Physico-chemical features and catalytic properties of V-MCF materials obtained by direct synthesis

The samples directly synthesized exhibited higher specific surface areas (up to 820 m2g-1 for V-SBA-15 and 925 m2g-1 for V-MCF) as compared to impregnated ones. Raman spectroscopy of V-SBA-15 and V-MCF showed V=O stretching modes of isolated V species in tetrahedral coordination (ca. 1035 cm-1), whereas typical bands of polymeric VOx and micro-crystalline V2O5 were observed with both V-SBA-15-i and V-MCF-i samples. Likewise, TEM images revealed the presence of V-based clusters only at the surface of impregnated samples. IR spectroscopy showed that more abundant and acidic sites were obtained by impregnation. Better catalytic performances in both selective and total oxidation reactions were achieved with V-SBA-15 and V-MCF as compared to impregnated ones, as consequence of a better V dispersion. Interestingly, higher selectivity to propene (ODH of propane, Figure 1A) was obtained over V-MCF as compared to V-SBA-15, whereas better dichloromethane conversions (Figure 1B) were achieved with V-SBA-15 instead of V-MCF (a similar trend was observed with impregnated samples). This behaviour is assigned to the different porous network of the two systems. SBA-15 has monodimensional mesopores that may favour longer residence times of molecules, unlike the 3-D ultra-large pores of MCF and therefore deeper oxidations occur in SBA-15 systems. On the other hand, a non-porous sample (V-SiO2), with V species well dispersed and incorporated into the silica framework, exhibited lower activity than mesoporous samples, confirming the important role of mesoporosity in oxidation reactions.