[Show abstract][Hide abstract] ABSTRACT: We have synthesized nano-sized zeolitic imidazolate frameworks (ZIF-8) and prepared polybenzimidazole (PBI)/ZIF-8 mixed matrix membranes (MMMs) with uniform morphology comprising ZIF as high as 58 wt% by directly mixing the as-synthesized ZIF-8 suspension into the polymer dope for pervaporation dehydration of ethanol, isopropanol (IPA) and butanol. Pervaporation test data show that the water permeability of PBI/ZIF-8 (1:1) MMMs is about one order higher than the original PBI membrane (14,000–22,000 vs. 1200–2300 Barrer). The apparent activation energy calculated from the Arrhenius equation indicates that there is a 29.2% reduction of energy barrier for penetrant transports for PBI membranes consisting of 33.7 wt% ZIF-8 nano-particles. As a result, the 33.7 wt% ZIF-8 in PBI membrane increases water permeability four times without much decrease in selectivity for water. The 58.7 wt% ZIF-8 in PBI membrane has a very high water permeability but a relatively low selectivity. Furthermore, the MMMs also show effectiveness in suppressing ethanol-induced swelling on the polymeric matrix owing to its inorganic properties. Vapor sorption studies confirm that the enhancement of pervaporation performance is primarily attributed to the significant improvement in diffusivity selectivity. With high thermal stability, chemical resistance and excellent compatibility with PBI, ZIF-8 nano-particles are promising fillers for enhancing the overall separation performance of low permeability but high performance materials like PBI.
Journal of Membrane Science 10/2012; s 415–416:577–586. DOI:10.1016/j.memsci.2012.05.052 · 5.06 Impact Factor
[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 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.
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