Basic Properties of the Mixed Oxides Obtained by Thermal Decomposition of Hydrotalcites Containing Different Metallic Compositions

Carbonated layered double hydroxides (LDHs) containing Al, Fe, or Cr in a Mg(OH)2 matrix or Al dissolved in hydroxides of Mg, Cu, Ni, Co, or Zn are used as precursors of basic catalysts. Decarbonation is studied by thermal analysis. The average basic strength, evaluated by the decarbonation temperature, is related to the partial charge of oxygen in the LDHs obtained from the Sanderson theory of electronegativity. The enthalpy of adsorption of CO2 on the resulting mixed oxides is measured by calorimetry. A homogeneous surface is generally observed for CO2 adsorption, with initial heats of adsorption close to those reported for MgO. The number of sites determined by this method is proportional to the rate constants for β-isophorone isomerisation, suggesting that both techniques measure surface properties. The layered structure in which OH− is the compensating anion can be re-formed by hydration. This process does not appreciably change the adsorption of CO2; thus, oxygens and hydroxyls show similar basic strengths in this case.