Layered Double Hydroxides and the Environment: An Overview - Advanced Materials for Agriculture, Food and Environmental Safety - page 4

1.1 Introduction

Layered double hydroxides (LDHs) have been known for a very long
time. Around 1842, naturally forming LDHs minerals were discovered in
Sweden. Crushing these minerals leads to a white powder similar to talc.
h ese materials were i rst synthesized by a German scientist, W. Feithnecht
(1942), through reaction between dilute solutions of metals with bases,
which he named “doppelschichtstrukturen” or double-sheet structure.
h e LDHs are also known as hydrotalcite-like compounds (HTLCs).
Hydrotalcite (HT) is a hydroxycarbonate of magnesium and aluminium
which occurs in nature in foliated and contorted plates or i brous masses.
During the discovery of hydrotalcite another hydroxycarbonate of mag-
nesium and iron was found, which was called pyroaurite. Pyroaurite was
later recognized to be isostructural with hydrotalcite and other minerals
containing dif erent elements, all of which were recognized as having simi-
lar features. Hydrotalcites have been studied for their use as catalysts and
precursors to various other catalysts as early as 1970 [1, 2].
Allman and Taylor studied single crystal X-ray dif raction on mineral
samples which revealed the main structural entities of LDHs and disproved
Feitknecht's theory. h ese studies showed that the two cations were in fact
located in a single layer and the interlayers were composed of water and car-
bonate ions. Although the main entities of LDHs have been elucidated, Evans
and Slade [3] have suggested that several intrinsic details still remain to be
fully understood. h ese include the possible stoichiometric range and compo-
sition, and the position and arrangement of metals within each cationic layer.
Prior to the study by Evans and Slade, Miyata and Okada [4-6] described
many structural features of LDHs/HTLCs which have dif erent guest anions.
Layered double hydroxide materials appear in nature and can be readily
prepared in the laboratory. In nature they are formed from the weathering
of basalts [7, 8] or precipitation [9] in saline water. All natural LDH miner-
als have a structure similar to hydrotalcite, which has the formula [Mg 6 Al 2
(OH) 16 ] CO 3. 4H 2 O. Unlike clays, however, layered double hydroxides are
not discovered in large, commercially exploitable deposits [9]. h e LDHs
have been prepared using many combinations of divalent and trivalent cat-
ions including magnesium, aluminium, zinc, nickel, chromium, iron, cop-
per, indium, gallium and calcium [10-31].

1.2 Structure of Layered Double Hydroxides

Layered double hydroxides (LDHs) are also known as hydrotalcite-like
compounds (due to their structural similarities to that mineral) or anionic