Study On The Dissolution Behavior And Structure-activity Relationship Of Hydrogen Chloride In Ionic Liquids And Deep Eutectic Solvents

Hydrogen chloride(HCl)is a byproduct of chlorination reactions,with over 8 million tons annual output in China.The HCl-containing exhaust gas is usually treated by water absorption,resulting in huge amount of low-grade hydrochloric acid.In contrast,high-purity HCl gas is an important chemical material.Currently,dry HCl is produced by distillation of concentrated hydrochloric acid.Such a recovery process is problematic for its complicated processes,serious corrosion to the units,high energy consumption,high cost and high pollution.Therefore,a non-volatile solvent for HCl gas adsorption is worthy of investigation and development.In this regard,this article proposes a new HCl absorption flash desorption process based on ionic liquids(ILs)and deep eutectic solvents(DESs).At present,research on the solubility of HCl gas in ILs and DES is limited,and research on its dissolution mechanism and thermodynamics is even rarer.Therefore,studying the absorption behavior and structure-activity relationship of ILs and DES for HCl is crucial.This study will expand the new applications of ILs and DES in gas separation,promote the study of molecular thermodynamics in ILs systems and utilization of HCl,which has important theoretical and practical significance.Considering the countless species of ILs,it is important to screen suitable IL absorbents through thermodynamic model.At first,this paper predicted the solubility of HCl in 450 ILs by COSMO-RS model and verified the feasibility through experimental data.The solubility of HCl in ILs and its variation with temperatures,pressures,and structure of ILs were systematically studied.The structure-activity relationship of ILs and the its dissolution behavior for HCl was obtained.The results show that the free volume of the ILs can be adjusted effectively by the structure of the cations,but its influence on the solubility of HCl is limited.In contrast,the nature of the anions plays a leading role.The reversible solubility of HCl can be improved effectively by adjusting the alkalinity of anions and the number of their substituents.The ILs with strong hydrogen(H)bonding or acid-base interaction between anions and HCl are not suitable for HCl absorption.Based on the structure-activity relationship of ILs and their dissolution behavior for HCl,some representative ILs were selected for in-depth study.The dissolution mechanism was revealed by Fourier transform infrared spectroscopy(FT-IR),nuclear magnetic resonance spectroscopy(NMR)spectra,density functional theory(DFT)and visual analysis of weak interaction.And the existence form and chemical equilibrium of HCl in solution were determined.The results indicate that the solubility of HCl is mainly dominated by the van der Waals interaction for the no-coordinative ILs,and dominated by the H-bond effect for the coordinative ILs.The ILs with medium alkalinity and strong H-bonding ability especially Cl~-are most likely the potential solvent for the absorptive separation of HCl.Through theoretical screening combined with other characteristics of ILs(cost,viscosity,stability,etc.),1-butyl-3-methylimidazole chloride(Bmim Cl)was ultimately selected as the HCl absorbent,and the corresponding solubility data were measured.Aspen Plus simulation was used to compare and evaluate the technical and economic performance of the HCl separation processes based on Bmim Cl and water absorption/desorption.The results show that the Bmim Cl-based process has a 16.1%lower energy consumption and a 23.9%lower economic cost compared to the water absorption process,which has potential application value.Secondly,six choline chloride(Ch Cl)based deep eutectic solvents(DESs)were designed and synthesized.Their absorption behavior for HCl gas was studied at varying temperature and pressure,and analyzed in terms of their free volume,H-bond donors(HBDs)’structure,interaction energy,various instrumental characterization,and changes of viscosity and density in the absorption process.The dissolution and desorption behavior of HCl in Ch Cl,HBD,and their corresponding DESs,was analyzed,a new thermodynamic model(PCAM)for the absorption process was proposed,and the contribution of physical and chemical interactions to solubility was clarified.The dissolution mechanism of HCl in Ch Cl-based DESs was proposed by combining FT-IR,NMR,and DFT calculations.The results indicate that Ch Cl-EG(1:2)is a potential HCl absorbent for its lowest viscosity(～33.18 m Pa·s),and highest absorption capacity of 0.97 mol/mol(0.403 g/g)at 303.15 K and 101.3 k Pa.Finally,in order to further improve the solubility of HCl,Bmim Cl with stronger alkalinity was selected as the HBA,and six Bmim Cl based DESs with different alkalinity were designed and synthesized.Their dissolution behavior and structure-activity relationship were studied from the perspectives of structure,alkalinity,free volume,intermolecular interaction energy,and absorption enthalpy.The relationship between intermolecular interactions and dissolution phase transition during the dissolution process of HCl was discussed in detail.The dissolution mechanism of HCl and the dynamic evolution of the HB network were verified by FT-IR and NMR spectra and quantum chemical calculation.The results show that it is the high free volume,suitable absorption enthalpy and the competitive HB network in Bmim Cl-TAA system that promotes the dissolution of HCl,reduces the absorption enthalpy,and renders a highest reversible absorption of 1.76 mol/mol(0.51 g/g).Compared with Bmim Cl,the absorption enthalpy of HCl in Bmim Cl-TAA(1:1)is reduced by25%and the reversible solubility increased by 50%.This result further proves that DES is more suitable as a HCl absorbent compared to ILs,and the research results have guiding significance for the rational design of efficient DES absorbents.