Effective ONIOM schemes for modeling MCM-22 zeolite

In order to describe the structural and energetic properties of Al-MCM-22 zeolite effectively and reasonably, ONIOM2 and ONIOM3 models as well as full B3LYP/6-31G(d) optimization on different sized clusters have been tested. On the basis of the computed bond lengths, substitution energies, proton affinities and O-H stretching frequencies for the Al1-O3H-Si4 and Al2-O9H-Si5 acid sites, three-layered ONIOM schemes (B3LYP/6-311G(d,p):HF/3-21G(d):MNDO) with 8T high-layer up to cluster sizes of 10 Å have the nearly same results as compared with those obtained from full B3LYP/6-31G(d) optimizations on 8 Å clusters. The computed O-H stretching frequencies, adsorption energy of ammonia, and Al···H1 distances agree well with the available experimental data.