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Thesis Format

Monograph

Degree

Master of Science

Program

Chemistry

Supervisor

Huang, Yining

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.

Summary for Lay Audience

Metal-organic frameworks (MOFs), an emerging class of solid porous materials, have exhibited enormous potential for carbon dioxide (CO2) capture and storage. In this thesis, MOF UTSA-74, a framework isomer of MOF-74, is studied as a promising material for CO2 storage. It features 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. Therefore, UTSA-74 is structurally more advantageous than MOF-74-Zn because UTSA-74 has two binding sites available per Zn2 metal ion, whereas only one binding site is accessible per Zn ion in MOF-74-Zn. This extra binding site in UTSA-74 significantly enlarges the potential in gas adsorption capacities. In this thesis, the behavior of 13CO2 in UTSA-74 at different 13CO2 loading (low, moderate, high) levels are investigated by using variable-temperature (VT) 13C static SSNMR spectroscopy because it offers the most direct insight into behavior of 13CO2 within the framework as well as the guest-host interactions. The results reveal that all 13CO2 molecules undergo localized wobbling around each Zn2 adsorption site. At low loading, some 13CO2 molecules jump among three nonadjacent 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.

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