Synthesis and characterization of layered double hydroxides (LDHs) with intercalated chromate ions

Chromate intercalated layered double hydroxides (LDHs) having the formula $M^II_6M'^III_2(OH)₁₆CrO_4.4H_2O$ $(M^II = Ca, Mg, Co, Ni, Zn$ with $M^'{III}_=Al$ and $M^II = Mg, Co, Ni$ with $M'^III = Fe)$ have been prepared by coprecipitation. The products obtained are replete with stacking disorders. DIFFaX simulations show that the stacking disorders are of three kinds: (i) turbostratic disorder of an originally single layered hexagonal (1H) crystal, (ii) random intergrowth of polytypes with hexagonal (2H) and rhombohedral (3R) symmetries and (iii) translation of randomly chosen layers by (2/3, 1/3, z) and (1/3, 2/3, z) leading to stacking faults having a local structure of rhombohedral symmetry. IR spectra show that the $CrO_4^\hspace{2mm}{2-}$ ion is incorporated either in the $T_d$ or in the $C_3v$ symmetry. The interlayer spacing in the latter case is 7.3 \AA characteristic of a single atom thick interlayer showing that the $CrO_4^\hspace{2mm}{2-}$ ion is grafted to the metal hydroxide slab. On thermal treatment, the $CrO_4^\hspace{2mm}{2-}$ ion transforms into Cr(III) and is incorporated into the spinel oxide or phase separates as $Cr2O_3$. In the LDH of Mg with Al, Cr(III) remains in the MgO lattice as a defect and promotes the reconstruction of the LDH on soaking in water. In different LDHs, 18–50% of the $CrO_4^\hspace{2mm}{2-}$ ion is replaceable with carbonate anions showing only partial mineralization of the water-soluble chromate. The extent of replaceable chromates depends upon the solubility of the corresponding LDH, which in turn is determined by the solubility of the $MCrO_4$. These studies have profound implications for the possible use of LDHs for chromate amelioration in green chemistry.