DFT Studies on Thermal and Oxidative Degradation of Monoethanolamine

Abstract

Thermal and oxidative degradation of monoethanolamine (MEA) represents a major problem for modern day carbon capture technologies. Here, we report on a series of density functional theory (DFT) calculations investigating the possible chemical pathways leading to the formation of the most commonly observed degradation products. 2-Oxyzolidinone (OZD) can be formed from ring closure reactions of carbamates, carbamic acids, or isocyanates. The latter, itself, formed by dehydration of MEA. N-(2hydroxethyl)ethylenediamine (HEEDA), 1-(2-hydroxyethyl)imidazolidone (HEIA), and N-(2-aminoethyl)-N′-(2-hydroxyethyl)imidazolidin-2-one (AHEIA) are all hypothesized to form favorably from degradation reactions of OZD.MEA can undergo oxidative degradation to form imines and hydroperoxides. This work details the mechanistic steps leading to the formation of these species that could help in the location of new compounds that aim to prevent their formation in future systems. Moreover, the thermochemical data will aid in the construction of a chemical kinetic mechanism to rationalize the rate of formation of all the species in real systems.

Modeling AMP Degradation Product Formation

“…In each case, reaction free energies are exergonic. Similar results were recently reported for radical abstraction from MEA …”

“…As can be seen, the reaction has a relatively high free activation energy and is overall endergonic. Similar results were recently reported for MEA . While the formation of the diol is not strictly prohibited, its formation is unlikely.…”

“…This complex is perfectly orientated to allow for hydrogen transfer from the zwitterion nitrogen to the other AMP molecule. The free activation energy is very low (0.6 kcal mol –1 ), which is consistent with recently reported data for MEA (1.2 kcal mol –1 ) . The reaction energy is also exergonic at −6.5 kcal mol –1 , suggesting that the formation of an ion pair should be relatively facile.…”

“…Overall, this reaction forms DMOZD, water, and AMP. The activation free energy is 34.9 kcal mol –1 , which is lower than the recently reported value for OZD formation in MEA degradation (39.3 kcal mol –1 ) and is consistent with a more nucleophilic amine …”

“…All calculations used the B3LYP functional, , and the cc-pVTZ basis set was used for all elements . This setup was chosen to be consistent with previously published work within the group and to allow for comparisons with that work . Moreover, benchmarking studies have shown this setup to be similar in quality to that afforded by CCSD­(T) calculations and can be conducted in a more timely manner. , Calculations were carried out at 298.15 K. Empirical dispersion corrections through the GD3 keyword were applied to all calculations.…”