(494i) Tailoring Carbon Molecular Sieve (CMS) Membranes through Free Volume Modification of PIM Based Precursors.

Membrane-based processes could potentially reduce industrial energy consumption by 90% compared to traditional separation methods, however, polymeric membranes face inherent challenges: a tradeoff between permeability and selectivity, and poor stability at extreme temperatures. Carbon molecular sieve (CMS) membranes offer a promising solution to these limitations. While most CMS membranes are made from commercial dense polymeric materials, this research investigates CMS membranes derived from the pyrolysis of a classic polymer of intrinsic microporosity (PIM-1) and compares findings with variations of PIM-1 with diverse labile functional groups, including -NH₂ and -tBOC functionalization. Removing functional groups during pyrolysis leads to unique free volume architectures.

A series of functional PIM polymers were carbonized at different pyrolysis temperatures (600 ^oC and 750 ^oC) and the effect of the changes in fractional free volume (FFV) of the precursor were observed using X-ray scattering. The gas permeation properties indicate that these CMS membranes are highly hydrogen selective, crossing the permeability-selectivity performance upper bounds for many gas pairs such as H₂/CH₄, O₂/N₂ and C₃H₆/C₃H₈. The best performing membranes have H₂/CH₄ selectivity of over 1000, with the permeability of H₂ being more than 2000 barrer, indicating potential use case in hydrogen separations. These gas-transport properties are correlated with the structure of the membranes as found through XRD which indicates formation of graphene like peaks. BET (with CO₂) also gives insight into the structure, with the carbonized membranes having twice the surface area of the initial polymer. XPS is also used to map the elemental compositions. Development of the CMS structure is evaluated in situ through TGA-MS and XRD using the actual pyrolysis conditions. It is seen that the ramping conditions affects the char yield of the CMS as obtained through TGA-MS. In situ XRD indicates that development of the CMS structure happens during the ramp phase especially for a polymer like PIM-1 without a detectable glass transition temperature_.

This study opens up space for a fundamental understanding of CMS structure development and relates the gas permeation properties of CMS membranes engineered by FFV modification to this structural development.