Carbon Molecular Sieve Membranes Derived from 6FDAbased Polyimides for the Separation of Hexane Isomers

Abstract

Conventional separation processes require huge amount of energy input because it separate one from another by phase change of materials. Since membrane-based separation process does not possess any phase change across the barrier layer, it provides significantly enhanced energy efficiency in large-scale chemical industry. In downstream process, the separation of alkane isomers is critically important to produce gasoline with high quality, but especially difficult due to similarity between sizes of molecules with similar physical properties (e.g., polarizability). Carbon molecular sieve (CMS) with bimodal pore structure consists of ultra-micropore (< 0.7 nm) and micropore (< 2.0 nm) could show excellent size selectivity for the hexane isomers and shape selectivity based on entropic contribution. Herein, the CMS membranes are fabricated with 6FDA-based polyimides with different pore openness: 6FDA/DAM (closed pore structure), 6FDA/DAM:DABA(3:2) (open pore structure), 6FDA:BPDA(1:1)/DAM (rigid pore structure). Both flat sheet and hollow fiber membranes of these polymers are fabricated and further pyrolyzed in oxygen-free conditions. Several kinds of pyrolysis protocols are used to examine the change in sorption selectivity and diffusivity of the hexane isomers via vapor sorption equipment.