(687d) Methane Adsorption on Zeolitic Imidazolate Framework-8 (ZIF-8)

ZIFs constitute a class of MOFs that have a high degree of thermal and chemical stability.¹ This makes them appealing candidates for practical application. In this study we report on methane adsorption on Zeolitic Imidazolate Framework-8 (ZIF-8) at low temperatures.

ZIF-8 is a porous MOF that has the sodalite structure. Structural studies at room temperature and high pressures, using a diamond anvil cell, found that at 1.47 GPa ZIF-8 undergoes a structural phase transition.² What occurs is that the pressure-transmitting fluid is forced into the ZIF-8 resulting in a reorientation (gate-opening) of the imidazolate windows in the ZIF-8 that increases their size. The larger windows allow more fluid to get into the ZIF-8; this additional fluid produces an expansion of the ZIF-8. ZIF-8 at 1.47 GPa has the same space-group symmetry that it had before any fluid was loaded in it.² Fairen-Jimenez and his collaborators conducted a combined structural, adsorption isotherm, and computer simulation study of N₂ sorption in ZIF-8 at 77 K.³ Using in situ XRD measurements they found that the same gate-opening transition that was observed in ZIF-8 at room temperature and high pressures was also present upon N₂ sorption in the ZIF-8 at low temperatures (77 K) and much lower pressures³. The 77 K N₂ in ZIF-8 isotherm exhibits a substep (i.e. a region of sharp increase in the loading with little pressure change in the isotherm). The GCMC simulations agree with the adsorption isotherm data only if the structure for the empty ZIF-8 (i.e., the as-produced or AP structure) is used for the simulation of the lower loadings, and, the structure of ZIF-8 after the transition at room temperature (i.e., the high-pressure or HP structure) is used for the simulation of the higher loadings. The isotherm substep falls between these two regimes; hence, the substep was associated with the occurrence of the structural transition.³

Several other gases (including Ar,⁴ Xe,⁵ CO^4,6 and O₂^4,7) sorbed in ZIF-8 have substeps in their adsorption isotherms measured at low temperatures; while other sorbents, e.g., CH₄⁸ and CO₂⁹, do not exhibit substeps in the isotherms that have been reported. The nature of the substeps present in the adsorption isotherms for some sorbates in ZIF-8 has been the subject of much study and some controversy. ^1,3,10

Methane was one of the first sorbates studied in ZIF-8. Yildirim’s group reported adsorption isotherms for temperatures between 125 and 300 K and pressures of up to 65 bar.⁸ Fairen-Jimenez’ group compared Yildirim’s experimental data at 125 K to their GCMC computer simulations.¹¹ Fairen-Jimenez used both the AP and the HP structures of ZIF-8 in their simulations. They found that the best agreement between the experimental data and simulations was obtained when the AP-structure was used for lower loadings and the HP structure was used for higher loadings. This led Fairen-Jimenez to conclude that gate-opening was occurring for CH₄ in ZIF-8, even if there were no subteps in the isotherm data.¹¹

For Xe adsorbed in ZIF-8 we found that there are substeps associated with gate-opening in isotherms measured at low temperatures, and that as the isotherm temperature increases eventually the substeps are no longer present.⁵ The relative pressure of the substeps (i.e. P_substep/P_o where P_o is the saturated vapor pressure and P_substep is the pressure at the midpoint of the substep) increases as the isotherm temperature increases, eventually reaching P_substep/P_o = 1 (i.e., no substep in the isotherm).⁵ We note that for Xe adsorbed in ZIF-8 other experimental techniques show that gate-opening is occurring at the higher temperatures.^12,13 We wanted to explore whether a similar behavior was present in CH₄ adsorbed in ZIF-8; so we explored CH₄ adsorption in ZIF-8 at temperatures lower than those which had been previously studied.

We will present the results of low temperature adsorption isotherms of methane in ZIF-8 over a range of temperatures between 85 K and 107 K. We have found that all five isotherms measured (85.00 K, 92.12 K, 97.49 K, 106.31 K and 107.37 K) exhibit substeps. The loading interval for the substep for all five isotherms was approximately 2000 μmoles/g. We have observed that the relative pressure of the substep approaches 1 as the isotherm temperature increases.

In addition to presenting results for the isotherm characteristics as a function of temperature we will present the loading dependence of the isosteric heat of adsorption for CH₄ in ZIF-8. At saturation the isosteric heat of adsorption for a system should be equal to the bulk latent heat for the corresponding transition. We have verified that this is the case for our CH₄ adsorption data on ZIF-8; the value of the isosteric heat of adsorption at saturation agrees with methane’s bulk latent heat of vaporization.¹⁴

We have also explored the kinetics of adsorption for methane in ZIF-8 as a function of loading. Results on the loading dependence of the equilibration time for a fixed temperature will be presented.

We will compare our results for methane with those that others and us have obtained for the sorption of other gases in ZIF-8.

References

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14. https://webbook.nist.gov/cgi/cbook.cgi?ID=C74828&Mask=4#Thermo-Phase