Theoretical Study On The CO₂ Adsorption Mechanism And Proton Transfer Of Bifunctional Ionic Liquids

The burning of fossil fuel is still an important way to produce energy even today and probably in the next several decades but it is associated with the emission of greenhouse gases?GHG?,such as carbon dioxide.The development of efficient methods for capturing CO₂,from power plant flues and for disposing of,or utilizing the product is highly demanded.The traditional technology for the capture of CO₂ in industry is chemical adsorption by an aqueous solution of amine,which is highly energy intensive owing to the thermodynamic properties of water and high enthalpy of absorption.Ionic liquids?ILs?,however,do not require water as a diluent or carrier and have remarkable properties,such as:?nearly?negligible volatility,high thermal stability,nonflammability,tunability,solvation properties,and most importantly high CO₂ solubility.These unique properties make ILs promising gas absorbents.In this work,we aim to study the mechanisms of the interactions that occur between ionic liquids and gases via the ab initio methods at the density functional theory?DFT?and second-order M?ller-Plesset perturbation theory?MP2?levels.In the first part,We describe a class of novel ionic liquids?ILs?formed between the 1-ethyl-3-methylimidazolium cation?[emim]+?and the aprotic heterocyclic anions?AHA?,and present a molecular level description of the interactions of ILs with CO₂.Minimum energy structures obtained using density functional theory based calculations suggest the dominance of Lewis acid–base interactions between the species,and the anion basicity has promotional effect on the solubility of CO₂.Not only that,the basicity of the anion also plays a key role in the ability to remove the acidic hydrogen to generate the nucleophilic carbon.The complexity of N-heterocyclic carbenes can be effectively tuned by appropriate choice of counteranions with varying bases.Additionally,the elongation and red shift of the C-H stretching frequency are explained by calculating the difference electron density coupled with the natural bond orbital?NBO?analyses.In the second part,the imidazolium ionic liquids with different substituent length have been investigated.The compute results indicate that the increase of alkyl chain has little effect on the binding energy of ionic liquid with CO₂.However,the [Cnmim][3-Triaz] series exhibited a binding energy decreasing with an increase of the alkyl chain,which means the proton transfer between anions and cations getting more unfavorable.Moreover,we find that the stronger the electron-withdrawing ability of the substituent on the anion,the lower the binding energy of CO₂ absorption as well as CO₂ absorption capacity.