Chemical composition and interlayer arrangement of polytype 3R2 Mg–Al layered double hydroxides

The polytype 3R2 of Mg–Al layered double hydroxide (LDH) can be synthesized either directly by hydrothermal treatment of magnesium oxide (MgO) and aluminium trihydroxide (ATH) at 170 °C or by a two-step synthesis. By the latter method, first polytype 3R1 is hydrothermally formed with a Mg/Al ratio of 2.2 at 90 °C, which is subsequently converted at 170 °C into polytype 3R2 with a Mg/Al ratio of 1.1, by addition of ATH. The chemical composition and interlayer arrangement of this polytype were investigated. XRPD confirmed the formation of polytype 3R2 Mg–Al LDH by the strong (10 l) over (01(l + 1)) (l = 3n + 1, n = integer) reflections. A unique feature of polytype 3R2, which is the presence of tetrahedrally coordinated aluminium ions, was revealed by FT-IR and 27-Al MAS NMR measurements. These aluminium ions, present as aluminates, act as counter ions in the interlayer of polytype 3R2. From the STEM-EDX result, it is concluded that the molar Mg/Al ratio in the octahedral layer is approximately 2.0. If all the charges in the octahedral layer are compensated by aluminate ions, the overall Mg/Al ratio would be 1.0. 27-Al NMR however, measured only about 40% tetrahedrally coordinated aluminium ions instead of 50%.