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Introduction

1.1 Hydrotalcite-Brief history

1.1.1 Structural features

1.1.2 Catalysis

1.1.2.1 Homogeneous catalysis

1.1.2.2 Heterogeneous catalysis

1.1.2.3 Solid base catalysts

1.1.2.4 Hydrotalcite in base catalysis

1.1.3 Ion-exchange

1.1.3.1 Hydrotalcite as anion exchangers

1.1.3.2 Anionic pollutants

1.1.3.3 Hydrotalcite in noxious anion removal

1.2 Synthesis of hydrotalcites

1.2.1 Co-precipitation under low supersaturation

1.2.2 Hydrolysis methods

1.2.3 Miscellaneous methods

1.3 Physicochemical characterization

1.3.1 Powder X-ray diffraction (PXRD)

1.3.2 Fourier transformed infrared (FT-IR) spectroscopy

1.3.3 Thermo gravimetric analysis (TGA)

1.3.4 ICP-OES Elemental analysis

1.3.5 BET adsorption measurements

1.3.6 Scanning electron microscopy (SEM)

1.3.7 Transmission electron microscopy (TEM)

1.3.8 UV-visible spectrophotometry

1.3.9 CHNS analysis

1.3.10 Temperature programmed desorption (TPD)

1.3.11 Temperature programmed reduction (TPR)

1.3.12 Nuclear magnetic resonance (NMR)

1.4 Scope and objectives of the work

1.5 References

Chapter 1 Introduction

Ph.D Thesis 1

1.1 Hydrotalcite-Brief history

In last three decades, hydrotalcite-like (HT-like) materials otherwise known as

layered double hydroxides (LDHs) have received diverse attention such as in catalysis

[1], adsorption[2], sensing [3], polymer blends [4], medicine [5], human health [6],

biological applications [7, 8], thin films for advanced materials [9, 10], nanoscale

research [11] and so forth. Hydrotalcites (HTs) belong to the large class of anionic

clays, and will be taken as a reference name for many other isomorphous and

polytype compounds. Hydrotalcite-like anionic clays as materials form a subset of

clay minerals. Clay minerals and clays constitute the world’s largest and mostly used

material with versatile features [12]. Clays find their potential application in ceramics,

building materials, adsorbents, ion-exchangers, sensors, decolorizing agents and

catalysis [13]. The two main features that evoke the interest on clays are their

common availability and their extraordinary properties [14]. Clay is defined as

materials with particle size less than 2 μm, although the Wentworth scale defines clay

as materials finer than 4 µm [15, 16]. The clays and clay minerals, either as such or

after modification, are recognized as the materials of the 21st century, because they

are abundant, inexpensive, and environment friendly [14]. Nowadays, clay minerals

are source for the preparation of nanostructured advanced materials including

catalysts, adsorbents, organoclays, pillared clays, intercalation compounds, polymer-

clay nanocomposites, agriculture, civil engineering, etc. [17]. The first and most

widely known application of clay in catalysis is French Houdry cracking process

developed in 1930 [18]. Clay minerals with its different and interesting set of

properties are very effective for wide range of organic reactions. The clay materials as

catalysts, exhibit excellent Bronsted and Lewis acid-base properties upon

pretreatment. The high surface area of clays also means that they can act as effective

supports for (usually inorganic) reagents bringing the benefits of heterogeneous

catalysis to several important reactions [19]. According to Lazlo the catalytic property

of clays is not only due to their surface areas and chemical nature. The main features

that contribute to good heterogeneous catalytic ability for clay minerals are local

concentration effects and low dimensionality (<4 µm). The former decides the

strength of adsorption of reactants on the surface and later gives more interaction for

reactant molecules within the surface, which increases the activity [20]. The

adsorptive power and high water retention capacity of clays are responsible for their

Chapter 1 Introduction

Ph.D Thesis 2

extensive applications. Clays and clay minerals can also be classified as layered

materials

Clays are classified into two categories:

1. Cationic or smectite type of clays having layered lattice structure in which

two dimensional oxy-anions are separated by layers of hydrated cations.

2. Anionic or brucite type of clays in which the charge on the layer and the

gallery anion is reversed, complementary to smectic-type clays [21].

Anionic clays, synthetic and natural layered mixed metal hydroxides

containing exchangeable anions, less well known and diffuse in nature than cationic

clays. Hydrotalcites is magnesium/aluminium hydroxylcarbonate layered material

which can be easily crushed into white powder was first discovered in Sweden in

1842 [22]. The stoichiometry of HT, [Mg6Al2(OH)2]CO3.4H2O, was first correctly

determined by Manasse in 1915, who was also the first to recognized that carbonate

ions were essential for its structure [23]. In 1930 Aminoff and Broome [24]

recognized the existence of two polytypes of HTs with rhombohedral and hexagonal

symmetry (proposed by Manasse) through X-ray investigation. In 1942, through the

synthesis of large number of compounds of HT-like structures, Feitknecht proposed

that the structure was assumed to be exist of consecutive layers of brucite [Mg(OH)2],

and aluminum hydroxide [Al(OH)3] and gave the name double sheet structures [25,

26]. In 1969, Allmann and Taylor showed that all the cations were located in the same

layer, and anions together with water molecules lies in the interlayer region through

single crystal X-ray diffraction (XRD) [27-29]. The first patent on HT as catalyst

came in 1970 as an optimal precursor for the preparation of hydrogenation catalysts

[30]. Nearly 40 years later, still many of the fine details of the structure such as the

range of possible compositions and stoichiometry, the extent of ordering of metal

cations within the layers, the stacking arrangements of the layers, the arrangement of

the anions and water molecules in the interlayer galleries are not fully understood. A

literature search of layered materials, over the period of 2000-2006 (limiting to

English as language), revealed nearly 20,000 papers which includes layered

perovskites (LP), pillared clays (PILC) and LDH materials, of which about 85% were

dealing on catalysis . It is also evidenced from the literature that the similar layered

hydroxides like layered perovskites (LP) and pillared clays (PILC) materials are still

mainly at the lab-scale development stage, while HTs find a broad range of

application [31]. This shows the importance of working on clays and clay based

Chapter 1 Introduction

Ph.D Thesis 3

compounds. Hydrotalcites constitute a class of extremely useful materials for

scientists especially in field like catalysis, and ion-exchange. ISI Web of knowledge

[v. 5.5] database search on these materials with the topic search keyword of

“Hydrotalcite* OR Layered double hydroxide*” showed number of articles published

on HTs and LDHs up to February 29th

-2012 has 10,608 records, (Figure 1.1).

Number of articles published up to 2000 was around 1150 and in the new century

literature says that LDHs emerged as one among the promising materials that have

diverse applications. In 2001, 246 articles got published and it increased to 783 in

2010 and increased further to 841 by Dec-2011; this shows the importance of working

on these materials [32].

1998 2000 2002 2004 2006 2008 2010 2012

0

100

200

300

400

500

600

700

800

900

1000

1100

Up

to

19

99

Art

icle

s a

nd

Pate

nts

Year

Figure 1.1 Number of articles and patents published in HTs and LDHs

1.1.1 Structural Properties

Hydrotalcite-like anionic clays are synthetic or natural crystalline materials

consisting of positively charged two-dimensional sheets with water and exchangeable

charge compensating anions in the interlayer region. The identities and ratio of the di-

and trivalent cations and the interlayer anion may be varied over a wide range, giving

rise to a large class of isostructural materials with varied physicochemical properties.

