Impact of partial oxidation in dense CO2 on the solid-state properties of (VO)2P2O7catalyst

Abstract

(VO)₂P₂O₇ was used as a catalyst for the partial oxidation of propane to oxygenates (acrylic acid and acetic acid) in dense carbon dioxide (2–10 MPa) at 673 K using air as the oxidant. The catalytic studies showed a decreasing yield of acrylic acid with higher reaction pressure, with the opposite effect found for acetic acid. One of the reasons for such behaviour has recently been found to be a stronger interaction of the catalyst and the olefinic double bond via catalyst Lewis sites. Another reason could be changing vanadium oxidation state in the solid catalyst. Therefore, detailed XPS and ³¹P NMR spectroscopic studies using the spin echo mapping technique were carried out to reveal more information on the possible pressure dependency of the vanadium oxidation state. The investigations clearly revealed an increasing vanadium oxidation state in the direction of V⁵⁺ with increasing reaction pressure. This might be due to a higher oxygen concentration on the catalyst surface. The decrease in acrylic acid yield also seems to be due to the observed increase in the vanadium oxidation state leading to over-oxidation of a greater extent.