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

Abstract

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 CO within the pores of Zn(dobpdc) (dobpdc = 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 CO 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 CO diffusion through the channels parallel to the crystallographic c axis (self-diffusion coefficient D = (5.8 ± 0.1) × 10 m s at a pressure of 625 mbar CO), we unexpectedly find that CO is also able to diffuse between the hexagonal channels in the crystallographic ab plane (D = (1.9 ± 0.2) × 10 m s), 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 CO transport in a metal-organic framework with nominally one-dimensional porosity.

Revisiting Anisotropic Diffusion of Carbon Dioxide in the Metal-Organic Framework Zn2(dobpdc)

“…18,48−56 By analyzing the line shape changes as a function of the applied pulsed field gradient, we obtained values for the self-diffusion parallel (D ∥ ) and perpendicular (D ⊥ ) to the MOF channels. 22 The obtained nonzero value of D ⊥ was unexpected given the onedimensional channels in the crystal structure obtained by single-crystal X-ray diffraction, and we suggested that structural defects are responsible. In our previous work we noted, "The dominant source of error in our analysis is most likely systematic error arising from the finite peak line widths that arise from homogeneous broadening, such that for a given δ, crystals at a range of θ values contribute to the signal," 22 where θ is the angle between the crystallographic c-axis and the applied magnetic field, B 0 .…”

“…We recently used this latter approach to study the anisotropic diffusion of CO 2 in the MOF Zn 2 (dobpdc) ( Figure 1) (dobpdc 4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), 22 a material from a wider MOF family that has shown promise for CO 2 capture from power station flue gases. 2,38−47 The residual chemical shift anisotropy for pore-confined CO 2 rendered the chemical shift dependent on crystal orientation, 22 as in several other NMR studies of CO 2 in MOFs. 18,48−56 By analyzing the line shape changes as a function of the applied pulsed field gradient, we obtained values for the self-diffusion parallel (D ∥ ) and perpendicular (D ⊥ ) to the MOF channels.…”

Solid‐State NMR Spectroscopy: A Powerful Technique to Directly Study Small Gas Molecules Adsorbed in Metal–Organic Frameworks

“…Long‐ and short‐range CO 2 dynamics can both be studied using SSNMR spectroscopy. Even though long‐range studies can provide important information on diffusivity, local CO 2 dynamics has been the main focus for many of the recent SSNMR investigations. Hence, we restrict our current discussion to local CO 2 dynamics studies.…”

“…MOFs are promising CO 2 adsorbents, [13] and SSNMR spectroscopy has been employed to studyC O 2 chemisorptiona nd physisorption in severalM OFs. [12] This section focuses on physisorptiona nd demonstrates how guest dynamics, [14][15][16][17][18][19][20][21][22] MOF-CO 2 interaction strengths [16] and binding sites [16,17] can be investigated.…”

“…Long-and short-range CO 2 dynamics can both be studied using SSNMR spectroscopy.E ven thoughl ong-ranges tudies can provide important information on diffusivity, [19,22] local CO 2 dynamics has been the main focus for many of the recent SSNMR investigations.Hence, we restrict our currentd iscussion to local CO 2 dynamics studies. Various SSNMR experiments (e.g.,e xchange spectroscopy, [20] T 1 relaxation studies, [21] variable-temperature (VT) experiments [14][15][16]18] )h ave been used to probe CO 2 motions, and static VT 13 Ce xperiments are one of the most commonly employed techniques.…”