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

Abstract

Zr-containing layered double hydroxides (LDHs) with variable xZr_solid = 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 ≤ xZr_solid ≤ 0.5 show only X-ray reflexes typical for pure LDH compositions, while products of syntheses with xZr_solid > 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 xZr_solid and the stoichiometry of brucite-like layers corresponds to [Mg_3–2xAl_1–xZr_x]. 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 [Mg_3–2xAl_1–xZr_x]-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 ≤ xZr_solid ≤ 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.