Determination of Locations and Dynamics of Adsorbed CO₂ in MIL-53(Al) Using Solid-State Nuclear Magnetic Resonance Analysis and Theoretical Calculations

Understanding the phenomena of CO₂ adsorption in porous materials is crucial for designing new materials and developing applications for gas separation. By combining molecular motion analysis using solid-state nuclear magnetic resonance (NMR) spectroscopy with CO₂-loaded structural prediction by density functional theory (DFT) calculations, we investigated the locations and dynamics of adsorbed CO₂ in a flexible metal-organic framework (MOF), MIL-53(Al), exhibiting a narrow pore (np)-large pore (lp) structural transition upon CO₂ sorption. We analyzed the CO₂ dynamics by ¹³C NMR line shape simulations based on CO₂ locations in the DFT-calculated structures. Our analysis revealed that two types of CO₂-loaded pore structures coexist in the np form and four CO₂ species are captured by MOF-CO₂ and CO₂-CO₂ interactions in the lp form. CO₂ undergoes a hopping motion among the adsorbate locations. Additionally, the CO₂ wobbling motion was evaluated using ¹³C line shape analysis and molecular dynamics simulations.