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离子液体预处理秸秆生物质及其机理研究
徐俊丽
学位类型博士
导师吕兴梅
2015-09
学位授予单位中国科学院研究生院
学位授予地点北京
学位专业化学工艺
关键词生物质 秸秆 离子液体 预处理 机理
其他摘要

作为一种蕴藏量大、分布广的可再生能源,生物质已被广泛应用于制备液体燃料和高附加值化学品。然而,生物质各组分间的结构复杂,使得生物质直接利用率较低。生物质预处理过程可以有效去除木质素、降低纤维素结晶度,大幅度提高纤维素酶解效率和生物质利用率。因此,生物质预处理已成为其高效利用的关键步骤。作为一种新型介质,离子液体具有良好的溶解性和稳定性,近年来逐渐被应用到生物质预处理中。清晰认识离子液体预处理生物质的机理,寻找高效的预处理方法是目前离子液体预处理生物质的两大难点。本论文以玉米和水稻秸秆为原料,采用离子液体复合体系预处理玉米秸秆,获得富纤维素材料(CRM),后经酶解或转化制备高附加值化学品;利用激光共聚焦显微镜、原子力显微镜、核磁共振等方法研究不同尺度上离子液体与水稻秸秆相互作用,揭示离子液体与秸秆的作用机理。主要研究内容及结果如下:(1) 开展了离子液体+添加剂二元复合体系预处理玉米秸秆的研究。研究发现,在1-烯丙基-3-甲基咪唑醋酸盐([Amim][OAc])中加入1, 5, 7-三氮杂二环[4.4.0]癸-5-烯(TBD)可有效预处理玉米秸秆,获得良好的CRM。考察了离子液体种类,添加剂类型,预处理时间及添加剂用量对玉米秸秆预处理的影响。当加入1.0 wt% TBD时,CRM中纤维素含量为39.12%,木质素含量为6.74%。改变TBD用量至0.1 wt%,CRM中木质素含量降低至2.06%。同时CRM中纤维素酶解率可达98%。密度泛函理论计算结果表明TBD的加入可使[Amim]+和[OAc]-相互作用能从99.1 kcal·mol-1降低至89.2 kcal·mol-1,利于[Amim]+和[OAc]-与秸秆组分相互作用。此外,TBD碱性以及N原子暴露的孤对电子,可促使TBD与木质素β-O-4键发生有效作用,从而打破木质素结构,降解木质素为小分子物质,最终达到去除木质素的效果。(2) 开展了两步离子液体法预处理玉米秸秆的研究。研究发现氢氧化胆碱(ChOH)与1-乙基-3-甲基咪唑磷酸二甲酯([Emim][DMP])相结合的两步法可以有效地预处理秸秆。考察了两步法中离子液体种类、温度和时间等因素对预处理玉米秸秆的影响,结果表明利用50 wt% ChOH水溶液常温浸泡秸秆3 h,不溶物可在[Emim][DMP]中130 oC下10 min内完全溶解,再生CRM去除木质素率达77.28%,纤维素量达52.14%。将CRM催化转化制备5-羟甲基糠醛,产率可达87.21%。密度泛函理论计算揭示了ChOH阴阳离子的协同作用打破了木质素β-O-4键,可以有效地去除木质素;同时[Emim][DMP]有效降低了再生纤维素的结晶度。该离子液体两步法中ChOH具有生物可降解性,[Emim][DMP]易于合成、价格低廉且可循环利用;同时该两步法操作简单,有效地缩短了高温溶解时间,降低了预处理能耗,为解决生物质预处理提供了新思路,具有重要的工业前景。(3) 开展了离子液体与秸秆相互作用的机理研究。微米尺度上,1-乙基-3-甲基-咪唑醋酸盐([Emim][OAc])预处理水稻秸秆切片细胞,厚壁细胞最先溶胀,维管束细胞溶胀次之;薄壁细胞溶胀同时发生扭曲。细胞壁溶胀是其溶解的关键步骤,细胞壁溶胀变化值接近原细胞壁厚度时,开始出现溶解现象。传统酸碱溶液可通过溶解水稻材料的木质素和半纤维素来破坏细胞结构,1-丁基-3-甲基咪唑六氟磷酸盐([Bmim][PF6])不能溶胀水稻秸秆切片细胞。纳米尺度上,水稻微纤维在[Emim][OAc]作用下先溶胀后溶解;微纤维溶胀直径变化值接近原微纤维直径时,开始出现溶解现象;同时,溶胀微纤维逐渐剥离出较细的微纤维和单根微纤丝。酸碱水溶液和[Bmim][PF6]对微纤维基本没有溶胀效果;纯冰醋酸可较小程度溶胀却不能溶解微纤维。微纤维溶胀是其溶解的关键步骤,不能有效溶胀微纤维的体系难以溶解微纤维。分子尺度上,核磁和模拟计算表明离子液体阴阳离子可与纤维素葡萄糖链形成较强离子液体氢键,不同于常规氢键;模拟计算表明离子液体阴离子起主导作用,核磁位移变化表明离子液体阳离子可与葡萄糖链发生作用。核磁位移变化与低聚糖链长有关;离子液体阴阳离子尺寸及形成的氢键位置等因素对溶解纤维素具有较大影响。 归纳以上三种尺度研究结果,提出以下可能作用机理:1)离子液体阴阳离子首先与葡萄糖链外裸露的羟基形成离子液体氢键,从而削弱葡萄糖链间的内部氢键网络;2)葡萄糖链间的内部氢键网络松动增大了原有葡萄糖链间空隙,离子液体阴阳离子可以插入到空隙之间,离子液体氢键“空间效应”增强了离子液体与葡萄糖链间的氢键作用,阴阳离子“尺寸效应”进一步扩大空隙,逐渐撑开葡萄糖链间网络结构,发生微纤维溶胀现象;3)离子液体氢键寿命长,产生离子液体氢键“时间效应”,离子液体氢键同时作用可打开葡萄糖链间氢键网络结构,微纤维开始逐渐溶解;4)微纤维溶胀和溶解引起细胞壁溶胀和溶解,打破纤维素结晶区,有效提高纤维素酶解率。

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As a sustainable and renewable energy resource with large reserves and wide distribution, biomass has been widely applied in the preparation of liquid fuel and high value-added chemicals. However, the complex network structures among the compositions of biomass make the biomass direct utilization rate very low. Biomass pretreatment could effectively remove lignin, lower cellulose crystallinity, and greatly increase the enzymolysis rate and improve biomass utilization significantly. Therefore, pretreatment procedure has become the key step in biomass efficient utilization. As a new medium,Ionic liquids (ILs) possess good solubility and stability, and has been gradually applied in biomass pretreatment in recent years. Improving biomass pretreatment efficiency and understanding the pretreatment mechanism clearly are two challenges in the biomass pretreatment with ILs. In this work, cornstalk and rice straw were used as raw material; cornstalk was pretreated by different composite ILs systems to obtain cellulose rich materials (CRM), which would be used to prepare high