Evolution of structure and performance of Cu-based layered double hydroxides

A new series of Cu/Zn/Mn/Fe/Al hydrotalcite-like layered double hydroxides (LDHs) with the Cu/(Zn + Mn)/(Fe + Al) atomic ratios of 1/1/1 in synthesis mixture were synthesized by the coprecipitation method. The chemical composition of multicomponent precursors was identified by chemical analyses. The thermal stability, structure, and texture changes of these as-synthesized LDHs were studied by in situ high-temperature X-ray diffraction (HT-XRD), thermogravimetric-differential thermal analysis combined with mass spectrometry (TG-DTA-MS) in different atmosphere, transmission electron microscopy (TEM), and N2 adsorption–desorption experiments. The results exhibit that the incorporation of Fe3+ and Mn2+ into the lattices of Cu-containing LDHs in sequence decreases the crystallinity, water content, and thermal stability of corresponding compounds, and the thermal treatment of LDHs results in the formation of thermodynamically stable composite metal oxide associated with a small amount of simple metal oxide and also changes in texture of calcined solid. Under mild experimental conditions (atmospheric pressure and 25 °C), the catalytic liquid-phase oxidation of aqueous phenol solutions is related to the composition, oxidation states, composite forms and synergy of transition metal cations in calcined LDHs, and calcined LDH with Cu/Zn/Mn/Fe/Al atomic ratio of 1/1/0/0.3/0.7 at 500 °C achieves the highest conversion of phenol mainly owing to the formation of a larger amount of composite metal oxide with some residual carbonate in the solid.