Hydrogenation by CoMo/Al2O3 catalyst. (Part 3). Effect of H2S on hydrogenation of monoaromatic hydrocarbons.

Abstract

Reactions of monoaromatic hydrocarbons over the presulfided CoMo/Al₂O₃ catalyst were examined, and the effect of gas-phase H₂S on the reactions was studied in connection with the extent of sulfidation of the catalyst. The reactions were confined to hydrogenation forming the corresponding ring saturated compounds and subsequent skeletal isomerization resulting in ring contraction. The addition of H₂S to the gas-phase inhibited hydrogenation of benzene, but accelerated hydrogenation of o-xylene. No effect was observed for hydrogenation of toluene. It was further observed that in the hydrogenation of o-xylene, the promoting effect of H₂S varied according to its concentration, and the addition of 5% H₂S increased twice the conversion in H₂ flow. As to the skeletal isomerization, however, the promoting effect was observed in all three reactants when H₂S was added to the gas-phase. The extent or levels of sulfidation of the presulfided catalyst were measured thermogravimetrically, and they were found to vary according to the concentration of gas-phase H₂S. 30% of the total sulfur contained in the presulfided catalyst, corresponding to 0.6 atom S per Mo atom, was found to be mobile. Combining it with the activity test, this mobile sulfur seems to be effective for hydrogenation. The promoting effect of H₂S is ascribed to the formation of new active sites which can exist only in the H₂S/H₂ atmosphere, and the inhibiting effect is ascribed to the competitive adsorption of H₂S and monoaromatic compounds.