value-added chemicals through enzymolysis or catalytic conversion; meanwhile, laser scanning confocal microscope (CLSM), atomic force microscope (AFM) and nuclear magnetic resonance (NMR) were used to observe or characterize the interaction bewteen rice straw and ILs in different scales, and the mechanism of rice straw dissolution in ILs was obtained. The main contents and results are as follows:(1) Study on cornstalk pretreatment in binary compound system of IL + additives. The delignification of cornstalk was efficiently accomplished by using 1, 5, 7-triazabicyclo [4.4.0]dec-5-ene (TBD) as an additive in 1-allyl-3-methylimidazolium acetate ([Amim][OAc]), and good CRM was obtained. The influences of ILs types, additives kinds, pretreatment time, and TBD loading on cornstalk pretreatment were also investigated. When 1.0 wt% TBD was added to [Amim][OAc], the cellulose and lignin contents of CRM were achieved to be 39.12% and 6.74%, respectively. With the addition of 0.1 wt% TBD to [Amim][OAc], the lignin content of CRM could be reduced to 2.06%. Simultaneously, the CRM regenerated from the system of [Amim][OAc] + TBD was effectively hydrolyzed by cellulase with 98% enzymatic hydrolysis yield. The density functional theory (DFT) calculations indicated that the interaction energy of [Amim]+ and [OAc]- decreased from 99.1 kcal·mol-1 to 89.2 kcal·mol-1 with the addition of TBD, which made the [Amim]+ and [OAc]- easier to interact with the cornstalk components. Besides, the alkalinity and exposed nitrogen atoms made TBD efficient in cleaving the β-O-4 ether bond of lignin, thus the structures of lignin were disrupted and lignin was degraded into small molecular substances, eventually the enhanced delignification was achieved. (2) Study on cornstalk pretreatment with two-step ILs method. A two-step ILs method combining choline hydroxide (ChOH) and 1-ethyl-3-methylimidazolium dimethyl phosphate ([Emim][DMP]) for cornstalk delignification was developed. The influences of ILs types, temperature and time on cornstalk pretreatment were investigated. In the two step ILs method, cornstalk sample was first soaked in ChOH aqueous solution for 3 h, and the residue was dissolved completely by [Emim][DMP] in 10 min at 130 oC. The results showed that 77.28% lignin was removed and cellulose content was increased to 52.14% in the CRM. Simultaneously, the CRM could be catalyzed and converted into 5-Hydroxymethylfurfural (HMF) with a high yield of 87.21%. The DFT calculations indicated that the synergy of Ch+ and OH- is helpful for delignification, meanwhile, the cellulose crystalline structures were disrupted by [Emim][DMP]. In this two step method, the ChOH is biodegradable, and the [Emim][DMP] can be easily synthesized and recycled; simultaneously, the dissolution time at high temperature was reduced and the energy consumption was lowered. Therefore, this two-step method is promising for biomass pretreatment and provides a new way of thinking for biomass industrial utilization.