13C NMR study of diffusion anisotropy of carbon dioxide adsorbed in nanoporous DMOF-1

• CO2 mobility in nanochannels of MOF Zn2(bdc)2dabco (DMOF-1) observed by 13C NMR.
• Residual 13C chemical shift represents orientation of DMOF-1 channels with respect to B→0.
• Orientational dependent signal attenuation by pulsed field gradients gx and gz.
• Axisymmetrical diffusion tensor, CO2 diffusion anisotropy D‖/D⊥≈3D‖/D⊥≈3.

DMOF-1 (Zn2(bdc)2dabco) is a metal–organic framework with tetragonal symmetry. It contains parallel one-dimensional channels with a diameter of about 0.75nm interconnected by smaller windows. For carbon dioxide adsorbed in the nanochannels the chemical shift tensor and the diffusion tensor were investigated by 13C NMR. The spectra acquired under static conditions reveal a powder pattern with a residual chemical shift anisotropy of 〈Δδ〉=-55〈Δδ〉=-55 ppm. The collinearity of the axisymmetrical residual chemical shift and the diffusion tensors was utilized to assess the diffusion anisotropy via the diffusion attenuation of the 13C PFG NMR powder pattern. The results show that carbon dioxide is highly mobile in DMOF-1. The apparent diffusion coefficient (the trace of diffusion tensor) is (6.2±1.0)×10-9m2s-1 at 298 K and the corresponding anisotropy expressed as D‖/D⊥D‖/D⊥ is approximately equal to 3.

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