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

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 Tiup arrowMCM-41 oxidation catalysts. The formation of a number of different Ti(eta(2)-peroxo) and Ti(eta(1)-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(eta(1)-OO(t)Bu) complex is 33 kJ mol(-1) lower in energy than the Ti(eta(2)-OO(t)Bu) complex which is contrary to previous estimates in the literature and we propose that 6-coordinate Ti(eta(1)-OOR) complexes, where R is the peroxide substituent (i.e. R=H for H(2)O(2) and R=(t)Bu 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/Tiup arrowMCM-41 systems are 6-coordinate. Furthermore, our DFT cluster calculations and X-ray absorption analysis collectively reveal the presence of 6-coordinate 'Ti(eta(2)-OO(t)Bu)(OHR)' and 'Ti(eta(1)-OO(t)Bu)(OHR).H(2)O' complexes in TBHP/Tiup arrowMCM-41 catalysts.