Ionic liquid facilitated melting of the metal-organic framework ZIF-8

Vahid Nozari,Thomas D Bennett,Lothar Wondraczek,Courtney Calahoo,Joshua M Tuffnell,David A Keen

doi:10.1038/s41467-021-25970-0

Abstract

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.

Highlights

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
zeolitic imidazolate framework (ZIF) are a subset of MOFs having similar topologies as those which are found in inorganic zeolites[10,11,12]
We hypothesize that melting is achieved by reducing the melting temperature of ZIF-8 to below its thermal decomposition temperature, using electrostatic interactions of the ionic liquid (IL) at the ZIF-8 pores’ interior surface in order to stabilize the rapidly dissociating ZIF-8 linkers upon heating

Summary

Results

[EMIM][TFSI] was chosen since it is a hydrophobic IL, enabling incorporation into the hydrophobic pores of ZIF-831. Ag(IL@ZIF-8-LT) contained weak diffuse scattering, alongside Bragg peaks reminiscent of the starting crystalline phase This suggests that treatment temperature and time are important parameters in the formation of IL@ZIF-8 glasses and crystal-glass composite samples, as indicated in Supplementary Fig. 3 for a range of tested synthesis conditions. The variety of electronic environments found in broad NMR peaks is assumed to be from varying bond angles and bond lengths, and strongly indicates a system without long-range order This loss of crystallinity is confirmed for the ag(IL@ZIF-8-LT), and ag(IL@ZIF-8-HT) by both XRD and pair distribution function (PDF) measurements, as well as by single-pulse 13C NMR Returning to more quantitative single-pulse 13C NMR, in Supplementary Fig. 25, we find that for the C2 carbon in the LT sample roughly 21, 58, and 21% can be identified as crystalline, amorphous, and free linker, respectively

C1: CH3 Ca

Discussion

Findings

Methods