Influence of Pore Size on Carbon Dioxide Diffusion in Two Isoreticular Metal–Organic Frameworks

Abstract

The rapid diffusion of molecules in porous materials is critical for numerous applications including separations, energy storage, sensing, and catalysis. A common strategy for tuning guest diffusion rates is to vary the material pore size, although detailed studies that isolate the effect of changing this particular variable are lacking. Here, we begin to address this challenge by measuring the diffusion of carbon dioxide in two isoreticular metal-organic frameworks featuring channels with different diameters, Zn2(dobdc) (dobdc 4-= 2,5dioxidobenzene-1,4-dicarboxylate) and Zn2(dobpdc) (dobpdc 4− = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate), using pulsed field gradient NMR spectroscopy. An increase in the pore diameter from 15 Å in Zn2(dobdc) to 22 Å in Zn2(dobpdc) is accompanied by an increase in the self-diffusion of CO2 by a factor of 4 to 6, depending on the gas pressure. Analysis of the diffusion anisotropy in Zn2(dobdc) reveals that the self-diffusion coefficient for motion of CO2 along the framework channels is at least 10,000 times greater than for motion between the framework channels. Our findings should aid the design of improved porous materials for a range of applications where diffusion plays a critical role in determining performance.

Buffered Coordination Modulation as a Means of Controlling Crystal Morphology and Molecular Diffusion in an Anisotropic Metal–Organic Framework

“…Morphological control of MOF crystals has direct consequences on system-level adsorbent performance. For instance, the diffusion of guest molecules within M 2 (dobdc) frameworks is highly favored along the c-axis (i.e., through the hexagonal pores shown in Figure 1), 53 and thus the methods introduced in this study to control crystal aspect ratio should enable direct control over the diffusional path length under conditions wherein the limiting mass transfer resistance is intracrystalline diffusion. Specifically, we anticipated that gas diffusion will be faster in crystals with shorter dimensions along the c-axis because those crystals would have a shorter diffusional path length (see the c-axis shown in Figure 3).…”

“…37,[39][40][41][42] The traditional solvothermal routes used to prepare these frameworks generate either significantly polycrystalline samples or long rod-like crystallites, wherein the longest crystallite direction aligns with the one-dimensional channels propagating along the c-axis. [43][44][45][46][47][48][49][50][51][52] Guest transport along the one-dimensional channels of the long crystals has been shown to be much faster than diffusion across the channels, 53 rendering much of the external surface inaccessible to molecules. Thus, developing a controlled means of synthesizing these crystals with optimal morphologies presents a highly desirable target.…”

“…The well-known series of frameworks M 2 (dobdc) (M = Mg, Mn, Fe, Co, Ni, Cu, Zn, Cd; dobdc 4– = 2,5-dioxido-1,4-benzenedicarboxylate; Figure ) − present an interesting test case in this regard. These materials feature a high density of coordinatively unsaturated metal­(II) sites that can interact strongly and selectively with a variety of guest molecules and have accordingly been demonstrated as promising adsorbents for numerous gas separations applications. ,− The traditional solvothermal routes used to prepare these frameworks generate either significantly polycrystalline samples or long rod-like crystallites, wherein the longest crystallite direction aligns with the one-dimensional channels propagating along the c -axis. − Guest transport along the one-dimensional channels of the long crystals has been shown to be much faster than diffusion across the channels, rendering much of the external surface inaccessible to molecules. Thus, developing a controlled means of synthesizing these crystals with optimal morphologies presents a highly desirable target.…”

Preferential adsorption sites for propane/propylene separation on ZIF-8 as revealed by solid-state NMR spectroscopy

“…Additionally, pulsed field gradient (PFG) NMR spectroscopy facilitates the extraction of the self-diffusion coefficients for guest molecules trapped within MOF cages. 64–71 One-dimensional (1D) and two-dimensional (2D) spin diffusion MAS NMR has been successfully utilized to establish the polarization transfer from hydrocarbons to MOFs. 72–74 Recently, spin diffusion solid-state NMR is utilized to investigate the adsorption selectivity and competitive adsorption of ethane and ethylene mixtures on ZIFs.…”

S-Mg2(dobpdc): a metal–organic framework for determining chirality in amino acids

“…The grafting of chiral 1,2-diaminocyclohexane enabled the solid-state NMR characterisation of the ammonium carbamate chains formed upon the adsorption of CO 2 to yield chemical shift differences of up to 2.0 ppm. Mg 2 (dobpdc) (and its Zn II analogue) have been extensively studied by solid-state NMR, [14][15][16][17][18][19] including in experiments that probe molecular dynamics using 15 N NMR 20 and investigations of local disorder by 25 Mg NMR. 21,22 The breadth of information concerning the behaviour of Mg 2 (dobpdc) makes it a valuable target for the study of chiral elucidation by solid-state NMR.…”