【作者】 张晓春；

【导师】 汪文川； 刘志平；

【作者基本信息】 北京化工大学 ， 化学工程， 2009， 博士

【摘要】 由于大气中二氧化碳（CO₂）气体含量的增加而引起的“全球气候变暖”问题,已经引起了全世界的关注。众多的研究发现离子液体不但是捕捉CO₂的良好的吸收剂,而且还是固定及转化CO₂的优良的溶剂或催化剂。但是现已合成出大量的离子液体,如果通过实验去逐一的测定CO₂在每种离子液体中的溶解度的方法来寻找适合吸收CO₂的离子液体,将会需要大量的研究周期。本文针对以上问题,对使用功能化离子液体吸收CO₂进行了研究。主要内容如下:1.首先采用一种基于量子化学计算的筛选方法——COSMO-RS方法,预测在298.2 K时CO₂在408种离子液体中的亨利常数。筛选结果表明阴离子为三（五氟乙基）三氟磷酸（[FEP]）的离子液体能够吸收更多的CO₂。然后针对COSMO-RS方法筛选出的离子液体,利用IGA-003型重量分析仪测定了在283.2 K、298.2 K和323.2 K压力最高达1.8 MPa下的CO₂在1-己基-3-甲基咪唑（[hmim]）[FEP]、1-丁基-1-甲基吡咯（[bmpyrr]）[FEP]和S-乙基-N,N,N’,N’-四甲基异硫脲（[ETT]）[FEP]中的溶解度。实验结果表明[hmim][FEP]分别要比[hmim][Tf₂N]和[hmim][PF₆]多吸收15%和70%（摩尔分数）的CO₂。这种先通过预测方法筛选出高效吸收CO₂的离子液体,然后再通过实验测定CO₂溶解度的方法,相比于直接通过实验方法去从大量的离子液体中筛选出较高CO₂的溶解度的离子液体,具有成本低、周期短的优点。2.COSMO-RS是一种有效的预测CO₂在离子液体中溶解度的方法,但是它不能研究具体的溶解机理。为了探索[hmim][FEP]比[hmim][PF₆]能够溶解更多CO₂的机理,对[hmim][FEP]-CO₂和[hmim][PF₆]-CO₂的混合物进行了分子模拟研究。首先,开发了[FEP]阴离子的联合原子力场,由力场模拟得到的[hmim][FEP]的密度值与实验所测得的密度值吻合较好,证明了所开发的力场的准确性。用所构建的力场,通过连续分数组成蒙特卡罗（Continuous Fractional Component Monte Carlo-CFC MC）方法模拟了在298.2 K和323.2 K压力最高达到20 bar下CO₂在[hmim][FEP]中的溶解度。结果表明模拟得到的CO₂的溶解度的趋势与实验的吻合很好。因此,通过本文开发的[hmim][FEP]的力场和CFC MC方法,可以预测在较广温度和压力范围内的CO₂在[hmim][FEP]中的溶解度。3.运用分子动力学模拟（MD）方法研究了CO₂与[hmim][FEP]、[hmim][PF₆]的混合物。通过分析点对点径向分布函数发现,虽然CO₂在[PF₆]阴离子周围有较强的分布,但是有更多的CO₂存在于[FEP]阴离子的第一溶剂化层内,这主要是因为[FEP]阴离子尺寸较大且本身又是一个不对称的结构。采用MC和MD两种方法计算了[hmim][FEP]和[hmim][PF₆]吸收CO₂后的能量。结果表明,对于尺寸小且结构对称的[PF₆]阴离子来说,主要是通过静电力吸收CO₂;而相对于大尺寸且结构不对称的[FEP]阴离子,则主要是通过范德华力与CO₂作用。因此,本文的工作为今后设计高效吸收CO₂的离子液体提供了重要的信息。4.在离子液体应用的过程中,当有水等杂质存在时会对离子液体的性质有影响。本文通过MD方法研究了水（H₂O）、甲醇（CH₃OH）和甲醚（CH₃OCH₃）与[hmim][FEP]的作用机理。通过计算这三种混合物的超额摩尔体积、超额摩尔混合焓和扩散系数,分析点对点径向分布函数,以及相对应的配位数,结果表明H₂O、CH₃OH和CH₃OCH₃上的O原子与[hmim]阳离子上的H5之间存在氢键;这三种小分子主要分布在[FEP]阴离子周围;[hmim][FEP]与这三种小分子作用的强弱顺序是CH₃OCH₃＞CH₃OH＞H₂O。5.由于离子液体黏度较高,因此限制了其走向实际应用。本文初步进行了[hmim][FEP]固定化的研究。通过物理吸附固定化离子液体的方法将[hmim][FEP]和[bmim][BF₄]固定在硅胶上。对[hmim][FEP]的红外光谱特征峰进行了指认。通过测定硅胶固定化[hmim][FEP]和[bmim][BF₄]前后的BET比表面的变化,表明[hmim][FEP]要比[bmim][BF₄]更容易被固定在硅胶上。通过分析硅胶固定化[hmim][FEP]前后的红外光谱图,进一步说明[hmim][FEP]已成功地固定在硅胶上。本文的工作为今后研究应用固定化的[hmim][FEP]吸收CO₂和广泛应用[hmim][FEP]奠定了坚实的基础。更多还原

