Heterogeneous alkene epoxidation: past, present and future

Recent progress in the metal-catalysed heterogeneous epoxidation of alkenes is reviewed with respect to key mechanistic issues, drawing on results obtained with both single crystals and practical dispersed catalysts. New data are also presented and future prospects examined. It is shown that on silver surfaces oxygen adatoms are responsible for all the chemistry-both combustion and selective oxidation. In particular, the minimum necessary and sufficient conditions for epoxidation to occur are pi-adsorbed alkene and O-a. Studies with co-adsorbed F, Cl, Br, and I demonstrate that promotion of epoxidation selectivity correlates with halogen electronegativity-supporting the view that this is an electronic effect involving control of the valence charge density on Oa. The observations that (i) selectivity markedly increases with increasing Oa coverage and (ii) alkalis strongly decrease selectivity are in excellent accord with this view. In the presence of alkali, addition of NO., to the reaction gas leads to formation of absorbed nitrate species that open an ultra-selective epoxidation channel that involves transfer of oxygen from submonolayer quantities of adsorbed alkali nitrate to alkene, probably by a direct mechanism. Our comprehension of silver surface chemistry suggests that copper should also be an epoxidation catalyst-an expectation borne out in practice. In fact, Cu is more selective than Ag. When allylic hydrogen atoms are present (propene, allylbenzene and its isomers), Cu surpasses Ag by an even greater margin: copper continues to catalyse selective oxidation effectively, whereas silver simply bums these alkenes. The proximity of the C=C bond to the Cu surface is critically important in determining the degree to which epoxidation occurs. (C) 2004 Elsevier B.V. All rights reserved.