Rapid, selectivity, and reversibility absorption of SO₂ via purine-based deep eutectic solvents and thermodynamic analysis

Since the advantages of simple preparation, low-priced, environmental friendliness, and high absorption capacity, deep eutectic solvents (DESs) are considered to have eminent application potential in terms of SO₂ absorption. However, the absorption rate, selectivity, and reversibility of DESs urgently need to be further improved to meet the requirements of industrialization. In this work, five purine-based DESs were designed and synthesized through the use of 1-ethyl-3-methylimidazolium chloride (EmimCl) as hydrogen bond acceptors (HBAs) plus 6-aminopurine (6-AmP), 6-hydroxypurine (6-HoP), and 6-chloropurine (6-ChP) as hydrogen bond donors (HBDs), respectively. The results indicated that the optimal molar ratio of HBAs to HBDs is 7:1, and the absorption capacity of EmimCl + 6-AmP-7 can reach up to 18.118 mol/kg, at 298.15 K and 1.0 bar. Notably, the present purine-based DESs not only achieve gas-liquid equilibrium within 40 s, but also exhibit outstanding reversibility (absorb-desorb more than 30 times) and remarkable selectivity of SO₂/CO₂. Furthermore, a reaction equilibrium thermodynamic model (RETM) equation was employed to investigate the absorption behavior by combining the absorption data under different SO₂ partial pressures and temperatures. Finally, Fourier-transform infrared (FT-IR) spectroscopy and ¹H nuclear magnetic resonance (NMR) were conducted to explore further the formation and SO₂ absorption mechanism of purine-based DESs. It is revealed that the former is mainly hydrogen bonding interaction among HBAs and HBDs, and the latter is mainly Lewis acid-base interaction plus strong charge-transfer interaction among DESs and SO₂. Based on the obtained data, it could be confirmed that the SO₂ absorption includes both physical and chemical absorption.