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

  • Author(s): Chen, JJ
  • Mason, JA
  • Bloch, ED
  • Gygi, D
  • Long, JR
  • Reimer, JA
  • et al.

We describe a robust screening technique that correlates the surface area of metal-organic frameworks to the proton T 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 T of the pore-confined solvent, allowing for a direct porosity analysis of the T 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 T relaxation exchange (REXSY) experiments; increases in the pore size also increases the T 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. 2 2 2 2 2

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