Mixed-linker Zeolitic Imidazolate Frameworks Research Articles

Effect of Solvent Assisted Linker Exchange (SALE) and De Novo Synthetic Routes on CO2 Uptake and Fixation by Mixed-Linker Zeolitic Imidazolate Frameworks

Direct mixing (de novo) and Solvent Assisted Ligand Exchange (SALE) are the main methods used for the synthesis of Mixed-Linker Zeolitic Imidazolate Frameworks (ML-ZIFs). ML-ZIFs with combined –NO2 and –Br/–Cl functionalities were prepared via both synthetic routes. Thereafter the CO2 uptake of the ML-ZIFs were compared, as well as their abilities to fixate CO2 with epoxide substrates. The de novo synthesis resulted in ML-ZIFs with SOD topologies, 60: 40 (–NO2: –Br/–Cl) functionality ratios, higher porosities, better thermal stability and higher CO2 uptake than equivalent SALE products. SALE resulted in smaller ML-ZIF crystallites, only ~ 10% incorporation of –Br/–Cl functionalized imidazolate linkers, and phase change during activation. ML-ZIF-7Cl, obtained from direct mixing, resulted in the highest CO2 uptake (90 cm3 g−1), in line with its higher porosity. ML-ZIF-7Cl, in combination with tetrabutylammonium bromide (TBAB), showed a high catalytic activity (TOF of 446 h−1) for the fixation of CO2 with propylene oxide and was reusable for up to 4 cycles without loss in activity.Graphical

Hybrid Benzimidazole-Dichloroimidazole Zeolitic Imidazolate Frameworks Based on ZIF-7 and Their Application in Mixed Matrix Membranes for CO2/N2 Separation.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) with the sodalite (sod) topology type and based on ZIF-7 have been prepared by direct synthesis from the mixtures of benzimidazole (BzIm) and 4,5-dichloroimidazole (dcIm). Incorporation of dcIm into the ZIF-7 structure gives ZIF-7/COK-17 hybrids with rhombohedral symmetry that do not show the “open-to-closed form” structural transition upon solvent removal exhibited by ZIF-7. They show Type I isotherms for low molecular weight gases and high affinity for CO2 even at low partial pressures. Synthesis under mild conditions gives ZIF nanoparticles (250–400 nm) suitable for incorporation into mixed matrix membranes (MMMs): these were prepared with both glassy (Matrimid) and rubbery (PEBAX 1657) polymers. Permeation tests at 298 K and 1.2 bar reveal that the incorporation of Zn(BzIm0.55dcIm0.45)2 nanoparticles at up to ca. 12 wt % gives defect-free membranes with enhanced CO2 permeability in both polymer matrices, with retention of selectivity (Matrimid) or with an enhancement in selectivity that is most pronounced for the smaller nanoparticles (PEBAX). The membrane with the best performance exhibits a selectivity of ca. 200 for CO2/N2 (a 4-fold increase compared to the pure polymer) and a CO2 permeability of 64 Barrer. At the relatively low loadings investigated, the MMMs’ performance obeys the Maxwell model, and the intrinsic property of diffusivity of the ZIFs can be extracted as a result.

Selective adsorption of CO2 and SF6 on mixed-linker ZIF-7–8s: The effect of linker substitution on uptake capacity and kinetics

