Carbon membranes for challenging gas separations

Carbon molecular sieving membranes are new materials that have established their validity by surpassing the upper bound limit constraining traditional polymer membranes. These materials are chemically robust and their controllable molecular sieving ability can be used to perform challenging separations. Carbon material applications have recently extended to the separation of important hydrocarbons such as C₃H₆/C₃H₈. The study of this pair along with other gas pairs such as O₂/N ₂ and CO₂/CH₄ has allowed understanding of ways to tailor the carbon structure to obtain the best gas transport properties for specific separations.; Experimental pyrolysis parameters affect the carbon molecular sieve properties significantly. The final pyrolysis temperature, the polymer precursor used and the “thermal soak” length are variables of importance. Permselectivity tends to increase with higher final pyrolysis temperature. At increased pyrolysis temperatures, the ultramicropores of the carbon material that cause molecular sieving decrease in size, therefore for a particular separation, the diffusion of the larger penetrant is restricted causing an increase in selectivity. The length of ‘thermal soak’ which is the amount of time that the material is exposed to the final pyrolysis temperature, controls the time material undergoes structural evolution. The polyimide precursors used in this study to from the carbon materials differed in their structure. Matrimid ® has relatively more tightly packed polymer chains than 6FDA/BPDA-DAM formed carbon materials with a higher selectivity under equivalent processing conditions.; Gas sorption and pure and mixed gas permeation experiments were used primarily to characterize the carbon materials. Several traditional material science characterization techniques were also used to investigate the morphology of the carbon materials. However, because of the amorphous nature of the carbon the results were not conclusive. Gas transport is the most effective means for determining the critical “pore mouth” dimension that controls molecular sieving. Homogeneous, thin, dense films were used to measure intrinsic gas transport properties without the complexities of an asymmetric structure. Processing difficulties due to their rigid nature can ultimately be overcome by using a mixed matrix material, which consists of flakes of carbon material in a polymer matrix.