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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.

Liquid-phase hydrogenation of toluene in isooctane, n-heptane, and cyclohexane was studied on a commercial Ni/Al 2 O 3 catalyst at 100-200°C and 20-40 bar. None of the solvents was adsorbed on the catalyst. The variation in the hydrogenation rates is explained by the variation in the hydrogen solubility: the reaction rates in isooctane and n-heptane were similar, whereas the reaction rate in cyclohexane was lower, especially at higher temperatures. The reaction order for toluene was close to 0, while the reaction order with respect to hydrogen increased from near 0 to 1 with increasing temperature. A kinetic model based on the multiplet theory describes well the reaction orders and gives a good fit with physically reasonable parameters.

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Deactivation of palladium and platinum catalysts due to coke formation was studied during hydrogenation of methyl esters of sunflower oil. The supported metal catalysts were prepared by impregnating γ-alumina with either palladium or platinum salts, and by impregnating α-alumina with palladium salt. The catalysts were reused for several batch experiments. The Pd/γ-Al 2 O 3 catalyst lost more than 50% of its initial activity after four batch experiments, while the other catalysts did not deactivate. Samples of used catalysts were cleaned from remaining oil by repeated extractions with methanol, and the amount of coke formed on the catalysts was studied by temperature-programmed oxidation. The deactivation of the catalyst is a function of both the metal and the support. The amount of coke increased on the Pd/γ-Al 2 O 3 catalyst with repeated use, but the amount of coke remained approximately constant for the Pt/γ-Al 2 O 3 catalyst. Virtually no coke was detected on the Pd/α-Al 2 O 3 catalyst. The formation of coke on Pd/α-Al 2 O 3 may be slower than on the Pd/γ-Al 2 O 3 owing to the carrier's smaller surface area and less acidic character. The absence of deactivation for the Pt/γ-Al 2 O 3 catalyst may be explained by slower formation of coke precursors on platinum compared to palladium. , temperature-programmed oxidation (TPO). FIG. 2. (A) Measured coke content on samples contacted with oil for various times and temperatures. (B) Coke formation on the catalysts vs. time on-stream. Each symbol indicates one batch. The coke concentration was not analyzed for the Pd-α-Al 2 O 3 catalyst after batches 2 and 3. The other two curves represent catalysts on γ-Al 2 O 3 support.

The hydrogenation reactivity of some aromatic compounds used to model diesel fractions was examined under sulfur-free conditions. Reactions of toluene, 1,2,3,4-tetrahydronaphthalene (tetralin), and naphthalene, separately and as mixtures, were studied using a commercial Ni/ Al 2 O 3 catalyst. The reactivity decreased in the following order: naphthalene . tetralin > toluene. Because of competitive adsorption and subsequent inhibition, naphthalene severely reduced the other hydrogenation rates in mixtures, whereas the hydrogenation rate of naphthalene was little affected by the concentration of toluene or tetralin. A kinetic model based on the general form of the Langmuir-Hinshelwood equation was developed. The activation energies of toluene, tetralin, and naphthalene were found to be 52.9, 40.4, and 58.7 kJ/mol, respectively, and the reaction orders of the monoaromatic and diaromatic compounds were about 1.4 and 2.1, respectively. Furthermore, simulations showed that the reaction kinetics in mixtures can be successfully described with models based on single-compound experiments, if surface-concentration terms (K i c i ) for all aromatics are included in the rate equations.

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