Unexchangeable Interlayer Anions; Synthesis and Characterization of Zn/Al- and Mg/Al-Layered Double Hydroxides with Interlayer Alizarin red S

Zn/Al- and Mg/Al-layered double hydroxides with interlayer 9,10-anthraquinone-1,2-dihydroxy-3-sulfonate (Alizarin red S anion abbreviated as ARS) are synthesized (products are abbreviated as M/Al/ARS-LDH where M=Mg or Zn) and the solid-state properties are characterized. In powder X-ray diffraction measurements, basal spacings of the layered compounds are elucidated on the basis of the molecular size of ARS and its conformation at the interlayer gallery region. 27Al MAS NMR spectra reveal that the Al3+ ions in the layers of Zn/Al/ARS-LDH or Mg/Al/ARS-LDH locate in the octahedral hexa-coordination sphere whereas some of the metal cations in the calcined Zn/Al/CO3-LDH, the precursor in the synthesis of Zn/Al/ARS-LDH, locate in the penta- or the tetra-coordination sphere. FT-IR spectra and 13C CP/MAS NMR spectra illustrate that the interlayer ARS anions are stable in both layered compounds and the coordination bond is formed in Zn/Al/ARS-LDH between the sulfonate of ARS and Al3+ in the layer. The interlayer ARS anions are not eliminated from the interlayer region of Zn/Al/ARS-LDH when the solid is immersed in an aqueous solution containing carbonate; the unexchangeable nature of the interlayer ARS with carbonate results from the coordination bond observed in FT-IR and 13C CP/MAS spectra.