MOF–polymer enhanced compatibility: post-annealed zeolite imidazolate framework membranes inside polyimide hollow fibers

Abstract

Thermal annealing, a commonly used procedure for improving the performance of polymeric membranes, is in this work exploited in the presence of a metal–organic framework (MOF) supported layer. MOFs and polymers are materials with a common organic character, suggesting an enhanced affinity between them when used together in membrane separation. Zeolite-like imidazolate frameworks (ZIFs) ZIF-8 and ZIF-93 with sod and rho structures and pore apertures of 0.34 and 0.36 nm, respectively, have been grown inside 356 μm OD co-polyimide P84 hollow fibers by microfluidics, leading to continuous supported membranes. When these membranes were thermally in situ annealed below the glass transition temperature, while monitoring both H₂ and CH₄ permeances, the MOF–polymer adhesion was enhanced. Thus the gas separation selectivity increased without any significant reduction in the gas permeance, and H₂/CH₄ and CO₂/CH₄ maximum selectivities of 103 and 18 (ZIF-8) and 101 and 20 (ZIF-93) were respectively measured. The good compatibility between MOF and polymer made improvements possible in the annealing of the membrane once it was prepared. If the annealing of the polymer was carried out before the MOF synthesis, the polymer chain rearrangement and surface smoothing prevented an optimum MOF–polymer interaction and the separation performance worsened. These results proved the compatibility between both materials and their synergistic contribution to gas selective transport.