Tunable Polyimide and Resulting Carbon Molecular Sieve Membranes for Gas Separation

Polymer membranes have been used for natural gas and olefin-paraffin gases separations successfully. However, the separation performance is limited by a productivity and efficiency trade-off upper bound. Therefore, advanced carbon molecular sieves (CMS) materials were developed. These materials show separation performance that exceeds the polymer “upper bound” for gas separations, while potentially maintaining economically attractive manufacturing. CMS membranes can be formed by thermal decomposition of polymer precursors, and contain “micropores” of 6–20 Å connected by smaller so-called “ultramicropores”. The combination of ultramicropores and micropores provides the molecular sieving function and high permeability characteristics of these unusual CMS membranes.

We synthesized diverse but systematic sets of co-polyimides, containing symmetric and asymmetric polymer backbone structure, to explore effects of matrix rigidity and packing of precursor polyimides on final CMS properties. We also explored the interaction of pyrolysis conditions on final CMS properties.   Permeation was tested using both pure gases (He, O₂, N₂, CO₂, CH₄, C₂H₄, C₂H₆, C₃H₆, C₃H₈) and mixed gases for both the polyimide precursor membranes as well as the resulting CMS membranes. We correlated separation performance of precursor polyimides and formation conditions with the CMS membranes’ gas separation performance. This research provides an improved fundamental framework to guide the systematic understanding and fabrication of CMS membranes for important energy intensive separations.