Effects Of Precursor Chemical Structure On The Microstructure And Gas Permeability Of Carbon Membrane

Carbon membrane is a novel carbon-based membrane material with the high gas permeability and for gas separation and broke through the "Robesen upper bound" compared to traditional polymeric membrane materials. However, how to choose the precursor materials is a critical factor to obtain a carbon membrane with a high gas separation performance. At present, the precursor materials of carbon membrane mainly include polyimide, poly-furfuryl alcohol, phenolic resin, polyether sulphone ketone and polystyrene ether polymer. In these materials, PI is the best precursor material for the preparation of carbon membrane with good gas separation performance owing to its good thermal stability and chemical resistance.In this paper, five polyimide precursors with different monomer structure such as PDA-PMDA, TMPPD-PMDA, ODA-PMDA, BAPP-PMDA and BDAF-PMDA were designed and synthesized for the preparation of carbon membrane. The chemical structures of different precursors and their changes during the pyrolysis were investigated by thermogravimmetric analysis (TGA), dynamic scanning calorimetry (TG-MS), infrared spectrometer (FT-IR), and X photoelectron spectrometry (XPS). The microstructures of carbon membrane were measured by X-ray diffraction (XRD). The effects of the chemical structure and pyrolysis temperature on the gas separation performance of carbon membrane were also investigated by gas permeation tests of pure gas and the mixed gas.The results show that as the increase of fractional free volume of the precursor, the gas permeability of carbon membrane increases, however, gas separation performance decreases. The carbon membranes prepared from the polyimides of ODA-PMDA, BAPP-PMDA and BDAF-PMDA have much higher gas permeability, which have broad prospects in gas separation of carbon membrane. The introduction of fluorine groups can greatly improve the gas permeability of carbon membrane. Therefore, the permeability of BDAF-PMDA type PI carbon membrane carbonized at 700℃for pure gas O2 and N2 are 1867 and 431Barrer (1 Barrer=10-10 cm3(STP)·cm·cm-2·s-1·cmHg-1), and O2/N2 separation selective is 4.2.The permeability of CO2 of BAPP-PMDA and BDAF-PMDA type PI carbon membrane carbonized at 700℃is higher than H2, and both of them has priority penetration of CO2 in the mixed gas CO2/H2. Although BAPP-PMDA and BDAF-PMDA type PI carbon membrane have larger microstructure size, the gas separation mechanism of them is mainly surface diffusion with the molecular sieve. With the rise of pyrolysis temperature, the gas permeability of all the carbon membrane decreases. When the carboned temperature is 800℃, the permeability of ODA-PMDA type PI carbon membrane for pure gas O2 and N2 drop to 5 Barrer below, and the permeability of CO2 of BAPP-PMDA type PI carbon membrane is lower than that of H2, and now the gas separation mechanism of BAPP-PMDA type PI carbon membrane is mainly by molecular sieve. The permeability of BDAF-PMDA type PI carbon membrane carbonized at 900℃is still larger and the permeability of CO2 is still higher than that of H2, but CO2/H2 separation selective reduced to 1.7, which indicates that the gas separation mechanism of it gradually transforms the surface diffusion to the molecular sieve.