Hybrid glasses from melt-quenched metal-organic frameworks (MOFs) have been emerging as a new class of materials, which combine the functional properties of crystalline MOFs with the processability of glasses. However, only a handful of the crystalline MOFs are meltable. Porosity and metal-linker interaction strength have both been identified as crucial parameters in the trade-off between thermal decomposition of the organic linker and, more desirably, melting. For example, the inability of the prototypical zeolitic imidazolate framework (ZIF) ZIF-8 to melt, is ascribed to the instability of the organic linker upon dissociation from the metal center. Here, we demonstrate that the incorporation of an ionic liquid (IL) into the porous interior of ZIF-8 provides a means to reduce its melting temperature to below its thermal decomposition temperature. Our structural studies show that the prevention of decomposition, and successful melting, is due to the IL interactions stabilizing the rapidly dissociating ZIF-8 linkers upon heating. This understanding may act as a general guide for extending the range of meltable MOF materials and, hence, the chemical and structural variety of MOF-derived glasses.

Here, we present a new concept of a core-shell type ionic liquid/metal organic framework (IL/MOF) composite. A hydrophilic IL, 1-(2-hydroxyethyl)-3-methylimidazolium dicyanamide, [HEMIM][DCA], was deposited on a hydrophobic zeolitic imidazolate framework, ZIF-8. The composite exhibited approximately 5.7 times higher CO uptake and 45 times higher CO/CH selectivity at 1 mbar and 25 °C compared to the parent MOF. Characterization showed that IL molecules deposited on the external surface of the MOF, forming a core (MOF)-shell (IL) type material, in which IL acts as a smart gate for the guest molecules.

Twenty-nine different imidazolium ionic liquids (ILs) were combined with two different metal−organic frameworks (MOFs), ZIF-8 and CuBTC, and the resulting IL/MOF composites were characterized in detail by combining X-ray diffraction (XRD), scanning electron microscopy (SEM), Brunauer−Emmett−Teller (BET), and Fourier transform infrared (FTIR) spectroscopy. Characterization data illustrated that MOFs remained structurally intact upon combining them with ILs. Thermogravimetric analysis performed on IL/MOF composites showed that most of the composites have lower thermal stabilities compared to the bulk ILs and pristine MOFs, whereas composites with ILs having a functional group in their anions showed thermal stability limits higher than those of bulk ILs. The derivative onset temperatures representing the maximum tolerable temperatures of the composites were analyzed based on the structural differences in MOFs and ILs, such as the changes in the alkyl chain length, methylation on the C2 site, and functionalization of the cation and the size/electronic changes on the anion. Data illustrated that the maximum tolerable temperatures of IL/MOF composites decrease with an increase in the alkyl chain length on the IL's imidazolium ring. Substitution of the alkyl group with functionalized groups in the IL's imidazolium ring also led to a decrease in the maximum tolerable temperatures of the composites. Whereas, fluorination of the anion resulted in an increase in the thermal stability limits of the corresponding IL/MOF composites. Furthermore, ILs having a dicyanamide, acetate, and phosphate anion also showed an increase in their maximum tolerable temperatures when combined with CuBTC compared to their bulk counterparts. Moreover, simple structural descriptors for each cation and anion were defined by means of the density functional theory (DFT) calculations and used in the quantitative structure−property relationship (QSPR) analysis to correlate the maximum tolerable temperatures of IL/MOF composites to the IL's cation and anion structure. Results presented in this study will provide a guideline for the selection of proper IL−MOF pairs according to the application temperature of IL/MOF composites in various fields.

One of the structural factors controlling the extent of interactions between ionic liquids (ILs) and metal–organic frameworks (MOFs) in IL/MOF composites is elucidated. Results showed that the thermal stability limits and adsorption performances of the IL/MOF composites can be tuned by the interionic interaction energy of bulk ILs, which can be probed spectroscopically via C2H infrared stretching frequency.

Metal-organic frameworks (Mofs) are intriguing host materials in composite electrolytes due to their ability for tailoring host-guest interactions by chemical tuning of the Mof backbone. Here, we introduce particularly high sodium ion conductivity into the zeolitic imidazolate framework ZIF-8 by impregnation with the sodium-salt-containing ionic liquid (iL) (na 0.1 eMiM 0.9)tfSi. We demonstrate an ionic conductivity exceeding 2 × 10 −4 S • cm −1 at room temperature, with an activation energy as low as 0.26 eV, i.e., the highest reported performance for room temperature na +-related ion conduction in Mof-based composite electrolytes to date. partial amorphization of the Zif-backbone by ball-milling results in significant enhancement of the composite stability towards exposure to ambient conditions, up to 20 days. While the introduction of network disorder decelerates IL exudation and interactions with ambient contaminants, the ion conductivity is only marginally affected, decreasing with decreasing crystallinity but still maintaining superionic behavior. This highlights the general importance of 3D networks of interconnected pores for efficient ion conduction in MOF/IL blends, whereas pore symmetry is a less stringent condition. Crystalline metal-organic frameworks (MOFs) consist of metal nodes as coordination centers and organic linkers which self-assemble to form a three-dimensional network. Chemical tailoring of both the inorganic node and the organic linker enables property design for a wide range of applications such as gas storage, gas separation, catalysis and ion conduction 1,2. An alternative route to tune the properties of a given MOF is post-synthetic modification, for example, by applying pressure, temperature or other exogenous stimuli 3. Depending on stimulus intensity, such post-treatment can lead to structural collapse and solid-state amorphization of the framework 4-7. The formation of amorphous MOFs through solid-solid transitions (or, similarly, through quenching of MOF-liquids) is of particular interest due to the distinct variations in chemical, mechanical and physical properties which can be obtained as a result of structural disorder 8. Amorphization of MOFs can be achieved via different techniques, including pressure-induced structural collapse, ball-milling, melt-quenching, hot-pressing, and re-melting 8-10. Of these, ball-milling, or mechanosynthesis, which can also be used to synthesize crystalline MOFs, is the most universally applicable route. The low minimum shear moduli of MOFs have previously been shown to be responsible for facile collapse of systems such as UiO-66 ([Zr 6 O 4 (OH) 4 (1,4-BDC) 6 ], BDC = benzenedicarboxylate) 11. Using calcein as a model drug incorporated into crystalline UiO-66, it was demonstrated that amorphization via ball-milling leads to delayed release of the guest molecule: the timescale of release was increased from ~2 days in the crystalline structure to one month in the amorphous composite as a result of structural collapse 12. Here, we investigate how st...