On the Correlation of Structure and Catalytic Performance of VPO Catalysts

Correlations are reported between structure and performance of current, state-of the art, industrial maleic anhydride production catalysts. Characterization was done with fully equilibrated catalysts to achieve reliable correlations of structure and activity. The kinetic analysis of the investigated VPO catalysts revealed differences in activities but comparable activation energies indicating active on-surface sites of similar nature but of different concentrations. One major crystalline component, VPP, and six minority compounds were identified, hence, VPP alone cannot be reasonably correlated with catalytic activity. XPS revealed elevated V5+ concentrations, differences in the P/V ratios, and the P surface content correlates negatively with catalytic activity. Hence, all crystalline phases must have noncrystalline, surface-layers containing V5+ species and varying P amounts. Deconvolution of Raman spectra identified four major vanadyl phosphate species of changing concentrations. One species, alpha(I)-VOPO4, was absent from the XRD patterns of the two most active catalysts, indicating that it is noncrystalline and, hence, integral part of the amorphous surface layers. These spectral/structural differences correlated with catalyst activity. The most active catalysts contain relatively more crystalline VPP, in accordance with literature. These active catalysts also have a higher amount of the amorphous V5+ surface species, alpha-VOPO4. All other V5+ species detected, delta- and (g)-VOPO4 are crystalline and negatively affect catalyst activity. Again in agreement with literature, the surface P/V ratio negatively correlates with catalyst activity, while the reported correlation of the V5+/V4+ ratio with activity was not observed. The multi-method approach chosen here, always strongly favored by H. Knozinger, generated a detailed picture of the compositional differences between the bulk and surface regions of the investigated catalysts and revealed how these differences correlate with catalytic performance.