Cobalt-substituted polyoxometalate pillared hydrotalcite: Synthesis and catalysis in liquid-phase oxidation of cyclohexanol with molecular oxygen

Abstract

Cobalt-substituted polyoxometalate (POM), α-[SiW 9O 37{Co(H 2O)} 3] 10−, was heterogenized for its effective use as a solid catalyst in the liquid-phase reaction by inserting it in between the layers of basic Mg 3Al-hydrotalcite. It was characterized by FT-IR, UV–vis, X-ray diffraction, N 2 adsorption–desorption and elemental analysis. A POM cluster was intercalated into the interlayer of layered double hydroxide (LDH), and the integrity of the cluster [SiW 9Co 3] was maintained when the hydrotalcite precursor, Mg 3Al-LDH-carbonate, was transformed into a mixed-oxide solid solution, followed by the formation of Mg 3Al-LDH-hydroxide through hydrolytic reconstruction and Mg 3Al-LDH-adipate via anion exchange by organoacid precursor before its use in the pillaring reaction under a stringent N 2 atmosphere. The intercalated cobalt-substituted POM cluster showed high catalytic activity and reusability in the liquid-phase selective oxidation of cyclohexanol to cyclohexanone using 1 atmosphere of molecular oxygen under mild reaction conditions.