Selective Cleavage Of The ?-O-4 And C_??C_? 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 ?-O-4 and C_??C_? 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 ?-O-4 bond of lignin model compound by ethanolysis below 230 °C in the absence of catalyst.The ?-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 ?-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 ?-O-4 bonds in lignin model compounds without the C_??OH were cleaved directly and quickly to form two aromatic monomers.The ?-O-4 bond cleavage of lignin model compounds with the C_??OH followed three reaction routes.Firstly,dehydroxylation of the C_??OH occurred into C_??H,followed by a fast cleavage of the ?-O-4 bond.Secondly,dehydrogenation of the C_??OH occurred into C_?=O,followed by a slow cleavage of the ?-O-4 bond.And the dehydrogenation was a primary reaction during the hydrogenolysis.Thirdly,the ?-O-4 bond was cleaved directly,and the reaction route tended to occur on the ?-O-4 bond linked with S unit.The Caryl?OCH3 had no impact on product distribution,but it promoted the cleavage of the ?-O-4 bond.The cleavage reactivity of the ?-O-4 bond linked with S unit was the highest,and the ?-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 ?-O-4 bonds in lignin model compounds with the C_??OH.An easy,efficient,and selective catalytic methylation of the C_??OH with methanol solvent into C_??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_??OH,and the methylation only occurred at C_??OH rather than C??OH or Caryl?OH.Methylation of the C_??OH increased the cleavage reactivity of the ?-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_??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_??C_? 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_??OH with methanol solvent into methylated product.A further elimination of the Caryl?OCH3 with C_??H in methylated product occurred to form alkenyl product containing the C_?=C_? 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_??C_? bond.Microwave heating facilitated the cleavage of the C_??C_? bond in lignin and contributed to 96% cleavage of the C_??C_? bond in lignin.Microwave-assisted cleavage of the C_??C_? bond narrowed the distribution of aromatic monomers which mainly contained vanillin,syringaldehyde,methyl vanillate,and methyl syringate.The oxidization of the C_??OH to C_?=O increased decomposition rate constant and decreased reaction barrier of decomposition-determining step,resulting in an improvement of the C_??C_? bond cleavage.The absence of the C_?=C_? bond in lignin model compound whose C_??OH was reduced to C_??H resulted in the C_??C_? bond cleavage being hindered.The methylation of the C_??OH to C_??OCH3 had no effect on the C_??C_? bond cleavage.Lignin model compound containing the C_?=O produced aromatic ester,while aromatic aldehyde was obtained from lignin model compounds containing the C_??OH or C_??OCH3.The microwave-assisted cleavage of the C_??C_? bond followed a first-order kinetic and radical reaction mechanism.