Carbon molecular sieve (CMS) membranes, derived from the pyrolysis of polymer precursors, possess multi-modal pore structures with size-sieving bottlenecks, leading to excellent gas separation properties. In this study, we show that the scalable vapor-phase infiltration (VPI) technique can fine-tune pore sizes and enhance size-sieving ability. Specifically, a polyimide, 6FDA-DAM, was carbonized at 500 °C and then treated with the VPI process by exposure to trimethylaluminum (TMA) vapor and water vapor alternately. The TMA infiltrates into the CMS, reacts with F-containing groups, and forms AlOx on the micropore surface after reacting with water vapor. Increasing the VPI cycle number decreases the porosity and gas permeability but increases gas selectivity, especially for C3H6/C3H8 separation. For example, increasing VPI cycles from 0 to 10 cycles increases C3H6/C3H8 selectivity from 16 to 67 at 35 °C while decreasing C3H6 permeability from 570 to 80 Barrer. Similarly, CO2/CH4 selectivity increases from 38 to 50, and O2/N2 selectivity increases from 4.9 to 7.7 when 10-cycle VPI treatment is conducted. Notably, the VPI treatment mainly affects gas diffusion, and diffusion selectivity for gas separations. Additionally, the VPI can be coupled with low-temperature carbonization in making size-sieving CMS membranes while retaining good mechanical properties. The successful fine-tuning of micropores enables high-performance CMS membranes by low-temperature carbonization and also underscores the versatility of the VPI process in engineering porous membranes.
