Selective Propylene Oxidation to Acrolein by Gold Dispersed on MgCuCr2O4 Spinel

Gold nanoparticles supported on a MgCuCr2O4 spinel catalyze the aerobic oxidation of propylene to acrolein. At 200 °C, the selectivity is 83% at a propylene conversion of 1.6%. At temperatures above 220 °C, propylene combustion dominates. The good performance of Au/MgCuCr2O4 in selective propylene oxidation is due to the synergy between metallic Au and surface Cu+ sites. Kinetic experiments (H2 addition, N2O replacing O2) show that the reaction involves molecular oxygen. DFT calculations help to identify the reaction mechanism that leads to acrolein. Propylene adsorbs on a single Au atom. The adsorption of propylene via its π-bond on gold is very strong and can lead to the dissociation of the involved Au atom from the initial Au cluster. This is, however, not essential to the reaction mechanism. The oxidation of propylene to acrolein involves the oxidation of an allylic C–H bond in adsorbed propylene by adsorbed O2. It results in OOH formation. The resulting CH2–CH–CH2 intermediate coordinates to the Au atom and a support O atom. A second C–H oxidation step by a surface O atom yields adsorbed acrolein and an OH group. The hydrogen atom of the OH group recombines with OOH to form water and a lattice O atom. The desorption of acrolein is the most difficult step in the reaction mechanism. It results in a surface oxygen vacancy in which O2 can adsorb. The role of Cu in the support surface is to lower the desorption energy of acrolein.