Preparation Of Novel Vanadium Phosphate Catalysts For Catalytic Oxidation Reactions

Vanadium oxyphosphate（VPO）catalysts are widely used in C2-C4 selective oxidation reactions.Its composition structure is very complicated,but generally takes+4 vanadium as the basic active center.In the past,VPO catalysts with+4 valence vanadium used phosphoric acid as the phosphorus source and added a reducing agent（such as alcohols,etc.）to the synthesis method.This method has a complicated reaction system and requires high temperature conditions.Some scholars proposed to use phosphite as both reducing agent and phosphorus source,so that the reaction system was simpler.However,due to the relatively weak reduction of phosphite(HPO₃^2-,phosphorus is+3 valence state),the reaction requires higher reaction temperature（above 145°C）and longer reaction time（above 72 h）to complete the redox reaction with V₂O₅.In this paper,hypophosphite（H₂PO₂^-,phosphorus is+1 valence state）with strong reduction was innovatively used as reducing agent and phosphorus source.The results showed that V⁵⁺could be reduced to V⁴⁺or even V³⁺by hypophosphorous acid at 100°C.Therefore,the preparation of the V⁴⁺valence vanadyl phosphate material can be realized under the condition of normal pressure liquid phase reflux in the aqueous solution,which has a great technological breakthrough significance.In this paper,a series of VPO catalysts were prepared by using water,low eutectic mixture and ionic liquid as solvent,hypophosphite as reducing agent and phosphorus source,vanadium pentoxide as vanadium source.The composition,morphology and structure of the prepared materials were studied in detail,and their catalytic performance in the synthesis of cyclohexanol and cyclohexanone by the oxidation of cyclohexane was studied.The specific research content and conclusions are as follows:（1）VPO catalysts was synthesized by using deionized water as reaction solvent,H₃PO₂ as reducing agent and partial phosphorus source,H₃PO₄ as external phosphorus source and V₂O₅ as vanadium source.At different temperatures（100^oC,110^oC,120 ^oC）and different vanadium-phosphorus ratios(V⁵⁺:P¹⁺:P⁵⁺=1:1:1,V⁵⁺:P¹⁺:P⁵⁺=1:2:1,V⁵⁺:P¹⁺:P⁵⁺=1:3:1).XRD characterization showed that under normal pressure conditions,a better crystalline（VO）₂P₂O₇ catalyst can be synthesized at as low as100^oC,indicating that+5 valence V can be reduced to+4 valence V at 100^oC,and the purity was high.When the ratio of vanadium to phosphorus is V⁵⁺:P¹⁺:P⁵⁺=1:1:1,the synthesized catalyst is（VO）₂P₂O₇ crystal phase,as the vanadium-phosphorus ratio increase,the V of+5 is reduced to+3 price,and synthesized catalyst changes from（VO）₂P₂O₇ to V_1.28（PO₄）（OH）_0.84（H₂O）_0.37 crystal phase.Rod-shaped VPO-W-1 and flake-shaped VPO-W-150 catalysts were used for cyclohexane oxidation reaction,the cyclohexane conversion rate of VPO-W-1 catalyst can reach 91.25%,and the selectivity of KA oil can reach above 80%.The cyclohexane conversion rate of the VPO-W-150catalyst is 88.71%,and the KA oil selectivity is 76.62%.（2）A series of VPO catalysts were synthesized by using different eutectic mixtures as reaction solvents,H₃PO₂ as reducing agent and partial phosphorus source,H₃PO₄ as external phosphorus source and V₂O₅ as vanadium source.X-ray diffraction characterization showed that the catalysts synthesized with these three eutectic mixtures were amorphous.It can be seen from the measurement of the specific surface area that the specific surface area of the catalyst obtained by using the eutectic mixture as the reaction solvent is slightly larger than that of the catalyst synthesized by the hydrothermal method,which may provide more surface active sites.The VPO-DES-3catalyst showed the best catalytic effect in the synthesis of KA oil from cyclohexane.The conversion of cyclohexane reached 94.52%,and the selectivity of KA oil was85.09%,which was better than that of the catalyst synthesized by hydrothermal method.（3）A series of VPO catalysts were synthesized by using H₃PO₂ as reducing agent and partial phosphorus source,H₃PO₄ as external phosphorus source and V₂O₅ as vanadium source.By adjusting the ratio of ionic liquid（1-butyl-3-methylimidazolium bromide（[Bmim]Br））and deionized water in the reaction solvent.XRD characterization showed that the crystalline phase of the catalyst synthesized with ionic liquid as solvent is basically amorphous.Scanning electron microscopy showed that as the proportion of ionic liquid increases,the morphology of the VPO catalyst precursor changes from large flakes（VPO-20）to small particles（VPO-100）.Due to the unique physical and chemical properties of ionic liquids,the specific surface area of VPO-100is increased to 13.8 m²/g compared with that of VPO-W-1 synthesized by hydrothermal method,which may expose more active sites.The results showed that the catalyst VPO-100 synthesized under pure ionic liquid has the best catalytic performance.When used in cyclohexane oxidation reaction,the cyclohexane conversion rate was 89.41%,and the KA oil selectivity was the largest,which was 80.99%.（4）Toluene was used as the internal standard,the cyclohexane and the target product were qualitatively and quantitatively analyzed by the internal standard method.The catalytic performance of VPO-W-1 catalyst synthesized by hydrothermal method in cyclohexane oxidation was systematically studied.The ratio of the fixed tert-butyl hydroperoxide to cyclohexane was 5:1.The effects of reaction temperature,reaction time and catalyst dosage on the conversion of cyclohexane and the selectivity of the target product KA oil were investigated.The optimal reaction conditions were as follows:the optimal reaction time was 9 h,the optimal reaction temperature was 80^oC and the optimal amount of catalyst was 0.25 g.