Synthesis and characterization of porous vanadium silicates in organic medium

Porous vanadium silicates have been synthesized in ethanolic medium by base hydrolysis of a mixture of tetraethoxysilicon(IV) and cetyltrimethylammonium vanadate precursors. The materials are similar to mesoporous silicates such as MCM-41 but have smaller pores and specific surface areas ranging from 270 to 1100 m(2)/g for samples with vanadium concentrations from 32 to 8 at. %. Using a variety of techniques, it is shown that the synthesis method results in isolated tetrahedral vanadate units homogeneously incorporated into and on the silicate matrix. On calcination, a similar vanadium coordination is found, but adsorption of water causes cleavage of Si-O-V bonds to give a mixture of isolated tetrahedral vanadium species in addition to polymeric vanadates and vanadium oxides. Nevertheless, the vanadium in these samples appears to be in a highly dispersed state. Silicon-29 MAS NMR of uncalcined samples shows that the degree of SiO(4) condensation decreases with increasing vanadium content. The thermal stability appears to depend on establishing a well-condensed pore wall that occurs for samples containing up to 11 at. % vanadium. For samples containing vanadium concentrations greater than about 15 at. % the pore structure is unstable and ruptures on exposure to moist air. The vanadium silicates are compared to vanadia-silica aerogels and MCM-41.