Reactions of 2- and 3-Methylpentane, Methylcyclopentane, Cyclopentane, and Cyclohexane on Activated Mo2C

Abstract

Molybdenum carbide, prepared by a new synthetic route and further activated either by reductive or by oxidative treatments, has been studied as a catalyst for isomerization of 2- and 3-methylpentane and also for hydrogenolysis of cyclopentane, cyclohexane, and methylcyclopentane. The nature of this activating pretreatment dramatically modifies both the activity and the selectivity of the catalyst. Reductive pretreatments lead either to inactive, or to active but poorly selective, catalysts (Mo metal on the surface or pure Mo carbide), whereas oxidative treatment first inhibits the hydrogenolysis reaction present on oxygen-free molybdenum carbide, and second, enhances the isomerization rate of the catalyst. The branched molecules isomerize to give mostly acyclic isomers. No deactivation is observed with time on stream during isomerization of the branched molecules. The rate of hydrogenolysis of methylcyclopentane and cyclopentane is about four times lower than the rate obtained with branched molecules, and the catalyst is strongly deactivated. This phenomenon is attributed to the formation of highly dehydrogenated polyaromatics, and to the formation of CH4, by demethylation of methylcyclopentane, which can irreversibly reduce the oxicarbide phase. The deactivated catalyst can easily be regenerated by a short oxidative treatment at 350°C. The results, in terms of both activity and isomer selectivity, obtained for n-hexane, 2-methylpentane. and 3-methylpentane isomerization are attributed to a bond-shift mechanism involving a metallocyclobutane ring intermediate which would take place on an oxicarbide phase.