Multi-quaternary Ammonium-based Polymeric Ionic Liquids And Their Applications In Separating CO₂ By Pressure Swing Absorption

With the rapid development of the world economy,increasing consumption of fossil fuels has raised global concerns for their massive CO2 emission which has resulted in increasing atmospheric temperature.Thus,it is extremely important to recover CO2 from industrial waste gases in view of both reutilization of CO2 as a carbon resource and environmental preservation.Poly(ionic liquid)s(PILs)have attracted much attention in CO2 capture due to its high thermal stability,designed anions and cations,and high CO2 selectivity.In our previous studies,a quaternary ammonium-based poly(ionic liquid)was proposed and showed good performance in CO2 capture and syngas separation.In this study,a series of multi-quaternary ammonium-based(a IL monomer containing multi-quaternary ammonium moieties)cross-linked PILs with improved hydrophobicity and higher CO2 absorption were proposed and prepared.The performance of separating CO2 from a simulated syngas was studied by using the selected PILs.A double-quaternary ammonium-based IL monomer was synthesized by the quatemization reaction of 4-vinylbenzyl chloride and N,N,N',N'-tetramethyl-1,3?propanediamine.After ion-exchange and free-radical polymerization of the monomer,the PIL[PVBp-PF6]with improved hydrophobicity was successfully prepared.Different drying methods were employed to obtain different pore structure of PILs.These materials were further characterized by infrared spectroscopy,X-ray diffraction,N2 adsorption-desorption,contact angle test,scanning electron microscopy and differential scanning calorimeter.A high-pressure spring quartz method was employed to measure the performance of PILs in CO2 capture.Results showed that compared with other drying methods,supercritical drying(SCD)effectively protected the pore structure of PIL[PVBp-PF6-SCD].[PVBp-PF6-SCD]exhibited good affinity and selectivity to CO2 because of its lower crystallinity and higher specific surface area.It also exhibited good regeneration,thermal stability,suggesting its potential in industrial application.Another two multi-quaternary ammonium-based IL monomers were synthesized by the quaternization reaction of N,N-dimethyl-1-(4-vinylphenyl)methanamine with 1,3,5-tris(bromomethyl)benzene and 1,2,4,5-tetrakis(bromomethyl)benzene,respectively.After ion-exchange,free-radical polymerization and subsequent SCD of the monomers,the PILs[PVBDTB-n-PF6](n=3,4)with improved hydrophobicity and abundant pore structure were successfully prepared.Through a series of characterization and absorption tests,results showed that[PVBDTB-4-PF6]possessed higher BET surface area and higher carbon dioxide capture capacity than[PVBDTB-3-PF6].It also exhibited good regeneration,thermal stability,suggesting its potential in industrial application.[PVBp-PF6-SCD]and[PVBDTB-4-PF6]on the absorption and separation of CO2 from a simulated high-pressure syngas(H2/CO/CO2)were studied by a pressure swing absorption device.Results showed that[PVBp-PF6-SCD]could reduce CO2 from 19.0%to 1%at 303.0 K and 4.0 MPa,while[PVBDTB-4-PF6]could reduce CO2 from 19.0%to 0%.Moreover,[PVBDTB-4-PF6]could reduce CO2 from 19.0%to 1.8%at 303.0 K and 3.0 MPa.Therefore,[PVBDTB-4-PF6]is superior to[PVBp-PF6-SCD]in CO2 absorption and separation.