The mechanism of the oxidation of propene to acrolein over copper oxide

Abstract

The oxidation of stereo-labeled propene to acrolein over copper oxide was studied to obtain information about the nature of the intermediates involved in the second hydrogen abstraction and oxygen incorporation process. Experiments with (E)---CHD=CHCH3 and (Z)---CHD=CHCH3 gave deuterated acrolein product ratios which were independent of the stereochemistry of the starting material. The amounts of (E-acrolein-3-d1, (Z)-acrolein-3-d1, and acrolein-1-d1 were in the ratio of 1:1:1.6 which are consistent with irreversible conversion of a [pi]-allyl to a [sigma]-allyl intermediate. They exclude a geometric discrimination effect in the second H abstraction step. The reaction of (E)-CHD=CDCH3 was also consistent with irreversible formation of a [sigma]-allyl species and gave a discrimination isotope effect of 0.37 +/- 0.06 for the second H abstraction. The results from the oxidative dehydrogenation of D- and 18O-labeled allyl alcohol show no end-to-end deuterium scrambling and partial loss of 18O in the acrolein. This suggests that a [sigma]-allyl species bonded to the surface through oxygen does not equilibriate between the end carbon atoms before desorbing to acrolein. The differences in behavior of the [sigma]-allyl intermediates over copper oxide and molybdate catalysts are discussed.