Preparation And Performance Of Moisture Swing Sorbents For Direct Air Capture Of CO₂

With distributed emission sources such as transportation,residential construction accounting emissions CO₂ into the atmosphere directly for nearly 30%～50%of the global artificial emissions every year,direct air capture of CO₂（DAC）is rapidly emerging as a necessary complement to negative this part of CO₂ emissions.Renewable adsorbents play the key role in DAC technology,the reduction of water molecules in the nanosystem will actively promote the hydrolysis of CO₃^2-to HCO₃^-and OH^-,leading nanostructured CO₂adsorbent with water swing.When the ambient air is dry,the adsorbent will spontaneously combine with CO₂ in the ambient air,and release CO₂ when the ambient air is wet.As it trades the input of heat in a thermal swing or the mechanical energy in a pressure swing of the traditional sorbents,against the consumption of water,the energy input for CO₂ capture is very low.This brings about an inexpensive and efficient direct air capture technology for mitigating the greenhouse problem.The sorbents are usually heterogeneous sheets composed of amine-based anion exchange resins and a matrix polymer.In order to obtain a hierarchical microporous structure to allow sufficient access of the air to the ion-exchange resins buried inside the matrix,previously,hydrothermal pretreatment of the resin sheets was employed before put into operation.However,the desorption ratio（desorption quantity/adsorption quantity）of the material is only～30%when desorption is activated by exposing to bulk water.The present work aims to reduce the energy consumption for pretreatment and improve the desorption performance.The hydrothermal pretreatment time and temperature are studied systematically.In addition,the humidity sensitivity of materials is very high by moisture swing sorbents to direct air capture of CO₂.However,the concentration of CO₂should be higher than that in the air in some actual working conditions.Therefore,it is necessary to systematically study the dynamics of H₂O molecules in practical application conditions.In this thesis,the adsorption/desorption performance of anion exchange resin membrane material for CO₂ in the preparation of air capture functional membrane was systematically studied under different hydrothermal pretreatment temperature.It was found that the room-temperature soaking pretreatment instead of hydrothermal pretreatment of the resin sheets,can also lead to excellent microporous structures and carbon capture performances,concerning both the capture capacity and kinetics.More importantly,based on the infiltration mechanism of gas and liquid into nanopores,we found that the micron-sized water particles produced by ultrasonic atomization could greatly promote the desorption ratio,from～30%to～60%,compared to that by exposing the sorbent to bulk water,moreover the CO₂ adsorption/desorption capacity of resin performed steadily in the cycle test,the carbon emission of 30.5kg can be saved every ton of CO₂ capture,which greatly saved the cost of air capture;These optimizations made in the present work further pave the way for large-scale engineering implementation of direct air capture of CO₂.In addition,ambient humidity could made extremely influence the adsorption kinetic of the resin in ambient air,when humidity reaches more than 95%in the environment,the resin would desorb CO₂.But the humidity sensitivity of the resin gradually decreases as the ambient CO₂ concentration is increasing,even when the ambient humidity reaches saturation,due to the combined effect of chemisorption and physisorption,the materials can all achieve the same adsorption saturation（equilibrium adsorption capacity/saturated adsorption capacity）under different humidity environment.