[Show abstract][Hide abstract] ABSTRACT: The framework hydrophobicity and flexibility of ZIF-8 are investigated by a detailed adsorption and diffusion study of a series of probe molecules including ethanol, 1-butanol, water, hexane isomers, xylene isomers, and 1,2,4-trimethylbenzene. The prospects for using ZIF-8 in biofuel recovery and hydrocarbon separations are discussed in terms of adsorption or kinetic selectivities. ZIF-8 shows extremely low water vapor uptakes and is especially suitable for vapor phase butanol-based biofuel recovery. The extraordinary framework flexibility of ZIF-8 is demonstrated by the adsorption of hydrocarbon molecules that are much larger than its nominal pore size, such as m-xylene, o-xylene and 1,2,4-trimethylbenzene. The calculation of corrected diffusion coefficients reveals an interesting spectrum of promising kinetic hydrocarbon separations by ZIF-8. These findings confirm that a molecular sieving effect tends to occur in the sorbate molecular size range of 4–6 Å rather than around the nominal ZIF-8 pore size of 3.4 Å, due to its surprising framework flexibility.
[Show abstract][Hide abstract] ABSTRACT: We studied molecular sieving properties of zeolitic imidazolate framework-8 (ZIF-8) by estimating the thermodynamically corrected diffusivities of probe molecules at 35 °C. From helium (2.6 Å) to iso-C4H10 (5.0 Å), the corrected diffusivity drops 14 orders of magnitude. Our results further suggest that the effective aperture size of ZIF-8 for molecular sieving is in the range of 4.0 to 4.2 Å, which is significantly larger than the XRD-derived value (3.4 Å) and between the well-known aperture size of zeolite 4A (3.8 Å) and 5A (4.3 Å). Interestingly, because of aperture flexibility, the studied C4 hydrocarbon molecules that are larger than this effective aperture size still adsorb in the micropores of ZIF-8 with kinetic selectivities for iso-C4H8/iso-C4H10 of 180 and n-C4H10/iso-C4H10 of 2.5 × 106. These unexpected molecular sieving properties open up new opportunities for ZIF materials for separations that cannot be economically achieved by traditional microporous adsorbents such as synthetic zeolites.
[Show abstract][Hide abstract] ABSTRACT: The ability to obtain a maximum loading of inorganic nanoparticles while maintaining uniform dispersion in the polymer is the key to the fabrication of mixed-matrix membranes with high pervaporation performance in bioalcohol recovery from aqueous solution. Herein, we report the simultaneous spray self-assembly of a zeolitic imidazolate framework (ZIF)–polymer suspension and a cross-linker/catalyst solution as a method for the fabrication of a well-dispersed ZIF-8–PDMS nanohybrid membrane with an extremely high loading. The ZIF-8–PDMS membrane showed excellent biobutanol-permselective pervaporation performance. When the ZIF-8 loading was increased to 40 wt %, the total flux and separation factor could reach 4846.2 g m−2 h−1 and 81.6, respectively, in the recovery of n-butanol from 1.0 wt % aqueous solution (80 °C). This new method is expected to have serious implications for the preparation of defect-free mixed-matrix membranes for many applications.
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