrapore condensation of H₂O (30–75 kPa), therefore, reflect the kinetic relevance of ethanol-water clusters containing at least three H₂O molecules (n ≥ 3). DFT-calculated apparent activation free energies are consistent with energetically-accessible associative dehydration pathways through intermediates with (m = 1) or without (m = 0) co-adsorbed H₂O, suggesting the most-abundant reactive intermediates may include as many as four H₂O molecules (n = 4). Such values of n may differ from those predicted in pure-water systems because co-adsorbed ethanol molecules disrupt hydrogen-bonding networks in H₂O clusters. The clustered nature of adsorbed H₂O at hydroxyl groups is consistent with volumetric H₂O adsorption isotherms (293 K) that are proportional to H⁺ content, and infrared spectra of adsorbed H₂O with ν(OH) peak centers that shift to lower frequencies as clusters with greater extents of hydrogen bonding form at H⁺ or Si-OH groups within H-Al-Beta-F and dealuminated (Si-OH)₄-Beta-F analogs, respectively, but do not form within hydrophobic, pure-silica Si-Beta-F zeolites. In situ IR spectra collected at 373 K with and without co-fed C₂H₅OH (P_H2O = 10–75 kPa) possess ν(OH) peak center shifts similar to those measured at 293 K under the same relative pressure regimes (P/P_sat = 0.1–0.75), and indicate similar extents of H₂O clustering under temperature and pressure conditions of turnover rate measurements. The clustered nature of reactive intermediates and the interplay of reactant-water co-adsorption in forming them at Brønsted acid sites under conditions approaching intrapore condensation of water provide insights relevant to aqueous-phase reaction conditions.

References

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