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

Abstract

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 ; M 2+ = Co 2+ , Zn 2+ ; 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 C 3 and C 4 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.

A MOF Glass Membrane for Gas Separation

“…The pore size radius of ZIF‐62 glass (3.16 Å) was much larger than that of crystalline ZIF‐62 (2.66 Å). The result was in line with the previous report …”

“…The ZIF‐62 crystals melted at 434 °C, consistent with prior observations . The melting temperature ( T m ) of ZIF‐62 crystals can be engineered by adjusting the molar ratio of Im to Bim . Thermogravimetric (TG) analysis (Figure S1) exhibited no obvious weight loss during the whole melting process, suggesting the excellent thermal stability of ZIF‐62.…”

Unraveling the effects of linker substitution on structural, electronic and optical properties of amorphous zeolitic imidazolate frameworks-62 (a-ZIF-62) glasses: a DFT study

“…As the bIm ligands occupy much more space than the Im ligands, the porosity decreases with the increase of the bIm ligands, which is consistent with the experiments in ref. 35. And the glass network is still well preserved as shown in Fig.…”

“…In ref. 35, Frentzel-Beyme et al found that there was a minor loss of porosity upon melting and forming a-ZIF-62 glasses. And the gas (n-butane, propane and propylene) sorption isotherms decrease with increasing bIm concentration, which indicates that the pore aperture size also reduces.…”

A Co-MOF with a (4,4)-connected binodal two-dimensional topology: synthesis, structure and photocatalytic properties

“…The central ions and the organic ligand or ligand combination play the main role in the assembly of MOFs. Porous MOFs can capture guest molecules and show potential applications for storage, separation and purification (Kubas 2007;Ye et al, 2019;Frentzel-Beyme et al, 2019). Low-dimensional MOFs can construct high-dimensional supramolecular architectures via hydrogen-bonding interactions (Liu et al, 2009).…”