Investigation of porosity of carbon materials and related effects on gas separation properties

Carbon molecular sieving membranes are chemically robust materials with tailorable gas transport properties for O₂/N₂, CO₂/CH₄ and C₃H₆/C₃H₈ separations. Such carbon materials were formed in this study by the pyrolysis of polyimide precursors. The final pyrolysis temperature was varied to alter the carbon structure, which changed the average pore size. Characterization of the porosity of these materials and how this feature changes when pyrolysis conditions are varied could guide the systematic control of these materials. However, the carbon is an amorphous, microporous material, which makes it difficult to characterize compared to crystalline materials. From separation studies of penetrants on these materials it appears that these materials have both ultramicropores (<7 Å) and larger micropores. The ultramicropores are believed to be mainly responsible for molecular sieving while the micropores provide negligible resistance to diffusion but provide high capacity sorption sites for penetrants. Techniques such as wide angle X-ray diffraction and the analysis of carbon dioxide adsorption isotherms using density functional theory were employed to characterize the microporosity of the material. The small dimensions of the key ultramicropores make accurate determination of their pore size distribution difficult. Therefore, to effectively discuss the differences in transport properties when different pyrolysis temperatures are used as well as penetrants with different dimensions, a hypothetical ultramicropore size distribution was used as a tool to discuss and interpret a combination of parameter effects and trends of separation properties.