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)
(dobdc4– = 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 10000 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.