Ultra-selective molecular-sieving gas separation membranes enabled by multi-covalent-crosslinking of microporous polymer blends

Abstract

High-performance membranes exceeding the conventional permeability-selectivity upper bound are attractive for advanced gas separations. In the context microporous polymers have gained increasing attention owing to their exceptional permeability, which, however, demonstrate a moderate selectivity unfavorable for separating similarly sized gas mixtures. Here we report an approach to designing polymeric molecular sieve membranes via multi-covalent-crosslinking of blended bromomethyl polymer of intrinsic microporosity and Tröger's base, enabling simultaneously high permeability and selectivity. Ultra-selective gas separation is achieved via adjusting reaction temperature, reaction time and the oxygen concentration with occurrences of polymer chain scission, rearrangement and thermal oxidative crosslinking reaction. Upon a thermal treatment at 300 °C for 5 h, membranes exhibit an O₂/N₂, CO₂/CH₄ and H₂/CH₄ selectivity as high as 11.1, 154.5 and 813.6, respectively, transcending the state-of-art upper bounds. The design strategy represents a generalizable approach to creating molecular-sieving polymer membranes with enormous potentials for high-performance separation processes.

Fig. 1. Schematic of crosslinking reactions between PIM-BM and TB.

a Chemical structure of PIM-BM and TB. b Proposed crosslinking mechanisms between PIM-BM and TB.

Fig. 2. SEM images and photos of polymer membranes after thermal crosslinking at varied temperatures.

a, c, e Cross-sectional images of PIM-BM/TB, PIM-BM/TB-250 °C-10 h and PIM-BM/TB-300 °C-5 h. b, d, f Surface images of PIM-BM/TB, PIM-BM/TB-250 °C-10 h and PIM-BM/TB-300 °C-5 h. g Photos of PIM-BM/TB treated at different temperatures.

Fig. 3. Structural characterizations of fresh and thermally crosslinked PIM-BM/TB.

a XPS spectrum of Br 3d of fresh PIM-BM/TB. b XPS spectrum of Br 3d of PIM-BM/TB-120 °C-20 h. c XPS spectrum of Br 3d of PIM-BM/TB-200 °C-20 h. d XPS spectrum of Br 3d of PIM-BM/TB-250 °C-10 h. e XPS spectrum of Br 3d of XPIM-BM/TB-300 °C-5 h. f FTIR spectra of fresh and thermally crosslinked PIM-BM/TB.

Fig. 4. Properties of crosslinked microporous membranes.

a Thermal stability of PIM-BM/TB and crosslinked PIM-BM/TB. b XRD profiles. c Mechanical properties of polymer membranes.

Fig. 5. Characterization of PIM-BM/TB and XPIM-BM/TB membranes physical structures.

a Representative chain conformations in crosslinked PIM-BM/TB from computer modeling results. b 3-D view of crosslinked PIM-BM/TB modeling structure in an amorphous cell (300 °C-5 h) (cell size: 30 × 30 × 30 A; density: ~1.223 g/cm³; Gray–Van der Waals surface; dark gray–Connolly surface with pore radius of 1.45 A). c Pore size distribution from PALS.

Fig. 6. Gas transport properties.

a Gas permeabilities as a function of kinetic diameters. b CO₂/CH₄ separation with Robeson upper bound. c H₂/CH₄ separation with upper bounds.

Fig. 7. Gas transport properties of membranes prepared under various conditions.

a Gas permeability and b gas selectivity as a function of reaction temperature. All samples were thermally treated at set-point temperature for 10–20 h under 200 ppm of O₂, except that the sample at 300 °C was annealed for 5 h. c Gas permeability and d gas selectivity as a function of reaction time treated at 250 °C of crosslinked membranes. e Gas permeability and f gas selectivity as a function of oxygen concentrations for membranes treated at 250 °C of 10 h.

Fig. 8. Effects of feed pressures and physical ageing on membranes.

a Pure CO₂ permeability versus feed pressure. b CO₂/CH₄ ideal selectivity versus feed pressure. c CO₂ permeability versus feed pressure for separating equimolar CO₂/CH₄ gas mixtures. d CO₂/CH₄ selectivity versus feed pressure for separating equimolar CO₂/CH₄ gas mixtures. e Pure gas permeability versus aging time of XPIM-BM/TB- 250 °C-10 h. f Gas selectivity versus aging time of XPIM-BM/TB- 250 °C-10 h.