Synthesis and characterization of new Mg2Al-paratungstate layered double hydroxides

Layered double hydroxides (LDHs, or hydrotalcites) with Mg2+ and Al3+ cations in the mixed metal hydroxide layer and paratungstate anions in the interlayer have been prepared. Different methods have been followed: anion exchange with Mg,Al LDHs originally containing nitrate or adipate, reconstruction of the LDH structure from a mildly calcined Mg2Al-CO3 LDH, and coprecipitation, In all cases, the tungsten precursor salt was (NH4)(10)H2W12O42, The prepared solids have been characterized by elemental chemical analysis, powder X-ray diffraction (PXRD), FT-IR spectroscopy, thermogravimetric (TG) and differential thermal (DTA) analyses, scanning electron microscopy (SEM) with EDX (energy-dispersive X-ray analysis), and nitrogen adsorption at -196 degreesC for surface area and surface texture. Most of the synthesis methods used, especially anion exchange starting from a Mg2Al-NO3 precursor at low temperature and short reaction times, lead to formation of a hydrotalcite with a gallery height of 9.8 Angstrom; increasing the reaction temperature to 70-100 degreesC and maintaining short contact times leads to a solid with a gallery height of 7.8 Angstrom. Both phases have been identified as a result of the intercalation of W7O246- species in different orientations in the interlayer space. If the time of synthesis or the temperature is increased, a more stable phase, with a gallery height of 5.2 Angstrom corresponding to a solid with intercalated W7O246-, is formed, probably with grafting of the interlayer anion on the brucite-like layers. All systems are microporous. Calcination at 300 degreesC leads to amorphous species, and crystallized MgWO4 is observed at 700 degreesC.