Confined conversion of hydrogen-bonded organic framework into carbon molecular sieve-interlocked-substrate membrane for efficient CO2/CH4 separation

Abstract

Carbon molecular sieve (CMS) membranes, derived from the crystalline porous and solution-processed hydrogen-bonded organic frameworks (HOFs), possess narrow pore size distributions for efficient gas molecular sieving. However, the small monomer molecules of HOF precursors (unlike polymer chains) can more easily infiltrate the porous α-Al₂O₃ substrates. Their conversion to CMS would lead to increased mass transfer resistance and reduced gas permeation. In this work, we turn this necessity into an advantage by fabricating CMS-interlocked-substrate membranes. This is achieved by infiltrating and confining a HOF-8 precursor within the surface voids of the substrate, followed by its subsequent conversion into CMS to serve as the selective layer. The derived CMS layers, with a thickness of ∼14.2 ​μm, are tightly interlocked with the substrates, as observed via scanning electron microscopy and energy-dispersive spectrometry. Consequently, the optimal CMS-interlocked-substrate membranes inherit the pore characteristics of their ordered porous precursors, featuring a narrower pore size distribution with a significant proportion of 3–4 ​Å ultramicropores, which yield an exceptional CO₂/CH₄ selectivity of 254.6. By eliminating the mass transfer resistance of continuous surface CMS layer, the membranes exhibit an enhanced CO₂ permeance of 137.3 GPU, surpassing our previous results of 47.6 GPU. Moreover, the membranes maintain stable separation in long-term permeation test and after surface sanding. These findings offer new perspectives on the fabrication of advanced CMS membranes.