Characterization of Acidity in [B], [Al], and [Ga] Isomorphously Substituted ZSM-5: Embedded DFT/UFF Approach

The structure and electronic properties of the Bronsted 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. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 111: 2275-2282, 2011