The hydrogenation of toluene and o-, m-, and pxylene over palladium : I. Kinetic behavior and o-xylene isornerization

Abstract

The vapor-phase hydrogenation of toluene and o-, m-, p-xylene was studied over Pd powder and Pd dispersed on MgO, SiO 2, Al 2O 3, SiO 2Al 2O 3, and TiO 2. As with benzene, the use of the last two acidic oxides can markedly enhance activities, but the relative change diminishes as methyl groups are added; for example, the average enhancement in turnover frequency was 8 for toluene while the lowest was 3 for m-xylene. The turnover frequency decreased in the following order: benzene > toluene ≥ p-xylene ≥ m-xylene> o-xylene; however, the differences were less pronounced for Pd powder and Pd/MgO compared to Pd/SiO 2Al 2O 3 and Pd/TiO 2. This trend is attributed to an electronic effect on the aromatic ring related to the addition of methyl groups although steric hindrance also appears to be responsible for the consistently low rates for o-xylene hydrogenation. All reactions were first order in H 2 and zero order in the aromatic compound, and activation energies were near that of benzene and did not vary significantly, as average values were 11.8, 12.0, 12.7, and 13.7 for toluene, m-xylene, o-xylene, and p-xylene, respectively. Isomerization of o-xylene to m-xylene was observed over all the Pd catalysts, but rates were fivefold higher on the most acidic Pd/SiO 2Al 2O 3 and Pd/TiO 2 catalysts thus verifying the presence of acid sites on the support.