Electrochemical Oxidation of Dibenzothiophene in Acetonitrile and Acetonitrile-Water Mixtures

Crude oil is basically formed of a mixture of many hydrocarbons containing a wide variety of heteroatoms like sulfur and nitrogen. The content of sulfur in crude oil can vary from 0.05% to 10% [1]. This percentage is low; however, this amount is particularly important due to environmental implications [2, 3]. The major sulfur compounds present in crude oil are thiophenes, benzothiophenes, dibenzothiophenes, which are carcinogens. For that reason, the hydrodesulfurization [4], which is the method used in refineries, does not remove all the sulfur compounds, mainly dibenzothiophene and derivatives. To reach this objective, several methods have been proposed to remove the sulfur compounds by converting them into hydrogen sulfide or more polar compounds [5]. For that reason, it has been of interest to explore the electro-oxidative behavior of these compounds to convert them to more polar compounds that are easier to extract with water as solvent [6].

It is known that refractory organosulphur compounds such as dibenzothiophene (DBT) can be oxidized to dibenzothiophenesulfoxide (DBTO) and dibenzothiophenesulfone (DBTO₂), that can be removed by physical methods such as extraction and adsorption using polar solvents. Therefore, in this work the electrochemical oxidation of a model sulfur aromatic compound such as dibenzotiophene (DBT) was studied in acetonitrile in the presence of low ( < 0.1 M ) and high ( > 1 M)  content of water  on carbon electrodes. This study is focused to obtain mechanistic information about the products formed.

Cyclic voltammetry of DBT in acetonitrile in the presence of < 0.1 M water on glassy carbon electrode showed three chemically irreversible anodic peaks. The first and second peaks involve a global transfer of one electron which allows respectively the formation of sulfoxide and sulfone derivatives as well as the release of one equivalent of protons, according to the global proposed reaction.

DBT – e^- + H₂O = DBTO + H⁺ + 1/2H₂

When water was added to the acetonitrile solution ( >1 M ), the voltammetric pattern was maintained, however the global mechanism giving rise to the sulfoxide and sulfone derivatives was modified to two electrons with the release of two equivalents of protons.

DBT – 2e^- + H₂O = DBTO + 2H⁺

The proton formation was confirmed by cyclic voltammetry using acetate ions as proton probe while the products dibenzothiophene sulfoxide and dibenzothiophene sulfone were prepared by constant potential electrolysis and characterized by HPLC-MS-TOF and ¹H and ¹³C NMR experiments. The analysis of the variation of the peak potential and half-peak width with the scan rate was used to establish the relevant role of water on the reaction mechanism, which changes from an ECCCC to an ECCEC pathway when water is present in excess

According to the above mentioned results, different oxidation products can be formed depending on the electrolysis conditions and for this reason, this work deals  with the anodic oxidation process of DBT in acetonitrile to determine how the intermediates and the composition of the electrolyte solution govern the formation of the oxygenated products.  Cyclic voltammetry, sampled current chronoamperometry, coulometry, preparative electrolysis, chromatography coupled with mass spectrometry and nuclear magnetic resonance (NMR) spectroscopy were used as analytical techniques

Acknowledgements

The authors acknowledge CONACYT for financial support through the project 221548. E.M.A also acknowledges B.R. Díaz and G. Cuellar for their assistance in some electrochemical, NMR and MS analysis respectively.

References

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