Restricting Lattice Flexibility in Polycrystalline Metal-Organic Framework Membranes for Carbon Capture.
basic_science · Level V
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- Record sourced from PubMed, PMID 31087696.
- Also identified by DOI 10.1002/adma.201900855.
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Abstract
Although polycrystalline metal-organic framework (MOF) membranes offer several advantages over other nanoporous membranes, thus far they have not yielded good CO<sub>2</sub> separation performance, crucial for energy-efficient carbon capture. ZIF-8, one of the most popular MOFs, has a crystallographically determined pore aperture of 0.34 nm, ideal for CO<sub>2</sub> /N<sub>2</sub> and CO<sub>2</sub> /CH<sub>4</sub> separation; however, its flexible lattice restricts the corresponding separation selectivities to below 5. A novel postsynthetic rapid heat treatment (RHT), implemented in a few seconds at 360 °C, which drastically improves the carbon capture performance of the ZIF-8 membranes, is reported. Lattice stiffening is confirmed by the appearance of a temperature-activated transport, attributed to a stronger interaction of gas molecules with the pore aperture, with activation energy increasing with the molecular size (CH<sub>4</sub> > CO<sub>2</sub> > H<sub>2</sub> ). Unprecedented CO<sub>2</sub> /CH<sub>4</sub> , CO<sub>2</sub> /N<sub>2</sub> , and H<sub>2</sub> /CH<sub>4</sub> selectivities exceeding 30, 30, and 175, respectively, and complete blockage of C<sub>3</sub> H<sub>6</sub> , are achieved. Spectroscopic and X-ray diffraction studies confirm that while the coordination environment and crystallinity are unaffected, lattice distortion and strain are incorporated in the ZIF-8 lattice, increasing the lattice stiffness. Overall, RHT treatment is a facile and versatile technique that can vastly improve the gas-separation performance of the MOF membranes.