Unexpected Diffusion Anisotropy of Carbon Dioxide in the Metal-Organic Framework Zn₂(dobpdc)

Metal-organic frameworks are promising materials for energy-efficient gas separations, but little is known about the diffusion of adsorbates in materials featuring one-dimensional porosity at the nanoscale. An understanding of the interplay between framework structure and gas diffusion is crucial for the practical application of these materials as adsorbents or in mixed-matrix membranes, since the rate of gas diffusion within the adsorbent pores impacts the required size (and therefore cost) of the adsorbent column or membrane. Here, we investigate the diffusion of CO₂ within the pores of Zn₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) using pulsed field gradient (PFG) nuclear magnetic resonance (NMR) spectroscopy and molecular dynamics (MD) simulations. The residual chemical shift anisotropy for pore-confined CO₂ allows PFG NMR measurements of self-diffusion in different crystallographic directions, and our analysis of the entire NMR line shape as a function of the applied field gradient provides a precise determination of the self-diffusion coefficients. In addition to observing CO₂ diffusion through the channels parallel to the crystallographic c axis (self-diffusion coefficient D_∥ = (5.8 ± 0.1) × 10^-9 m² s^-1 at a pressure of 625 mbar CO₂), we unexpectedly find that CO₂ is also able to diffuse between the hexagonal channels in the crystallographic ab plane (D_⊥ = (1.9 ± 0.2) × 10^-10 m² s^-1), despite the walls of these channels appearing impermeable by single-crystal X-ray crystallography and flexible lattice MD simulations. Observation of such unexpected diffusion in the ab plane suggests the presence of defects that enable effective multidimensional CO₂ transport in a metal-organic framework with nominally one-dimensional porosity.