Layered Double Hydroxide
The Layered double hydroxides (LDHs) belong to anionic clays, which themselves are part of phyllites family. Clay minerals and clays are very common compounds on the earth surface, so they constitute the main component of soils and sedimentary rocks. The brucite-like sheets can stack one on the other with two different symmetries: rhombohedral and hexagonal. The LDH specimens with rhombohedral symmetry have mainly been found in nature, while the hexagonal symmetry may be the high-temperature form of the rhombohedral one. LDHs are important catalysts in organic reactions due to their basic character but also, for the transition metal-containing LDHs, due to their redox properties; they can be used in the as-synthesized LDH form or after different controlled thermal treatments. The LDHs are reliable materials as adsorbents and anion-exchangers due to the presence of anions in the interlayer region and to the thickness inter-layer modification ability vs. dimension of guest anions.