Synthesis Of Imide-based Anion-functionalized Ionic Liquids For The Absorption Of Carbon Dioxide

CO₂ emissions from the burning of fossil fuels results in an increasing CO₂ concentration in the atmosphere,which leads to a series environmental problems,such as green house effect,ocean acidification,and climate change.Therefore,reducing CO₂ emissions and increasing CO₂ capacity are very important for the sustaiable development of our society.Recently,the technology used for industrial CO₂ capture is using aqueous alkanolamine solutions as CO₂ absorbents.However,the disadvantages of this technology,including high energy consumption and unrecyclable performance should not be ignored.Therefore,the development of efficient and reversible absorbent is a major challenge in the field of CO₂ capture.Ionic liquids,especially functionalized ionic liquids,have unique physicochemical properties,such as high thermal stability,low vapor pressure,wide liquid range and strong structural tunability.Therefore,ionic liquids have been used as acidic gas absorbents and showed good capture performance.However,the conventional ionic liquids used for CO₂ capture usually have some problems such as low absorption capacity,dramatic increase in viscosity during capture,and difficulty in regeneration of ionic liquids,which become the biggest drawback for the application of ionic liquids in industry at present.Thus,in this work,a series of imide-based anion-functionalized ionic liquids were designed and synthesized.The performance of thses ionic liquids for CO₂ capture was investigated,and the interaction between these ionic liquids and CO₂ was studied systamatically,which opens a door for the design of new functionalized ionic liquids for efficient and reversible CO₂ capture.The main contents are as follows:?1?A series of imide-based anion-functionalized ionic liquids composed of imide-based anions and phosphonium cations were designed and synthesized.The basic physical properties such as glass transition temperature,thermal decomposition temperature,density and viscosity of representative ionic liquids were determined.The effect of water in the reaction medium on impurities in the ionic liquids was investigated by FT-IR and ¹H NMR spectroscopy.Based on these data,the low temperature performance and thermal ability of the ionic liquids were evaluated,and the temperature dependence of density and viscosity were examined.These investigations lay the foundation of the next studies.?2?The effects of preorganization and cooperative interactions on the low-concentration CO₂ capture by imide-based anion-functionalized ionic liquids were investigated.Through a combination of quantum chemical calculations,NMR and FT-IR spectroscopy,the mechanism of CO₂ capture was studied.It was found that there were two stable cis-N-C=O-active sites in the preorganized cyclic succinimide anion[Suc],which could react with two CO₂ molecules through cooperative interactions,resulting in significant increase of CO₂ capture capacity.?3?Based on the tunable structure of the ionic liquids,the electron donating group?cycloalkyl?and electron withdrawing group?phenyl?were grafted on the imide-based anion[Suc]as a substitute,CO₂capture capacity could be tuned by different substitutes on the[Suc]anion of ionic liquids.The mechanism of CO₂ capture by[Cy-Suc]based anion-functionalized ionic liquids was investigated by quantum chemical calculation,NMR and FT-IR spectroscopy.It was shown that the electron donating substituent?cycloalkyl?on the anion increased the electron cloud density of the active sites?-CO-N-CO-?in the anion,leading to the increased CO₂ capture capacity through N···CO₂,N···CO₂···O,and C–H···O interactions between[Cy-Suc]and CO₂.?4?Considering the importance of thermodynamic property of CO₂ capture in evaluating new absorbents and guiding the design of related process engineering,solubility of CO₂ in four kinds of imide-based anion-functionalized ionic liquids was determined at different temperatures and different CO₂partial pressures.Through the modified“deactivated IL”model,the thermodynamic properties,such as reaction Gibbs energy,enthalpy,and entropy in the chemical absorption were calculated.These properties support the capture mechanism proposed in this chapter:there were strong interactions between the active sites of ionic liquids and CO₂,and ionic liquid-CO₂ complex was formed.It was also found that reaction enthalpy was the main driving force for CO₂ capture.