EFFECT OF PROMOTERS ON THE SELECTIVE OXIDATION OF N-BUTANE WITH VANADIUM-PHOSPHORUS OXIDE CATALYSTS

The effect of additives on the catalytic performance of vanadium phosp hate catalysts is described and discussed in detail for materials prep ared using HCl as the reducing agent for V2O2 in water or isobutanol s olvents. Performance data are compared on the basis of a simplified ki netic model for n-butane oxidation involving a series-consecutive reac tion scheme of pseudo first-order reactions (where C4H2O3 signifies ma leic anhydride): [GRAPHICS] Using this simplified kinetic scheme it is possible to calculate three factors: (a) the rate constant for butane conversion corrected for surface area (k(1) + k(2))/sa which is a mea sure of the specific activity; (b) the primary selectivity S-0 (select ivity extrapolated to zero conversion); and (c) the relative rates of oxidation of n-butane and maleic anhydride k(3)/(k(1) + k(2)), the lat ter two factors being a measure of the selectivity of the catalyst. It is proposed that these can be used to determine the relative catalyti c performance of promoted catalysts. Hence an improved catalyst would require high specific activity (i.e., a high value of (k(1) + k(2))/sa ) together with high maleic anhydride selectivity (i.e., a high value of S-0 and a low value of k(3)/[k(1) + k(2)]) For catalysts prepared b y the isobutanol/HCl method the addition of Co is found to promote the specific activity while Mo is found to promote both the specific acti vity and the selectivity to maleic anhydride at high conversion (>90%) . However, for most additives no effect on the specific activity is ob served although certain additives are found to act as poisons (Ag, Cs) while others promote the oxidation of maleic anhydride (Cs, Pd, Ru, Z n). The specific activity of catalysts prepared using water/HCl with a n additional water-washing step was found to be promoted by the additi on of a range of transition metal additives and the effect is describe d in detail with Co promoted catalysts. The mechanism of the promotion is discussed in terms of structural and electronic effects. (C) 1996 Academic Press, Inc.