Oxidative Study of Amino Polymer Supported Adsorbents for CO2 Capture

Increasing levels of CO₂ in the atmosphere, currently at 400 ppm, have been correlated with global climate change. Of the methods to capture CO₂, solid adsorbents are a promising option because of their ease of synthesis and use and high adsorption capacities. For this study, branched poly(ethylenimine) (PEI) and poly(allylamine) (PAA) supported on mesoporous γ-alumina are evaluated for their oxidative stability under various conditions. Adsorbents containing PEI are composed of primary, secondary, and tertiary amines, whereas PAA contains only primary amines. A thermal oxidation study of PEI impregnated alumina demonstrates a reduction in degradation of PEI under humid conditions versus dry conditions. Using 21% O₂, PEI experiences an 89.5% and 37.7% reduction in CO₂ capacity under dry conditions at 110^oC and 70^oC; whereas under humid conditions a 70.1% and 33.4% reduction in CO₂ capacity is observed at the same temperatures. Next, the oxidative stability of PEI and PAA are compared. Under the harshest conditions, 110^oC and 21% O₂, it is observed that the PEI impregnated adsorbents undergo considerable degradation, and degradation of the PEI is reduced with a lower temperature and O₂ concentration, 70^oC and 5% O₂. In contrast, the use of PAA impregnated alumina showed much increased oxidative stability with only a 10.9% reduction in CO₂ capacity after treatment with 21% O₂ at 110^oC. Use of FTIR, FT-Raman, and ¹³C NMR spectroscopy to characterize the oxidized sorbents shows key differences between the PEI and PAA samples. Of most interest is the carbonyl (C=O) peak, which is present in the oxidized PEI but not PAA samples. This is consistent with previous work that demonstrated that secondary amines, such as those within PEI, contribute to the lack of oxidative stability of amine-based sorbents, which PAA does not contain. Thus PAA can be used in development of adsorbents with increased oxidative stability for use in CO₂ capture.