Chapter 1 Introduction

Transcription

1 Chapter - 1

2 1.1 Background The term lamellar double hydroxide (LDHs) (1) is used to designate synthetic or natural lamellar hydroxides with two kinds of metallic cations in the main layers with interlayer domains containing anionic species. This wide family of compounds is also referred to as anionic clays, by comparison with the naturally occurring cationic clays whose interlamellar domains contain cationic species. Material having Mg and Al cations with carbonate as intercalated anion are known as hydrotalcite. Therefore, LDHs are also known as hydrotalcite like compounds by reference to one of the polytypes of the corresponding [Mg-Al] based minerals. Layered double hydroxides (LDH), hydrotalcite-like (HTl), hydrotalcite-type (HTt), anionic clays, are the common names used for a wide family of layered materials. 1.2 History The first natural mineral belonging to this family of materials was discovered in Sweden in 1842, known as hydrotalcite, and was given the general formula Mg 6 Al 2 (OH) 6 CO 3 4H 2 O. The first study on the synthesis, stability, solubility and structure determination dates back to 1930 and was mostly carried out by Feiknecht (1-2). Feiknecht described the materials as having a double layered structure and layered double hydroxide is perhaps a misinterpretation of this term. Around 1967, several groups (3-5) correctly identified the layered structure. 1.3 Structure of layered double hydroxide The structure of the layered double hydroxide is very similar to that of brucite-like Mg(OH) 2 where an isomorphous substitution of Mg 2+ by a trivalent element M 3+ occurs. In brucite, each magnesium cation is octahedrally surrounded by hydroxyls. The resulting octahedron shares edges to form infinite sheets with no net charge. When Mg 2+ ions are replaced by a trivalent ion, a positive charge is generated in the brucite sheet. The positive charge is 2

3 compensated by anions in the interlayer, in the free space of which the water of crystallization also finds a place (Figure 1) (6-8). LDHs are generally expressed by the following formula: [(M 2+ 1-xM 3+ x (OH) 2 ) +x. (A n- x/n.m H 2 O) x- ] Where M 2+ = Mg 2+, Ni 2+, Zn 2+, Cu 2+, Mn 2+ etc. M 3+ = Al 3+, Fe 3+, Cr 3+ etc. A n- = CO 2-3, SO 2-4, NO - 3, Cl -, OH - etc x = M(III)/M(II)+M(III) x is normally between 0.17 to 0.33 but LDH with higher cation molar ratio are also reported (9-11) Figure 1. Structure of Layered Double Hydroxide 3

4 1.3.1 Metal cations in layer The divalent and trivalent metal cations found in LDHs belong mainly to the third and fourth periods of the periodic classification of elements. The ionic radii are in the range of Å for divalent cations and Å for trivalent cations (With the main exception, Al: 0.50 Å). Higher ionic radii (Ca, Cd and Sc, La) seem to be incompatible with the formation of true brucite like layers. LDHs can also be obtained with a Li-Al monovalent-trivalent (12) and Co-Ti, Zn-Ti divalent-tetravalent associations (13-14). Presence of more than two different cations in the layer is generally observed in LDH minerals containing variable amounts of minor elements (15). LDHs exist as minerals with different names depending on their composition and the symmetry of polytypes as shown in Table 1. Table 1. Different LDHs containing different cations Hydrotalcite, Manasseite Mg, Al Sjogrenite, Pyroaurite, Coalingite Mg, Fe Stichtite, Babertonite Mg, Cr Takovite Ni, Al or Ni, Ni Reevesite Ni, Fe Woodwardite Cu, Al Interlayer anions In LDHs, the interlamellar domains contain anions, water molecules and sometimes other neutral or charged moieties. One major characteristic of LDHs is that, in most cases, only 4

