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 st udied. Using hydrated ion-exchanged samples, a pronounced catalytic ac tivity, 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, l ike cyclohexyl hydroperoxide and t-butyl hydroperoxide, a product dist ribution 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 decompos ition mechanism to be at the basis of the observed activity in autoxid ation reactions, which is confirmed by kinetic analysis. The observed order of activity is indicative of a charge-transfer mechanism. Inhibi tion is rationalized in terms of heterolytic peroxide decomposition, i n the case of acidic samples, and strong peroxide adsorption, followed by intrazeolitic decomposition, in the case of the calcined samples. (C) 1998 Academic Press.