Influence of the Mg2+ or Mn2+ contents on the structure of NiMnAl and CoMgAl hydrotalcite materials with high aluminum contents

• Pure NiAl- and CoAl-terephthalate hydrotalcite type materials with x >0.33.
• CoMgAl- and NiMnAl-terephthalate trimetallic hydrotalcites were prepared.
• EXAFS confirm high aluminum content in CoAl- and NiAl-terephthalate LDHs.
• Mg and Mn substitute Co and Ni up to 40% in CoAl- and NiAl-terephthalate LDHs.

In this work, layered double hydroxides with high aluminum content (aluminum atom fractions 0.3 and 0.5, relative to total metal cations) were successfully synthesized in the systems CoAl, CoMgAl, NiAl and NiMnAl, using a co-precipitation method with controlled pH. Terephthalate was used as the charge compensation anion. Different characterization techniques (XRD, FTIR, ICP, AAS, TGA, CHN and EXAFS) showed that for these high-aluminum LDHs, the terephthalate ions are vertically placed within the interlayer galleries and the cations were effectively included in the layers. The unusually large limiting value for the intralayer aluminum proportion is possibly associated with the larger length and polarizability of the terephthalate anion as compared to the usual small inorganic anions (as carbonate, nitrate or hydroxyl). However, in the trimetallic samples (CoMgAl and NiMnAl) a segregated aluminum phase was found for higher Mg or Mn contents. With high Mn content a manganese terephthalate phase was also observed.

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