The mechanism of the oxidation of propene to acrolein over antimony-tin mixed oxide catalysts

Abstract

The oxidation of propenes such as 13CH2=CH---CH,3, CH2=CH-CD3, cis---CHD=CD---CH3, and CH2---CH---CH3 was studied over Sb6O13, SnO2, and Sb---Sn mixed oxide catalysts. The results with 13CH2=CH---CH3 and CH2=CH---CD3 were consistent with a [pi]-allyl intermediate. The isotope effect for allylic hydrogen abstraction was 110.55 (kHlkp) over the Sb---Sn oxide catalysts, indicating that this is the slowest step in the formation of acrolein as with other catalyst systems. The oxidation of CHD=CH---CH3 did not exhibit a marked isotope effect for the second hydrogen abstraction. This is inconsistent with a fast [pi]-allyl to [sigma]-allyl equilibration process or the irreversible [pi]-allyl to [sigma]-allyl conversion observed over other metal oxide catalysts. The absence of an isotope effect is similar to oxidations over rhodium. The roles of Sn and Sb ions in the oxidation are also discussed