Selective Oxidation Of Lignin Catalyzed By Polyoxometalate Ionic Liquids

As the only renewable carbon resource in nature,the efficient utilization of biomass is one of the solutions that can sustain the long-term human needs for energy with minimized environmental impact.Lignin is the most abundant renewable aromatic polymer in nature,so it has been considered as a promising feedstock for producing high-quality fuels and value-added platform chemicals.Nevertheless,the effective utilization of lignin remains a major challenge because of its complex molecular structure and the recalcitrant chemical nature,which generally result in poor yields and selectivities of products.Therefore,development of novel and efficient lignin conversion processes plays an important role in the high-value utilization of lignin.In this work,we propose an efficient catalytic process for the selective oxidation of lignin to high value-added chemicals using polyoxometalate ionic liquid functioned with acidity and oxidizability.Moreover,a plausible pathway for this process is proposed.The detailed studies are as follow:?1?Combined with the advantages of polyoxometalates and ionic liquid,a series of polyoxometalate ionic liquids were designed and synthesized.Then the catalytic performance of the selective oxidation of lignin to diethyl maleate was investigated.Under the optimized condition?180?,5 h?,65.5%of the alkali lignin could be converted over[BSmim]Cu H₂PMo₁₀V₂O₄₀,affording 1243.6 mg·g^-1 yield and 84.3%selectivity of diethyl maleate.Moreover,[BSmim]Cu H₂PMo₁₀V₂O₄₀simplifies the recovery process through self-separation with temperature control,and shows good recyclability,where a satisfactory catalytic activity is demonstrated even after six consecutive runs.Furthermore,the structure evolution of the alkali lignin was also investigated via the comparative characterization of the original and regenerated lignin using FT-IR,2D HSQC NMR,GPC and elemental analysis.It shows that polyoxometalate ionic liquids could effectively promote the oxidation of structural units in lignin,especially G unit and S unit,thereby achieving high conversion of lignin with high yield and selectivity of products.?2?In order to understand the reaction mechanism of lignin oxidation catalyzed by polyoxometalate ionic liquids,4-methoxy-?-[?2-methoxyphenoxy?methyl]-benzenemethanol,the representative lignin model compound contained?-O-4 linkage was chosen as the feedstock.Under the optimized condition?180?,5 h?,98.5%of lignin model compound could be converted through the cleavage of C-O bond and benzene ring over[BSmim]Cu H₂PMo₁₀V₂O₄₀,affording 84.3 wt.%yield and 62.2%selectivity of diethyl maleate.Furthermore,a plausible pathway for this process is proposed.Moreover,polyoxometalate ionic liquid demonstrates satisfactory catalytic activity on the oxidative conversion of lignin model compounds contained various chemical linkages.Therefore,this process provides a novel and potential strategy for the high-value utilization of lignin.