Coverage effects on adsorption of water in faujasite : an ab initio cluster and embedded cluster study

Quantum cluster and embedded cluster approaches were used to investigate the proton transfer reaction for a series of model clusters of zeolite/(H2O)n ; n = 1, 2, 3, and 4, using the B3LYP/6-31G (d,p) level of theory. For both calculations, without promoted water, the hydrogen-bonded dimer of the zeolite/water 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 the addition of two promoted molecules, ZO(H3O)H2O(H2O)2, must be involved for complete proton transfer from zeolite to H2O. Inclusion of the Madelung potential was found to increase the acidity of the Brønsted acid site, yielding the complete proton transfer from zeolite to H2O in the presence of the third promoted water, ZO(H3O)(H2O)2. The distance between the oxygen of the hydronium ion and the zeolitic acid site oxygen is predicted to be 2.512 Å for the embedded cluster which is in good agreement with the experiment.