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 (III) 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, 13C- and 29Si-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.

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► Two novel biomimetic catalysts immobilized on clay minerals were synthesized.
► Enhanced reaction rate of the catechol oxidation.
► Immobilized heterogeneous catalysts were more efficient than the free homogeneous catalyst.
► Effective heterogeneous catalysis for at least two sequential reaction cycles.
► Catechol oxidation depended on the amount of immobilized catalyst.