离子液体溶解生物质的分子模拟研究
发布时间：2018-08-19 20:30
【摘要】：木质纤维素生物质是地球上储量最丰富的可再生资源,其开发利用是未来能源的重要发展方向。它主要由纤维素、木质素和半纤维素组成,其中,纤维素是含量最高的部分,由于其结构的稳固性,需要经过溶解等预处理过程,才能进行开发利用。离子液体是近年来纤维素预处理中的"明星溶剂",具有优良的溶解性和稳定性。然而,离子液体为何能够溶解纤维素是当今的难点问题,目前仍然缺乏系统性解释。本文构建了纤维素微丝体系,通过大规模分子动力学模拟获得完整的溶解过程,揭示了不同种类离子液体对纤维素的作用机制。并且研究了阳离子饱和性对离子液体溶解纤维素的影响机制,探索了软木木质素与离子液体的相互作用。本文的研究将为认识微观溶解过程和开发新型离子液体溶剂提供理论依据。论文的主要内容及结论如下:(1)离子液体溶解纤维素束的模拟研究。针对7*8(7根纤维素单链,聚合度为8)纤维素束,在4种溶剂([Emim][Cl],[Emim][OAc],[Bmim][Cl]和H2O)中进行长时间的分子动力学模拟。研究发现,离子液体溶解纤维素的速度快慢顺序是[Emim][OAc][Emim][Cl][Bmim][Cl],与实验相符。通过氢键分析,发现[OAc]-能够在纤维素链之间形成特定的氢键构象,这种构象能有效地分开相邻纤维素链,从而加速溶解过程。另外,提出了阴阳离子是以协同的方式溶解纤维素:阴离子首先插入纤维素表面的外层链之间,与周围的羟基形成氢键,随着阴离子与纤维素束的充分接触,更多的阴离子与内部的羟基形成氢键,阳离子由于阴离子负电荷的吸引以及与糖环的范德华相互作用,也进入微丝之中,进而剥离出纤维素单链。(2)离子液体溶解纤维素微丝的模拟研究。针对36*40的纤维素微丝(36根纤维素单链,聚合度为40)体系,在[Emim][OAc]中进行长时间(3μs)的模拟。分析了纤维素微丝在溶解过程中的结构变化,发现了逆时针的扭曲,且扭曲发生在溶解过程之前。考察了微丝的溶解方式,发现微丝是以单根链剥离的形式逐渐溶解于离子液体中,且微丝中亲疏水面交界处的链最先剥离,另外剥离从还原性末端开始。最后对阴阳离子和纤维素的相对位置进行分析,提出离子液体与纤维素的作用模式,[OAc]-阴离子主要在与纤维素链平行的方向上,和纤维素的羟基形成大量的氢键,而[Emim]+阳离子更多地分布在疏水面上,与纤维素之间主要为范德华作用。(3)阳离子饱和性对离子液体溶解纤维素的控制机理。模拟了7*8纤维素束在四种离子液体([Bmim][OAc],[Bpyr][OAc],[Bpy][OAc]和[Bpip][OAc])中的变化过程,最终纤维素只能溶解在阳离子含不饱和杂环的离子液体[Bmim][OAc]和[Bpy][OAc]中,与实验相吻合。通过动力学模拟研究了阳离子和纤维素的相互作用,通过量化分析研究了不饱和杂环的影响,另外考察了体系传质性质对溶解的影响规律。研究发现,不饱和杂环的作用机制主要包括结构和传质两方面:首先,不饱和杂环由于π电子离域,能够增强阳离子与纤维素的作用,也能稳定阴离子与纤维素形成的氢键,且含不饱和杂环的阳离子体积较小,在溶解过程中更容易进入纤维素内部;另外,不饱和的[Bmim][OAc]和[Bpy][OAc]相比于饱和的[Bpyr][OAc]和[Bpip][OAc],具有更快的传质特性,阴阳离子能更充分地与纤维素发生相互作用,进而促进溶解。(4)离子液体和木质素的作用机理及其界面结构。针对软木木质素建立了一条长链模型,研究其与[Emim][Cl],[Bmim][Cl],[Emim][OAc],[Choline][OAc],[Choline][Gly]五种离子液体的相互作用。研究发现,离子液体能够在木质素周围形成相对稳定的结构,阴离子分布在第一溶剂化层,与木质素有较强的静电相互作用;阳离子分布在第二溶剂化层,与木质素有较强的范德华作用。阴阳离子作用于木质素不同的位置,共同溶解木质素。[OAc]-与木质素形成较多的氢键,[Gly]-与木质素形成更多高氢键构象,因此,[Emim][OAc],[Choline][OAc]和[Choline][Gly]对木质素的溶解效果要强于[Emim][Cl]和[Bmim][Cl]。
[Abstract]:Lignocellulose biomass is the most abundant renewable resource on the earth, and its development and utilization is an important development direction of energy in the future. It mainly consists of cellulose, lignin and hemicellulose. Cellulose is the most abundant part of lignocellulose. Because of its structural stability, lignocellulose needs to be dissolved and other pretreatment processes before it can be developed. Ionic liquids (ILs) are the "star solvents" in cellulose pretreatment in recent years, which have excellent solubility and stability. However, it is still a difficult problem to explain why ILs can dissolve cellulose. A cellulose microfilament system has been constructed in this paper, which is integrated by large-scale molecular dynamics simulation. The dissolution process reveals the mechanism of different kinds of ionic liquids on cellulose. The influence mechanism of cationic saturation on the dissolution of cellulose by ionic liquids is studied. The interaction between cork lignin and ionic liquids is explored. This study will provide a basis for understanding the micro-dissolution process and developing new ionic liquids solvents. The main contents and conclusions of this paper are as follows: (1) Simulation of cellulose beam dissolution by ionic liquids. For 7*8 (7 cellulose single chains, degree of polymerization 8) cellulose beam, long-term molecular dynamics simulations were carried