Understanding The Fascinating Origins of CO2 Adsorption and Dynamics in MOFs

Metal–organic frameworks (MOFs) have shown great promise for the adsorption and separation of gases, including the greenhouse gas CO2. In order to improve performance and realize practical applications for MOFs as CO2 adsorbents, a deeper understanding of the number and type of CO2 adsorption mechanisms must be unlocked, along with fine details of CO2 motion within MOFs. Using several complementary characterization methods is a promising protocol for comprehensively investigating the various host–guest interactions between MOFs and CO2. In this work, a combination of solid state NMR (SSNMR) and single crystal X-ray diffraction (SCXRD) has been utilized to reveal both the location and dynamics of adsorbed CO2 within the related PbSDB and CdSDB MOFs, as well as to probe the role of metal centers in CO2 adsorption. 13C SSNMR experiments targeting CO2 reveal the number of unique adsorption sites and the types of CO2 dynamics present, as well as their associated motional rates and angles. 111Cd and 207Pb SSNMR methods are used to probe the influence of CO2 adsorption on the MOF metal centers, and also to investigate the possibility of any metal–guest interactions. SCXRD experiments yield the exact locations and occupancies of adsorbed CO2 in both MOFs; by pairing this information with SSNMR data, a comprehensive model of CO2 adsorption and dynamics in PbSDB and CdSDB has been established. Both MOFs share a common adsorption site in the V-shaped "π-pocket" formed by the phenyl rings of an individual V-shaped organic linker, while CdSDB also features an additional π-pocket adsorption site arising from the phenyl rings of two linkers joined by Cd. SCXRD and SSNMR data indicate that CO2 adsorbed at the SDB-based π pocket in both MOFs exhibits a local rotation or "wobbling" at an individual adsorption site, as well as a nonlocalized jumping or "hopping" between symmetry-equivalent adsorption sites. The combined analysis of SCXRD and SSNMR data has the potential to yield rich information regarding guest dynamics, adsorption locations, and host–guest interactions in many MOFs.