Mixed Hierarchical Local Structure in a Disordered Metal–Organic Framework

Abstract

Amorphous metal–organic frameworks (MOFs) are an emerging class of materials. However, their structural characterisation represents a significant challenge. Fe‑BTC, and the commercial equivalent Basolite® F300, are MOFs with incredibly diverse catalytic ability, yet their disordered structures remain poorly understood. Here, we use advanced electron microscopy to identify a nanocomposite structure of Fe‑BTC where nanocrystalline domains are embedded within an amorphous matrix, whilst synchrotron total scattering measurements reveal the extent of local atomic order within Fe‑BTC. We use a polymerisation-based algorithm to generate an atomistic structure for Fe-BTC, the first example of this methodology applied to the amorphous MOF field outside the well-studied zeolitic imidazolate framework family. This demonstrates the applicability of this computational approach towards the modelling of other amorphous MOF systems with potential generality towards all MOF chemistries and connectivities. We find that the structures of Fe-BTC and Basolite® F300 can be represented by models containing a mixture of short- and medium-range order with a greater proportion of medium-range order in Basolite® F300 than in Fe-BTC. We conclude by discussing how our approach may allow for high-throughput computational discovery of functional, amorphous MOFs.

Multivariate Analysis of Disorder in Metal–Organic Frameworks

“…2c]. 13,16,35,52,53 crystalline In this study we explore how PCA can be applied to characterise and quantify disorder in the MIL-100 and Fe-BTC family, with a particular focus on components that encode distortions in the structure, which have, to date, received less attention in PCA studies. By studying the distortions, we are able to directly fingerprint the motifs of structural disorder.…”

“…8,9 Characterising disorder is challenging; detecting the presence of defects or determining the structure of an amorphous framework, for example, are both non-trivial endeavours. [10][11][12][13] However, such knowledge is crucial in order to truly take advantage of the functionality that disorder can impart. Unfortunately, the use of traditional crystallographic methods -sensitive only to the average structure -are largely blind to structural disorder.…”

Gas adsorption in the topologically disordered Fe-BTC framework

“…28 More recent investigations, using the harsher drying conditions employed in this study, gave rise to powdered Fe-BTC samples that were essentially non-porous to nitrogen at 77 K, yet still retained the same local structure as MIL-100. 29 Basolite® F300, whose exact synthetic route remains undisclosed, has a BET surface area in the range 1300 to 1600 m 2 g À1 as reported by the manufacturers. Independent experimental measurements have reported the BET surface area to be around 685 to 840 m 2 g À1 , with a total pore volume of 0.29 cm 3 g À1 and pore size of 2.2 nm.…”

“…1e]. 28,29 X-ray absorption near edge structure analysis of Fe-BTC revealed the presence of octahedrally coordinated Fe(III) ions at ambient temperature. 30 Furthermore, analysis of the extended X-ray absorption ne structure region revealed Fe-BTC possessed the same trimer unit structure present in MIL-100.…”

“…Synchrotron X-ray pair distribution function analysis revealed the presence of mixed hierarchical local structure within Fe-BTC, conrming the trimer unit's existence and the presence of tetrahedral assemblies as observed in MIL-100. 29 This characterisation was used to produce the rst atomic-scale model of Fe-BTC.…”