Reactions of n-Hexane on Pt Catalysts: Reaction Mechanism as Revealed by Hydrogen Pressure and Compensation Effect

The reactions of n-hexane + hydrogen have been studied using two Pt black catalysts of different preparation (Pt–N, reduced by hydrazine; Pt–HCHO, reduced by formaldehyde). The activities and selectivities were compared with those of EUROPT-1 (6.3% Pt/SiO2). Maximum rates were observed as a function of hydrogen pressure for the overall reaction as well as for hydrogenolysis, isomerization, C5-cyclization, and aromatization. Dehydrogenation to hexenes decreased with increasing H2 pressure. Both Pt black catalysts showed lower turnover frequencies than did EUROPT-1 and the maximum rates appeared at higher hydrogen pressure. The values of Arrhenius parameters calculated in the negative and in the positive hydrogen order range were different for the overall reaction as well as for the individual processes. The apparent activation energies showed compensation effects. Different compensation lines and isokinetic parameters were obtained in the range of positive and negative hydrogen order. The isokinetic parameters indicated that the rate-determining step in the positive hydrogen order may have been a Pt—C bond breaking (“hydrogenative product desorption”), while a C—C bond rupture could be the rate-limiting step in excess hydrogen. The compensation effects of different reactions under different conditions contributes to our knowledge of the reaction mechanism.