Efficient and reversible absorption of HCl gas by ChCl-based deep eutectic solvents-Insights into the absorption behavior and mechanism

Efficient separation and reclaim of HCl from its effluent gases is vital for its high value resource utilization. For this, six choline chloride (ChCl) based deep eutectic solvents (DESs) were designed and synthesized. Their absorption behavior for HCl gas was studied at varying temperature and pressure, and analyzed in terms of their free volume, hydrogen bond donors (HBDs)' structure, interaction energy, various instrumental characterization, and changes of viscosity and density in the absorption process. The dissolution mechanism of HCl was proposed, and a new 3-parameter physical chemical absorption model (PCAM) was developed. The solubility of HCl in DESs is reversely proportional to the interaction energy between ChCl and HBD, and followed the order of ChCl-polyol (ethylene glycol (EG), glycerol) > ChCl-hydroxyacid (glycolic acid, lactic acid) > ChCl-carboxylic acid (oxalic acid, malonic acid). ChCl-EG (1:2) is the best solvent here for its lowest viscosity (∼33.18 mPa·s), and highest absorption capacity of 0.97 mol·mol−1 (0.403 g·g−1) at 303.15 K and 101.3 kPa. The interaction energy of ChCl-EG (1:2) (-95.99 kJ·mol−1) is comparable to its interaction energy with HCl (-54.68 kJ·mol−1), which promotes the complete desorption of HCl under mild conditions compared to the partial desorption from its neat constituents. DES is proved to be a viable solvent for HCl gas absorption in terms of its least volatility, low cost and energy consumption, and high efficiency and reversibility.