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.