Effects of the Aluminum Content of a Zeolite Framework:  A DFT/MM Hybrid Approach Based on Cluster Models Embedded in an Elastic Polarizable Environment

We report the first computational study with a sophisticated quantum mechanics/molecular mechanics (QM/MM) technique that addresses the effect of the aluminum content on the properties of acidic zeolites. To account for both electrostatic and mechanical interaction between the QM cluster and its MM environment, we used cluster models embedded in the covalent variant of the elastic polarizable environment (covEPE) [Nasluzov, V. A.; Ivanova, E. A.; Shor, A. M.; Vayssilov, G. N.; Birkenheuer, U.; Rösch, N. J. Phys. Chem. B 2003, 107, 2228]. For the practical application of the covEPE method, it was necessary to develop a new force field for Al containing zeolites. Two types of zeolite materials, FAU and MFI, were employed as examples. We modeled the variation of the Al content both in the MM environment and in the QM cluster, and we studied pertinent properties of bridging OH groups of the zeolite frameworks, OH vibrational frequencies, and deprotonation energies. The computational results suggest that the local structure and the location of the OH groups exert a stronger effect than the variation of the Al content of the framework.