【Abstract】 The "global climate warming" problem caused by the increasing of carbon dioxide（CO₂） in the atmosphere has been paid much attention all over the world.A lot of studies have found that ionic liquids（ILs） is not only a good absorbent for CO₂ capture,but also a good solvent or catalyst for CO₂ reacting with other compounds.However,a large number of ionic liquids have been synthesized at present,and apparently,it will cost too much research period to select a suitable ionic liquid for absorbing CO₂ by using experimental methods to measure the solubility of CO₂ in every kind of ionic liquid.To address above questions,using functional ionic liquids for absorbing CO₂ was studied in this work.The main contents and findings are summarized as follows.1.First,a screening method-COSMO-RS method,based on quantum chemistry calculation,was implemented to predict the Henry’s law constants of CO₂ in 408 ILs at 298.2 K.It was found that the ILs with the anion tris（pentafluoroethyl）trifluorophosphate（[FEP]） can absorb more CO₂ by the screening method.Then,aiming at the ionic liquids selected by COSMO-RS method,the solubilities of CO₂ in 1-hexyl-3-methylimidazolium （[hmim]）[FEP],1-butyl-1-methylpyrrolidinium（[bmpyrr]）[FEP]and S-ethyl-N,N,N’,N’-tetramethylthiouronium（[ETT]）[FEP]at 283.2,298.2 and 323.2K,up to the pressure of 1.8 MPa were measured by the intelligent gravimetric analyzer-003（IGA-003,）.The experimental data show that the solubility of CO₂ in[hmim][FEP]is about 15%and 70%（mole fraction） higher than that in 1-hexyl-3-methylimidazolium bis（trifluoromethylsulfonyl）imide（[hmim][Tf₂N]） and 1-hexyl-3-methylimidazolium hexafluorophosphate（[hmim][PF₆]）, respectively.The screening method is implemented before doing experiment has the characteristics of lower cost and shorter period,when compared with the selected ionic liquid of high CO₂ absorption capacity from a large number of ionic liquids by experiment directly.2.COSMO-RS is an efficient method for predicting the solubility of CO₂ in ionic liquids,but it provides no detailed mechanistic explanation.In order to find mechanisms of[hmim][FEP]could dissolve more CO₂ than [hmim][PF₆],the mixtures of[hmim][FEP]-CO₂ and[hmim][PF₆]-CO₂ were studied by molecular simulation.First,a united atom force field for the[FEP] anion was developed.The predicted density from our proposed force field is in good agreement with the experimental data for[hmim][FEP],which proves the accuracy of the proposed[hmim][FEP]force field.By the developed force field of[hmim][FEP],the Continuous Fractional Comoonent Monte Carlo （CFC MC） method was used to predict CO₂ absorption isotherms in [hmim][FEP]at 298.2 and 323.2K and pressures up to 20.0 bar.The simulated isotherms capture the experimental trends quite well.Therefore,the solubilities of CO₂ in[hmim][FEP]could be predicted in a wider range of temperature and pressure by the proposed[hmim][FEP]force field and CFC MC method in this work3.Molecular dynamics（MD） simulations were performed to study the mixtures of CO₂ absorption in[hmim][FEP]and[hmim][PF₆].The site-site radial distribution functions（RDFs） show that although CO₂ is highly organized around the[PF₆]anion,more CO₂ can be found in the first coordination shell of[FEP]due to its bigger size and asymmetry structure. Both MC and MD methods were used to calculate the energy between [hmim][FEP]-CO₂ and[hmim][PF₆]-CO₂.The results show that for[PF₆] anion,which is small and symmetrical,mainly absorbs CO₂ by electrostatic force.In contrast,for[FEP]anion,which is larger and asymmetrical,mainly absorbs CO₂ by VDW interactions.Thus,this work provides important information for designing the ionic liquids which can highly absorb CO₂.4.During the application of ILs,other matters,such as water,are usually present in them,which can influence the properties of ILs.The interaction mechanism of water（H₂O）,methanol（CH₃OH） and dimethyl ether（CH₃OCH₃） in[hmim][FEP]were studied by molecular dynamic simulation in this work. By calculating the excess molar volume,excess molar enthalpy,diffusion coefficients,and analyzing the site-site RDFs and the corresponding coordination numbers of the three mixtures,it was found that there exist strong hydrogen bond between O atoms of H₂O,CH₃OH and CH₃OCH₃ and H5 of the cation;the three solutes mainly distribute around[FEP]anion;the sequence of[hmim][FEP]interact with the three solutes is CH₃OCH₃＞CH₃OH＞H₂O.5.Industrial application of ionic liquids has been limited due to its higher viscosity.Immobilization of[hmim][FEP]was studied in this work. [hmim][FEP]and[bmim][BF₄]were immobilized on the porous silica gel by physical absorption.The Fourier transform infrared（FT-IR） spectrum of [hmim][FEP]was assigned.By comparing the BET surface area of silica gel before and after immobilization of[hmim][FEP]and[bmim][BF₄],it was found that[hmim][FEP]is more easier to be immobilized on silica gel than [bmim][BF₄].By analyzing the FT-IR spectrums of silica gel before and after immobilization of[hmim][FEP],it was found that[hmim][FEP]has been successfully immobilized on the silica gel.This work has laid a solid foundation for using immobilized[hmim][FEP]to absorb CO₂ and widely application of[hmim][FEP].更多还原