On the catalytic dehydrogenation of naphthenes I. Kinetic study

Abstract

The kinetics of the dehydrogenation of naphthenes over a chromium oxide supported on alumina catalyst at temperatures of 400° C and above has been investigated. Cyclohexene has been detected and estimated in the products from cyclohexane, and it has been shown that side reactions producing olefine other than cyclohexene proceed at only 0.01 of the speed of the main reaction at 450° C. If the contact time is sufficiently long a stationary concentration of olefine is established. The linear nature of the plot of the reciprocal of the rate against the reciprocal of the pressure for cyclohexane and methyl cyclohexane has been demonstrated experimentally for the stationary state conditions, while the intercepts and slopes have been identified with functions of the rate constants. As a result of the study of the rate of dehydrogenation of cyclohexene it is concluded that the loss of the first pair of hydrogen atoms from cyclohexane is the slow step in the reaction. An activation energy of 36 kcal./g. mol. has been obtained for this step. The variation of the stationary olefine con­centration with temperatures indicates that for cyclohexane and methyl cyclohexane the loss of the first pair of hydrogen atoms is the step with the highest activation energy. The variation of rate with pressure and the behaviour when nitrogen and benzene are mixed with the reactants all prove that benzene and hydrogen act mainly as diluents. Evidence is presented from the failure to attain thermodynamic equilibrium between ethyl benzene, styrene and hydrogen during the dehydrogenation of ethyl cyclohexane for a stepwise mechanism in which a large proportion of the molecules evaporate from the surface after the loss of the first pair of hydrogen atoms.