Kinetics and Mechanism of Hydrogen Sulfide Oxidation to Elemental Sulfur over a Redox-Active [BMIM][FeCl4] Ionic Liquid

Hydrogen sulfide (H2S) removal with simultaneous sulfur recovery is essential for natural gas, refinery, and biogas processing because of the toxicity and corrosivity of H2S. However, conventional Claus sulfur recovery is energy-intensive, operates at high temperatures, and requires multiple processing units, creating a need for efficient low-temperature alternatives. In this study, the redox-active ionic liquid 1-butyl-3-methylimidazolium tetrachloroferrate ([BMIM][FeCl4]) was investigated as both an absorbent and catalyst for the direct oxidation of H2S to elemental sulfur. Experiments were conducted in a laboratory-scale semi-batch single-bubble reactor at 25–100 °C and 500–3000 ppm H2S. Under the optimum operating conditions, H2S conversion exceeded 99%, with removal efficiency increasing from 95.0% at 25 °C to 99.1% at 100 °C within 60 min. Kinetic analysis revealed first-order reaction kinetics with respect to H2S and Fe(III), with apparent rate constants increasing from 0.0506 to 0.0636 min−1 over the investigated temperature range and an apparent activation energy of 2.78 kJ mol−1. Hatta number analysis confirmed that the process operated predominantly in the reaction-controlled regime. Raman spectroscopy showed the attenuation of [FeCl4]− vibrational bands together with the appearance of characteristic S8 bands, while X-ray diffraction (XRD) verified the formation of crystalline elemental sulfur. CHNS analysis provided complementary elemental evidence for sulfur formation. The ionic liquid exhibited excellent thermal stability (>320 °C) and retained 92–95% of its initial activity after repeated regeneration cycles. These findings demonstrate that [BMIM][FeCl4] effectively integrates H2S absorption and catalytic oxidation within a single liquid phase, providing a promising low-temperature and energy-efficient alternative to conventional sulfur recovery technologies.