Study On The Thermodynamic Properties And Carbon Capture Performance Of DBU-type Ionic Liquids

Ionic liquids（ILs）are a class of solvent proposed in the context of green chemistry,which are composed of anions and cations.They have many unique physical and chemical properties,such as negligible vapor pressure,high solubility,good thermal stability and wide liquid range.Accordingly ILs have been widely used in the chemistry and chemical industry fields such as electrochemistry,catalysis,biomass conversion and organic synthesis.In recent years,due to the rapid development of industry,the environmental and ecological problems caused by the increasing carbon dioxide（CO₂）in the atmosphere have attracted global attention.The unique properties of ionic liquids（adjustable structure,higher solubility,better stability,etc.）provide a new approach for the CO₂ capture.To this end,a series of DBU-based ionic liquids were designed and synthesized in this paper.On the one hand,the micro-structure of binary solutions of ionic liquids were studied by measuring their thermodynamic properties（such as conductivity,viscosity and density）and 1H NMR chemical shifts.On the other hand,ionic liquids were applied to capture CO₂,and the effects of temperature,pressure and different anions on the absorption of CO₂ were investigated.In addition,the reversibility and stability of ionic liquids were explored through absorption-desorption recycle experiments.Finally,the absorption mechanism was proposed based on quantum chemical calculation and spectra analysis.In this paper,the target ionic liquids[HDBU][La],[HDBU][Im]and[HDBU][Triz]were sysnthesized by acid base neutralization reaction（one-step synthesis）.Different solvents were selected to prepare ionic liquids/solvent binary systems.The thermodynamic properties of the solution at different concentration were determined.Take[HDBU][Im]as an example,the density,viscosity and conductivity of[HDBU][Im]/ethanol,[HDBU][Im]/DMF,[HDBU][Im]/DMSO were measured at different composition.Conductivity of solutions have been fitted using Casteel-Amis equation.Excess properties V^E andΔG*^E can be calculated by using density and viscosity data,which can be fitted by Redlich-Kister equation.It can be shown that the order of influence of three solvents on the conductivity of[HDBU][Im]is ethanol>DMF>DMSO,which is consistent with results of V^E andΔG*^E analysis.Secondly,this paper investigates the micro-structure and properties of DBU ionic liquid by using local component model（LC）and ¹H NMR chemical shifts.At 298.15K,the ¹H NMR chemical shifts of[HDBU][Im]/ethanol,[HDBU][Im]/DMF and[HDBU][Im]/DMSO were measured at different concentrations,which can be correlated by LC model.The results show that the interaction of solvent with ionic liquids can destroy the original network structure of ionic liquids.The concentration of ionic liquids will affect the strength of the interaction between ILs and the solvent.Within the ILs-high concentration range,the self-association of ILs dominates,while when the ILs concentration is low,the interaction between the solvent and ILs is enhanced,which could induce the dissociation of ILs.The order of the three solvents’ability to destroy self-association of ILs structure is as follows:ethanol>DMF>DMSO.Ethanol is more likely to destroy ILs network structure,because ethanol has the strongest ability to form hydrogen bonds and the volume of ethanol is smaller.Finally,the application of ionic liquids in CO₂ capture was investigated.Taking[HDBU][Triz]as an example,influence of temperature,pressure,humidity and flow rate on CO₂ capture were studied.In addition the reversibility of[HDBU][Triz]was verified by five absorption-desorption recycle experiments.It can be found that[HDBU][Triz]can capture CO₂ efficiently.Temperature,pressure,humidity and other external conditions will affect the absorption rate and maximum absorption capacity.The absorption rate and absorption capacity of[HDBU][Triz]gradually decrease with the increase of temperature in 35℃～50℃.And CO₂ capture is proved to be an exothermic process,which is consistent with the effect of temperature on CO₂ capture.At 35℃,with the increase of partial pressure of CO₂,the absorption rate and absorption capacity of[HDBU][Triz]are gradually improved.The study also found that wet[HDBU][Triz]absorption efficiency was greatly reduced,which could be due to the interaction of hydrogen bonds between anions and water molecules.Futhermore the absorption mechanism of CO₂ capture by[HDBU][Triz]was revealed by DFT calculations:two active sites（N1 and N4）were co-chemically absorbed to form carbamate carbonyl carbon atoms in N₁-coo^-and N₄-coo^-,which can be demonstrated by ¹³C NMR and FT-IR spectra.Finally,absorption-desorption cycle showed that[HDBU][Triz]could be used repeatedly and have good reversibility.In conclusion,the micro-structure and properties of binary systems of DBU-based ILs were investigated by combining NMR and LC model as well as thermodynamic properties.Furthermore,the effciency of DBU-based ILs in the CO₂capture was studied,which can broaden the knowledge for application of ionic liquids in absorption of other acidic gases.