Synthesis Of Polyamide Containing Fluorene Groups For CO₂ Separation

CO₂ emissions lead to some global problems,such as greenhouse effect,ocean acidification,and endangered species.Simultaneously,CO₂is also the main pollutants of the petroleum chemical industry,and CO₂not only influences the product quality but also forms acid solution that will corrode equipments and pipes.Therefore,CO₂ separation has become the research focus.The membrane separation technology possesses some advantages such as low operation cost,flexible in operation,low operating area,low energy consumption and low polution.Thus,membrane separation technology is a potential CO₂ separation technology.The membrane material itself has a take-off relationship between permeability and selectivity,which limits the application of gas separation membranes.Currently,preparation of gas separation membranes with high permeability and selectivity has become the research focus.The amide bond of polyamide facilitates CO₂ separation,and polyamide possesses good film formation,thermal stability,and mechanical properties.In this study,we prepare polyamides containing fluorene groups by polycondensation reaction and use simple blend method to improve gas separation properties.Specific research work and conclusions are as follows:（1）A new 4,4’-（9-fluorenylidene）dianiline（9FDA）monomer containing the bulky fluorene groups was prepared by the reaction of9-fluorenone and aniline.Polyamides（PAs）were prepared by polycondensationreactionbetween9FDAandtwodiacid（4,4’-（hexafluoroisopropylidene）bis（benzoicacid）（HFA）and4,4’-oxybis（benzoic acid）（OBA））,respectively.Copolyamides（CPAs）were synthesized through polycondensation reaction among 9FDA、HFA and OBA by adjusting the molar ratio of HFA and OBA.PAs and CPAs membranes were fabricated by using 8%（w/v）of polymer solution with N-methylpyrrolidone（NMP）as solvent.The characterization by FTIR and ¹H NMR proved the successful synthesis of PAs and CPAs.The characterization by TGA-DSC displayed high thermal stability(T_d15>378°C)and glass transition temperature（Tg,Tg>345°C）.The single gas test was performed with single gases（H₂,CO₂,O₂,CH₄,and N₂）at different temperatures（25°C,35°C,45°C and 55°C）and 1.0 atm.Results showed that with the increase in operating temperature,CO₂permeabilty gradually increased,while CO₂/N₂ and CO₂/CH₄ selectivities gradually decreased.PA-1 membrane containing fluorene groups and bulky-CF₃ groups exhibited the highest permeability(P_CO2=12.26 Barrer)and good selectivity（α（CO₂/N₂）=26.65 andα（CO₂/CH₄）=22.70）at 25℃and 1.0 atm.（2）A series of poly（amide-co-poly（propylene glycol））random and block copolymers（RPA-PPGs and BPA-PPGs）were synthesized by polycondensation reaction between 9FDA,OBA,and polypropylene glycol（PPG）.RPA-PPGs and BPA-PPGs membranes were fabricated by using 8%（w/v）polymer solution with NMP as solvent.RPA-PPGs and BPA-PPGs were characterized by FTIR and ¹H NMR,suggesting the successful synthesis of RPA-PPGs and BPA-PPGs.RPA-PPGs and BPA-PPGs were characterized by UV and XRD,indicating that the distance between polymer chains gradually decreased as PPG content increased.The effects of chemical structure,content of PPG,and operating temperature on the gas permeation properties of RPA-PPGs and BPA-PPGs membranes were discussed.Results showed that RPA-PPGs membranes had a better gas permeation property than that of BPA-PPGs membranes.With the increase in content of PPG,CO₂ gas permeability gradually decreased,and CO₂/CH₄ and CO₂/N₂ selectivities gradually increased.However,with the increasing operating temperature,CO₂ gas permeability gradually increased,and CO₂/CH₄ and CO₂/N₂ selectivities gradually decreased.Polyamide random copolymer containing 20 wt%PPG（RPA-PPG（20%））exhibited excellent gas separation preperties(P_CO2=5.972 Barrer,α（CO₂/CH₄）=38.28 andα（CO₂/N₂）=21.41)at 25℃and 1.0 atm.（3）A series of poly（amide-co-poly（propylene glycol））copolymers（PA-PPGs）with different content of PPG were synthesized by polycondensation reaction between HFA,9FDA,and PPG.These PA-PPGs were blended with Pebax 2533.The characterization by FTIR demonstrated the successful synthesis of PA-PPGs and the successful fabrication of Pebax/PA-PPG blend membranes.These blend membranes were characterized by XRD,suggesting Pebax 2533 microstructure change from microcrystalline structure to amorphous structure.The characterization by SEM exhibited the good compatibility between Pebax2533 and PA-PPGs.The effects of content of PA-PPGs and operating temperature on gas permeation properties of Pebax/PA-PPG blend membranes were investigated.Results showed that CO₂ gas permeability gradually increased with the increase of operating temperature,while CO₂gas permeability gradually decreased with the increase in content of PA-PPGs.CO₂/N₂ selectivity gradually increased with the increase of PA-PPGs content.In particular,Pebax/PA-PPG（50）-60%blend membranes exhibited the best CO₂ separation performance(P_CO2=79.7Barrer,α（CO₂/N₂）=34.7 andα（CO₂/CH₄）=8.2)at 25℃and 1.0 atm.