Synthesis and characterization of hydrotalcite-like compounds containing V3+ in the layers and of their calcination products

The synthesis and full characterization of a new hydrotalcite-like compound with the formula [Mg-0.71 V-0.29(OH)(2)](CO3)(0.145). 0.72H(2)O and with V3+ in the layers are described. The influence of hydrothermal treatment and drying rate on the crystallinity of the materials obtained is discussed. The evolution to mixed oxides upon calcination at different temperatures (448, 548, 773, 1023, and 1273 K) under different atmosphere environments (air or nitrogen) for 2 h has been studied. Characterization of the original layered materials and of the calcination products has been carried out by X-ray diffraction, thermal analysis, Fourier-transform infrared spectroscopy, BET specific surface area determination, temperature-programmed reduction, and transmission electron microscopy. X-ray absorption spectroscopies (XANES and EXAFS) have also been used to assess the local geometry of vanadium ions in the different compounds prepared. All experimental data agree with a well-crystallized hydrotalcite-like compound after thermal treatment, and also a minor effect of the drying rate on the crystallinity has been found. Thermal decomposition yields poorly crystalline layered compound at 448 K that undergoes transformation to mostly amorphous materials when calcined at 548--773 K, finally leading to a mixture of MgO and Mg3V2O8, which has increasing crystallinity as the calcination temperature increases. XAS results indicate the presence of V3+ ions in an octahedral coordination in the parent sample calcined at 448 K and tetrahedrally coordinated V5+ species for samples calcined at higher temperatures, calcination giving rise to a better ordering of the second coordination sphere. Similar results were found when calcination was performed in nitrogen, although higher temperatures were needed to achieve the same result.