Cyclohexane dehydrogenation kinetics on a platinum‐rhenium/aluminium oxide catalyst with hydrogen and inert diluents

Abstract

AbstractThe dehydrogenation of cyclohexane to benzene on a platinum‐rhenium/aluminium oxide (Pt‐Re/Al2O3) catalyst was investigated using both pulse and continuous flow techniques with hydrogen and inert (N2, He, Ar) diluents at the following conditions: 1 and 4 atm (1 atm = 101 kPa) total pressure and 432–623 K. Experiments performed with hydrogen as a carrier in the pulse mode showed anomalous behaviour as cyclohexane conversion was observed to increase with pulse size/pulse width and decrease with temperature and contact time. The dehydrogenation reaction was first‐order with respect to cyclohexane for both the pulse and continuous flow experiments. Lower activation energies were observed for the inert carriers; activation energies were between 37.7–54.4 kJ mol<−1 for inert diluents. With a hydrogen carrier, the activation energies were found to be 87.9 and 121.3 kJ mol−1 for T<553 K and <553 K, respectively. The difference in activation energy between hydrogen and the inert carriers was attributed to a change in mechanism on the coked catalyst surface. Surface conversion of cyclohexane to adsorbed cyclohexane was the slowest step in a hydrogen carrier. For the inert carriers, it was postulated that surface diffusion of adsorbed intermediate for for subsequent conversion was rate determining.