Comparison between MeIIMg/Al hydrotalcites and hydrotalcite-supported Me(II) acetylacetonates (Me(II) = Co, Cu or Ni) catalysts for the epoxidation of cyclohexene with molecular oxygen

The paper presents a comparison between the catalytic performances of cobalt, copper and nickel modified hydrotalcites in the epoxidation of cyclohexene using molecular oxygen, isobutyraldehyde as reductant, at 25 °C and 1 atmosphere pressure of oxygen in acetonitrile as solvent. The modified hydrotalcites have been obtained using different methods: i) the introduction of the modifier in the brucite-type layer by co-precipitation using MeII(NO3)2 as transition metal source, which yielded MeIIMg/Al catalysts (MeII = Co2+, Ni2+ or Cu2+); and ii) the impregnation of the hydrotalcite (HT) support with acetone solutions of bivalent transition metal acetylacetonates MeII(acac)2, which yielded Me(acac)2/HT catalysts. The catalysts were characterized by XRD, DR-UV–Vis, DRIFTS, and texture measurements. The most active catalyst was Co(acac)2/HT. The correlation between the catalytic tests results and the interactions of the metal acetylacetonates with the support showed that in this catalyst the cobalt acetylacetonate species are preserved while in the case of supported Ni(acac)2 and Cu(acac)2 there is a partial loss of the (acac) ligand from the complex due to a partial ligand exchange with the OH groups of the carrier. For Co(acac)2 precursor, the effect of the support has been also investigated using as carriers the mixed oxide derived from the calcinations of HT, (CHT) and the reconstructed hydrotalcite (RHT) obtained by rehydration of CHT.

Research Highlights
► HT-supported MeII(acac)2 (MeII= Co, Ni, Cu) were prepared by impregnation.
► MeII(acac)2 complexes are mainly deposed on the outer surface of the support.
► Upon impregnation, Co(acac)2 species are better preserved than Ni or Cu homologues.
► MeII(acac)2/HT are better catalysts than MeIIMg/Al with identic MeII content (2 wt%).
► The catalytic activity for MeII(acac)2/HT varies depending on MeII: Co>Ni>Cu.