Ultra-selective carbon molecular sieve membranes and methods of making

Embodiments of the present disclosure are directed to a process for making a carbon molecular sieve membrane having a desired permselectivity between a first gas species and a second gas species, in which the second gas species has a larger kinetic diameter than the first gas species. The process comprises providing a polymer precursor and pyrolyzing the polymer precursor at a pyrolysis temperature that is effective to selectively reduce the sorption coefficient of the second gas species, thereby increasing the permselectivity of the resulting carbon molecular sieve membrane.