Breaking The Permeance-Selectivity Tradeoff for Post-Combustion Carbon Capture: A Bio-Inspired Strategy to Form Ultrathin Hollow Fiber Membranes.

Thin film composite (TFC) hollow fiber membranes with ultrathin selective layer are desirable to maximize the gas permeance for practical applications. Herein, a bio-inspired strategy is proposed to fabricate sub-100-nm membranes via a tree-mimicking polymer network with amphipathic components featuring multifunctionalities. The hydrophobic polydimethylsiloxane (PDMS) brushes act as the roots that can strongly cling to the gutter layer, the PDMS crosslinkers function as the xylems to enable fast gas transport, and the hydrophilic ethylene-oxide moieties (brushes and mobile molecules) resemble tree leaves that selectively attract CO2 molecules. As a result, a ≈27 nm-thick selective layer can be attached to the hollow fiber-supported PDMS gutter layer through a simple dip-coating method without any modification. Furthermore, a CO2 permeance of ≈2700 GPU and a CO2 /N2 selectivity of ≈21 that is beyond the permeance-selectivity upper bound for hollow fiber membranes is achieved. This bio-inspired concept can potentially open the possibility of scalable hollow fiber membranes production for commercial applications in post-combustion carbon capture and beyond.

Delayed Linker Addition (DLA) Synthesis for Hybrid SOD ZIFs with Unsubstituted Imidazolate Linkers for Propylene/Propane and n-Butane/i-Butane Separations.

We present a novel synthesis strategy termed delayed linker addition (DLA) to synthesize hybrid zeolitic-imidazolate frameworks containing unsubstituted imidazolate linkers (Im) with SOD topology (hereafter termed Im/ZIF-8). Im linker incorporation can create larger voids and apertures, which are important properties for gas storage and separation. To date, there have been only a handful of reports of Im linkers incorporated into ZIF-8 frameworks, typically requiring arduous and complicated post synthesis approaches. DLA, as reported here, is a simple one-step synthesis strategy allowing high incorporation of Im linker into the ZIF-8 framework while still retaining its SOD topology. We fabricated mixed-matrix membranes (MMMs) with 6FDA-DAM polymer and Im/ZIF-8 obtained via DLA as a filler. The Im/ZIF-8-containing MMMs showed excellent performance for both propylene/propane and n-butane/i-butane separation, displaying permeability and ideal selectivity well above the polymer upper bound. Moreover, highly detailed molecular simulations shed light to the aperture size and flexibility response of Im/ZIF-8 and its improved diffusivity as compared to ZIF-8.

Linker-Doped Zeolitic Imidazolate Frameworks (ZIFs) and Their Ultrathin Membranes for Tunable Gas Separations.

Zeolitic imidazolate frameworks (ZIFs) have gained much interest due to their potentials in gas separations. A hybrid approach by mixing metals and/or linkers has been recently investigated to fine-tune the ZIF framework porosity and surface properties and potentially extending their separation applications for many important gas mixtures. In general, the hybrid approach requires mixing of isostructural ZIFs to maintain their topology, thus limiting the options of linkers and metal centers to obtain hybrid ZIFs. Linker-doping, as reported here, can be a strategy to expand the option of imidazolate linkers to obtain mixed-linker hybrid ZIFs. Two linkers are investigated to be doped into the ZIF-8 framework: 2-ethylimidazole (eIm) and 2-phenylimidazole (phIm). The linker-doping strategy is shown to tune the "stiffness" of Zn-N bonding as characterized by FT-IR, thereby the linker flip-flopping motion of ZIF-8, which is analyzed through gas adsorption isotherms. Furthermore, well-intergrown ultrathin eIm-doped ZIF-8 membranes are grown on α-Al2O3 substrates, in which the incorporation of eIm affects the morphology and thickness of the polycrystalline membranes, improving the permeance of propylene and propane molecules.

Nano-gate opening pressures for the adsorption of isobutane, n-butane, propane, and propylene gases on bimetallic Co-Zn based zeolitic imidazolate frameworks.

In this article, zeolitic-imidazolate framework-8 (ZIF-8) and its mixed metal CoZn-ZIF-8 were synthesized via a rapid microwave method. The products were characterized by Raman spectroscopy, XPS, XRD, EDX, TEM, NanoSEM, TGA, and DSC. The gas adsorption properties of samples were determined using C3 and C4 hydrocarbons, including propane, propylene, isobutane and n-butane at a temperature of 25 °C. The adsorption equilibrium and kinetics of these gases on various ZIFs were studied. It was noted that ZIF-8 and mixed metal CoZn-ZIF-8 samples start to adsorb these gases after certain pressures which are believed to result in the opening of their nano-gates (i.e., 6-membered rings) to allow the entry of gas molecules. The nanogate opening pressure value (p0) for each ZIF towards different gases was determined by fitting adsorption equilibrium data against a modified form of the Langmuir adsorption isotherm model. It was observed that the value of p0 differs significantly for each gas and to various extents for various ZIFs. Therefore, it is possible that the distinct values of p0 afford a unique technique to separate and purify these gases at the industrial scale. The overall mass transfer coefficient values of the adsorption process were also investigated.

