Identification of desulfurization products in the photochemical desulfurization process for benzothiophenes and dibenzothiophenes from light oil using an organic two-phase extraction system

The desulfurization products of benzothiophenes (BTs) and dibenzothiophenes (DBTs), produced during a new photochemical desulfurization process, employing an organic two-phase liquid-liquid extraction system, have been identified. The acidic products were converted to their corresponding methyl esters by reaction with diazomethane and were then analyzed. In acetonitrile, BT is converted first to benzothiophene-2,3-dione, followed by hydrolysis, loss of carbon monoxide, and oxidation of the sulfur atom, to be converted finally to 2-sulfobenzoic acid. 3-Methyl BT and 2,3-dimethyl BT produce 2-sulfobenzoic acid and two other products, benzenesulfonic acid and 2-acetylbenzenesulfonic acid. DBT is first photooxidized to DBT sulfoxide, which is then further oxidized to form DBT sulfone or dibenz[c,e][1,2]oxathiin-6-oxide. The latter is then oxidized and finally converted to 2-sulfobenzoic acid via dibenz[c,e][1,2]oxathiin-6,6-dioxide. Benzothiophene-2,3-dicarboxylic acid is likely to be formed directly by the oxidation of DBT by a singlet oxygen, generated by photosensitization with DBT sulfone. 4-Methyl DBT and 4,6-dimethyl DBT finally produce the corresponding sulfobenzoic acid, dicarboxylic acid, and sulfone. The yields of these products are seriously affected by the presence of methyl substituents.