Physicochemical and Structure–Property Relationship of Alkylamine Hydrochlorides-Phenol Deep Eutectic Solvents for NH ₃ Absorption

This work elucidates the structure–property relationships of alkylamine hydrochloride-based deep eutectic solvents (DESs) for efficient NH3 capture from industrial gas streams. Through systematic variation of hydrogen bond acceptors (HBAs) including amine type (primary/secondary/tertiary) and alkyl chain length (C1–C3). Three key structural parameters governing absorption performance: (1) amine substitution state (primary > secondary > tertiary), (2) alkyl chain length (C1 > C2 > C3), and (3) HBA: HBD molar ratio (1:7 > 1:5 > 1:3 > 1:2). Primary amine-based DESs achieve optimal performance, exhibiting exceptional NH3 capacities (8.32 mol/kg at 313.2 K, 101.4 kPa) coupled with low viscosities (9.6–13.9 mPa·s). Spectroscopic and computational studies reveal a cooperative dual-site absorption mechanism involving both phenolic –OH and ammonium protons of alkylamine hydrochlorides. The optimized solvents display outstanding NH3/CO2 selectivity (>170) and cycling stability (>88% retention after 5 cycles) compared to many reported absorbents, while comprehensive physicochemical characterization provides critical data for industrial implementation, such as viscosity, density, and thermal stability. These findings establish molecular design principles for NH3 capture solvents that simultaneously address capacity, selectivity, and transport property requirements.