Their general formula is as;

[M(II)1- x M(III)x(OH)2]x+

[Ax/n] n-

]x-

.mH2O

where M(II) and M(III) represent divalent and trivalent metal ion respectively, A is

the interlayer anion with charge n, x represents M(II)/M(III) ratio and m, the water of

crystallization. The value of ‘x’ can be in the range 0.2-0.4. For values outside this

Chapter 1 Introduction

Ph.D Thesis 4

range, pure hydroxides or other compounds with different structure are obtained [33,

34]. The main criteria for elements to crystallize in this network are their ionic radius.

M(II) and M(III) having an ionic radius not too different from that of Mg2+

may be

accommodated in the octahedral sites of the close-packed configuration of the OH-

ions in the brucite-like layers to form HT-like compounds. The ionic radii of M(II)

cation between 0.65 to 0.80 Å and M(III) cation of 0.62 to 0.69 Å (with the main

exception of Al 0.53 Å) can form LDHs [33]. Higher ionic radii (Cd and Sc) seems to

be incompatible with the formation of true brucite-like layers. Recently Guo et al.

reported CdCr and ZnCdCr LDHs. The material showed magnetic properties and also

delaminated in formamide [35]. LDHs can also be obtained with a Li-Al monovalent-

trivalent, [36] Co-Ti divalent-tetravalent [37] and Zn-Mo associations [38]. The single

phase formation of hydrotalcite occurs, after proper selection of M(II) and M(III)

ions. The water molecules are localized in the interlayer sites, which are not occupied

by anions. The amount of water accommodated in the interlayer can be calculated on

the basis of number of sites present in the interlayer.

Structurally, hydrotalcites possess a brucite-like (Mg(OH)2) layered network

wherein a partial substitution of bivalent ion by trivalent ion, say Al3+

, occurs and the

resulting excess positive charge in the layers is compensated by anions located in the

interlayer [39, 40]. The affinity for the interlayer anions can be derived for both mono

and divalent anions (the divalent anions are more strongly held in the interlayer than

the monolayer anions, and carbonate is held most strongly) [41]. The schematic

representation of the hydrotalcite structure is shown in Figure 1.2. The affinity of

interlayer anions within the interlayer of hydrotalcite is:

CO32-

>> SO42-

>> OH- > F

- > Cl

- > Br

- > NO3

- > I

-

Hydrotalcites 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 the plane. The layers then

stack on top of one another to form the three dimensional structure. The metal cations

occupy the octahedral holes between alternative pairs of OH planes and thus occupy a

triangular lattice identical to that occupied by the OH- ions.

Chapter 1 Introduction

Ph.D Thesis 5

Figure 1.2 Schematic representations of hydrotalcites

The brucite-like sheets can stack on one over the other with two different

symmetries, rhombohedral or hexagonal. The brucite-like layers in LDHs may be

stacked in different ways, which gives raise to variety of possible polytype structures.

LDHs usually crystallize in two different polytypes, one with two-layer hexagonal

stacking sequence (polytype 2H) and another with a three-layer rhombohedral

sequence (polytype 3R). All sites in the (110) plane of the close packed hydroxide

layers may be represented as A, B or C related by lattice translations of (1/3, 2/3, 0) or

(2/3, 1/3, 0) and the locations of octahedral holes occupied by metal cations can be

described analogously as a, b or c. Thus single brucite layer can be represented as

AbC (since, if close packed hydroxyl groups occupy A and C sites, the cations must,

of necessity, occupy b sites). AbC layers may be stacked in various ways giving rise

to a large number of possible polytypes. These polytypes may be classified in terms of

the number of sheets stacked along the c axis of the unit cell. If the opposing OH

groups of adjacent layers lie vertically above one another (say both in C sites), a

trigonal prismatic arrangement (denoted by =) results; if the hydroxyls are offset (say

one layer in C sites and those of an adjacent layer in either A or B sites) then six OH

groups form an octahedral arrangement (denoted by ~). Thus brucite itself can be

denoted as …AbC~AbC~… or 1H, where “1” denotes a one layer polytype and the

“H” denotes a stacking sequence with hexagonal symmetry. Bookin and Dirts [42, 43]

have systematically derived all of the possible polytypes for other stacking sequences.

There are three possible two layer polytypes, each of which has hexagonal stacking of

layers, which can be denoted as,

Chapter 1 Introduction

Ph.D Thesis 6

….AbC=CbA=AbC…. 2H1

….AbC~AcB~AbC…. 2H2

….AbC~BcA=AbC…. 2H3

The interlayers in the 2H1 polytype are all prismatic and those in the 2H2 polytype are

all octahedral, whilst in the 2H3 polytype both types of interlayers are present. There

are nine possible three-layer polytypes. Two of these have rhombohedral symmetry

(3R):

….AbC=CaB=BcA=AbC…. 3R1

….AbC~BcA~CaB~AbC…. 3R2

the remaining seven have hexagonal symmetry:

….AbC~AcB~AcB~AbC…. 3H1

….AbC~AcB~CaB~AbC…. 3H2

….AbC~AcB=BcA~AbC…. 3H3

….AbC~AbC=CbA=AbC…. 3H4

….AbC~AcB=BaC~AbC…. 3H5

….AbC~AcB~CbA=AbC…. 3H6

….AbC~AbA~BcA=AbC…. 3H7

For the 3R1 polytype, the interlayers are all prismatic and in the case of 3R2, 3H1 and

3H2 they are all octahedral; other polytypes involve both types of interlayers. Bookin

and Drits have also described the large number of possible six-layer polytypes, some

of which have rhombohedral symmetry (6R) and the remaining hexagonal symmetry

(6H).

Likewise in the case of hydrotalcite, if some Mg2+

ions are replaced by a

higher valent ion having similar radius different anionic clay minerals can be

synthesized (such as Fe3+

in pyroaurite and Cr3+

in stichtite) [44]. Crystallographic

parameters of different known anionic clay minerals with similar stacking order of are

given in Table 1.1. The parameter ‘a’ gives the cation-cation distance, typical for

hydrotalcite-like materials [45, 46]. The value of ‘a’ parameter has no relation with

the interlayer anions and doesn’t change with anions. However for ‘c’ parameter,

which corresponds to the thickness of the layers, the value depends on the change and

size of interlayer anions. Variation of ‘c’ parameter with different interlayer anion is

given in Table 1.2 [41]. The thickness of the corresponding interlayer region for HT is

the difference between ‘c’ and the thickness of the brucite-layer (4.8 Å.) [12]. A wide

variety of anions like inorganic anions (CO32-

, SO42-

, NO3-, OH

-, CrO4

2-, WO4

2-,

Chapter 1 Introduction

Ph.D Thesis 7

S2O32-

etc.), isopoly anions (V10O282-

, Mo7O242-

etc.), heteropoly anions (PMo12O403-

,

PW12O403-

etc.), complex anions (Fe(CN)63-

, Fe(CN)64-

, IrCl62-

etc.) and organic

anions (carboxylates, porphyrins, pharmaceutically active functional anions and alkyl

sulfates etc.) can be intercalated in the LDH layers.

