Adsorption of n-alkanes in ZIF-8: Influence of crystal size and framework dynamics

Abstract

Due to its exceptional chemical and thermal stability, ZIF-8 is one of the most promising representatives of nanoporous metal-organic frameworks. In this work, we investigate adsorption properties of this material both experimentally and theoretically. The experiments were carried out on 8 preparations differing in morphology of the crystals. Adsorption was studied in isothermal approach exploiting standard adsorbates, such as N 2 or CO 2 , as well as in isobaric regime with C5–C9 linear alkanes. The latter were performed with the novel quasi-equilibrated temperature-programmed desorption and adsorption (QE-TPDA) technique showing that a complexity of n-alkanes adsorption mechanism in ZIF-8 depends on the nature of adsorbate. Unexpectedly, for adsorption of C7–C9 n-alkanes a two-step process was found. QE-TPDA yielded high quality adsorption isobars which were successfully reproduced by Grand-Canonical Monte Carlo molecular simulations. The calculations showed that the specific adsorption behaviour of ZIF-8 is due to the fact that its structure undergoes conformational changes in order to adapt to the guest molecules. QE-TPDA measurements with n-nonane were performed at conditions close to saturation of the adsorbate. This allowed to observe surface-related adsorption on the ZIF-8 crystals, which was correlated with their size. • Combined theoretical and experimental studies on adsorption of C5–C9 n-alkanes in ZIF-8. • Investigations on the influence of crystal size on sorption properties. • Two-step adsorption of specific n-alkanes influenced by framework flexibility. • Packing of n-alkanes similar as for LTA-type zeolites. • Hyperbolic trend between surface adsorption amount and the size of the crystals.