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Chapter 1
INTRODUCTION
1.1 Layered solids
Layered structures have been said to form an awkward bridge between simple
compounds with a high degree of ionicity like NaCl and less ionic compounds with
considerable covalency but with similar structure like AgCl, and solids such as HgCl2
and Al2Br6 wherein the presence of discrete molecules is apparent [1]. A solid is
assumed to exhibit a layered structure only when the bonds among atoms of the same
plane are much stronger than the interactions among atoms of adjacent planes [2].
Majority of the typical layered solids such as anionic and cationic clays, graphite, M IV
phosphates and phosphonates have covalent bonds between atoms of the same layer
and weak van der Waals forces between atoms of the adjacent layers. These layered
compounds can therefore be visualized as solids in which a single layer is a giant
planar macromolecule and the solid is a molecular crystal formed by the stacking of
these layers. Due to the anisotropy in bonding, the layered solids usually exhibit a
high degree of anisotropy in their physical properties. The structure of a typical
layered solid is schematically represented in Figure 1.1.
Figure 1.1 Schematic of a typical layered solid
The distance between the barycenters of two adjacent layers of a layered solid
is called the interlayer (or interlamellar) distance (d) which is synonymous with
„interlayer spacing‟, basal spacing or d-spacing. When the thickness of the layer is
subtracted from the interlayer distance, the „free distance‟ between adjacent layers,
2
(df), is known as the „gallery height‟. The space between two adjacent layers is
usually called the „interlayer region‟.
1.2 Classification of layered solids
There are several ways of classifying layered solids. A classification may be
made on the basis of the inorganic or organic nature of the constituent layers, which is
easy if the nature of the layer is completely organic or inorganic, but gets tricky if it is
of mixed nature [2]. A second classification made by S. A. Solin [3] on the basis of
the thickness of the constituent layers of the layered solids. According to him layered
solids can be classified into three subclasses: Class I layered solids are formed from
atomic monolayers; typical examples for this class are graphite which is homopolar
and boron nitride which is a binary layered material. Class II layered solids are
formed from layers which are a few atoms thick. There are many examples of such
compounds, which include layer dichalcogenides such as MoS2, TiS2 and HfS2,
FeOCl-type compounds and metal chlorides such as FeCl2 and CoCl2. Class III
layered solids are formed from layers which are many atoms thick. The prototypical
examples of Class III layered solids are the layered silicate clays. This classification is
directly related to the rigidity of the layered solids, which increases as we move from
Class I to Class III layered materials.
Figure 1.2 Representative examples of layered solids classified into different classes
based on layer thickness
Finally the layered solids can be classified on the basis of the
presence/absence of fixed charges on the layers of the planar macromolecules. In this
classification, the layered solids may be subdivided into two large classes:
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„uncharged‟ (or electrically neutral) and „charged‟ layered solids [2]. Electrically
neutral layers can be further subdivided into conducting and non-conducting (or
insulator) layered solid. Typical examples of non-conducting layered solid are nickel
hydroxide, α-metal (IV) phosphonates, α-metal (IV) phosphites, and clays like
kaolinite, serpentine, pyrophyllite and dickite. Electrically conducting layers are
present in layered solids like graphite, transition metal dichalcogenides, transition
metal oxyhalides and metal phosphorus chalcogenides. Host guest interactions in the
insulating layers is either by van der Waals interactions and/or hydrogen bonding,
while in the electrically conducting layers the host-guest interactions mainly are of
ionic type. The class of charged layered solids can be further sub divided into cationic
layered solids with negatively charged layers like montmorillonite, saponite,
vermiculite (also called as cationic clays) and anionic layered solids with positively
charged layers like hydrotalcite and hydrotalcite-like compounds (also called as
anionic clays). The charge on the layer can arise due to a partial substitution of a
divalent metal ion with a trivalent metal ion as in the case of hydrotalcite or due to the
presence of monovalent metal ions in the voids as in the case of gibbsite derived
LDHs or by partial hydroxyl deficiencies as in the case of α–hydroxides or by
substitution of divalent metal ions in tetrahedral vacancies one above and one below
an octahedral vacancy as in the case of zinc hydroxysalts.
Cationic Layered Solid
Neutral Layered Solid
Anionic Layered Solid
Figure 1.3 Layered solids classified based on layer charge
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1.3 Anionic clays
1.3.1 Layered double hydroxides
Layered double hydroxides (LDHs) are found in nature as minerals or can be
readily synthesized in the laboratory. In nature, they are formed from the weathering
of basalts or precipitation in saline water sources [4]. The term LDHs is used to
designate synthetic or natural layered hydroxides with two or more kinds of metallic
cations in the main layers and hydrated interlayer domains containing anionic species.
This large family of compounds is also commonly referred to as hydrotalcite–like
compounds or anionic clays, the latter term indicating its complementarity with the
more common cationic clays like montmorillonite/saponite, whose interlayer domains
contain cationic species [5]. All LDH minerals found in nature have a structure
similar to that of hydrotalcite or its hexagonal analog, manasseite. Hydrotalcite, a
mineral that can be easily crushed to a white powder and is similar to talc, was
discovered in Sweden around 1842. The first exact formulae for hydrotalcite and
other isomorphous minerals were proposed by Manasse [6], a few of which are given
in Table 1.1 [4, 7]. The general formula for the members of the family based on a
combination of divalent and trivalent metal cations, can be written as [MII1-xMIIIx
(OH)2x]x+(An-)x/n·mH2O, where MII represents a divalent metal which is usually Ca+2,
Mg+2, Ni+2, Zn+2, Mn+2, Fe+2, Cu+2 or Co+2, while MIII is a trivalent metal like Al+3,
Mn+3, Fe+3, Ni+3 or Co+3 and An- is a suitable anion. These LDHs are referred to as the
II–III LDHs. Another class of LDHs derived from aluminum hydroxide having a
general formula [LiAl2(OH)6]+(An-)1/n·mH2O, are classified as I–III LDHs.
The basic layered structure of LDHs is closely related to that of brucite,
Mg(OH)2 which has a CdI2 type structure [4, 8]. In a brucite layer, each Mg2+ ion is
approximately octahedrally surrounded by six OH- ions, these octahedral units form
infinite two-dimensional layers by edge-sharing, with the hydroxide ions sitting
perpendicular to the plane of the layers. The octahedron is slightly flattened, with the
distance between OH- neighbors on the same side of the layer being 0.314 nm. While
the distance between OH- neighbors on opposite sides of the sheet is only 0.270 nm.
The bond length of Mg–O is ca. 0.207 nm, and the repeat distance, or layer thickness,
is 0.478 nm. The distortion of the brucite layers does not change the hexagonal
symmetry (ao = bo = 0.314 nm, co = 0.478 nm, γ = 120◦) and has a Rm space group.
