On the structure and coordination of the oxygen-donating species in Ti↑MCM-41/TBHP oxidation catalysts: a density functional theory and EXAFS study

Abstract

We present a combined density functional theory and EXAFS investigation into the geometry and coordination of the oxygen-donating species in surface grafted, tert-butyl hydroperoxide (TBHP) exposed Ti↑MCM-41 oxidation catalysts. The formation of a number of different Ti(η²-peroxo) and Ti(η¹-peroxo) type species, including radical and anionic complexes, has been examined, arising from the attack of peroxide (where both hydrogen peroxide and TBHP have been studied) on a tetrahedral Ti^IV site, together with and without one molecule of water. BP86/DZVP calculations show that the Ti(η¹-OO^tBu) complex is 33 kJ mol⁻¹ lower in energy than the Ti(η²-OO^tBu) complex which is contrary to previous estimates in the literature and we propose that 6-coordinate Ti(η¹-OOR) complexes, where R is the peroxide substituent (i.e. R = H for H₂O₂ and R = ^tBu for TBHP), are the oxygen-donating species in peroxide/titanosilicate mixtures. X-ray absorption analysis is in good agreement with our DFT predictions that Ti–peroxo species in TBHP/Ti↑MCM-41 systems are 6-coordinate. Furthermore, our DFT cluster calculations and X-ray absorption analysis collectively reveal the presence of 6-coordinate ‘Ti(η²-OO^tBu)(OHR)’ and ‘Ti(η¹-OO^tBu)(OHR)·H₂O’ complexes in TBHP/Ti↑MCM-41 catalysts.