Design, Synthesis, and Characterisation of Metal–Organic Framework Crystal–Glass Composites (MOF CGCs)

Abstract

Metal–organic frameworks (MOFs) are a highly topical class of three-dimensional porous materials proposed for applications such as gas storage, separations, and catalysis. Typically, MOFs are synthesised as microcrystalline powders of nanometer- to millimetre-sized particles ill-suited to industrial settings without prior processing. However, recent research has revealed solid-liquid transitions within the family, which is used here to create a class of functional, stable and porous composite materials. Described herein is the design, synthesis and characterisation of MOF crystal–glass composites formed by dispersing crystalline MOFs within a MOF glass matrix. The first of these novel materials incorporates MIL 53 within a ZIF 62 glass matrix where the crystalline phase’s coordinative bonding and chemical structure are preserved. Whilst the phases are separated, the interfacial interactions between the proximate microdomains improve the mechanical properties of the glass composite. More significantly, the high-temperature, open-pore phase of MIL 53, which spontaneously transforms to a narrow pore phase upon cooling in the presence of water, is stabilised at room temperature in the crystal–glass composite. This leads to a significant improvement in CO2 adsorption capacity. This enhancement is further explored and maximised by synthesising a compositional series of composites. The distribution and integrity of the crystalline component in this series were determined, and these findings were used to identify the maximum crystalline loading and maximum CO2 adsorption capacity. In addition to the study of MIL 53, other MOF crystal-glass composite (MOF CGC) systems were explored, and the thermal stability considerations in the formation of MOF CGCs are highlighted. Resultantly, two separate MOFs were identified, MIL 118 and UL MOF 1, with which MOF CGCs were successfully synthesised. These new materials, alongside the prototypical MOF CGC, formed using MIL 53, were studied using scanning electron microscopy, powder X-ray diffraction, and gas sorption techniques to reveal an approximate kinetic diameter limitation of gases that may permeate through the glass matrix. Furthermore, the thermal expansion behaviour of these three MOF CGCs was investigated. Specifically, variable-temperature powder X-ray diffraction data and thermomechanical analysis show the suppression of thermal expansivity in each of these three crystalline MOFs when suspended within a ZIF 62 glass matrix. In particular, for the two flexible frameworks, the average volumetric thermal expansion (αV) was found to be near-zero in the crystal–glass composite.