MECHANISMS OF FORMATION OF EXTRAFRAMEWORK AL2O3 IN ZEOLITES

Extraframework alumina in zeolites is considered by some authors to pl ay a crucial role in the establishment of their reactivity. Charge tra nsfer molecular dynamics simulations (CTMD) in the microcanonical ense mble of zeolite Y confirmed that catalytically active sites in zeolite s are produced by the presence of extraframework alumina, and indeed, that it also is necessary to stabilize the zeolite lattice. In this wo rk we present a plausible model for the formation of extraframework al umina based on the results of CTMD simulations of a Y zeolite with com pensation sodium cations, and give the physicochemical arguments for t he stabilization role that it plays. The simulated system consisted of 653 particles representing 165 silicon atoms, 414 oxygen atoms, 47 al uminum atoms, and 27 sodium atoms. Radial distribution functions, the i.r. spectrum, and the n.m.r. predictions reproduced fairly well the c orresponding available experimental data. (C) Elsevier Science Inc. 19 97