Table 1.1 Comparison of composition, crystallographic parameters, and symmetry for

different anionic clays [23, 45]

Mineral Chemical composition Unit cell

parameters

Symmetry

a (Å) c (Å)

Hydrotalcite Mg6Al2(OH)16CO3·4H2O 3.054 22.81 3R

Manasseite Mg6Al2(OH)16CO3·4H2O 3.10 15.6 2H

Pyroaurite Mg6Fe2(OH)16CO3·4H2O 3.109 23.41 3R

Sjogrenite Mg6Al2(OH)16CO3·4H2O 3.113 15.61 2H

Stichtite Mg6Cr2(OH)16CO3·4H2O 3.10 23.4 3R

Barbertonite Mg6Cr2(OH)16CO3·4H2O 3.10 15.6 2H

Takovite Ni6Al2(OH)16CO3·4H2O 3.025 22.59 3R

Reevesite Ni6Fe2(OH)16 CO3·4H2O 3.081 23.05 3R

Meixnerite Mg6Al2(OH)16(OH)2·4H2O 3.046 22.92 3R

Desautelsite Mg6Mn2(OH)16CO3·4H2O 3.114 23.39 3R

Table 1.2 Values of ‘c’ with different interlayer anions [23, 47]

Anion OH- CO3

2- F

- Cl

- Br

- I

- NO3

- SO4

2- ClO4

-

c(Å) 7.55 7.65 7.66 7.86 7.95 8.16 8.79 8.58 9.20

1.1.2 Catalysis

Catalysis is one of the most important fields in chemistry, which has very

good applications in industrial research [48]. Catalysis, derived from the Greek word

‘kata’ (cata) means down, and ‘lyein’ (lysis) means loosen. The Swedish chemist

Berzelius (1779-1848) first used the word katalysis meaning breaking down or

loosening in 1836 [49]. During that period in 1814, it was observed by other

researchers that in the presence of acid, the conversion of starch to sugar enhances

and in presence of finely powdered platinum, alcohol got oxidized to acetic acid [50].

Berzelius correlated these observations made by other chemists [51, 52] and

introduced the concept, catalysis. According to German chemist Ostwald who first

scientifically defined catalysis in 1894, catalyst is a substance which alters the rate of

Chapter 1 Introduction

Ph.D Thesis 8

approaching of chemical equilibrium without itself being changed or substantially

consumed in the process [53]. In a reaction the catalyst generally enters into chemical

combination with the reactants but ultimately regenerated so that the amount of

catalyst remains unchanged. The first application of catalysis was done in 1820s by

Dobereiner, who introduced “tinderbox” which was commercially used for the

purpose of lightning fires and smoking pipes. A jet of hydrogen produced by zinc and

sulphuric acid was directed on to the supported platinum where it catalytically

combined with oxygen to yield gentle flame (Million of tinder boxes sold in 1820).

Followed by this several important industrial applications on catalysis emerged such

as;

Industrial oxidation of HCl to Cl2 using clay brick impregnated with cupric

salt as catalyst (Deacon process 1871).

Karlsruhe Fritz Haber prepared copious quantities of ammonia from nitrogen

and hydrogen in presence of a reduced magnetite (Fe3O4) catalyst using high

pressure apparatus (1909).

Industrial synthetic production of methanol using zinc oxide–chromium oxide

catalyst at 400 oC and 200 bar pressure (1923).

Synthetic zeolites were first reported for the selective isomerization of

hydrocarbons in 1960.

Over a billion (109) kilograms of fructose were produced in USA for soft drink

from corn syrup using immobilized glucose isomerase as catalyst (1980).

Some of the path breaking highlights in catalysis reactions are Haber process

for the production of ammonia from gas phase nitrogen [54], Ziegler-Natta catalysts

for the polymerization of olefins [55], Ostwald process for the oxidation of ammonia

to nitric oxide for production of nitric acid [56] and the introduction of Fischer-

Tropsch process [57]. For a good catalyst it must have the following properties;

High and stable activity

High and stable selectivity

Controlled surface area and porosity

Good resistance to high temperature and pressure

High mechanical strength

No uncontrollable hazards

Chapter 1 Introduction

Ph.D Thesis 9

Commercially catalyst makes it possible for the reaction to proceed at rates

high enough to permit their commercial exploitation on a large scale, resulting in

economic benefits. It is also interesting to note that over 90% of industrial processes

involve catalysts in one form or another and the number is rising; interestingly most

of the metals available in nature are involved in catalytic systems in one way or other.

Catalysts are broadly classified into two namely homogeneous catalysts

(dissolve in the liquid reaction mixture) and heterogeneous catalysts (in the form of

insoluble solids).

1.1.2.1 Homogeneous catalysis

Homogeneous catalysis is wherein the reactant and catalyst are in the same

phase, which is usually the liquid phase. In general, homogeneous catalysts are more

active and more selective; it is due to the homogeneity of the active site. But it has

several demerits also. The separation of the catalyst is difficult and hence purification

becomes tough and energy intensive and expensive to separate catalysts from

products.

1.1.2.2 Heterogeneous catalysis

In heterogeneous catalysis, the catalyst and the reactant may be in different

phase. The catalyst will be in solid phase and the reactants may be in liquid or gas

phases. Therefore, heterogeneous catalysts are also broadly referred as solid catalysts.

Heterogeneous catalysts are widely used in refining, petrochemical industry and

pharmaceutical industries. Eventhough it is less active and selective than

homogeneous catalyst, it is economical because of easy separation of products and

reusable. The main advantages of solid catalysts over conventionally used

homogeneous catalysts are: environmental friendly, reusable, non-corrosive and easily

separable from product mixture and also possess higher activity, selectivity, and

longer catalyst life. Heterogeneous catalysts allow high regio and chemo selectivity

due to shape selectivity of porous solids, and poly functionality (acid, base, redox

etc.) allowing multi step reactions.

Heterogeneous catalytic processes can broadly be classified into redox

catalysts and acid-base catalysts [58]. In redox catalysis, the catalyst influence the

bond breaking of reactant molecules with the formation of unpaired electrons, and

further formation of new bonds with the participation of electrons [59]. Acid-base

Chapter 1 Introduction

Ph.D Thesis 10

catalysts either posses a tendency to donate a proton/to accept an electron pair, or

accept a proton/to donate an electron pair as per Bronsted (-Lowry) concept for acids

& bases. These definitions are adequate for an understanding of the acid-base

phenomena shown by various solids, and are convenient for a clear description of

solid acid and base catalysis. However, it should be noted that the same site could

serve as a Bronsted base as well as a Lewis base, depending on the nature of the

adsorbate in a reaction [60].

Solid acid and base catalysis are fast growing research area due to increasing

environmental awareness and concerns. Up to now, more than hundreds of solid acid

and base catalysts are reported in the literature; among them, most are concerned with

or pertain to solid acid catalysts. The reason for rapid growth in the area of solid acid

catalysis is due to great progress/developments in refining and petrochemical

industries in the last 50 years, for example in cracking process. The range of such

materials available include the acidic forms of ion-exchanged resins, zeolites and

mesoporous silicates, modified oxides such as zirconia, immobilized forms of Lewis

acids such as metal halides, Bronsted acids including phosphoric, triflurosulphonic

acids and heteropoly acids (HPAs). Fewer efforts have been made on heterogeneous

base-catalyzed reactions when compared to acid catalysis. Tanabe and Hoelderich

made a statistical survey till 1999, over different type of catalysts (Figure 1.3) in

industrial processes [61]. The total number of commercial processes related to solid

base (8%) and bi-functional catalysts (11%) is much less than that of solid acid

catalysts (81%).

Solid base catalysts

Solid acid-base bifuctional catalysts

Solid acid catalysts

81%

11% 8%

Figure 1.3 Industrial processes based on the heterogeneous catalysts [61]

Chapter 1 Introduction

Ph.D Thesis 11

Nevertheless, the application of heterogeneous base catalysts to the synthesis

of fine and specialty chemicals is receiving increased attention as it is a route to the

design of safer, cleaner, and more sustainable environment friendly industrial

processes. The commercial processes those used solid base catalysts are shown in

Table 1.3.

