Model Vanadium-Phosphorus-Oxygen Catalysts for the Selective Oxidation of C sub 4 Hydrocarbons to Maleic Anhydride

摘要

Two model vanadium-phosphorus-oxygen catalysts, beta -VOPO sub 4 and (VO) sub 2 P sub 2 O sub 7 , were investigated for the selective oxidation of C sub 4 hydrocarbons to maleic anhydride. In situ laser Raman spectroscopy was used. Complementary techniques including x-ray powder diffraction and x-ray photoelectron spectroscopy were used for characterization of the fresh and used catalytic materials. The direct observation of phase stability and the participation of lattice oxygen during catalysis was possible using the in situ Raman technique. In particular, beta -VOPO sub 4 and (VO) sub 2 P sub 2 O sub 7 demonstrated bulk structural integrity during n-butane oxidation. The relatively greater reducing capacity of 1-butene induced the beta -VOPO sub 4 to (VO) sub 2 P sub 2 O sub 7 phase transformation. Direct structural identification of catalytically active centers for paraffin and olefin oxidation were investigated using an sup 18 O-enriched beta -VOPO sub 4 phase catalyst. Active sites responsible for complete combustion (Site I) and selective oxidation (Site II) were identified. The selective route for 1-butene oxidation involved predominantly Site II centers, while Site I centers were associated with complete combustion. In contrast, n-butane oxidation required the highly active Site I centers for initial activation and for the formation of an intermediate containing two oxygen atoms. Raman band assignments indicated these oxygen sites were associated with PO sub 4 units in the beta -VOPO sub 4 lattice. Maleic anhydride conversion was particularly sensitive to the catalytic phase present. Significant combustion activity was observed when maleic anhydride was fed directly to an integral flow reactor charged with model catalysts. The combustion activity was least for (VO) sub 2 P sub 2 O sub 7 , nominally a V(IV) phase. The beta -VOPO sub 4 catalyst, nominally a V(V) phase, resulted in increased conversions of maleic anhydride. (ERA citation 12:035323)