Secondary Amine Chain Length Governs CO2 Absorption in DEEA-Based Blends: Screening, Equilibrium, Thermodynamic, and Speciation

The effect of secondary amine alkyl chain length on CO2 absorption was investigated in DEEA blends with MAE, EAE, and BAE. Rapid screening identified a 2.5:2.5 molar ratio as the representative formulation based on CO2 loading, saturation behavior, viscosity, and regeneration performance. Equilibrium measurements showed that increasing secondary amine chain length decreased CO2 loading but accelerated saturation, with only a minor effect on the apparent heat of absorption. A Deshmukh–Mather thermodynamic model accurately represented VLE data, with relative average deviations of 4.08%, 3.81%, and 3.14% for the three blends. 13C NMR and species distribution analysis showed that carbamate dominated at low loading, while bicarbonate became prevalent at higher loading. Electrostatic potential analysis further indicated that DEEA promotes proton transfer and early carbamate formation. These results establish a structure–property relationship linking secondary amine chain length with CO2 capacity, absorption kinetics, and reaction speciation in DEEA-based blended amines.