A series of mixed-linker Zeolitic Imidazolate Framework(ZIF)-7–8s constructed from different amounts of benzimidazolate (bIm) and 2-methylimidazolate (mIm) linkers were synthesized in this study. We demonstrated that by carefully controlling the ratios between bIm and mIm linkers, the particle morphology and pore size of ZIF-7–8 can be tailored to selectively adsorb CO2 or SF6 with significantly enhanced uptake capacity. ZIF-7–8 with 90% bIm and 10% mIm linkers showed a CO2 uptake of 1.44 mmol g−1 at 293 K (1 bar) and a CO2/N2 selectivity of over 30. On the other hand, ZIF-7–8 with 26% bIm and 74% mIm linkers had a high SF6 uptake of 2.08 mmol g−1 at 293 K (1 bar) as well as a high SF6/N2 selectivity of over 40. Isosteric enthalpy of adsorption calculations and cyclic adsorption experiments confirmed that both CO2 and SF6 were physisorbed on ZIF-7–8. CO2 was found to adsorb rapidly on ZIF-7–8, with over 80% of the total uptake capacity reached within 30 s. Detailed kinetics analysis concluded that the adsorption of CO2 and SF6 on these ZIF-7–8s could be described by pseudo-second order kinetics and the diffusion was found to be governed by a mixture of different mechanisms. The highly tunable sorption properties, high uptake capacity and high selectivity of ZIF-7–8 render them as interesting candidate sorbents for different greenhouse gases.

Structural integrity, meltability, and variability of thermal properties in the mixed-linker zeolitic imidazolate framework ZIF-62.

Metal-organic framework (MOF) glasses have emerged as a new class of melt-quenched glasses; however, so far, all MOF glass production has remained at lab-scale; future applications will require large-scale, commercial production of parent crystalline MOFs. Yet, control of synthetic parameters, such as uniform temperature and mixing, can be challenging, particularly, when scaling-up production of a mixed-linker MOF or a zeolitic imidazolate framework (ZIF). Here, we examine the effect of heterogeneous linker distribution on the thermal properties and melting behavior of ZIF-62. X-ray diffraction (XRD), Raman, and 1H nuclear magnetic resonance spectroscopies revealed little discernable structural difference between samples of ZIF-62 synthesized in our lab and by a commercial supplier. Differential scanning calorimetry and variable temperature/isothermal XRD revealed the samples to have significantly different thermal behavior. Formation of ZIF-zni was identified, which contributed to a dramatic rise in the melting point by around 100 K and also led to the alteration of the macroscopic properties of the final glass. Parameters that might lead to the formation of unexpected phases such as an uneven distribution of linkers were identified, and characterization methods for the detection of unwanted phases are provided. Finally, the need for adequate consideration of linker distribution is stressed when characterizing mixed-linker ZIFs.

Meltable Mixed-Linker Zeolitic Imidazolate Frameworks and Their Microporous Glasses: From Melting Point Engineering to Selective Hydrocarbon Sorption.

Porous glasses from metal-organic frameworks (MOFs) represent a new class of functional inorganic-organic materials, which have been proposed for applications ranging from solid electrolytes to radioactive waste storage. So far, just a few zeolitic imidazolate frameworks (ZIFs), a subset of MOFs, have been reported to melt and the structural and compositional requirements for MOF melting and glass formation are poorly understood. Here, we show how the melting point of the prototypical ZIF-4/ZIF-62(M) frameworks (composition M(im)2-x(bim)x; M2+ = Co2+, Zn2+; im- = imidazolate; bim- = benzimidazolate) can be controlled systematically by adjusting the molar ratio of the two imidazolate-type linkers im- and bim-. By covering the entire range from x = 0 to 0.35, we unveil a delicate transition from ZIF materials showing sequential amorphization/recrystallization to derivatives exhibiting coherent melting and a liquid phase that is stable over a large temperature window. The melting point of this ZIF system is a direct function of x and can be lowered from ca. 430 °C to only 370 °C, by far the lowest melting point reported for a three-dimensional porous MOF. On the basis of our results, we postulate compositional requirements for ZIF melting and glass formation, which may guide the search for other meltable ZIFs. Moreover, gas physisorption experiments establish that the ZIF glasses adsorb technologically relevant C3 and C4 hydrocarbons. Importantly, the adsorption kinetics are much faster for propylene compared to propane and are also dependent on the im-:bim- ratio, thus demonstrating the potential of these ZIF glasses for applications in gas separation.

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.

