The vapor phase conversion of maleic anhydride was studied using two model vanadium-phosphorus-oxygen (V-P-O) catalysts, β-VOPO4 and (VO)2P2O7, and a series of industrial-type catalysts having P-to-V ratios of 0.9, 1.0, 1.1 and 1.2. Maleic anhydride was fed directly to an integral flow reactor using a molten maleic anhydride saturator system. Direct evidence for the combustion of maleic anhydride has been observed with the simultaneous production of maleic acid. The conversion of maleic anhydride increased with temperature, oxygen partial pressure, and residence time. The incorporation of "excess" phosphorus in industrial catalysts suppressed maleic anhydride combustion by stabilizing the (VO)2P2O7 phase.
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