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. Al-27 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 C-13 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 C-13 CP/MAS spectra.