Structural Changes of Surface Layer of Vanadyl Pyrophosphate Catalysts by Oxidation−Reduction and Their Relationships with Selective Oxidation of n-Butane

The surface structure of vanadyl pyrophosphate ((VO) 2 P 2 O 7 ) and its changes upon controlled oxidation and reduction have been investigated comprehensively by means of Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), thermogravimetry (TG), EXAFS, X-ray diffraction (XRD), transmission electron diffraction (TED), and “micropulse” reaction of n -butane. Oxidation of a well-defined (VO) 2 P 2 O 7 with O 2 (1 atm) at 733 K formed “X 1 phase” as a thin surface overlayer on (VO) 2 P 2 O 7, where X 1 phase is a phase reported previously (Shimoda, T.; Okuhara, T.; Misono, M. Bull. Chem. Soc. Jpn . 1985, 58, 2163−2171) and similar to δ-VOPO 4 . By the repeated micropulse reactions of n -butane, the surface X 1 phase was gradually reduced back to (VO) 2 P 2 O 7, showing that reversible redox reactions between X 1 phase and (VO) 2 P 2 O 7 occur by the reactions with butane and oxygen. XRD, EXAFS, and TED demonstrated that X 1 phase has a structure analogous to (VO) 2 P 2 O 7, in both of which V−O−V pair sites exist. The micropulse reaction of n -butane with the surface X 1 phase produced maleic anhydride with a significantly higher selectivity (∼40%) than that with β-VOPO 4 (<10%). These results indicate that X 1 phase is the real active phase involved in the redox cycle for the oxidation of n -butane to maleic anhydride over (VO) 2 P 2 O 7 .