Table 1.3 Commercial processes using solid base catalyst [62]

Process Catalyst year

Alkylation of phenol with methanol MgO 1970

Iso-Butyraldehyde to iso-butylisobutyrate ZrO2 1974

Dehydration of 1-hexylethanol ZrO2 1986

Alkylation of cumene with ethylene Na/KOH/Al2O3 1988

Isomerization of safrole to iso-safrole Na/KOH/Al2O3 1988

Isomerization of 2,3-dimethyl-1-butene Na/KOH/Al2O3 1988

Isomerization of 3,5-vinylbicyclo[2.2.1]heptene Na/KOH/Al2O3 1988

Reduction of carboxylic acid to aldehyde ZrO2-Cr2O3 1988

Thiols from alcohols with hydrogen sulfide Alkali/ Al2O3 1988

Dehydration of propylamine-2-ol ZrO2-KOH 1992

Esterification of ethylene oxide with alcohol Hydrotalcite 1994

Cyclization of imine with sulfur dioxide Cs-zeolite 1995

Alkylation of o-xylene with butadiene Na/ K2CO3 1995

Isomerization of 1,2-propadiene to propyne K2O/ Al2O3 1996

Dehydrotrimerization of iso-butyraldehyde BaO-CaO 1998

1.1.2.3 Solid base catalysts

Base catalysts play a decisive role in a number of reactions essential for fine-

chemical synthesis as compared to acid catalysts which finds applications largely in

petroleum refining. Solid-base catalysts find application in reactions including

isomerization, aldol condensation, Knoevenagel condensation, Michael condensation,

oxidation, and Si-C bond formation [63]. Base catalyzed condensation is one of the

well known methods for C-C bond formation. The first study of solid base catalyst

includes sodium dispersed on alumina in 1950s for double bond isomerization of

alkenes [64], calcium oxide and magnesium oxide for 1-butene isomerization [60].

Later Zeolites that have been ion exchanged with alkali metal salts show weak

activity whose base strength can be increased by increase in alkali weight percentage

Chapter 1 Introduction

Ph.D Thesis 12

[65]. MgO was also used in series of base catalyzed reactions [66]. The study on the

developments of solid base catalysts were extended to single metal mixed oxide up to

1970s. Busca recently reviewed the basic properties of different solids which found

application in industrial catalysis [67] and one among them is LDHs. Different solid

base catalysts available in literature are given in Table 1.4.

Table 1.4 Types of solid base catalysts [61-63]

Typical catalyst Details of the catalyst

Single metal

oxide

MgO, CaO, SrO, BaO, Al2O3, La2O3, YbO2, ZrO2

Mixed oxides MgO-Al2O3, Al2O3-B2O3, ZrO2-MgO, ZrO2-NaOH, ZrO2-KOH,

SiO2-Al2O3, Al2O3-NaOH-Na, Al2O3-KOH-K, MgO-TiO2

Non-oxides KF/Al2O3, KNH2/Al2O3, Lanthanide imide, nitride on zeolite,

Si2N2O, AlPON, ZrPON, AlGaPON

Zeolites K, Rb and Cs–exchanged zeolite X, Y; nitriles impregnated on

zeolites

Supported alkali

metal ions

On silica, alumina, alkaline earth oxide

Mesoporous

materials

Functionalized by amino groups, MgO/SBA–15

Basic supported

catalyst

KF/ Al2O3, Na/NaOH/ Al2O3, Na/MgO, Na/ K2CO3

Clay and modified

clay

Hydrotalcite, Chrysotile, Sepiolite

Other Modified natural phosphate (NP), Calcined NaNO3/NP, Chitosan

1.1.2.4 Hydrotalcites in base catalysis

Compared to other basic materials like sepiolite, modified zeolites and organic

resins, LDH derived materials show improved thermal stability and diffusion

resistance [68]. Specific metals/anions can be incorporated in the octahedral layer and

thus impart catalysis activity [69, 70]. The anion-exchange strategy is especially

successful in the context of heterogenizing homogeneous catalysts [71]. These

materials on calcination (weight loss of up to 40%) leads to formation of highly

dispersed, large surface area and porous nano dimensional mixed metal oxides, which

can be potentially used for many base catalyzed reactions [72]. The hydrotalcite (HT)

Chapter 1 Introduction

Ph.D Thesis 13

as such is a solid base or, depending on the elemental composition of its octahedral

layers, may have redox properties. HTs are excellent materials to design bifunctional

redox-base catalysts [58, 73-75]. Potential applications of HTs range from the

production of large-scale basic chemicals to the synthesis of small-scale specialty

chemicals [12].

A particular advantage for HTs as base catalysis is that the number and

strength of basic sites can be tuned precisely to a specific reaction. The basic property

of these materials was initially envisaged by Nakatsuka et al. [76], Reichle [77] and

Laylock et al. [78], for catalytic polymerization and aldol condensation reactions. The

basicity of LDH is affected by the calcination procedure, typically at 673-773 K and

by structural and compositional parameters [79, 80]. Cations like Co, Zn or Ni give

less basicity than Mg; less basic catalysts are also obtained from Cl- or SO4

2-

precursors than from CO32-

or OH-containing materials [77] and the basicity also

depends on the Mg/Al ratio [81]. The correlation of the LDH basic properties with the

Mg/Al ratio, however, is not always straightforward [77].

The basicity for hydrotalcite in the as-synthesized form is due to hydroxyl

groups (OH-) that provide Bronsted basic sites [82-84]. In case of calcined LDHs, the

basicity is due to the presence of strong O2-

Lewis basic sites, Mn+

Lewis acid sites,

and Mn+

-O2-

pairs [85]. Figure 1.4 represents schematic picture of catalytic sites

available in hydrotalcite-like materials.

Different alternate synthetic procedures like microwave and sonication leads

to materials with different basic characters [86-88]. Homogenous particles of ~80 nm

average particle size and with higher defects was produced using sonication showed

high basicity [88]. Sol-gel derived materials showed higher surface area than the co-

precipitated materials that are beneficial in catalysis [89]. Reconstruction of thermally

treated hydrotalcite (also known as memory effect) increased the basicity with an

increase in hydroxide ions [90]. Ebitani et al. showed that reconstructed LDH found

to be active for the aldol reactions to produce α-hydroxy carbonyl derivatives in the

presence of water and also can promote the aqueous Knoevenagel and Michael

reactions using nitriles [91].

Chapter 1 Introduction

Ph.D Thesis 14

Figure 1.4 Catalytic sites available in hydrotalcite and derived forms

The basicity of hydrotalcite-like materials was assessed using different

techniques. Among these techniques for correlating basicity with activity, most

common ones are CO2-TPD, calorimetry, NMR and FT-IR spectroscopy using NH3

and pyrole as probe molecules [92-94]. Despite many reports on calcined forms,

necessary correlation between the activity and basicity of as-synthesized LDHs is

scarce and challenging [95]. Earlier attempts for finding the basicity of calcined HT-

like materials include decomposition of 2-methyl-3-butyn-2-ol, 2-propanol,

isophorone isomerization, phenol adsorption, allylbenzene isomerization [96-101].

The presence of both Lewis and Bornsted type basic sites in LDHs and ability to host

different metal ions extended its applications in both base catalyzed reactions and

oxidation reactions [102].

Corma et al. studied the basic property of the materials by carrying out the

condensation of benzaldehyde with ethyl acetoacetate in presence of Mg-Al LDH

catalyst. They found that this material shows basic sites with pKa values up to 16.5,

which are normally uncommon among the other commercial basic zeolite materials

Chapter 1 Introduction

Ph.D Thesis 15

[80]. With zeolite as catalyst, the only reaction observed was Knoevenagel

condensation, while calcined LDHs showed other reactions like Michael-type addition

and Claisen condensation, which requires stronger basic sites. The total amount of

basic sites and their strength distribution in the material were determined by carrying

out the Knoevenagel condensation reaction with methylenic groups of different pKa

values in presence of increasing amount of benzoic acid. By increasing the Mg/Al

ratio in the LDH, the number of basic sites with 9.0 ≤ pKa ≤ 13.3 increases, whereas

the amount of basic sites within 13.3 ≤ pKa ≤ 16.5 decreases.

