Kinetics of the HDS of 4,6-dimethyldibenzothiophene and its hydrogenated intermediates over sulfided Mo and NiMo on γ-Al2O3

To study the hydrodesulfurization (HDS) reaction network of 4,6-dimethyldibenzothiophene (DMDBT), three hydrogenated intermediates—tetrahydro-, hexahydro-, and dodecahydro-DMDBT—were synthesized, and their HDS was investigated over sulfided Mo and NiMo on γ-Al2O3 catalysts at 300 °C and 5 MPa. Tetrahydro-DMDBT reacted by hydrogenation to hexahydro-DMDBT, which in turn reacted to dodecahydro-DMDBT by hydrogenation and to 3,3′-dimethylcyclohexylbenzene by desulfurization. All four diastereoisomers of hexahydro-DMDBT were observed, all of which interconverted rapidly during HDS. Dodecahydro-DMDBT reacted by desulfurization to 3,3′-dimethylbicyclohexyl. The rate constants of all steps in the kinetic network of the HDS of DMDBT could be measured over Mo/γ-Al2O3, and those of some steps could be measured over NiMo/γ-Al2O3. The first step—hydrogenation of DMDBT to tetrahydro-DMDBT—is the slowest, rate-determining step, but the hydrogenation of and the sulfur removal from hexahydro-DMDBT are also slow. Opening of the sulfur-containing ring in DMDBT and its hydrogenated intermediates occurs by CS hydrogenolysis rather than by elimination.