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

Bates, J. S., Purdue University
Bukowski, B. C., Purdue University
Greeley, J., Purdue University
Gounder, R., Purdue University
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 (PC2H5OH/PH2O < 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 (PH2O = 10–75 kPa) possess ν(OH) peak center shifts similar to those measured at 293 K under the same relative pressure regimes (P/Psat = 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.


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