1. Introduction
The economically sustainable chemical model is an attractive trend to develop the environmental-friendly chemical industry with high economic benefits 1. Impurity such as hydrogen sulfide (H2S) exists diffusely in energy gas and is recognized as major contaminants that can reduce operation safety. In the related research of H2S, scholars mainly focus on the removal of acidic gas by the way of absorption 2-4, adsorption5,6, and membrane separation 7,8, etc. However, the utilization of highly purified H2S separated from the above-mentioned technology remains to be a tough issue. Conversion of H2S into value-added organic compounds should be a promising strategy, which usually requires chemical complexation to activate H2S over a low energy barrier.
As a kind of novel green solvent, ionic liquids (ILs) have attracted much attention due to their unique properties (negligible volatility, high thermal stability, structural variation, and excellent affinity to acidic gas, etc.). The research of ILs mainly focused on catalytic synthesis 9-14, energy 15-17, solvents 18-22, materials 23-26, etc. Protic ionic liquids (PILs) are a class of cost-effective ILs for the removal of acidic gas, which can be easily prepared from simple materials through the direct neutralization between the corresponding acid and base. In addition to the application in absorption separation, PILs can provide hydrogen bonding in the process of reaction to activate epoxide27.
After years of in-depth research in the field of H2S capture by functionalized ILs 2-4,28-30, our group has started to explore the resource utilization of H2S in a green medium. We firstly reported the ILs and deep eutectic solvents (DESs) mediated Claus process with excellent H2S conversion under ambient conditions 30,31. This finding pioneered a promising method for the capture and conversion of H2S through green and sustainable mediums. Subsequently, Wu et al. reported ILs-formulated solvents to promote Claus reaction32. Additionally, [Bmim][FeCl4] has been tested as an alternative medium for the oxidation of H2S to produce sulfur33. Notably, the above H2S conversion target is bulk chemical sulfur. Recently, we disclosed a kind of task-specific hydrophobic PILs for the efficient capture and conversion of H2S into value-added mercaptan acids28. These studies demonstrate that ILs are promising media for the capture and conversion of H2S.
Except for the conversion of H2S into sulfur or mercaptan acids, is there some other way to reuse H2S? As is well known, H2S can be activated by the tertiary amine group to split into nucleophilic SH group 28. The epoxide is effortless to be ring-opening in presence of a nucleophilic group because of its instability. It is believed that H2S can react with epoxide mediated in tertiary amine-functionalized ILs.
To prove the argument above and screen another H2S conversion way, we develop a novel, inexpensive, and environmentally friendly H2S utilization process through the addition of H2S and epoxide into mercaptan alcohols mediated in tertiary amine-functionalized PILs without any metal or halogen additive under mild condition (The chemical structure of PILs are presented inScheme 1 ). The effect of cation and anion, reaction temperature, water content, and ILs loading were systematically studied. Moreover, quantum chemistry calculation was utilized to calculate the barriers of ring-opening reaction. Water extraction was employed to reuse the catalysts. It is believed that this work could provide a distinctive insight to convert H2S.
Scheme 1 . Chemical structures of the PILs.