ISOMERIZATION OF N-HEPTANE ON AN OXYGEN-MODIFIED MOLYBDENUM CARBIDE CATALYST

The isomerization of n-heptane in the presence of hydrogen has been ca rried out over a molybdenum carbide catalyst modified by an oxygen tre atment. Heptane was isomerized selectively to isoheptanes, a reaction which is difficult over traditional bifunctional catalysts due to exte nsive cracking. The C7 products were mainly monomethylhexanes, 2-methy lhexane and 3-methylhexane, in close to equilibrium ratios. A typical bifunctional catalyst (Pt supported on an acidic zeolite) gave similar isomerization products, but mostly propane and isobutane as the crack ed products. The selectivity over the oxidized carbide was found to be a function of pressure but independent of the conversion; increased t he hydrogen pressure led to a decrease in the C7 selectivity. This was found to be different from the Pt/zeolite catalyst, over which the se lectivity was a function of the conversion; a high selectivity was onl y obtained at low conversions. The active carbide-based catalyst was p robably an oxycarbide of molybdenum. The results obtained over the oxi dized carbide catalyst are discussed in terms of a bond-shift mechanis m via a metallocyclobutane intermediate.