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

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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 TiIV site, together with and without one molecule of water. BP86/DZVP calculations show that the Ti(η1-OOtBu) complex is 33 kJ mol−1 lower in energy than the Ti(η2-OOtBu) 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-OOtBu)(OHR)’ and ‘Ti(η1-OOtBu)(OHR)·H2O’ complexes in TBHP/Ti↑MCM-41 catalysts.

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