Mechanism of the desulfurization route of the reduction of SO2 on carbons. Dimerization of Disulfur and Tetrasulfur

• Published in: Journal of physical organic chemistry 2024-07, Vol.37 (7), p.n/a
• Main Authors: Humeres, Eduardo, Correia, Marília Isabel Tarnowski, Debacher, Nito Angelo, Moreira, Regina de F. P. M.
• Format: Article
• Language: English
• Subjects: Absorbance; Activated carbon; Carbon; Chemical reduction; Cyclohexane; Decay; desulfurization of carbons; Desulfurizing; Dimerization; Disulfur; Graphene; reduction mechanism on carbons; Sulfur dioxide; Tetrasulfur; Ultraviolet spectra
• ISSN: 0894-3230; 1099-1395
• DOI: 10.1002/poc.4610

The desulfurization of carbons modified with SO2 was studied as a dispersion in boiling cyclohexane (81°C) using activated carbon (mAC) and graphene oxide (mGO), modified by SO2. The steady‐state species in the carbon matrix after the catalytic reduction of SO2 was considered a trisulfane. For mAC, there was a burst of a sulfur species identified as S2 by UV spectrum with a maximum at 217 nm (εM at 217 nm = 2.56 × 103) that showed a second‐order decay of absorbance with kS2 = 47.29 M‐1·sec‐1 ( ΔG354‡= 18.1 kcal·mol‐1). The product was postulated to be S4. No other consecutive reaction was observed because of the possible adsorption of S4 in the carbon matrix. The desulfurization of mGO was shown by XPS and the kinetics were a second‐order decay up to 16 min ( kS2= 18.41 M‐1·sec‐1; ΔG354‡= 18.8 kcal·mol‐1) followed by a second‐order increase of absorbance with kS4 = 3.84 M‐1·sec‐1 ( ΔG354‡= 19.9), where the product showed a double maximum at 260‐285 nm typical of S8. These results are consistent with a mechanism of consecutive thermodynamically favorable dimerizations of S2 and S4 and with the desulfurization mechanism that has been previously postulated. The desulfurization of carbons modified with SO2 occurred in boiling cyclohexane from a trisulfane inserted in the carbon matrix, with a burst of disulfur S2 that dimerized to tetrasulfur and finally to S8. The desulfurization path is largely more energetically favorable than the decarboxylation route.