Electronic Effect of Substituents on the Hydrodesulfurization of Ring-Substituted Benzenethiols

Abstract

In order to clarify the effect of electron-releasing property of substituents on the hydrodesulfurization, the ortho, meta and para isomers of aminobenzenethiol, methoxybenzenethiol and toluenethiol were hydrogenolyzed by a batch method at 160∼240°C and H₂ (40 atm) over molybdenum(M) sulfide catalyst. Benzothiazole and phenothiazine were also reacted un der the similar conditions.
Hydrogenoly ses of aminobenzenethiols at 160°C (Table 2), methoxybenzenethiols at 200°C (Table 3), and toluenethiols at 240°C (Table 4) yielded appreciable amounts of aniline, anisole/phenol, and toluene, respectively, as the main hydrodesulfurization products. Among the isomers of these ring-substituted benzenethiols, the easiness of the hydrodesulfurization by the C-S bond cleavage was found to be in the order of para>ortho>meta isomer, approxiMately. Furthermore, in the case of the para-substituted benzenethiols, the easiness of the hydrodesulfurization resulted in the following order: p-aminobenzenethiol>>p-methoxybenzenethiol>p-toluenethiol=benzenethiol at 160°C, and p-toluenethiol>>benzenethiol at 240°C. These results indicate that the hydrodesulfurization of the ring-substituted benzenethiols are promoted to a great extent when high electron-releasing substituents like amino and methoxy l group exist in the para or ortho position of benzenethiols. The promoting effect of the substituents on the reaction might be attributed to the increase of the electron density on the electron acceptable sulfur atom by the presence of the electron-releasing substituents making these substrates to be easily adsorbed on the anion vacancy of the catalytic surface through the sulfur atom.
In the hydrogenoly s es of benzothiazole and phenothiazine, the selective cleavage of C-S bond on benzene ring took place easily at 280°C (Table 5) in a similar manner as the above stated hydrodesulfurization of p- or o-aminobenzenethiol. It was suggested that the hydrodesulfurization of these heterocyclic compounds were extremely accelerated by the presence of ring-substituted nitrogen atom in the ortho position, compared with the hydrodeoxygenation of oxygen analogues presented before.