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 M g(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 temper ature, is related to the partial charge of oxygen in the LDHs obtained from the Sanderson theory of electronegativity. The enthalpy of adsorption of C O2 on the resulting mixed oxides is measured by calorimetry. A homogeneous surface is generally observed for CO2 adsorption, with initial heats of ads orption close to those reported for MgO. The number of sites determined by this method is proportional to the rate constants for beta-isophorone isome risation, suggesting that both techniques measure surface properties. The l ayered 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. (C) 2000 Academic Press.