The O–H bonds are directed along the threefold axis towards the vacant tetrahedral
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site in the adjacent layers. The layers then stack on top of one another to form the
three-dimensional structure. From the point of view of close-packing, the structure
can be said to be composed of close-packed planes of hydroxyl anions that lie on a
triangular lattice. The magnesium ions occupy the octahedral holes between alternate
pairs of OH planes and thus occupy a triangular lattice identical to that occupied by
the OH ions. In actual fact, both the local geometry around the metal and the closepacking of the hydroxyl anions are strongly distorted away from the idealized
arrangements.
Partial isomorphous substitution of Mg2+ ions by Al3+ gives the brucite-like
layers a positive charge and the resulting negative charge deficiency is relatively
delocalized with respect to the inter-lamellar plane. In hydrotalcite the positive charge
is balanced by carbonate anions, which are located in the interlayer region between
the two brucite-like layers (Figure 1.4.). This gallery also contains water molecules,
hydrogen bonded to layer OH and/or to the interlayer anions. The electrostatic
interactions and hydrogen bonds between the layers and the contents of the gallery
hold the layers together, forming the three-dimensional structure.
Layer
Thickness
Interlayer
Region
Interlayer
Spacing
Gallery
Height
- Water Molecule
-Carbonate Anion
Figure 1.4 Structure of hydrotalcite
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Table 1.1: Some minerals related to hydrotalcite and their composition
Common Name
Chemical Composition
System
c (nm)
Hydrotalcite
Mg6Al2(OH)16CO3·4H2O
Trigonal
2.28
Desautelsite
Mg6Mn2(OH)16CO3·4H2O
Trigonal
2.34
Iowaite
Mg4Fe(OH)10Cl·3H2O
Trigonal
2.42
Meixnerite
Mg6Al2(OH)16(OH)2·4H2O
Trigonal
2.29
Pyroaurite
Mg6Fe2(OH)16CO3·4.5H2O
Trigonal
2.34
Reevsite
Ni6Fe2(OH)16CO3·4H2O
Trigonal
2.30
Stichtite
Mg6Cr2(OH)16CO3·4H2O
Trigonal
2.34
Takovite
Ni6Al2(OH)16CO3,OH·4H2O
Trigonal
2.26
(Ca,Mg)7(Al,Fe)2(OH)18
Wermlandite
(SO4)2·12H2O
Trigonal
2.26
Manasseite
Mg6Al2(OH)16CO3·4H2O
Hexagonal
1.56
Barbertonite
Mg6Cr2(OH)16CO3·4H2O
Hexagonal
1.56
Brugnatellite
Mg6Fe(OH)13CO3·4H2O
Hexagonal
1.60
Sjögrenite
Mg6Fe2(OH)16CO3·4.5H2O
Hexagonal
1.56
Woodwardite
Cu4Al2(OH)12SO4·4H2O
Hexagonal
1.09
1.3.2 Hydroxy double salts
Hydroxy double salts (HDS) are a class of anionic clays which can generally be
represented by the formula [M3OO Zn2T(OH)8]2+(An- )2/n·xH2O, where M is a divalent
metal ion, An- is an anion with valency „n‟, O and T refer to octahedral and tetrahedral
sites and  refers to vacancies. The structure of hydroxy double salts can be regarded
as a variation of the hypothetical C6 or CdI2-type Zn(OH)2 structure [9-11]. If one
quarter of the zinc atoms are removed from the octahedral sheet, so that each filled
zinc octahedron has common edges with two unoccupied and four other occupied
octahedra, the resulting [Zn3OO(OH)8]2- sheet becomes negatively charged. To
satisfy this charge, tetrahedrally coordinated zinc atoms are located above and below
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the unoccupied octahedra. Three corners of the tetrahedron are occupied by hydroxide
ions belonging to the sheet described above and the fourth corner by a water
molecule.
The
resulting
complex
sheet
is
positively
charged,
[Zn3OO(OH)8ZnT(H2O)2]2+. Anions get incorporated in the interlayer region to
compensate the positive charge on the layers. The anion can coordinate with both the
octahedral and tetrahedral zinc atoms as in case of hydrozincite [9] or the anion can be
coordinated only to the tetrahedral zinc as in the case of zinc hydroxy chloride [10] or
the anion may be coordinated to neither the octahedral nor tetrahedral zinc atoms as in
zinc hydroxynitrate [11]. HDS can be represented by the general formula
[M3OZn2T(OH)8](Am-)2/m·yH2O.
Figure 1.5 Structure of zinc hydroxynitrate
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1.4 Synthesis of Anionic Clays
Anionic clays can be synthesized in many ways. Most commonly used method
is the direct precipitation method. This method involves nucleating and growing the
metal hydroxide layer by mixing an aqueous solution containing the salts of two metal
ions, in the presence of the desired anion and a base. An inherent limitation of this
technique is that the LDH/HDS synthesis can be carried out only if the desired
interlayer anion is at least as tightly held as the counter ion of the metal salts used. For
this reason, metal chlorides and nitrates are widely preferred for the synthesis, while
sulfates are generally avoided. A more serious limitation is that the anion to be
incorporated in the LDH/HDS should not too readily form insoluble salts with the
constituent cations. LDH phosphates, for example, cannot be prepared by this method.
One common refinement of this technique is precipitation at constant pH, also
referred to as coprecipitation, suggesting that all the cations precipitate
simultaneously in a ratio fixed by the starting solution. This method, involves the
steady addition of a solution of cations and a solution of base simultaneously to a
flask with vigorous mixing, with the relative rates of addition regulated so that the
overall pH is steady. The final precipitate will be formed at the nominal reaction pH
only if at least one of two conditions is satisfied: the rate of mixing is greater than the
rate of conversion of initial precipitate to LDH, or the mixture is left stirring long
enough for any LDH formed in the mixing zone to redissolve as part of a process of
Ostwald ripening. The pH chosen for precipitation must be higher than that necessary
for LDH formation, but lower than that required for the precipitation of the metal
hydroxides themselves or their basic salts. Carbonate contamination can result from
the absorption of carbon dioxide from atmosphere into the solution, especially under
basic pH conditions. Such uptake of carbon dioxide, giving rise to carbonate-LDH, is
a major nuisance in LDH chemistry. Carbonate is among the most strongly held
anions within the LDH lattice, and it is very efficiently incorporated during initial
precipitation and final washing. It is therefore desirable to exclude carbon dioxide
from within the reaction vessel if another anion is to be intercalated [4].
Anionic clays like layered double hydroxides, hydroxy double salts, and
hydroxy (basic) chlorides and nitrates can also be prepared by reacting a solid oxide
with an aqueous solution of a metal salt [12]. Reacting a metal oxide, MO with
another divalent metal ion Me2+ solution or a solution containing the same divalent
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metal ion M2+, HDSs. Typically of the compositions [(M,Me)2(OH)3(An-)1/n] and
[(M,Me)5(OH)8(An-)2/n] are formed. In some cases M=Me, i.e., when the solution of
the same divalent metal is used. Similarly when a metal oxide, MO is reacted with a
solution containing trivalent metal ions, Me3+ solution, an LDH is formed. Conversion
of an oxide into a layered double hydroxide was first described by Boehm et.al [13].
