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

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– = 2,5-dioxidobenzene-1,4-dicarboxylate) and Zn₂(dobpdc) (dobpdc⁴⁻ = 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.