Improved solvation routes for the Bunsen reaction in the sulphur iodine thermochemical cycle: Part I – Ionic liquids

The Bunsen reaction is central to the Sulphur Iodine cycle, however large excesses of both water and iodine are currently employed to enable phase separation of the two acids produced. This causes separation issues later in the cycle and induces a large thermal burden for water evaporation. The use of solvents in the reaction has the potential to reduce these large excesses, thereby increasing the cycle efficiency. This paper investigates ionic liquids as solvents for the Bunsen reaction. Several potential ionic liquids are identified based on their anion properties. The extraction of HI into the ionic liquid is then investigated experimentally. [FAP]− ionic liquids were examined but their extreme hydrophobicity prevented water being taken up into the organic phase, severely retarding the extraction of acid by the solvent. Results for the [TMPP]− ionic liquid showed discrepancies in the component balance and it is thought that the solvent may be susceptible to hydrolysis. The extraction of acid by the [Tf2N]− ionic liquids was more promising, the amount of acid extracted being of the order of 20%. However, the amount of protons and iodide ions extracted by the solvents were not equivalent and evidence is presented demonstrating the presence of an ion exchange mechanism. None of the ionic liquids tested are therefore suitable for use in the Bunsen reaction, however the properties of an ionic liquid can be tailored by the choice of anion and cation. Further investigation of ionic liquids is necessary before they can be conclusively ruled out.