Formulation-driven optimization of a hydrated choline chloride-DETA deep eutectic system for direct air capture of CO2

초록

This study reports the design and systematic evaluation of CO2 absorbents based on hydrated deep eutectic solvents formulated from choline chloride (Chl) and N-(2-aminoethyl)ethane-1,2-diamine (DETA). A series of formulations (DxWy) are prepared by varying the DETA: Chl weight ratio (“x” = 0–8, with a fixed Chl at 10 mg) and water volume (“y” = 1–5 mL). In a flow-dynamic system, the CO2 absorption breakthrough of DxWy absorbents is assessed under varying inlet concentrations (1000-10,000 ppm), where the effect of composition on capacity, thermodynamics, and kinetics is examined. At 1000 ppm CO2, D5W4 exhibits superior performance for CO2 capture, with an equilibrium absorption capacity (Q) of ∼ 126.7 mg/g (Chl-DETA) and a Henry's solubility constant of 0.0293 mol/kg.Pa. Notably, the Q value of D5W4 is ∼ 1.25 times higher than that of the amine-deficient D1W4 formulation, underscoring the dominant role of amine active site in CO2 capture under fixed hydration conditions. As the inlet CO2 concentration rises to 10,000 ppm, the Q value of D5W4 significantly increases, peaking at 489 mg/g, consistent with improved mass transfer and reactivity, driven by a higher partial pressure. Thermodynamic analysis yields an exothermic enthalpy change (ΔH° = −21.9 kJ/mol), corroborating chemisorption-dominated uptake. Density functional theory (DFT) calculations further confirm that chemisorption proceeds primarily through the thermodynamically favored bicarbonate formation pathway, which exhibits a significantly lower activation-energy barrier (19 kcal/mol) than the carbamate formation route (38 kcal/mol). Preliminary three-cycle absorption-regeneration tests retain about 93% of the initial Q, supporting the short-term recyclability of the optimized formulation under the tested regeneration conditions. Global reactivity descriptors from DFT, including electrophilicity index (ω) of 1.091 eV for DETA and 1.821 eV for Chl, remain consistent with the experimental results toward low-concentration CO2 capture applications.

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