Mechanisms of formation of extraframework Al2O3 in zeolites

Extraframework alumina in zeolites is considered by some authors to play a crucial role in the establishment of their reactivity. Charge transfer molecular dynamics simulations (CTMD) in the microcanonical ensemble of zeolite Y confirmed that catalytically active sites in zeolites are produced by the presence of extraframework alumina, and indeed, that it also is necessary to stabilize the zeolite lattice. In this work we present a plausible model for the formation of extraframework alumina based on the results of CTMD simulations of a Y zeolite with compensation sodium cations, and give the physicochemical arguments for the stabilization role that it plays. The simulated system consisted of 653 particles representing 165 silicon atoms, 414 oxygen atoms, 47 aluminum atoms, and 27 sodium atoms. Radial distribution functions, the i.r. spectrum, and the n.m.r. predictions reproduced fairly well the corresponding available experimental data. (C) Elsevier Science Inc. 199