Kustrowski et al. studied the acidic and basic properties of the MgAl mixed

oxides derived from LDHs with different interlayer anions using both NH3 and CO2

TPD [103]. These results showed that concentration of basic sites are in the order of

CO32-

> Cl- > HPO4

2- > Terephthalate > SO4

2- and the acidic sites are in the order of

CO32-

> Terephthalate > Cl- > HPO4

2- > SO4

2-. Both CO3

2- and Cl

- showed higher

basic sites whereas HPO42-

, SO42-

and terephthalate showed weak basic sites.

Kustrowski et al. also studied the variation of Mg/Al ratio over the acidic and basic

property of the LDH using NH3 and CO2 TPD [104]. Materials calcined at different

temperatures showed different acid-base behaviours. For materials calcined at 550 oC

the concentration of acidic sites are in the order of 3.5:1 > 3:1 > 4:1 > 2:1 Mg:Al

ratios, whereas concentration of basic sites are in the order of 2:1 > 3:1 > 3.5 :1 > 4:1.

Choudhary et al. attempted to increase the basicity of MgAl hydrotalcite by

incorporating tert-butoxide and was reported that so formed MgAl-O-t-Bu was active

for base catalyzed organic transformations like cyanoalkylation, Henry reaction,

transesterification, and aldol condensation. They reported that the basicity of MgAl-

O-t-Bu catalysts was much higher than as-synthesized, calcined and rehydrated MgAl

hydrotalcites. The tert-butoxides which is an organic base provides the basicity [105-

108].

Chimentao et al. studied the effect of using mechanical stirring or ultrasound

during reconstruction of the mixed oxides and showed that this leads to an

enhancement in the catalytic activity. Modifications in the structure and basicity of

the resulting materials, together with an increased surface area and improved

accessibility to the active sites lead to higher activity. Increasing the rehydration time

during stirring or sonication also strongly affects the final catalyst and the

performance of these materials has been disclosed for the epoxidation of styrene

[109].

Chapter 1 Introduction

Ph.D Thesis 16

Figueras et al. conducted detailed investigation on the basic properties of

hydrotalcite like materials and published two reviews on the basic properties and the

catalytic applications of hydrotalcite-like materials for fine chemical synthesis. The

authors examined the efficiency of HT for different organic transformations like

aldolization, condensation, hydrogenation, oxidation, and so forth [110, 111].

Onda et al. used activated hydrotalcites for the lactic acid production from D-

glucose in flow reactor at 323 K in aqueous media. The number of accessible

Bronsted-base sites was determined by the ion-exchange method with sodium salts,

based on the OH/Al ratio. The catalytic activity for the lactic acid production showed

a linear increase with the number of the Bronsted-basic sites [112].

Kantam et al. reported an eco-friendly, simple and efficient catalytic system

for selective aerobic oxidation of alcohol to aldehyde. The racemic-BINOL over

CuAl-HT in presence of K2CO3 as base gave selectively aldehyde under mild

conditions [113]. In another study they supported lithium diisopropylamide (LDA)

over MgAl3-HT and studied for aldol, Knoevenagel, Henry, Michael,

transesterification and epoxidation reactions. They reported that these composite

catalysts were effective for C-C and C-O coupling reactions [114].

Recently Kaneda et al. used silver nanoparticles supported on LDH that

efficiently catalyzed chemoselective reduction of nitroaromatic compounds into the

corresponding amines using CO/H2O as a reductant. The basicity of LDH facilitates

the formation of active AgH- species [115]. Solid base-metal combination of copper

nano particles supported on LDH was used for oxidant free alcohol dehydrogenation

[116]. LDH supported gold nanoparticles acted as a reusable catalyst for the synthesis

of lactones from diols using molecular oxygen as an oxidant under mild conditions

and reported that the basicity of supports and the size of the Au particles are key

factors in promoting the above oxidative lactonization [117].

Zeng et al. showed that calcined LDH with Mg/Al molar ratio of 4.0 exhibited

the highest catalytic activity in the synthesis of propylene glycol methyl ether from

etherification of propylene oxide with methanol. The catalytic activity of the material

was correlated with the amount of the basic sites determined by Hammett indicators

[118].

Our group in CSMCRI, has expertise in exploiting LDHs for various base

catalyzed transformations such as condensation [119], double bond isomerization of

alkenyl aromatics [120-122] and also in redox calaysis. Several reports have been

Chapter 1 Introduction

Ph.D Thesis 17

published recently on hydroxylation of phenol and benzene using various transition

metals containing multi-metallic hydrotalcites [123-127].

1.1.3 Ion-exchange

Ion-exchange may be defined as reversible interchange of ions between a solid

phase and a solution phase, where an atom or molecule from solution is exchanged for

a similarly charged ion attached to an immobile solid particle. Ion-exchange process

was first studied over inorganic solids like soil, rocks, clays, zeolite and so forth

[128]. In most cases the term ion-exchange is used to denote the processes of

purification, separation, and decontamination of aqueous and other ion-containing

solutions with solid. Materials having ion-exchange capability possess wide spread

application especially in waste-free technologies as well as in fields like nuclear

chemistry and polymeric materials.

The important features for material to be good ion-exchangers are,

The structure should be hydrophilic

Controlled and effective ion exchange capacity

Rapid rate of exchange

Chemical stability

Physical stability in terms of mechanical strength and resistance

Consistent particle size and effective surface area

Reproducibilty

Among ion-exchangers, cationic exchangers like aluminosilicates possess

great industrial importance [129, 130]; while that for anionic exchangers are less

familiar. Anion exchangers are a class of ion-exchangers consist of fixed ions and

anions that can undergo exchange. Anion exchange materials are classified as either

weak base or strong base depending on the type of exchange group. In general these

are solid bases (Bronsted or Lewis) over wide range of pH [128]. An anion exchange

reaction can be written as:

The effectiveness of anion-exchangers is expressed in terms of anion exchange

capacity (AEC). AEC is a measure of total content of exchangeable ions, and is

Chapter 1 Introduction

Ph.D Thesis 18

conventionally expressed in terms of the total number of equivalents of ion, usually in

milli-equivalents per gram, meq/g.

1.1.3.1 Hydrotalcite as anion exchangers

Hydrotalcite-like materials in the current research arena form widespread class

of anion exchangers. Ion-exchange process in HT is given schematically in Figure

1.5. The structural features of LDHs makes it a good exchanger for anions through

different mechanism like anion exchange with interlayers, reconstruction of heat

treated materials, surface precipitation, surface adsorption and so forth. Anion

exchange capacity (AEC) of hydrotalcite-like (HT-like) materials usually are around

2-3 meq/g (much less than theoretical maximum of 3.6 meq/g [131]), which is

comparable to that of commercial anion exchangers [132].

Figure 1.5 Typical ion-exchange process over HT

The AEC of LDHs has reliance on the interlayer anions which undergoes

substitution during the anion exchange; usually hydrotalcites with carbonate as the

interlayer anion shows less adsorption capacity due to the high affinity of carbonate

with the layers. Generally carbonate anion is almost impossible to remove by ion-

exchange which has large preference over other anions in layers. However,

hydrotalcites with nitrates or chlorides in the interlayer generally bestow relatively

good anion exchange capacity (than carbonate containing LDHs) and are well

explored for the removal of contaminants [133, 134]. The literature also says that the

calcined hydrotalcites shows good removal capacity for toxic anions through

reconstruction mechanism than as-synthesized forms [135]. It is also known that

anions with higher charge density exchange more strongly than monovalent anions.

