Selective alkene epoxidation by molecular oxygen in the presence of aldehyde and different type catalysts containing cobalt

Abstract

This chapter discusses a comparative study of the catalytic properties of different type cobalt-containing compounds in alkene epoxidation by dioxygen in the presence of iso-butyraldehyde (IBA) and provides some data that allows clarifying the reaction mechanism and the nature of the catalytic action of cobalt compounds. Here, the catalytic properties of cobalt-containing compounds having different nature have been discussed. These include the simple salt, Co(NO 3 ) 2 .6H 2 O, tetra-n-butylammonium salts of PW 11 CoO 59 3− (PW 11 CO) and CoW 12 O 40 −6 (CoW 12 ) heteropolyanions (HPA), CoNaY zeolite, and Co(II) phtalocyanine (CoPc), in alkene epoxidation by the O 2 /IBA system. It has been found that various alkenes can be converted to the corresponding epoxides with good-to-high selectivity (80%–99%) at complete alkene conversion at ambient conditions. Neither allylic oxidation nor epoxide ring cleavage products were detected for all alkenes tested except for cyclohexene. The nature of catalyst does not considerably affect the selectivity of the epoxidation that depends mainly on the olefin structure. Some decrease of the selectivity was generally observed at high cobalt concentrations. The results obtained in this investigation prove that alkene epoxidation by O 2 in the presence of iso-butyraldehyde (IBA) and cobalt catalysts proceeds via radical chain mechanism. Acylperoxy radicals act most likely as the main epoxidizing species although some other species, for example, coordinated to the metal center acylperoxy radicals, may contribute into the epoxidation process when catalysts with low redox potentials are used. Superior catalytic activity of cobalt compounds in alkene epoxidation by O 2 /IBA system is because of the high ability of cobalt to catalyze the chain branching and promote the chain initiation rather than the ability of cobalt to activate dioxygen via its coordination.