Controlling SO2 Poisoning, Pore Size and Oxidative Degradation of Amine-Containing Adsorbents to Enhance CO2 Capture

Abstract

The increasing concentration of atmospheric CO2 is a key contributor to global climate change. With global mitigation efforts already falling short of critical targets, the development of efficient and durable carbon capture technologies has become imperative to reduce CO2 emissions on a global scale. Among the various strategies that have been explored, supported amine-based adsorbents have emerged as a highly promising solution due to their ability to capture CO2 selectively and reversibly. However, the technical route is not wholesome yet, because the CO2 uptake capacity and long-term stability of these adsorbents are significantly compromised by fundamental challenges, such as SO2 poisoning and oxidative degradation. This thesis is therefore aimed at addressing these challenges while also focusing on the rational design of adsorbents to optimize CO2 uptake. Overall, this work comprises three interconnected research projects. The first one focuses on mitigating SO2 poisoning, the second one develops strategies for slowing down oxidative degradation, and the third one is dedicated to tailoring the pore structure to enhance adsorption performance. SO2 poisoning is a major challenge, as amines form stable salts even at trace concentrations of SO2, resulting in a substantial decline in CO₂ adsorption capacity. To address this, silica-supported polyethylenimine (PEI) adsorbent, functionalized with glycidol (GD-PEI/S), was developed. This material contains only tertiary amines, which are selective for SO2 over CO2. The GD-PEI/S adsorbent was characterized using infrared (IR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and thermogravimetric analysis (TGA). Column breakthrough experiments demonstrated that the material is highly selective for SO2 over CO2, even in gas streams where CO2 concentration was significantly higher. Moreover, the SO2 uptake of the adsorbent nearly doubled under humid conditions, likely due to enhanced SO2 diffusion facilitated by increased moisture content. Furthermore, the adsorbent showed good reversibility under dry and humid recycling conditions. Oxidative degradation is another limitation for deploying amine adsorbents on an industrial scale, where adsorbents are inevitably exposed to air at elevated temperatures during cooling stage after CO2 regeneration. This oxidative degradation leads to formation of less degradation products along with scission of amine chain, which significantly reduces CO2 uptake and hence compromising adsorbent lifespan. To enhance oxidation stability, hydroxyethyl starch (HES) was co-impregnated with PEI on a support as an additive (HES-PEI/S). The performance of HES-PEI/S was evaluated under various oxidation conditions using CO2 uptake measurements and mass spectrometry, with the findings supported by FTIR and NMR analyses. The enhancement in oxidation stability by HES was compared to other hydroxyl-containing additives, such as polyvinyl alcohol (PVA) and polyethylene glycol (PEG), as well as to 1,2-epoxy butane (EB). The results demonstrated that HES-PEI/S showed oxidation stability comparable to EB-PEI/S, while maintaining twice the CO2 uptake based on PEI content. Additionally, HES-PEI/S showed significantly higher oxidation stability than PEI/S and other co-impregnated adsorbents, such as PVA-PEI/S and PEG-PEI/S. While the previous projects addressed challenges such as SO2 poisoning and oxidative degradation to enhance the stability of the adsorbent, consequently improving CO2 uptake, third project presents a strategic innovation to optimize CO2 uptake of the adsorbents. The project focused on maximizing CO2 uptake of amine adsorbents by leveraging the relationship between relative humidity (RH) of the feed gas, pore size of the material, and CO2 uptake. To study the relationship, periodic mesoporous silicas with 3 to 9 nm pore sizes and consistent morphology were synthesized, followed by grafting with comparable triamine loadings. H2O and CO2 adsorption isotherms as a function of RH were obtained for adsorbents with different pore sizes using Dynamic Vapor Sorption (DVS) analyzer. The adsorption isotherms showed pronounced maxima at the RH corresponding to water capillary condensation within the adsorbent, which was attributed to the formation of ammonium bicarbonate facilitated by the presence of liquid-like water. Furthermore, the results revealed that the RH of the water capillary condensation and hence the optimum CO2 uptake of the adsorbent shifted to higher RH as the pore size of the adsorbent increased. This pore-size-dependent behavior of maximum CO2 uptake allows precise tailoring of adsorbents for feed gases with different RHs.