Effects of surface treatments and annealing on carbon-based molecular sieve membranes for gas separation

Abstract

A new method in preparing carbon-based molecular sieve (CMS) membranes for gas separation has been proposed. Carbon-based films are deposited on porous Al 2O 3 disks using hexamethyldisiloxane (HMDSO) by remote inductively coupled plasma (ICP) chemical vapor deposition (CVD). After treating the film with ion bombardment and subsequent pyrolysis at a high temperature, carbon-based molecule sieve membranes can be obtained, exhibiting a very high H 2/N 2 selectivity around 100 and an extremely high permeance of H 2 around 1.5 × 10 −6 mol m −2 s −1 Pa −1 at 298 K. The O 2/N 2 selectivity could reach 5.4 with the O 2 permeance of 2 × 10 −7 mol m −2 s −1 Pa −1 at 423 K. During surface treatments, HMDSO ions were found to be more effective than CH 4, Ar, O 2 and N 2 ions to improve the selectivity and permeance. Short and optimized surface treatment periods were required for high efficiency. Without pyrolysis, surface treatments alone greatly reduced the H 2 and N 2 permeances and had no effect on the selectivity. Besides, without any surface treatment, pyrolysis alone greatly increased the H 2 and N 2 permeances, but had no improvement on the selectivity, owing to the creation of large pores by desorption of carbon. A combination of surface treatment and pyrolysis is necessary for simultaneously enhancing the permeance and the selectivity of CMS membranes, very different from the conventional pore-plugging mechanism in typical CVD.