out in four solvents ([Emim] [Cl], [Emim] [OAc], [Bmim] [Cl] and H2O). It was found that the dissolution rate of cellulose by ionic liquids was high. The order of speed is [Emim] [OAc] [Emim] [Cl] [Bmim] [Cl] [Cl] [Cl] [Cl], which agrees with the experiment. Through hydrogen bond analysis, it is found that [OAc] - can form a specific hydrogen bond conformation between cellulose chains. This conformation can effectively separate adjacent cellulose chains, thus speeding up the dissolution process. Firstly, the outer chains of the cellulose surface were inserted to form hydrogen bonds with the surrounding hydroxyl groups. With the full contact between the anions and the cellulose bundles, more anions formed hydrogen bonds with the inner hydroxyl groups. (2) Simulation of dissolving cellulosic microfilaments in ionic liquids. A long-term simulation of cellulosic microfilaments (36 cellulose single chains, degree of polymerization 40) in [Emim] [OAc] was carried out. Structural changes of cellulosic microfilaments during dissolution were analyzed and counterclockwise distortion was found. Previously, the dissolution of microfilaments was investigated. It was found that the microfilaments were gradually dissolved in ionic liquids in the form of a single strand peeling. The chain at the junction of hydrophilic and hydrophobic surfaces was first peeled off, and the peeling started at the reductive end. In the model, [OAc] - anions mainly form hydrogen bonds with the hydroxyl groups of cellulose in the direction parallel to the cellulose chain, while [Emim]+ cations are more distributed on the hydrophobic surface and mainly van der Waals interaction with cellulose. The changes of four ionic liquids ([Bmim] [OAc], [Bpyr] [OAc], [Bpy] [OAc] and [Bpip] [OAc]) were studied. The cellulose could only be dissolved in cationic ionic liquids containing unsaturated heterocycles [Bmim] [OAc] and [Bpy] [OAc], which were in agreement with the experiment. The interaction between cations and cellulose was studied by kinetic simulation, and the quantitative analysis was carried out. The mechanism of unsaturated heterocycles mainly includes structure and mass transfer. Firstly, unsaturated heterocycles can enhance the interaction between cations and cellulose and stabilize the hydrogen formed by anions and cellulose because of the delocalization of PI electrons. In addition, unsaturated [Bmim] [OAc] and [Bpy] [OAc] have faster mass transfer characteristics than saturated [Bpyr] [OAc] and [Bpip] [OAc], and anions and cations can interact more fully with cellulose, thus promoting dissolution. A long chain model of cork lignin was established to study the interaction between cork lignin and five ionic liquids, namely [Emim] [Cl], [Bmim] [Cl], [Emim] [OAc], [Choline] [OAc], [Choline] [Gly]. The cations are distributed in the second solvation layer and have a strong van der Waals effect on lignin. The anions and cations act on different positions of lignin to dissolve lignin together. [OAc] - Form more hydrogen bonds with lignin, [Gly] - Form more hydrogen bonds with lignin. Thus, [Emim] [OAc], [Choline] [OAc] and [Choline] [Gly] are more effective in dissolving lignin than [Emim] [Cl] and [Bmim] [Cl].