Zinc chromium LDH [Zn2Cr(OH)6]NO3·2H2O was prepared by reacting ZnO with a
solution of Cr(NO)3·6H2O. LDHs can also be prepared from mixed metal oxides
formed from the thermal decomposition of another LDH, usually a carbonated LDH,
this method is known as the regeneration method.
Synthesis of layered solids can also be carried out in a homogeneous solution
by inducing precipitation using suitable precipitating agents which releases the base
slowly (provides the hydroxyl anions) upon heating, like urea and hexamine. LDHs
have been synthesized by Costantino et al. [14] by adding solid urea to 0.5 mol/dm3
metal chloride solutions, having molar fraction M(III)/[M(III)+M(II)] equal to 0.33,
until the molar ratio urea/[M(II)+M(III)] reached the value 3.3. The clear solutions
were heated, under stirring, at temperatures between 60 and 100°C.
HDSs can also be prepared from the hydrothermal hydrolysis of a mixed metal
salt solution [15]. Here the authors have subjected a mixture of metal acetates of Ni
and Zn to hydrothermal reaction to get HDSs with different orientations on the glass
substrate.
The above described methods are some of the generally used methods and do
not list all the available methods for the synthesis of layered solids. The methods
described above have their own advantages or disadvantages and a proper choice has
to be made according to the property and crystal quality of the anionic clay required.
1.5 Intercalation chemistry of anionic clays
Intercalation is defined as the reversible insertion of a mobile guest species
(atoms, molecules or ions) in the interlayer region or the gallery of the layered solid
[16]. The driving force of an intercalation process in the interlayer region is the
interaction of the guest species with the „active sites‟ or binding sites, which are
usually present on the surface of the layers. These sites may be crystallographic
position or groups bearing negative or positive charge or having a basic or acidic (or
in general, polar) character. In layered solids, guest molecules access intracrystalline
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binding sites via intercalation reactions. In most cases, intercalation reactions are
unselective processes which are driven by, oxidation-reduction, ion-exchange, acidbase, or donor-acceptor reactions [17].
Historically, intercalation chemistry began more than a century ago, when
Schauffautt [18] first reported, in 1841, the intercalation of sulphuric acid in graphite.
Intercalation gives an easy method to change the chemical, catalytic, optical,
electronic and magnetic properties of the layered solids [19]. There are various
methods to carry out the intercalation in anionic clays, but the choice of the method
depends upon the anion to be intercalated.
1.5.1 Intercalated anions in anionic clays
The intercalation chemistry of anionic clays is extensive and there seems no
significant restriction to the nature of anion that can be intercalated into the anionic
clay. A large variety of anions ranging from simple inorganic anions, coordination
complexes, organic anions and biomolecules have been incorporated into the anionic
clays. The intercalation chemistry of LDHs and HDSs has been studied extensively
and an overview of the anions intercalated into the anionic clays [4] is given below.
1. Common inorganic anions: halides (X-), CO32-, NO3-, OH-, SO42-, ClO4-, TcO4, ReO4-, MnO4-, Al(OH)4-, CrO42-, TcO4-, VO43-, CrO4-, Cr2O72-, MoO42-,
H2PO4-, PO33-, PO43-, P2O72-, silicate anions, borate, tetra borate, thiosulfate
and C60 anion.
2. Organic anions: carboxylates, dicarboxylates, benzene carboxylates,
alkylsulfates, alkanesulfonates, chlorocinnamates, glycolate, glycerolate,
organic dyes and drug molecules.
3. Polymeric anions: Polyvinylsulfonate, polystyrenesulfonate, polyacrylate,
polyaniline, ionized poly vinyl alcohol, polyethylene glycol, and polystyrene
oligomer anion.
4. Complex anions: CoCl42-, NiCl42-, IrCl62-, Fe(CN)64-, Fe(CN)63-, Mo(CN)83Cu(CN)64- and Co(CN)635. Macrocyclic ligands and their metal complexes: porphyrin and phthalocyanine
derivatives and their Cu2+, Co2+, Mn3+, Zn2+ complexes and cyclodextrins.
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6. Iso-and heteropolyoxometalates (POMs): Mo7O246-, W7O246-, H2W12O406-,
V10O286-, Keggin anion α-(XM12O40)n- (X = H, Si, P, Ge, etc; M = Mo, V, W,
etc).
7. Biochemically important anions: various amino acids, DNA, CMP, AMP,
GMP, ATP, ADP and related species, vitamins, enzymes and phospholipids.
LDHs find applications in various fields as catalysts, scavengers, biosensors,
polymer stabilizers to name a few. Intercalation of inorganic anions has been more
widely studied than that of organic anions in layered solids. Preparation of a
composite involving a layered solid usually requires monolayers of the layered solid
well dispersed in a solid i.e. a delaminated colloidal solution of the layered solid.
Delaminating a layered solid in a suitable solvent was initially achieved by
intercalating organic anions within the layered solid making the interlayer
organophilic, which would then enable the layered solid to be delaminated easily in an
organic solvent.
Intercalation of carboxylic acids into LDHs by various methods has been
reviewed by S. Carlino [20] and he presented five different methods which can be
employed for the intercalation of aliphatic and aromatic mono- and dicarboxylic acids
into LDHs and their calcined oxides (LDOs).
1. The direct ion exchange method: The LDH is shaken with a suitably concentrated
solution of the desired carboxylic acid or its salt in an aqueous medium or by
shaking the LDH with aliquots of the desired acid in alcoholic solution.
2. The coprecipitation method: In this method the (MII/MIII) mixed metal salt
solution is added to a base solution (or vice versa) containing the carboxylate
anion.
3. The rehydration method: In this method the LDH is calcined at a suitable
temperature for a given period of time to give rise to mixed metal oxide, which is
added to a solution containing the carboxylate anion of the carboxylic acid and
stirred.
4. The thermal or „melt‟ reaction method: The organic acid is intimately mixed with
either the LDH or the mixed metal oxide prior to heating at a ramp-rate not greater
than 10 oC min-1 up to a temperature which is approximately 10 oC above the
melting point of the particular mono- or dicarboxylic acid used. The mixture is
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then maintained at this temperature for 8 h in order to attain equilibrium and then
cooled to room temperature at a ramp-rate of 10 oC min-1.
5. Glycerol effected exchange: Glycerol mediated exchange can be carried out in
two different ways. Firstly, the LDH may be heated in glycerol (or glycol) vapors
in order pre-swell the interlayer region. This pre-swelled LDH is then reacted with
an appropriate solution of the carboxylic acid to be exchanged. In the second
method, the LDH or LDO is stirred with carboxylic acid or its salt present in a
glycerol (or glycol) solution.
1.5.2 Factors affecting intercalation in anionic clays
There are various factors which determine the ease with which an anion can be
intercalated into the anionic clay, some of which are briefly discussed here.
1.5.2.1 Affinity of incoming anion (Host-Guest and Guest-Guest interactions)
Generally, the exchange ability of the incoming anion increases with
increasing charge and decreasing ionic radius, this can be related to the equilibrium
constant for the incoming anion which increases with increasing charge density.
