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 (T1 rho) by solid-state NMR. Rotational correlation times (tau(C)) were also estimated using spin relaxation models based on the Bloch, Wangsness, and Redfield and the Bloembergen-Purcell-Pound theories. As expected, the tau(C )values for the two physisorbed CO2 species are considerably shorter (32 and 20 mu s) than for the three identified chemisorbed CO2 species (162, 62, and 123 mu s). 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 tau(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-1 rho and tau(C) values compatible with a CO2 layer of weakly interacting molecules with the silica surface. The values for chemical shift anisotropy (CSA) and H-1-C-13 heteronuclear dipolar couplings have also been estimated from T-1 rho 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.



subject category

Chemistry; Science & Technology - Other Topics; Materials Science


Fonseca, R; Vieira, R; Sardo, M; Marin-Montesinos, I*; Mafra, L*

our authors


This work was developed within the scope of the project CICECO-Aveiro Institute of Materials, UIDB/50011/2020 and UIDP/50011/2020, financed by national funds through the Portuguese Foundation for Science and Technology/MCTES. We also acknowledge funding from the project PTDC/QUI-QFI/28747/2017 (GAS2MAT-DNPSENS-POCI-01-0145-FEDER-028) , financed through FCT/MEC and cofinanced by FEDER under the PT2020 Partnership Agreement. The NMR spectrometers are part of the National NMR Network (PTNMR) and are partially supported by Infrastructure Project 022161 (cofinanced by FEDER through COMPETE 2020, POCI, PORL, and FCT through PIDDAC) . This work has received funding from the European Research Council (ERC) under the European Union?s Horizon 2020 Research and Innovation Program (grant agreement 865974) . FCT is also acknowledged by R.V. for a Junior Researcher Position (CEECIND/02127/2017) and by M.S. for an Assistant Research Position (CEECIND/00056/2020) .

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