Surface structure modification on the gas separation performance of carbon molecular sieve membranes

Carbon molecular sieve (CMS) membranes for separating gases were deposited on porous Al₂O₃ disks using CH₃COCH ₃+CH₄, by a remote inductively coupled-plasma (ICP) chemical vapor deposition (CVD). The new method for preparing membranes with a high H₂/N₂ selectivity without any reduction of the permeance was introduced by combining the surface treatment with high-energy ion bombardment and subsequent high-temperature pyrolysis study. The H ₂/N₂ selectivity reached 50 with an extremely high permeance of approximately 1.6×10^-6 mol m^-2 s ^-1 Pa^-1 at 423 K. The O₂/N₂ selectivity reached 2.5 and the O₂ permeance was approximately 2×10^-7 mol m^-2 s^-1 Pa^-1 at 398 K. Short and optimized surface treatment periods were required to ensure high efficiency. Without pyrolysis, surface treatments alone markedly reduced the H₂ and N₂ permeances but did not affect selectivity. Besides, without any surface treatment, pyrolysis alone markedly increased the H₂ and N₂ permeances, but did not improve selectivity, because the desorption of carbon left large pores. Surface treatment must be combined with pyrolysis to enhance simultaneously the permeance and selectivity of CMS membranes, in a manner very different from that associated with conventional pore-plugging mechanism in typical CVD.