Modelling the effect of defects and disorder in amorphous metal−organic frameworks

Abstract

Amorphous metal−organic frameworks (aMOFs) are a class of disordered framework materials with a defined local order given by the connectivity between inorganic nodes and organic linkers, but absent longer-range order. The rational development of function for aMOFs is hindered by our limited understanding of the underlying structure-property relationships in these systems, a consequence of the absence of long-range order, which makes experimental characterization particularly challenging. Here, we use a versatile modelling approach to generate in-silico structural models for an aMOF based on Fe trimers and 1,3,5-benzenetricarboxylate (BTC) linkers, Fe-BTC. We build a phase space for this material that includes nine amorphous phases with different degrees of defects and local order. These models are analyzed through a combination of structural analysis, pore analysis and pair distribution functions. Therefore, we are able to systematically explore the effects of the variation of each of these features, both in isolation and combined, for a disordered MOF system, something that would not be possible through experiment alone. We find that the degree of local order has a greater impact on structure and properties than the degree of defects. The approach presented here is versatile and allows for the study of different structural features and MOF chemistries, enabling the development of design rules for the rational design of aMOFs.

Mapping nanocrystalline disorder within an amorphous metal–organic framework

“…Using this knowledge we produced several candidate models for the average structure of Fe-BTC using an adapted polymerisation algorithm. 33,34 We found that a model comprised solely of trimer units did not account for the medium-range order in Fe-BTC, while a model built entirely from tetrahedral assemblies overestimated the porosity. Instead, a model containing a mixture of both individual trimer units and assembled tetrahedra captured our experimental results best [Supplementary Figs.…”

“…The slight reduction in peak intensity of some peaks in the ePDF of Fe-BTC compared to those in MIL-100 is consistent with its more defective nature, particularly for peaks that are dominated by Fe-O correlations (2.0 Å, 3.3 Å). 34 To obtain a representative ePDF from the corresponding crystalline domains in Fe-BTC, sufficient orientations would need to be sampled within the data. Unfortunately, obtaining a fully representative crystalline ePDF was not possible due to limited crystal orientations observed in Fe-BTC.…”

Computational modeling to assist in the discovery of supramolecular materials

“…[70][71][72] We have also recently extended our approach to the simulation of the structures of amorphous MOFs, allowing an explanation of the level of disorder and defects through comparison to experimental scattering measurements 73,74 and unique insight into the effect of defects and disorder on properties that would not be possible through experiment alone. 75 Beyond insight into structure-property relationships in porous polymer membranes, we would ideally be able to rapidly predict properties from knowledge of the monomer structures alone, rather than need to first predict structure. While one can imagine the use of ML for this task, we do not currently have the scale of data needed available to train these models.…”

“… 70 , 71 , 72 We have also recently extended our approach to the simulation of the structures of amorphous MOFs, allowing an explanation of the level of disorder and defects through comparison to experimental scattering measurements 73 , 74 and unique insight into the effect of defects and disorder on properties that would not be possible through experiment alone. 75 …”