Effect of chemical vapor deposition of toluene on gas separation performance of carbon molecular sieve membranes

A carbon molecular sieve (CMS) membrane is a microporous membrane that can serve as a molecular sieve. Tuning the pore size of a CMS membrane to improve the selectivity of desirable gas pair is highly necessary. Herein, we report the first study on the toluene vapor modification of a CMS membrane that shows high H 2 selectivity. A commercial polyimide hollow fiber membrane was used as the polymeric precursor, and it was pyrolyzed at different temperatures from 625 to 700 ℃ for up to 120 min in an inert nitrogen atmosphere. Toluene vapor was introduced into the nitrogen stream for different durations (10–50 min) at the final pyrolysis temperature. The permeance of the resulting CMS membranes for H 2 , CO 2 , O 2 , N 2 , and CH 4 gases was evaluated at 35 ℃ under 1 atm pressure. The gas permeance and selectivity could be readily controlled by adjusting the pyrolysis temperature and duration of toluene vapor addition for membrane modification. Upon comparing samples with similar H 2 permeances, the toluene-vapor-modified membrane was found to exhibit a higher selectivity for H 2 /CO 2 , H 2 /N 2 , and H 2 /CH 4 . This could not be realized simply by increasing the pyrolysis temperature without toluene vapor addition. For instance, a CMS membrane pyrolyzed at 675 ℃ with 20 min of toluene vapor addition showed H 2 /CH 4 selectivity of 4200 and H 2 permeance of 270 GPU, while a CMS membrane pyrolyzed at 700 ℃ without using toluene vapor showed a lower H 2 /CH 4 selectivity of 1100 and a similar H 2 permeance of 250 GPU. The optimal pyrolysis conditions of the CMS membrane subjected to toluene vapor modification depend on the targeted gas pair. The proposed toluene vapor modification process can be used as a simple and effective additional treatment for the preparation of CMS membranes.