مسعود مفرحی

Understanding the thermodynamics of CO₂ reactive amines is important for clarifying reaction pathways relevant to solvent regeneration and degradation in carbon capture. In this work, the thermodynamics of oxazolidinone formation from ethanolamine and N-methylethanolamine are investigated to quantify the thermodynamic tendency of intramolecular cyclization in these two aminoalcohol systems. Equilibrium constants are derived from Gibbs reaction energies, with standard enthalpies of formation obtained by combustion calorimetry and standard entropies determined from quantum chemical calculations combined with vaporization data. In the gas phase, N-methylethanolamine exhibits a stronger thermodynamic driving force for intramolecular cyclization (K°≈10⁻²) than ethanolamine (K°≈10⁻⁴). This indicates that N-methylation enhances thermodynamic favorability for oxazolidinone formation. Yet, the absolute gas phase equilibrium constants remain low for both systems. In contrast, in the liquid phase a significant increase in cyclization equilibrium constants can be observed (Kf/a = 84.2 for MEA and 0.902 for NMEA at 293 K). Both reactions display pronounced exothermicity, which leads to decrease in equilibrium constants by three orders of magnitude during the temperature change from 293 K to 403 K. These results demonstrate a pronounced phase dependence of oxazolidinone formation. N-methylation increases the thermodynamic favorability of the gas phase cyclization reaction whereas, MEA shows a stronger thermodynamic driving force for the same process in the liquid phase. Within this scope, MEA shows stronger liquid phase thermodynamic favorability for oxazolidinone formation, whereas NMEA shows lower stabilization of the cyclic product, which may be relevant to pathway specific reversibility.