Selective Cleavage Of The β-O-4 And C_α─C_β Bonds In Lignin Model Compounds

Lignin is mainly composed of p-hydroxyphenyl(H),guaiacyl(G),and syringyl(S)precursors linked via C─O and C─C linkages,and it is an abundant renewable feedstock for the production of aromatic monomers and advanced liquid fuels.Highly random and branched three-dimensional polyphenolic structure of lignin results in a relatively low yield and a complicated distribution of aromatic monomers using conventional depolymerization technologies.The selective cleavage mechanisms of the β-O-4 and C_α─C_β bonds were investigated using lignin model compounds in order to increase the yield and regulate the distribution of aromatic monomers from lignin depolymerization.It was difficult to cleave the β-O-4 bond of lignin model compound by ethanolysis below 230 °C in the absence of catalyst.The β-O-4 bond of lignin model compound was selectively cleaved by Pd/C cooperated with HCOO-via a heterolytic process,and the corresponding aromatic monomers were obtained.Both microwave heating and an increase of reaction temperature contributed to an acceleration of the β-O-4 bond cleavage.The condensation of the products with the Caryl─OH occurred into higher molecular weight products with high dosage of formic acid(> 0.8 mmol),long reaction time(> 20 min),or high reaction temperature(> 160 °C).In Pd/C hydrogenolysis system,the β-O-4 bonds in lignin model compounds without the C_α─OH were cleaved directly and quickly to form two aromatic monomers.The β-O-4 bond cleavage of lignin model compounds with the C_α─OH followed three reaction routes.Firstly,dehydroxylation of the C_α─OH occurred into C_α─H,followed by a fast cleavage of the β-O-4 bond.Secondly,dehydrogenation of the C_α─OH occurred into C_α=O,followed by a slow cleavage of the β-O-4 bond.And the dehydrogenation was a primary reaction during the hydrogenolysis.Thirdly,the β-O-4 bond was cleaved directly,and the reaction route tended to occur on the β-O-4 bond linked with S unit.The Caryl─OCH3 had no impact on product distribution,but it promoted the cleavage of the β-O-4 bond.The cleavage reactivity of the β-O-4 bond linked with S unit was the highest,and the β-O-4 bond cleavage reactivity of lignin precursor was in order of S > G > H.The Caryl─OH had no impact on product distribution,and it hindered the cleavage of the β-O-4 bonds in lignin model compounds with the C_α─OH.An easy,efficient,and selective catalytic methylation of the C_α─OH with methanol solvent into C_α─OCH3 was achieved,and microwave heating improved the methylation reactivity and selectivity.Both Fe2(SO4)3 and Al2(SO4)3 exhibited an excellent catalytic methylation reactivity of the C_α─OH,and the methylation only occurred at C_α─OH rather than Cγ─OH or Caryl─OH.Methylation of the C_α─OH increased the cleavage reactivity of the β-O-4 bond in lignin model compound by 55.9%,and allowed for an improvement of product selectivity that 15 products were decreased to four products.Methylation pretreatment of the C_α─OH in lignin increased the degree of lignin depolymerization by 25~29%,increased the yield of aromatic monomers by 43~55%,and reduced the oxygen content of alkyl side chain in aromatic monomers.The cleavage of the C_α─C_β bonds on phenyl C3 side-chain in lignin model compounds was oriented by microwave irradiation cooperated with Fe3+.The acidity promoted a methylation of the C_α─OH with methanol solvent into methylated product.A further elimination of the Caryl─OCH3 with C_β─H in methylated product occurred to form alkenyl product containing the C_α=C_β bond,and the formation of alkenyl product was a decomposition-determining step.Density functional theory(DFT)calculations indicated that the phenolic lignin model compound was more thermodynamically favorable to form alkenyl product than non-phenolic lignin model compound,so the Caryl─OH promoted the cleavage of the C_α─C_β bond.Microwave heating facilitated the cleavage of the C_α─C_β bond in lignin and contributed to 96% cleavage of the C_α─C_β bond in lignin.Microwave-assisted cleavage of the C_α─C_β bond narrowed the distribution of aromatic monomers which mainly contained vanillin,syringaldehyde,methyl vanillate,and methyl syringate.The oxidization of the C_α─OH to C_α=O increased decomposition rate constant and decreased reaction barrier of decomposition-determining step,resulting in an improvement of the C_α─C_β bond cleavage.The absence of the C_α=C_β bond in lignin model compound whose C_α─OH was reduced to C_α─H resulted in the C_α─C_β bond cleavage being hindered.The methylation of the C_α─OH to C_α─OCH3 had no effect on the C_α─C_β bond cleavage.Lignin model compound containing the C_α=O produced aromatic ester,while aromatic aldehyde was obtained from lignin model compounds containing the C_α─OH or C_α─OCH3.The microwave-assisted cleavage of the C_α─C_β bond followed a first-order kinetic and radical reaction mechanism.