JO URNA L O F MO LEC ULA R C A TA LYSIS ELSEVIER Journal of Molecular Catalysis 9 1 ( 1994) 343-352 Is metallic palladium formed in Wacker oxidation of alkenes? Milan Hronec, Zuzana CvengroSov& hefan Holotlk Department of Organic Technology, Faculty of Chemzcal Technology, Slovak Technical University, 812 37 Bratislava, Slovak Republic (Received November 24, 1993; accepted February 4, 1994) Abstract The oxidation of cyclohexene with molecular oxygen catalyzed by solid Pd/C or Pd” acetate- hydroquinone-iron phthalocyanine gives cyclohexane, benzene and oxygenated products. Oxygen pressure and solvent used influence significantly the distribution of the products and at the pressure above 2 atm no cyclohexane is formed. Over Pd/C catalyst in the absence of oxygen, disproportion- ation to cyclohexane and benzene (the ratio is nearly 2 : 1) proceeds exclusively. Under comparable conditions 2-cyclohexenol is disproportionated to cyclohexanol and phenol and some of it rearranges to cyclohexanone. The explanation for the disproportionation of cyclohexene under the Wacker conditions is that Pd” centres intermediately formed after stoichiometric oxidation of cyclohexene by Pd” are not completely reoxidized, but depending on the reaction conditions, they can partly aggregate and then, similarly to metallic surfaces, dehydrogenate cyclohexene to benzene. The hydrogen species formed migrate on the palladium surface and hydrogenate cyclohexene or at sufficient oxygen pressure they are oxidized to water. Key words: alkenes; cyclohexene; liquid phase; oxidation; palladmm 1. Introduction Apart from the production of bulk chemicals e.g., different monomers, palladium catalysts are very active, selective and stereospecific in many hydrogenation, oxi- dation, carbonylation and other reactions orientated toward preparation of chemical specialities [ 11. Numerous advances have been made in both the homogeneous and heterogeneous catalytic oxidation of various hydrocarbons, alcohols, carbo- hydrates, where the oxidation state of the catalytically active palladium sites and *Corresponding author; fax. ( +42-7)495381. 0304.5102/94/$07.00 0 1994 Elsewer Science B.V. All rights reserved SSD10304-5102(94)00043-U