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
Abstract
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.