Steam dealkylation of aromatic hydrocarbons: II. Role of the support and kinetic pathway of oxygenated species in toluene steam dealkylation over group VIII metal catalysts

Abstract

The role of the support in steam dealkylation (SDA) is studied on a series of Group VIII metal catalysts supported on alumina, silica, and titania. When possible turnover frequencies are given on the basis of the free metal fraction during the reaction. The values are generally constant with time-on-stream and represent the actual turnover frequency of the catalyst. Metals can be classified into two groups, namely, support-sensitive metals (Pt, Rh, Pd) and support-insensitive metals (Ni, Co, Ru, and to a certain extent Ir). Support sensitivity is related to the oxidizability of the metallic surface. For metals of the first group, the reaction is probably governed by a noncompetitive mechanism in which the metal coverage by the oxygenated species is negligible. Kinetic derivation leads to a rate law where there is at once intervention of the support site concentration and of the specific perimeter of the metal/support interface. One can thus explain the support effect for this metal group and the slight sensitivity to the crystallite size observed in the Rh Al 2O 3 series. For metals of the second group, a competitive mechanism probably takes place on the metal. Kinetic derivation leads to a rate law independent of the support site concentration and accounting for the slight negative order with respect to toluene as previously reported. The conspicuous parallelism between the selectivities of the various metals in SDA, in hydrodealkylation, and in hydrogenolysis is also discussed. In addition to the metal, the support and the crystallite size are determining factors of the selectivity to benzene in SDA.