Gas-Phase Kinetic Studies of Tetralin Hydrogenation on Pt/Alumina

The hydrogenation of aromatic compounds is a reaction of relevance in the improvement of diesel fuel quality. In this work, the kinetics of tetralin (1,2,3,4-tetrahydronaphthalene) hydrogenation over Pt/Al2O3 catalysts have been studied in an integral fixed bed reactor at 500 psig in the temperature range 553-573 K. A semiempirical Langmuir-Hinshelwood equation has been used to model the kinetics. The values of the kinetic parameters were determined by a nonlinear fitting, while the heats of adsorption for the hydrocarbons involved (tetralin and cis- and trans-decalin) were experimentally obtained by temperature-programmed desorption. The adsorption competition between tetralin and the hydrogenated products (cis- and trans-decalin) for surface active sites can be quantitatively traced to the higher heat of adsorption of tetralin compared to those of cis- and trans-decalins. As a result, while the activation energy for the cis-trans isomerization is significantly lower than the one for tetralin hydrogenation and its intrinsic reaction rate much higher than that of hydrogenation, the isomerization reaction can only be observed when the concentration of tetralin in the gas phase is very much reduced. In consequence, at low conversions, the trans/cis ratio remains unchanged, and it only increases to its equilibrium value at very high tetralin conversions.