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Part 1
FUNDAMENTAL METHODOLOGIES
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Chapter 1
Layered Double Hydroxides and the Environment: An Overview
Amita Jaiswal* Ravindra Kumar Gautam and Mahesh Chandra Chattopadhyaya*
Environmental Chemistry Research Laboratory, Department of Chemistry, University of Allahabad, Allahabad, India
Abstract
Due to their versatility, hundreds of millions of tons of clay minerals currently find applications not only in ceramics, building materials, paper coating and fillings, drilling muds, foundry molds, pharmaceuticals, etc., but also as adsorbents, catalysts or catalyst supports, ion exchangers, etc., depending on their specific properties. There are two broad classes of clays: Cationic clays (or clay minerals), widespread in nature, and Anionic clays (or layered double hydroxides), more rare in nature but relatively simple and inexpensive to synthesize on a laboratory or industrial scale. Cationic clays have negatively charged alumina-silicate layers with small cations in the interlayer space to balance the charge, while anionic clays have positively charged brucite type metal hydroxide layers with balancing anions and water molecules located interstitially. The layered double hydroxides (LDHs) belonging to the general class of anionic clay minerals can be of both synthetic and natural origin. Also known as hydrotalcite-like compounds (HTLCs), these materials are interesting because their layer cations can be changed among a wide selection, and the interlayer anion can also be freely chosen. Like cationic clays, they can be pillared and can exchange interlayer species, thus increasing applications and making new routes to synthesize the derivatives.
This chapter deals with the brief history of layered double hydroxides, their structure, properties, synthesis by different methods and characterization, along with their applications mainly in the environmental field.
Keywords: Layered double hydroxides, anionic clays, cationic clays, brucite, interlayer species, heavy metals, dyes, greenhouse gases, surfactants
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. These materials were first 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. The 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 fibrous masses.
During the discovery of hydrotalcite another hydroxycarbonate of magnesium and iron was found, which was called pyroaurite. Pyroaurite was later recognized to be isostructural with hydrotalcite and other minerals containing different elements, all of which were recognized as having similar 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 diffraction on mineral samples which revealed the main structural entities of LDHs and disproved Feitknecht’s theory. These studies showed that the two cations were in fact located in a single layer and the interlayers were composed of water and carbonate 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. These include the possible stoichiometric range and composition, 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 different 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 minerals have a structure similar to hydrotalcite, which has the formula [Mg6Al2 (OH)16] CO3. 4H2O. Unlike clays, however, layered double hydroxides are not discovered in large, commercially exploitable deposits [9]. The LDHs have been prepared using many combinations of divalent and trivalent cations including magnesium, aluminium, zinc, nickel, chromium, iron, copper, 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 clays and host-guest layered materials [1, 3, 32–35], which are quite rare in nature. Most LDHs are synthetic phases and their structure resembles the naturally occurring mineral hydrotalcite [Mg6Al2(OH)16] CO3. 4H2O, having the general formula of [M(II)1−x M (III)x (OH)2] (Yn−)x/n. YH2O, where, M(II), M(III) = divalent and trivalent metals respectively, 0.2 < x < 0.33, and Yn− = the exchangeable anions between the layers [10, 36, 37].
The basic layer structure of LDHs is based on brucite [Mg (OH)2], typically associated with small polarizing cations and polarizable anions. It consists of magnesium ions surrounded approximately octahedrally by hydroxide ions. These octahedral units form infinite layers by edge-sharing with the hydroxide ions sitting perpendicular to a plane of the layers. The layers then stack on top of one another to form a three-dimensional structure.
When Mg2+ is replaced by a trivalent cation similar in radius, an overall positive charge results in the hydroxyl sheets and counter balance is provided by carbonate ions which are positioned within the hydroxyl interlayer. In addition to carbonate ions, water molecules are found in the interlayer gallery. The nature of the interlayer anion and the extent of hydration often determine the layer spacing between each brucite-like sheet [38]. The brucite-like sheets may occur in two different symmetries, namely rhombohedral and hexagonal. In nature, the rhombohedral symmetry is widespread. However, in mineral samples, the hexagonal symmetry is seen to favor the interior of the crystallite samples, while the rhombohedral symmetry is found on the exterior. This is a result of cooling during crystallite transformation, in which the extrerior surface of the crystallite cools much quicker than the interior and hence the interior hexagonal form cannot transform due to a higher energy transformation barrier at lower temperature. From these observations, it has been deduced that the hexagonal symmetry is favored by high temperature [1, 4]. Naturally occurring minerals that exhibit a LDH structure include manasseite, pyroaurite, sjogrenite, bar-betonite, takovite, reevesite, desautelsite and stichtite. They differ from one another in the stacking arrangement of the octahedral layers [1, 39].
Conventionally synthesized LDHs are strongly hydrophilic materials, either amorphous or microcrystalline with hexagonal habit, with the dominant faces developed parallel to the metal hydroxide layers. Adjacent layers are tightly bound to each other. Figure 1.1 shows the structure of layered double hydroxides.
One of the advantages of LDHs among layered materials is the great number of possible compositions and metal–anion combinations that can be synthesized. Layered double hydroxides (LDHs) have high charge density. The charge density is dependent on the metal ratio. Since it comprises a divalent and trivalent metal cation, their ratio affects charge density of the layers. A lower divalent/trivalent ratio results in a higher charge density.
1.3 Properties of Layered Double Hydroxides
Layered double hydroxides (LDHs) display unique physical and chemical properties close to those of clay minerals. Some interesting properties of these materials summarized by Del Hoyo [40] are:
- High specific surface area (100±300 m2/g)
- Memory effect
- Anion exchange capacities
- Synergistic effects
The LDHs exhibit anion mobility, surface basicity and anion exchangeability due to their positively charged layered structure. The anions and water, which fill the interlayer space, are labile. Therefore a variety of inorganic and organic anions can be intercalated in the interlayer of LDHs through anion exchange reactions [33]. The mixed metal oxides obtained on calcination of LDH usually exhibit properties such as high surface area, surface basicity and formation of homogeneous mixture with small crystallite size when heated to higher temperature [1]. The LDHs as well as the oxides obtained from them exhibit excellent catalytic activity. Structure reconstruction, or so called “memory effect,” is another important property of LDHs which is unique to this class of layered solids. Structure reconstruction is usually achieved by first decomposing the LDH at suitable high temperature followed by treating the resultant mixed metal oxides with a solution containing a suitable anion [41]. These materials have a high capacity for adsorbing anions as well as cations [38, 42]. Magnetic properties of the LDHs depend on the space between the layers. This space can be adjusted by insertion of organic anions with different chain lengths. This suggests that these hybrid materials would work as tunable magnets [43]. The LDHs intercalated with long-chain surfactant molecules such as dodecyl sulphate have the ability to swell in organic solvents. This property of delamination is exploited in the preparation of monolayers, which are used extensively in the synthesis of nanohybrids and nanocomposites [44].
The interlayer anions present in LDHs can be exchanged by other anions. The order of preference for some common inorganic anions is as follows:
NO3− is an anion which can be easily replaced by a more strongly held one like CO32−. Therefore when preparing precursor for interaction, nitrate salts are preferred ...
