Investigating the structure of Metal-Organic Frameworks and the behaviors of adsorbed guest molecules in MOFs via Solid-state NMR Spectroscopy

Yihao Shen, The University of Western Ontario

Abstract

Metal-organic frameworks (MOFs) are a class of hybrid organic-inorganic microporous materials with lots of significant applications including gas capture and storage, gas separation, batteries, catalysis, ion exchange and drug delivery. MOFs based on s-block metals are important due to their many practical applications. These MOFs are also very interesting because the s-block MOFs often exhibit unpredictable behavior of coordination. In this thesis, the structures of six s-block metal (Li, Na, Cs, Mg, Ca, Sr) based MOFs involving V-shaped link, 4, 4′- sulfonyldibenzoate (SDB) are characterized by various techniques including, powder X-ray diffraction, TGA and solid-state NMR spectroscopy. ¹³C CPMAS NMR was used to probe the local structure around SDB linkers. ⁷Li, ²³Na, ¹³³Cs, and ²⁵Mg SSNMR were utilized to directly characterize the metal center surroundings. Explicitly, variable temperature (VT) ¹³C SSNMR spectroscopy is used to investigate the behavior of ¹³CO₂ in four selected MOFs with porous channels. The analysis of ¹³C static spectra reveals that CO₂ molecules undergo localized wobbling and non-localized hopping within these MOFs. Additionally, all these s-block MOFs based on SDB share the common adsorption site (π-pocket of SDB ligands). Moreover, another novel MOF, UTSA-280, with Ca center and C₄O₄ ligands is also studied. Based on its rigid channel size, UTSA-280 shows a high separation performance of CO₂ from CH₄ and C₂H₄ from C₂H₆ under ambient condition. In this thesis, the behaviors of ¹³CO₂ and C₂D₂ within UTSA-280 are investigated via VT ¹³C wide-line SSNMR experiments and VT ²H wide-line SSNMR experiments, respectively. The results show a tight guest-host interactions. Meanwhile, with the same ligands as UTSA-280, MOF (Zn(bipy)(C₄O₄)(H₂O)₂) is briefly studied as well.