Encapsulation of transition metal tetrahydro-Schiff base complexes in zeolite Y and their catalytic properties for the oxidation of cycloalkanes

Abstract

A variety of transition metal (M = Cu, Co, Fe and Mn) tetrahydro-Schiff base complexes (denoted as M–[H 4]Schiff with Schiff = salen, salpn and salicyhexen, see Scheme 1) have been encapsulated in zeolite Y with flexible ligand method for the first time. The prepared samples were characterized with X-ray diffraction, diffuse reflectance UV–vis spectroscopy, Infrared spectroscopy, N 2 adsorption/desorption at −196 °C, elemental analysis, as well as thermo-gravimetry and differential-thermal analysis techniques. The formation of tetrahydro-Schiff base ligand was confirmed with 1H NMR and IR spectroscopy before complexing. The prepared M–[H 4]Schiff/Y catalysts such as Cu–[H 4]salen/Y showed much higher conversion in the oxidation of cyclohexane than the corresponding M–Schiff analogues encapsulated in zeolite Y as a result of the incorporation of larger amounts and the stronger oxidation ability of tetrahydro-Schiff base complexes. In addition, the M–[H 4]Schiff/Y catalysts were also active for the oxidation of other cycloalkanes although their activity depended on the reaction conditions as well as the central transition metals of the complexes and the substrate molecules. It was shown that the catalytic reaction occurred via a radical mechanism. The time for the addition of H 2O 2 significantly affected the catalytic performance. Earlier (e.g. 20 min) addition of H 2O 2 than substrate molecules to the reaction system led to a drastic reduction in the catalytic activity.