| The three-dimensional dense and complex structure of lignocellulosic materials hinder the transformation and utilization for each component.Therefore,it is necessary to pretreat it to deconstruct and separate the components for utilization,thereby elaborating the comprehensive utilization rate of raw materials and producing substantial value-added chemicals.Recently,ionic liquids have become a potential green solvent for biomass pretreatment owing to their good performances.As a similar solvent of ionic liquids,deep eutectic solvent(DES)is preferred in virtue of wide source,facile recovery,environmentally friendly and low toxicity properties.However,some research reveal its low efficiency in pretreatment process.Among the organic solvent,1,4-dioxane is regarded as an excellent solvent for biomass pretreatment taking advantages of low boiling point,good lignin compatibility,easy recovery and mild reaction conditions.Based on the above considerations,this paper developed a pretreatment technology of DES collaborated with acidic dioxane for poplar conversion,which not only efficiently separated the components of poplar fiber,but also greatly improved the efficiency of enzymatic glycation of the pretreated residues(cellulose components).The research is mainly discussed in the following aspects:(1)System screening of the pretreatment solvent,including selecting optimal DES and synergetic effect with acid dioxane for poplar conversion.Firstly,four common DESs were prepared:choline chloride/formic acid(CC/FA),choline chloride/acetic acid(CC/AA),choline chloride/lactic acid(CC/LA)and choline chloride/glycerol(CC/GL).Then,the depolymerization of poplar in different DESs synergistic acidic dioxane solvent systems was tested.The comprehensively evaluation was based on the effect of different DESs pretreatment system on the poplar components and the enzymatic hydrolysis saccharification rate of pretreated residues,and the physiochemical characterization of each DES.Finally,the solvent systems of choline chloride/acetic acid(CC/AA)and choline chloride/glycerol(CC/GL)combined with acidic dioxane were selected as the experimental system in the follow-up research.(2)Choline chloride/acetic acid(CC/AA)and choline chloride/glycerol(CC/GL)combined with acidic dioxane were used as pretreatment solvent systems,respectively,and the pretreatment system conditions were optimized via orthogonal experimental design three facots:temperature,time and hydrochloric acid mass fraction,and the optimal pretreatment conditions were obtained based on the enzymatic hydrolysis saccharification rate of the poplar residues.In the synergistic pretreatment system of CC/AA DES,the optimal conditions were 1%hydrochloric acid mass fraction,110℃ pretreatment temperature and 70 min pretreatment time,and the corresponding enzymatic hydrolysis saccharification rate was 69.75%.In the CC/GL DES synergistic pretreatment system,the optimal experimental conditions were 1.2%hydrochloric acid mass fraction,140℃ pretreatment temperature and 70 min pretreatment time,and the corresponding enzymatic saccharification rate was 89.60%.(3)On the basis of the orthogonal optimization experiments of the two synergistic pretreatment systems,the poplar residues pretreated under optimal conditions were analyzed by XRD,FT-IR,TG,SEM and XPS characterization methods.The relationship between the enzyme hydrolysis characteristics of poplar and the changes of its components were studied structurally.The results indicated that the pretreatment mainly broke the dense structure of poplar fiber and removed most of the lignin and hemicellulose in the poplar,further enhanced the accessibility of cellulase to substrate and facilitated the enzymatic hydrolysis and saccharification rate.(4)On the basis of the orthogonal optimization experiments of the two synergistic pretreatment systems,the lignin components of poplar in the pretreatment process were separated and extracted under optimal conditions.The lignin components were characterized by UV,FT-IR,TG,2D-HSQC,and Py-GCMS,and the structural characteristics of lignin were systematically analyzed The analysis results showed that the lignin samples had high thermal stability and were rich in β-O-4 ether bonds inside their structures.And the samples could produce a rich variety of aromatic compounds during thermal cracking,which were conducive to the downstream conversion of lignin into small molecule aromatic chemicals.Therefore,the lignin components extracted from the pretreatment system of this study have the potential to be converted into aromatic platform chemicals with high value. |
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