Reactivity of the Thermally Stable Intermediates of the Reduction of SO₂ on Carbons and Mechanisms of Insertion of Organic Moieties in the Carbon Matrix

Abstract

The reduction of SO 2 on carbons proceeds through reactive intermediates bound to the carbon matrix, which were postulated to be 1,2-oxathiene 2-oxide (or sultine), and 1,3,2-dioxathiolane that decomposes to produce an episulfide and CO 2 . The reactivity of these intermediates was studied in this work through several reactions, using XPS and NMR spectra to postulate their mechanisms. When modified activated carbon obtained after reaction with SO 2 at 630 °C was heated at 900 °C, it was observed that the changes of the XPS spectrum resulted from the forward reaction of decomposition of the oxidized intermediate with S-transfer to produce the episulfide and CO 2 and the reverse reaction with expulsion of SO 2 . Strong bases hydrolyzed the dioxathiolane intermediate and the episulfide. The thiolysis, aminolysis, and reaction of alkyl halides with modified activated carbon occurred with the insertion of the organic moiety in the carbon matrix. Laser photolysis at 266 nm in t-butanol showed insertion of t-butoxide on the matrix. Consistent mechanisms for these reactions were postulated. These results provide additional evidence on the mechanism of reduction of SO 2 on carbons and the chemical nature of the intermediates, offering a new method to modify the physical and chemical properties of a carbon matrix by functionalization with an organic moiety.

Interconversion and selective reactivity of sulfur dioxide reduction intermediates inserted on graphene oxide

“…MPGO presented only non-oxidized sulfur before the reaction, and after the thiolysis performed refluxing in DMSO (200°C), the interconversion to oxidized intermediates was observed. [19] Interconversion of the intermediates during thermal treatment is plausible SO 2 because according to the theoretical calculations [5] and experimental results [18] , the primary product episulfide is more stable than the oxidized intermediates, and the energetic barrier, as shown in this work, is only 25.81 kcal mol…”

“…[18][19][20] In all cases, the thiolysis proceeded through the oxidized intermediate, while reaction with amine took place through the primary product episulfide. These results suggest the possibility of a double functionalization of the matrix through an amino-thiolysis reaction if both reactive sites would be near enough as assumed by the primary mechanism.…”

Reactivity of the intermediates of the reduction of SO2. Functionalization of graphite, graphite oxide and graphene oxide

“…The reaction proceeds through a primary mechanism (Scheme ) where after the adsorption of SO 2 on the carbon, a 1,3,2‐dioxathiolane 1 and 1,2‐oxathietane 2‐oxide 2 were formed and decomposed to produce an episulfide 3 and CO 2 …”

“…The initial spectra and the spectra calculated through the series of reactions using the atom inventory method, were used to postulate the mechanisms involved.…”

“…The characterization of the intermediates of the reduction of SO 2 on activated carbon was carried out through the study of its reactivity with respect to the basic hydrolysis, thiolysis, aminolysis, reaction with alkyl halides, and photolysis in tert‐butanol . The reactions mechanisms were postulated from the XPS spectra after the reaction by the atom inventory method and were consistent with the simultaneous presence of the dioxathiolane, sultine, and episulfide on the surface of the modified activated carbon.…”