A Theoretical Study On Choline Chloride-based Deep Eutectic Solvents And Their Mechanism Of SO₂ Capture

The severe air pollution is a big problem currently in China,in which sulfur dioxide(SO2)is one of the major sources and caused by heavily coal-depentant energy structure.It is practically important to absorbe SO2 in large amount and to recycle the sulfur resources.Ionic liquid(IL)is a kind of promising solvents.However,the cost of IL is typically too to be applied in large scale.Deep eutectic solvent(DES),as an alternative of IL,is formed by the direct mixing of hydrogen bond acceptor(HBA)and hydrogen bond donor(HBD)in a certain proportion.DESs are much cheaper than ILs because of the easy preparation by abundant sources.The solvents prepared by mixing choline chloride and different HBD ligands are the most common DESs.Their application in SO2 absorption is also reported experimentally,but the mechanism is not clear.In this work,we applied the state-of-art methods in quantum chemistry.In this work,quantum chemistry calculation is used to understand the nature of the interactions among SO2 and various species in DES,which is important to uncover the mechanisms of SO2 absorption in DESs and also helpful to the molecular design of new solvents.The studied in this work include:(1)We employed various calculation methods in quantum chemistry to study the interactions among different species in DES,because the accuracy is heavily dependant on both the methods and the systems.Calibrated by the CCSD(T)/CBS interaction energies,the M06-2X/6-311++G(d,p)is used in this work,which is a balance of accuracy and computational cost.(2)Six kinds of cholind chloride(ChCl)based DESs are studied,including ethylene(EG),glycol(GLY),thiourea(THIO),urea(UREA),malonic acid(MA)and lactic acid(LAC).The interactions between chloride and different ligands are found quite different,which is characterized by structure,electrostatic potential,charge transfer,binding energy and RDG and AIM analyses.The clusters of two ratios of ChCl and ligand(1:1 and 1:2)are optimized as the start point to absorbe SO2.It is found the latter one is more reasonable.The relationship between charge transfer and melting point is also discussed.(3)We studied the strength of interaction between SO2 and different species in DES to locate the driving force in absorption.It is found that the major interaction is come from that beteewn the chloride anion and SO2,which will compete to the hydrogen bonds between the anion and HBD ligands.On the other hand,the O in SO2,as HBA,can form weak HBs with various hydrogens in the ligands(COOH,OH,NH2,CH2),which contribute to the binding energy.As regards THIO,the moderately strong interaction is formed between S in THIO and S in SO2.The binding energy results indicate a very different mechanism,which is induced by ligand-SO2 interaction,rather than Cl-SO2 interation.(4)The clusters of DES and multiple SO2 are analyzed to understand the absorption mechanism at infinite concentration.It is found that the ligand can provide additional sites to absorbe SO2 in all the DESs studied.Moreover,we also studied the role of water on SO2 capture because of the hygroscopic nature of DES.