Synthesis of Nordstrandite and Nordstrandite‐Derived Layered Double Hydroxides of Li and Al: A Comparative Study with the Bayerite Counterpart

Phase Chemistry for Hydration Sensitive (De)intercalation of Lithium Aluminum Layered Double Hydroxide Chlorides

Lithium aluminum layered double hydroxide chlorides (LADH-Cl) have been widely used for lithium extraction from brine. Elevation of the performances of LADH-Cl sorbents urgently requires knowledge of the composition-structure-property relationship of LADH-Cl in lithium extraction applications, but these are still unclear. Herein, combining the phase equilibrium experiments, advanced solid characterization methods, and theoretical calculations, we constructed a cyclic work diagram of LADH-Cl for lithium capture from aqueous solution, where the reversible (de)hydration and (de)intercalation induced phase evolution of LADH-Cl dominates the apparent lithium "adsorption-desorption" behavior. It is found that the real active ingredient in LADH-Cl type lithium sorbents is a dihydrated LADH-Cl with an Al:Li molar ratio varying from 2 to 3. This reversible process indicates an ultimate reversible lithium (de)intercalation capacity of ∼10 mg of Li per g of LADH-Cl. Excessive lithium deintercalation results in the phase structure collapse of dihydrated LADH-Cl to form gibbsite. When interacting with a concentrated LiCl aqueous solution, gibbsite is easily converted into lithium saturated intercalated LADH-Cl phases. By further hydration with a diluted LiCl aqueous solution, this phase again converts to the active dihydrated LADH-Cl. In the whole cyclic progress, lithium ions thermodynamically favor staying in the Al-OH octahedral cavities, but the (de)intercalation of lithium has kinetic factors deriving from the variation of the Al-OH hydroxyl orientation. The present results provide fundamental knowledge for the rational design and application of LADH-Cl type lithium sorbents.

Synthesis and Structure Refinement of [Co-Al4-X] LDHs (X = NO3 - and SO4 2-) from Nordstrandite

[M-Al₄-X] LDHs (M = divalent metal and X = anion) are a class of aluminum-rich layered double hydroxides synthesized from both the gibbsite and bayerite polymorphs of Al(OH)₃. Henceforth, "g" and "b" are used to indicate gibbsite and bayerite. Despite the differences in the stacking arrangement of the hydroxyl layers in the precursor polymorphs, [M-Al₄-X] LDHs whether synthesized from gibbsite or bayerite were seen to be structurally the same. In this work, we report the first ever synthesis of [M-Al₄-X] LDHs (M = Zn, Ni, and Co and X = NO₃ ^- and SO₄ ^2-) from nordstrandite, which is yet another polymorph of Al(OH)₃. Hereafter, "n" represents the nordstrandite precursor. We report that n-[M-Al₄-X] LDHs do not differ structurally from those prepared from gibbsite and bayerite. We also report the structural refinement of n-[Co-Al₄-X] LDHs, where X = NO₃ ^- and SO₄ ^2-. This work is also significant as it gives for the very first time the refined structure of a [Co-Al₄-NO₃] LDH, though there are earlier reports on the synthesis of this LDH from both gibbsite and bayerite. The NO₃ ^- ion in the interlayer makes an angle of ∼48° with the plane of the metal hydroxide layer, and its symmetry reduces from D _3h to C _2v . Similarly, the change in the symmetry of the SO₄ ^2- ion in the interlayer is from T _d to C _3v .

Dehydration-Rehydration Studies on Polytypes of Chloride and Nitrate Layered Double Hydroxides of Nordstrandite and Bayerite: a Comparative Study

In this work, we report for the first time, the dehydration-rehydration studies of nordstrandite-derived layered double hydroxides (LDHs) of Li and Al, n-[Li-Al-X] (X = Cl^- and NO₃ ^-) (n-nordstrandite derived). n-[Li-Al-NO₃], an orthorhombic phase, dehydrated at 180 °C to a monoclinic phase. Refinement placed the NO₃ ^- ions parallel to the hydroxide layers. The dehydration showed no change in basal spacing. The monoclinic n-[Li-Al-Cl] dehydrated at 160 °C with a 0.49 Å compression in basal spacing to an orthorhombic polytype. We compared our results with the published results of their bayerite counterparts b-[Li-Al-X] (b-bayerite derived) and observed that though n-[Li-Al-X] and b-[Li-Al-X] LDHs have similar structures, their dehydrated phases are structurally different. We also report the refinement of b-[Li-Al-Cl] (DH). Previous studies attribute the basal spacing values to the (i) degree of hydration and (ii) orientation of anions in the interlayer. We observe that basal spacing is a manifestation of the symmetry of the crystal. Dehydration of nitrate intercalated LDH, which proceeds from an orthorhombic symmetry to a monoclinic symmetry with no decrease in the interlayer spacing, is attributed to sliding of the hydroxyl layers in the ab-plane due to the increase in the β value. This sliding stabilizes the interlayer through weak long-range electrostatic forces that mainly contribute to the stabilization of the layered structure at separations much larger than the effective radius of hydrogen bonds. Such stabilization would negate the need for the layers to compress, thus conserving the basal spacing in n-[Li-Al-NO₃] (DH).