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

摘要

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) exosed Ti arrow up MCM-41 oxidation catalysts. The formation of a number of different Ti(eta~2-peroxo) and Ti(eta~1-perroxo) 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~tBu) complex is 33 kJ mol~(-1) lower in energy than the Ti(eta~2-OO~tBu) 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_2O_2 and R=~tBu for TBHP), are the oxygen-donating apecies in peroxide/titanosilicate mixtures. X-ray absorption analysis collectively reveal the presence of 6-coordinate 'Ti(eta~2-OO~tBu)(OHR)' and 'Ti(eta~1-OO~tBu)(OHR) centre dot H_2O' complexes in TBHP/Ti arrow up MCM-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~tBu)(OHR)' and 'Ti(eta~1-OO~tBu)(OHR) centre dot H_2O' complexes in TBHP/Ti arrow up MCM-41 catalysts.