The anion exchange capacity for HT has been affected by anion species in the

interlayer and the layer charge or ratio of divalent and trivalent metal ions. Miyata and

Chapter 1 Introduction

Ph.D Thesis 19

co-workers have done extensive studies on anion exchange properties of hydrotalcite-

like materials. They determined the ion-exchange equilibrium constants of HTs with

anions in the sequence as; CO32-

>> SO42-

>> OH- > F

- > Cl

- > Br

- > NO3

- > I

- [41].

The anion-exchange capacity in the case of hydrotalcite-like materials is calculated by

the formula [136],

where x stands for M(II)/M(III) atomic ratio and FW is formula weight

1.1.3.2 Anionic pollutants

Increase in the concentration of harmful ions (both anions and cations) in

water bodies causes environmental concern (or pollution) that not only threatens the

human life but also put at risk the life of aquatic organisms. The main protocol of the

governing bodies around the globe in this 21st century is the preservation of nature, or

other way controlling air, land and water pollution. The present scenario mainly aims

on water pollution which is a major problem to our society as we know how important

water to mankind. The increased industrialization and inadequate disposal of waste is

a serious problem that we are facing in the environmental perspective. Hence the

removal of hazardous anions is a big challenge towards environmental view point.

Number of processes is available for decontamination of noxious anions from

polluted water bodies, mainly coagulation [137], surface precipitation [138], ion-

exchange through resins [139], magnetic separation [140], bio sorption [141], and

sorption process [2]. Sorption process has its own advantages towards water

remediation due to less waste disposal. The water remediation properties of LDHs are

due to the inherent nature of material that have large surface area for the adsorption of

noxious anion, facile exchange of interlayer anion, and tailorable

hydrophobic/hydrophilic nature.

1.1.3.3 Hydrotalcite in noxious anion removal

For the uptake of toxic anions, various materials like carbons, zeolites, carbon

nanotubes, chitin, metal oxides, metal hydroxides, polymer composites, clays as

adsorbent are available [142-150]; among them layered double hydroxides or

hydrotalcite-like materials received more attention towards removal of toxic anion

(oxy-anions) in last few decades. Hydrotalcites as anion exchangers have also gained

Chapter 1 Introduction

Ph.D Thesis 20

significant progress in the research and development for the removal of organic,

inorganic and nuclear wastes from contaminated waters [2]. Hydrotalcites are widely

used for the removal of inorganic monovalent anions like nitrate, fluoride, chloride,

bromide, iodide, divalent oxy-anions like phosphate, chromate, selenate, arsenate,

borate, organic pesticides, dyes, and so forth (Table 1.5).

Table 1.5 Uptake of pollutants (anions) over various hydrotalcite-like materials

Toxic

anions

Hydrotalcite (HT) References

Arsenite As-sythesized and calcined MgAl, hydrocalumite [151, 152]

Arsenate As-sythesized and calcined MgAl, hydrocalumite, MgAl-

NO3

[153-155]

Chromate As-synthesized Ni–Fe, as-synthesized and calcined

MgAl, ZnAl

[134, 135,

156-158]

Phosphate As-synthesized and calcined ZrZnAl, ZnAl [138,

159-162]

Selenite ZnAl, MgAl, ZnFe LDHs, MgAl and ZnAl [163-165]

Selenate As-sythesized and calcined MgAl [165, 166]

Borate As-synthesized and calcined ZnAl, calcined MgAl,

MgAl-NO3, MgAl and MgFe

[167-170]

Nitrate MgAl, CoFe, NiFe, and MgFe, as-synthesized MgAl,

CoFe, NiFe, and MgFe-Cl

[171, 172]

Iodate As-synthesized MgAl LDHs and Mg-Al-NO3 [173]

Fluoride MgAl, NiAl, and CoAl, ZnAl, calcined MgAl [174-177]

Iodide Calcined MgAl [178]

Bromide As-synthesized and calcined MgAl, MgAl [179, 180]

Molybate As-synthesized and calcined MgAl, hydrocalumite [181-183]

Vanadate Calcined MgAl [184, 185]

Dyes MgAl-NO3 [186]

Pesticides MgAl-Cl, MgFe-Cl, MgAlFe-Cl, calcined MgAl [187]

This doctoral work is designed after understanding the importance of

hydrotalcite-like materials in the field of catalysis and ion-exchange. They were used

for industrially important organic transformation exploiting its basicity and as

exchangers for the removal of toxic chromate anions from aqueous conditions.

Chapter 1 Introduction

Ph.D Thesis 21

1.2 Synthesis of hydrotalcites

Hydrotalcite like materials can be prepared by different methodologies,

namely, precipitation at constant pH, precipitation at variable pH, deposition

precipitation reactions, hydrothermal synthesis, anion exchange, sol-gel, structure

reconstruction, electro chemical methods and hydrolysis reactions [188].

1.2.1 Co-precipitation under low super saturation

Conventionally MgAl hydrotalcites are synthesized by co-precipitation of

aqueous alkaline solutions of Mg and Al salts (nitrates or chlorides) at fixed pH under

stirring and in some cases in inert atmosphere (Figure 1.6). Classically, an aqueous

NaOH or KOH solution is used to adjust the pH and an aqueous Na2CO3 or K2CO3

solution is added as the carbonate source [189]. After aging of the slurry, the resulting

material is filtered, washed with deionised water and dried (~373 K). In the case of

nitrate containing HTs, NaNO3 along with NaOH was used as precipitating agent and

all the process like metal addition and aging was done under N2 atmosphere. The

aging step may itself be used as a tool to change the properties of the final material.

For example, microwave irradiation may be used during aging in order to provoke a

reduction in the size of the particles, an increase in the specific surface areas and an

increase in the basicity [190].

M2+ + M3+

Solution (A)

NaOH + Na2CO3

Solution (B)

Slurry

Crystallized Gel

Dry Sample

110°C, 12h Drying

65°C, 18h

Aging

Figure 1.6 Schematic sketch of synthesis of hydrotalcites by coprecipitation at fixed

pH

Hydrotalcites based on other metals (Co, Ni, Cu, etc.) can be synthesized in

similar way by introducing appropriate nitrate salt [191,192]. However it is necessary

Chapter 1 Introduction

Ph.D Thesis 22

to precipitate at a pH higher than that of pH of precipitation of metal hydroxides.

Materials synthesized by this method has several advantages like high surface area,

multiple metal ions containing LDHs can be synthesized simply by taking different

metal nitrate precursors, highly dispersed metal ions containing LDHs and

crystallinity of the material can be tuned by varying the aging time and temperature.

Several tetravalent metal cations containing LDHs were also synthesized successfully

using this method [159, 193].

1.2.2 Hydrolysis methods

Urea on hydrolysis at high temperature releases ammonia which precipitates

the metal cations in the synthesis of LDHs. The urea method was initially developed

by Costantino et al. and has become very popular and undergone several

modifications [194]. A typical synthetic procedure is as follows: An aqueous stock

solution of urea (1.0 M), magnesium chloride (0.1 M) and aluminium chloride (0.1

M) are mixed together at the molar Mg/Al/urea ratio of 4:1:10 with magnetic stirring

at room temperature. After cooling to room temperature, the solid precipitate is

collected by centrifugation and washed with deionized water subsequently. In

aqueous solutions, urea decomposes on heating to give ammonia and HNCO. In

acidic or neutral media, HNCO is converted into CO2, and ammonia takes up a proton

to give NH4+. Both these steps lead to consumption of H

+ and hence increase the pH

of the medium [195, 196]. The so formed NH4+

precipitates the metal nitrates. Urea

hydrolysis is found to be one of the best methods to produce the highly crystalline

LDHs [197].

