Subtle penetrant size effects on separation of carbon molecular sieve membranes derived from 6FDA:BPDA-DAM polyimide

Economically scalable carbon molecular sieve (CMS) hollow fiber membranes rely upon a tunable bimodal pore morphology to separate gas pairs using angstrom-level size discrimination. Freshly formed CMS membranes experience self-retarding physical aging, reflected by permeance losses and selectivity gains that stabilize over short times compared to the lifetime of the membrane. Self-retarding morphology rearrangements are of special interest here due to tightening of the largest size range of ultramicropores, thereby causing aging for polyimide-derived CMS. We report effects of such aging for two A/B penetrant pairs, C3H6/C3H8 and CO2/CH4. These two pairs not only have different average sizes, (d) over bar (A/B), but also different size differences, Delta d(A/B), between the members in each pair. We focus primarily on the C3H6/C3H8 pair, which is the most difficult of the two pairs to separate, and use some CO2/CH4 results as a comparison case. We study CMS derived from 6FDA:BPDA-DAM (1:1) polyimide by pyrolysis at 550 degrees C, 600 degrees C and 650 degrees C. We suggest conditions and physical causes that expedite, retard or even suppress the aging process within CMS membranes. Finally, we suggest how analysis of CMS derived from other precursors and other penetrant pairs can be generalized by understanding the results for the 6FDA:BPDA-DAM (1:1) polyimide derived CMS. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Economically scalable carbon molecular sieve (CMS) hollow fiber membranes rely on tunable bimodal pore morphology to separate gas pairs based on angstrom-level size discrimination. The self-retarding physical aging process of CMS membranes affects gas separation efficiency, particularly in the tightening of ultramicropore size ranges. Understanding and controlling the aging process within CMS membranes can be generalized to other precursor-derived CMS membranes for improved gas separation performance.