Versatile heterogeneous dipicolinate complexes grafted into kaolinite: Catalytic oxidation of hydrocarbons and degradation of dyes

• Ni-, Co- and Mn- dipicolinate-kaolinite heterogeneous catalysts were prepared.
• The catalysts effectively catalyzed cis-cyclooctene oxidation, 55% yield and total selectivity to the epoxide.
• The catalysts effectively catalyzed cyclohexane oxidation, 22% yield and total selectivity towards cyclohexanone.
• The catalysts effectively catalyzed the decolorization of metanil yellow, methylene blue and green light solutions (70–100%).
• Catalysis was always truly heterogeneous, involving high-valent metal-oxo as active catalytic species. The catalysts were easily reused.

New heterogeneous catalysts were prepared by immobilization of Me(II)-dipicolinate complexes (Me = Co, Mn or Ni) on kaolinite (Ka). The precursor material was kaolinite grafted with dipicolinic acid (dpa) obtained via melting of the acid. The catalysts were prepared by suspending the Ka-dpa precursors (lamellar or exfoliated) in Me2+ solutions with a cation/ligand ratio of 1:3. The grafted complexes were characterized by thermal analyses, X-ray diffraction, UV/Vis and infrared spectroscopies, and transmission electron microscopy. The catalysts were tested in three reactions: (i) epoxidation of cis-cyclooctene to cis-cyclooctenoxide reaching 55% yield, with total selectivity to cyclooctenoxide; (ii) oxidation of cyclohexane, reaching 22% yield, with total selectivity towards cyclohexanone; and (iii) Fenton-like decolorization of the dyes metanil yellow, methylene blue and green light, reaching 70–100% decolorization, the maximum effectiveness being observed for methylene blue degradation. The high conversion in the oxidation reactions and the high levels of decolorization of dyes confirmed the versatility of these heterogeneous catalysts based on kaolinite.

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