Flux melting of metal-organic frameworks.

Louis Longley,Nicholas J Terrill,Jingwei Hou,Andrew J Smith,Cara M Doherty,,Aaron W Thornton,Seth M Cohen,Paul A Midgley,Shane G Telfer,Shichun Li,Glen J Smales,Sean M Collins,David A Keen,Ang Qiao,Ilknur Erucar,Xiao Yu,Anita J Hill,Thomas D Bennett

doi:10.1039/c8sc04044c

Abstract

Recent demonstrations of melting in the metal-organic framework (MOF) family have created interest in the interfacial domain between inorganic glasses and amorphous organic polymers. The chemical and physical behaviour of porous hybrid liquids and glasses is of particular interest, though opportunities are limited by the inaccessible melting temperatures of many MOFs. Here, we show that the processing technique of flux melting, 'borrowed' from the inorganic domain, may be applied in order to melt ZIF-8, a material which does not possess an accessible liquid state in the pure form. Effectively, we employ the high-temperature liquid state of one MOF as a solvent for a secondary, non-melting MOF component. Differential scanning calorimetry, small- and wide-angle X-ray scattering, electron microscopy and X-ray total scattering techniques are used to show the flux melting of the crystalline component within the liquid. Gas adsorption and positron annihilation lifetime spectroscopy measurements show that this results in enhanced, accessible porosity to a range of guest molecules in the resultant flux melted MOF glass.

Highlights

Porous three dimensional materials formed by the self-assembly of inorganic nodes connected by organic ligands or, as they are commonly known, metal–organic frameworks (MOFs),[1] remain of extreme interest to the scienti c community
A suitable combination was found (Fig. 1a) using: (i) the comparatively low Tm of ca. 710 K established for Zeolitic imidazolate frameworks (ZIFs)-62 [Zn(C3H3N2)1.75(C7H5N2)0.25],46 and (ii) the relatively high thermal decomposition (Td) of ZIF-8.21
Our computational approach represents the rst instance where accurate predictions for the gas adsorption performances of ZIF–ZIF crystalline mixture absorbents and ZIF–ZIF glassy ux melts have been made. These results show that the concept of ux melting, that is, the use of a molten salt as a solvent, may be applicable to MOF chemistry

Summary

Introduction

Porous three dimensional materials formed by the self-assembly of inorganic nodes connected by organic ligands or, as they are commonly known, metal–organic frameworks (MOFs),[1] remain of extreme interest to the scienti c community. Gas adsorption and positron annihilation lifetime spectroscopy measurements show that this results in enhanced, accessible porosity to a range of guest molecules in the resultant flux melted MOF glass. A sample of pure ZIF-8 was ball-milled for 5 minutes and heated to 773 K, subsequently cooled to room temperature (Fig. S3†).

Results

Conclusion