Ionic Liquid-Assisted Synthesis of Vanadium Phosphate Catalysts from Phosphorous Acid for Selective Oxidation Reactions

Vanadium oxyphosphate (VPO) with a high surface area was synthesized in a mixed solvent of ionic liquid [Bmim]Br and water, using phosphorous acid to reduce V5+ to V4+ and as a partial phosphorus source, and V2O5 as the vanadium source. The effects of ionic liquids on the synthesis process were investigated in detail. Using pure water as the solvent, the product was a vanadium phosphate compound with large flakes. With the increase in the amount of ionic liquid, the size of the large flakes of vanadium phosphate gradually decreased, and the morphology was changed to spherical particles. When the proportion of ionic liquid was 80% or the ionic liquid was solely used as the solvent, the VPO showed completely spherical particle morphology, and the specific surface area was the highest. X-ray diffraction analysis showed that the crystal phase of VPO changed from vanadium (IV) hydrogenphosphate hemihydrate (VOHPO4 center dot 0.5H(2)O) to an amorphous state with an increase in ionic liquid ratio. All of the above results show that ionic liquids play an important role in the synthesis of VPO materials. After being calcined in an n-butane/air atmosphere, the precursors were transformed into the vanadium pyrophosphate phase. The active VPO catalysts were used in the selective oxidation of cyclohexanol, and the yield of cyclohexanone was 57.98% for the VPO-80 catalyst synthesized using the ionothermal method but only 21.41% for the VPO-W catalyst synthesized using a traditional hydrothermal method, which shows the advantage of ionic liquid synthesis for VPO catalysts. Graphic Abstract Vanadium oxyphosphate material with high surface area was synthesized in the mixed solvent of ionic liquid [Bmim]Br and water. Ionic liquid plays an important role in the morphology and properties of synthesized VPO materials. The active VPO catalysts were used in the selective oxidation of cyclohexanol, and the yield of cyclohexanone was 57.98% for VPO-80 catalyst synthesized by ionothermal method, but only 21.41% for VPO-W catalyst synthesized by traditional hydrothermal method, which exhibited the advantages of ionic liquid synthesis for VPO catalysts. [GRAPHICS] .