Producing elemental sulfur from SO2 by calcium loaded activated coke: Enhanced activity and selectivity

[Display omitted] •A simple ultrasound assisted liquid phase impregnation method.•CaCl2 loading greatly enhances the reaction activity and selectivity.•C-O-Ca structure reduces the energy needed for O transfer by about 100 kJ mol−1.•The catalyst CaCl2 can be recovered and recirculated. SO2 carbothermal reduction is one of the promising strategies to convert SO2 to elemental sulfur, while the long-term challenge is to simultaneously improve the reactivity and selectivity. In this work, we demonstrate an effective approach for enhancing both the activity and selectivity of activated coke (AC) for SO2 carbothermal reduction by controllably loading catalytic Ca species. Ca-loaded ACs were prepared via a simple ultrasound assisted liquid phase impregnation method, in which the pore structure as well as CaCl2 distribution in Ca-loaded ACs can be optimized by tuning CaCl2 loading content. Dynamic experiments show that CaCl2-loaded ACs exhibit much higher SO2 removal rates as compared with pure AC, indicating the promoting role of Ca loading in SO2 carbothermal reduction activity. Such enhanced activity of CaCl2-loaded ACs is attributed to the decrease of activation energy according to the kinetic parameters calculated from Random Pore Model (RPM). By calculating and correlating the amounts of consumed reactants and reaction products, AC loaded with 30- wt% CaCl2 can achieve a record-high elemental sulfur production selectivity of 80%, which should be contributed by both the reactions of SO2 + C → CO2 + S and 2CO + SO2 → 2CO2 + S. Chemical environment investigations and density functional theory (DFT) calculations further demonstrate that the C-O-Ca structure in Ca-loaded AC can weaken the C-O bond in reaction intermediates and thereby reduce the energy needed for O transfer, making SO2 reduction feasible to occur. In addition, we also evaluate the recyclability of catalyst CaCl2 that is proven to be easily recovered and recirculated. This work provides a facile strategy to improve the reactivity and selectivity for SO2 carbothermal reduction, holding great application potentials in flue gas desulfurization.