Gopal H. Singhal

Hydrodesulfurization of sulfur heterocyclic compounds: Kinetics of dibenzothiophene

A kinetic analysis of the hydrodesulfurization (HDS) of dibenzothiophene (DBT), a representative model of the condensed sulfur heterocyclics found in heavy petroleum feeds, has been carried out. Kinetic data were generated over standard Co-Mo/..gamma..-Al/sub 2/O/sub 3/ catalyst at temperatures of 285 to 350/sup 0/C by varying hydrogen partial pressure, liquid hourly space velocity, and DBT feed concentration. From the data, models obeying Langmuir-Hinshelwood kinetics were developed to account for the rate of disappearance of DBT as well as for the selectivity of product formation. The models imply that two types of sites on the catalyst surface are involved in the HDS reaction, on one of which dibenzothiophene and the reaction products biphenyl, cyclohexylbenzene, and hydrogen sulfide competitively adsorb and a second site on which hydrogen adsorbs. The kinetic models developed to account for the appearance of the products indicate that product formation proceeds predominantly by a parallel path mechanism. However, there is a small contribution to the cyclohexylbenzene production from the sequential hydrogenation of biphenyl which becomes significant at higher temperatures.

Hydrodesulfurization of sulfur heterocyclic compounds: Reaction mechanisms

Abstract The hydrodesulfurization of sulfur heterocyclic compounds over a commercial Co-Mo γ- Al 2 O 3 catalyst has been studied under close to commercial conditions (300–350 °C, 20–40 atm). The compounds were selected on the basis of stability, steric requirement, and resonance interactions and included thianthrene, 1,4,6,9-tetramethylthianthrene, tetraphenylthiophene, dibenzothiophene, and some of its potential intermediates. The study strongly suggests that the pathways followed by the compounds are structure dependent and can be divided into two categories: (1) in compounds where the electron density is localized on the sulfur atom, desulfurization by direct coordination of the sulfur to a catalyst site can take place giving only the aromatic products; (2) in compounds such as dibenzothiophene in which the electron density of the sulfur atom is delocalized over an extensive π system and steric crowding around the sulfur atom is present, the first step in the reaction pathway is probably a π-complex formation which is followed by hydrogenation and desulfurization. In this case, both aromatic and partially saturated compounds are produced. It is proposed that these products result from a common intermediate suggesting a primarily parallel rather than sequential path for the formation of the hydrogenated products.