5 weak bonding occurs between these lamellar ions or molecules and the host structure. Where the interlamellar anions are mainly carbonate, and sometimes sulphate or chloride but a great variety of anionic species can be located between the layers during the formation of the lamellar structure, or by further anionic exchange. There are several reports in literature on LDH with varied intercalated anions other then carbonate. These anions can be halides, oxoanions (16), oxo (16-17) and polyoxo-metallates (18-20), silicates (21-22), anionic complexes (23), carboxylate anions (24-26), EDTA (27-29), ferrocyanide anion (30-32), nitrate anion (33-34) and organic moieties (35-36). A number of reported LDHs (37) (synthetic or naturally occurring) having varied cations and anions are summarized in Table 2. Table 2. Summary of the commonly reported LDHs (37) M 2+ M 3+ Interlayer anion Chemical composition Ca Al CO 3 [Ca 0.66 Al 0.33 (OH) 2 ](CO 3 ) 0.17 nh 2 O Ca Al NO 3 [Ca 0.66 Al 0.33 (OH) 2 ](NO 3 ) H 2 O Ca Al OH [Ca 0.66 Al 0.33 (OH) 2 ](OH) 0.33 nh 2 O Cd Al CO 3 [Cd 0.67 Al 0.33 (OH) 1.67 ](CO 3 ) H 2 O Cd Al NO 3 [Cd 0.67 Al 0.33( OH) 1.67 ](NO 3 ) H 2 O Co Al Cl [Co 0.66 Al 0.33 (OH) 2 ]Cl 0.33 nh 2 O Co Al OH [Co 0.75 Al 0.25 (OH) 2 ](OH) 0.13 nh 2 O Cu Al [Fe(CN) 6 ] [Cu 0.88 Al 0.5 (OH) 2 ][Fe(CN) 6 ] H 2 O Cu Cr C 6 H 4-1,4-(CO 2 ) 2 [Cu 0.6 Cr 0.4 (OH) 2 ][C 6 H 4-1,4-(CO 2 ) 2 ].2 0.6H 2 O 5

6 Cu Cr Cl [Cu 0.69 Cr 0.31 (OH) 2 ]Cl H 2 O Cu Cr Dodecyl-sulfate [Cu 0.66 Cr 0.33 (OH) 2 ](dodecylsulfate) 0.33 nh 2 O Li Al Br [Li 0.33 Al 0.66 (OH) 2 ]Br 0.33?nH 2 O Li Al Cl [Li 0.33 Al 0.66 (OH) 2 ]Cl 0.33?nH 2 O Li Al Cl (rhomb) [Li 0.33 Al 0.66 (OH) 2 ]Cl 0.33?nH 2 O Li Al CO 3 [Li 0.33 Al 0.66 (OH) 2 ](CO 3 )0.16nH 2 O Li Al NO 3 [Li 0.33 Al 0.66 (OH) 2 ](NO 3 )0.33nH 2 O Li Al OH [Li 0.33 Al 0.66 (OH) 2 ](OH) 0.33 nh 2 O Li Al SiO(OH) 3 [Li 0.33 Al 0.66 (OH) 2 ][(OH) 3 SiO] 0.33 nh 2 O Li Al SO4 [Li 0.33 Al 0.66 (OH) 2 ](SO 4 ) 0.17 nh 2 O Mg Al Cl [Mg 0.66 Al 0.33 (OH) 2 ]Cl 0.33 nh 2 O Mg Al ClO 4 [Mg 0.75 Al 0.25 (OH) 2 ](ClO 4 ) 0.18 (CO 3 ) H 2 O Mg Al CO 3 [Mg 0.66 Al 0.33 (OH) 2 ](CO 3 ) 0.17 nh 2 O Mg Al Fe(CN) 6 [Mg Al (OH) 2] [Fe(CN) 6 ] (CO 3 ) H 2 O Mg Al NO3 [Mg 0.66 Al 0.33 (OH) 2 ](NO 3 ) 0.33 nh 2 O Mg Al SiO(OH) 3 [Mg 0.75 Al 0.25 (OH) 2 ][(OH) 3 SiO] 0.25 nh 2 O Mg Cr CO 3 [Mg 0.71 Cr 0.29 (OH) 2 ](CO 3 ) H 2 O Mg Cr Fe(CN) 6 [Mg 0.56 Cr 0.44 (OH) 2 ][Fe(CN) 6 ] H 2 O Mg Cr Oxalate [Mg 0.61 Cr 0.39 (OH) 2 ](C 2 O 4 ) H 2 O Mg Fe CO 3 [Mg 0.75 Fe 0.25 (OH) 2 ](CO 3 ) H 2 O 6

