The use of hydrogen as an energy carrier to be used for various purposes has been strongly considered, including for such applications as transportation fuels and residential/commercial/industrial combined heat-and-power.  Of the many ways to produce hydrogen one can count processes such as water electrolysis and steam reformation of fossil fuels.  One method that shows the promise of high effciciency coupled with the use of sustainable energy resources, such as nuclear or solar heat, is the Sulfur-Iodine thermochemical cycle, where water is thermolysed in a cascade of chemical reactions to produce hydrogen and oxygen. 

A fundamental step of this chemical reaction cascade is a reaction where sulfur dioxide is oxidized by elemental iodine in the presence of water known as the Bunsen reaction, highlighted in yellow in Scheme 1 below. 

Scheme 1.  Summary of Sulfur-Iodine thermochemical cycle with Bunsen reaction highlighted in yellow.

Currently, however, this reaction is carried out in a medium with a vast excess of water, due to the fact that the aqueous products, namely HI and SO₃ form a biphasic aqueous system, with the sulfur bearing stream spontaneously separating from the hydrogen iodide stream.  While this simplifies this separation, it creates a need for the removal of the excess of water from the system, which becomes a critical problem.  Several approaches to achieve this water removal have been proposed including reactive distillation and electrodialysis.

A cursory examination of the boiling points of the various reactants and products, listed in Table 1, suggests a different approach to the separation of the sulfur bearing stream from the iodide bearing one:  taking advantage of the vapour pressure of the pure products. 

Table 2.  Boiling points of the chemical species involved in the Sulfur-Iodine cycle and its variants.

To enable this, the reaction must be carried out without the excess of water present, and in a solvent with negligible vapor pressure between the reaction temperature, occurring at close to room temperature, to the temperature at which the sulfur bearing species evolves from solution, which is up to 400°C.  Our team has elected to investigate the feasibility of this reaction in an ionic liquid such as an alkyl-methyl-immidazolium salt of tetrafluoroborate or hexafluorophosphate. 

These ionic liquids are miscible with water, excellent solvents, inert towards oxidation/reduction under the conditions described, and exist as stable liquids from well below -20°C to over 400°C. 

The presentation will present initial feasibility results for this process.