A Combination of the Monte Carlo Method and Molecular Mechanics Calculations:  A Novel Way To Study the Ti(IV) Distribution in Titanium Silicalite-1

Abstract

A combination of the Metropolis Monte Carlo method and molecular mechanics calculations is used to study the Ti(IV) distribution in titanium silicalite-1 (TS-1). Calculations are carried out in which the Ti atoms are placed at crystallographically different T-sites rather than at symmetry-related T-positions. In this way, also the effect of already incorporated Ti atoms on the accommodation of new Ti atoms can be studied. It is shown that the Ti atoms are distributed over all crystallographically different lattice positions rather than located at one preferred T-site. The distribution, however, is not random: T2 and T12 are clearly favored. Modeling of the Ti distribution at loadings above the experimentally determined maximum limit of 2 1 / 2 Ti atoms per unit cell seems questionable. The framework symmetry is related to both the location of the Ti atoms and the Ti loading. There are much less Ti atoms present as neighbors than can be expected from a random substitution, which might indicate that Loewenstein's rule is dependent not only on electrostatic effects but also on framework deformations.