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

Abstract

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.