Structural and Mechanistic Differences in Mixed-Linker Zeolitic Imidazolate Framework Synthesis by Solvent Assisted Linker Exchange and de Novo Routes.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are a subclass of metal-organic frameworks (MOFs) amenable to significant property tuning by altering the functional groups on the imidazolate linkers. Solvent assisted linker exchange (SALE) and de novo synthesis of mixed-linker ZIFs have been demonstrated, but the differences in structural properties-most importantly the linker distributions-and synthesis mechanisms of these two different types of hybrid ZIFs are unknown. In this work, a combination of 1H NMR combined rotation and multiple pulse spectroscopy (CRAMPS), water adsorption, and nitrogen measurements reveal distinct differences in linker mixing between SALE and de novo ZIF-8-90 hybrids. Native-fluorescence confocal microscopy is shown to provide a direct means to visualize these differences. The effects of crystal size, temperature, and SALE duration were studied in detail, and a generalizable mechanism for SALE processes in ZIFs is proposed. The SALE process is found to follow a diffusion-limited behavior leading to core-shell morphologies. Under harsher SALE conditions, deviations from diffusion-limited behavior are found due to etching and partial dissolution of the initial ZIF-8 crystals. With the selection of appropriate reaction conditions, SALE processes appear to be capable of generating controlled core-shell ZIF structures of good morphological quality that complement the well-mixed structures obtained by de novo methods.

Structure Elucidation of Mixed-Linker Zeolitic Imidazolate Frameworks by Solid-State (1)H CRAMPS NMR Spectroscopy and Computational Modeling.

Mixed-linker zeolitic imidazolate frameworks (ZIFs) are nanoporous materials that exhibit continuous and controllable tunability of properties like effective pore size, hydrophobicity, and organophilicity. The structure of mixed-linker ZIFs has been studied on macroscopic scales using gravimetric and spectroscopic techniques. However, it has so far not been possible to obtain information on unit-cell-level linker distribution, an understanding of which is key to predicting and controlling their adsorption and diffusion properties. We demonstrate the use of (1)H combined rotation and multiple pulse spectroscopy (CRAMPS) NMR spin exchange measurements in combination with computational modeling to elucidate potential structures of mixed-linker ZIFs, particularly the ZIF 8-90 series. All of the compositions studied have structures that have linkers mixed at a unit-cell-level as opposed to separated or highly clustered phases within the same crystal. Direct experimental observations of linker mixing were accomplished by measuring the proton spin exchange behavior between functional groups on the linkers. The data were then fitted to a kinetic spin exchange model using proton positions from candidate mixed-linker ZIF structures that were generated computationally using the short-range order (SRO) parameter as a measure of the ordering, clustering, or randomization of the linkers. The present method offers the advantages of sensitivity without requiring isotope enrichment, a straightforward NMR pulse sequence, and an analysis framework that allows one to relate spin diffusion behavior to proposed atomic positions. We find that structures close to equimolar composition of the two linkers show a greater tendency for linker clustering than what would be predicted based on random models. Using computational modeling we have also shown how the window-type distribution in experimentally synthesized mixed-linker ZIF-8-90 materials varies as a function of their composition. The structural information thus obtained can be further used for predicting, screening, or understanding the tunable adsorption and diffusion behavior of mixed-linker ZIFs, for which the knowledge of linker distributions in the framework is expected to be important.

Selective Solvent-Assisted Linker Exchange (SALE) in a Series of Zeolitic Imidazolate Frameworks.

Solvent-assisted linker exchange (SALE) has recently emerged as an attractive strategy for the synthesis of metal-organic frameworks (MOFs) that are unobtainable via traditional synthetic pathways. Herein we present the first example of selective SALE in which only the benzimiadazolate-containing linkers in a series of mixed-linker zeolitic imidazolate frameworks (ZIF-69, -78, and -76) are replaced. The resultant materials (SALEM-10, SALEM-10b, and SALEM-11, respectively) are isostructural to the parent framework and in each case contain trifluoromethyl moieties. We therefore evaluated each of these materials for their hydrophobicity in condensed and gas phases. We expect that selective SALE will significantly facilitate the design of improved, and potentially complex, MOF materials with new and unusual properties.

