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The liquid-phase hydrogenation kinetics of benzene and three monosubstituted alkylbenzenes, toluene, ethylbenzene, and cumene, was determined in a semibatch reactor operating at hydrogen pressures of 20−40 atm and at temperatures of 95−125 °C. Commercial preactivated catalyst particles of nickel−alumina were used in all experiments. The hydrogenation activity of the compounds decreased in the order benzene ≫ toluene > ethylbenzene > cumene. The main reaction product was always the completely hydrogenated cycloalkane, whereas only trace amounts of cycloalkenes were detected. The hydrogenation rates had a slow initial period followed by a period of a virtually constant rate, which decreased at the end of the reaction. The analysis of the data with a reaction−diffusion model revealed that the kinetics was influenced by pore diffusion. Rate equations based on a sequential addition mechanism of adsorbed hydrogen to the aromatic nucleus were derived, and the kinetic parameters were estimated from the reaction−diffusion model with nonlinear regression analysis. The rate equations were able to describe all the features of the experimental data.

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The kinetics of the liquid-phase hydrogenation of naphthalene and tetralin (1,2,3,4-tetrahydronaphthalene) in decane was studied on a commercial nickel catalyst at 80−160 °C and 20−40 bar in a CSTR. The proposed kinetic model assumes three adsorption modes (π-, π/σ-, and σ-adsorption), of which two are associative and one is dissociative. The associatively adsorbed aromatic compounds are assumed to be active in hydrogenation, whereas the dissociative adsorption leads to coke formation. Moreover, it is proposed that naphthalene adsorption occurs on a single active site, whereas tetralin adsorption requires an ensemble of Ni atoms. This explains the nonlinear decrease in the tetralin hydrogenation rate with catalyst deactivation, whereas the naphthalene hydrogenation decreases linearly. The proposed reaction and deactivation mechanism is able to describe the main features of the observed kinetics, including the formation of octalins (octahydronaphthalene), changes in the cis-to-trans selectivity of decalin (decahydronaphthalene), and the difference between the naphthalene and tetralin hydrogenation and deactivation rates.

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