Metal oxide (TiO₂, MoO₃, WO₃) substituted silicate xerogels as catalysts for the oxidation of hydrocarbons with hydrogen peroxide

TiO₂, MoO₃, and WO₃ have been dispersed in amorphous silica using the low temperature sol-gel procedure for xerogel preparation. These simply prepared amorphous compounds are proposed as possible alternatives to metal-substituted crystalline molecular sieves in H₂O₂ oxidations. The metallosilicate compounds are catalytically active in the 30% aqueous H₂O₂ oxidation of alkenes and alcohols provided the metal oxide precursor in the xerogel synthesis is a metal-dichlorodialkoxy compound yielding MO_x(Cl)-SiO₂, and not the tetraalkoxy derivative yielding MO_x-SiO₂. Catalyst efficiency is increased by using low loading of metal oxide in the silica framework. Excess H₂O₂ reduces yield due to the detrimental effect of water, so more hydrophobic silicates with phenyl-silicon units increases catalyst efficiency. IR studies show that in the xerogels, the absorption at ∼950 cm^-1 is mainly due to the Si-OH vibrations in (SiO)₃Si-OH units and not (SiO)₃Si-OM as has often been reported in studies of titanium-substituted zeolites. ²⁹Si MAS NMR spectra, sensitive to second neighbor atoms, of catalytically active MO_x(Cl)-SiO₂ versus inactive MO_x-SiO₂ reveals that the former have larger Q₃ peaks and therefore more (SiO)₃Si-OM units, indicating higher molecular dispersion of the metal oxide in the xerogels. Diffuse reflectance UV-vis measurements indicate, however, that this molecular dispersion is not complete as absorptions attributable to polymeric forms of metal oxide are observable. ESR spectra of the metal oxide substituted silicates in the presence of hydrogen peroxide or in the reduced form are not useful in differentiating between active and inactive xerogel compounds. Atomic force microscopy imaging of the xerogels at ∼10 nm resolution shows that the xerogel has a basically smooth surface. Large cylindrical pits of 500-700 nm diameter and depth of 15-40 nm are also observable as imperfections in the xerogel. There is also formation of small silicate droplets on the surface with dimensions similar to that of the pits. The catalytic xerogels are microporous with an average pore diameter of 15 Å and a surface area of 750 m²/g.