Highly tunable molecular sieving and adsorption properties of mixed-linker zeolitic imidazolate frameworks.

Nanoporous zeolitic imidazolate frameworks (ZIFs) form structural topologies equivalent to zeolites. ZIFs containing only one type of imidazole linker show separation capability for limited molecular pairs. We show that the effective pore size, hydrophilicity, and organophilicity of ZIFs can be continuously and drastically tuned using mixed-linker ZIFs containing two types of linkers, allowing their use as a more general molecular separation platform. We illustrate this remarkable behavior by adsorption and diffusion measurements of hydrocarbons, alcohols, and water in mixed-linker ZIF-8(x)-90(100-x) materials with a large range of crystal sizes (338 nm to 120 μm), using volumetric, gravimetric, and PFG-NMR methods. NMR, powder FT-Raman, and micro-Raman spectroscopy unambiguously confirm the mixed-linker nature of individual ZIF crystals. Variation of the mixed-linker composition parameter (x) allows continuous control of n-butane, i-butane, butanol, and isobutanol diffusivities over 2-3 orders of magnitude and control of water and alcohol adsorption especially at low activities.

Mixed-linker zeolitic imidazolate framework mixed-matrix membranes for aggressive CO2 separation from natural gas

Zeolitic imidazolate framework (ZIF) materials are a promising subclass of metal–organic frameworks (MOF) for gas separations. However, due to the deleterious effects of gate-opening phenomena associated with organic linker rotation near the limiting pore apertures of ZIFs, there have been few demonstrations of improved gas separation properties over pure polymer membranes when utilizing ZIF materials in composite membranes for CO2-based gas separations. Here, we report a study of composite ZIF/polymer membranes, containing mixed-linker ZIF materials with ZIF-8 crystal topologies but composed of different organic linker compositions. Characterization of the mixed-linker ZIFs shows that the mixed linker approach offers control over the porosity and pore size distribution of the materials, as determined from nitrogen physisorption and Horváth–Kawazoe analysis. Single gas permeation measurements on mixed-matrix membranes reveal that inclusion of mixed-linker ZIFs yields membranes with better ideal CO2/CH4 selectivity than membranes containing ZIF-8. This improvement is shown to likely occur from enhancement in the diffusion selectivity of the membranes associated with controlling the pore size distribution of the ZIF filler. Mixed-gas permeation experiments show that membranes with mixed-linker ZIFs display an effective plasticization resistance that is not typical of the pure polymeric matrix. Overall, we demonstrate that mixed-linker ZIFs can improve the gas separation properties in composite membranes and may be applicable to aggressive CO2 concentrations in natural gas feeds.

Tunable CO2 Adsorbents by Mixed-Linker Synthesis and Postsynthetic Modification of Zeolitic Imidazolate Frameworks

The incorporation of accessible amine functionality in zeolitic imidazolate frameworks (ZIFs) is used to improve the adsorption selectivity for CO2/CH4 gas separation applications. Two synthetic approaches are described in this work to introduce functionality into the ZIF: (i) mixed-linker ZIF synthesis with 2-aminobenzimidazole as a substitution linker and (ii) postsynthetic modification of a mixed-linker ZIF with ethylenediamine. Using 2-aminobenzimidazole, a linker with a primary amine functional group, substitution of the ZIF-8 linker during synthesis allows for control over the adsorption properties while maintaining the ZIF-8 structure with up to nearly 50% substitution in the mixed-linker ZIF framework, producing a material with tunable pore size and amine functionality. Alternatively, postsynthetic modification of a mixed-linker ZIF containing an aldehyde functional group produces a ZIF material with a primary amine without detrimental loss of micropore volume by controlling the amount of function...