Quadrupolar magic angle spinning NMR spectra fitted using the Pearson IV function

Synthesis and Thermal Degradation of MAl4(OH)12SO4·3H2O with M = Co2+, Ni2+, Cu2+, and Zn2

The synthesis and thermal degradation of MAl₄(OH)₁₂SO₄·3H₂O layered double hydroxides with M = Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺ ("MAl₄-LDH") were investigated by inductively coupled plasma-optical emission spectroscopy, thermogravimetric analysis, powder X-ray diffraction, Rietveld refinement, scanning electron microscopy, scanning tunnel electron microscopy, energy-dispersive X-ray spectroscopy, and solid-state ¹H and ²⁷Al NMR spectroscopy. Following extensive synthesis optimization, phase pure CoAl₄- and NiAl₄-LDH were obtained, whereas 10-12% unreacted bayerite (Al(OH)₃) remained for the CuAl₄-LDH. The optimum synthesis conditions are hydrothermal treatment at 120 °C for 14 days (NiAl₄-LDH only 9 days) with MSO₄(aq) concentrations of 1.4-2.8, 0.7-0.8, and 0.08 M for the CoAl₄-, NiAl₄-, and CuAl₄-LDH, respectively. A pH ≈ 2 for the metal sulfate solutions is required to prevent the formation of byproducts, which were Ni(OH)₂ and Cu₃(SO₄)(OH)₄ for NiAl₄- and CuAl₄-LDH, respectively. The thermal degradation of the three MAl₄-LDH and ZnAl₄-LDH in a nitrogen atmosphere proceeds in three steps: (i) dehydration and dehydroxylation between 200 and 600 °C, (ii) loss of sulfate between 600 and 900 °C, and (iii) formation of the end products at 900-1200 °C. For CoAl₄-LDH (ZnAl₄-LDH), these are α-Al₂O₃ and CoAl₂O₄ (ZnAl₂O₄) spinel. For NiAl₄-LDH, a spinel-like NiAl₄O₇ phase forms, whereas CuAl₄-LDH degrades by a redox reaction yielding a diamagnetic CuAlO₂ (delafossite structure) and α-Al₂O₃.