Preparation of Zr-containing layered double hydroxides and characterization of the acido-basic properties of their mixed oxides

Multicomponent M(II)/Al/Zr layered double hydroxides (LDHs) with M(II) = Mg, Ni, and Zn, exhibiting single hydrotalcite-like phases by XRD, were obtained by coprecipitation at constant pH = 10. The cationic contents were close to those of the solutions, excepted for Zr, for which 95, 70, and 65 mol % of the amount in solution were introduced in the Mg-, Ni-, and Zn-containing LDHs, respectively. This suggests either an easier accommodation of Zr when the size of the divalent cation decreases or a different behavior of the solutions during precipitation at pH = 10. The introduction of Zr4+ of high ionic size induces a distortion of the brucite-like sheets and a heterogeneous distribution of charges, as evidenced by a lowering of the symmetry of the intercalated species. The lattice a parameter follows the Vegard correlation as a function of the (Al + Zr)/[M(II) + Al + Zr] ratio when M(II) = Mg and Ni, but it is nearly constant in Zn-containing LDHs for varying (Al + Zr)/(Zn + Al + Zr). This suggests, in these latter samples, the formation of pure lamellar phases with similar (Al + Zr)/(Zn + Al + Zr) ratios, regardless of the composition in solution, with an additional Zn-containing phase. At the same time, the presence of an excess of carbon in these samples, in addition to the compensating CO32- species belonging to the layer structure, suggests that hydrozincite Zn5(CO3)2(OH)6 is the excedent phase. The introduction of Zr in the layers increases the basicity of the poorly basic Zn-containing mixed oxides but not the basicity of the Zn-containing LDHs. The basicity of the lamellar structure decreases as a result of the lowering of the partial charge of the oxygen atoms, whereas that of the mixed oxide increases through the formation of Zr4+-O2- acid-base pairs.