NMR relaxation and exchange in metal–organic frameworks for surface area screening

• A porosity screening tool has been developed for metal–organic framework characterization.
• High-porosity frameworks exhibit characteristics of simultaneous relaxation and exchange.
• Kinetics models can be used to derive pore volumes for large-pore frameworks.
• The NMR-derived pore volumes agree well with Langmuir surface areas up to ∼5000 m2/g.

We describe a robust screening technique that correlates the surface area of metal–organic frameworks to the proton T2 relaxation behavior of imbibed solvent at low field (13 MHz). In frameworks with small pore sizes (<1 nm) or strong solvent-framework interactions, diffusional exchange between the pore-confined and inter-particle solvent populations remains slow compared to the T2 of the pore-confined solvent, allowing for a direct porosity analysis of the T2 spectrum obtained from Laplace inversions. Increases in framework pore-size (>1 nm) lead to corresponding increases in the rate of solvent exchange, as confirmed by T2 relaxation exchange (REXSY) experiments; increases in the pore size also increases the T2 of the pore-confined solvent. The combination of these two effects results in comparable rates of relaxation and exchange, which precludes the direct analysis of Laplace inversions. Thus, two- and three-site kinetics models were applied to extract porosity from relaxation decays, thereby improving the utility of the porosity screening tool.

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