Task-specific design of protic metal-based deep eutectic solvents for efficient separation of HCl/SO2

Abstract

Efficient separation and recovery of hydrogen chloride (HCl) from HCl tail gas containing impurities such as sulfur dioxide (SO2) is very important from the viewpoint of economy and environment. However, due to the similar dissolution ability of SO2 and HCl, most solvents, including organic solvents, ionic liquids and deep eutectic solvents (DESs), show low SO2/HCl selectivity near 1.0 and are thus incompetent to separate SO2/HCl. To address this, we firstly reported the task-specific design of a novel class of protic metal-based DESs (PM-DESs) containing hydrochloride, metal chloride and sulfolane, which have good SO2 affinity but much inhibited HCl uptake, for the separation of HCl/SO2. The solubility data of SO2 and HCl in PM-DESs were determined and correlated by Krichevsky-Пiinskaya (K-I) equation. Among these PM-DESs, N,N-dimethylaniline hydrochloride (DMA·HCl) + zinc chloride (ZnCl2) + sulfolane exhibits satisfactory recyclability and selectivity up to 2.669 (100 kPa, 313.2 K), which is the highest value reported so far. Absorption of HCl/SO2 (50/50) mixed gas was also tested, with a high SO2/HCl selectivity of 3.250 obtained. In addition, the NMR and FTIR spectroscopic characterizations paired with quantum chemistry calculation were utilized to investigate the absorption mechanism from the perspective of atomic level. The DESs developed in the present work have the potential to separate and recycle HCl from SO2 owing to their good selectivity and desirable reversibility, and the strategy proposed in this work may offer new guidance for designing green media for HCl recovery.