Computational Assessment of Relative Sites Stabilities and Site-Specific Adsorptive Properties of Titanium Silicalite-1

Abstract

Titanium Silicalite-1 (TS-1), because of its crystalline structure and its well-defined Ti sites, represents the prototype of a single site catalyst. According to this fundamental aspect and to the relevant role of TS-1 as selective catalyst in important industrial partial oxidation reactions, TS-1 has been widely characterized through both experimental and computational techniques. Still, several fundamental aspects of its structural and catalytic properties have to be addressed. Among these, an intriguing topic is the Ti location in the various sites of the MFI framework. The independent sites are generally considered to be 12, following the Pnma space group of TS-1 at high Ti loading. However, when Ti loading is lower than 2 atoms per unit cell, diffraction showed that the system must be described by the P21/n space group, thereby allowing 24 independent sites. With respect to previous studies, this work aims to exploit this datum to give a more accurate description of the TS-1 system at low Ti loadings, adopting a state of the art methodology (all electron periodic B3LYP-D2 calculations). The relative stabilities of the 24 Ti sites have been evaluated, showing a good agreement with previous studies. The simulation of adsorption energies for ammonia (present as reactants in some of the most important industrial reactions catalyzed by TS-1) over the most stable sites have been computed as well, in order to validate the obtained models. Additionally to binding energies, adsorption enthalpies and Gibbs free energies have been obtained through an approximate reduced Hessian scheme. The improved local description of the Ti sites (in combination with the adducts stabilities given by the energetic data) allowed the deep understanding of subtle effects, such as the number of molecular ligands each Ti atom can actually host upon adsorption. These results, showing only few sites can efficiently host two ligands in the neighborhoods of STP conditions, allowed for the first time the heterogeneity in the experimental outcomes reported over the last two decades to be rationalized.