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

Barker, CM; Gleeson, D; Kaltsoyannis, N; Catlow, CRA; Sankar, G; Thomas, JM; (2002) On the structure and coordination of the oxygen-donating species in Ti up arrow MCM-41/TBHP oxidation catalysts: a density functional theory and EXAFS study. PHYS CHEM CHEM PHYS , 4 (7) 1228 - 1240. 10.1039/b104735n.

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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 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)-(OOBu)-Bu-t) complex is 33 kJ mol(-1) lower in energy than the Ti(eta(2)-(OOBu)-Bu-t) 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 H2O2 and R=Bu-t 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)-(OOBu)-Bu-t)(OHR)' and 'Ti(eta(1)-(OOBu)-Bu-t)(OHR).H2O' complexes in TBHP/Tiup arrowMCM-41 catalysts.

Type:Article
Title:On the structure and coordination of the oxygen-donating species in Ti up arrow MCM-41/TBHP oxidation catalysts: a density functional theory and EXAFS study
DOI:10.1039/b104735n
Keywords:TERT-ALKYL HYDROPEROXIDES, X-RAY-ABSORPTION, HYDROGEN-PEROXIDE, TITANIUM-SILICALITE, FRAMEWORK TI(IV), TI-SILICALITE, ZEOLITE-BETA, TITANIUM(IV)-CATALYZED EPOXIDATION, TITANOSILICATE CATALYSTS, EXCHANGE-ENERGY
UCL classification:UCL > School of BEAMS > Faculty of Maths and Physical Sciences > Chemistry

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