7 Mg Al, Fe CO 3 [Mg 0.74 Fe 0.11 Al 0.15 (OH) 2 ](CO 3 ) H 2 O Mg, Pd Al CO 3 Atomic Ratios 66 : 5 : 29 Mg, Pt Al CO 3 Atomic Ratios 67 : 4 : 29 Mg,Al Rh CO 3 Atomic Ratios 71 : 24 : 5 Mg Al Ir CO 3 Atomic Ratios 71 : 24 : 5 Mg Al Ru CO 3 Atomic Ratios 71 : 24 : 5 Mg Al Zr CO 3 Atomic Ratios 3 : 0.52 : 0.50 Mg V CO 3 [Mg 0.52 V 0.48 (OH) 2 ](CO 3 ) H 2 O Mn Al Cl [Mn 0.66 Al 0.33 (OH) 2 ]Cl 0.33 nh 2 O Ni Al Cl [Ni 0.75 Al 0.25 (OH) 2 ]Cl 0.25 nh 2 O Ni Al CO 3 [Ni 0.75 Al 0.25 (OH) 2 ](CO 3 ) nh 2 O Ni Al NO 3 [Ni 0.67 Al 0.33 (OH) 2 ](NO 3 ) 0.20 (CO 3 ) H 2 O Ni Co CO 3 [Ni 0.75 Co 0.25 (OH) 2 ](CO 3 ) H 2 O Ni Fe CO 3 [Ni 0.75 Fe 0.25 (OH) 2 ](CO 3 ) H 2 O Ni Fe SO 4 [Ni 0.7 Fe 0.3 (OH) 2 ](SO 4 ) 0.17 nh 2 O Ni V CO 3 [Ni 0.82 V 0.18 (OH) 2 ](CO 3 ) H 2 O Ni, Cu Al CO 3 [Ni 0.30 Cu 0.38 Al 0.32 (OH) 2 ](CO 3 ) H 2 O Zn Al Cl [Zn 0.66 Al 0.33 (OH) 2 ]Cl H 2 O Zn Al CO 3 [Zn 0.75 Al 0.25 (OH) 2 ](CO 3 ) 0.13 nh 2 O Zn Al NO 3 [Zn 0.66 Al 0.33 (OH) 2 ](NO 3 ) H 2 O 7

8 Zn Cr Br [Zn 0.66 Cr 0.33 (OH) 2 ]Br H 2 O Zn Cr Cl [Zn 0.66 Cr 0.34 (OH) 2 ]Cl H 2 O Zn Cr ClO 4 [Zn 0.66 Cr 0.33 (OH) 2 ](ClO 4 ) H 2 O Zn Cr F [Zn0.66Cr0.33(OH)2]F0.33?0.66H 2 O Zn Cr HPO 4 [Zn 0.66 Cr 0.33 (OH) 2 ](HPO 4 ) H 2 O Zn Cr I [Zn 0.66 Cr 0.33 (OH) 2 ]I H 2 O Zn Cr NO 3 [Zn 0.66 Cr 0.33 (OH) 2 ](NO 3 ) H 2 O Zn Cr SO 4 [Zn 0.66 Cr 0.33 (OH) 2 ](SO 4 ) H 2 O Zn Cr n-cah 2 a11so 4 [Zn 0.66 Cr 0.33 (OH) 2 ](n-cah 2 a11so 4 ) H 2 O Orientation of anions in interlayer The anions with different structure, dimensions and charges can be hosted between the brucite-like layers. This feature is quite generally reflected in the interlayer spacing or in the gallery height. In general the anions are oriented in order to maximize the interaction with their surroundings. For example, planar carbonate anions are usually positioned in parallel to the brucite like layer in order that the three oxygen atoms can interact well with the layer hydroxyl groups by forming hydrogen bond. This orientation also maximizes the electrostatic interaction between carbonate anions and the positively charged layer since the gallery height is minimized corresponding to the thickness of the planar carbonate (38). 8

