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

The synthesis and thermal degradation of MAl4(OH)(12)SO4 center dot 3H(2)O 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 H-1 and Al-27 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 degrees 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 approximate to 2 for the metal sulfate solutions is required to prevent the formation of byproducts, which were Ni(OH)(2) and Cu-3(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 degrees C, (ii) loss of sulfate between 600 and 900 degrees C, and (iii) formation of the end products at 900-1200 degrees C. For CoAl4-LDH (ZnAl4-LDH), these are alpha-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 alpha-Al2O3.