Based on the calculation of the equilibrium constants, the order for simple inorganic
anions was found to decrease in the order CO32- > C10H4N2O8S2- (NYS2-) > SO42- and
for divalent anions and OH- > F- > Cl- > Br- > NO3-> I- for monovalent anions [21] and
HPO42- > HAsO42- > CrO42- > SO42- > MoO42- for divalent anions [22].
Dermot O‟hare [23] and coworkers studied the intercalation of all the
geometrical isomers of the benzoate derivatives and found a general order of
preference 4-isomer > 3- isomer > 2-isomer. The LDH host showed remarkable
intercalation preference for those isomer anions in ethanol/water solution, for which
there was a large difference in dipole moment. They reasoned that the host selects
anions based on the ability of the anion to interact with the positively charged layers if
other conditions remain the same. Among all the interactions, the interaction between
carboxylate oxygen atoms and the layers, which includes both hydrogen bonding and
electrostatic interactions, was of primary importance; the interaction between dipole
moments and layers was of secondary importance. Furthermore, other interactions
such as hydrogen bonding from non-carboxyl-oxygen atoms, and interactions among
anions (eg. aromatic stacking effect) also play a role.
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1.5.2.2 Exchange medium
The interlayer space of LDHs can be expanded to some extent in a suitable
solvent medium, which favors the ion exchange process. An aqueous medium, for
example favors the exchange by inorganic anions, whilst an organic solvent favors
exchange by organic anions. The selective intercalation of 4-nitrophenolate (4-NP)
and 2,4-dinitrophenolate (2,4-DNP) in [LiAl2(OH)6]Cl·xH2O was studied in solvent
mixtures (50:50 V/V) of water and a miscible polar solvent such as THF, acetone,
ethanol, acetonitrile and DMSO. It was found that the percentage of intercalated 2,4DNP with respect to 4-NP at 80 oC increases proportional to the dielectric constant of
the solvent system [24]. The two NPs were equally intercalated in THF whereas in
water 70% of 2,4-DNP is selectively intercalated into Li-Al-Cl LDH. The solvation of
4-NP which has a higher dipole moment increases as the dielectric constant of the
solvent increases and thus it is more favorable for 4-NP to remain in the liquid phase.
Fogg et al [17] intercalated 1,5-naphthalenedisulphonate (1,5-NS) and 2,6naphthalenedisulphonate (2,6-NS) in [LiAl2(OH)6]Cl·H2O via direct ion-exchange
and studied their competitive intercalation, they found that there was a high
preference for 1,5-NS over 2,6-NS in the ratio 99:2, when the reaction was carried out
in water at 100 oC. They observed that the preference order was reversed when the
reaction was carried out in 50% water/acetonitrile mixture, with 60% of the 2,6isomer getting intercalated from an equimolar mixture of the isomers at 100 oC.
1.5.2.3 pH
The pH of intercalation is usually decided by the pH at which the desired
anion to be intercalated exists in a suitable form in solution. Rojas et al [25] carried
out intercalation of vandate species in layered NiZn hydroxyacetate at different pH
(4.5, 8.5 and 9.5) using NaVO3 (sodium metavanadate) at 60 oC, hydrothermally and
layered solid preswelled with caprylate anions as direct exchange at room temperature
was not possible. At a low pH (4.5), the intercalated species was α-VO3- chains while
at higher pH (9.5) V2O74- (vanadate) species was intercalated and at a pH of 8.5 both
the species (phases) were present. Aisawa et al [26] found that the amount of amino
acid, phenylalanine (Phe) intercalated into Zn-Al LDH at pH 7.0 increased gradually
with time and reached an equilibrium after 3 days, However, when the intercalation
was carried out at pH 10.5 equilibrium was quickly attained after 12 h, but the amount
of Phe intercalated was relatively lower.
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1.5.2.4 Chemical composition of the layer
The chemical composition of the layers (LDH sheets) influences the charge
density of the sheets and the hydration state, thereby affecting the ion exchange
process. Rives and co-workers [27] have observed that when the divalent metal to
trivalent metal ratio, MII:MIII, here Mg:Al ratio, was varied there was a change in the
charge density of the layers, leading to change in the orientation of the intercalated
nitrate anion from „flat‟ for higher Mg:Al ratio to „tilted‟ for lower Mg:Al ratio. They
observed that the samples with the „tilted‟ nitrate exchange for larger ions like borate
and silicate, while the „flat‟ oriented nitrate samples did not undergo anion exchange
with borate and silicate.
1.5.2.5 Temperature
It is usually accepted that higher temperatures favors a better ion exchange.
Fogg et al [17] intercalated 1,5-naphthalenedisulphonate (1,5-NS) and 2,6naphthalenedisulphonate (2,6-NS) in [LiAl2(OH)6]Cl·H2O via direct ion-exchange
and studied their competitive intercalation, they found that there was a high
preference for 1,5-NS over 2,6-NS in the ratio 99:2, when the reaction was carried out
at 100 oC with an equimolar solution of the two isomers. However, the preference
changes dramatically with decreasing temperature in favor of 2,6-NS, and at 20 oC the
ratio of the amounts of 1,5-NS versus 2,6-NS was found to be 27:73, they observed
that crossover of preference in water occurs at about 45 oC.
Newman et al. [28] observed that it was not possible to intercalate acetate,
benzoate or terephthalate ions into La(OH)2NO3·H2O at room temperature, but anion
exchange reactions were successful, when these reactions were carried out at 65 oC
for 1 week.
1.5.2.6 Reaction time
Intercalation reactions are usually very fast and occur within a few minutes but
in some cases there needs to be sufficient time given before the reaction can be seen
to occur. When Cu2(OH)3(OCOCH3)·H2O was investigated for its selectivity for
anions, it was observed that only one kind of anion was selectively exchanged from a
mixture of equimolar solution of various anions [29]. The products which selectively
exchanged for nitrate and perchlorate could be exchanged reversibly for the acetate
ions, while the chloride and bromide ions could not be re-exchanged with acetate after
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one day of exchange time; the same exchange reaction when carried out for four days
with replacement of the supernatant after every 24 h resulted in an acetate exchanged
sample. The authors observed that anion-exchange process for acetate (CH3COO-)
intercalated Ni/Zn HDS to nitrate (NO3-) intercalated Ni/Zn HDS took about 50 min,
while for the reverse anion exchange it took only 10 min.
It is an important characteristic of thin films that the anion exchange takes
place more rapidly in them than in the case of the same sample exchanged in the
powdered form [30]. Also if the anion to be exchanged is strongly held within the
interlayer then its exchange with another anion would proceed slowly as in the case of
exchange of dodecylsulphate with acetate anion which required 8 days for complete
exchange, with the supernatant being replaced daily by a fresh acetate anion solution
[31].