Hexamethylenetetramine (HMT), also called as hexamine, found to hydrolyze

at higher temperature in aqueous solution release, ammonia similar to urea, which

makes the solution alkaline. In a typical procedure metal nitrates or chlorides with

hexamine in molar ratio of (M2+

+ M3+

): HMT with 0.15 M: 0.45 M were taken in a

Teflon inner vessel with a stainless steel outer vessel and allowed to react at 140 oC

for 24 h. Pioneering synthesis of LDH using hexamine was done by Iyi et al. [198]

and they have achieved hexagonal crystalline MgAl-LDH. Hexamine on hydrolysis

gives ammonia and formaldehyde; formaldehyde on reductive amination with

ammonia (Leuckart reaction) gives formic acid and subsequently leads to carbonate

containing LDH. It was also proposed that slow release of ammonia leads to highly

crystalline hexagonally shaped LDH. Followed by Iyi, several works were reported on

Chapter 1 Introduction

Ph.D Thesis 23

the synthesis of LDH as well as brucite-like mixed hydroxides using HMT hydrolysis

[199, 200].

1.2.3 Miscellaneous methods

Apart from these three important methods, several other methods are also

reported. The sol-gel synthesis of materials based on the hydrolysis and condensation

of molecular precursors is used to prepare a wide range of inorganic materials. This

procedure gives sols and these sol colloidal particles suspended in a liquid, progress

through a gelation process to form two interpenetrating networks between the solid

phase and the solvent phase. This technique limits the amount of alkali required and

thus sometimes preferred for industrial synthesis processes [201, 202]. The synthesis

procedure is as follows: Magnesium ethoxide is dissolved by acid hydrolysis with

HCl (35% in water) or with HNO3 (65%) in 120 ml of ethanol; the solution is refluxed

at 353 K, under constant stirring. A second solution, containing a suitable amount of

aluminium acetylacetonate in 80 ml of a mixture acetone/ethanol 1:1 in order to

obtain Mg2+

/Al3+

ratios in the range 3-6, is then slowly added, and the pH is adjusted

to about 10 with NH4OH. The solution is then refluxed at 353 K for 17 h, until the gel

was formed. Gel is then repeatedly washed with ethanol and dried overnight at 353 K

and gel can also be exchanged with Na2CO3 aqueous solution to get carbonate in the

interlayers of LDH [203]. Sol-gel synthesis of ZnAl-LDH without impurity phase was

prepared [204] which were not possible previously. Recently, Valente et al.

synthesized the novel multi-metallic LDHs with nanocapsular morphology using sol-

gel method [205, 206] and one of the advantages here is that these materials shows

high surface area than both co-precipitation and urea/hexamine hydrolysis materials.

Other methods include synthesis of LDH nanomaterials with uniform

crystallite size through separate nucleation and aging steps [207] and explored for

various applications. NiAl-LDH was synthesized by Mizuhata et al. using liquid

phase deposition (LPD) using aluminium metal [208]; highly stable LDH obtained

through this method can be applied in nickel-metal hydride batteries. Davila et al.

synthesized MgAl-mixed oxides using combustion method using sugar as fuel [209]

and was further converted to LDH using memory effect. Recently O’Hare synthesized

LDHs with unique morphologies using reverse microemulsion method [210]; the

surfactant/water ratio enables them to obtain nanometer sized LDH platelets typically

with 40-50 nm diameter and 10 nm thickness. MgAl and ZnAl-LDH were synthesized

Chapter 1 Introduction

Ph.D Thesis 24

using lazer ablation in water [211] and are also synthesized through electrochemical

methods [212].

1.3 Physicochemical characterization

1.3.1 Powder X-ray diffraction (PXRD)

The powder X-ray diffraction (PXRD) is a diagnostic tool for the phase

identification of these compounds. The PXRD patterns of all clay minerals possesses

layered structure that generally show sharp, symmetric peaks at lower angels (2θ) and

broad, asymmetric peaks at higher diffraction angles. The thickness of the brucite-

like layers (4.8 Å) [213], and the interlayer space vary depending on the size and

orientation of anion (2.8 Å shows presence of carbonate anion with its molecular

plane parallel to the brucite-like layers). Lattice parameter ‘a’ is the distance between

the neighboring cations in the brucite-like layers, which can be estimated from the

ionic radii of the cations in the brucite-like lattice [191] and their molar fractions in

the samples while the parameter ‘c’ is three times the distance between the adjacent

brucite-type layer, controlled mostly by the size (and orientation) of the interlayer

anion and the electrostatic forces operating between the interlayer anion and the

layers. PXRD was carried out in a Philips X’Pert MPD system using Cu K radiation

( = 1.5406 Å). The operating voltage and current were 40 kV and 30 mA,

respectively. A step size of 0.04˚ with a step time of 2 seconds was used for data

collection. The data were processed using the Philips X’Pert (version 2.2e) software.

Identification of the crystalline phases was made by comparison with the JCPDS files

[214]. PXRD of some of the samples was also carried out in Rigaku-Miniflex II using

Cu K radiation ( = 1.5406 Å). The operating voltage and current were 30 kV and 15

mA, respectively. A step size of 0.04˚ with a step time of 2 seconds was used for data

collection (scan speed, 1.2 deg/min).

1.3.2 Fourier transformed infrared (FT-IR) spectroscopy

Although infrared (IR) analysis is not a primary tool for the characterization of

LDHs, yet it has been routinely used especially for the identification of the foreign

anions in the interlayer space and its interaction with the brucite-like sheets. Besides

that, information about the type of bonds formed by the anions and about their

orientation can also be discerned. FT-IR absorption spectra of the samples were

recorded in a Perkin-Elmer FT-IR spectrometer (Model-FT-1730). The powdered

Chapter 1 Introduction

Ph.D Thesis 25

samples were ground with KBr in 1:20 ratio and pressed into pellets for recording the

spectra. 64 spectra (recorded with a nominal resolution of 4 cm-1

) were accumulated

and averaged to improve the signal-to-noise ratio.

1.3.3 Thermogravimetric analysis (TGA)

The thermal behavior of LDHs is generally characterized by two transitions:

The first one being reversible, endothermic, at low temperature corresponds to the

loss of interlayer water, without collapsing the structure and the second one

endothermic, at higher temperature is due to the loss of hydroxyl group from the

brucite-like layer as well as of the anions. The nature of these two transitions depends

on many factors such as: M(II)/M(III) ratio, type of anions, low temperature treatment

(hydration, drying etc.) and atmosphere (in case of oxidizable element such as

Cr(III)). Thermogravimetric analysis (TGA) was carried out in Mettler TGA/SDTA

851e and the data were processed using Star

e software, in flowing nitrogen at a flow

rate of 60 ml/min and at a heating rate of 10 oC/min.

1.3.4 ICP-OES elemental analysis

Elemental chemical analyses of the samples were determined using

inductively coupled plasma optical emission spectrometry (ICP-OES; Perkin Elmer,

OES, Optical 2000 DV). The samples were digested in minimum amount of

concentrated HNO3 and diluted using milli Q water (conductivity ~ 18 m Ohm) and

analyzed.

