Quantitative Structure-activity Relationship And Mechanism Of SO₂ Captured By Ionic Liquids

With the rapid development of the economy,the emission of SO ₂ and acid rain has affected human health and the ecological environment severely.ionic liquids has many merits such as negligible vapor pressure,high thermal stability,high gas solubility and designable characteristics,which gives them the potential to be used as a new green and efficient SO₂ collector.Due to the wide variety of ionic liquids,the current development and expe rimental research of ionic liquid collectors mainly use trial and error methods.In addition,the mechanism of capture SO ₂ by some ionic liquids is not clear.Revealing its capture mechanism is of great significance to the development of new ionic liquid c ollectors and subsequent gas desorption.In response to these problems,this paper uses a combinat ion of density functional theor,molecular dynamics simulation and quantitative structure-activity relationship to design new ionic liquid collectors.To obtain the SO₂ capture performance and mechanism of midazolium,pyridinum,guanidinium and ether-functionalized ionic liquids and provide theoretical guidance for design ing ionic liquid collectors.The relation between the microstructure parameters of 45 kinds of ionic liquids and the SO₂ solubility was analyzed thtough a combination of density functional theor and quantitative structure-activity relationship,and a good correlation was obtained.The results show that with the methylation of the imidazole ring and the pyridine ring,the extension of the alkane side chain or the increase of the number of ether bonds,the absolute value of the molecular interaction energy,the energy gap difference and the electrophilic index decrease,the polarizability and dipo le moment increase.As a result,the bond between anion-cation was weakened,the ability of attracting electrons is reduced,the electron donating ability and polarity of molecules are getting stronger,the reactivity is getting better and better,and the solubility of SO₂ in the ionic liquids is increased.The increase of oxygen atoms in the anion increases the absolute value of molecular interaction energy,energy gap difference,polarizability and dipole moment,which leads to the increase of molecular a ctivity,dipole moment,and electron donating capacity,which increases the solubility of SO ₂in ionic liquids.7 kinds of new ionic liquid collectors were designed and the solubility predictions were probvided:1-nonae-thylene glycol monomethyl ether-2,3-dimethylimidazolium methanesulfonate([E₉DMIM]MeSO₃)is 7.017 mol SO₂?mol IL^-1,1-nonae-thylene glycol monomethyl ether-3-methylimidazole chloride([E₉MIM]Cl)is 5.698 mol SO₂?mol IL^-1,1-octaethylene glycol monomethyl ether-2,3-dimethyl-imidazolium methanesulfonate([E₈DMIM]MeSO₃)is 6.382 mol SO₂?mol IL^-1,1-hexyl-3-methylimidazolium thiocyanate([C₆MIM]SCN)is 3.080 mol SO₂?mol IL^-1,1-nonae-thylene glycol monomethyl ether-3-methyl-imidazolium methanesulfonate([E₉MIM]MeSO₃)is 6.512 mol SO₂?mol IL^-1,1-hexyl-2,3dimethylpyridinium thiocyanate([C₆^{3 3}Py]SCN)is 3.760 mol SO₂?mol IL^-1 and 1-hexyl-3-methyl-imidazolium methyl sulfate([C₆MIM]MeSO₄)is 2.932 mol SO₂?mol IL^-1.The interaction between four kinds of ionic liquids and capturing SO₂was studied by combination of density functional theor and molecular dynamics simulation to study:1-ethyl-3-methylimidazolium chloride([C ₂MIM]Cl),1-butyl-3-methylimidazo-lium chloride([C₄MIM]Cl),1-hexyl-3-methylimidazolium chloride([C₆MIM]Cl)and 1-octyl-3-methylmidazolium chloride([C₈MIM]Cl).The results show that SO₂ is closest to the anion and has the highest affinity.Cations are also important in the process of capturing SO ₂ by ionic liquids.The cations provide space for SO₂ and further capture SO₂ through hydrogen bond interactions.The extension of the alkane side chain enhances the charge transfer between the ion pair and SO ₂,and effectively improves the solubility of SO ₂ in the ionic liquids.The density value of the simulated ionic liquids is in good agreement with the experimental value.The calculated radial distribution functio ns,coordination number,spatial distribution functions and interaction energy analysis show that as the length of the alkane side chain increases,the interaction between anion and cation is weakened,and the cation-SO₂ and anion-SO₂ are enhanced the interaction between them improves the solubility of SO₂ in the ionic liquids.With the length of the alkane side chain increases,the diffusion speed of SO ₂ becomes slowe and the viscosity decreases,which is beneficial to the diffusion and mass transfer of SO ₂.The interaction,microstructure and capture mechanism of four kinds of guanidinium ionic liquids for capturing SO₂ were study by combination of density functional theor and molecular dynamics simulation 1,1,3,3-tetramethyl-guanidinium acetate([TMG]Ac),1,1,3,3-tetramethylguanidinium lactate([TMG]L),1,1,3,3-tetramethylguanidiniu m propionate([TMG]P)and 1,1,3,3-tetramethyl-guanidinium trifluoroacetate([TMG]Tf)with different substituents on the anions to capture SO₂.The results show that the O at om on the anionic carbonyl group is the main interaction site for capture SO ₂,while the cation mainly capture SO ₂ through-NH...O.The change of different groups on the Ac^-increases the charge of the O atom on the anion carbonyl,which enhances the inter action between the ionic liquid and SO₂.The density value of the simulated ionic liquids is in good agreement with the experimental value.The calculated radial distribution functions,coordination number,spatial distribution functions and interaction energy analysis show that the changes of different groups on the Ac^-weaken the interaction between the anion and the cation in the system,and strengthen the interaction between the anion and SO ₂.The interaction between the ionic liquids is conducive to capturing more SO₂.The results of the diffusion properties of the components in the system obtained by the study show that the diffusion rate of the anions is faster than the cations when the different groups on the anions change,and the interaction betwee n the anions and SO₂ is stronger than the cation.The decrease in the viscosity of the system during the gas capture process will facilitate gas diffusion and mass transfer.The order SO₂soubility in new ionic liquids was verified via density functional theor and molecular dynamics stimulaition and the capturing mechanism of SO₂ was revealed also.The SO₂ solubility of 7 kinds of new ionic liquids is predicted.The interaction energy sequence of the new ionic liquid collector and SO ₂ is consistent with the quantitative structure-activity relationship prediction sequence.The results show that there is a strong electrostatic interaction between anions and SO ₂,and a strong van der waals(or electrostatic)interaction between cations and SO ₂.The combined effect of anion and cation is mainly responbisble for the high solubility of SO₂ in these new ionic liquid collectors.