Reactivity of Ti(IV) sites in Ti-zeolites: An embedded cluster approach

We report a complete cluster/embedded cluster study by means of ab initio methods in the ONIOM scheme, as implemented in GAUSSIAN 98 code, of the reactivity towards water and ammonia of Ti(IV) centers in zeolitic frameworks. For water adsorption, we observe a remarkable increment of the binding energies by moving from 2.1 kJ mol⁻¹ for the unconstrained ${\mathrm{Ti(OSiH}}_3{)}_4$ cluster to 16.9 kJ mol⁻¹ for the ${\mathrm{TiSi}}_{17}{\mathrm{O}}_{26}{\mathrm{H}}_{20}$ cluster, obtained by cutting a portion of the MFI framework. The same holds for ammonia, where the binding energy increases from 17.4 to 35.4 kJ mol⁻¹, all reported values being BSSE corrected. These results underline the fundamental role played by zeolitic framework constraints, in enhancing the reactivity of Ti(IV) centers towards both ${\mathrm{H}}_2\mathrm{O}$ and ${\mathrm{NH}}_3$ probes. On the geometrical ground the Ti–O distance of bare clusters and its modification in complexes are in good agreement with the first shell EXAFS data analysis.