Oxidative degradation characteristics of silica-supported amine sorbents with varying amounts of tetraethylenepentamine (TEPA) and polyethylene glycol (PEG; P200 or P600 represents PEG with molecular weights of 200 or 600) have been studied by IR and NMR spectroscopy. Thermal treatment of the sorbents and liquid TEPA at 100 °C for 12 h changed their color from white to yellow. The CO2 capture capacity of the TEPA/SiO2 sorbents (i.e., SiO2-supported TEPA with a TEPA/SiO2 ratio of 25:75) decreased by more than 60 %. IR and NMR spectroscopy studies showed that the yellow color of the degraded sorbents resulted from the formation of imide species. The imide species, consisting of NH associated with two C—O functional groups, were produced from the oxidation of methylene groups in TEPA. Imide species on the degraded sorbent are not capable of binding CO2 due to its weak basicity. The addition of P200 and P600 to the supported amine sorbents improved both their CO2 capture capacities and oxidative degradation resistance. IR spectroscopy results also showed that TEPA was immobilized on the SiO2 surface through hydrogen bonding between amine groups and the silanol groups of SiO2. The OH groups of PEG interact with NH2/NH of TEPA through hydrogen bonding. Hydrogen bonds disperse TEPA on SiO2 and block O2 from accessing TEPA for oxidation. Oxidative degradation resistance and CO2 capture capacity of the supported amine sorbents can be optimized through adjusting the ratio of hydroxyl to amine groups in the TEPA/PEG mixture.

CO2 adsorption/desorption onto/from tetraethylenepentamine (TEPA) films of 4, 10, and 20 μm thicknesses were studied by in situ attenuated total reflectance (ATR) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) techniques under transient conditions. Molar absorption coefficients for adsorbed CO2 were used to determine the CO2 capture capacities and amine efficiencies (CO2/N) of the films in the DRIFTS system. Adsorption of CO2 onto surface and bulk NH2 groups of the 4 μm film produced weakly adsorbed CO2, which can be desorbed at 50 °C by reducing the CO2 partial pressure. These weakly adsorbed CO2 exhibit low ammonium ion intensities and could be in the form of ammonium-carbamate ion pairs and zwitterions. Increasing the film thickness enhanced the surface amine-amine interactions, resulting in strongly adsorbed ion pairs and zwitterions associated with NH and NH2 groups of neighboring amines. These adsorbed species may form an interconnected surface network, which slowed CO2 gas diffusion into and diminished access of the bulk amine groups (or amine efficiency) of the 20 μm film by a minimum of 65%. Desorption of strongly adsorbed CO2 comprising the surface network could occur via dissociation of NH3(+)/NH2(+)···NH2/NH ionic hydrogen bonds beginning from 60 to 80 °C, followed by decomposition of NHCOO(-)/NCOO(-) at 100 °C. These results suggest that faster CO2 diffusion and adsorption/desorption kinetics could be achieved by thinner layers of liquid or immobilized amines.

CO2 adsorption on fresh and oxidatively degraded tetraethylenepentamine (TEPA) and polyethylene glycol (PEG-200) supported on amorphous silica (SiO2) was studied by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Fresh TEPA sorbents adsorb CO2 in the form of ammonium ions, carbamates, and carbamic acid of which infrared features and binding strength strongly depend on the type of amine sites (i.e., primary or secondary) and their densities. CO2 adsorbed on the primary amine sites exhibited a strong IR intensity of carbamate in the 1680–1200 cm–1 region, and CO2 adsorbed on the secondary amine sites showed a high IR intensity of ammonium ion in the 2800–2100 cm–1 region. Oxidative degradation of the fresh sorbents in the presence of air at 100 °C for 12 h converted a high fraction of secondary amines in TEPA to amide and imide species, isolating the primary amine sites and decreasing the overall density of amines in the sorbent. The isolated primary amine sites of the degraded sorbents bind weakly adsorbed CO2 in the form of carbamic acid and carbamate. Carbamic acid can be removed from the sorbent at a higher rate than carbamate by flowing inert gas or air at 25 °C. High amine density (i.e., loading) sorbents containing multilayers of amines bind strongly adsorbed CO2 which can only be thermally desorbed from the sorbents.