The role of V and Mn incorporated mesoporous molecular sieves was investigated for the vapor phase oxidation of o-xylene. Mesoporous monometallic V-MCM-41 (Si/V = 25, 50, 75 and 100), Mn-MCM-41 (Si/Mn = 50) and bimetallic V-Mn-MCM-41 (Si/(V + Mn) = 100) molecular sieves were synthesized by a direct hydrothermal (DHT) process and characterized by various techniques such as X-ray diffraction, DRUV-Vis spectroscopy, EPR, and transmission electron microscopy (TEM). From the DRUV-Vis and EPR spectral study, it was found that most of the V species are present as vanadyl ions (VO2+) in the as-synthesized catalysts and as highly dispersed V5+ ions in tetrahedral coordination in the calcined catalysts. The activity of the catalysts was measured and compared with each other for the gas phase oxidation of o-xylene in the presence of atmospheric air as an oxidant at 573 K. Among the various catalysts, V-MCM-41 with Si/V = 50 exhibited high activity towards production of phthalic anhydride under the experimental condition. The correlation between the phthalic anhydride selectivity and the physico-chemical characteristics of the catalyst was found. It is concluded that V5+ species present in the MCM-41 silica matrix are the active sites responsible for the selective formation of phthalic anhydride during the vapor phase oxidation of o-xylene.
1. HorvathIT (2003) Encyclopedia of catalysis, vol 6, Wiley Interscience Publication, p 141.
HorvathIT (2003) Encyclopedia of catalysis, vol 6, Wiley Interscience Publication, p 141.)| false