An investigation of active and selective oxygen in vanadium phosphorus oxide catalysts for n-butane conversion to maleic anhydride

Abstract

<p>The role of lattice oxygens in two model catalysts, [beta]-VOPO[subscript]4 and (VO)[subscript]2P[subscript]2O[subscript]7, was investigated for the selective and nonselective oxidation of C[subscript]4 hydrocarbons to maleic anhydride and combustion products. Specific catalytic oxygen sites in each model catalyst were labeled with specific amounts of [superscript]18O. Labeled sites were identified by laser Raman spectroscopy and Fourier transform infrared spectroscopy. The level of [superscript]18O enrichment in each site was estimated from the laser Raman spectra and the stoichiometry of reactions involved in the synthesis of the labeled catalysts. Products of the anaerobic C[subscript]4 hydrocarbon oxidation and, in the case of (VO)[subscript]2P[subscript]2O[subscript]7, alternating pulses of oxygen with pulses of hydrocarbon, over labeled catalysts were monitored by quadrupole mass spectrometry. These studies reveal that, on both catalysts, more than one path to CO[subscript]2 exists involving lattice oxygens. The consecutive combustion of maleic anhydride involves mainly V=O oxygen, while all other oxygen sites are utilized to form small amounts of CO[subscript]2. In both catalyst systems, P-O-V sites are the source for selective oxygens. Additionally, specific selective oxygen sites could be related to mechanistic steps in the conversion of n-butane to maleic anhydride for both model catalysts. The results of this work indicate that the initial selective interaction of n-butane with the surface of the catalyst results in an intermediate which is very different than that of the other C[subscript]4's studied, and that this strongly adsorbed intermediate is constrained to react with specific P-O sites. Additional evidence suggests that [beta]-VOPO[subscript]4 is not the V(V) species formed in the oxidation-reduction couple on the surface of (VO)[subscript]2P[subscript]2O[subscript]7.</p>