Portal de Periódicos da CAPES

Relations between active phase characteristics and catalytic properties in supported metal oxide catalysts: the dehydrogenation of cyclohexane on molybdena catalysts supported on γ-Al2O3 doped with alkali cations

1985; Elsevier BV; Volume: 15; Issue: 2 Linguagem: Inglês

10.1016/s0166-9834(00)81844-8

1873-3867

Christos Kordulis, Alexis Lycourghiotis, S. Voliotis, P.J. Pomonis,

Catalytic Processes in Materials Science

The dehydrogenation of cyclohexane to benzene on molybdena catalysts supported on γ-Al2O3 modified by various amounts of alkali cations has been studied. Kinetic experiments were carried out on the reduced form of these catalysts using a flow bed reactor. These allowed the determination of the fractional conversions in the temperature range 410-490°. It was found that the conversion increased with time of reduction while one “plateau” of constant catalytic activity was attained after 5 h. Moreover, it was found that the modification of the y-A1203 causes a decrease in the catalytic activity. Conductivity measurements obtained at various temperatures allowed the determination of the activation energy of conduction for the catalysts studied. No relation exists between this parameter and kind or content of the modifier. D.R.S. spectra of some catalysts obtained after catalytic tests for 12 h allowed the estimation of the concentration of the supported Mo ions with valence lower than six. Modification by Li+ cations does not practically affect this concentration, whereas doping by the remaining alkali cations inhibits the transformation of Mo(VI) into Mo with valence lower than six. Using Arrhenius law as a criterion the mechanistic scheme of the reaction has been established. On the base of this scheme apparent activation energies were determined for the catalytically active specimens. In our system, it was demonstrated that the apparent activation energy is not an appropriate measure of the catalytic activity. The mechanism of the reaction was found to be independent of the geometrical and structural characteristics of the molybdenum phase as well as of the semi-conducting properties of the specimens. The catalytic activity of the catalysts examined depends almost exclusively on the valence of the supported molybdenum. The dispersion of the molybdenum phase does not relate to the catalytic activity. Moreover, no relation exists between the semiconducting properties of the specimens as estimated by the activation energy of conduction and the catalytic activity.