Self-Assembled Na12[WZn3(ZnW9O34)2] as an Industrially Attractive Multi-Purpose Catalyst for Oxidations with Aqueous Hydrogen Peroxide

Eleven W-based catalyst systems for alkene epoxidation with aqueous H 2 O 2 were compared under identical conditions and at equal level of 0.1 mol % W-atoms. Of these, those based on a combination of H 2 WO 4 and a methyltrioctylammonium phase transfer catalyst turned out to be most active in particular systems that contain a source of phosphate. Evidence is presented that under our conditions the actual epoxidizing species in H 2 WO 4 -based catalyst systems without phosphate source is mononuclear [WO(OH)(O 2 ) 2 ] - rather than binuclear [{WO(O 2 ) 2 } 2 O] 2 - that is usually thought to be active. For large-scale applications, however, the polyoxometalate Na 12 [WZn 3 (ZnW 9 O 34 ) 2 ] (NaZnPOM) in combination with a suitable phase transfer catalyst such as methyltrioctylammonium chloride is preferred over H 2 WO 4 -based catalysts. This preference results from the fact that use of H 2 WO 4 requires a catalyst activation step that is troublesome on a large scale, whereas epoxidations catalyzed by NaZnPOM start without induction period on addition of H 2 O 2 . Optimizations of epoxidations catalyzed by QCl/NaZnPOM or QCl/H 2 WO 4 have shown that the optimum Q/W ratio depends on the alkene that is epoxidized and differs from that expected from catalyst stoichiometry. An attractive feature of NaZnPOM from the viewpoint of industrial applicability is that epoxidations and other reactions with H 2 O 2 are efficiently catalyzed by a readily available aqueous solution of NaZnPOM prepared through self-assembly. A 1 mol scale example is provided of an epoxidation catalyzed by a combination of self-assembled NaZnPOM and Luviquat mono CP as a multifunctional cocatalyst with emulsifying, buffering, and phase-transferring properties.