Design And Synthesis Of Keggin Type Hetero-polymolybdate Composite Catalysts And Its Catalytic Oxidation Performance For 5-hydroxymethylfurfural

5-Hydroxymethylfurfural（HMF）,converted from sustainable carbohydrates,is considered as one of the most promising multifunctional biomass-based platform molecules.2,5-Diformylfuran（DFF）is a multifunctional platform molecule converted from HMF with promising applications in pharmaceuticals and polymers.And selective oxidation of HMF to DFF is an ongoing challenge.Polyoxometalates（POMs;abbreviated as polyacids）are a class of polymetallic oxygen cluster compounds formed from former transition metals（such as W,Mo,V,Nb,and Ta）linked by oxygen.Due to the unique structure and tunability of POMs,they are highly favored by researchers in the fields of catalysis,magnetism,medicine,photochemistry,and energy.However,POMs are extremely soluble causing that the products are difficult to be separated and purified in catalytic systems.They can only be used as homogeneous catalysts,which is not in line with the concept of green catalysis.Therefore,it is imperative to develop easily recoverable and recyclable POMs-based catalysts for practical applications in industry.Therefore,in this paper,the design and synthesis of Keggin-based heteropolymolybdate composite catalysts and their catalytic oxidation performance for HMF were carried out,using vanadium-substituted Keggin-type heteropolymolybdate(PMo₁₀V₂)as the object of study,selecting metal organic framework materials（MOFs）as the precursors to prepare the carriers using pyrolysis,and finally preparing POMs-based composite catalysts using hydrothermal synthesis method and applying them to the catalytic oxidation reaction of HMF.The main research work is summarized as follows:（1）Using Co₃O₄/C material prepared by high-temperature calcination of ZIF-67 as a carrier to load PMo₁₀V₂,the PMo₁₀V₂@Co₃O₄ catalyst was designed and synthesized.When it was used for HMF catalysis,the optimum conditions were:DMSO as the reaction solvent,130°C of reaction temperature,0.8 MPa of oxygen pressure,6 h of reaction time,15 mg of catalyst dosage.The conversion of HMF under the optimal conditions reached 98.7%and the selectivity of DFF was up to 100%because of the strong interaction between PMo₁₀V₂ and Co₃O₄/C,which had high stability during the catalytic process and maintained 98.7%conversion after 5 cycles.The catalytic mechanism was explored using tests such as XPS and in situ DRIFTS,and the results showed that Mo and V in PMo₁₀V₂ and Co in the Co₃O₄/C carrier were involved in the reaction during the catalytic oxidation process.A proton coupled electron transfer（PCET）catalytic mechanism can be proposed containing polymetallic centers during the reaction process.（2）The MIL-88A（Fe）was calcined in nitrogen atmosphere to obtain Fe₃O₄/C support,and PMo₁₀V₂ was loaded onto the Fe₃O₄/C support,successfully preparing PMo₁₀V₂@Fe₃O₄composite catalyst.The catalyst maintains the original spindle structure of MIL-88A（Fe）.The catalytic activity and cycling stability of the catalysts were investigated comprehensively and systematically.The optimum conditions were:DMSO as the reaction solvent,130°C of reaction temperature,0.8 MPa of oxygen pressure,6 h of reaction time,15 mg of catalyst dosage.The conversion of HMF under the optimal conditions reached 99.1%and the selectivity of DFF was up to 100%.The recyclable cycle was up to 10 times.The catalytic mechanism was explored using tests such as XPS and in situ DRIFTS,and the results showed that V⁵⁺,Mo⁶⁺and Fe³⁺were involved in the reaction,and HMF could be oxidized to DFF by PCET after adsorption on the surface of PMo₁₀V₂@Fe₃O₄/C,accompanied by a redox cycle with multiple electron transfers in this catalytic process.（3）The composite catalyst PMo₁₀V₂@Co Fe₂O₄/C was constructed by loading PMo₁₀V₂into a weakly basic Co Fe₂O₄/C carrier made by roasting MIL-88A（Fe/Co）,and the catalyst was systematically characterized using various testing methods.The effect of different conditions on the oxidation of HMF was examined to obtain the optimal catalytic reaction conditions,and the cycling stability of the catalyst was also investigated.The optimal conditions were:DMSO as the reaction solvent,reaction temperature of 120°C,oxygen pressure of 0.4 MPa,reaction time of 4 h,and catalyst dosage of 15 mg.The conversion of HMF under the optimal catalytic reaction conditions reached 99.6%and the selectivity of DFF was up to 100%.The composite catalyst had high stability,maintaining a high conversion of 99.6%after 10 cycles.Finally,the catalytic mechanism was explored using XPS and in situ DRIFTS.There was a strong interaction between PMo₁₀V₂ and Co Fe₂O₄/C,and the Mo and V in PMo₁₀V₂ and Fe and Co in the Co Fe₂O₄/C carrier were involved in the catalytic oxidation process.