Selective oxidation of n-butane on a V-P-O catalyst: Improvement of the catalytic performance under fuel-rich conditions by doping

The oxidation of n-butane to maleic anhydride is possible under fuel-rich c onditions (nC(4)/O-2 = 0.6) at 400 degrees C by modifying the vanadium phos phorus oxide (V-P-O) catalysts by doping with Co or Mo; it is impossible on the undoped V-P-O catalyst, which deactivates under the same conditions. T he doped precursors were prepared by reduction of the dihydrate VOPO4, 2H(2 )O, and dopants were introduced at different moments of the preparation fro m cobalt acetylacetonate and ammonium heptamolybdate solutions, respectivel y. The Co-doped V-P-O catalyst (Co/V = 0.77%) conserves a higher capacity f or reoxidation of the surface (higher V5+/N4+ ratio) and maintains a higher surface distribution of the V, P, and O atoms under fuel-rich conditions: Oz consumption is not total. Maleic anhydride is then produced and the cata lysts do not deactivate. The Mo-doped V-P-O catalysts (Mo/V from 0.88% up t o 2.56%) do not give performances as good as those for the Co-doped V-P-O c atalyst. It appears that it is necessary to dope at a higher percentage, Mo /V = 2.56%, in order to achieve higher selectivity for MA. The correspondin g catalyst is less active (5.3% nC(4) conversion instead of 25.5%) than the Co-doped V-P-O catalyst. This study demonstrates that the nature of the do pant is more important than its way of introduction into the V-P-O matrix. It opens new routes for adapting V-P-O catalysts for oxidizing n-butane to maleic anhydride under fuel-rich conditions and for developing new technolo gies. (C) 2000 Academic Press.