Promoting the hydrogen sulfide–iodine thermochemical cycle with deep eutectic solvents: An experimental and theoretical study

Handling and processing hydrogen sulfide (H₂S), a highly toxic and corrosive byproduct of the oil and gas industry, remains a critical environmental and safety challenge. Converting this waste gas into valuable intermediates and products, such as hydrogen (H2), is essential for transitioning toward a circular economy. This study investigates the H₂S–iodine (I₂) thermochemical cycle to produce hydrogen iodide (HI), a key intermediate for potential clean H2 recovery, using deep eutectic solvents (DES) as both catalysts and reaction media. We present a combined experimental and theoretical evaluation of choline chloride–ethylene glycol (ChCl:2EG, 1:2 molar ratio) relative to traditional aqueous media and alternative DESs formulations such as ChCl:2Urea, and ChCl:pTSA. Experimentally, the aqueous ChCl:2EG mixture (70:30 v/v, DES:H₂O) exhibited enhanced phase stability and significantly higher H⁺ production than pure aqueous solutions, with H⁺/H₂S molar ratios approaching the theoretical maximum of 6.0 for the low H₂S concentration mechanism. Control experiments confirmed that both H₂S and I₂ are required for H⁺ production and that ChCl and EG act synergistically. Independent iodometric titration validated that HI accounts for >90% of the measured H⁺, with dissolved SO₂ as sulfurous acid contributing the remaining minor fraction, consistent with the proposed mechanism. FTIR-ATR and ¹H NMR analyses of pre- and post-reaction samples confirmed the structural integrity of the DES, supporting its role as a regenerable catalyst rather than a consumed reactant. Quantum chemistry calculations at the M06–2X/def2-TZVP level reveal that ChCl:2EG in an aqueous phase actively catalyzes the initiation and rate-limiting steps via a “6-membered ring” mechanism involving a double hydrogen-atom transfer reaction. Our findings position the aqueous ChCl:2EG mixture as a highly effective medium for the H₂S-I₂ thermochemical cycle, enabling toxic H₂S conversion into HI as a key intermediate for potential downstream H2 recovery.