9 1.3.4 Water in the interlayers Water molecules normally occupy the sites available in the interlayer that are not occupied by the anions. Each OH group in brucite-like layers provides one such site. In general the water molecules are held through the formation of hydrogen bonds with the hydroxide layer OH and/or the interlayer anions. The inelastic neutron scattering investigation have shown that the water molecules in the interlayer are not fixed in one position but rotate freely and move about hydroxide oxygen sits (8, 39). It has been shown using Raman spectroscopy that there are three types of structured water; a) water hydrogen bonded to the interlayer carbonate ion, b) water hydrogen bonded to the hydrotalcite hydroxyl surface, and c) interlamellar water. This water lost over a temperature range of C, depending on the strength of the interactions (39). Heating up to C can push out the water and leave the framework unchanged. The dehydrated LDHs can also reabsorb water while cooling in humid surroundings (8). In addition, a certain amount of water may be physically absorbed on the surface of small crystallites. This weakly absorbed water can be removed by heating at 100 C Staking and Polytypes Structural disorders are known to occur in LDHs, among which stacking disorders are common (40-41). As the layered sheet of metal hydroxide can be stacked in various ways, a number of polytypes can be formed. Naturally occurring LDHs (manasseite) are known to have either a triple-layer cell with rhombohedral symmetry designated as 3R polytype or a double-layer cell with hexagonal symmetry designated as 2H polytype (43) One-layer (1H) (41) and six-layer (6R) polytypes (43) have also been reported. Furthermore, polytypes 9

10 having equal number of layers per unit cell, however, differ substantially in layer arrangements are theoretically possible for LDH materials (43). 1.4 Synthesis of Layered double hydroxides LDH materials occur in nature as naturally occurring minerals and can also be readily prepared in the laboratory. In nature, they are formed from the weathering of basalts (43) or precipitation in saline water sources (44). All natural LDH minerals have a structure similar to hydrotalcite, which has the formula [Mg 6 Al 2 (OH) 16 ]CO 3 4H 2 O. Unlike clays, however, LDHs are not discovered in large, commercially exploitable deposits (45). LDHs are simple and inexpensive to synthesize on laboratory and industrial scales (47). These were first prepared in the laboratory in 1942 when Feitknecht reacted dilute aqueous metal salt solutions with base (2, 48). Several methods have been used to synthesize LDHs with tailored physical and chemical properties suitable for many applications, as discussed below: Direct methods Co-precipitation LDHs are commonly prepared by co-precipitation method. In co-precipitation method, LDHs are generally precipitated from a mixture of metal salts and the interlamellar anion of interest under basic conditions either at constant or at increasing ph. In this titration or variable ph co-precipitation method, M(III) hydroxides or hydrous oxides are initially formed, and further addition of base results in co-precipitation or conversion into LDH. To obtain LDH with high chemical homogeneity, co-precipitation at constant ph is recommended. It allows the preparation of a large number of LDH with CO 2-3, Cl -, or NO - 3 anions as precursors for subsequent reactions (2,48-50). Several factors are important in the precipitation of LDH 10

11 compounds, such as the nature of the cations, their ratio, nature of the anions, ph, temperature and aging Urea method In the standard constant-ph co-precipitation method, the precipitation of hydroxide particles is initiated from the beginning of the reaction, then nucleation and particle growth overlap resulting in a broad distribution of particle size. In order to prepare mono-dispersed particles, it is necessary to avoid the overlapping of these two steps and shorten the germination period. The method of precipitation using temperature controlled urea hydrolysis as base retardant production is properly adapted for this purpose. A reaction temperature above 60 C produces the progressive decomposition of urea in ammonium hydroxide leading to a homogeneous precipitation. This method has been already employed for the synthesis of well crystallized [LiAl-CO 3 ] (51), [MgAl-CO 3 ] (52), [ZnAl-CO 3 ] (53) and [NiAl-CO 3 ] (52) LDHs Sol-gel technique The sol-gel process was first explored by Lopez et al. (54) to prepare Mg-Al LDH samples. This process involves formation of a mobile colloidal suspension (sol) that then gels (gel) due to internal cross linking. The preparation of LDH is the result of the hydrolysis and polymerization of a solution of metal alkoxides. The alkoxides are first dissolved in an organic solvent and refluxed. To this solution water is slowly added, causing to occur cross linkage. Materials prepared by this technique exhibit good homogeneity, relatively good control of stoichiometry, and high surface and high porosity characteristics. The sol-gel hydrotalcite shows thermal stability up to 550 C (55). However, LDH samples prepared by co-precipitation are more crystalline than those prepared by the sol-gel method (15,56). The marked increase in specific surface area is ascribed to the increase in mesopore volume for 11

