Catalytic Conversion Of Lignin Into High Value-added Chemicals

As one of main components of inedible biomass,lignin is recognized as the only renewable resource of aromatic chemicals.Catalytic conversion of lignin for production of fuels and value-added chemicals has become the hotspot of academic research.“Lignin first”biofinery strategy has drawn much attention because lignin can be depolymerized in situ and thus the utilization efficiency of three main components can be promoted without exception.In this study,fractionation of corn stover over tungstic acid,zeolites and Ni-based catalysts was investigated.The catalytic activity of HTc-derived re-Ni Al-LDO was also tested in the one-pot conversion of corn stover in CH₃OH/H₂O mixed solvents.Fractionation of corn stover was examined over a commercial tungstic acid catalyst.Lignin component was converted into aromatic monomers among which methyl p-coumarate and methyl ferulate were the main products with the yield of82.2%and delignification of 98.5%in the optimal reaction condition.Cellulose retention and hemicellulose removal rate were as high as 82.2%and 78.7%respectively.Methanol acted as reactant in the solvolysis of in the first step of fractionation and H-donor in the second step for the stabilization of reactive intermediates through side-saturation even without hydrogenation catalysts.Tungstic acid promoted the cleavage of C-O bonds and the esterification of p-coumaric acid and ferulic acid hence inhibited the decarboxylation reaction.Higher temperature was in favor of the cleavage of C-O bonds at the expense of lower cellulose retention.Effect of types of solid acids and solvents were also investigated in the fractionation of corn stover.Owing to the small pore size,zeolite can only promote delignification process indirectly through promoting the depolymerization of resulted lignin oligomers.Higher yield of 4-ethylguaiacol can be obtained over zeolites due to its lower activity of esterification.Methanol featured high lignin solubility,unique H-donation ability as well as lower steric hindrance,making it as the optimal solvent in this reaction system.Besides,higher alkylphenols were observed in the product distribution in ethanol and THF.The activity of Ni-based catalysts was also examined in the reductive fractionation of corn stover.A total of 27.7%lignin monomers were achieved with the delignification of 92.4%and the polysaccharide retention of 83.6%.Ni-based catalysts facilitated the cleavage of C-O bonds in lignin oligomers and the stabilization of reactive intermediates in high-pressure H₂atmosphere.For the tested Ni-based catalysts,re-Ni Al-LDO performed similar activity of hydrogenation and deoxygenation as noble metal catalysts.Cocatalyst Zn Cl₂can not only promote solvolysis process in the first step to release lignin component but also favor the cleavage of C-O bonds in lignin monomer in the second step.2D-HSQC-NMR and MALDI-TOF MS suggested almost complete disappearance of C-O bonds in lignin oil in the optimal reaction condition.One-pot conversion of corn stover was also tested through the addition of H₂O into the last reductive fractionation system.Complete liquefaction of corn stover can be achieved in the optimal reaction condition.The yield of diols and its derivatives was as high as 38.8%.Hydrogen bonds in cellulose were broken by H₂O resulting in the high conversion of cellulose,while methanol was responsible for the solubilization of lignin component.Conversion of corn stover was mainly affected by temperature and ratio of CH₃OH/H₂O.High-pressure H₂atmosphere was in favor of the production of ethylene glycol,1,2-propylene glycol and 1,2-butylene glycol.Cocatalyst Zn Cl₂facilitated the retro-aldol reaction.After used for 1^strun,deactivation occurred to re-Ni Al-LDO because of carbon deposition according to Raman spectrum.