Heterogenized redox catalysts on the basis of the chitosan matrix 1. Copper complexes

Series of heterogenized copper complexes are prepared by either coprecipitation or adsorption of Cu(II) on the bulk chitosan and composite supports (egg-shell type chitosan/SiO2 and chitosan/MCM-41 systems). The morphology and properties of the catalysts are studied by FTIR, UV-Vis, SEM, and ESR methods, and the activity of the redox sites immobilized by the chitosan matrix is tested in oxidation of o- and p-dihydroxybenzenes in water. The estimate of the copper concentration derived from the ESR data shows that essentially all Cu(II) ions introduced in low-loaded samples (less than or equal to1.5 wt.% Cu) are ESR-visible, with the symmetry of isolated Cu2+-sites in chitosan approximating the square-planar coordination. Redox transformations of the active sites in the course of catalytic tests or prolonged boiling in water are not accompanied by copper leaching from the chitosan matrix, and the catalyst reoxidation by H2O2 leads to quantitative restoration of the Cu(II)-ESR signal. Thus, the matrix of chitosan is able to stabilize and retain isolated Cu2+ ions in highly coordinatively unsaturated state. In contrast with a homogeneous copper-chitosan system demonstrating copper-hydroquinone complexation that suppresses dramatically the yield of the oxidation products, the heterogenized samples are active and stable catalysts in the process of quinone formation. The binary composite system, with a thin film of low-loaded Cu-chitosan supported on macroporous SiO2, demonstrates significantly higher activity in oxidation of hydroquinone, as compared with the bulk Cu/chitosan sample. This approach to catalyst design opens up a promising way for synthesis of the supported chitosan catalysts with a very low content of noble metals. (C) 2003 Elsevier Science B.V. All rights reserved.