Preferential adsorption sites for propane/propylene separation on ZIF-8 as revealed by solid-state NMR spectroscopy

Solid-state NMR spectroscopy in conjunction with theoretical calculation was employed to investigate the adsorbent-adsorbate host-guest interactions during propane/propylene separation on ZIF-8. 1H NMR chemical shifts of free gaseous and adsorbed propane/propylene are unambiguously assigned with the assistance of two-dimensional (2D) 1H-1H correlation spectroscopy (COSY) MAS NMR spectra. Meanwhile, the adsorption selectivity for propane/propylene mixtures on ZIF-8 at a pressure in range of 1.9-9.6 bar is quantitatively determined using 1H MAS NMR experiments, which agreed well with the ideal adsorbed solution theory (IAST) predictions. The preferential adsorption of propane compared with propylene on ZIF-8 is directly visualized from the 2D 1H-1H spin diffusion homo-nuclear correlation (HOMCOR) MAS NMR spectroscopy. Moreover, the preferential adsorption sites for propane and propylene are deduced from the 1H-1H spin diffusion buildup curves, which is further confirmed by DFT theoretical calculations. This work provides insights to understand the structure-property relationship during the propane/propylene separation on ZIF-8 as adsorbent.