Theoretical And Experimental Studies On CO₂ Adsorption By Ammonium-based Polymeric Ionic Liquids

Research and development on novel CO₂ sorbents to mitigate climate change have attracted intense attention recently.Most of the developed solid sorbents have unique interfacial properties,such as porous structures and modifiable functional groups.In this scene,polymeric ionic liquids?PILs?were proposed to capture CO₂ for that they possess characteristics of ionic liquids and macromolecular architecture together.However,the research on PILs is still at its early stage.The structure-property relationship and its underlying mechanism is still unrevealed.The existing report on the thermodynamic properties of PILs focused on adsorption capacity,and the research on the kinetic properties is inadequate.In this thesis,the microscopic mechanism of CO₂ adsorption,the structure-property relationship and dynamic adsorption characteristic were studied through combining the theoretical analysis and experiment.This study provides a quantitative microscopic insight into the role of the anion in CO₂ adsorption and paves the way toward the optimal PIL structure for CO₂ capture under specific circumstances.In this section,density functional theory?DFT?is employed to study the underlying mechanism of CO₂ adsorption of P[VBTEA][CO32-].The optimized structures of ion pairs,the reaction pathways of CO₂ adsorption and thermodynamic equilibrium of PILS-CO₂-H2O system are obtained to reveal the multi-roles of hydrated water on CO₂ adsorption.The adsorption/desorption equilibrium would shift to desorption by adding water,which constitutes the theoretical basis for humidity swing.The proton transfer process from water to carbonate at sorbent interface is successfully depicted with an intermediate which has higher energy state.By determining the CO₂ adsor:ption energy and activation energy at different hydration state,it is discovered that water could promote CO₂ adsorption by reducing energy barrier of proton transfer.By analyzing the hydrogen bond and structure of water molecules,it is interesting to find that the CO₂ adsorption weakens the hydrophilicity of sorbent and results in release of water.The requirement of latent heat for the phase change of water could significantly reduce the heat of adsorption.The special"self-cooling" effect during gas adsorption can lower the temperature of sorbent and benefit CO₂ adsorption.To obtain the structure-property relationship of quaternary-ammonium based PILs,PILs with different counter anions for CO₂ capture are systematically investigated through DFT calculations.Three aspects of the sorbent design were studied:the suitability of the CO₂ affinity for different applications?e.g.,direct air capture or flue gas capture?,capability for moisture-swing adsorption and activation energy of CO₂ adsorption reaction.CO₂ affinity was found to positively correlate with the pKa value of the counter anion,except for fluoride,which has an intrinsic character of attracting protons.The moisture swing capacity is determined by the difference between the hydration energies of the reactant and product after CO₂ adsorption and followed the order CO32->F->Ac-.Further investigations revealed that the repulsion between the two quaternary ammonium cations could promote the dissociation of hydrated water,which results in the lowest activation energy for CO₂ adsorption for the PIL with carbonate.Therefore,the PIL with carbonate is potentially a desirable candidate for air capture and moisture-swing regeneration,while the PIL with acetate is suitable for CO₂ capture under high partial pressure and regeneration through conventional approaches.This study provides a quantitative microscopic insight into the role of the anion in CO₂ adsorption and paves the way toward the optimal PIL structure for CO₂ capture under specific circumstances.Based on the unique characteristics of PILs,which is ion diffusion and moisture swing adsorption,a CO₂ adsorption model was developed.The mathematical expressions of general system,as well as some simplified cases are derived from basic diffusion-reaction equations.With,the help of the in-house code based on Scilab,the numerical solutions of the model are obtained.The dynamic adsorption process is studied by conducting the moisture-swing frequency response?MSFR?experiments and fitting with the simulation results by the built model.The mass transfer inside the PILs was emphasized and the results show that the gas diffusion is as important as ion diffusion inside the PILs.The humidity has a great influence on kinetic parameters,such as diffusion coefficient.At last,a new concept of CO₂ collection device,"CO₂ pump",is proposed.The water gradient creates a CO₂ gradient in the device and as water goes out of the device,CO₂ migrates the opposite way.The simulation results have proved that the device is feasible in principle.Next step is to synthesize appropriate material for "CO₂ pump"?such as tube or membrane?,and to verify it by experiment.