All the data reported previously have been reevaluated to determine the quantitative reaction network for dibenzothiophene hydrodesulfurization at about 100 atm and 300/sup 0/C. This network shows the high selectivity of the catalyst for simple hydrodesulfurization (biphenyl + H/sub 2/S formation). When methyl groups are present in the 4 and 6 positions of dibenzothiophene, the primary hydrogenation reaction is approximately as fast as the primary hydrodesulfurization reaction. Ni and Mo or Ni and W used instead of Co and Mo in the catalyst, or H/sub 2/S in the reactant mixture, also favor hydrogenation relative to hydrodesulfurization. Previous results have been evaluated in light of the recent literature to provide a new interpretation of the reaction mechanism of catalytic hydrodesulfurization on surfaces of sulfided Co-Mo/Al/sub 2/0/sub 3/. 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 deuterium-exchange results, the reactivities of benzothiophene and dibenzothiophene, and the small steric effects of methyl substituents affecting the adsorption and reaction of compounds in the thiophene, benzothiophene, and dibenzothiophene families …

Flow reactor studies of HDS of dibenzothiophene at about 300/sup 0/C and 100 atm have shown a new dependence of catalyst activity on H/sub 2/S concentration at low values. We infer that the catalyst undergoes structural changes during reaction, sometimes becoming sulfur deficient and relatively inactive. It is evidently necessary to maintain the catalyst in a properly sulfided state during reactor startup and operation by avoiding its contacting too little H/sub 2/S. The relative activities of Ni--Mo/Al/sub 2/O/sub 3/, CO--Mo/Al/sub 2/O/sub 3/, and Ni--W/Al/sub 2/O/sub 3/ catalysts for HDN of acridine have been determined, showing that Ni--Mo/Al/sub 2/O/sub 3/ is the most active. The results are interpreted inthe context of the acridine reaction network, in which both the hydrogenation and hydrogenolysis (cracking) reactions are kinetically important.