A substrate-versatile catalyst for the selective oxidation of light alkanes - I. Reactivity

The reactivity of niobium and pyridine-exchanged molybdophosphoric (NbPMo(12)pyr) and molybdovanadophosphoric (NbPMol(11)Vpyr) acid catalysts are investigated for the selective oxidation of light alkanes. Productivities and selectivities are presented for the reactions of propane to acrylic acid and n-butane to maleic acid. Pretreatment of the material to 420 degreesC in an inert atmosphere is required to form an active catalyst, and the combination of both niobium and pyridine is necessary for obtaining maximum activity. Vanadium is not essential for the selective formation of maleic acid from butane, but does appear to affect the selective formation of acrylic acid from propane. Other metal cations can be used instead of niobium; however, none are as effective as niobium. Comparison of activity results to those from other molybdate-based structures indicates that high activity requires the use of a reducible molybdenum species. In addition, the central phosphorus heteroatom of the polyoxometalate is important for high activity. Structures without phosphorus or with other heteroatoms have lower activity. The catalysts are effective under both hydrocarbon-rich and oxygen-rich conditions. For butane, the productivity to maleic acid exceeds that of the current industrial standard vanadium phosphorus oxide. With propane, the productivity to acrylic acid is above that of the MoVNbTe mixed-metal oxide. In addition to efficiently catalyzing the selective oxidation of propane and butane, the catalysts are active with feeds containing other substrates such as ethane, isobutane, and toluene. (C) 2003 Elsevier Inc. All rights reserved.