Phosphonium Ionic Liquids-based Membranes For CO₂ Separation

The large consumption of fossil fuels such as petroleum,coal,and natural gas has led to a dramatic increase of greenhouse gases,especially CO₂ in the atmosphere,which caused many serious problems like the greenhouse effect.Therefore,the efficient capture and separation of CO₂ is great significant in the fields of ecological environment,energy and chemical industries.Ionic Liquids?ILs?,as a new green medium,have many features such as negligible vapor pressure,good stability,structural design and lower desorption energy consumption.Therefore,it is widely used in gas separation?CO₂,SO₂,NH₃ etc.?,especially the CO₂ separation and purification.However,there are still some challenges,like high viscosity and high cost,which limits their industrial applications.Immobilizing ILs onto inorganic or organic solid materials is one of the effective ways to solve these problems.Among them,the ILs based membranes have the advantages of both ILs and membranes.Meanwhile,the addition of ILs can improve the membranes separation performance.In this work,a series of quaternary phosphonium ILs were synthesized and blended with a polyether copolyamide?Pebax2533?to prepare IL/Pebax composite membranes.Besides,ZIF-8 was added to improve the stability and gas permeability of membranes.The membranes were characterized and their gas separation performance were systematacially studied.The main research and results were concluded as follows:Three kinds of quaternary phosphonium ILs([P₆₆₆₁₄][NTf₂],[P₆₆₆₁₄][DCA]and[P₆₆₆₁₄][Triz])were successfully synthesized,and the IL/Pebax composite membranes were prepared.FTIR,TGA,DSC,and other characterization analysis revealed that both the IL and the polymer were physically blended and exhibited good thermal stability.The influence of the anion and the content of ILs on the membrane separation performance was studied.The results showed that the[P₆₆₆₁₄][NTf₂]/Pebax composite membrane has the best performance,with the CO₂ permeability reached 244.3 Barrer,when the IL content reached 40 wt%.With the increase of IL content,the CO₂permeability of[P₆₆₆₁₄][NTf₂]/Pebax composite membrane increased,while the other two IL composite membranes decreased firstly and then increasesed.ZIF-8/Pebax composite membranes were prepared,and the effect of ZIF-8 content on the separation performance were investigated.The result showed that with the addition of ZIF-8 up to 10 wt%,the CO₂ permeability of the membrane increased to267.1 Barrer.This is because the addition of ZIF-8 promotes the diffusion of gas in the membrane.However,with the further increase of ZIF-8 content,obvious agglomeration occurred in the composite membrane,resulting in no increase in gas permeability.Therefore,the content of ZIF-8 in the three-component composite membrane was selected as 10 wt%.A series of[P₆₆₆₁₄][NTf₂]/ZIF-8/Pebax three-component composite membranes have been prepared.The structure of the composite membrane was characterized by FTIR,TGA,SEM and XRD.The gas separation performance of the composite membrane was studied systematically.The results showed that the gas permeability of the three-component composite membrane increases as the content of[P₆₆₆₁₄][NTf₂]increases.When the content of[P₆₆₆₁₄][NTf₂]reaches 30 wt%and the content of ZIF-8reaches 10 wt%,the CO₂ permeability of the membrane reaches 386.8 Barrer.This is because the addition of IL increases the free volume of the membrane and reduces the agglomeration of ZIF-8,which promotes gas diffusion,hence improves the CO₂permeability of the membrane.