Creation of Well‐Defined “Mid‐Sized” Micropores in Carbon Molecular Sieve Membranes

Abstract

Carbon molecular sieve (CMS) membranes are candidates for the separation of organic molecules due to their stability, ability to be scaled at practical form factors, and the avoidance of expensive supports or complex multi‐step fabrication processes. A critical challenge is the creation of “mid‐range” (e.g., 5–9 Å) microstructures that allow for facile permeation of organic solvents and selection between similarly‐sized guest molecules. Here, we create these microstructures via the pyrolysis of a microporous polymer (PIM‐1) under low concentrations of hydrogen gas. The introduction of H2 inhibits aromatization of the decomposing polymer and ultimately results in the creation of a well‐defined bimodal pore network that exhibits an ultramicropore size of 5.1 Å. The H2 assisted CMS dense membranes show a dramatic increase in p‐xylene ideal permeability (≈15 times), with little loss in p‐xylene/o‐xylene selectivity (18.8 vs. 25.0) when compared to PIM‐1 membranes pyrolyzed under a pure argon atmosphere. This approach is successfully extended to hollow fiber membranes operating in organic solvent reverse osmosis mode, highlighting the potential of this approach to be translated from the laboratory to the field.

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

“…As illustrated in Supplementary Fig. 13 , the existence of oxygen-containing functional groups (such as carboxylic acid, carbonyl, and epoxy groups) between the ordered layers can strongly retard CO 2 transport, as reported by Kim et al 5 Besides, the sp 2 -hybridized carbon formed at a higher carbonization temperature provides a more ordered graphitic carbon structure, which is beneficial to the packing of the carbon strands and induces the formation of narrower ultramicropores 11 . On the other hand, the micropores existing between the aromatic carbon plates are more prone to compaction, due to the reduced content of three-dimensional sp 3 -hybridized carbon.…”

“…During the carbonization process, the entangled polymeric precursor chains are transformed into rigid carbonized aromatic strands, and thereafter, form organized plates to approach higher entropy in the system 11 , 23 . A proposed mechanism for the transformation of cellulose precursors to carbon membranes is summarized in Supplementary Note 3 and Supplementary Fig.…”

“…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 .…”

“…Tailoring the ultramicropores in CMS membranes would provide a powerful approach to obtain CMS membranes with higher selectivities for H 2 /CO 2 separation. Recently, Ma et al reported an H 2 -assisted method to create “mid-sized” ultramicropores (5–7 Å) in CMS membranes by introducing H 2 into the carbonization environment 11 . 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.…”

“…Carbon molecular sieve (CMS) membranes have rigid pore structures and are fabricated by controlled carbonization of polymeric precursors at high temperature. 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 .…”