Knowledge Management System Of Institute of process engineering,CAS
|关键词||离子液体 纤维素 木质素 分子动力学模拟 溶解机理|
木质纤维素生物质是地球上储量最丰富的可再生资源，也是未来能源的重要开发方向。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，纤维素在离子液体中的溶解速度快慢是[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]。
As one of the most abundant renewable resources on the earth, lignocellulosic biomass is an important direction of future energy development. It is mainly composed of cellulose, lignin and hemicellulose, in which cellulose has the highest part of the content. Due to the structural robustness, pretreatment process such as dissolution is needed before its conversion and utilization in the next step. Ionic liquid (IL) is regarded as the "star solvent" for cellulose these years, with plenty of appealing properties as solubility and stability. However, the reason why ILs can dissolve cellulose is a thorny issue, and there is still no systematic explanation. In this work, realistic cellulose models were constructed and complete dissolution process was obtained through large-scale molecular dynamics simulation, through which the mechanism of different kinds of ILs dissolving cellulose was revealed. The effects of cation saturation on cellulose dissolution and the interaction between IL and lignin were also explored. It is hoped that this study can provide a theoretical basis for understanding the microscopic dissolution process and developing new IL solvents. The main contents and results are as follows: (1) Simulation of ILs dissolving cellulose bunch. In this part, a cellulose bunch containing 7 single glucose chains (degree of polymerization = 8) was constructed, and molecular dynamics simulations were carried out in [Emim][Cl], [Emim][OAc], [Bmim][Cl] and H2O for 500ns. It was found that the dissolution rate of cellulose in IL was [Emim][OAc] > [Emim][Cl] > [Bmim][Cl], which was in agreement with the experiment. [OAc]- can form three different kinds of H-bonds within cellulose chains which can provide enough gaps for separation. [Cl]- cannot effectively divide the cellulose chains and this is why [OAc]- is more effective. In addition, a synergistic mechanism of cation and anion dissolving cellulose was proposed. The anions initially form H-bonds with hydroxyl groups with an insertion into the cellulose strands and cations stack to the side face of the glucose rings. As more and more anions bind to the cellulose chains, cations start to intercalate into cellulose bunch due to their strong electrostatic interaction with anions and Van-der-waals interaction with the cellulose bunch, then cellulose dissolution begins. (2) Simulation of IL dissolving cellulose microfibril. A cellulose microfibril containing 36 single chains (degree of polymerization = 40) was set up and simulated in [Emim][OAc] for 3μs. Through analysis of structure changes of cellulose microfibril during dissolution, an anticlockwise twist was observed, and the twist would occur in a very short period of time, earlier than dissolution. The dissolution route was also investigated, and cellulose microfibril was dissolved in IL through a gradual peeling-off process of single chains. The chain at the junction of water and hydrophobic surface was firstly peeled off, starting from the reducing end. The energy and position information of ILs and cellulose was analyzed, and then the interaction model of IL and cellulose was put forward. The [OAc]- anion interacts with cellulose hydroxyl groups in the horizontal direction, and the [Emim]+ cation preferred to in contact with the hydrophobic surface of cellulose microfibril. (3) The controlling mechanism of the unsaturated structure of cations on the dissolution of cellulose in ILs. The changing process of the cellulose bunch in [Bmim][OAc], [Bpyr][OAc] , [Bpy][OAc] and [Bpip][OAc] were simulated. It was found that cellulose can only be dissolved in ILs containing unsaturated cations, in accordance with experimental results. The interaction between different cations and cellulose was studied. The influence of heterocyclic ring was analyzed by quantum chemistry calculation and the effect on the mass transfer was also investigated. It is found that the mechanisms of unsaturated heterocyclic structure include two aspects. One is the structure factor: the π electron delocalization of unsaturated heterocyclic ring makes the cation more active to interact with cellulose and provides more space for acetate anions to form hydrogen bonds (H-bonds) with cellulose. The other is the dynamic effect: the larger volume of cations with saturated heterocyclic ring result in a slow transfer of both cations and anions, which is not beneficial to the dissolution of cellulose. (4) The interaction and interface structure of ILs and lignin. A softwood lignin model based on experimental data was established, and simulation of the lignin model with [Emim][Cl], [Bmim][Cl], [Emim][OAc], [Choline][OAc] and [Choline][Gly] was carried out. It is found that ILs can form a relatively stable distribution around lignin. The distribution of anion was in the first solvation shell, and the main interaction energy comes from electrostatic interaction; the cation distributed in the second solvation shell, interacting with lignin mainly through Van-der-waals interaction. The cation and anion interact with lignin at different positions and dissolve lignin together. IL containing [OAc]- can form more H-bonds with lignin and IL containing [Gly]- can form a wide range of conformation with large H-bond number, which is why [Emim][OAc], [Choline][OAc] and [Choline][Gly] dissolve lignin better than [Cl]- based ILs.
|李垚. 离子液体溶解生物质的分子模拟研究[D]. 北京. 中国科学院研究生院,2017.|
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