Two open metal sites on the same metal: Dynamics of CO2 in MOF UTSA-74

Metal-organic frameworks (MOFs) are an emerging class of porous materials with many unique properties that make them promising candidates for carbon dioxide (CO2) capture and storage. A better understanding of the behavior of CO2 adsorbed inside MOF will enable researchers to develop the ability of designing new MOF based materials with high CO2 adsorption capabilities. It is well known that presence of open metal sites (OMSs) can greatly enhance the gas adsorption capability of MOFs. MOF UTSA-74, a framework isomer of well-known MOF-74 has a unique feature that upon activation, a single metal ion has two OMSs. Therefore, it presents a unique opportunity to probe the dynamics of CO2 molecules adsorbed on these OMSs. In this work, the CO2 adsorptive properties of MOF UTSA-74 are examined with particular attention being paid to the dynamics of the CO2 adsorbed on these OMSs. Specifically, variable temperature 13C static solid-state nuclear magnetic resonance (SSNMR) experiments were conducted to directly monitor the behavior of 13CO2 in UTSA-74 at different loadings. All CO2 molecules undergo localized wobbling. At low loadings, some CO2 molecules jump among three OMSs from three different Zn atoms in the cross-section of the channel. Others hop back and forth between the two neighboring OMSs. At high loading, the three-site jump has ceased, but two-site hopping persists. The dynamical behavior of CO2 in UTSA-74 results from the unique Zn coordination environment. It was discovered that CO2 is less mobile in UTSA-74 than in its framework isomer, MOF-74-Zn.