Electron spin resonance and diffuse reflectance ultraviolet-visible spectroscopies of vanadium immobilized at surface titanium centers of titanosilicate mesoporous TiMCM-41 molecular sieves

A series of mesoporous titanosilicate TiMCM-41 molecular sieves with variable amounts of framework titanium have been hydrothermally synthesized using H2O2 as a phase stabilizer. Some structurally analogous materials containing framework zirconium (ZrMCM-41), aluminum (AlMCM-41), or only silicon (MCM-41) have also been synthesized. These materials are calcined to decompose the organic structure-directing agent and used as vanadium supports. Vanadium is loaded onto the supports by incipient-wetness impregnation of a dilute (0.001-0.017 M) aqueous vanadyl sulfate solution. The chemical environment of vanadium in these materials is investigated by electron spin resonance and diffuse reflectance ultraviolet-visible spectroscopy. Both techniques reveal that in freshly prepared siliceous MCM-41 or AlMCM-41 with either low or high vanadium loading only mobile vanadyl VO2+ ions are detected. By contrast, in freshly prepared TiMCM-41 or ZrMCM-41 with low vanadium loading, vanadium exists as monatomically dispersed square-pyramidal VO2+ ions and tetrahedral V5+ ions. This indicates that surface titanium or zirconium centers immobilize these vanadium species and promote the oxidation of VO2+ to V5+. Thus, a strong interaction between vanadium and surface titanium or zirconium centers is suggested. As the vanadium loading in TiMCM-41 increases beyond a critical V/Ti ratio of about 0.17, VO2+ clusters develop, and some square-pyramidal and distorted octahedral V5+ ions occur simultaneously by additional coordination to water molecules. This suggests that the accumulation of more than two vanadium ions at a single titanium center occurs at higher vanadium loading. Upon calcination and rehydration in moist air, vanadium is completely oxidized to tetrahedral V5+ when the vanadium loading is below a critical V/Ti ratio of 0.33 or to some square-pyramidal and octahedral V5+ at higher vanadium loading. Such an increase in the critical V/Ti ratio on calcination suggests that calcination gives rise to a greater dispersion of the surface vanadium species on the wall surfaces of TiMCM-41.