Self-diffusion of pure and mixed gases in mixed-linker zeolitic imidazolate framework-7-8 by high field diffusion NMR.

Self-diffusion of pure gases including carbon dioxide, methane, ethylene, ethane, and xenon as well as selected two-component mixtures was studied in hybrid zeolitic imidazolate framework-7-8 (ZIF-7-8) crystals using pulsed field gradient (PFG) NMR. This material was formed by mixing 2-methylimidazolate (ZIF-8 linker) and bulkier benzimidazolate (ZIF-7 linker) in the same framework. The intracrystalline diffusion data measured in mixed-linker ZIF-7-8 was compared with the corresponding data in the parent ZIF-8 material. It was found that under the same or comparable experimental conditions the intracrystalline gas diffusion was always slower in ZIF-7-8 than in ZIF-8. This observation is consistent with the expected lower pore aperture size in ZIF-7-8 than in ZIF-8. At the same time, the ethane/ethylene diffusion selectivity was found to be similar in both ZIFs. It was also observed that for the pure studied gases larger than carbon dioxide the diffusivity ratios in ZIF-8 and ZIF-7-8 do not increase with increasing gas size at all loading pressures used. All these data are attributed to greater framework flexibility effects in ZIF-7-8 than ZIF-8. Such effects manifest themselves in a distortion and/or increase in the aperture size in the presence of large sorbates due to linker flexibility.

Ethane diffusion in mixed linker zeolitic imidazolate framework-7-8 by pulsed field gradient NMR in combination with single crystal IR microscopy.

Pulsed field gradient (PFG) NMR was used in combination with single crystal IR microscopy (IRM) to study diffusion of ethane inside crystals of a mixed linker zeolitic imidazolate framework (ZIF) of the type ZIF-7-8 under comparable experimental conditions. These crystals contain 2-methylimidazolate (ZIF-8 linker) and benzimidazolate (ZIF-7 linker). It was observed that the PFG NMR attenuation curves measured for ethane in ZIF-7-8 exhibit deviations from the monoexponential behaviour, thereby indicating that the ethane self-diffusivity in different crystals of a crystal bed can be different. Measurements of the ethane uptake curves performed by IRM under the same conditions in different ZIF-7-8 crystals of the bed yield different transport diffusivities thus confirming that the rate of ethane diffusion is different in different ZIF-7-8 crystals. The IRM observation that the fractions of ZIF-8 and ZIF-7 linkers are different in different ZIF-7-8 crystals allowed attributing the observed heterogeneity in diffusivities to the heterogeneity in the linker fraction. The quantitative comparison of the average ethane self-diffusivities measured by PFG NMR in ZIF-7-8 with the corresponding data on corrected diffusivities from IRM measurements revealed a good agreement between the results obtained by the two techniques. In agreement with the expectation of smaller aperture sizes in ZIF-7-8 than in ZIF-8, the average ethane self-diffusivities in ZIF-7-8 were found to be significantly lower than the corresponding self-diffusivities in ZIF-8.

Rapid One-Pot Microwave Synthesis of Mixed-Linker Hybrid Zeolitic-Imidazolate Framework Membranes for Tunable Gas Separations.

The relatively slow and complex fabrication processes of polycrystalline metal-organic framework (MOF) membranes often times restrict their way to commercialization, despite their potential for molecular separation applications. Herein, we report a rapid one-pot microwave synthesis of mixed-linker hybrid zeolitic-imidazolate framework (ZIF) membranes consisting of 2-methylimidazolate (ZIF-8 linker) and benzimidazolate (ZIF-7 linker) linkers, termed ZIF-7-8 membranes. The fast-volumetric microwave heating in conjunction with a unique counter diffusion of metal and linker solutions enabled unprecedented rapid synthesis of well-intergrown ZIF-7-8 membranes in ∼90 s, the fastest MOF membrane preparation up to date. Furthermore, we were able to tune the molecular sieving properties of the ZIF-7-8 membranes by varying the benzimidazole-to-2-methylimidazole (bIm-to-mIm) linker ratio in the hybrid frameworks. The tuning of their molecular sieving properties led to the systematic change in the permeance and selectivity of various small gases. The unprecedented rapid synthesis of well-intergrown ZIF-7-8 membranes with tunable molecular sieving properties is an important step forward for the commercial gas separation applications of ZIF membranes.