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

Abstract The polytype 3R 2 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 3R 1 is hydrothermally formed with a Mg/Al ratio of 2.2 at 90 °C, which is subsequently converted at 170 °C into polytype 3R 2 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 3R 2 Mg–Al LDH by the strong (10 l ) over (01( l + 1)) ( l = 3n + 1, n = integer) reflections. A unique feature of polytype 3R 2 , 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 3R 2 . 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%.