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Selective Propylene Oxidation to Acrolein by Gold Dispersed on MgCuCr<sub>2</sub>O<sub>4</sub> Spinel
Gold
nanoparticles supported on a MgCuCr<sub>2</sub>O<sub>4</sub> 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/MgCuCr<sub>2</sub>O<sub>4</sub> in selective
propylene oxidation is due to the synergy between metallic Au and
surface Cu<sup>+</sup> sites. Kinetic experiments (H<sub>2</sub> addition,
N<sub>2</sub>O replacing O<sub>2</sub>) 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 O<sub>2</sub>. It results in OOH formation. The resulting CH<sub>2</sub>–CH–CH<sub>2</sub> 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
O<sub>2</sub> can adsorb. The role of Cu in the support surface is
to lower the desorption energy of acrolein