Reversible hydration of the perchlorate-intercalated layered double hydroxides of Li and Al: structure models for the dehydrated phases

Abstract

Imbibition of lithium sulphate into aluminium hydroxide is known to result in a sulphate-intercalated layered double hydroxide (LDH) of Li and Al. The perchlorate ion has the same size and molecular symmetry as the sulphate ion, but only half its charge. Consequently, twice the number of $\hbox {ClO}_{4}^{-}$ ions is needed to balance LDHs the charge on the metal hydroxide layer, compared to the $\hbox {SO}_{4}^{2-}$ ions. In this work, the $\hbox {ClO}_{4}^{-}$-intercalated LDHs were obtained from both the bayerite and gibbsite precursors. Inclusion of the hydration sphere along with the $\hbox {ClO}_{4}^{-}$ anion, induced turbostratic disorder in the stacking of the metal hydroxide layers. Temperature-induced dehydration ($T \sim 100$–$140{^{\circ }}\hbox {C}$) brought about a partial ordering in the interlayer region and the $\hbox {ClO}_{{4}}^{{-}}$ ion oriented itself with one of its $C_{{2}}$-axes parallel to the metal hydroxide layer. The close packing of $\hbox {ClO}_{4}^{-}$ ions could be realized by the complete dehydration of LDH and the distribution of the $\hbox {ClO}_{4}^{-}$ ions in all the available interlayer sites. In contrast, within the crystal of the sulphate analogue, the sulphate ions occupy only half the number of interlayer sites. The other half is occupied by the residual water molecules, as the sulphate analogue does not fully dehydrate even at elevated temperatures. This difference in the behaviour of the two LDHs has its origin in the large difference in the hydration enthalpies of the two anions.