INSTITUTO DE FISICA LA PLATA
Unidad Ejecutora - UE
congresos y reuniones científicas
Relationship between Oxygen Coordination Environment and catalytic properties in Grafted Ti-Calixarenes catalysts probed by Ti-K XANES studies
L. ANDRINI; J.M. NOTESTEIN; E. IGLESIA; F.G. REQUEJO
Otro; Reunião Anual de Usuários (RAU); 2007
Highly dispersed Ti centers dispersed onto or within SiO2 frameworks are active catalysts for selective oxidations and other molecular rearrangements catalyzed by Lewis acids because the resulting four-coordinate Ti centers are electron-deficient and can expand their coordination to bind oxidants or substrates and to activate oxidants towards attack by electron-rich substrates. We report here the use of X-ray absorption methods on surface-grafted Ti-SiO2 catalysts based on calixarene-Ti complexes, which catalyze epoxidation reactions with turnover rates independent of Ti surface density. Ti K-edge X-ray absorption near-edge spectroscopy (XANES) allow us the determination of the Ti coordination number and density of unoccupied electronic states at Ti 3d-level in these materials as a function of treatment temperature, Ti surface density, Ti precursor (TiClS4 or TiOiPr4), and catalyst use in epoxidation reactions. The Ti K-XANES experiments were realized in the D04B-XAS beamline of the LNLS (Laboratorio Nacional do Luz Síncrotron, Campinas, Brazil). Ti K-edge XANES spectra of all dried materials based grafting calixarene-Ti complexes indicate Ti with increased Ti 3d occupancy based on comparison to known Ti-SiO2 catalysts and soluble calixarene-metal species with known single crystal X-ray diffraction structures. In contrast to the similar catalytic activity and structure of materials based on calixarene-Ti, materials based on homoxacalixarene-Ti possess an average Ti coordination number of 5 and this coordination number is persistent to 523 K eliminating the possibility of simple coordination sphere expansion by a physisorbed species.