High-performance carbon molecular sieve membranes for ethylene/ethane separation derived from an intrinsically microporous polyimide

• Published in: Journal of membrane science Vol. 500; pp. 115 - 123
• Main Authors: Salinas, Octavio, Ma, Xiaohua, Litwiller, Eric, Pinnau, Ingo
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 15.02.2016
• Subjects: artificial membranes; carbon; Carbon molecular sieve; diffusivity; ethane; ethylene; Ethylene/ethane separation; micropores; Mixed-gas permeation; permeability; Polyimide of intrinsic microporosity; polymers; porous media; pyrolysis; solubility; sorption; temperature; Carbon molecular sieve; Mixed-gas permeation; Polyimide of intrinsic microporosity; Ethylene/ethane separation
• ISSN: 0376-7388; 1873-3123
• DOI: 10.1016/j.memsci.2015.11.013

An intrinsically microporous polymer with hydroxyl functionalities, PIM-6FDA-OH, was used as a precursor for various types of carbon molecular sieve (CMS) membranes for ethylene/ethane separation. The pristine polyimide films were heated under controlled N2 atmosphere at different stages from 500 to 800°C. All CMS samples carbonized above 600°C surpassed the polymeric ethylene/ethane upper bound. Pure-gas selectivity reached 17.5 for the CMS carbonized at 800°C with an ethylene permeability of about 10Barrer at 2bar and 35°C, becoming the most selective CMS for ethylene/ethane separation reported to date. As expected, gravimetric sorption experiments showed that all CMS membranes had ethylene/ethane solubility selectivities close to one. The permselectivity increased with increasing pyrolysis temperature due to densification of the micropores in the CMS membranes, leading to enhanced diffusivity selectivity. Mixed-gas tests with a binary 50:50 v/v ethylene/ethane feed showed a decrease in selectivity from 14 to 8.3 as the total feed pressure was increased from 4 to 20bar. The selectivity drop under mixed-gas conditions was attributed to non-ideal effects: (i) competitive sorption that reduced the permeability of ethylene and (ii) dilation of the CMS that resulted in an increase in the ethane permeability. [Display omitted] •First report of ethylene/ethane separation of a PIM-polyimide.•PIM-6FDA-OH-derived CMS (pyrolized at 800°C) exhibits the highest pure-gas selectivity reported to date (αC2H4/C2H6 ~18).•Permselectivity of the CMS membranes is completely dominated by diffusivity selectivity.•Mixed-gas ethylene/ethane permselectivity decreased from 14 at 4 bar to 8.3 at 20 bar.