Oxidation of propene by means of SnO 2-MoO 3 catalysts : III. Kinetic hydrogen isotope effects

Abstract

A mixture of 8.9% of propene-2- d (CH 2CDCH 3) and 91.1% of ordinary propene was oxidized over an SnO 2-MoO 3 catalyst (at 370 °C and 1 atm; air and water vapor present; continuous flow). The percentage of propene-2- d in the nonconverted propene ( 2 3 of the initial quantity) was 10.8. It is concluded that ordinary propene reacts about 2.2 times faster than propene-2- d. In another test, ordinary propene was first supplied and then propene-2- d; the degree of conversion was rather low. The formation of acetone, acetic acid, acetaldehyde and maleic acid was attended by kinetic isotope effects; those relating to acetone and acetic acid differed appreciably, but the average was 1.8, smaller than the 2.2 cited above. The difference must be due to a partly rate-determining step, which does not manifest itself in the isotope effect when the two propenes are introduced as a mixture. Kinetic experiments suggest that this step is one of those enabling oxygen to take part in the oxidation process and also that the large difference between the isotope effects involved in the formation of acetone and acetic acid is to be ascribed to the behavior of the oxygen. Experiments have also been undertaken with propene in which most of the methyl- and methylene-hydrogen atoms had been replaced by D atoms (the percentage of CD 2 CHCD 3 in this propene was 55). These runs formed part of a series performed with all four combinations of deuterated propene and ordinary propene, on the one hand, and D 2O and H 2O on the other. On introduction of deuterium into the propene, the collective rate of formation of acetone, acetic acid and acetaldehyde remained unchanged, but decreased upon a change-over from H 2O to D 2O. This can be explained by means of a reaction model in which propene is chemisorbed in the form of isopropyl groups. There was no kinetic isotope effect in the formation of acrolein (a byproduct) from propene-2- d, but one was observed from propene whose methyl and methylene groups had been deuterated, in agreement with the literature. The mechanisms responsible for the formation of acetone and related substances, on the one hand, and acrolein, on the other, differ widely, but they agree on the point that in both of them abstraction of a hydrogen atom constitutes the primary oxidation step. Using other catalysts for oxidizing propene to acetone and acetic acid (i.e., V 2O 5 and Fe 2O 3-MoO 3), a kinetic isotope effect was noted when the H atom bound to the second carbon atom of the propene was replaced by a D atom.