Investigating the Behavior of Adsorbed CO2 in Metal-Organic Frameworks via 13C Solid-state NMR Spectroscopy

Yingxian Li, The University of Western Ontario

Abstract

Metal-organic frameworks (MOFs) are a class of microporous materials with lots of unique properties that make them promising candidates for carbon dioxide (CO₂) capture and storage. In this thesis, the adsorption behavior of CO₂ in MOF UTSA-74 (a framework isomer of a well-known MOF, MOF-74-Zn) is studied at a molecular level as it is a promising material for CO₂ storage. It has a distinct binuclear secondary building unit (SBU) that one of metal ions (Zn1) is in a tetrahedral coordination with no binding sites, while the other one (Zn2) is in an octahedral geometry with two open metal sites (OMSs) upon activation. Explicitly, variable temperature (VT) ¹³C static solid-state nuclear magnetic resonance spectroscopy is used to investigate the behavior of ¹³CO₂ in UTSA-74 at low, moderate and high loading levels of ¹³CO₂ (i.e. 0.30, 0.54, 0.90 and 1.48 ¹³CO₂/ Zn2). The results reveal that all ¹³CO₂ molecules undergo localized wobbling. At low loading, some ¹³CO₂ molecules jump among three Zn2 OMSs in the cross-section of the channel, while others hop back and forth between the two neighbouring OMSs. At high loading, the three-site jumping has ceased, but two-site hopping persists. The dynamical behavior of ¹³CO₂ in UTSA-74 results from the unique Zn2 coordination environment. It was also discovered that ¹³CO₂ is less mobile in UTSA-74 than in its framework isomer, MOF-74-Zn.