Enhanced catechol oxidation by heterogeneous biomimetic catalysts immobilized on clay minerals

Two novel heterogeneous biomimetic catalysts were synthesized by immobilizing a meso-tetra(2,6-dichloro-3-sulfonatophenyl)porphyrinate of manganese (111) chloride on both kaolinite and montmorillonite clay minerals, previously functionalized with a molecular spacer whose terminal nitrogen atom coordinates the metal in the porphyrin ring. The clay functionalization by a 3-(1-imidazolyl)propylcarbamoyl-3'-aminopropyl-triethoxysilane spacer was proved by X-ray diffraction, elemental analysis, DRIFT-IR, C-13- and Si-29-CPMAS-NMR spectroscopies, and the percent of manganese-porphyrin immobilized by the spacer coordination calculated by spectrophotometry. The activity of the novel catalysts was evaluated in the oxidative coupling reaction of catechol, a humic phenol, using H2O2 as oxygen donor. The rate of catechol oxidation catalyzed by both heterogeneous catalysts was about four times as fast as that catalyzed by the free manganese-porphyrin, and depended on the amount of catalyst immobilized on clay minerals. Moreover, the activity of the heterogeneous catalysts remained effective for at least two sequential reaction cycles, although at a decreasing rate. We showed that the immobilization of a biomimetic catalyst on clay minerals increased the catalytic efficiency and allowed the catalyst recycling and reuse for additional reactions. Metal-porphyrins immobilized on clay minerals through a flexible spacer may act as environment-friendly heterogeneous catalysts to control phenolic molecules in environmental humus. (C) 2013 Elsevier B.V. All rights reserved.