Significantly enhanced gas separation properties of microporous membranes by precisely tailoring their ultra-microporosity through bromination/debromination

Tailoring the ultra-microporosity of polymer and carbon molecular sieve membranes (CMSM) is vital to their gas separation properties. Herein, we adopted a simple novel bromination/debromination method to tune the ultra-microporosity and gas separation properties of a triptycene-based polyimide of intrinsic microporosity (PIM-PI) and its derived CMSM. The brominated PIM-PI showed higher free volume, larger surface area, and ∼ 8 times gas permeability without scarifying selectivity. This is due to the bromine introduction in the main chain induced huge increased ultra-microporosity that caused strong molecular sieving effect. After debromination at 550 °C, the CMSM from the brominated PIM-PI displayed larger pore volume, narrower pore size, ∼ 9-fold enhanced gas permeability and same selectivity than the non-brominated counterpart. The corresponding DBPI-550 demonstrated an unprecedented CO2 permeability of 20639 Barrer combined with CO2/N2 and CO2/CH4 selectivity of 29.2 and 30.9, respectively. Thermal dynamic results showed that this main chain bromination enhanced separation property by improving the activation energy of diffusion (Ed). The diffusion transition state theory suggested this is contributed by the energetic factor of selectivity (Eenthalpic). The debromination improved activation energy of permeation originated from its decreased heat of sorption (ΔHs). This bromination/debromination technique provided a facile method to editing the ultra-microporosity of the resulting membranes, and shed light on the future designing of high performance industrial gas separation membranes.