Newman et al. [28] studied the intercalation of terephthalate, benzoate and
acetate ions into various layered solids. In the case of Zn5(OH)8(NO3)2·2H2O,
terephthalate ion required only 24 h to get intercalated, while for the complete
intercalation of benzoate ion it required 72 h. They observed that it was not possible
to intercalate acetate under the experimental conditions used. In Cu2(OH)3NO3
intercalation of acetate and terephthalate occurred within 24 h, it required much
longer duration [28, 32] for the benzoate to get intercalated. In La(OH)3NO3·H2O
intercalation of all the carboxylate anions were observed when the reaction was
carried out for 1 week at 65 oC only, any shorter time duration lead to incomplete
intercalation.
1.5.2.7 Type of anion already present in the interlayer
Some anions are more strongly held in the interlayer then others. It is difficult
to replace such strongly held anions by other anions through simple exchange.
Carbonate is held tenaciously in the interlayer of anionic clays due to its high charge
density and its ability to form H-bonding with the hydroxyl groups. It is impossible to
carry out carbonate LDHs by the usual anion exchange procedures. Preferential
exchange of organic and inorganic ions with organic/inorganic ions was observed by
Feng et al [33] who showed selective intercalation of adipate and fluoride into
second-staged LDHs. They synthesized Zn2Cr LDH with second-staging for
succinate/Cl and tartrate/Cl, which were then reacted with adipate solution. First the
16
organic (hydrophobic) part was replaced by adipate to give adipate/Cl- second stage
part. As the reaction progressed Cl- was replaced by adipate, giving rise to adipate
intercalated Zn2Cr LDH. Similarly when these second-staged Zn2Cr LDHs were
reacted with fluoride, the Cl- inorganic (hydrophilic) part was first exchanged giving
rise to Zn2Cr succinate/F or Zn2Cr tartrate/F.
1.5.2.8 Concentration of the anion
The anion exchange reaction is an activity driven process and so higher the
concentration of the incoming anion better is the anion exchange. For example,
Suzuki et al [34] found that the degree of intercalation of Co(CN)63- ion in MgAl-Cl
LDH increases as the milliequivalents (meq) of Co(CN)63-was increased. 74% of the
interlayer chloride was exchanged when 3.25 meq of Co(CN)63- was added to the
solution, which is equal to the amount of interlayer chloride, while 83% of the
interlayer chloride was exchanged when 19.5 meq of Co(CN)63- was added. The
degree of anion exchange varied from 79% to 90% in MgAl-NO3, MgAl-Cl, MgAl
SO4, MgAl-CrO4 and ZnAl-NO3 LDHs, but the exchange was only 21% in the case of
ZnCr-NO3 LDH. Hussein et al. [35], found that it was not possible to intercalate acid
fuchsin, an organic dye into Mg-Al-LDH if the concentration of acid fuchsin was <
6.25 mM.
1.5.3 Selective Intercalation
During intercalation the interlayer gallery expands to accommodate the guest
species resulting in very little size- or shape-selectivity to the reaction. However there
have been reports of layered double hydroxide (LDH) that exhibit shape-selective
intercalation, which to some extent can be controlled by altering the conditions in
which the intercalation is carried out like, temperature and solvent of the intercalation
reaction. Selective or preferential intercalation studies have been carried out with an
interest of using the layered solids in separation process.
Tagaya
et
al
[36]
studied
the
competitive
intercalation
of
naphthalenecarboxylates in Mg-Al-CO3 and Zn-Al-CO3 LDHs via calcinationrehydration method. When an equimolar mixture of 2-naphthoicacid (2-NA), 2,6naphthalenedicarboxylicacid (2,6-NDCA) and 1,4,5,8-naphthalenetetracarboxylicacid
(1,4,5,8-NTCA) was reacted with calcined Mg-Al-LDH, they found that the
percentages of the naphthalenecarboxylates in the product was 87.1% 1,4,5,8-NTCA,
17
12.3% of 2,6-NDCA and 0.1% of 2-NA. They also found that 2-NA is preferentially
intercalated when compared to 1-naphthoic acid (1-NA). The ratio of 2-NA/1-NA was
28.1/13.8 after 1 hour of reaction time. Similarly, when 2,6-NDCA and 1,8naphthalenedicarboxylic acid (1,8-NDCA) were reacted with calcined Mg-Al-LDH,
14.2% of 2,6-NDCA and 53.3% of 1,8-NDCA were found to be intercalated after 15
min of reaction time. When 2,6-NDCA and 2,7-naphthalenedicarboxylic acid (2,7NDCA) were reacted with calcined Mg-Al-LDH, 30.3% of 2,6-NDCA and 4.4% of
2,7-NDCA was found to be intercalated after 15 min of reaction time, the preferential
intercalation of the same isomer in Zn-Al-LDH was much more pronounced with
more than 90% of 2,6-NDCA and 22% of 2,7-NDCA was found to be intercalated
into the layers. Tagaya et al. [37] have also studied preferential intercalation between
1-naphthoic acid and 2-naphthoic acid in hydroxy double salts (HDS) of zinc and
copper, and found that 2-naphthoic acid intercalates preferentially into the zinc and
copper HDSs. They also observed that 2-naphoic acid is preferentially intercalated
even in the presence of 2,7-naphthalene dicarboxylic acid indicating that charge
densities of the carboxylic acids were not so important for incorporation of anion into
the HDSs.
Williams et al [38] observed that LDHs with a general formula
[MAl4(OH)12](NO3)2·yH2O
(M = Zn, Cu, Ni and Co) showed preferential
intercalation towards various isomeric pairs. During the competitive intercalation of
1-naphthalenesulfonate (1-NS) and 2-naphthalenesulfonate (2-NS) the latter was
preferentially intercalated with a selectivity >85% into the LDHs. In the case of 1,5naphthalenedisulphonate (1,5-NDS) and 2,6-naphthalenedisulphonate (2,6-NDS), the
selectivity was found to be much lower varying between 45% to 90% depending upon
the intercalation conditions. While during the competitive intercalation of 1,2benzenedicarboxylate (1,2-BDC) and 1,4-benzenedicarboxylate (1,4-BDC), 1,4-BDC
is preferentially intercalated (> 90%) in all the LDHs.
O‟hare
et
al
[39]
studied
the
competitive
intercalation
benzenedicarboxylate (1,2-BDC), 1,3-benzenedicarboxylate (1,3-BDC)
of
1,2-
and 1,4-
benzenedicarboxylate (1,4-BDC) in [LiAl2(OH)6]Cl·H2O. They found that they could
selectively intercalate 1,4-BDC from a mixture of the three isomer (> 95%). While
during the competitive intercalation of 1,5- and 2,6-napthalenedicarboxylates and they
found that 2,6-napthalenedicarboxylate was intercalated with over 99% selectivity
18
from the mixture of the two isomers in water at 100 oC. The same group has also
carried out selective ion exchange of 1,4- and 1,2-benzenedicarboxylate anions.
