Carbon Molecular Sieve Membranes Derived From Polyimides For Gas Separation

Carbon membrane,as a carbon-based membrane material,exhibits excellent gas separation performance because of its nanoscale microporous structure.However,the low gas permeance of current carbon membranes hasn't satisfied the needs of industrial applications.And then,increasing the fractional free volume of precursors is an important strategy to improve the gas permeability of carbon membranes.In this paper,a series of novel polyimide precursors with rigidity and high free volume referred to as BAF:BAHF(1:1)-6FDA,BAF-6FDA,BATPPP-6FDA,BAPPP-6FDA,BAF-CBDA,BAF-BPDA and BAF-ODPA were designed and synthesized for the preparation of carbon membranes.The pyrolytic characteristics of polyimides and chemical structures and microstructures of derived carbon membranes were characterized by thermogravimetric analysis(TG),Flurier transform infrared spectroscopy(FT-IR),X-ray diffraction(XRD)and high-resolution transmission electron microscopy(HRTEM).The gas separation performance of carbon membranes was evaluated using pure gases H2,O2,N2,CO2 and CH4.And then the effects of polyimide structures on the gas separation properties of derived carbon membranes were discussed.Based on above preliminary study,the effects of pyrolysis temperature on the microstructures and gas transport properties of BATPPP-6FDA and BAPPP-6FDA based carbon membranes were investigated in detail.The evolution of BAF:BAHF(1:1)-6FDA precursor and effects on the microstructures and gas transport properties of derived carbon membranes were also investigated by other technologies such as thermogravimetry coupled with mass spectrometry(TG-MS)and nitrogen adsorption.Results indicated that space configuration and free volume of polyimide precursors significantly affected the microstructures and gas separation performance of derived carbon membranes.The higher the structure rigidity of polyimide was,the higher its FFV was,the microstructure of prepared carbon membrane should become looser and ultramicropore size become larger,resulting in the higher gas permeability of carbon membrane.Wherein,combined action of rigid groups such as fluorene,phthalide with hexafluoroisopropyl in polyimides could effectively disrupt the packing between the molecular chains and enhance the FFV of polyimides significantly.The derived carbon membranes with looser microstructures exhibited excellent gas permeability.As the pyrolysis temperature increasing,the gas permeability of carbon membranes based on BATPPP-6FDA and BAPPP-6FDA decreased continuously,but selectivity increased firstly and then decreased.And all of carbon membranes pyrolyzed at different temperatures showed good gas separation performance,which could apply to different separating targets.BAF:BAHF(1:1)-6FDA copolyimide was converted to PBO-PI precursor through thermal rearrangement around 480 oC,which disrupted the chains packing and led to a further increment of FFV on the basis of original high FFV.And thus the formation of PBO-PI precursor increased the gas permeability of derived carbon membranes with very loos microstructures and large pore sizes significantly,while maintaining comparable selectivity at the same time,solving the low permeability in traditional carbon membranes.Especially,the carbon membrane prepared from BAF:BAHF(1:1)-6FDA at 550 oC exhibited the highest CO2 permeability of 24770 Barrer,and thus achieved CO2 separation and capture.In brief,all of polyimide precursors in this paper had high FFV,although the precursor structures were different,resulted in that derived carbon membranes exhibited good gas separation performance,exceeding the Robeson upper bound.Among them,BAF:BAHF(1:1)-6FDA based carbon membranes showed the best separation performance,suggesting the attractive potential in carbon dioxide removal from natural and flue gas,air gas separation.