A molecular sieve inorganic membrane with a uniform pore diameter $(d_p)$ may achieve significant "size$-$selectivity" between two nonadsorbing molecules of different kinetic diameters $(d_k)$ when

when $d_p$ is more than two times the $d_k$ of both molecules.

when the molecule with a greater molecular weight $(M_w)$ has a smaller $d_k.$

$d_k$ of the larger molecule approaches the $d_p.$

when $d_k$ of the smaller molecule exceeds the $d_p.$