CREATION OF VOX SURFACE SPECIES ON PURE SILICA MCM-48 USING GAS-PHASEMODIFICATION WITH VO(ACAC)(2)

Pure silica MCM-48 is prepared by a novel synthesis method, using the 18H37N+(CH3)(2)-(CH2)(12)-N+(CH3)(2)C18H37].2Br(-) surfactant, abbrevi ated as GEMINI 18-12-18. The MCM-48, obtained after careful calcinatio n, is a highly crystalline, mesoporous material with the characteristi cs of the Ia3d cubic phase, a surface area exceeding 1000 m(2)/g, and a narrow mesoporous pore size distribution (r = 1.4 nm; fwhh < 0.2 nm) . This MCM support is grafted with VOx species using a designed disper sion of VO(acac)(2) in a gas-deposition reactor. In the first step, th e complex is anchored to the support. In a subsequent step the adsorbe d complex is thermolyzed to yield chemically bonded VOx surface specie s, The final material contains 1.7 mmol V/g (8.7 wt % V) and still has a narrow pore-size distribution and a surface area of 800 m(2)/g. It is observed that all silanols are consumed during the adsorption of th e VO(acac)(2) complex to the MCM support. Therefore, the maximum achie vable number of surface V species is limited by the silanol number and not by the geometrical surface, which has a higher capacity, After ca lcination of the adsorbed complex, the supported VOx species are prese nt in a strictly tetrahedral configuration, mainly as chains of linked tetrahedra and not as isolated species.