Precursor dependence of the nature and structure of non-stoichiometric magnesium aluminium vanadates

Hydrotalcite-like compounds containing (a) Mg-II; Al-III and V-III in the layers and carbonate in the interlayers, or (b) Mg-II and Al-III in the layers and decavanadate in the interlayers, have been synthesized and characterized using powder X-ray diffraction, thermal analysis, FTIR spectroscopy, temperature-programmed reduction, and specific surface-area determination. The crystalline phases formed after calcination in air at 700 degrees C have been identified using these same experimental techniques. It has been found that the nature of the precursor determines the nature of the Mg-V-O phases formed; thus, when starting from (a), Mg-3(VO4)(2) containing isolated [VO4] units is formed, while when starting from (b), MgV2O6, containing pairs of edge-sharing [VO6] octahedra, is formed preferentially. Such a reactivity appears to depend on the nature of the support, and not on the stoichiometry (i.e., Mg/V atomic ratio) of the starting materials.