Comparison of all sites for Ti substitution in zeolite TS-1 by an accurate embedded-cluster method.

We studied the preferential location of Ti centers in the framework of the Ti-containing MFI zeolite TS-1 using a hybrid DFT/MM embedding method developed recently. This "covalent elastic polarizable environment" (covEPE) cluster embedding allows a complete and self-consistent treatment of solid covalent systems such as zeolites. For the present study, we used a gradient-corrected density functional approach. The resulting structural features of both Si- and Ti-substituted forms of the zeolite framework fit well with available experimental information. The calculated substitution energy of Ti at the 12 crystallographically different tetrahedral sites of the MFI structure vary within 19 kJ/mol with T12 and T2 as most and least preferred sites, respectively. On the basis of these computational results and the preferential sites for Ti substitution reported from different experimental investigations, we concluded that the Ti distribution in the TS-1 framework is not governed by the thermodynamic stability of the pure material.