Study of the formation of the layered double hydroxide [Zn-Cr-Cl]

The synthesis of the layered double hydroxide (LDH) [Zn-Cr-Cl], well-known for its ability to be synthesized only with the ratio Zn:Cr = 2:1, has been followed by UV-visible and extended X-ray absorption fine structure (EXAFS) spectroscopy in order to understand the mechanisms of its formation. The UV-visible spectrum reveals first the hydrolysis of the Cr(III) monomer units, favored by the acidity conditions, followed by the formation of trimer and tetramer species. The EXAFS experiments at the Cr K-edge confirm the formation of the oligomeric species before the LDH signature. At the Zn K-edge, the results show that hexaaquozinc(II) complexes are present before the formation of the LDH phase. During LDH synthesis, oligomeric chromium(III) species and hexaaquozinc(II) complexes coexist in solution with the LDH solid. A structural pathway based on direct condensation of hexaaquozinc(II) complexes with deprotonated Cr(III) monomeric species is proposed to explain the formation of LDH solid with ratio Zn:Cr = 2. The structural pathway strongly supports the concept of cationic order in the [Zn-Cr-Cl] LDH sheets suggested by several authors but not demonstrated until now. Nevertheless we cannot exclude some faults during condensation as incorporation of a few oligomers such as dimers, trimers, and planar tetramers, but this should be very exceptional since the condensation of chromium oligomers with zinc monomers would lead to an LDH solid with a Zn:Cr ratio less than 2.