離子液體溶解生物質的分子模擬研究
發(fā)布時間：2018-08-19 20:30
【摘要】：木質纖維素生物質是地球上儲量最豐富的可再生資源,其開發(fā)利用是未來能源的重要發(fā)展方向。它主要由纖維素、木質素和半纖維素組成,其中,纖維素是含量最高的部分,由于其結構的穩(wěn)固性,需要經(jīng)過溶解等預處理過程,才能進行開發(fā)利用。離子液體是近年來纖維素預處理中的"明星溶劑",具有優(yōu)良的溶解性和穩(wěn)定性。然而,離子液體為何能夠溶解纖維素是當今的難點問題,目前仍然缺乏系統(tǒng)性解釋。本文構建了纖維素微絲體系,通過大規(guī)模分子動力學模擬獲得完整的溶解過程,揭示了不同種類離子液體對纖維素的作用機制。并且研究了陽離子飽和性對離子液體溶解纖維素的影響機制,探索了軟木木質素與離子液體的相互作用。本文的研究將為認識微觀溶解過程和開發(fā)新型離子液體溶劑提供理論依據(jù)。論文的主要內容及結論如下:(1)離子液體溶解纖維素束的模擬研究。針對7*8(7根纖維素單鏈,聚合度為8)纖維素束,在4種溶劑([Emim][Cl],[Emim][OAc],[Bmim][Cl]和H2O)中進行長時間的分子動力學模擬。研究發(fā)現(xiàn),離子液體溶解纖維素的速度快慢順序是[Emim][OAc][Emim][Cl][Bmim][Cl],與實驗相符。通過氫鍵分析,發(fā)現(xiàn)[OAc]-能夠在纖維素鏈之間形成特定的氫鍵構象,這種構象能有效地分開相鄰纖維素鏈,從而加速溶解過程。另外,提出了陰陽離子是以協(xié)同的方式溶解纖維素:陰離子首先插入纖維素表面的外層鏈之間,與周圍的羥基形成氫鍵,隨著陰離子與纖維素束的充分接觸,更多的陰離子與內部的羥基形成氫鍵,陽離子由于陰離子負電荷的吸引以及與糖環(huán)的范德華相互作用,也進入微絲之中,進而剝離出纖維素單鏈。(2)離子液體溶解纖維素微絲的模擬研究。針對36*40的纖維素微絲(36根纖維素單鏈,聚合度為40)體系,在[Emim][OAc]中進行長時間(3μs)的模擬。分析了纖維素微絲在溶解過程中的結構變化,發(fā)現(xiàn)了逆時針的扭曲,且扭曲發(fā)生在溶解過程之前�？疾炝宋⒔z的溶解方式,發(fā)現(xiàn)微絲是以單根鏈剝離的形式逐漸溶解于離子液體中,且微絲中親疏水面交界處的鏈最先剝離,另外剝離從還原性末端開始。最后對陰陽離子和纖維素的相對位置進行分析,提出離子液體與纖維素的作用模式,[OAc]-陰離子主要在與纖維素鏈平行的方向上,和纖維素的羥基形成大量的氫鍵,而[Emim]+陽離子更多地分布在疏水面上,與纖維素之間主要為范德華作用。(3)陽離子飽和性對離子液體溶解纖維素的控制機理。模擬了7*8纖維素束在四種離子液體([Bmim][OAc],[Bpyr][OAc],[Bpy][OAc]和[Bpip][OAc])中的變化過程,最終纖維素只能溶解在陽離子含不飽和雜環(huán)的離子液體[Bmim][OAc]和[Bpy][OAc]中,與實驗相吻合。通過動力學模擬研究了陽離子和纖維素的相互作用,通過量化分析研究了不飽和雜環(huán)的影響,另外考察了體系傳質性質對溶解的影響規(guī)律。研究發(fā)現(xiàn),不飽和雜環(huán)的作用機制主要包括結構和傳質兩方面:首先,不飽和雜環(huán)由于π電子離域,能夠增強陽離子與纖維素的作用,也能穩(wěn)定陰離子與纖維素形成的氫鍵,且含不飽和雜環(huán)的陽離子體積較小,在溶解過程中更容易進入纖維素內部;另外,不飽和的[Bmim][OAc]和[Bpy][OAc]相比于飽和的[Bpyr][OAc]和[Bpip][OAc],具有更快的傳質特性,陰陽離子能更充分地與纖維素發(fā)生相互作用,進而促進溶解。(4)離子液體和木質素的作用機理及其界面結構。針對軟木木質素建立了一條長鏈模型,研究其與[Emim][Cl],[Bmim][Cl],[Emim][OAc],[Choline][OAc],[Choline][Gly]五種離子液體的相互作用。研究發(fā)現(xiàn),離子液體能夠在木質素周圍形成相對穩(wěn)定的結構,陰離子分布在第一溶劑化層,與木質素有較強的靜電相互作用;陽離子分布在第二溶劑化層,與木質素有較強的范德華作用。陰陽離子作用于木質素不同的位置,共同溶解木質素。[OAc]-與木質素形成較多的氫鍵,[Gly]-與木質素形成更多高氫鍵構象,因此,[Emim][OAc],[Choline][OAc]和[Choline][Gly]對木質素的溶解效果要強于[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].
【學位授予單位】：中國科學院大學(中國科學院過程工程研究所)
【學位級別】：博士
【學位授予年份】：2017
【分類號】：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].
【學位授予單位】：中國科學院大學(中國科學院過程工程研究所)
【學位級別】：博士
【學位授予年份】：2017
【分類號】：TQ413.2
【參考文獻】
相關期刊論文 前3條
1 姚瑩瑩;李W，
本文編號：2192749
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