Partial Hydrogenation of Benzene to Cyclohexene on Ruthenium Catalysts Supported on La2O3-ZnO Binary Oxides

Sung Cheng Hu, Yu Wen Chen

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67 Scopus citations

Abstract

The partial hydrogenation of benzene to cyclohexene at 2.34-5.10 MPa of hydrogen, 125-175 °C, was studied over Ru catalysts supported on lanthanum and zinc binary oxides with various atomic ratios in a stirred autoclave and in the presence of an aqueous sodium hydroxide solution. Due to the very small particle size of the catalyst, the rate of benzene or cyclohexene conversion was not affected by mass-transfer limitations, so that the measured kinetic data directly reflect the reaction on the catalyst surface. It was found that a binary oxide of lanthanum and zinc as a support material for the Ru catalyst is more selective and always gets a much higher yield of cyclohexene than that of either La2O3 or ZnO alone in this reaction. In addition, the catalyst with the oxide composition of La/Zn = 1/5 (atomic ratio) gives the highest yield of cyclohexene. Therefore, it is a potential candidate for producing cyclohexene from the industrial viewpoint. With zinc oxide as a major component for the support material, the ruthenium catalyst, originally hydrophobic, is rendered hydrophilic. With water in the reaction mixture, the Ru catalyst could be surrounded with a stagnant water film (i.e., a large amount of water adsorbed on the surface so as to become a water film), and also because of the very low solubilities of the reactants in the water, a slow adsorption process on the catalyst surface results. Water then expels easily the formed cyclohexene from the catalyst surface, which slows down the subsequent hydrogenation of cyclohexene. The observed reaction rate suppression for all binary-oxide-supported catalysts could be ascribed to this slow adsorption effect. The rate limitation at elevated temperature could also be explained by decreasing the surface coverages of benzene and hydrogen. Reactants such as hydrogen, cyclohexene, and benzene compete to a certain extent on the catalyst surface; this can explain why the initial reaction rate diminishes on raising the hydrogen pressure beyond the certain limit corresponding to the equal surface coverages of benzene and hydrogen.

Original languageEnglish
Pages (from-to)5153-5159
Number of pages7
JournalIndustrial and Engineering Chemistry Research
Volume36
Issue number12
DOIs
StatePublished - Dec 1997

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