Hyperaging Tuning of a Carbon Molecular‐Sieve Hollow Fiber Membrane with Extraordinary Gas‐Separation Performance and Stability

Abstract

This study reports 6FDA:BPDA‐DAM polyimide‐derived hollow fiber carbon molecular‐sieve (CMS) membranes for hydrogen and ethylene separation. Since H2/C2H4 selectivity is the lowest among H2/(C1‐C3) hydrocarbons, an optimized CMS fiber for this gas pair is useful for removing hydrogen from all‐cracked gas mixtures. A process we term hyperaging provides highly selective CMS fiber membranes by tuning CMS ultramicropores to favor H2 over larger molecules to give a H2/C2H4 selectivity of over 250. Hyperaging conditions and a hyperaging mechanism are discussed in terms of an expedited physical aging process, which is largely controlled by the hyperaging temperature. For the specific CMS material considered here, a hyperaging temperature beyond 90 °C but less than 250 °C works best. Hyperaging also stabilizes CMS materials against physical aging and stabilizes the performance of H2 separation over extended periods. This work opens a door in the development of CMS materials for the separation of small molecules from large molecules.

Carbon hollow fiber membranes for a molecular sieve with precise-cutoff ultramicropores for superior hydrogen separation

“…The same trend of a narrowing pore width indicates that the pore size of the selective layer, which is responsible for the molecular sieving mechanism, can be finely tuned by the carbonization temperature. Except for a similar bimodal PSD compared to carbon membranes reported in the literature 12,13,31 , the present cellulosebased CHFMs exhibit a much stronger peak for the ultramicropores in the range of 3-4 Å, which is in the size range needed to allow molecular sieving between H 2 (2.9 Å) and other larger gas molecules (e.g., CO 2 , N 2 , and CH 4 ). With the increase of the carbonization temperature, the pore peaks of >5 Å are weakened, while that of the pores (<5 Å) increases, which indicates that the average pore size decreases for the CHFMs carbonized at higher temperatures.…”

“…The ultramicropores are the slits or the smaller spaces between highly aromatic strands of carbon. The ultramicropores govern the gas pair selectivity, while the micropores, formed by voids between aromatic carbon plates, contribute to high gas permeance [11][12][13] . The cellulose-based CMS membranes reported high gas selectivities for O 2 /N 2 20 and CO 2 /CH 4 19 .…”

“…The introduction of H 2 during the carbonization process was found to inhibit aromatization during thermal decomposition of the polymer network, resulting in a structure with wider ultramicropores compared with the CMS membranes made using argon atmosphere. Introducing an additional thermal treatment step for the freshly-prepared CMS membranes at a temperature range of 90-250°C, referred to as "hyperaging treatment" to accelerate aging, was shown to create smaller ultramicropores as reported by Qiu et al 13 . However, the CMS membranes reported so far still present relatively larger ultramicropores, which does not allow precise gas sieving between H 2 and CO 2 .…”

“…CMS membranes are promising candidates as temperature- and pressure-resistant materials when fabricated into hollow fibers suitable for membranes modules 11 , 12 . The bimodal pore structure of CMS membranes, which comprises ultramicropores and micropores provides favorable gas selectivity in H 2 -related separations such as H 2 /CH 4 12 and H 2 /C 2 H 4 13 . However, due to the strong adsorption between the carbon surface and CO 2 molecules, relatively low H 2 /CO 2 selectivities of <10 were usually reported 12 , 14 , 15 .…”

Inorganic microporous membranes for hydrogen separation: Challenges and solutions

“…For the physical aging, Qiu et al performed post-pyrolysis hyperaging treatment from 90 to 200 °C. This treatment led to the smaller ultra-micropore and improved H2/C2H4 selectivity while maintaining high H2 permeance [67].…”