(3) The dissolution mechanism of rice straw in ILs was studied from micron scale to nanoscale. In the micron scale, the three major cell types in the cell walls, i. e. the sclerenchyma cells, tracheids, and parenchyma cells are all first swelled and then dissolved in 1-ethyl-3-methyl-imidazolium acetate ([Emim][OAc]), but the rates of swelling and dissolving are variously, sclerenchyma cells were first swelled, vascular bundle cells took second place, and the parenchyma cells were swelled and distorted at the same time. Swelling is the key step in the cell walls dissolution, when the thickness changes of cell walls was close to the original cell wall thickness, the cell walls started to be dissolved. Lignin and hemicellulose can be quickly dissolved in traditional acid and alkali solution, thus the cellular structures were destroyed; 1-butyl-3-methylimidazolium hexafluorophosphate ([Bmim][PF6]) could not swell the cell walls of rice straw. In the nanoscale, the microfibres were swelled and dissolved in [Emim][OAc]. When the change of microfibre diameter was close to the original microfibre diameter, the microfibers began to be dissolved, meanwhile, thinner microfiber and single microfibril were peelled from microfibers gradually. Acid/alkali aqueous solution and [Bmim][PF6] could not swell the microfibres, microfibers could be swelled to a little degree but can't be dissolved in pure glacial acetic acid. Therefore, swelling is the key process for microfibers dissolution, microfibers would not be dissolved in the system that could not effectively swelling microfibers. In the molecular scale, the NMR characterizations and simulation results indicated that ILs hydrogen bonds (ILs-HBs), which were different from conventional hydrogen bonds, were formed among the anions, cations of ILs and glucose chains of cellulose. Simulation results shows that ILs anions play a leading role, NMR chemical shift changes suggest that ILs cations could interact with glucose chains. The changes of NMR chemical shifts of ILs have close relationship with the chain length of the oligosaccharides. Cellulose dissolution in ILs is great influenced by the size of cations and anions of ILs and the position of hydrogen bonds that formed with glucose chains. According to the results above mentioned, the possible mechanism can be summarized as followed: 1) the anions and cations of ILs firstly formed strong ILs-HBs with the exposed hydroxyl of glucose chains, the hydrogen bonds networks among the glucose chains were weakened; 2) the loose hydrogen bonds networks increased the gaps among the original glucose chains, which would induce more anions and cations of ILs to be inserted into the spaces to form more ILs-HBs with the hydroxyl of glucose chains, the “space effect” of ILs-HBs enhanced the interaction between ILs and glucose chains; the “size effect” of anions and cations of ILs further expanded the gaps among the original glucose chains, and gradually disrupted the hydrogen bonds networks among the glucose chains, then the microfibrils were swelled; 3) the “time effect”and the consistency of the ILs-HBs, which was attributed to the long lifetime of the ILs-HBs, would disrupt the hydrogen bonds among glucose chains completely, and the microfibers were dissolved in ILs gradually; 4) the microfibers swelling and dissolution induced the cell walls swelling and dissolution; meanwhile, the cellulose crystalline was destructed and ultimately increased the biomass enzymolysis. 

语种中文
文献类型学位论文
条目标识符http://ir.ipe.ac.cn/handle/122111/21359
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徐俊丽. 离子液体预处理秸秆生物质及其机理研究[D]. 北京. 中国科学院研究生院,2015.
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