Quantum Chemical Calculation For Mechanism Of Volatile Organic Compounds Absorption By Phosphonium Ionic Liquids

The emissions of volatile organic compounds（VOCs）in the field of chemical engineering and energy industry account for a large proportion of the total VOCs emissions in China.As green solvents,ionic liquids show great potential in the field of VOCs absorption and treatment.Compared with other ionic liquids,phosphonium ionic liquids（PILs）have high stability,large free volume and low price.In this study,the methods of simulation and calculation will be used to explore the absorption mechanism of all kinds of VOCs by PILs.8 kind of PILs:trimethyl（butyl）phosphonium tetrafluoroborate([P₁₁₁₄][BF₄]),trimethyl（butyl）phosphoniumhexafluorophosphate([P₁₁₁₄][PF₆]),trimethyl（butyl）phosphonium bis（trifluoromethylsulfonyl）imide([P₁₁₁₄][Tf₂N]),trimethyl（butyl）phosphoniumtetrafluoroacetate([P₁₁₁₄][CF₃COO]),tributyl（propyl）phosphoniumtetrafluoroborate([P₄₄₄₃][BF₄]),tributyl（propyl）phosphoniumhexafluorophosphate([P₄₄₄₃][PF₆]),tributyl（propyl）phosphonium bis（trifluoromethylsulfonyl）imide([P₄₄₄₃][Tf₂N]),tributyl（propyl）phosphonium tetrafluoroacetate([P₄₄₄₃][CF₃COO])and their absorption for 34 kinds of VOCs including alcohols,aldehydes,ethers,ketones,aromatics,cycloalkanes,alkanes,alkenes,alkynes,chlorinated hydrocarbons and heterocycles were simulated and calculated at the level of DNP/GGA/PW91 by the quantum chemistry calculation module DMol3 in the Materials Studio software.The research content mainly includes the binding energy,hydrogen bond and charge of PILs ion pairs,as well as the active sites of PILs for VOCs absorption,absorption energy(E_abs),hydrogen bond,charge,frontier molecular orbitals（FMOs）and so on.The research focuses on the interaction between PILs and VOCs,the effect of different anions and cations of PILs in the absorption of VOCs,and the absorption mechanism of PILs with multiple active sites.The main conclusions are as follows:（1）Accompanied by charge transfer,hydrogen bonds are formed between anions and cations of PILs.The binding energy of ion pairs with[P₁₁₁₄]⁺is higher than that with[P₄₄₄₃]⁺in the same anion,indicating that the side chain growth of the cation will weaken its interaction with the anion.When the cations are the same,the binding energy order is the same as the order of the anion basic size,which is[CF₃COO]^->[BF₄]^->[PF₆]^->[Tf₂N]^-.（2）The leading role in absorbing different types of VOCs by the PILs is different.There always be ions that play a major role in the absorption structure.Meanwhile,the strength of the absorption,namely the absorption energy(E_abs),is significantly different.In the process of absorbing VOCs by PILs,cations play a major role in ketones,aldehydes,aromatic hydrocarbons,ethylene oxide,ethers,and olefins absorption,and can form C-H…πinteractions with aromatic hydrocarbons or form hydrogen bonds with ketones,aldehydes,epoxy,ethane and ethers.Anions play a major role in alcohols,chlorinated alkanes,acetylene,chlorinated alkenes,cyclohexane,and alkanes absorption by PILs,and can form hydrogen bonds with alcohols,chlorinated alkanes,acetylenes,and chlorinated alkenes.At the same time,alcohols can also form hydrogen bonds with cations.According to the type of VOCs,the E_abs order of VOCs absorption by PILs is as the following:alcohols>ketones>aldehydes and chlorinated alkanes>aromatics and ethylene oxide>ethers,acetylene and chlorinated olefins>olefins and cyclohexane>alkanes.（3）In the process of PILs absorbing VOCs and under different dominant effects,anion and cation have different effects on the absorption.When the VOCs and cations are the same,if the absorption is dominated by anions,the order of E_abs in absorbing VOCs by PILs with different anions is almost the same as that of anion basicity,and the E_abs of VOCs absorption by PILs with[CF₃COO]^-having much larger basicity than the other three anions is far ahead.If the absorption is dominated by cations,the difference in E_abs of VOCs absorption by PILs with different anions is smaller,and the order has no association with the basicity of the anions.In addition,under the same VOCs and anion,if the absorption process is dominated by cations,the E_abs of VOCs absorption by PILs with[P₄₄₄₃]⁺is slightly lower than that with[P₁₁₁₄]⁺.If the absorption process is dominated by anions,the side chain length of cations has no effect on E_abs.（4）Ion pair of PILs generally has 3 similar active sites for absorption,and the space for accommodating VOCs molecules is mainly provided by cations.The active site of the VOCs absorption by PILs based[P₁₁₁₄]⁺at single site is usually near the internal hydrogen bond of the ion pair,consisting of basic sites of anions andα-H atoms in cations.The E_abs of the VOCs absorption by PILs based[P₁₁₁₄]⁺at multiple sites has a little change comparad with that at single site,and the absorption sites are basically unchanged,which shows the ion pairs have a better multi-site absorption potential.The simulation calculation of 2 ion pairs shows that the absorption sites are easily occupied by the hydrogen bonds formed by the anions and cations in the PILs based[P₁₁₁₄]⁺having small anion and cation volume difference and large symmetry,hence the PILs based[P₁₁₁₄]⁺cannot provide space for effectively accommodating VOCs molecules.The E_abs difference between the VOCs absorption by PILs based[P₄₄₄₃]⁺at single site and multiple sites is small.The difference of active sites of VOCs absorption by PILs based[P₄₄₄₃]⁺is small compared with that based[P₁₁₁₄]⁺.The main difference is that someβ-H atoms of long chain cations become absorption sites.The study of 2 ion pairs shows that the symmetry of the PILs based[P₄₄₄₃]⁺is small because the[P₄₄₄₃]⁺cation volume is larger than the anion,hence the absorption sites are not easily covered by the hydrogen bonds.The PILs composed of weakly basic and larger anions are more likely to be exposed absorption sites.The PILs based[P₄₄₄₃]⁺have a larger free volume than that based[P₁₁₁₄]⁺,but their space to accommodate VOCs is still limited.It is predicted that PILs will provide the largest absorption space when the length of the cationic side chain is around like[P_10,10,10,5]⁺.There are 104 figures,37 tables,and 127 references in this paper.