The effect of hydration on structure and location of Ti-sites in Ti-silicalite catalysts. A computational study

Abstract

A novel force field for titanosilicates, of the ion-pair shell-model type, has been derived from ab initio calculations on cluster models. This force field is used within a combined quantum mechanics/molecular mechanics (QM/MM) embedded cluster approach to examine several possible Ti sites in silicalite and their interaction with one or two water molecules. This method takes the full periodic zeolite structure into account. Periodic QM calculations on the model system Ti-chabasite show that it converges to the true periodic QM value when large enough QM regions are used. In the dehydrated state the stability differences between structures with Ti in different crystallographic positions are typically between 0 and 10 kJ mol⁻¹. In agreement with previous studies no pronounced site preferences are predicted. In the hydrated state one or two water molecules form stable complexes with the Ti sites, extending their oxygen coordination from 4 to 5 or 6. The binding energies per water molecule range from 0 to 31 kJ mol⁻¹ and strongly depend on the site location. This makes the stability difference between different Ti sites much larger (up to 40 kJ mol⁻¹) than in the dehydrated state and also the order of stability of Ti sites in Ti-silicalite changes by hydration. This demonstrates that hydration has to be taken into account when discussing Ti siting in zeolites. Hydrolysis of Ti–O–Si bridges is endothermic, although the hydrolyzed structures represent stable structures. The reaction energies are strongly site- and framework-dependent. The hydrolyzed structures show a peculiar bridging hydroxy group, Ti–O(H)–Si, similar to that observed in acidic zeolites. The silanol group acts as an oxygen ligand for Ti and extends its coordination from 4 to 5 or 5 to 6. Complete hydrolysis of a tetrahedral Si or Ti site, which gives rise to the so-called ‘‘ hydrogarnet defect’’, is calculated to be exothermic on silicalite-1 but endothermic on Ti-silicalite-1. This is in accord with the stability of the Ti centres and explains the observed ability of mono-nuclear Ti species to heal hydroxylated defects in zeolite frameworks.