| Lignin is a low-cost and abundant by-product from the pulp and paper industry., However, a major part of industrial lignin is incinerated as a fuel for the production of energy. Only a small part of lignin separated from spent pulp liquors is used for the preparation of high value-added chemical products. In this paper, different raw materials and types of lignin were degraded by using different ionic liquids as the oxidation system and molecular oxygen as the oxidant at a certain temperature and oxygen pressure. In addition, surface morphology and structural characterization of degradation lignin were investigated. This paper proposed a new way for the high-value industrial lignin utilization of preparing aromatic products.Based on ionic liquid/oxygen as oxidative degradation system, five major aromatic monomer products could be obtained after degradation of eucalyptus kraft lignin: vanillic acid, vanillin, syringic acid, syringaldehyde, acetosyringone and a small amount of acetyl acetovanillone. Yield of lignin oxidation product in ionic liquid reaction system was significantly higher than the yield in sodium hydroxide solution system. Adding sodium hydroxide into the ionic liquid system could significantly improve the yield of aromatic monomer products. With the reaction temperature increasing, yield of the aromatic products increased. The Oxygen pressure had a great influence on the formation of aromatic monomer products, and most products significantly increased with the oxygen pressure increasing. For the same temperature and oxygen pressure, yield of syringaldehyde and acetosyringone decreased with the reaction time increasing, but other products would reduce after the first increase. Compared with other metal oxides such as Co2O3, Fe2O3 and Mn O2, Cu O was the only effective catalyst.In order to improve the extraction yield of the aromatic product, seven common organic solvents were chosen as the extraction solvent. The results showed that ethyl acetate was the most efficient extraction solvent. It may be related to the polar of extraction solvent. In addition, prior to extraction, adding water to the reaction system could improve the extraction efficiency of the organic solvent obviously. It had a greater impact on the concentration of extracted products, which may be due to that water can significantly change the partition coefficient of the products in the two phases. The concentration of extracted products increased with the molar ratio of water and ionic liquid increasing. The activity coefficient of products in the ionic liquid and organic solvent phase would be changed by the addition of water. Extraction conditions also have impact on the concentration of the extracted products. Compared with the extraction time, temperature had a greater impact on the concentration of aromatic monomers. The performance of the recovered ionic liquids was similar to the unreacted ionic liquids, the structure of the ionic liquids didn’t changed even after 5 cycles.Different kraft lignins were prepared from different types of materials. Lignins from different materials were characterized and degraded. After the degradation, the bagasse, douglas fir and reed lignins had the highest yield of vanillin which was up to more than 19mg/g. Meanwhile, the eucalyptus, bagasse and reed lignins had the highest yield of Syringaldehyde more than 14mg/g. The lignin surface of different materials changed obviously before and after the degradation in ionic liquids. Surface of douglas fir and eucalyptus lignin residues appeared porous structure and no longer smooth. Compared with the kraft lignin and enzymatic hydrolysis lignin, dioxane lignin has significantly lower concentration of aromatic products.Lignin degradation was studied in different ionic liquids reaction systems. The results showed that lignin degradation reaction in all ionic liquids were similar and all aromatic monomer products generated fast as linearly at the initial stage of the reaction. The rate of syringaldehyde and syringic acid could achieve 5 or more after lignin reacted in the [BMIM]Cl O4 ionic liquid. The concentration of syringaldehyde could reach to 32mg/L which was much higher than that in [BMIM][Eto SO3] and [BMIM][(Buto)2PO2]. From the FTIR and 1H-NMR, the structure of ionic liquids did not change significantly and remain the same after recycle.The effect of reaction conditions on the oxidative conversions of three lignin model compounds was studied in this paper. The results showed that temperature had a great influence. The conversion of alcohols was slow so as to the concentration of aldehydes and acid was lower at 90oC. When the temperature increased, the concentration of alcohol linearly reduced, but the aldehydes and acids gradually increased. With the reaction, the conversion rate of alcohol increased and its molar concentration reduced in the reaction system. The rate of alcohol conversion didn’t change obviously and the concentration of alcohol gradually stabilized after the time was 60 min. Conversion of alcohols increased rapidly before 2MPa of oxygen pressure meanwhile the decrease of alcohol concentration was significantly than beyond 2MPa. Acid and aldehyde also increased slowly when oxygen pressure reached 2MPa. The catalyst with Co, Mn, Fe and Cu could promote the conversion of alcohol to some extent. It could catalyze the oxidation of alcohol in a short time and increase its conversion rate. In the ionic liquids, the simple dimer model lignin which containing β-O-4 structure could be oxidized to corresponding ketone but couldn’t be further oxidized to the corresponding aldehyde or acid. |
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