Study On Selective Liquefaction Of Lignin To Produce Aromatic Compounds

Lignin,as the only renewable aromatic polymer on the earth,can obtain abundant phenols and aromatic hydrocarbons by effective thermal-chemical depolymerization,which is of great significance to alleviate energy pressure and promote the sustainable development of the green chemical industry.However,the complexity and diversity of lignin depolymerization bio-oil pose a great challenge to the subsequent processing and utilization.Based on liquid phase depolymerization in supercritical ethanol system,phenolic oils were enriched as platform compounds,and then bio-oils rich in aromatic hydrocarbons were prepared by catalytic hydrogenation.Firstly,lignin with high purity was separated from rice husk in ethanol/water system,and the effects of technological parameters on lignin extraction rate and structural characteristics were explored.The results showed that when the ratio of material to liquid was 1:6 and the concentration of ethanol was 70%,the lignin extraction rate was 83.25%with a residence time of 2 h at 200oC.FTIR characterization showed that the structure of lignin extracted by ethanol method was less modified and could be used for subsequent liquid phase depolymerization research.Subsequently,lignin-based phenolic oils were prepared by directional depolymerization in CO₂-assisted ethanol system.The effects of reaction temperature on the two-step liquefaction behavior at different residence time were studied.After the first step of supercritical CO₂-subcritical ethanol pretreatment,the pore diameter and total pore volume of lignin were increased by 123%and 116%,respectively,which improved the mass transfer efficiency of hydrogen free radicals.Compared with the direct liquefaction of bio-oil at 320oC for 15 min and 30 min,the yield of bio-oil increased by 15.48 wt%and 21.41 wt%.Reaction temperature was the main factor affecting the bio-oil yield,while residence time as a secondary factor plays a synergistic role and was weakened with the increase of temperature.At the same time,raising reaction temperature enhanced alkylation reaction and hydrogen supply deoxidation reaction,enriched oxygen-containing substituted phenols to alkyl phenols,and obtained the highest target phenol content of 56.78%at 320oC for 30 min.Under this reaction condition,bio-oil had the highest H/C of 1.343 and the lowest O/C of 0.213,and the calorific value of 32.05 MJ/Kg.Finally,HZSM-5 was used as carrier for desilication by alkali treatment,and stable polymetallic catalysts were prepared by impregnation.Lignin-based phenolic oil was used as platform compound to prepare bio-oil rich in aromatic hydrocarbons.The results show that the yield and conversion of phenolic oil can reach 80.40%and 98.21%by catalytic modification with Cu·Ni·Ru/Na OH*and the physical and chemical properties of bio-oil,such as water content,viscosity and acid value,have been significantly improved.Among them,the deoxidation of phenolic compounds to aromatic hydrocarbons was mainly accomplished through the alkyl transfer of methoxy groups or the reaction of demethoxylation and dehydration.Bio-oil products contain 28.59%aromatic hydrocarbons and a small amount of naphthenic hydrocarbons,and the phenolic content decreases from 56.78%to 35.29%.At the same time,the content of carbon and hydrogen increased while the content of oxygen decreased,and the highest deoxidization rate reached 47.01%.The final calorific value of bio-oil increased from 29.40 MJ/Kg to 35.22 MJ/Kg,which increased by 19.80%.