Carbon molecular sieve membranes derived from Matrimid® polyimide for nitrogen/methane separation

A commercial polyimide, Matrimid® 5218, was pyrolyzed under an inert argon atmosphere to produce carbon molecular sieve (CMS) dense film membranes for nitrogen/methane separation. The resulting CMS dense film separation performance was evaluated using both pure and mixed N₂/CH₄ permeation tests. The effects of final pyrolysis temperature on N₂/CH₄ separation are reported. The separation performance of all CMS dense films significantly exceeds the polymer precursor dense film. The CMS dense film pyrolyzed at 800 °C shows very attractive separation performance that surpasses the polymer membrane upper bound line, with N₂ permeability of 6.8 Barrers and N₂/CH₄ permselectivity of 7.7 from pure gas permeation, and N₂ permeability of 5.2 Barrers and N₂/CH₄ permselectivity of 6.0 from mixed gas permeation. The temperature dependences of permeabilities, sorption coefficients, and diffusion coefficients of the membrane were studied, and the activation energy for permeation and diffusion, as well as the apparent heats of sorption are reported. The high permselectivity of this dense film is shown to arise from a significant entropic contribution in the diffusion selectivity. The study shows that the rigid ‘slit-shaped’ CMS pore structure can enable a strong molecular sieving effect to effectively distinguish the size and shape difference between N₂ and CH₄.