Physical Mechanism And Regulation Of CO₂ Adsorption By Ionic Liquids

A large quantity of greenhouse gases in atmosphere,such as CO2 emitted from industries,has caused serious global warming problems.A series of environmental problems have appeared such as the rising earth temperature and sea level due to the excess CO2 emissions.Therefore,in order to alleviate the worsening environmental problems,exploring the measures with efficient CO2 capture and low regeneration cost has become an important topic at the international level.Ionic liquids(ILs)have emerged as potential candidates for CO2 capture and separation owing to their unique molecular structures consisting cations and anions,and special functional groups,as well as of the correspondingly outstanding properties,such as low vapor pressure,low volatility,environmentally friendly,designability,higher CO2 solubility and selectivity,when compared to the conventional organic solventsHowever,there are countless types of ionic liquids,the cost of experimental synthesis is high,and the period of consumption is long.Albeit a large number of works that have been conducted experimentally,the effective CO2 capture and storage are still challenges:the impact of the ionic liquid microstructure on the adsorption performance still lacks systematic theoretical guidance.It is inefficient to screen out suitable ionic liquids for CO2 capture only through experimental method.Along with the society progress and development,molecular dynamics simulation has received more and more attention and application in scientific research.It is very meaningful to screen out suitable ionic liquids by molecular dynamics simulation method for characteristic study.In this work,the solvation process of CO2 by several ILs is studied by atomic molecular dynamics(MD)simulations.Four experimentally widely-studied ILs and the corresponding hydrated states were adopted as the representative solvents.Several key characters of ILs,including the specific types of cations and anions,the length of the alkyl chain,etc.,have been systemically assessed on CO2 solvation.To verify the validity of our present models of CO2 and ILs,the solvation free energy of CO2 in pure water is 1.80 kJ/mol at 300 K,this agrees well with the experimental value of 1.005 kJ/mol.In addition,we have calculated the solvation free energy of CO2 in pure[BMIM][BF4]at the 323 K and obtained a value of-1.00 kJ/mol.This is well consistent with experimental value of-1.005 kJ/mol.For cations,the alkyl chain dominants CO2 capture because longer alkyl chain could effectively enhance CO2 solvation.While for anions,the hydrophobic species are proved to favor CO2 solvation.From free energy decomposition analyses,this is attributed to the interacting character between CO2 and the ILs which is mainly driven by the van der Waals attractions.The existence of water seriously abates CO2 solvation energy for all the ILs which is attributed to the weakened anion-CO2 interactions.More importantly,it is found that ILs with hydrophobic anions are more resistant to the existence of water to capture CO2 than ILs with hydrophilic anions.The experimental evidences have demonstrated that addition of two dimensional(2D)nanomaterials into ILs effectively improved the CO2 capturing capability.However,the:in-depth mechanism of how 2D nanomaterials regulate ILs and CO2 interactions is still poorly documented.In this work,the absorptions of CO2 by a representative 1-ethyl-3-methyl-imidazole-tetrafluoroborate([EMIM][BF4]),which is coated on Nitrogenized graphene(C3N)and graphene(GRA,the prototype 2D nanomaterial)were investigated by molecular dynamics simulations.The influences of the thickness of IL coating have been systematically compared.Simulation results clearly indicate that the IL at the air interface undergoes a clear structure change which becomes more attractive to CO2 molecules,resulting in enhanced CO2 accumulation at the interface.At the IL/C3N and IL/GRA interfaces,only slight enhancement was observed for CO2 accumulation which is believed to be negligible for CO2 capture and storage.Structural analyses reveal that,the orientations of cations and CO2 were seriously influenced by the thickness of ILs.Quantitative calculations of the potential of mean force for CO2 inside the IL coating further support the dynamics simulation results.Our simulation work provides a deep microscopic understanding of CO2 capturing by ILs which could benefit the design and fabrication of high performance CO2 capturing and storage medium through the synthetic effects of ILs and 2D nanomaterial.