| Lignin is a three-dimensional polymer formed by complex linkages of three phenolic monomers,and high-value phenolic compounds can be obtained by transformation.Liquefaction of lignin has great research value due to that it can make use of lignin into high-quality phenolic compounds.Microwave heating method has the advantages of uniform heating and high efficiency,but the research on the mechanism of liquid degradation of lignin under microwave is still little.In this paper,lignin was selected as the raw material,and the product distribution of lignin(catalyzed)alcoholysis and its reaction mechanism by microwave heating were studied,which laid a theoretical foundation for the preparation of high value phenolic compounds by lanolinysis.Firstly,the degradation mechanism of lignin assisted by microwave heating in isopropanol solvent system was studied.The effects of reaction temperature(100-180?)and reaction time(5-60 min)on the product distribution(liquid product,char and residual lignin)were investigated.The study found that the yield of liquid product was the highest at 120?and 30 min,reached 45.35%,the yield of char was 38.65%and the residual lignin yield was 14.73%.The liquid products were characterized by Gas Chromatography-Mass Spectrometer(GC-MS)?Matrix-Assisted Laser Desorption/Ionization Time-Of-Flight Mass Spectrometry(MALDI-TOF MS).The results of GC-MS show that the reaction temperature and reaction time had a significant effect on the distribution of monomer compounds in the liquid product,and the yield of vanilla ketone and acetosyringone was the highest in the monomer product.The MALDI-TOF MS results show that the molecular weight of the oligomers in the liquid product is mainly between 200-500 m/z,and the aromatic ring oligomers with a molecular weight of 274,290,316,332,362,406,454 are the highest.And its distribution at different reaction temperature and reaction time were analyzed.(13C-NMR)was used to characterize the distribution of the bond in the liquid product.The results show that ?-O-4,?-?,5-5 and?-5 are the most.The possible molecular structure of 7 kinds of oligomers in the liquid product and the distribution of the bondages were deduced,and the matrix model was established to calculate the possibility of different molecular structures.Based on the previous deduction of the main monomer and oligomer structure in lignin liquid degradation products,the possible reaction mechanism of lignin was explored.In the first stage,the depolymerization of lignin occurs in the interfacial bonds(such as ?-O-4 and ?-5),resulting in oligomeric products(such as oligomers III,V and IV);The second stage oligomers can be converted into monomeric compounds by secondary depolymerization(e.g.,vanillin ketone and acetosyringone),or a condensation reaction takes place to form a solid residue.According to the product distribution of lignin under the microwave alcoholysis,the highest yield of high-quality phenolic compounds,vanillin ketone and acetosyringone were determined as the target product,and the effects of different metal catalysts(Co/Al2O3,Co/ZrO2,Co/CeO2 and Co/MgO)on the yield of target products were studied.The results show that the acid catalyst Co/Al2O3 is more susceptible to the cleavage of the bond between the lignin and the C-H bond and C-O bond of the oligomer product,which is favorable for the formation of the target monomer compounds.Due to the addition of the auxiliary agent Fe in the Co/Al2O3 catalyst and the optimum working conditions(Co loading of 5%,Co/Fe molar ratio of 1:0.3,catalyst dosage:0.8 g,reaction temperature:140 ?,reaction time:15 min),the yields of vanillin ketone and acetosyringone are significantly improved. |
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