Reactions of Propylene Oxide on Supported Silver Catalysts: Insights into Pathways Limiting Epoxidation Selectivity

The reactions of propylene oxide (PO) on silver catalysts were studied to understand the network of parallel and sequential reactions that may limit the selectivity of propylene epoxidation by these catalysts. The products of the anaerobic reaction of PO on Ag/alpha-Al2O3 were propanal, acetone and allyl alcohol for PO conversions below 2-3%. As the conversion of PO was increased either by increasing the temperature or the contact time, acrolein was formed at the expense of propanal, indicating that acrolein is a secondary reaction product in PO decomposition. With addition of oxygen to the feedstream the conversion of PO increased moderately. In contrast to the experiments in absence of oxygen, CO2 was a significant product while the selectivity to propanal decreased as soon as oxygen was introduced in the system. Allyl alcohol disappeared completely from the product stream in the presence of oxygen, reacting to form acrolein and CO2. The product distribution may be explained by a network of reactions involving two types of oxametallacycles formed by ring opening of PO: one with the oxygen bonded to C1 (OMC1, linear) and the other with oxygen bonded to C2 (OMC2, branched). OMC1 reacts to form PO, propanal, and allyl alcohol. OMC2 can give rise to acetone and PO. (DFT) calculations have verified accessibility to the two oxametallacycle structures from propylene and PO, and have provided energy barriers for each of the steps involved in PO isomerization. This work illustrates the complex manifold of sequential reactions that contribute to the difficulty of achieving high selectivity in direct propylene epoxidation with silver catalysts.