The stereochemistry of propylene-1,2-d2 epoxidation over silver catalysts

The stereochemistry of the epoxidation of cis-propylene-1,2-d₂ to propylene oxide over three unsupported and one supported silver catalysts as well as over Ag₂O and AgO was studied by microwave spectroscopy. Variations of the stereochemistry with N₂O as the oxidant and with catalysts heavily moderated by C₂H₄Cl₂ were also explored. In the absence of Cl moderators, the equilibration in the product ranged between 66 and 99% and was similar to a previous study with ethylene. Unlike ethylene, the use of Cl moderators had a large effect and equilibration varied between 37 and 65%. Substitution of methyl hydrogen atoms with deuterium, where trans-propylene-1,2,3,3,3-d₅ was used, did not change the equilibration kinetics even though the selectivity was increased. Three theses are developed from the results. (1) The predominant epoxidation mechanisms for ethylene and propylene are similar for unmoderated catalysts. (2) The randomization kinetics upon epoxidation are uncoupled from the parallel production of CO₂. (3) Chloride moderation alters the randomization kinetics for propylene perhaps by stabilizing a secondary radical or otherwise increasing the rotational barrier about the CC bond of an adsorbed intermediate.