Vanadium oxide incorporated into mesoporous titania with a BET surface area above 1000 m2•g-1:: Preparation, spectroscopic characterization, and catalytic oxidation

To reveal the potential properties of mesoporous titania with a BET surface area of more than 1000 m(2.)g(-1), we incorporated vanadium into mesoporous titania by co-condensation (direct incorporation, V-meso TiO2) and by postsynthesis impregnation (impregnation, V/meso TiO2). The mesostructure remained until the loading reached 5 wt % as V metal for V-meso TiO2. The catalysts were characterized by transmission XANES and room-temperature ESR spectroscopies. The pre-edge absorption in V K edge spectra of V-meso TiO2 demonstrated that the valence of V is 4+ under atmospheric conditions (i.e., exposed to air and at room temperature). Ti K edge XANES showed that 5-fold coordinated Ti, which originally comprised 37% of the total titanium in the mesoporous titania, was substituted by V. In contrast, V in V/meso TiO2 was assigned to a valence of 5+. In ESR spectra, both of these catalysts showed strong hfs multiples characteristic of isolated vanadyl species in either square pyramidal or distorted octahedral coordination. The pattern of V4+ in V-meso TiO2 did not change by evacuation at 473 K, implying an extremely stabilized tetravalent state in mesoporous titania. Both V-meso TiO2 and V/meso TiO2 were highly active in a propene oxidation reaction. The oxidation rates per unit V were much enhanced from the activity of highly dispersed V/JRC-TIO-4 (conventional nonporous titania support).