Synthesis and characterization of mixed Ga/Al-containing layered double hydroxides:: study of their basic properties through the Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate, and comparison to other LDHs

Two series of mixed Ga/Al-containing LDHs, [MgR-Ga/Al alpha-CO3] and [MgR-Ga/Al alpha-NO3] (where R=M(II)/M(III)=3 and alpha = Ga/(Al+Ga)=0, 0.25, 0.50, 0.75, 1.0 in the starting solutions) were prepared following the coprecipitation method at constant pH. Their structural, thermal and textural properties were examined. When compared with the composition of the starting solutions, the solids show an incomplete incorporation of gallium probably due to the solubility of Ga(OH)(3) species at the coprecipitation pH (9-10) used. The linear variation of the lattice parameters a and c vs. x=(Al+Ga)/(Mg+Al+Ga) clearly indicates that Al3+ and Mg2+ cations are isomorphically substituted by Ga3+ cations in the hydrotalcite structure. No influence of the relative concentrations of the cations on the thermal behavior was observed here as x only varies from 0.19 to 0.28. Nevertheless, a few differences are noted depending on the interlamellar anion. On the other hand, the basic reactivity of these new materials and their calcined derivatives was evaluated through the Knoevenagel condensation of ethyl cyanoacetate with benzaldehyde. Other LDHs containing: divalent and trivalent cations of different electronegativity (M(II)=Mg2+, Cu2+, Zn2+; and M(III)=Al3+, Sc3+, Mn3+, Ga3+) as well as diverse simple anions (CO32-, NO3-, F-), were also tested in order to examine the relationship between the composition and the basic properties of LDHs. In addition, the number of base sites were quantified from the adsorption isotherms for phenol. Both calcined and uncalcined Mg,Ga/Al-hydrotalcites exhibit high reactivity. For the calcined LDHs, it appears that the basicity order found from the catalytic activity is related to the base site density and the average electronegativity of the solid, i.e., the basic strength. (C) 1999 Elsevier Science Ltd. All rights reserved.