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 MOx(Cl)-SiO2, and not the tetraalkoxy derivative yielding MOx-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 MOx(Cl)-SiO2 versus inactive MOx-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.
|Number of pages||12|
|Journal||Journal of Catalysis|
|Publication status||Published - 1997|
ASJC Scopus subject areas
- Process Chemistry and Technology