Catalytic oxidation over molecular sieves ion-exchanged with transition metal ions VI. A kinetic study of the oxidative dehydrogenation of cyclohexane over Y sieve ion-exchanged with cupric ion

Abstract Kinetics and adsorption in the oxidative dehydrogenation of cyclohexane over Cu(II)-Y were studied to elucidate the reaction mechanism and the key to the high selectivity of Cu(II)-Y catalyst among the molecular sieves ion-exchanged with transition metal ions. Rate equations for benzene and carbon dioxide formation were V( benzene ) = k 1 p 2 , and V( CO 2 ) = k 2 p 2 1 2 (p2, partial pressure of oxygen) under partial pressures of 0.05 to 0.95 (oxygen) and 0.01 to 0.18 atm (cyclohexane) at 320 °C. Adsorption of reactants measured by means of the pulse gas Chromatographic technique revealed the following results: (a) an oxygen pulse converted preadsorbed cyclohexane exclusively into carbon dioxide, but a cyclohexane pulse was converted by preadsorbed oxygen into benzene; (b) adsorption of oxygen was saturated, whereas that of cyclohexane increased with partial pressure in the same pressure range as used in the kinetic measurements; (c) reactivity of adsorbed oxygen was much higher than that of adsorbed cyclohexane; (d) no adsorbed oxygen was observed during the oxidation reaction in spite of (b) above. The results indicate that adsorption of oxygen is rate determining in the oxidative dehydrogenation for benzene formation. The high benzene yield of Cu(II)-Y is assumed due to its high adsorption ability for oxygen. Catalytic activities of the Y sieves ion-exchanged with cupric ion at several levels were also studied to discuss the location of the active site for this oxidation.