NMR relaxation and diffusion studies of methane and carbon dioxide in nanoporous ZIF-8 and ZSM-58

• Comparative study of relaxation and diffusion of CO2 and CH4 in nanopores.
• Adsorbed phase T1 and T2 in ZIF-8 exceed bulk gas phase data.
• Surface relaxivities in ZIF-8 are smaller than in ZSM-58.
• 8-Ring windows of ZSM-58 reduce CH4 self-diffusion stronger than 6-ring windows of ZIF-8.

The 1H and 13C relaxation times of methane and carbon dioxide adsorbed in nanoporous ZIF-8 increase with loading and exceed the values of the bulk gases at the corresponding equilibrium pressure. The surface relaxation for the adsorbed molecules was determined by plotting the relaxation rates as function of inverse density. Longitudinal surface relaxivities of (3.1±0.7)×10-10m·s-1 for CH4 at 125 MHz and of (1.22±0.07)×10-10m·s-1 for 13CO2 at 100.6 MHz were obtained in ZIF-8. This approach applied to zeolite ZSM-58 yields much higher, density dependent surface relaxivities of 0.7-5.4×10-8m·s-1 for methane. In ZIF-8, the self-diffusion coefficients of both gases are in the range of 10-10m2·s-1 and do not depend significantly on density. In ZSM-58, there is a two orders of magnitude difference in the diffusivities of both gases, which is caused by the small 8-ring windows of this zeolite.

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