(119d) Effects of Intrapore Hydroxyl Density on Confined Water Structures and Ethanol Dehydration Kinetics within Microporous Brønsted Acids
Bimolecular dehydration turnover rates (373 K, per H+) measured on a suite of H-Al-Beta-F zeolites (Si/Al = 23â220; 0.1â2.0 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 â n. Measured values of â3 under conditions approaching intrapore condensation of H2O (30â75 kPa), therefore, reflect the kinetic relevance of ethanol-water clusters containing at least three H2O molecules (n â¥ 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 Î½(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 = 10â75 kPa) possess Î½(OH) peak center shifts similar to those measured at 293 K under the same relative pressure regimes (P/Psat = 0.1â0.75), 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 BrÃ¸nsted acid sites under conditions approaching intrapore condensation of water provide insights relevant to aqueous-phase reaction conditions.
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