SELECTIVE OXIDATION OF n-BUTANE TO MALEIC ANHYDRIDE UNDER OXYGEN-DEFICIENT COMDiTIONS OVER V-P-O MIXED OXIDES

The selective oxidation of n-butane to maleic anhydride over V-P-O mixed oxides was studied under oxygen deficient conditions. The mixed oxides were prepared with P/V atomic ratios ranging from 0.7 to 1.0. Catalysts with P/V (1.0 did not show any selectivity to maleic anhydride formation, regardless of whether or not (VO)2P207 was present. For catalysts with P/V = 1.0, containing by &e actual surface V '/V4 (VO) P207 and/or the so-cfled f*-phase, the selectivity was strongly influenced ratio. This ratio is determined by the temperature, the crystal phases present in the catalyst and the composition of the gas mixture. Optimal selectivity was obtained at 425°C with 15% butane in air and a butane/oxygen ratio of 0.9. INTRODUCTION Benzene'has for many years been the most important feedstock for the production of maleic anhydride (MA). The search for a more economical feedstock was initiated by the rapid increase in benzene prices caused by the increase in oil prices and to the increased use of aromatics in unleaded automobile fuels, this trend being stimulated by tighter benzene-emission control standards. Thus the benzene process has been replaced gradually in the last ten years by the selective oxidation of n-butane (or butylene). The catalysts used are mostly V-P-O mixed oxides, materials which are known to be active and selective for this oxidation reaction [1,2]. fke relationship between the methods of preparation, the catalyst structure and the selectivity for MA production has been reviewed recently by Hodnett [3]. For the active catalysts the P/V atomic ratio is typically in the range 1.0-1.2 and the average vanadium oxidation state is in the range 4.1-4.3. It appears, in spite of much research effort, that the efficiency of n-butane-based plants is still only 60-70X of that of benzene-based plants. Commercial butanebased plants work with a gas-mixture of 1.5% butane in air and have a maximum yield of 50-60% at a conversion of 90-100%. Attempts have been made to prepare catalysts with higher surface areas than the conventionally used catalysts by preparing the precursor from non-aqueous solutions [4,5] in order to attain higher conversions at lower temperatures [6] 0166~@834/?37/$03.50