Design And Synthesis Of Polyoxometalates And Their Activities In Lignocellulose Conversion

With the depletion of fossil fuels and the increasing problems on global warming,many researchers are constantly searching for environmentally friendly and sustainable energy sources.Lignocellulose has been paid more attention owing to its wide sources and renewable,which can not only be converted into fuels and high value-added fine chemicals and reduce dependence on fossil fuels,but also do not compete with people for food rations.Lignocellulose has reticular structure,which is connected by lignin,hemicellulose and cellulose.Although,the diversity of component offers possibility for the production of different chemicals,the efficient conversion is difficult owing to its solid“reinforced cement”structure,such as low conversion efficiency,poor product selectivity,harsh reaction conditions and so on.So far,the study of high efficiency and simultaneous depolymerization to high-value chemicals is the focus in lignocellulose conversion.Therefore,the design and synthesis of high efficiency catalysts is an important research aspect.Polyoxometalates（abbreviated as POMs,known as heteropolyacid）are considered to be one of the most promising catalysts in lignocellulose conversion owing to their diverse structure,controllable acidity,rapid multi-electron transfer and reversible redox properties.It was reported that POMs were mostly used for oxidation or acid-catalyzed conversion of single component of lignocellulose.There are few reports on the multicenter POMs in converting lignocellulose stage-by-stage.In this paper,multicenter POMs were designed and synthesized,and the catalytic performance of the catalysts were investigated in converting lignocellulose stage-by-stage.1.A series of POMs with different composition and structure(H₃PW₁₂O₄₀,H₄SiW₁₂O₄₀,H3PMo12O40,H5PMo10V2O40 with Keggin structure and H6P2W18O62,H6P2Mo18O62 with Dawson structure)were synthesized and applied in the oxidation ofβ-O-4 lignin model compound 2-phenoxy-1-phenylethanol（abbreviated as PP-ol）.The activity was related to the redox ability and Br?nsted acidity of POMs.H₆P₂Mo₁₈O₆₂ showed highest actvivity in PP-ol oxidation.When the reaction conditions were 140℃,10 h,0.5 MPa O₂ and molar ratio of catalyst to substrate=1:3,the conversion of PP-ol was 68.9%,and the selectivities of phenol and acetophenone were 72.3%and 59.9%,respectively.2.Synthesis and catalytic performance of[MIMPS]_nH_6-nP₂Mo₁₈O₆₂:A series of ionic liquid temperature-controlled catalysts[MIMPS]_nH_6-nP₂Mo₁₈O₆₂(abbreviated as[MIMPS]_nH_6-nP₂Mo₁₈,n=1-6)were synthesized by using temperature-controlled ionic liquids 1-（3-sulfonic group）propyl-3-methyl imidazolium（abbreviated as MIMPS）and H₆P₂Mo₁₈O₆₂,and the catalytic performance in PP-ol oxidation were investigated.It was found that[MIMPS]₂H₄P₂Mo₁₈ showed the best activity owing to the synergistic effect of its suitable Br?nsted acidity,redox property,adsorption of PP-ol and O₂ and temperature-response.When the reaction conditions were 140℃,10 h,0.5 MPa O₂ and molar ratio of catalyst to substrate=1:3,94.6%PP-ol conversion,88.3%phenol selectivity and 75.2%acetophenone selectivity were obtained.Based on the above results,two kinds of natural lignocellulose（pine and poplar）were used to be investigated on the mechanism and optimal reaction conditions in converting lignocellulose stage-by-stage,which depolymerized lignin firstly,and subsequently hydrolyzed hemicellulose and cellulose.Moreover,the catalyst showed desirable stability and can be reused for 10 times in PP-ol oxidation without any significant decrease in activity.3.Synthesis and catalytic performance of Be_nH_6-nV₂Mo₁₈O₆₂:A series of nanometer micellar catalyst BenH6-nV2Mo18O62（abbreviated as BenH6-nV2Mo18,n=1-6）were designed and synthesized by using betaine(C₅H₁₁NO₂,abbreviated as Be)and H₆V₂Mo₁₈O₆₂.The catalyst was self-assembled into a microreactor to enrich PP-ol and O2 to the active site and improve the mass transfer.As a result,the synergistic effects of acidity,suitable redox ability and concentration effect made the high efficiency in PP-ol oxidation,in which BeH5V2Mo18 showed the best activity in converting PP-ol to phenol acetophenone.When the reaction conditions were140℃,8 h,0.8 MPa O2 and molar ratio of catalyst to substrate=1:3,the conversion of PP-ol was 92.9%,and the selectivities of phenol and acetophenone were 92.9%and 83.1%,respectively.Pine and poplar were used to be investigated on the mechanism and optimal reaction conditions of nanometer micellar catalyst in converting lignocellulose stage-by-stage.Moreover,the catalyst kept good activity after being reused for 10 times in PP-ol oxidation.4.Synthesis and catalytic performance of[NLL]_nH_6-nV₂Mo₁₈O₆₂:A series of acid-base-oxidation triple active sites catalysts[NLL]nH6-nV2Mo18O62（abbreviated as[NLL]n H6-nV2Mo18,n=1-6）were designed and synthesized by introducing amino acid surfactant(N-（N’-Laruoyl-L-lysine,C18H36N2O3,abbreviated as NLL）into H6V2Mo18O62 and applied in PP-ol oxidation.The structure-activity relationship between the catalyst and PP-ol oxidation was investigated systematically.It was found that the synergistic effect of acidity,basicity,redox and concentration effect of the catalyst made the efficient conversion of PP-ol.[NLL]H₅V₂Mo₁₈showed the best activity.When the reaction conditions were 130℃,10 h,1.0 MPa O₂ and molar ratio of catalyst to substrate=1:5,96.8%PP-ol conversion,91.2%phenol selectivity and83.3%acetophenone selectivity were obtained.Pine and poplar were used to be investigated on the synergistic effect and optimal reaction conditions of multicenter catalyst in converting lignocellulose stage-by-stage.Moreover,the catalyst kept good activity after being reused for10 times in PP-ol oxidation,and there was no obvious loss in activity.5.Synthesis and catalytic performance of H₆V₂Mo₁₈/PANI/V₂O₅（n）:A series of different loading amount catalysts H₆V₂Mo₁₈/PANI/V₂O₅（n）(n=10,20,30,40 and 50 wt%,n represented the loading amount of H₆V₂Mo₁₈)were designed and synthesized by H₆V₂Mo₁₈O₆₂(abbreviated as H₆V₂Mo₁₈)dopped in polyaniline（abbreviated as PANI）and metal oxide V₂O₅.The effects of physical and chemical properties with acidity,basicity,redox ability and concentration effect of the catalysts in PP-ol oxidation were investigated systematically.The interaction between PANI/V₂O₅ and POM can promote the electron transfer,thus increase the redox potential of the catalysts.H₆V₂Mo₁₈/PANI/V₂O₅（40）showed the best activity.When the reaction conditions were 130℃,8 h,0.8 MPa O₂ and 0.03 g catalyst,the conversion of PP-ol was 93.2%,and the selectivities of phenol and acetophenone were 90.1%and 81.4%,respectively.The optimal reaction conditions were investigated in converting pine or poplar stage-by-stage,which depolymerized lignin firstly,and subsequently hydrolyzed hemicellulose and cellulose.Moreover,the catalyst kept good stability and activity after being reused for 10 times in PP-ol oxidation.