Vanadium-phosphorous-oxygen industrial catalysts for C4 hydrocarbon selective oxidation to maleic anhydride
The selective oxidation of n-butane to maleic anhydride by vanadium-phosphorus-oxygen (V-P-O) industrial catalysts varying in P-to-V ratio has been studied. V-P-O catalysts have been synthesized, characterized, and the reactivity of these catalysts has been studied in a fixed bed integral reactor system. Catalyst characterization studies including X-ray diffraction, laser Raman spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy, X-ray energy dispersive spectroscopy, and BET surface area measurements were used. A strong effect of P-to-V synthesis ratio on catalyst structure, catalyst morphology, vanadium oxidation state, and reactivity in n-butane selective oxidation was observed. A slight "excess" of catalyst phosphorus (P/V = 1.1 catalyst) was found to stabilize an active and selective (VO)(,2)P(,2)O(,7) phase. The mechanism of n-butane selective oxidation to maleic anhydride was studied by in situ infrared spectroscopy using n-butane, 1-butene, 1,3-butadiene, crotyl alcohol, maleic acid, crotonic acid, and maleic anhydride feeds. During paraffin selective oxidation, highly reactive olefin species and maleic acid were observed on the surfaces of V-P-O catalysts. Further evidence in support of conjugated or possibly strained olefin and maleic acid reaction intermediates in n-butane and 1-butene partial oxidation to maleic anhydride was gathered.