392294 Effects of Open Metal Site Availability on Adsorption Capacity and Olefin Selectivity in Coordinatively Unsaturated Cu3(BTC)2 Metal-Organic Frameworks
Several metal-organic frameworks (MOFs) have been identified that have metal centers that are not entirely coordinated to framework oxygen atoms. These coordinatively unsaturated MOFs have open metal sites which can be exposed by removing the coordinated solvent molecules during thermal activation. These open metal sites are attractive for electronegative molecules, such as olefins. MOFs such as Cu3(BTC)2 and MOF-74 are known to separate small olefins from paraffins in the gas phase due to the π-electron interaction with the open metal sites. However, the effectiveness of the open metal sites is strongly dependent on the synthesis and pre-activation of the MOF before its use as an adsorbent. These relationships are not well understood from previous work.
In this work, we study in detail the removal of solvent molecules from Cu3(BTC)2 MOF as a function of activation temperature, quantify the exposure of open metal sites, and characterize the effects of open metal site availability on the adsorption capacity and olefin selectivity. This study was conducted in a vapor breakthrough adsorption unit with binary mixtures of hexane and 1-hexene In the case of fresh Cu3(BTC)2 samples separately activated at different temperatures, we observe a trade-off between adsorption capacity and olefin selectivity as the activation temperature is increased. However, in the case of Cu3(BTC)2 samples cycled repeatedly through adsorption and desorption steps at successively increasing activation temperatures, we observed a different effect. The adsorption capacity and olefin selectivity remained nearly constant up to a limiting activation temperature, beyond which the total adsorption capacity decreased and the olefin selectivity increased dramatically. We discuss these findings, supported by X-ray diffraction, N2 physisorption, and FTIR spectroscopy, in terms of the accessibility of the pore space and open metal sites.