Characterization of acidity in [B], [Al], and [Ga] isomorphously substituted ZSM-5: Embedded DFT/UFF approach

Abstract

The structure and electronic properties of the Brønsted acid site in B, Al or Ga isomorphously substituted ZSM-5 zeolites were studied by using quantum cluster and embedded ONIOM approaches. In the former approach, zeolites are modeled by 5T and 12T quantum clusters, where T represents a Si or Al atom. In the latter model, called “Embedded ONIOM”, the long-range interactions of the zeolite lattice beyond the 12T quantum cluster is included via optimized point charges added to the ONIOM(B3LYP/6-31G(d,p):UFF). Inclusion of the extended zeolitic framework covering the nanocavity has an effect on the structure and adsorption properties. We found that the OH distances and v OH of the acidic proton in zeolite obtained from both models can predict the trend of acid strength as: B-ZSM-5 < Ga-ZSM-5 < Al-ZSM-5, which is in very good agreement with the experimental sequence. Furthermore, the PA data calculated from E-ONIOM is also consistent with the experimental trend: B-ZSM-5 < Ga-ZSM-5 < Al-ZSM-5. It has, therefore, been demonstrated that our embedded ONIOM model provides accurate performance and can be one of the useful and affordable methods for future mechanistic studies involving petrochemical reactions. © 2010 Wiley Periodicals, Inc. Int J Quantum Chem, 2011