Examining the Reactions of Ethanolamine’s Thermal Degradation Compounds in Carbon Capture through ¹H NMR and ¹³C NMR

In amine scrubbing carbon capture, concerns about amine solvent degradation include whether it can affect the ability of the solvent to capture CO₂. This study examines the interactions between the three most reported monoethanolamine (MEA) thermal degradation compounds─namely, oxazolidine-2-one (OZD), N-(2-hydroxyethyl)-ethylenediamine (HEEDA), and N-(2-hydroxyethyl)-imidazoline-2-one (HEIA)─with CO₂ in the presence and absence of MEA. We compared ¹H NMR, ¹³C NMR, and heteronuclear single-quantum coherence (HSQC) NMR for neat OZD, HEEDA, and HEIA samples with CO₂-loaded samples to observe changes in protonation and unique carbamate species formation. We found that OZD and HEIA did not directly react with CO₂ or undergo proton shifting based on our comparison of the neat OZD and HEIA samples with CO₂-loaded spectra. However, we observed that the OZD can protonate when sparged with CO₂ in the presence of MEA, which suggests that the OZD acts as an intermediate. The NMR spectra for HEEDA indicated that HEEDA directly reacts with CO₂ at both amino groups and can protonate. In the presence of MEA, HEEDA and MEA can act like a solvent blend, resulting in multiple carbamates forming within the solvent. The neat HEIA spectrum, compared with CO₂-loaded HEIA spectra, revealed similar results as OZD, where it does not react with CO₂ or protonate directly. However, HEIA does not protonate in the presence of MEA. These degradation compound reactions can increase the number of general equilibrium reactions during carbon capture and impact the model MEA solvent. This work helps provide a more complete picture of the reactions as the solvent degrades. Although this study examines CO₂ effects on thermal degradation products for MEA, other amines such as piperazine or 1-amino-3-propanol will degrade, and the degradation may speciate similarly with CO₂. This study can impact and improve process models by assessing the degradation compounds’ reactions with CO₂ and potentially incorporating them into the model based on a neat solvent.