Exploring Molecular Dynamics of Adsorbed CO 2 Species in Amine-Modified Porous Silica by Solid-State NMR Relaxation.

Previous studies on CO 2 adsorbents have mainly addressed the identification and quantification of adsorbed CO 2 species in amine-modified porous materials. Investigation of molecular motion of CO 2 species in confinement has not been explored in depth yet. This work entails a comprehensive study of molecular dynamics of the different CO 2 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 CO 2 species are considerably shorter (32 and 20 μs) than for the three identified chemisorbed CO 2 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 CO 2 species, the τ C values of the CO 2 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 CO 2 layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and 1 H- 13 C heteronuclear dipolar couplings have also been estimated from T 1ρ measurements, for each adsorbed CO 2 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 CO 2 dynamics in silica surfaces.