12 the LDHs prepared through sol-gel technique. The textural properties of the calcined samples are not appreciably influenced by the method of synthesis (56) Indirect methods Indirect methods include all syntheses that use an LDH as a precursor. Examples of these are all anion exchange and non anion exchange based methods Anion exchange based methods The lamellar structure of LDH, based on staking of positive layers trapping anionic species in the interlayer domains, is highly favorable to anion diffusion making LDHs one of the principal classes of inorganic ion exchangers. This property has been used in order to prepare new LDH phases by anionic exchange reactions. The anion exchange includes: Direct anion exchange (57) Anion exchange by acid attack with elimination of the guest species in the interlayer region (58) Anion exchange by surfactant salt formation (59) Non-anion exchange methods The non-anion exchange methods include the delamination-restacking method (60-61). LDHs are also able to form nanocomposites, by intercalating polymers in the interlayer. A way to facilitate this intercalation is to delaminate LDHs, previously synthesized by direct synthesis or hydrothermal treatment (6), making the total surface of the LDH accessible. Nevertheless, because of the strong interlayer electrostatic interactions between the sheets, exfoliation of the sheets in water or other non-aqueous solvents remains difficult (62-63). To overcome this 12

13 lack of accessibility to the interlayer space, researchers have made several attempts to weaken the stacking of the layers by exchanging the inorganic anions with organophilic ones, such as surfactants (60, 63). Another method includes LDH reconstruction method, in which after moderate calcination to pre-spinel oxide, a further step is the reconstruction of a LDH phase in a solution containing a new anion to be intercalated (64-65). The reconstruction process can also be performed in air, which supply water molecules and carbonate; in this case, the same anion being reintercalated, the only interest is to modify the textural properties of the primitive LDH. The textural properties are also modified by reconstruction in solvents other than water, thus evidencing a template effect of the solvent on the spatial organization of the regenerated micro-crystallites giving rise to meso-scale porosity Post synthesis treatments Post-preparative treatments are usually necessary to improve the crystallinity of the LDH materials prepared by conventional methods. Improved crystallinity is desirable in LDHs because it allows for greater stabilization of intercalated anions. Ageing the precipitation at or above room temperature is often used as a post-treatment method. In addition, hydrothermal, ultrasonic and microwave treatment techniques have also been employed as post treatments as described below: Hydrothermal Treatments Different in-situ or post-synthesis treatments were applied to the as prepared samples in order to control structural and textural properties. Hydrothermal treatment is used to improve the crystallinity of the compounds or to increase the anion exchange rate of low affinity anions such as alkyl carboxylates. Hydrothermal treatment has a strong effect on the chemical 13

14 composition (Mg 2+ /Al 3+ ) of synthetic hydrotalcite (66). Series of Ni Al Cr and Ni Al Fe HTlcs were prepared by co-precipitation at 60 C, followed by hydrothermal treatment at 150 C (67). Direct hydrothermal synthesis of Mg-Al-LDH and Mg-Cr-LDH compounds has also been reported (68-69) Ultrasound Irradiation Crystallinity of the phases can also be improved by ultrasound irradiation. Larger LDH crystallites with enhanced adsorption capacities were observed using ultrasonic technique (70). The crystallization processes and the properties of the crystals can be improved and widen the application areas of products. Narrow and small particles can also be prepared by this method (71) Microwave Irradiation The hydrotalcite synthesis method has been significantly improved substituting the conventional hydrothermal treatment step by microwave irradiation (72-73). The autoclave high temperature treatment is, in this way, avoided and the long crystallization time is substantially reduced. Microwaves were used during synthesis in order to accelerate both the growing and ageing steps. Microwave irradiation time results in a well crystallized material compared to conventional co-precipitation and extent of enhancement in crystallinity depends on the nature of the trivalent metal ion (74). In general, the surface area, porosity (75), and thermal stability (76) of the synthetic materials are increased with the duration of microwave exposure. 14

