Ion-exchanged Y zeolites as catalysts for the liquid phase autoxidation of cyclohexane

The effect of Zeolite Y in its acidic form or ion-exchanged with Na+-, Ba2+, Sr2+, or Ca2+ cations on the autoxidation of cyclohexane was studied. Using hydrated ion-exchanged samples, a pronounced catalytic activity, with a reactivity order CaY > SrY > BaY > NaY, can be observed. On the other hand, in the presence of acidic and/or calcined samples, inhibition occurs. In the decomposition of organic hydroperoxides, like cyclohexyl hydroperoxide and t-butyl hydroperoxide, a product distribution typical of homolytic peroxide decomposition is observed when using ion-exchanged Y samples, with the same order of reactivity CaY > SrY > BaY > NaY. This strongly suggests a homolytic peroxide decomposition mechanism to be at the basis of the observed activity in autoxidation reactions, which is confirmed by kinetic analysis. The observed order of activity is indicative of a charge-transfer mechanism. Inhibition is rationalized in terms of heterolytic peroxide decomposition, in the case of acidic samples, and strong peroxide adsorption, followed by intrazeolitic decomposition, in the case of the calcined samples. (C) 1998 Academic Press.