Molecular Design Of Functional Ionic Liquids For Absorbing CO₂

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].