On the structure and coordination of the oxygen-donating species in Ti↑ MCM-41/TBHP oxidation catalysts: a density functional theory and EXAFS study
Barker, Carolyn M, Gleeson, David, Kaltsoyannis, Nikolas, Catlow, C Richard A, Sankar, Gopinathan and Thomas, John Meurig (2002) On the structure and coordination of the oxygen-donating species in Ti↑ MCM-41/TBHP oxidation catalysts: a density functional theory and EXAFS study. Physical Chemistry Chemical Physics, 4. pp. 1228-1240.
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(Z2-peroxo) and Ti(Z1-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(Z1-OOtBu) complex is 33 kJ mol�1 lower in energy than the Ti(Z2-OOtBu) complex which is contrary to previous estimates in the literature and we propose that 6-coordinate Ti(Z1-OOR) complexes, where R is the peroxide substituent (i.e. R 1⁄4 H for H2O2 and R 1⁄4 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(Z2-OOtBu)(OHR) ’ and ‘ Ti(Z1-OOtBu)(OHR)�H2O ’ complexes in TBHP=Ti"MCM-41 catalysts.
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