Exploring Molecular Dynamics of Adsorbed CO₂ Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation

Previous studies on CO₂ adsorbents have mainly addressed the identification and quantification of adsorbed CO₂ species in amine-modified porous materials. Investigation of molecular motion of CO₂ species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO₂ species chemi- and physisorbed at amine-modified silica materials through the determination of the rotating frame spin-lattice relaxation times (T _1ρ) by solid-state NMR. Rotational correlation times (τ_C) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen-Purcell-Pound theories. As expected, the τ_C values for the two physisorbed CO₂ species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO₂ species (162, 62, and 123 μs). The differences in molecular dynamics between the different chemisorbed species correlate well with the structures previously proposed. In the case of the physisorbed CO₂ species, the τ_C values of the CO₂ species displaying faster molecular dynamics falls in the range of viscous liquids, whereas the species presenting slower dynamics exhibit T _1ρ and τ_C values compatible with a CO₂ layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and ¹H-¹³C heteronuclear dipolar couplings have also been estimated from T _1ρ measurements, for each adsorbed CO₂ species. The CSA tensor parameters obtained from fitting the relaxation data agree with the experimentally measured CSA values, thus showing that the theories are well suited to study CO₂ dynamics in silica surfaces.