Adsorption and Activation of CO2 by Amine-Modified Nanoporous Materials Studied by Solid-State NMR and 13CO2 Adsorption

The interaction of gaseous CO2 with the surface of amine-modified nanoporous clays has been studied. CO2 adsorption and adsorption microcalorimetry revealed high adsorption capacity and strong interaction with the surface at low pressures, due to the presence of amine groups. Considerable surface heterogeneity and high initial adsorption heat (125 kJ mol(-1)) have been observed, although the adsorption was reversible with hysteresis at low:pressures and very slow desorption kinetics. Interaction between (CO2)-C-13 and the surface of the nanoporous clay materials has been investigated by C-13 and N-15 magic-angle spinning (MAS) NMR. C-13 NMR resonances at ca. 164 and 160 ppm have been assigned to, respectively, carbamate and carbamic acid, and the stability of these species have been studied. Peak areas and the amount of (CO2)-C-13 adsorbed allowed the determination of the concentration of carbamate and carbamic acid. To the best of our knowledge, this is the first time that solid-state NMR is used to clearly establish the formation of amine-CO2 bonding at the surface of amine-modified nanoporous materials and to identify the nature of the species formed. The results presented here shed light on the mechanism of CO2 activation, since the CO2 adsorption on the surface of such materials is the activation step that allows further reactions to occur. The instability of the carbamate and carbamic acid species formed on the surface is important in explaining the reactivity of these intermediates and supports the possible application of these materials in CO2 activation.