Effect of high coverages on proton transfer in the zeolite/water system

The density functional theory and Hartree-Fock methods were used to investi gate the proton transfer reaction for a series of model clusters of zeolite /(H2O)(n); n = 1, 2, 3, and 4. Without promoted water, the hydrogen-bonded dimer of the water/zeolite system exists as a simple hydrogen-bonded comple x, ZOH.(H2O)(2), and no proton transfer occurs from zeolite to water. The t hird promoted water, ZOH(H2O)(2)H2O, was found to induce a pathway for prot on transfer, but at least addition two promoted molecules, ZO(H3O+)H2O(H2O) (2), must be involved for complete proton transfer from zeolite to H2O. The results show that the hydronium ion in water cluster adsorbed on zeolite, ZO(H3O+)(H2O)(3), can considerably affect the structure and bonding of the hydrogen-bonded dimer of water. The O . . .O distance is contracted from 2. 818 Angstrom found in the neutral complex, ZOH(H2O)4, to 2.777 Angstrom for ion-pair complex, ZO(H3O+)(H2O)(3). The distance between the oxygen of the hydronium ion and the zeolitic acid site oxygen is predicted to be 2.480 A ngstrom which is in good agreement with the experimentally observed value o f 2.510 Angstrom. The corresponding density functional adsorption energy of the high coverages of adsorbing molecules on zeolite is calculated to be - 9.14 kcal/mol per molecule at B3LYP/6-311 + G(d,p) level of theory and comp ares well with the experimental observation of -8.20 kcal/mol. (C) 2001 Els evier Science B.V. All rights reserved.