Characterization of mesoporous VO _x /MCM-41 composite materials obtained via post-synthesis impregnation

Spherical-particle MCM-41 was synthesized at room temperature, and, then, impregnated with aqueous solutions of NH ₄ VO ₃ to produce variously loaded VO _x /MCM-41 composite materials. Bulk and surface properties of the materials thus produced were characterized by means of X-ray powder diffractometry (XRD), infrared spectroscopy (FTIR), N ₂ sorptiometry and X-ray photoelectron spectroscopy (XPS). Results obtained indicated that subsequent calcination at 550 °C (for 2 h) of the blank and impregnated MCM-41 particles, results in materials assuming the same bulk structure of MCM-41, and exposing uniformly mesporous, high area surfaces (P _w = 2.0-2.3 nm; 974-829 m ² /g), except for the material obtained at 20 wt%-V ₂ O ₅ that was shown to suffer a considerable loss on surface area (down to 503 m ² /g). XPS results implied that the immobilization of the VO _x species occurs via interaction with surface OH/H ₂ O groups of MCM-41, leading to the formation of vanadate (VO ₃ ^- ) surface species, as well as minor V-O-Si and V ₂ O ₅ -like species. However, in all cases, the vanadium sites remained pentavalent and exposed on the surface.