Catalytic activity of reduced MoO3/alpha-Al2O3 for hexanes reforming - II.Catalytic activity and mechanistic approach using C-13 tracer studies and probe molecules

The catalytic activities of 4-methyl-1-pentene and of the corresponding alk ane, 2-methylpentane, are compared on two series of reduced MoO3 catalysts supported on alpha-type-Al2O3, prepared either by impregnation with Na2MoO4 or by the slurry impregnation method starting from MoO3. The reduction of these samples depends on the molybdenum oxide content and therefore on the interaction with the alumina surface. The use of probe molecules, such as 3 ,3-dimethylbut-1-ene (dibranched containing a quaternary carbon atom), 2,3- dimethylbut-2-ene (dibranched with two tertiary carbon atoms), 4-methyl-1-p entene (monobranched with a tertiary carbon atom), l-hexene(linear), and cy clohexene (cyclic), combined with C-13 tracer studies leads to the conclusi on that isomerization mainly occurs on acidic sites via surface alkoxy inte rmediates, resulting in protonated cyclopropane species. From the correlati on with XPS measurements, it can be concluded that Mo(V) species are respon sible for acidic isomerization and that Mo(IV) and Mo(II) species exhibit m etallic dehydrogenation properties. Nevertheless, metallic isomerization vi a metallocyclobutane intermediates should be considered when reduction at h igh temperature occurs. (C) 2000 Academic Press.