The
interaction between hydrogen sulfide and ZIF-8 was studied
via structural characterizations and guest molecule diffusion measurements.
It was found that hydrogen sulfide reacts with the ZIF-8 external
particle surface to form a surface barrier that excludes the uptake
of larger molecules (ethanol) and slows down the uptake of smaller
molecules (carbon dioxide). Nonetheless, bulk transport properties
were unaltered, as supported by pulsed field gradient nuclear magnetic
resonance studies. Dispersion-corrected density functional theory
calculations revealed that H2S is consumed by reactions
occurring at the ZIF external surface. These reactions result in water
and defect formation, both of which were found to be exothermic and
independent of both crystallographic facets ({001} and {110}) and
surface termination. We concluded that these surface reactions lead
to structural and chemical changes to the ZIF-8 external surface that
generate surface barriers to molecular transport.
Single-file diffusion (SFD) of CO/CH 4 and CO/CO 2 mixtures as well as the corresponding pure gases in channels of polycrystalline L-Ala-L-Val dipeptide was investigated by pulsed field gradient (PFG) NMR. The measured SFD mobilities of the mixtures, where each component was found to exhibit identical diffusion behavior, were compared with the SFD mobilities of the corresponding pure gases at the same or comparable total gas concentrations. Both studied mixtures were observed to diffuse faster than the slowest pure component forming the mixture. This observed behavior is in stark contrast to the trend often reported in the case of normal diffusion in microporous materials where the addition of a faster diffusing component to a slower diffusing component does not change significantly the diffusivity of the slower diffusing component. Molecular clustering in the studied single-file channels is proposed to explain the observed relationship between the mixture and one-component mobilities and to reconcile the experimental SFD data with the predictions of a random walk model reported earlier. For sufficiently large diffusion times, this random walk model predicts a transition from the SFD to the mechanism of center-of-mass diffusion, which is characterized by concerted movements of all molecules in each channel. This transition to center-of-mass diffusion was not observed experimentally for CO/CH 4 and CO/CO 2 mixtures.
High field NMR diffusometry reveals single-file diffusion of CO/CH4 mixture in dipeptide nanochannels with a coincident mobility for CO and CH4. In contrast to the relationship commonly observed for normal diffusion, this mixture mobility is only slightly smaller than that of pure CO which diffuses much faster than pure CH4.
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