The direct epoxidation of higher olefins using molecular oxygen

Recent developments in olefin epoxidation have shown promising results indicating that higher olefins can be directly epoxidized using molecular oxygen, or indirectly, by using molecular oxygen to generate an active and selective oxidant in situ during reaction. Indirect approaches have utilized bifunctional catalysts that combine new catalyst components that generate such oxidants as H2O2 in situ with the functional component that activates H2O2 for olefin epoxidation., This approach is currently limited by the low rates of in situ generation of H2O2 and subsequent low rates of olefin epoxide formation. Heavily-modified, silver catalysts have also shown promise as catalysts for propylene epoxidation. These catalysts contain much higher silver and alkali and alkaline earth metal loadings than their analogs used for ethylene epoxidation and are quite different in terms of their chemical and physical properties. Currently, these compositions exhibit activities and selectivities to propylene oxide that are too low for commercial application, although further research and development may further improve catalyst performance. Silver-based catalysts have also been used to epoxidize a wide variety of higher olefins, such as 1,3-butadiene, that do not contain allylic hydrogen atoms, or higher olefins that contain non-reactive allylic hydrogen atoms, such as norbornene. Silver-based catalysts used for selective epoxidation of non-allylic, or kinetically-hindered, olefins require promoters, typically cesium, rubidium, or thallium salts, to assist in the desorption of the olefin epoxide. Such catalysts are extremely active, selective, and stable under extended reaction conditions. (C) 2001 Elsevier Science BN. All rights reserved.