H. Kwart

Hydrodesulfurization of methyl-substituted dibenzothiophenes catalyzed by Co-Mo/gamma-Al2O3

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Hydrodesulfurization of thiophene, benzothiophene, dibenzothiophene, and related compounds catalyzed by sulfided CoO-Mo3/gamma-Al2O3 : low-pressure reactivity studies

Hydrodesulfurization experiments were carried out with a sulfided CoOMoO3γ-Al2O3 catalyst in a pulse microreactor operated at atmospheric pressure and temperatures of 350 to 450 °C. The reactants were hydrogen and pure sulfur-containing compounds (or pairs of compounds), including thiophene, benzothiophene, dibenzothiophene, several of their hydrogenated derivatives, and various methyl-substituted benzothiophenes and dibenzothiophenes. The aromatic compounds appeared to react with hydrogen by simple sulfur extrusion; for example, dibenzothiophene gave H2S + biphenyl in the absence of side products. The reactivities of thiophene, benzothiophene, and dibenzothiophene were roughly the same. Each hydrogenated compound (e.g., tetrahydrothiophene) was more reactive than the corresponding aromatic compound (e.g., thiophene). Methyl substituents on benzothiophene had almost no effect on reactivity, whereas methyl substituents on dibenzothiophene located at a distance from the S atom slightly increased the reactivity, and those in the 4-position or in the 4- and 6-positions significantly decreased the reactivity. In contrast to the observation of a near lack of dependence of low-pressure reactivity on the number of rings in the reactant, the literature shows that at high pressures the reactivity decreases with an increased number of rings. The pressure dependence of the structure-reactivity pattern is suggested to be an indication of relatively less surface coverage by the intrinsically more reactive compounds (e.g., thiophene) at low pressures but not at high pressures. The relative reactivities are also suggested to be influenced by differences in the structures of the catalyst at low and high hydrogen partial pressures, which may be related to the concentrations of surface anion vacancies and the nature of the adsorbed intermediates.

Hydrodesulfurization of thiophenic compounds: the reaction mechanism

Abstract It has often been assumed that catalytic hydrodesulfurization of thiophene and related compounds proceeds via a one-point “end-on” adsorption involving bonding of the sulfur atom with Mo ions at an anion vacancy on the catalyst surface. This interpretation is inadequate, failing to account for hydrogen-deuterium exchange results, the reactivities of benzothiophene and dibenzothiophene, and the small steric effects of methyl substituents on the adsorption and reaction of compounds in the thiophene, benzothiophene, and dibenzothiophene families of homologs. An alternative mechanism, presented here, involves a multipoint adsorption of the reactant, with a CC bond interacting with a Mo cation, and the S atom of the reactant interacting with a S ion on the surface. The new mechanism accounts for the observed deuterium exchange and hydrodesulfurization reactions and is consistent with the observed steric effects.

Hydrogenation of aromatic compounds catalyzed by sulfided CoOMoO3γ-Al2O3

Abstract Hydrogenation of aromatic compounds catalyzed by sulfided CoOMoO 3 γ- Al 2 O 3 was investigated with batch and flow reactors at 275–350 °C and pressures near 100 atm. The reactants included benzene, biphenyl, dibenzothiophene, naphthalene, 2-phenylnaphthalene, and benzo-[ b ]naphtho[2,3- d ]thiophene. The reactivity for ring saturation was about an order of magnitude greater for the latter three [(substituted) naphthalenes] than for the others [(substituted) benzenes]. Sulfur in a reactant molecule slightly increased the reactivity of a neighboring ring for hydrogenation. Dibenzothiophene hydrogenation was not inhibited by H 2 S, but biphenyl hydrogenation was moderately inhibited by H 2 S and dibenzothiophene hydrogenolysis was strongly inhibited by H 2 S. These and other results suggest that the reactant to be hydrogenated is π-bonded at exposed Mo cations, where H 2 S undergoes weak competitive adsorption and bases like acridine undergo strong competitive adsorption.