| With the continuous growth of worldwide population and rapid development of economy,the consumption of fossil fuels is increasing rapidly.The potential shortage of fossil fuels and the environmental pollution problems associated with using them force humans to search for clean and renewable alternative energy sources.At present,lignocellulose biomass is considered as the most potential alternative energy source for fossil fuels due to its renewable,high reserves and the greatest potential and the lowest cost in the field of conversion into biofuels.Lignin,which is the second largest abundant renewable aromatic polymer,is becoming a significant feedstock for fuels and high-value chemicals.Unfortunately,lignin degradation and effectively transform of lignin are still difficult to study,because of lignin has complex structure and stable chemical properties.Pretreatment method is necessary for efficient conversion of lignin.Ionic liquids are widely used in the dissolution or degradation of lignocellulose as a new green solvent.Based on the unique characteristics of ionic liquids and their wide application in lignocellulose pretreatment,ionic liquid pretreatment technology is believed to be a nascent technology with great promise.Therefore,in this paper we select different acidic ionic liquids to degrade kraft lignin and lignin model compounds and discuss the possible pathway of ether bond cleaving.The main research contents are as follows:1.Ionic liquid pretreatmentKraft lignin was treated with ionic liquid[DMEA][HSO4],[DMEA][OAc],[DMEA][Cl]in the range of 80-160 ℃ for 30-150 min,respectively.After that,hydrothermal liquefaction of regenerated lignin was performed at 280 ℃ for 4 h.The experimental results indicate that ionic liquids can destroy the macromolecular structure of lignin and bring its preliminary degradation to success.Pretreatment conditions and acidity of ionic liquids have great influence on lignin degradation rate and deoxidation effect.When the pretreatment temperature is low and the acidity of ionic liquids is weak,the degradation rate,deoxidation efficiency and vanillin selectivity are not satisfying.With the increase of pretreatment temperature,the rate lignin degradation and deoxidation were up to 25.58%and 37%respectively.Furthermore,the analysis of lignin degradation products showed that during the ionic liquids pretreatment process,we could not only obtain vanillin with high yield but also modify the structure of regenerated lignin.Combined with the relevant literature,we speculated that the ionic liquid could selectively break the aromatic ether bonds in the lignin structure.Considering the yield of vanillin(a high value-added chemical in degradation products)and repolymerization reaction of lignin,the best pretreatment conditions were determined as 160 ℃,60 min.2.Pretreatment of lignin model compounds in ionic liquidsFirst of all,we examined the effects of pretreatment conditions on lignin model compounds.The experimental results show that when the ionic liquid is[DMEA][HSO4],lignin model compounds were showing the highest degradation rate at 120 ℃ for 90 min,which was different from the optimum pretreatment conditions of lignin.A large amount of acetophenone and benzaldehyde compounds were detected in the degradation products of lignin model compounds,which further proved that the ionic liquid could selectively break the aromatic ether bonds in lignin structure.The selective breaking rule of theβ-O-4 bond and the distribution of the degradation products during the degradation of lignin model compounds were studied by analyzing the liquid products.Therefore,we analyzed liquid products to explore the possible approach of lignin degradation and provide an effective way for revealing how ionic liquids breaking the ether bonds.In a word,in this work we discussed the preliminarily degradation process of lignin in bio-based ionic liquids and provided some theoretical guidance for the efficient degradation of lignin and the conversion of high value-added chemicals. |
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