Selective oxidation over copper and manganese salens encapsulated in zeolites

Abstract

Copper and Mn(III)(X₂-salen) complexes, where salen^2-=N,N'-ethylenebis(salicylideneaminato) and X=H, Cl, Br or (NO₂), encapsulated in the cavities of zeolites NaX and NaY, have been synthesized and characterized by various physicochemical measurements. Samples obtained by synthesizing the complexes 'in situ' in the cavities of the zeolite by the 'flexible ligand' method contain a higher concentration of the complex than those obtained by synthesizing the zeolites around the preformed copper complex. Substitution of the aromatic hydrogen atoms of the salen ligand by electron-withdrawing groups like -Cl, -Br and NO₂ has two major effects: (1) retention and concentration of the copper complex in the zeolite cavities is enhanced (due to the larger size of the substituents); and (2) the spectral properties of the encapsulated complex are modified. Cyclic voltammetric data indicate that the zeolite matrix facilitates the reduction of Mn(III) to Mn(II), suggesting that it behaves like an electron-withdrawing substituent. The rates of decomposition of H₂O₂ and tert-butyl hydroperoxide as well as the selective, low-temperature oxidation of phenol, styrene and para-xylene are all enhanced by electron-withdrawing substituents on the salen ligand.