Two open metal sites on the same metal: Dynamics of CO2 in MOF UTSA-74
Yingxian Li, Wanli Zhang, Yining Huang
Abstract
Metal-organic frameworks (MOFs) are an emerging class of porous materials with many unique properties that make them promising candidates for carbon dioxide (CO 2 ) capture and storage. A better understanding of the behavior of CO 2 adsorbed inside MOF will enable researchers to develop the ability of designing new MOF based materials with high CO 2 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 CO 2 molecules adsorbed on these OMSs. In this work, the CO 2 adsorptive properties of MOF UTSA-74 are examined with particular attention being paid to the dynamics of the CO 2 adsorbed on these OMSs. Specifically, variable temperature 13 C static solid-state nuclear magnetic resonance (SSNMR) experiments were conducted to directly monitor the behavior of 13 CO 2 in UTSA-74 at different loadings. All CO 2 molecules undergo localized wobbling. At low loadings, some CO 2 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 CO 2 in UTSA-74 results from the unique Zn coordination environment. It was discovered that CO 2 is less mobile in UTSA-74 than in its framework isomer, MOF-74-Zn.