【学位授予单位】：中国科学院大学(中国科学院过程工程研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ413.2
[Abstract]:Lignocellulose biomass is the most abundant renewable resource on the earth, and its development and utilization is an important development direction of energy in the future. It mainly consists of cellulose, lignin and hemicellulose. Cellulose is the most abundant part of lignocellulose. Because of its structural stability, lignocellulose needs to be dissolved and other pretreatment processes before it can be developed. Ionic liquids (ILs) are the "star solvents" in cellulose pretreatment in recent years, which have excellent solubility and stability. However, it is still a difficult problem to explain why ILs can dissolve cellulose. A cellulose microfilament system has been constructed in this paper, which is integrated by large-scale molecular dynamics simulation. The dissolution process reveals the mechanism of different kinds of ionic liquids on cellulose. The influence mechanism of cationic saturation on the dissolution of cellulose by ionic liquids is studied. The interaction between cork lignin and ionic liquids is explored. This study will provide a basis for understanding the micro-dissolution process and developing new ionic liquids solvents. The main contents and conclusions of this paper are as follows: (1) Simulation of cellulose beam dissolution by ionic liquids. For 7*8 (7 cellulose single chains, degree of polymerization 8) cellulose beam, long-term molecular dynamics simulations were carried out in four solvents ([Emim] [Cl], [Emim] [OAc], [Bmim] [Cl] and H2O). It was found that the dissolution rate of cellulose by ionic liquids was high. The order of speed is [Emim] [OAc] [Emim] [Cl] [Bmim] [Cl] [Cl] [Cl] [Cl], which agrees with the experiment. Through hydrogen bond analysis, it is found that [OAc] - can form a specific hydrogen bond conformation between cellulose chains. This conformation can effectively separate adjacent cellulose chains, thus speeding up the dissolution process. Firstly, the outer chains of the cellulose surface were inserted to form hydrogen bonds with the surrounding hydroxyl groups. With the full contact between the anions and the cellulose bundles, more anions formed hydrogen bonds with the inner hydroxyl groups. (2) Simulation of dissolving cellulosic microfilaments in ionic liquids. A long-term simulation of cellulosic microfilaments (36 cellulose single chains, degree of polymerization 40) in [Emim] [OAc] was carried out. Structural changes of cellulosic microfilaments during dissolution were analyzed and counterclockwise distortion was found. Previously, the dissolution of microfilaments was investigated. It was found that the microfilaments were gradually dissolved in ionic liquids in the form of a single strand peeling. The chain at the junction of hydrophilic and hydrophobic surfaces was first peeled off, and the peeling started at the reductive end. In the model, [OAc] - anions mainly form hydrogen bonds with the hydroxyl groups of cellulose in the direction parallel to the cellulose chain, while [Emim]+ cations are more distributed on the hydrophobic surface and mainly van der Waals interaction with cellulose. The changes of four ionic liquids ([Bmim] [OAc], [Bpyr] [OAc], [Bpy] [OAc] and [Bpip] [OAc]) were studied. The cellulose could only be dissolved in cationic ionic liquids containing unsaturated heterocycles [Bmim] [OAc] and [Bpy] [OAc], which were in agreement with the experiment. The interaction between cations and cellulose was studied by kinetic simulation, and the quantitative analysis was carried out. The mechanism of unsaturated heterocycles mainly includes structure and mass transfer. Firstly, unsaturated heterocycles can enhance the interaction between cations and cellulose and stabilize the hydrogen formed by anions and cellulose because of the delocalization of PI electrons. In addition, unsaturated [Bmim] [OAc] and [Bpy] [OAc] have faster mass transfer characteristics than saturated [Bpyr] [OAc] and [Bpip] [OAc], and anions and cations can interact more fully with cellulose, thus promoting dissolution. A long chain model of cork lignin was established to study the interaction between cork lignin and five ionic liquids, namely [Emim] [Cl], [Bmim] [Cl], [Emim] [OAc], [Choline] [OAc], [Choline] [Gly]. The cations are distributed in the second solvation layer and have a strong van der Waals effect on lignin. The anions and cations act on different positions of lignin to dissolve lignin together. [OAc] - Form more hydrogen bonds with lignin, [Gly] - Form more hydrogen bonds with lignin. Thus, [Emim] [OAc], [Choline] [OAc] and [Choline] [Gly] are more effective in dissolving lignin than [Emim] [Cl] and [Bmim] [Cl].
【学位授予单位】：中国科学院大学(中国科学院过程工程研究所)
【学位级别】：博士
【学位授予年份】：2017
【分类号】：TQ413.2
【参考文献】
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1 姚莹莹;李W，
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