Millange et al [40] found that 1,4-BDC gets preferentially intercalated (>95%) in
Ca2Al(OH)6.NO3·2H2O layered double hydroxide even though both 1,4-BDC and 1,2BDC anions were observed to be intercalated initially, followed by an exchange of the
1,2-isomer for the 1,4-isomer from the solution. Cardoso et al [41] carried out
competitive intercalation studies between terephthalic acid (TA), 4-methyl benzoate
(mBA) and benzoic acid (BA) by regeneration/ memory effect of MgAl-CO3 LDH by
decomposing it at 773K for 4 h under an air flow rate of 150 cm3 per min. They found
that the preference of intercalation was in the order TA >> mBA > BA, which was
related to the capacity of each anion to stabilize the LDH layer. Further selectivity in
the isomeric benzoate ions was investigated by O‟hare and coworkers [42]. They
studied the intercalation of all the geometrical isomers of the benzoate derivatives,
XC6H4COO- (X=F, Cl, Br, OH, OCH3, NO2, COOCH3, NH2, N(CH3)2) into
[LiAl2(OH)6Cl]·H2O in 50% (v/v) water/ethanol solutions at 80 oC. Competitive
intercalation reactions were studied using any two or all three of the substituted
benzoic acids. They found that in all cases (except amino benzoate isomers) a general
preference of 4-isomer > 3-isomer > 2-isomer was observed. Also the 4-isomer
always produced the purest intercalation compound, while the 2-isomers produce the
most impure compounds with the largest amount of Al(OH)3 impurity.
Fogg et al [43] carried out selective intercalation of fumerate ions into Li/AlCl LDH from an equimolar mixture of disodium salts of maleate and fumerate.
Similarly they found that when a mixture of disodium salts of 1,2-, 1,3- and 1,4benzenedicarboxylicacid was stirred with Li/Al-Cl LDH, 1,4-benzenedicarboxylate
was preferentially intercalated into the LDH. Rhee et al [44] studied the intercalation
of (E, E)- and (Z, Z)-mucontes [2,4-hexadienedioates] into Li/Al-Cl LDH, followed
by the competitive intercalation of the muconates at varying mole fractions. They
found that at mole fractions of x(E,E) greater than 0.6 the (E, E) isomer was selectively
intercalated into the LDH, while at lower x(E,E) mole fractions both the isomers were
intercalated. Xu et al [45] carried out the intercalation of n-alkyl sulphonates from C-6
to C-8, during competitive intercalation of these sulphonates they observed that the
longer-chain sulphonate could preferentially or even exclusively be intercalated in
Mg2Al(OH)6Cl·H2O. This preferential intercalation of longer-chain sulphonate with
19
respect to a shorter-chain sulphonate was attributed to an increase in hydrophobic
interactions between the hydrocarbon chains.
Kuk et al [46] studied the selective intercalation of anthraquinone-1,5disulfonate (AQ15), anthraquinone-2,6-disulfonate (AQ26), and anthraquinone-2sulfonate (AQ2) in Zn2Al-LDH by co-precipitation at a pH of 7.50.2. They found
that the intercalation of the isomeric anthraquinone sulfonates in Zn/Al LDH was
found to be in the order AQ2 > AQ26 > AQ15. They found that during the synthesis
of Zn/Al LDH from a solution containing an equimolar mixture of all the three
isomeric anthraquinone sulfonates, 13.2% of AQ26 and 86.8% of AQ2 were
intercalated while AQ15 did not intercalate in the Zn/Al LDH. 88.5% of AQ26 and
11.5% of AQ15 were intercalated into the Zn/Al LDH when the synthesis was carried
out using an equimolar mixture of AQ15 and AQ26.
O‟Hare and co-workers [47] have studied the intercalation of all the six
isomers of pyridinedicarboxylate (2,3-PDA; 2,4-PDA; 2,5-PDA; 2,6-PDA; 3,4-PDA;
3,5-PDA) in [LiAl2(OH)6]Cl·H2O at 100 oC in water. Following an extensive series of
competitive intercalation reactions involving two component through to six
component mixtures they were able to determine the preference order for intercalation
of all six PDA guests as 2,5-PDA > 2,3-PDA > 2,4-PDA > 2,6-PDA >3,5-PDA > 3,4PDA in [LiAl2(OH)6]Cl·H2O. The same group [48] has studied the preferential
intercalation of the isomers of pyridinecarboxylate (PA) and toluate (TA) in
[LiAl2(OH)6]Cl·H2O.
They
found
that
the
preference
order
for
the
pyridinecarboxylate was 4-PA > 2-PA > 3-PA, while for the toluates it was 4-TA > 3TA > 2-TA. The same group has also studied the preferential intercalation of the
niotrophenolates in Li-Al-Cl LDH. Ragavan et al [49] carried out the intercalation of
4-nitrophenolate (4-NP) and 2,4-dinitrophenolate (2,4-DNP) in [LiAl2(OH)6]Cl·xH2O.
They were unable to intercalate (2-NP) and (3-NP) under any conditions and therefore
they achieved a separation of 4-NP from a mixture of 2-NP, 3-NP and 4-NP. They
studied competitive intercalation of 4-NP and 2,4-DNP too. It was found by time
resolved in situ energy dispersive X-ray powder diffraction (EDXRD) studies that the
intercalation of 4-NP is kinetically faster than that of 2,4-DNP. However when using
equimolar binary mixture of 2,4-DNP and 4-NP, it was found the 2,4-DNP
competitively intercalates over 4-NP into [Li-Al-Cl]. Ragavan et al [50] carried out
the intercalation of 4-chlorophenoxyacetate (4-CPA), 2,4-dichlorophenoxyacetic (2,4-
20
D) and 2,4,5-trichlorophenoxyacetate, (2,4,5-T) into [LiAl2(OH)6]Cl·xH2O ([Li–Al–
Cl] LDH) by ion-exchange. The kinetic preferential order of the intercalation of these
herbicides into [Li–Al–Cl] LDH was found to be 4-CPA > 2,4-D > 2,4,5-T, which
correlates to the size of the anions. In contrast, the thermodynamic guest preference,
as determined by performing competitive intercalation reactions using equimolar
mixtures of the herbicides was found to be in the order 2,4-D > 4-CPA > 2,4,5-T.
1.6 Interlayer reactions
The interlayer region has been used as a molecular container for storing or
transporting unstable chiral biomolecules or pharmaceutical agents and also to
enhance their stability or for a slow release of the anion from the interlayer. Wei et al
[51] intercalated L-tyrosine into various LDHs and found that intercalation could
inhibit racemization of L-tyrosine under the influence of sunlight, high temperature or
ultraviolet light. An organic dye like acid fuchsin has been intercalated into Mg-Al
LDH forming an organic–inorganic hybrid nanocomposite, which can be used for
coating or controlled release of the dye for slow dyeing process [35].
The interlayer region has also been visualized as a molecular reactor in which
controlled reactions can be carried out to get specific products. Reactivity of
carboxylate intercalated LDHs, towards alkyl and benzyl halides in dry media
conditions, i.e. without solvents, was studied by Ruiz-Hitzky and co-workers [52, 53].
