The Oxidative Depolymerization Of Lignin Over Polyoxometalate Catalysis

Lignin,as the most abundant aromatic compounds in nature,is of great significance for the preparation of fuels and fine chemicals,and the efficient depolymerization of lignin is the key.Finding efficient,stable and recyclable polyoxometalates(POMs)catalysts,studying the structure-activity relationship between lignin and catalysts in the degradation process and related mechanism,are important for achieving efficient depolymerization of lignin and expanding the application of POMs-based catalysts in the conversion of biomass.Based on the design of a series of POMs,several kinds of lignin and lignin model compounds were used to investigate the activity of POMs on the oxidative transformation of lignin under the different conditions,and the possible depolymerization mechanisms were also proposed,which were shown as follows:(1)Based on the Keggin-type heteropolytungstate,bioethanol lignin in biorefining residues were used to investigate the activities of different vanadium-substituted heteropolytungstate catalysts for the oxidation conversion of bioethanol lignin.It was shown that K6[SiV2W10O40]exhibited the best activity during the reaction.And the conversion efficiency of lignin reached 87%,while the yield of lignin oily products reached 53%at 150℃for 3 h.In addition,the products were mainly aromatic compounds such as vinyl guaiacol,p-hydroxybenzaldehyde,vanillin,and syringaldehyde.And the molecular weight of lignin was decreased from Mw=4200 g/mol to Mw=1300 g/mol during the reaction.(2)Aromatic compounds were prepared by degradation of alkali lignin with H3PMo12O40 as the catalyst.The yields of vanillin and methyl vanillate achieved the maximum of 3.9%and 3.6%at 120℃ for 1 h with the oxygen pressure of 1.5 MPa when H3PMo12O40 was used as the catalyst.The increase of the temperature and reaction time were beneficial for the improvement of the yield to some extent.However,higher temperature and longer reaction time could cause excessive oxidation of the lignin and the side reactions such as polycondensation between depolymerized products,which will lead to the decline of yields.And CH3· in the methanol/water system could combine with the intermediate fragments of lignin to prevent the repolymerization,which was conducive to the improvement of yields of lignin monomers.(3)Catalytic conversion of birch lignin and β-O-4 dimer lignin model compounds were investigated with CsnH5-nPMo10V2O40(n=1～5)as the catalysts.The results showed that Cs3H2PMo10V2O40 achieved the best catalytic activity for the effect of acidity and specific surface area,and the yield of oily products and lignin monomer reached 52.8%and 13.6%,respectively.The molecular weight of lignin decreased from Mw=12530 g/mol to Mw=1067 g/mol,while the structures β-O-4,β-β,and β-5 in the lignin damaged significantly during the reaction.Therefore,the degradation of model lignin showed that the phenolic dimer models were mainly degraded through the cleavage of the C-C bond,and only a small amount of C-O bonds were broken during the degradation.However,only C-C bond was broken during the oxidation of the non-phenolic dimer models.In addition,the introduction of Cs+ allowed the catalyst show excellent heterogeneity,and the yield of lignin oily product and lignin monomers achieved 48.9%and 10.8%after five recycles,showing high activity and stability.(4)The conversion of birch lignin and β-O-4 dimer model compound was also studied by using transition metal modified phosphomolybdenum POMs as the catalyst.It was shown that the introduction of transition metals could effectively adjust the physical and chemical properties of catalysts such as acidity,oxidation reducibility and specific surface area.Cs2.5CoH0.5PMo10V2O40(CsPMoV-Co)showed the best catalytic activity during the reaction and the yield of lignin oil and lignin monomer reached 55.5%and 9.6%,respectively.While the molecular weight of lignin decreased from Mw=12530 g/mol to Mw=1173 g/mol during the degradation.Moreover,the yield of vanillin,methyl vanillate and syringaldehyde achieved the highest yield of 3.8%,3.7%and 1.5%,respectively.For recycling,the yield of lignin oil product and lignin monomer only decreased 4.6%and 1.1%after five recycles.(5)The amphiphilic(CTA)nH5-nPMo10V2O40(n=1～5)were used as the catalysts to study the conversion of birch lignin and β-O-4 dimer model compounds.It was shown that the formation of micellar nanoclusters of(CTA)nH5-nPMo10V2O40 enabled the catalyst to achieve the excellent catalytic activity comparable to that of the homogeneous catalyst.The yield of lignin monomer and lignin monomer in the methanol/water system reached 48.8%and 13.6%,respectively,when(CTA),H4PMo10V2O40 was used as the catalyst and the O2 pressure was 1.5 MPa,at 150℃ for 3 h,and the molecular weight decreased from Mw=12530 g/mol to 920 g/mol.The amphiphilic micellar structure of(CTA)nH5-nPMo10V2O40 could increase the contact area between POMs and lignin and improve the reaction rate.At the same time,the long alkyl chains in the surfactant could also adsorb the small molecules of lignin generated during the reaction,reducing the excessive oxidation of lignin depolymerization products and preventing the repolymerization reaction between products,which was beneficial for the improving of the yield of lignin monomers.(6)Based on the high selectivity of 2,2,6,6-tetramethylpiperidine-l-oxyl(TEMPO)for Cαhydroxyl groups,the redox properties of POMs,and the stability of ionic liquids,multi-active sites Ionic Liquid-2,2,6,6-tetramethylpiperidine-l-oxyl-Polyoxometalates(IL-TEMPO-POMs)was synthesized for the first time.It was found that the yield of lignin oily products and lignin monomer yield could reach 50.0%and 13.4%when methanol/water was used as the solvent at 140℃ for 1 h.While the molecular weight of lignin decreased from Mw=12530 g/mol to 956 g/mol,and the β-O-4,α-O-4 and β-β linkages in the structure were significantly broken during the reaction.The conversion of the dimer lignin model verified the excellent selectivity of IL-TEMPO-POMs for β-O-4 bond cleavage and the catalyst showed superior heterogeneity during the reaction.During the reaction,the nitrosonium generated by the oxidation of nitroxyl radicals with POMs was of high oxidizing ability to the Ca hydroxyl group in the lignin side chain,which could reduce the bond energy of the lignin side chain,thereby achieving the efficiently broken β-O-4 bond on the side chain and completing the conversion of lignin from macromolecules to lignin monomers.