Fine-Tuning and Recycling of Homogeneous Tungstate and Polytungstate Epoxidation Catalysts

This chapter reviews work on epoxidations with aqueous dihydrogen peroxide catalyzed by tungstate and polytungstate catalysts. The activity of the sandwich polyoxometalate (sandwich POM) in cyclooctene epoxidation is compared to other W-based catalyst systems under ceteris paribus conditions. Catalyst systems based on H₂WO₄ display the highest activity of the 11 W-catalyst systems tested. Replacing H₂WO₄ with Na₂WO₄ in these systems results in a strong decrease of catalyst activity, and this points to the importance of catalyst acidity. An experimental screening of various carboxylic acids cocatalysts has generated a novel W-based epoxidation system. The sandwich POM is among the most efficient of the nonacidic W-based catalyst systems. Examples where the pH neutral nature of the sandwich POM is used advantageously in the preparation of acid-sensitive epoxides are given. Allylic alcohols are epoxidized efficiently with very high reactor yields and very low sandwich POM catalyst loadings. Allylic alcohol epoxidation also proceeds very efficiently with the vanadium containing catalyst and an organic hydroperoxide instead of H₂O₂ as the terminal oxidant. The diastereoselectivity of the epoxidation of chiral allylic alcohols is rationalized via 1,2- and/ or 1,3-allylic strain effects. Fine-tuning of catalyst activity and selectivity is achieved by varying the relative amount and nature of phase-transfer cocatalysts. Attention is paid to practical aspects that are of relevance for large-scale, industrial use of the sandwich POM catalyst, such as catalyst preparation, handling, and recycling. For the latter, an efficient method based on nanofiltration employing an a-alumina-supported g-alumina membrane has been developed.