Synthesis And Separation Property Of Porous Aromatic Frameworks (PAFs) Membranes

Membranes are widely used for desalination,dialysis,food processing,gas separation and other fields due to their low energy consumption,small capital cost and simple operation.In recent years,one popular application of membranes is in the environmental field such as carbon capture and liquid separation.Commonly,membrane separation is mainly based on molecular size difference and special surface property of membranes.Based on our current knowledge,the key points in design of new membranes at the molecular level are in the following:（i）proper pore size,（ii）narrow pore size distribution,（iii）thin membrane layer,（iv）strong interaction between the permeable component of interest and the membrane.These strategies aim to overcome the trade-off between selectivity and permeability that all current membranes face.Under the above background and theoretical basis,we have successfully designed and fabricated five porous aromatic frameworks（PAFs）membranes and they have been applied to separate gases or water desalination.The main work includes the following aspects:1.A transparent and freestanding porous promatic framework-97（PAF-97）membrane was successfully synthesized via a one-step acid-catalyzed reaction.Due to the introduction of ether groups,the obtained PAF-97 membrane possessed an enhanced structural flexibility,thus increasing the flexibility of the resulting membrane.This was proven by the result that the feeding pressure of the membrane reached as high as 0.55 MPa during the separation of gas mixtures.The Young’s moduli of the membrane were 6.615 GPa and 11.11GPa,either in dry or hydrated state respectively.To be highlighted,under a feed pressure of 0.36 MPa,the PAF-97 membrane rendered the permeance values of 2.90×10^-7,1.29×10^-8 mol m^-2 s^-1 Pa^-1for CO2 and CH4 respectively,with a CO2/CH4 permselectivity of 22.48.2.Due to the selection of a precisely designed monomer,the obtained PAF-98 membrane possessed microporous structure with specific pore properties.The PAF-98 membrane network displayed a significant performance for CO₂ separation from gas mixture with similar flue gas ratio.Notably,the PAF-98 membrane exhibited CO2/N2 ideal adsorption selectivity as high as 119.3.Furthermore,the PAF-98 membrane showed remarkable stability and reversibility.The CO₂ permeance showed no decay even after 120 h continuous test.3.PAF-97,PAF-121 and PAF-98 membranes were prepared using three similar monomers with different functional groups and were catalyzed by trifluoromethanesulfonic acid.Because of almost the same pore size of the three membranes,they showed similar rejection rates in pervaporation.The measured rejection rates were all over 96%for Na⁺,and the rejection rate of the PAF-98 membrane for Mg²⁺was 100%,and the rejection rate of PAF-121 for Ca²⁺andMg²⁺were also 100%.In the common effort of microporous structure,the three membranes showed different water permeability in pervaporation experiments.The variation of water permeability was consistent with that of the pore characteristics of the three membranes.At the same time,due to the salt precipitation and chemical corrosion,the three membranes had a certain aging phenomenon.In order to realistically simulate the desalination process,PAF-97 membrane as the representative,was tested in the reverse osmosis experiment.In the same solution,with the pressure increasing,the rejection rate of salt increased,and the flux of water increased gradually with the increase of pressure as well.Under the same pressure,with the increase of solution concentration,the salt rejection rate decreased gradually,and the water flux also decreased gradually.Synthetic porous membranes are capable of facilitating proton transport,promising for membrane engineering in many fields,for example,fuel cells,redox flow batteries,etc.Here we further tested the proton conduction of the three membranes at different temperatures in water.Among them,PAF-97membrane showed the best proton conductivityof 24 mS cm^-1 at 20°C.4.In order to widen the application of trifluoromethanesulfonic acid and broaden the membrane types,we used 7,7,8,8-Tetracyanoquinodimethane monomers and adjusted the ratio of monomer to trifluoromethanesulfonic acid,and then successfully prepared a new kind of membrane:tetracyanoquinodimethane（TCQDM）.We tested gas adsorption ability of the membrane for CO₂,C₂H₄ and C₂H₆.Although the new formed membrane contained more nitrogen active sites in the framework,the active sites were too dense thus limiting CO2molecules interaction with the membrane.The adsorption capacity for CO2 of this membrane was higher than other gases（C2H4,C2H6）.According to the Henry’s law,the adsorption selectivities of the membrane for CO2/C2H4 andCO2/C2H6 are 19.86 and 19.58 respectively.From the data,we found that the membrane was excellent for the separation of CO2/C2H4 and CO2/C2H6 gas mixtures.In addition,under trifluoromethanesulfonic acid catalysis,NMCZ-TM membrane was successfully prepared by the reaction of cyanuric chloride（TCT）with N-methyl carbazole.The adsorption capability of the membrane for CO2,N2,C2H4 and C2H6 was tested as well.It was found that,although more N active groups were introduced in the framework,grafted alkyl chains forced the interaction between CO2 and the membrane.Therefore,according to Henry’s law,the adsorption selectivities of the membrane for CO2/N2,CO₂/C₂H₄ and CO₂/C₂H₆ were 33.13,1.78 and 7.34,respectively.Obviously,it was not that good for the separation of the latter two mixtures.At the same time,we studied the water adsorption of TCQDM and NMCZ-TM membranes,and found that the water uptakes of these two membranes were higher than that of PAF-121.