New Insights into the Key Role of Thermal Treatment in V/P/O Catalysts for the Selective Oxidation of n-Butane to Maleic Anhydride

This work explores the thermal treatment of V/P/O catalyst precursors to achieve active and selective catalysts for the oxidation of n-butane to maleic anhydride (MA) in a continuous-flow fixed-bed reactor. Vanadyl pyrophosphate (V⁴⁺, VPP), the key catalyst component, is produced together with suitable V⁵⁺ vanadium orthophosphate (VOPO₄) allotropic forms by thermally treating vanadyl hydrogen phosphate hemihydrate (VHP) under various atmospheres and temperature ramps. The characterization conducted by using X-ray diffraction, Raman spectroscopy, and reaction testing allowed the identification of optimal conditions for active and selective catalysts. Oxygen is necessary for obtaining VPP and affects the vanadium oxidation state, which is a crucial parameter for selectivity. Water enhances the crystallinity and conversion of VHP to VPP. An optimized calcination atmosphere (6:10:84 mol % O₂/H₂O/N₂) ensures 70% MA selectivity at 50% butane conversion at 400 °C. VHP precursors characterized by higher P/V ratios allow us to obtain higher MA selectivity when treated under the same calcination conditions. This study provides valuable insights into the VPP production steps, representing the starting point for fine-tuning the calcination conditions based on the VHP properties (i.e., P/V ratio and carbon content).