SO₂ disproportionation for a sulphur-based thermochemical cycle studied in an operando Raman batch reactor

In situ and operando Raman spectroscopy were used to study the iodide-catalysed SO₂ disproportionation reaction at temperature of 118 °C and total pressures up to 9 bar. The disproportionation of SO₂ is one of the three process steps constituting a solar-aided thermochemical sulphur-based cycle for producing unlimitedly storable sulphur capable for on-demand consumption. A suitably designed quartz-made batch reactor cell was used enabling a special containment of SO₂, capable for in situ monitoring of the reaction progress in the liquid I^-/H₂O/SO₂ phase as well as for quantitative monitoring of the SO₂ pressure in the vapors thereof. The iodide catalyst content was varied in the 0.156-0.780 I^-/H₂O mol% range. The incorporation of SO₂ into the I^-/H₂O solution was facilitated by means of O₂S⋯I^- interactions resulting in formation of I(SO₂)_x^- adduct species. The rate of SO₂ consumption was accelerated with increasing I^- content and 5% average hourly rates of SO₂ consumption could be maintained after 6 h of reaction time. The mechanistic pathway of the iodide-catalysed SO₂ disproportionation was unravelled at the molecular level. Below a certain SO₂ threshold pressure and at high I^- content, formation of the undesired I₂ by-product takes place, which is known to severely complicate the post-batch product separation. The results offer insight into the SO₂ disproportionation and are discussed with relevance to implications for its technical integration into the sulphur based solar-aided thermochemical cycle.