| Due to serious environmental problems caused by the utilization of fossil fuels and decreasing reserves,renewable energy attracts worldwide attention.Wheat straw as non-food crop with the advantages of low price,high annual output and widespread distribution,is becoming one kind of the optimal raw material for biomass energy production.But it is difficult to utilize wheat straw to produce biofuel owing to its complex structure,so many researchers have started to study wheat straw pretreatment to promote yield.At present,ionic liquid has been widely recognized for wheat straw pretreatment,but the synthesis procedure of ionic liquid is complicated and the cost of ionic liquid is high,which hinders the industrialization of this technology.Therefore,reducing the cost of ionic liquid is crucial for its industrial application.This study solves these problems in two ways:firstly,developing ionic liquids-cosolvent mixtures to reduce the dosage of ionic liquids,thus reducing the cost of ionic liquids;secondly,developing a series of deep eutectic solvents(DESs)with the similar properties of ionic liquids but the easy synthesis procedure and cheap synthetic raw material,thus reducing the cost of ionic liquids.Firstly,[Bmim][Cl]as solvent coupled with three kinds of acidic catalysts respectively(N-methyl-2-pyrrolidonium-based ionic liquids,acidic DESs,72%H2SO4)has been used in pretreating microcrystalline cellulose in this study.The effect of pretreatment temperature,pretreatment time,the different kinds of acidic catalysts on reducing sugar yield was investigated.Secondly,five acidic and neutral DESs commonly used in biomass pretreatment were investigated for selecting suitable solvents to pretreat wheat straw,then the effects of the pretreatment temperature,pretreatment time,pH and viscosity of solvents on the lignin removal and polysaccharide conversion were studied.Thirdly,three basic DESs were developed and the effects of pretreatment temperature,pretreatment time,the different kinds of DESs,pH and viscosity of solvents on the lignin removal and polysaccharide conversions were investigated.Finally,a new type of cholinium amino acids ionic liquids([Ch][AA])-glycerol mixture as a solvent was developed for wheat straw pretreatment.The effects of pretreatment temperature,pretreatment time,content of ionic liquid in glycerol,pH of solvent were also studied.The main results are as follows:The results of cellulose pretreatment show that:[Hnmp][FeCl4]has the strongest ability to catalyze cellulose pretreatment.When pretreatment temperature is 100? and hydrolysis time is 1 h,reducing sugar yield can reach to 97.9%by using[Hnmp][FeCl4]as catalyst;which is similar with the results obtained by using other kinds of acidic ionic liquids reported in references.However,the estimated cost of[Hnmp][FeCl4]is obviously lower than other kinds of acidic ionic liquids.The effect of choline chloride:lactic acid(C:L,molar ratio 1:2)is more moderate on cellulose pretreatment than that of[Hnmp][FeCl4],so C:L is suitable for real materials pretreatment.The results obtained in acidic and neutral DESs pretreatment wheat straw reveal that C:L shows the strongest pretreatment capacity.When pretreatment temperature is 90 ?,pretreatment time is 12 h and liquid-solid mass ratio is 20:1,the lignin removal and xylan loss approch 49.0%and 80.4%respectively.After enzymatically hydrolyzing pretreated residues for 72 h,the cellulose and xylan conversion was 93.9%and 69.0%,respectively.The XRD,FT-IR and SEM analysis results of pretreated residues verify the lignin and xylan is removing during pretreatment to improve the effect of enzymatic hydrolysis.The results obtained in basic ethanolamine DESs pretreatment show that choline chloride:monoethanolamine(molar ratio 1:6,C:M)was the most effective solvent.When pretreatment temperature is 70?,pretreatment time is 9 h and liquid-solid mass ratio is 20:1,the lignin removal and cellulose reservation are 71.4%and 93.7%respectively.After enzymatically hydrolyzing pretreated residues for 72 h,the cellulose and xylan conversions approach 89.8%and 62.0%,respectively.The pretreatment ability of C:M is similar with traditional solvents and ionic liquids while C:M has several advantages,e.g.,lower cost with cheap materials and simple preparation procedure,mild pretreatment conditions and lower polysaccharide loss.After pretreatment,the XRD,FT-IR and SEM analysis results of pretreated residues verify that xylan and lignin are removing during pretreatment to improve the effect of enzymatic hydrolysis.The results obtained in[Ch][A A]-glycerol mixture pretreatment show that 50%cholinium alanine-glycerol(50%[Ch][Ala]-glycerol)is capable of removing 67.6%lignin while reserving 95.2%of cellulose(90?,L/S mass ratio of 20:1,6 h)and the cellulose and xylan conversions are 89.7%and 70.9%after enzymatically hydrolyzing pretreated residues for 72 h.The pretreatment ability of 50%[Ch][Ala]-glycerol is similar with traditional solvents and pure ionic liquids while 50%[Ch][Ala]-glycerol has desirable advantages,e.g.,biocompatibility,lower cost by adding glycerol than pure ionic liquid,much lower pretreatment temperature by using 50%[Ch][Ala]-glycerol(typically<100 ?)than that by using pure glycerol(typically>200?).[Ch][Ala]not only can remove lignin in wheat straw but also can swell cellulose while not remarkably dissolve cellulose resulting in high cellulose reservation,which favors the enzymatic hydrolysis. |
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