Ligand exchange post-processing is a classic strategy for precise structural control of synthesized MOFs. Studies have shown that the core-shell microstructure observed under certain conditions is mainly due to the faster ligand exchange rate than the diffusion rate in MOF.

In view of this, Adam J. Matzger and others at the University of Michigan in Ann Arbor took MOF-5 as the research object, systematically studied the influence of different solvent choices on the morphology of the core-shell structure, and developed corresponding control strategies.





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In particular, the researchers specifically studied the process in which terephthalic acid was replaced by deuterated terephthalate (bdc-d4) during the ligand exchange process from MOF-5→MOF-5-d4, and demonstrated Micro Raman spectroscopy systematically characterized the obtained structure.





Figure 1. Schematic diagram of ligand exchange



Researchers investigated the exchange performance of MOF ligands in 30 organic solvents, including amides (DMF, N,N-diethylformamide, N,N-dibutylformamide, N-methyl-2-pyrrolidone, 1-methylpyrrolidine, 1-formylpiperidine, N-formylmorpholine, N,N-dimethylacetamide, N-methylformamide and N,N-diethyl-3-methyl Benzamide), aldehyde/ketone (butanone, cyclohexanone, benzaldehyde); ether (THF, 2,5-dimethyltetrahydrofuran, dioxane, dibutyl ether, methyl tert-butyl ether, anisole) , 1,2-Dimethoxyethane and 2-methoxyethanol), esters (ethyl acetate, butyl acetate, tert-butyl acetate), carbonates (propylene carbonate and dimethyl carbonate), DMSO, sulfolane and acetonitrile. Before analysis, the solvent was shaken on the activated 4 Å molecular sieve for at least 24 h to remove the effects of trace water.



Figure 2. 30 solvents



Figure 3. The influence of 30 solvents



After a preliminary screening, the researchers narrowed the solvent range to 13 for more detailed testing. In short, the author synthesized and washed MOF, then matured in a 10 mM H2bdc-d4 solution for 24 h for ligand exchange, then activated and cleaved the MOF crystal, and characterized by Raman microscopy to achieve Visual distribution of ligand exchanges in the final structure. The effect of different solvents on the degree of exchange of the shell (0% means no exchange, 100% means complete exchange) and thickness (in the form of the full width at half maximum of the shell, FWHM) is clarified.



In order to explore the effect of mixed solvents on the ligand exchange of MOF-5→MOF-5-d4, the researchers also tested several binary solvent mixtures in the same way.



Figure 4. Relationship between maximum exchange degree and shell thickness





Figure 5. The effect of binary mixed solvents





These discoveries can provide new ideas and insights for adjusting the microstructure of MOF, and help MOF materials to be applied in more different fields.





references:

RyanA. Dodson, et al, Solvent Choice in Metal−Organic Framework Linker ExchangePermits Microstructural Control, J. Am. Chem. Soc., 2020

DOI: 10.1021/jacs.0c10224

https://pubs.acs.org/doi/10.1021/jacs.0c10224


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