Preparation, characterization and thermodynamic properties of Zr-containing Cl-bearing layered double hydroxides (LDHs)

Zr-containing layered double hydroxides (LDHs) with variable xZrsolid = Zr/(Zr + Al) mole fractions were synthesized by a co-precipitation method at ambient conditions. The chemical compositions of samples and corresponding aqueous solutions after syntheses were analyzed by ICP-OES, EDX (Mg, Al, Zr) and ion chromatography (Cl–). Results of PXRD technique demonstrated that solids with 0 ≤ xZrsolid ≤ 0.5 show only X-ray reflexes typical for pure LDH compositions, while products of syntheses with xZrsolid > 0.5 display additional patterns attributed to brucite. ICP-OES and EDX techniques shown that in pure Zr-containing LDHs the Mg/(Al + Zr) ratio is reducing with increase of xZrsolid and the stoichiometry of brucite-like layers corresponds to [Mg3–2xAl1–xZrx]. This fact may indicate that the incorporation of 1 Zr-containing specie results in the removal of 1 Al- and 2 Mg-containing species from the pure Mg-Al-composition. Such mechanism may be confirmed by the observation that measured a0 = b0 distances are generally consistent with theoretical estimates obtained from [Mg3–2xAl1–xZrx]-stoichiometry. The presence of predominant Mg2+, Al(OH)4– and Zr(OH)5– complexes in aqueous solutions after syntheses was established in thermodynamic calculations by applying GEMS-Selektor v.3. code and, therefore, the reaction: Mg3Al1(OH)8Cl1 + Zr(OH)5– = Mg1Zr1(OH)5Cl1 + Al(OH)4– + 2 Mg2+ + 4 OH– can describe a mechanism of Zr-substitution. Estimates of the molar Gibbs free energies of Zr-containing LDHs with 0 ≤ xZrsolid ≤ 0.5 show that the incorporation of Zr into the LDH increasing significantly their aqueous solubility. Thus, it is not possible to neglect that Zr can be partly localized as Zr(OH)5–-ligands in the interlayer space of the LDH structure