Spectroscopic Investigation into Oxidative Degradation of Silica-Supported Amine Sorbents for CO2 Capture

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 CO 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

In Situ ATR and DRIFTS Studies of the Nature of Adsorbed CO₂ on Tetraethylenepentamine Films

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