Investigating CO2 Adsorption Behavior in Metal-Organic Frameworks by Solid-state NMR
Abstract
With increasing demands in controlling carbon dioxide emissions, metal-organic frameworks (MOFs) are considered as promising candidates for CO2 capture due to their large CO2 adsorption capacity. In this study, the adsorption behavior of CO2 molecules in [Zn2(TRZ)2(NH2-BDC)] (TRZ=1,2,4-triazolate, NH2-BDC=2-amino-1,4-benzenedicarboxylic acid), and CALF-20 are studied. Moreover, water is ubiquitous in the atmosphere and could have a negative impact on the MOF’s gas adsorption. Therefore, understanding the behavior and dynamics of water and CO2 in MOFs is of fundamental importance. Solid-state nuclear magnetic resonance (SSNMR) is a powerful technique which reveals guest molecules’ behavior in MOFs. Herein, [Zn2(TRZ)2(NH2-BDC)] was loaded with pure CO2 and D2O, and CALF-20 was loaded with pure CO2 in this study. 2H and 13C variable temperature SSNMR was performed to investigate the behavior of CO2 and D2O in the framework. D2O exhibits restricted motions between 173-213K and becomes more mobile with increasing temperature and increasing concentrations in [Zn2(TRZ)2(NH2-BDC)]. CO2 also shows higher mobility upon temperature increase for both MOFs. NMR results suggest that D2O undergoes a π-flip flop motion and a 4-site jumping motion in [Zn2(TRZ)2(NH2-BDC)]. Additionally, CO2 undergoes C2 hopping and C3 wobbling motion for both MOFs. Once single component adsorption was examined, co-adsorption of CO2 and water was also investigated for [Zn2(TRZ)2(NH2-BDC)].