The mechanism of propylene oxidation to acrolein over bismuth molybdate, copper oxide, and rhodium catalysts

Abstract

The oxidation of (E)-propene-1-d1 to acrolein over bismuth molybdate, copper oxide, and rhodium catalysts was studied to determine if the reaction proceeded with (Z)-(E) randomization of the deuterium stereochemistry. Over Bi2Mo2O9 and Bi2MoO6, (E)-acrolein-3-d1:(Z)-acrolein-3-d1: acrolein-1-d1 was 1:1:1 consistent with a [sigma]-allyl intermediate which rapidly converts between two equivalent forms (*CHD---CH=CH2 h CHD=CH---*CH2). With copper oxide, (E)-acrolein-3-d1: (Z)-acrolein-3-d1: acrolein-1-d1, was 1: 1: 1.6. This points to a discrimination isotope effect consistent with a [sigma]-allyl intermediate without interconversion of equivalent forms. Over a Rh/[alpha]-Al2O3 catalyst, (E)-acrolein-3-d1: (Z)-acrolein-3-d1: acrolein-1-d1 was 1: 0.93: 0.89. This is consistent with an allyl intermediate along with a second minor nonallylic pathway which does not equilibrate the terminal carbon atoms. One or both of these processes occurred with some retention of the (E)-deuterium stereochemistry in the acrolein-3-d1. Over an unsupported Rh catalyst, (E)-acrolein-3-d1: (Z)-acrolein-3-d1: acrolein-1-d1 was 1: 0.68: 0.85. Only the allylic pathway is evident and the reaction process occurs with incomplete randomization (76 +/- 10%) of the (E)-deuterium stereochemistry in the acrolein-3-d1.