Steam activation of Mg–Al hydrotalcite. Influence on the properties of the derived mixed oxides

The influence of steam activation of Mg–Al hydrotalcite on the properties of the resulting mixed oxides has been investigated and compared with the typically performed thermal decomposition under dry conditions. The as-synthesized hydrotalcite was treated at 723 K and 1 bar in nitrogen flow containing steam partial pressures in the range of 0–800 mbar. Thermal analysis indicated that the hydrotalcite decomposition was not affected by the presence of steam in the feed gas. The average crystallite size and morphology of the dry and steam-activated materials, as determined by XRD and TEM, respectively, were very similar too. The porous properties of the resulting oxides present significant differences. The BET surface area decreased ca. 20% when small amounts of steam were present as compared to the dry-decomposed product, but was not further reduced upon increasing the steam partial pressure. This has been attributed to the disappearance of small mesopores (<4 nm) due to the hydrolysis of Al–O–Al bonds from adjacent layers and platelets.