1.3.5 BET adsorption measurements

Specific surface area and pore size analysis of the samples were measured by

nitrogen adsorption at -196 oC using a sorptometer (ASAP-2010, Micromeritics). The

samples were degassed under vacuum at 120 oC for 4 h prior to measurements in

order to expel the interlayer water molecules. The BET specific surface area (SA) was

calculated by using the standard Brunauer, Emmett and Teller method [215] on the

basis of adsorption data. Pore volume (PV), micropore area and mesopore area were

determined by using the t-plot method of De Boer [216]. Average pore size

distributions (APD) were calculated from the desorption branch of the isotherms

using the Barret, Joyner and Halenda (BJH) method [217].

Chapter 1 Introduction

Ph.D Thesis 26

1.3.6 Scanning electron microscopy (SEM)

The scanning electron microscopic studies were done in a scanning electron

microscope (Leo series VP1430) equipped with EDX facility (Oxford Instruments),

having silicon detector. The samples were coated with gold using sputter coating

before analysis to avoid charging effects during recording. Analyses were carried out

with an accelerating voltage of 20 keV and a working distance of 17 mm, with

magnification values up to 100,000x.

1.3.7 Transmission electron microscopy (TEM)

Transmission electron microscope (TEM) images were obtained with a JEOL

JEM-2100 microscope with acceleration voltage of 200 keV using carbon coated 200

mesh copper/gold grids. The samples were ultrasonically dispersed in ethanol for 5

min and deposited onto carbon film using capillary and dried in air for 30 min.

Elemental mapping analysis were done using STEM mode of HRTEM using an

energy dispersive X-ray (EDX) detector (Oxford EDX detector: Model INCAx-

Sight).

1.3.8 UV-visible spectrophotometry

UV-vis spectra were recorded following the reflectance technique in a

Shimadzu UV 3101PC instrument using 5 nm slits and BaSO4 as reference.

1.3.9 CHNS analysis

The elemental analysis was carried out using Perkin-Elmer CHNS/O analyzer

(Series II, 2400). The sample weighed in milligrams housed in a tin capsule is

dropped into a quartz tube at 1020 °C with constant helium flow (carrier gas). A few

seconds before the sample drops into the combustion tube, the stream is enriched with

a measured amount of high purity oxygen to achieve a strong oxidizing environment

which guarantees almost complete combustion/oxidation even for thermally resistant

substances. The combustion gas mixture is driven through an oxidation catalyst

(WO3) zone, then through a subsequent copper zone which reduces nitrogen oxides

and sulphuric anhydride (SO3) formed during combustion, to elemental nitrogen and

sulphurous anhydride (SO2) and retains the oxygen excess. The resulting four

components of the combustion mixture are detected by a thermal conductivity

detector in the sequence N2, CO2, H2O and SO2. In case of oxygen which is analyzed

Chapter 1 Introduction

Ph.D Thesis 27

separately, the sample undergoes immediate pyrolysis in a Helium stream which

ensures quantitative conversion of organic oxygen into carbon monoxide separated on

a GC column packed with molecular sieves.

1.3.10 Temperature programmed desorption (TPD)

Temperature programmed desorption (TPD) was carried out in an AutoChem

2910 (Micromeritics, USA) instrument. The sample was pretreated by passage of

high-purity (99.995%) helium (50 ml/min) at 100 oC for 2 h. After pretreatment, the

sample was cooled and started adsorption of a mixture of CO2-He (10 vol.% CO2) at

80 oC for 1 h and subsequently flushed with He (50 mL/min) at 105

oC for 2 h to

remove physisorbed CO2. TPD analysis was then carried out from ambient

temperature to 900 oC at a heating rate of 10

oC /min. The amount of CO2 desorbed in

the effluent stream was monitored and analyzed with the TCD and quantified by

deconvolution method.

1.3.11 Temperature programmed reduction (TPR)

Temperature programmed reduction (TPR) analysis was carried out in a

Micromeritics 2900 TPD/TPR instrument. The reducing agent was H2-Ar (5 vol. %)

from L’Air Liquide (Spain) and gas flow (50 ml min-1

), sample weight (15-20 mg)

and heating schedule (10 oC min

-1) were chosen according to literature to optimize

resolution of the curves. Calibration of the instrument was carried out with CuO (from

Merck).

1.3.12 Nuclear magnetic resonance (NMR)

Solid-state 7Li and

27Al NMR spectra were recorded on a Bruker Avance-II

500 MHz spectrometer equipped with a double resonance CPMAS probe. 27

Al spectra

was acquired using single pulse excitation method, with standard zg program, at MAS

frequency 12 kHz and for 7Li, 8 kHz.

27Al chemical shift was measured with reference

of Al(NO3), at 0 ppm and 7Li measured with reference LiCl, at 0 ppm.

Chapter 1 Introduction

Ph.D Thesis 28

1.4 Scope and objectives of the work

The thesis divulge two important applications of hydrotalcite-like (HT-like)

materials (otherwise known as layered double hydroxides; LDHs), namely for base

catalysis and ion-exchange. The inherent properties of these materials like high

surface area, acid-base functionality, ability to accommodate homogeneously different

metal cations in the layers and improved basic property through memory effect were

explored for catalysis. The superior anion exchange ability of these materials is

explored for the removal of toxic anionic pollutants for aqueous systems. In this

doctoral work, Chapter 2 & 3 deals with base catalysed isomerization reaction of

alkenyl aromatics under thermal and microwave irradiation. The iso products of

alkenyl aromatics have great commercial value and Ni and Mg containing HT-like

materials as solid base catalysts are potentially explored for these reactions, with an

endeavour to replace environmentally unfriendly conventionally practiced alkali-

based homogeneous catalysts. Kinetics of isomerization of eugenol is studied and

scale up of the reaction is endeavoured to have practical attractiveness. Chapter 4

deals with the structure-property-activity relationship study over hydrotalcite-like

materials for these reactions that are less traversed earlier. The science/chemistry

behind isomerization activity with the structure/property of hydrotalcite is discussed.

The operando/in situ DRIFT-FTIR spectroscopy is used for the first time as a tool to

deduce the surface-structure-activity relationship for NiCuAl catalysts for

isomerization reaction. Attempts are made to fill some gap in correlating the basicity

and activity for as-synthesized Mg and Ni containing hydrotalcites by studying

isomerization of allylbenzene. Further support on catalytic property with activity is

assessed by exploring LiAl-HTs synthesized through urea hydrolysis for base-

catalysed condensation reactions both in their as-synthesized and calcined forms.

Chapter 5 discusses the efficiency of NiAl HT-like materials for the removal of

chromate, a toxic pollutant, through anion exchange; kinetics and adsorption

equilibrium phenomenon are also studied. Chapter 6 discusses the utilization of

chromate uptake by CoAl HT-like materials and further use of adsorbed materials for

catalysis, a concept that is proposed for the first time. This ideology of combined

approach of using layered double hydroxides for both environmental remediation and

the derived material as heterogeneous catalyst is highly beneficial in green chemistry

perspective.

Chapter 1 Introduction

Ph.D Thesis 29

The objectives of the present thesis are:

Synthesis of hydrotalcites and their physiochemical characterization

Isomerization of alkenyl aromatics of potential interest over NiAl HTs

Kinetic studies for isomerization of eugenol over MgAl4 and scale up studies

Microwave assisted isomerization of alkenyl aromatics over MgAl and NiAl

HTs

Structure-surface-activity correlation studies over HT-like materials for base

catalyzed reactions

Operando DRIFT-FTIR studies for NiAl and NiCuAl HTs

Removal of toxic chromate using NiAlNO3 HTs as efficient adsorbents

Valorization of CoAlCrO4 HTs derived after chromate removal for selective

oxidation of benzyl alcohol

Chapter 1 Introduction

Ph.D Thesis 30

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