Cu-substituted molecular sieves as liquid phase oxidation catalysts

Abstract

Cu(II)-substituted molecular sieves were found to be efficient catalysts in liquid phase oxidation of aromatic compounds. CuAPO-5 was an efficient catalyst in hydroxylation of alkyl-substituted benzenes. For benzene itself as the reactant, a phenol selectivity of near 100% could be obtained using hydrogen peroxide as the oxidant at mild reaction conditions. Acetonitrile was the only effective solvent for this one-step oxidation of benzene to phenol. With the increase in steric hindrance of alkyl substitution on benzene, the oxidation reaction was driven toward benzylic oxidation instead of ring hydroxylation. However, the selectivities of the ring hydroxylation products were higher than those usually obtained in homogeneous radical reactions. On the other hand, Cu(II)-substituted MCM-41 of relatively larger pores was effective in catalyzing the oxidation of trimethylphenol to trimethylbenzoquinone at mild reaction conditions. The latter is an intermediate in manufacture of vitamin E. The optimal activity was obtained when MCM-41 was concomitantly substituted with small amounts of Cu(II) and Al(III). With acetonitrile as solvent, hydrogen peroxide and tert-butyl hydrogen peroxide were effective oxidants, but with benzaldehyde as solvent, oxygen gas was also an effective oxidant.