Effect of Co-Adsorbed Guest Adsorbates on the Separation of Ethylene/Ethane Mixtures on Metal-Organic Frameworks with Open Metal Sites.

Direct determination of the equilibrium adsorption and spectroscopic observation of adsorbent-adsorbate interaction is crucial to evaluate the olefin/paraffin separation performance of porous adsorbents. However, the experimental characterization of competitive adsorption of various adsorbates at atomic-molecular level in the purification of multicomponent gas mixtures is challenging and rarely conducted. Herein, solid-state NMR spectroscopy is employed to examine the effect of co-adsorbed guest adsorbates on the separation of ethylene/ethane mixtures on Mg-MOF-74, Zn-MOF-74 and UTSA-74. H MAS NMR facilitates the determination of equilibrium uptake and adsorption selectivity of ethylene/ethane in ternary mixtures. The co-adsorption of HO and CO significantly leads to the degradation of ethylene uptake and ethylene/ethane selectivity. The detailed host-guest and guest-guest interactions are unraveled by 2D H-H spin diffusion homo-nuclear correlation and static Mg NMR experiments. The experimental results verify HO coordinated on open metal sites can supply a new adsorption site for ethylene and ethane. The effects of guest adsorbates on the adsorption capacity and adsorption selectivity of ethylene/ethane mixtures are in the following order: HO>CO>O. This work provides a direct approach for exploring the equilibrium adsorption and detailed separation mechanism of multicomponent gas mixtures using MOFs adsorbents.