O-alkylation of the intercalated carboxylate ion (acetate or benzoate) was carried out
with organic bromides (1-bromooctane or benzyl bromide) by conventional thermal
treatment (100 oC) or by microwave (MW) irradiation giving the corresponding esters
with small amounts of other byproducts and bromide intercalated Zn/Cr-LDH, with
yields higher than 70%. Similarly, they have also carried out the O-alkylation reaction
of benzoate intercalated ZnCr, ZnAl, MgAl, CuCr, and CoAl-LDHs and sodium
benzoate supported on an LDH matrix under MW irradiation and classic thermal
treatment. In either case, the formation of the expected ester was always observed
with yields higher than 60%. They found that the use of intercalated LDH-benzoate,
where the anions were located within the layered structure of the inorganic host
lattice, improved the reaction yield. After the reaction, the compounds collected were
found to be bromide intercalated LDHs with adsorbed ester. The LDH could be
reused without any significant loss of activity.
21
Wei et al [54] carried out intercalation of two amino acids (L-cysteine and Lcystine) into Mg2Al-LDH. The amino acid L-cysteine intercalated into the LDH layer
was then oxidized using H2O2 (3 wt. %) and Br2 ([Br2]/[L-CysH] = 0.5 and 3). It was
found that when H2O2 is used as the oxidant, L-cysteine got oxidized to L-cystine in
the interlayer, same as the reaction occurring in an aqueous solution, but the interlayer
reaction leads to a decrease in the anionic species and the excess positive charge was
compensated by the incorporation of carbonate anions from the atmospheric carbon
dioxide which have a high affinity for the LDH layers. When bromine was used as the
oxidant, L-cysteine got oxidized to cysteic acid in the interlayer region, while in an
aqueous solution, the amount of bromine had a remarkable influence on the oxidation
product of L- cysteine, and cystine was obtained when a lower Bromine content was
used while higher bromine content gave cysteic acid as the product.
Whilton et al [55] intercalated aspartate in MgAl-LDH by co-precipitation
method and studied the in situ polymerization of aspartate in the aspartate intercalated
MgAl-LDH by heating to 200 oC followed by base hydrolysis giving rise to LDH
material containing poly (α,β-aspartate) and unreacted polyimide intermediates.
Preparation of manganese oxides in the interlayer region by suitable reactions
was carried out by Villegas et al [56] in which permanganate intercalated Mg-Al LDH
was reduced using glucose, ethanol and ascorbic acid. The product LDHs had reduced
concentration of Mn species. The product LDH had a mixture of manganese oxides
intercalated in them.
1.7 Composites based on anionic clays
Composites generally have been prepared to improve or enhance certain
properties which are of importance based on applications. For example, like thermal
stability and flame retardency of polymers can be increased by introducing fillers.
Layered solids can act as fillers in polymer matrix, in which they act as flame
retardants. For safety reasons it is important that polymers are able to tolerate high
temperatures. During the combustion/ exposure to flame anionic clays present within
the polymer matrix, decomposes resulting in release of water molecules, which helps
in retarding the flame and the polymer burns only partially, and the oxides produced
during the process can now act as thermal insulators.
22
Composite of layered solids can be prepared by adding mechanically a
relatively small amount (1–10%) of the anionic clay into the matrix of a polymer. The
clay is not dispersed uniformly during this method of preparation. A better method of
incorporating layered solids into the polymer matrix is to synthesize the composite by
exfoliation/delamination. In this process the anionic clay is made organophilic by the
intercalation of organic anionic followed by dispersion in a suitable medium
containing the polymer dispersed in, and the solvent is then removed to get the
composite in which ideally single layers are dispersed uniformly within the polymer
matrix. The materials so obtained may work as better flame retardants in polymers
without degrading or can even improve its other properties like mechanical strength.
A review of the polmer clay nanocomposites and their varied application has been
discussed by Paul et al [57].
Band gap of a quantum dot can be varied by altering the particle size of the
quantum dot or by changing the surface of the nanoparticle or by altering the
interaction of the particle with the surrounding medium. Venugopal et al [58] have
shown that optical property like, UV-Vis absorption maximum of CdSe nanoparticles
of uniform size distribution could be varied up to 45 nm by altering the interaction of
CdSe particles with the LDH matrix in CdSe-LDH composite by merely altering the
composition of the nanoparticle in the composite. LDHs containing drugs can act as
better substitutes than other drug formulation if they offer better properties like
stabilizing the drug, increasing its effectiveness by allowing targeting the drug to
specific cells, a controlled release of the drug within the vicinity of target cells rather
than the entire body or any other suitable advantage. Carja et al have synthesized
anionic clay composite with magnetic oxide/spinel intercalated with a drug, aspirin
showing good magnetic behavior enabling it for targeted delivery [59]. They have
also prepared LDH oxide nanoparticle composite for its use in catalysis [60].
Nanoparticles of iron oxide or vanadium oxide supported on the matrices of iron
substituted anionic clay was synthesized by regenerating the calcined iron LDH in
presence of FeSO4 or VOSO4 followed by ageing at 338K. These samples were then
calcined to obtain nanoparticles of iron or vanadium oxides on the iron LDH matrix,
which were used as catalysts for ethylbenzene dehydrogenation to styrene. Supported
vanadium oxides showed better catalytic performance for the above conversion in
presence of CO2.
23
Anionic clays like α-Ni hydroxide and α-Co hydroxide find application as
battery materials due to their α-phase. These anionic clays have a drawback that upon
ageing in alkaline medium (α-Ni hydroxide) one of the anionic clay leads to the
formation of its β phase while in (α -Co hydroxide) the other there is formation of
oxide both of which are undesirable in battery application. Nethravathi et al [61] have
prepared a randomly stacked composite of these two anionic clays, in which both the
anionic clays showed greater stability in the alkaline medium and also high discharge
capacity for alkaline battery application.
Venugopal et al [62] have prepared a composite of two different anionic clays
(MgAl and CoAl) via delamination restacking process. Here the thermal stability and
reconstruction behavior of CoAl-LDH is increased substantially in the composite.
Composite of anionic clay with graphite oxide was prepared by delamination
restacking process and the composite thus obtained when decomposed gave rise to
metal/metal oxide intercalated graphite [63]. When these samples were leached with
alkali selectively graphene-metal or graphene-metal oxide composites could be
obtained.
1.8 Scope of the thesis
This thesis deals with the intercalation of organic anions in anionic clays like
layered double hydroxides and hydroxy double salts. During the synthesis of anionic
clays the reactions conditions should be carefully controlled so as to avoid
contamination due to carbonate from the atmospheric carbon dioxide as carbonate
anion is very strongly held within the interlayer. We have carried out deintercalation
of carbonate with a simultaneous intercalation of anions which can then be easily
exchanged with anions of our interest. Competitive intercalation of organic anions
was widely studied but most of preferential intercalations were restricted to LiAl LDH
or CaAl LDH, so we ventured out to study if preferential intercalation with other
anionic clays was possible. Competitive intercalation study of nitrophenols had been
studied in LiAl LDH, so we wanted to see if there would be any difference in the
order of preferential intercalation of nitrophenols in a different anionic clay. We were
able to intercalate the three isomeric nitrophenols into MgAl-LDH and the
preferential intercalation studies carried out. We were able to achieve preferential
24
intercalation in MgAl-LDH, which can be used to separate the three isomeric
nitrophenols from a given mixture.
