Titanium Site Preference Problem in the TS-1 Zeolite Catalyst: A Periodic Hartree-Fock Study

Gordana Pirc and Jernej Stare
* National Institute of Chemistry, Ljubljana, Slovenia
* Corresponding author: E-mail: jernej@cmm.ki.si

The problem of titanium location within the silicalite framework of the well known TS-1 catalyst has been studied by means of periodic Hartree-Fock all-electron quantum calculations using split valence atomic basis functions. Periodicity of the system, including the fully featured space group symmetry, has been rigorously considered. Two distinct space groups have been employed in the calculations, namely the experimentally determined orthorhombic Pnma space group and its monoclinic P1121/a subgroup. The latter makes it possible to effectively reduce the titanium content to a slightly overestimated but still plausible value of 4.17 atomic %. In the first step, the structure of the MFI zeolite which is the parent structural template of TS-1 has been optimized, exhibiting very good agreement with the experimental structure. The order of site preference in TS-1 has been determined for both space groups and it has been found that the order is notably affected by symmetry restrictions imposed by the space group, particularly for the sites that lie close to the mirror plane of the Pnma group. In partial agreement with the experimental evidence and with the earlier periodical study, we found the T4, T5, T3, T12, T11 and T8 sites to be energetically most favorable, the respective energies spanning a narrow range of 1.37 kcal/mol in the P1121/a space group. In contrast to the previous experience, we found the otherwise preferred T10 site to be among the least favorable ones for Ti substitution. The calculated energy range of the isomers is relatively narrow (about 5 kcal/mol) and the energy differences calculated per one Ti site tend to decrease on the decreasing Ti concentration. Because of small energy differences between the site isomers, consideration of the free energy (entropy terms) seems to be a reasonable upgrade of the present approach and an important focal point of the future studies of the Ti-siting problem in TS-1.

Keywords: Periodic quantum-chemical calculations, titanosilicalite zeolites, TS-1, space groups.