Environmentally friendly liquid phase oxidation : enhanced selectivity in the aerial oxidation of alkyl aromatics, epoxidations and the Baeyer-Villiger oxidation using novel silica supported transition metal ions

Active transition metal species (Co, Cu, Cr, Ni or Mn) supported on a chemically modified silica gel are used as heterogeneous catalysts in a range of liquid phase oxidation reactions: alkyl aromatic side chain oxidations, epoxidations of alkenes and Baeyer–Villiger oxidations of linear ketones to esters and cyclic ketones to lactones. The catalyst employs metal centres bound to the silica surface via a hydrophobic spacer chain and is thus chemically robust and has a relatively high loading for a supported reagent (c 0.4 mmol g−1). The Cr version of the catalyst promotes the oxidation of ethylbenzene to acetophenone in a solvent-free system at a rate of 5.5% h−1 (>370 turnover h−1). It is also active for the oxidation of p-chlorotoluene and p-xylene to p-chlorobenzoic acid and p-toluic acid respectively. Cyclohexene is converted to its oxide at room temperature at a rate of c 28% h−1 (c 12 turnover h−1) using either the Ni or Cu versions of the catalyst. The room temperature Baeyer–Villiger oxidation of cyclohexanone is achieved at a rate of 44% h−1 (49 turnover h−1) using the Ni-containing catalyst. The same material also promotes the Baeyer–Villiger oxidation of linear aliphatic ketones and aromatic side chains. All the above systems use either air or molecular oxygen as the oxidant rather than peroxides or peracids. © 1999 Society of Chemical Industry