QUANTITATIVE ANALYSIS OF THE ELECTRON BEAM AND THERMAL DEGRADATION BEHAVIOUR OF METAL-ORGANIC FRAMEWORKS

Abstract

Metal-organic frameworks (MOFs) are an emerging class of crystalline materials with an ordered, intrinsic network of pores and channels as part of their structure. Their tuneable pore sizes, catalytically active sites, capacity for ion exchange, and ability to host guest molecules make them promising candidates for use in gas storage, fuel cells, catalysis, drug delivery, and many other applications. As their pore structures are critical to their function, characterisation techniques that probe their porosity, crystal structure, chemical environment, and stability are crucial. Traditionally, this is done using thermogravimetric analysis (TGA), gas adsorption isotherms, x-ray and neutron diffraction (XRD and ND), or various spectroscopic techniques. However, these techniques can provide only information averaged over bulk volumes of the sample. The emerging awareness of the importance of localised defects to the functionality of MOFs is also something that the aforementioned techniques will struggle to probe in much detail. Transmission electron microscopy (TEM) and its derivative techniques are able to provide localised high-resolution information about these materials, allowing for their pore and surface structures, local defects and domains, and the distribution of guest molecules to be studied. However, MOFs are highly susceptible to electron beam damage, making characterisation a difficult task. While technologies and techniques have been developed over this past decade that have allowed for more successful examples of TEM imaging of these materials, little progress has been made in understanding their damage mechanisms under electron beam irradiation. In this study, we quantitatively measured the electron beam stability of an isostructural family of MOFs, varying only the metal cation used (Al, Ga, In, and Cr). Electron diffraction patterns of each MOF were acquired until complete amorphisation was observed, and the change in diffraction intensity was measured as a function of the total electron exposure. A critical threshold was defined and extracted, then compared across crystals of different widths and for the different metal cations used. We found that the reflections in the diffraction pattern decayed anisotropically, with those corresponding to the metal cation chains being the most stable. This indicates that the nature of the bond in the MOF determines the stability of the structures associated with them. Thickness only had a weak effect on the critical threshold, indicating that other factors take precedence. For the different cations probed. Cr was the most stable, followed by Al, Ga, and then In, with the latter three showing that MOFs decreased in stability moving down the elements within the same group in the periodic table. When compared against their thermal stability as measured from temperature-programmed x-ray diffraction and TGA, however, we found that the order of stability differs from the electron beam stability, with Al being the most stable, followed by Cr, Ga, and then In. This change in order shows that thermal degradation and electron degradation occur via different pathways, and have to be considered separately. Instead, we observed that electron beam stability correlates well with the cation’s inertness in solution and with their structural stability under constant photocatalytic use, potentially indicating that electron beam stability and cation inertness can be used as predictors of MOF catalytic stability.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Sarah Haigh (Supervisor) & Grace Burke (Supervisor)