Metallosalen complexes immobilized in zeolite NaX as catalysts of aerobic oxidation of cyclooctane
Physicochemical properties of metallosalen complexes entrapped within NaX zeolite and their catalytic activities in the oxidation of cyclooctane with dioxygen in the absence of a reducing agent were studied, and compared with free metallocomplexes.Broadening of the zeolite structure-sensitive vibrations observed after the encapsulation gives the evidence of the zeolite framework changes due to the presence of the “ship-in-the-bottle” species. EPR spectroscopy evidences the inclusion of metallosalen complexes since it distinguishes between encapsulated metallocomplexes and those adsorbed on the zeolite external surface. UV–vis spectra confirm the immobilization and stabilization of the metallocomplexes inside the zeolite structure.The most active catalysts are free metallosalen complexes. We have found that the activity of these complexes increases in the order Ni(salen), Fe(salen), Mn(salen), Cu(salen) and Co(salen) with triple difference in the catalytic activity between the most and the least active catalyst. Upon encapsulation their activity drops about two times. Only Ni(salen) after heterogenization does not change its catalytic activity. The reduction of catalytic activity of the encapsulated catalysts is probably governed by two factors: constraints in the formation of the intermediate complex and change of the redox potential of the metal in the salen complex.
Journal: Applied Catalysis A: General - Volume 299, 17 January 2006, Pages 235–242