Structure and Catalytic Properties of VOx/MCM Materials for the Partial Oxidation of Methane to Formaldehyde

Novel VOx catalysts have been developed for the partial oxidation of methane to formaldehyde by air. High surface area mesoporous siliceous MCM-41 and MCM-48 materials have been used as supports to disperse vanadium oxide to prevalent monomeric, i.e., isolated vanadium oxide species, being necessary to minimize the consecutive oxidation of the desired product to carbon oxides. On these supports, a higher concentration of isolated active sites can be obtained than on conventional silica supports. Vanadium oxide was deposited by impregnation with aqueous solutions of NH4VO3 and VO(C2O4), respectively, followed by calcination of the precursor at 873 K for 16 h. The dispersity and the nature of the vanadium oxide species, i.e., their valence state, coordination, and location on and inside the pore walls, were studied by temperature-programmed reduction, potentiometric titration, solid state 51V NMR, ESR, and UV-Vis spectroscopy. FTIR spectroscopy was used to characterize the acidic porperties. Different vanadium oxide species were observed: (i) VIVOx and VVOx species anchored to the siliceous MCM walls and (ii) VIVOx and VVOx species incorporated into the pore walls. VVOx species anchored to the surface showed properties similar to those on conventional VOx/SiO2 catalysts. However, it was found that the VOx species carry acidic V–OH groups, at least partially. The catalytic properties of the catalysts were tested in a plug flow reactor. Reaction orders with respect to methane and oxygen, respectively, as well as the apparent activation energy were comparable with those reported for vanadium oxide supported on precipitated silica, pointing to similar active sites. However, significant higher space-time yields of formaldehyde could be achieved amounting to about 2.2 kg·kgcat−1·h−1. Increased activity during cofeeding of steam could be observed due to the generation of VVO(OH)x(OSi≡)3−x surface species or the corresponding “reduced sites” VIVO(OH)x(OSi≡)2−x supposed as genuine active sites.