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-G a/Al alpha-NO3] (where R=M(II)/M(III)=3 and alpha = Ga/(Al+Ga)=0, 0.25, 0.5 0, 0.75, 1.0 in the starting solutions) were prepared following the copreci pitation method at constant pH. Their structural, thermal and textural prop erties were examined. When compared with the composition of the starting so lutions, the solids show an incomplete incorporation of gallium probably du e to the solubility of Ga(OH)(3) species at the coprecipitation pH (9-10) u sed. 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 s ubstituted by Ga3+ cations in the hydrotalcite structure. No influence of t he relative concentrations of the cations on the thermal behavior was obser ved here as x only varies from 0.19 to 0.28. Nevertheless, a few difference s are noted depending on the interlamellar anion. On the other hand, the ba sic reactivity of these new materials and their calcined derivatives was ev aluated through the Knoevenagel condensation of ethyl cyanoacetate with ben zaldehyde. Other LDHs containing: divalent and trivalent cations of differe nt 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 f rom the adsorption isotherms for phenol. Both calcined and uncalcined Mg,Ga /Al-hydrotalcites exhibit high reactivity. For the calcined LDHs, it appear s that the basicity order found from the catalytic activity is related to t he base site density and the average electronegativity of the solid, i.e., the basic strength. (C) 1999 Elsevier Science Ltd. All rights reserved.