Selective Dehydrogenation of Cycloalkanes with Sulfur Tolerant Catalysts

Cyclohexane and other alkyl cyclohexanes were dehydrogenated selectively to benzene and alkyl benzenes with active carbon catalysts which supported metals such as vanadium, thorium, copper, cobalt and nickel on it. While metals such as copper, cobalt and nickel lost their original activity almost completely by being sulfided by H₂S, they exhibited excellent activities and selectivities when supported on active carbon. Especially, sulfided nickel (Ni (s)) on active carbon showed the best capability as the catalyst. The catalytic activity of Ni (s) on active carbon for cyclohexane dehydrogenation increased with increasing metal content in the catalyst up to 5 wt%. However, above the limit the activity was independent of the metal loadings. The reaction mechanism was explained by the “reverse spillover” mechanism, which comprises the transfer of hydrogen atoms from feed hydrocarbons to carbon surface, the migration of hydrogen atoms from carbon surface to metal or metal sulfide particles on it and the recombination of hydrogen atoms to hydrogen molecules to desorb into gas phase.<BR>The reactivity of mono-alkyl cyclohexane increased with the increase in the molecular weight of them, suggesting the role of van der Waals force in the adsorption of reactant onto active carbon.<BR>The reaction of hydrocarbon mixture which comprises normal hexane and methyl cyclohexane revealed that the dehydrogenation of normal hexane was suppressed markedly while that of methyl cyclohexane was not. The phenomenon are interpreted by that the active sites on carbon are covered by hydrogen from methyl cyclohexane.<BR>It was emphasized by the authors that the present sulfur tolerant catalysts (sulfided metal-active carbon) are favored for reforming coal derived naphtha which containes organo sulfur- and organo nitrogeno- compounds in high concentrations and thus is unsuitable for being treated with the conventional reforming catalysts.