MOFs for long-term gas storage: exploiting kinetic trapping in ZIF-8 for on-demand and stimuli-controlled gas release

In this study, we investigate the potential of metal-organic frameworks (MOFs) for long-term gas storage under ambient conditions. Specifically, we selected a MOF ZIF-8 (with a 0.34 nm large pore aperture), which exhibits a temperature- and pressure-regulated gating effect, and loaded it with sulphur hexafluoride (with a kinetic diameter of 0.55 nm). By optimising the loading conditions, we were able to achieve up to 33 wt% SF6 loading into the pores of ZIF-8. Although MOFs featuring gating effects are known to adsorb gases larger than the pore openings, herein, by applying high pressure (and optionally elevated temperature), kinetic trapping of the gas guest was also achieved. When investigating the gas release under ambient conditions, three MOF samples of different crystal sizes (ca. 45 nm, 1.5 & mu;m and 5 & mu;m) were examined. Remarkably, for the largest crystals, more than 86% of the initially loaded gas remained trapped in the pores even after being exposed to air for 100 days under ambient conditions. Our findings indicate that the extremely slow release of SF6 is due to the high activation energy for the guest diffusion through the narrow pore opening in ZIF-8, which was supported by both ab initio-based computational studies and experimental data including modulated thermogravimetric analysis. On the other hand, we also showed that the gas could be released on-demand by applying an elevated temperature or by exposing the MOF to an acidic environment, which opens possibilities for facile gas micro- and nano-dosing applications.

Automated summary

In this study, we investigated the potential of MOFs for long-term gas storage under ambient conditions. By loading a specific MOF, ZIF-8, with sulphur hexafluoride, we achieved high SF6 loading into the pores of ZIF-8 and demonstrated kinetic trapping of the gas guest. We also discovered that the slow release of SF6 from ZIF-8 was due to the high activation energy for guest diffusion.