Synthesis and Structure Refinement of [Co-Al4-X] LDHs (X = NO3- and SO42-) from Nordstrandite

Abstract

[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 SO42-) 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 SO42-. 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 D3h to C2v. Similarly, the change in the symmetry of the SO42- ion in the interlayer is from Td to C3v