Creation of VO_x Surface Species on Pure Silica MCM-48 Using Gas-Phase Modification with VO(acac)_2

摘要

Pure silica MCM-48 is prepared by a novel synthesis method, using the [C_(18)H_(37)N~+(CH_3)_2-(CH_2)_(12)-N~+(CH_3)_2C_(18)H_(37)]·2Br~- surfactant, abbreviated as GEMINI 18-12-18. The MCM-48, obtained after careful calcination, is a highly crystalline, mesoporous material with the characteristics 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 VO_x species using a designed dispersion of VO(acac)_2 in a gas-deposition reactor. In the first step, the complex is anchored to the support. In a subsequent step the adsorbed complex is thermolyzed to yield chemically bonded VO_x surface species. 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 the VO(acac)_2 complex to the MCM support. Therefore, the maximum achievable number of surface V species is limited by the silanol number and not by the geometrical surface, which has a higher capacity. After calcination of the adsorbed complex, the supported VO_x species are present in a strictly tetrahedral configuration, mainly as chains of linked tetrahedra and not as isolated species.