CO Guest Interactions in SDB-Based Metal-Organic Frameworks: A Solid-State Nuclear Magnetic Resonance Investigation

Metal-organic frameworks (MOFs) are promising materials for greener carbon monoxide (CO) capture and separation processes. SDB-based (SDB = 4,4^'-sulfonyldibenzoate) MOFs are particularly attractive due to their remarkable gas adsorption capacity under humid conditions. However, to the best of our knowledge, their CO adsorption abilities have yet to be investigated. In this report, CO-loaded PbSDB and CdSDB were characterized using variable-temperature (VT) ¹³C solid-state nuclear magnetic resonance (SSNMR) spectroscopy. These MOFs readily captured CO, with the adsorbed CO exhibiting dynamics as indicated by the temperature-dependent changes in the SSNMR spectra. Spectral simulations revealed that the CO simultaneously undergoes a localized wobbling about the adsorption site and a nonlocalized hopping between adjacent adsorption sites. The wobbling and hopping angles were also found to be temperature-dependent. From the appearance of the VT spectra and the extracted motional data, the CO adsorption mechanism was concluded to be analogous to that of CO₂. To gain a better understanding on the gas adsorption properties of these MOFs and their CO capture abilities, we subsequently compared the motional data to those reported for CO₂ in SDB-based MOFs and CO in MOF-74, respectively. A significant contrast in adsorption strength was observed in both cases because of the different physical properties of the guests (i.e., CO vs CO₂) and the MOF frameworks (i.e., SDB-based MOFs vs MOFs with open metal sites). Our results demonstrate that SSNMR spectroscopy can be employed to probe variations in binding behavior.