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

Abstract

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.