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 (τC) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen-Purcell-Pound theories. As expected, the τC 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 τC 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 τC 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.publishe