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

The density functional theory and Hartree–Fock methods were used to investigate 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 complex, ZOH.(H₂O)₂, and no proton transfer occurs from zeolite to water. The third promoted water, ZOH(H₂O)₂H₂O, was found to induce a pathway for proton 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 OO distance is contracted from 2.818 Å found in the neutral complex, ZOH(H₂O)₄, to 2.777 Å 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 Å which is in good agreement with the experimentally observed value of 2.510 Å. 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 compares well with the experimental observation of −8.20 kcal/mol.