Construction Of Novel Polyoxometalates On Aerobic Oxidation Of 5-hydroxymethylfurfural Combined With Micro-bubble Technique

Facing the increasingly serious depletion of petrochemical resources and the resulting environmental pollution,our country is expected to achieve the double-carbon target by2060.Among the many bio-platform compounds,5-hydroxymethylfurfural（5-HMF）has been identified as one of the eight intermediates by the U.S.Department of Energy.2,5-diformylfuran（DFF）is one of the attractive products from the oxidation of 5-HMF,which could be applied as a value-added chemical in synthesis of functional polymer,pharmaceutical compounds.The production of DFF from the selective oxidation of hydroxyl group in 5-HMF is significant in the transformation of biomass to chemicals.Polyoxometalates（POMs）have been widely used in the catalytic conversion of biomass due to their excellent acidity and oxidation.Heterogeneous catalysts showed a higher availability in targeted aerobic oxidation of 5-HMF to DFF.Micro-bubble technique has numerous advantages of small diameter,long residence time,stable phase interface and large specific surface area,which can effectively improve the gas utilization.Therefore,a series of heterogeneous catalysts with parent POMs of（NH₄）₅H₆PMo₄V₈O₄₀（PMo₄V₈）were synthesized to transform 5-HMF to DFF.1.A series of CTA_nH_11-nPMo₄V₈O₄₀(n=1,3,5,7,9,11,CTA_nH_11-nPMo₄V₈)were synthesized by in-situ phase transfer method with different ratios of Na₂Mo O₄,Na VO₃,H₃PO₄and cetyltrimethylammonium bromide（CTAB）.The catalytic activity and structure-activity relationship of the catalysts in oxidation of 5-HMF by oxygen under ambient pressure were investigated.The conversion of 5-HMF and DFF by CTA₅H₆PMo₄V₈reached 100%and 94.3%respectively in 126 mg of 5-HMF,80 mg of catalyst and 10 m L/min O₂at 110℃for 10 h.The activity of CTA₅H₆PMo₄V₈was mainly due to its self-assembly into reverse micelles in solvent dimethyl sulfoxide（DMSO）,which could aggregate 5-HMF molecules around the active site PMo₄V₈^11-by adsorbing hydroxyl groups in 5-HMF.At the same time,the oxidation product DFF was expelled from the reaction system,resulting in high selectivity of DFF.The introduction of micro-bubble greatly improved the reaction efficiency.TOF（Turnover frequency=[Conversion（%）×5-HMF（mmol）]/[Actual POMs（mmol）×Time（h）]）increased 1.24 times,representing the great increase of the utilization of O₂.CTA₅H₆PMo₄V₈O₄₀shows good stability and long duration in the two reaction models.2.PMo₄V₈was encapsulated within the cavities of the metal-organic（MOF）by in situ synthesis to prepare a series of catalysts PMo₄V₈（n）/MIL-101（Al）（n represents the load capacity of PMo₄V₈,n=10,20,30,35）.The catalysts with multiple active sites were used to in the oxidation of 5-HMF and monosaccharide.The composition and structure of PMo₄V₈（n）/MIL-101（Al）were characterized,which had acidic,alkaline and oxidation-reduction site at the same time.MIL-101（Al）could absorb the substrate and stabilize the micro-bubble,increasing the conversion of 5-HMF.Among them,PMo₄V₈（30）/MIL-101（Al）showed the best reaction activity,which obtained 97.3%5-HMF conversion and 93.1%DFF yield in 126 mg of 5-HMF,60 mg of catalyst and 20 m L/min O₂at 120℃for 8 h.PMo₄V₈（30）/MIL-101（Al）conducted the highly selective preparation of DFF from glucose and fructose in the one-pot conversion with the conversions of 93.7and 98.6%,and the DFF yields of 52.3 and 72.1%,respectively.The catalyst hybrid had good stability,which activity did not significantly decreased after 6 cycles.3.,The surfactant sodium dodecylbenzene sulfonate（SDBS）was introduced into PMo₄V₈（30）/MIL-101（Al）throughπ-πadsorption.A series of multifunctional catalysts PMo₄V₈（n）-SDBS（23）/MIL-101（Al）（x represents the content of SDBS,x=23,27,32wt%）were synthesized,and their activities in 5-HMF and monosaccharide oxidation were investigated.The results showed that SDBS can further effectively stabilize the microbubbles and promote the efficient oxidation of 5-HMF in the micro-bubble system.PMo₄V₈（30）-SDBS（23）/MIL-101（Al）shows the best reaction activity in 5-HMF,with the conversion of 95.7%,the yield of DFF of 92.0%in 126 mg of 5-HMF,50 mg of catalyst and 30 m L/min O₂at 110℃for 5 h.PMo₄V₈（30）-SDBS（23）/MIL-101（Al）was conducive to the highly selective preparation of DFF from glucose and fructose in the one-pot conversion method,with the conversions of 94.8 and 97.7%,the yields of DFF of 43.5 and70.4%,respectively.The composite catalyst could be recycled for more than 6 times.