Hydrogenolysis of alkanes. Part 3.—Hydrogenolysis of n-hexane and methylcyclopentane over variously treated Ru/TiO2 catalysts

Abstract

The hydrogenolysis and isomerisation of n-hexane and methylcyclopentane have been studied on 0.5, 1.0 and 5.0% Ru/TiO2 catalysts pretreated in air at 623 K and in hydrogen at 758 or 893 K. Catalysts reduced without a prior oxidation treatment have activities for the n-hexane reaction which are intermediate between those of oxidised catalysts and of catalysts which, after oxidation, are reduced a second time at 758 or 893 K. It is suggested that residual chloride on the catalysts after the first reduction diminishes the activity but prevents the creation of the strong metal–support interaction (SMSI) state, which is believed to be inactive for hydrogenolysis reactions. Re-oxidation eliminates most of the chloride and disperses the Ru. Reduction at 433 K leads to a very active catalyst. Subsequent high-temperature reduction in the absence of chloride produces a catalyst with a very low activity. A catalyst prepared from a chloride-free salt has intermediate properties. However, this catalyst still does not fully enter the SMSI state in the absence of a preoxidation treatment. The product distribution in the n-hexane hydrogenolysis reaction is almost invariant, irrespective of which catalyst or pretreatment is used. Catalysts after high-temperature reduction are selective for the isomerisation of n-hexane to 2- and 3-methylpentane. After the initial reduction, the activity of the 5% Ru catalyst for the hydrogenolysis of methylcyclopentane is almost identical to that of the oxidised catalyst. However, the selectivity in the ring-opening reaction to 2- and 3-methylpentane is very sensitive to the method of preparation. After a second reduction the activity decreases sharply but the selectivity is identical to that of the catalyst after the first reduction. The nature of the catalysts after each pretreatment and the effect of changes in the catalysts on the catalytic properties are discussed.