Reaction within the interlayer of a layered solid was gaining significance with
the recognition of the unique ability of the interlayer region to enhance the stability of
certain molecules or controlled release of the interlayer anion under suitable
conditions. The interlayer could also be visualized as a molecular container in which
suitable interlayer reaction could be carried out with a possibility of a difference in the
reaction mechanism / product formed due to the restrictions imposed by the interlayer
region. Therefore, we wanted to study the effect of the interlayer region, if any, on a
simple addition reaction. We were successful in anchoring isomeric geometrical
unsaturated dicarboxylates into the anionic clay through anion exchange reaction. The
anchored unsaturated dicarboxylate was subjected to a simple addition reaction like
bromination and the brominated product formed was then extracted from the anionic
clay by suitable anion exchange and compared with the product formed when a
similar reaction is carried out in absence of the anionic clay.
Composites of anionic clays and other materials like polymers, nanoparticles,
other layered solids etc. have been made and these find applications in various fields
like catalysis, solar cells etc. Composites of manganese oxide had been prepared with
potential applications in optical and electrochemical devices. A composite of anionic
clay and manganese oxide was prepared by dip-coating a suitable substrate with
colloidal solution of anionic clay and manganese oxide alternatively. A similar
composite was also prepared by reacting permanganate intercalated anionic clay with
various reducing agents, but unlike the composite obtained by LBL adsorption
technique, this composite had very little amount of manganese oxide component. We
intercalated a suitable anion into an anionic clay matrix which could undergo a redox
reaction and this could then be used to prepare a composite of manganese oxide and
the anionic clay wherein the composition could be controlled by the extent of the
redox reaction carried out thus providing another possible alternative route to prepare
composites via interlayer reactions.
Anionic clays also find application in the field of environmental amelioration
as sorbents to toxic organic and inorganic effluents. Sorption of inorganic metal ions
had been carried out by intercalating suitable organic complexing agents with anionic
clays, but after sorption these solids find no application and thus add to the problem of
25
disposing these solids suitably. We intercalated a suitable anion into the anionic clay
which could scavenge metal ions into the anionic clay and these metal ions could then
be reacted within the interlayer to generate suitable composites which can find
application like, in the field of catalysis, sensors etc.
1.9 Plan of the thesis
The thesis is divided into five chapters. The current chapter gives a brief
introduction to layered solids, classification of layered solids and in brief the
structure, preparation, properties and applications of anionic clays. Literature reports
on intercalation, factors affecting intercalation, intercalated anions, selective
intercalation of organic anions, interlayer reactions in anionic clays and composites
based on anionic clays are discussed in brief. The chapter concludes with the scope
and division of chapters of the present thesis.
Chapter 2 deals with reactions involving intercalation of an anion by suitable
intercalation reaction. This chapter is divided into two sections. In section A, we have
discussed decarbonative intercalation of halides, nitrate and sulfate into carbonated
LDHs by avoiding a direct use of acid for decarbonation. We have shown the
deintercalation of carbonate from MgAl and NiAl LDH with simultaneous
intercalation of monovalent anions like nitrate, chloride, bromide and iodide and a
divalent sulfate anion showing the versatility of the method. The method was found to
be superior compared to the existing methods since larger anions such as bromide and
iodide could also be intercalated completely. The solids thus obtained also showed
better order than solids got by other methods of preparation. In section B, we present
our study on intercalation of the three isomeric nitrophenolate (NP) ions in Mg-Al
LDH. While 2-NP and 4-NP could be intercalated in the LDH 3-NP could not be
intercalated. From a mixture of these isomeric anions 4-NP was preferentially
intercalated in Mg-Al LDH. The preference for intercalation of the three isomeric
nitrophenolates is in the order 4-NP > 2-NP >> 3-NP. While the fact that nitrophenols
can be intercalated can be made use of in the removal of these pollutants from water
bodies, the preferential intercalation offers a method to separate the three isomers
from their mixture.
Chapter 3 deals with intracrystalline reactions in anionic clays by intercalation
of a suitable organic anion which can then be subjected to an intracrystalline reaction
leading to the formation of suitable organic product or a suitable composite. This
26
chapter is divided into two sections. Section A discusses anionic clay-like nickel zinc
hydroxyacetate, Ni3Zn2(OH)8(OAc)2·2H2O exchanged with maleate and fumarate
ions. While the maleate enters as monoanion, fumarate enters as dianion. Also these
anions take up different orientations in the interlayer region. The intercalated organic
species could be reacted with bromine water in such a way that the brominated
product remains intercalated making the reaction a true intracrystalline reaction. The
stereochemistry of the reaction of the intercalated fumarate was identical to that of the
free fumarate ion – both yielding only anti addition product. While free maleate ion
yielded only anti addition product, the intercalated maleate ion yielded both syn and
anti addition products, the former forming only in minor quantities. The organic
products could be quantitatively recovered by anion exchange with oxalate ions.
Thus, anionic clays could be used as an anchor for an organic substrate in a reaction.
Section B deals with the reaction of oxalate intercalated anionic clay-like nickel zinc
with potassium permanganate in such a way that the layered manganese oxide formed
was within the interlayer region of the anionic clay resulting in a layered composite in
which the negative charges on the birnessite type manganese oxide layers compensate
the positive charges on the anionic clay layers. Birnessite to anionic clay ratio could
be varied by varying the reaction time or the amount of potassium permanganate used.
Thus, composites made of alternating negatively charged manganese oxide and
positively charged Ni-Zn hydroxide layers could be synthesized via an interlayer
reaction in the gallery of Ni-Zn hydroxyoxalate. The advantage of this reaction is that
the birnessite layers formed coexist with the anionic clay layers for charge
neutralization leading to good mixing of the two components of the composite.
Another feature of this method is that the birnessite to anionic clay ratio can be
controlled by altering either the amount of KMnO4 used or the reaction time.
In chapter 4, uptake of copper ions from an aqueous solution into the
interlayer of anionic clays followed by its immobilization in the form of either copper
(I) oxide or copper metal within the interlayer of the anionic clay by suitable
interlayer reaction are described. Citrate intercalated anionic clays when treated with
Cu2+ solution absorbed the Cu2+ ions by complexation with the intercalated anions.
When
the
Cu2+–complex
intercalated
anionic
clays
were
treated
with
glucose/hydrazine the complex got reduced to Cu2O/Cu respectively. Thus, anionic
clay-metal/metal oxide composites have been prepared from copper-citrate complex
intercalated LDH by interlayer reaction. This procedure is a general method and can
27
be suitably modified for other metal ions for its scavenging and conversion to useful
form which could find some useful application.
Chapter 5 lists out the major conclusions arrived at in this work. In addition,
we discuss the limitations and major failures of this work and highlight the scope for
future research in this area.
References
[1]
Inorganic Chemistry: Principles of structure and reactivity. James E. Huheey,
Ellen A. Keiter, Richard L. Keiter, Okhil K. Medhi , Second impression, 2007,
Pearson Education.
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