Depolymerization Of Alkaline Lignin Over KF/?-Al₂O₃ And MgO/?-Al₂O₃ Supported Solid Bases

As a kind of biomass resources,lignin is the only natural and rich renewable resource that can provide aromatic compounds.To realize lignin utilization commercially viable,it is necessary to depolymerize lignin efficiently.Herein,two solid base catalysts,KF/?-Al₂O₃ and MgO/?-Al₂O₃,were prepared by impregnation method and one-pot method respectively,and used to catalyze the depolymerization of alkali lignin in methanol system.The catalyst was characterized by X-ray diffraction,scanning electron microscopy,nitrogen adsorption-desorption and temperature-programmed desorption carbon dioxide.The results show that both catalysts have mesoporous structure and strong basicity,and KF/?-Al₂O₃ has stronger alkalinity than MgO/?-Al₂O₃.The effects of temperature,time and solid-liquid ratio on the depolymerization of lignin were studied.When no catalyst is added,the depolymerization rate of lignin will not change significantly with the change of temperature;after adding catalysts,the depolymerization rate of lignin increased with the rise of temperature.Under the same conditions,the depolymerization rate of lignin over KF/?-Al₂O₃ is higher than over MgO/?-Al₂O₃.Above the critical temperature of methanol,the depolymerization rate of lignin increases dramatically.The suitable time for the reaction is 2-4 h.The lignin residue was detected by infrared spectrometer?FTIR?and matrix assisted laser desorption ionization time-of-flight mass spectrometry.The results show that the residue is mainly unreacted lignin or lignin fragments,and their molecular weights are significantly lower than the raw material.The liquid products were analyzed by FTIR,GC/MS and quadrupole-electrostatic orbitrap high resolution mass spectrometry and found to be predominantly phenolic and non-phenolic aromatics.Above the critical temperature of methanol,the types of phenolic and non-phenolic aromatic compounds increase sharply,and there are also large differences with no catalyst and adding the catalysts.In addition,there are still oligomers with 300-500Da in the liquid products,the unsaturation of the oligomers is mainly 4 and 8,and the degree of unsaturation between the different oligomers is different by 4,indicating that the most of oligomers containing one or two benzene rings;and the carbon number is mainly concentrated between 15 to 30,revealing that the oligomer contains rich side chain groups.