Direct grafting of alkoxide promoters on vanadium hydrogen phosphate precursors: Improved catalysts for the selective oxidation of n-butane Original Research Article

New vanadyl phosphate (VPO) catalysts for the oxidation of n-butane to maleic anhydride were synthesized by grafting reactive metal alkoxides onto pre-formed vanadyl phosphorus oxide precursor, VO(HPO4)1/2H2O, followed by a calcination and activation protocol. A series of precursors grafted with these alkoxides was used to generate promoted catalysts containing molybdenum and mixtures of molybdenum with main group or transition metal promoters. For many catalysts, a large increase (greater than twofold) in oxidation activity for n-butane was observed and the high selectivity to maleic anhydride was not compromised. For these catalysts, the N2 BET surface areas do not change significantly with respect to unpromoted catalysts. Large reactivity enhancements were also obtained by contacting the alkoxides with vanadyl phosphate catalyst precursors which had been spray dried with polysilicic acid to form attrition resistant microspheres. Promoted VPO catalysts containing molybdenum combined with other promoters such as Sn, Ti and, in catalysts without silica binder, Fe, Mn, and Zn show further performance improvements compared to the single promoter system containing molybdenum. Hence, the relative ordering of molybdenum co-promoters for VPO catalysts without silica binder is Sn > Fe > Zn > Mn ∼ Bi > Co ∼ Ni in order of decreasing activity. For catalysts spray dried with silica binder, the relative ordering of co-promoters is Ti > Sn > Cu > Al ∼ Fe ∼ Zr. The reactive grafting technique of promoters with VPO is well suited for rapid, high throughput screening methods for the preparation of materials in which a vanadyl phosphate precursor “scaffold” is used to generate the new materials. This technique may be extended to generate a variety of new catalysts.