| Lignin is an abundant aromatic polymer in the world and an important component of biomass,the depolymerization of lignin into phenolic monomers is one of the main methods for its efficient utilization.However,due to its complex structure and low reactivity,achieving directional depolymerization and improving product yield and selectivity is considered to be one of the most important challenges in lignin applications.This paper overcomes the chanllege through the key scientific question of "separation and directed cleavage of chemical bonds during the depolymerization of aggregated lignin".In this paper,a comparative study of five solvents and two strategies for lignin separation was firstly conducted,and the efect of solvent separation on the structure of lignin was explored,a metal-supported molybdenum carbide catalyst was developed to depolymerize the separated lignin to achieve mild condition lignin depolymerization for the selective production of high value-added phenolic monomers.Finally,the depolymerization mechanism of lignin was studied by the depolymerization of the model compound and density functional calculation.The effects of different solvents(p-toluenesulfonic acid,formic acid,ethylene glycol,deep eutectic solvent and ionic liquid)and two separation strategies on lignin from corn straw were explored,and the comparative study on lignin structure was investigated by different characterization methods.The results showed that,due to its acidity and unique hydrotropic feature,99.42%of lignin in corn straw can be separated by using 60%p-toluenesulfonic acid water solution under the conditions of 70℃ and 20 min,which had a better separation effect than deep eutectic solvent,formic acid and ethylene glycol.While the strategy of using ionic liquid to separate hemicellulose and cellulose instead of lignin from coran straw can achieve efficient separation of hemicellulose and cellulose,but the purity of lignin in the residue was as low as 26.57%,which was not conducive to the subsequent utilization.The structural characterization of solvent lignin was further compared with lignin from different processes,and found that industrial lignin and hydrolyzed lignin have serious structural damage during the alkali or acid treatment,leading to abundant of β-O-4 bonds to be converted to C-C bonds,which made the structure more stubborn.However,the structure of PTS lignin did not change much compared with the native lignin in corn straw.Two-dimensional nuclear magnetic resonance analysis showed that the PTS lignin mainly contains guaiacyl units(69.07%)and syringyl units(30.93%).And the chemical bonds are dominated by β-O-4 and C=O at α-keto positions.Aiming at the problems such as high cost for using noble metal catalysts,low products selectivity and harsh reaction conditions in lignin depolymerization,a metal-supported molybdenum carbide catalyst was developed and used in the aqueous depolymerization of lignin.The characterization results of the catalysts showed that the Mo2C was successfully synthesized by impregnation coupled with carburizing reaction method.And the introduction of metal could not only promote the formation of β-Mo2C active catalytic center,but also improved the specific surface area and structure of the catalysts.Under the optimized conditions(260℃,4 h,20%methanol addition),the Ni-Fe supported molybdenum carbide catalyst can depolymerize lignin to obtain 89.56%liquid yield and 35.53%phenolic monomers,and among them the yield of 4-ethylphenol reached 14.77%.The results indicated that the efficient and selective depolymerization of lignin under mild conditions were realized.The depolymerization mechanism was studied by techniques such as NMR and HPLC-MS.The results showed that during the depolymerization of lignin,unstable intermediates were formed first.Afterwards methanol and active hydrogen would react with these intermediates to convert them into liquid products,thereby inhibiting the formation of coke.At the same time,Ni and Fe metals in the catalyst can inhibit the conversion of 4-ethylphenol to phenol by controlling the hydrogeno lysis reaction of the lignin side chain,thus achieving the purpose of selectively production of phenolic monomers.After the catalyst was recycled for five times,the yield of phenolic monomers did not decrease significantly,and the crystal structure,morphology and element distribution of the catalyst did not change,indicating that it has good recyclability and stability.The structure-activity relationship among the active components in the supported molybdenum carbide catalyst was studied by various characterization methods and depolymerization of lignin model compounds.Specifically,the Mo2C in the NixM10-xMo2C/AC catalysts mainly provided the acidity for the cleavage of β-O-4,while the alloy formed by bimetallic(Ni-Fe/Cu)can modify the hydrogenation performance of the catalyst.At the same time,there was a synergistic effect between Ni-Fe/Cu alloy and Mo2C,which can promote the depolymerization of model compounds.By optimizing the ratio of metals in the catalyst and the reaction conditions,the conversion rate of lignin β-O-4 model compound reached 100%.Moreover,the complete cleavage of β-O-4 and hydrodeoxygenation of C=O were achieved.The study of the reaction pathways revealed that Mo2C could control the cleavage of the β-O-4 prior to the hydrodeoxygenation of C=O,while methanol could provide alkyl side chains in the reaction to stabilize the intermediate.The optimized bimetallic catalyst was used for the depolymerization of PTS lignin,and the results showed that the highly acidic(1.3213 mmol/g)Ni7Fe3-Mo2C/AC catalyst could catalyze the depolymerization of lignin at 280℃ to obtain 85.11%of liquid products and 35.42%%of phenolic monomers,and the β-O4 and C=O in PTS lignin were completely cleaved and hydrodeoxygenated.The adsorption properties of the catalysts and the depolymerization mechanism of MME were investigated by density functional calculation(DFT).The results showed that the bimetallic NiFe(111)-Mo2C(001)catalyst had better adsorption effect for the model compound than Mo2C(001),and can reduce the bond energy by stretching the bond length of β-O-4,then promote the depolymerization reaction.At the same time,the catalysts had a lower adsorption energy for the partially hydrodeoxygenation products,which was beneficial for their rapid desorption from the catalysts surface,and then removed from the reaction.On the other hand,the catalysts had strong adsorption to the products containing C=O,which can facilitate the formation of target products by hydrodeoxygenation reaction.The study of the β-O-4 cleavage mechanism showed that the reaction energy barrier for H-adduct at the β position first followed by the β-O-4 cleavage was lower than that of the one-step hydrogenolysis of the β-O-4 bond,which was also beneficial for the hydrodeoxygenation of the products.For MME,the total reaction energy barrier of the depolymerization starting with the hydrogenation of C=O was higher than that of starting with the cleavage of β-O-4,indicating that the cleavage of β-O-4 prior to the hydrodeoxygenation of C=O is the most reasonable reaction route,which also verifies the experimental results from MME depolymerization. |
PDF Full Text Request