Vanadium-phosphorus-oxygen (V-P-O) catalysts, prepared in alcohol solution with P-to-V ratios from 0.9 to 1.2, were characterized by X-ray diffraction, infrared spectroscopy, laser Raman spectroscopy, scanning electron microscopy, and X-ray photoelectron spectroscopy. A strong effect of P-to-V synthesis ratio on catalyst structure, catalyst morphology, vanadium oxidation state, and reactivity in n-butane selective oxidation to maleic anhydride was observed. A slight "excess” of catalyst phosphorus (P/V =1.1 catalyst) was found to stabilize a (V0)2P207 phase which was active and selective in n-butane oxidation; a larger excess of phosphorus was found to enhance selectivity of the (V0)2P207 phase at the expense of catalyst activity. Used V-P-0 catalysts without “excess” phosphorus contained the active but nonselective α-VOP04 phase.
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