Study On Efficient Absorption And Conversion Of CO₂ By Phosphonium-Based Deep Eutectic Solvents

The rapid burning of fossil fuels and other human activities have released significant amounts of carbon dioxide（CO₂）into the atmosphere over the past few decades,posing serious threats to both the environment and human health.Therefore,developing effective CO₂capture and conversion utilization method is crucial.The existing CO₂capture process has problems such as high separation energy consumption and large solvent loss,and CO₂conversion and utilization also have problems such as harsh reaction conditions and low conversion efficiency,so it is urgent to develop new and efficient CO₂capture and conversion and utilization technologies.As a new type of material,deep eutectic solvents（DESs）have the advantages of simple synthesis,tunable structure,and high stability,and show broad application prospects in the field of CO₂capture and conversion.Based on this,this work designed and prepared a series of phosphonium-based functional DESs for CO₂capture and conversion to achieve efficient CO₂capture and mild conversion.The following is the paper’s specific research content:（1）Based on the hydrogen-electrostatic synergy of phosphonium-based cationic functional groups and anion to CO₂,quaternary phosphonium ionic liquids containing different functional groups are designed as hydrogen bond acceptors（HBAs）.By associating with glycol to form a new hydrogen bond network,the absorption performance of CO₂will be greatly improved,and ethylene glycol（EG）and diethylene glycol（DEG）are selected as hydrogen bond donors（HBDs）.Eight DESs are prepared,and structural characterization and physical property determination are carried out.The solubility of CO₂in eight DESs is determined by static method at a temperature of 303.15K～333.15 K and a pressure of 200 kPa～2500 kPa.The solubility data of the{CO₂+DESs}system are correlated using the NRTL model,and the average relative deviation between the calculated and experimental values was less than 5%.The experimental results show that 1-carboxylethyl tributylphosphoniumbromide-diethyleneglycol([P_4,4,4,2COOH][Br]-DEG)has the highest absorption of CO₂,with a mole fraction of 0.2167 for CO₂at 303.15 K and 1000 kPa.At the same time,[P_4,4,4,2COOH][Br]-DEG can maintain good CO₂absorption performance after five absorption and desorption cycles,indicating that the prepared DES has good cycle stability.In addition,the absorption mechanism is clarified through quantitative calculation and interaction region indicator function（IRI）analysis.The absorption of CO₂by the system was physically absorbed,and the absorption capacity is improved by hydrogen bonding.（2）Based on the promotion effect of high absorption capacity on CO₂conversion,the phosphonium-based functionalized DESs described above are used in the reaction of CO₂and epoxide cycloaddition to synthesizecarbonate.Afterevaluation,1-aminoethyl tributylphosphonium bromide-diethylene glycol([P_4,4,4,2NH₂][Br]-DEG)has the best catalytic effect,and under mild conditions（60℃,0.8 MPa,and 4 h）,the yield of propylene carbonate（PC）reaches 96%,and the selectivity exceeds 99%.Moreover,the catalyst showed a good catalytic effect on different epoxy substrates.In addition,the prepared DESs are easily recovered by ether extraction,and their catalytic properties remain unchanged after five cycles.Finally,the possible mechanism of[P_4,4,4,2NH₂][Br]-DEG catalyzed cycloaddition reaction is proposed using in situ infrared and quantitative calculations.Mechanistic studies show that the synergistic effect between the HBAs（quaternary phosphonium ionic liquids）and the HBDs（glycols）simultaneously activates epoxide and CO₂,and achieves a mild conversion of CO₂.