15 1.4.4 Synthesis of spinel form The thermal degradation of hydrotalcite and its conversion into spinel were reported in 1944 (77). Spinel form is crystalline mixed oxide formed after calcination of LDHs at higher temperature. Synthesis of highly reactive, phase-pure spinel powder seems to be a crucial step to the successful fabrication of dense ceramic materials at reasonable sintering temperatures. It is well accepted that the wet-chemical processing of multi-cation oxides provides considerable advantages of good mixing of the starting materials and excellent chemical homogeneity of the final product. In recent years, several types of wet-chemical techniques or wet-chemical related techniques have been developed and successfully used for the production of pure spinel powders at relatively low temperatures. These methods include hydroxide coprecipitation (78-79), sol gel of metal alkoxides or inorganic salts (80-81), spray-drying (82), freeze-drying (83), modified Pechini process (84), flame spray pyrolysis (85) and combustion synthesis (86). Magnesium aluminate spinel (MgAl 2 O 4 ) is an important ceramic material having high melting point (2135 C), high resistance against chemical attack (87), good mechanical strength both at room temperature and elevated temperatures (88), low dielectric constant (89) and excellent optical properties (90). Dense spinel ceramics could potentially and frequent applications in diverse engineering fields. Both direct and indirect methods have been reported for the synthesis of Mg-Al spinel form used as catalyst (91-92) Synthesis from waste The synthesis of hydrotalcite with various wastes has also been reported (93-94). The synthesis was carried out using reagents and the wastes discharged in an aluminium recycling process as a raw material (93). In an another report HT was synthesized using seawater and milk of lime in consideration for the utilization of plant-producing Mg(OH) 2. (94). 15

16 1.5 Characterization of layered double hydroxide Variety of techniques are used to characterize LDHs. Powder X-ray diffraction (PXRD) and infrared spectroscopy (IR), for example, are routinely used techniques whilst others such as ESR (95), XAS (96) and computer modelling (97) are less extensively employed. The characterization of LDH involves the examination of textural, structural and thermal properties. The textural properties include the surface area and structural properties are the d- spacing, crystallinity, crystallite size or disc diameter and lattice parameters. The thermal analysis of the LDH shows its thermal stability. Besides these, divalent, trivalent metal contents and water content are determined by the elemental analysis. In literature effect of different synthetic parameters, such as trivalent cations, and intercalation of different anions on their structural and textural properties have been studied in details using thermal and spectroscopic techniques (98-100). Morphology of LDH compounds have been studied in details using different techniques like Scanning electronic microscopy (SEM), X-ray diffractometer (XRD), Fourier transformer infrared (FTIR) and Raman spectroscopy which explains its different morphology such as sheet like (101), pillared (102), nanocapsular morphology (103), its delamination behavior (61) and surface porosity ( ). Various structural, textural, thermal, elemental and anion exchange properties are discussed briefly as below: 16

17 1.5.1 Structural properties Powder X-ray diffraction The lattice planes in a crystal are designated as (hkl) and characterized by Miller indices h, k and l, which are reciprocals of the intersections between the lattice plane and the three crystallographic axes that span the unit cell of the crystal. The incident X-ray beam (S o ) and the lattice planes have to be oriented in a certain angle (θ) to allow diffraction. Beams reflected at parallel lattice planes in the distance (d hkl ) interfere and give an intensity maximum, which occurs as peak in X-ray diffractogram, if their path difference 2d hkl is an integer of wavelength (λ). This condition has been described as Bragg s law, n.λ = 2. d hkl. Sin θ The X-ray diffractogram is the pattern obtained by plotting the intensity of the diffraction lines versus the angle 2θ. In powder samples with randomly oriented crystallites, the same amount of the crystallites has the right orientation for the diffraction for all lattice planes. If, instead, a certain pattern is preferred, the intensity of some reflections is lowered or increased. Information about the lattice parameters is available from peak position. The sharpness of the diffraction lines is determined from their intensity together with their breadth, which is called the full width at half maximum (FWHM). The breadth of the peak is increased with decreasing crystallite size. The XRD pattern of the sample shows the intensity of X-ray diffracted by different plane of the crystals at different angles. The crystal phase identification is based on the comparison of the set of reflections of the specimen with that of pure reference phases or with database such as JCPDS data files. The 17

18 crystallinity (%) of the phases is calculated by comparing the areas of the characteristic peaks of the phases. X-ray diffraction line broadening analysis is used to determine crystallite size of the crystalline phase using following Scherrer formula (107): Crystallite size = K. λ/ W.Cosθ Where, K = shape factor, λ = wavelength of X-ray radiation used, W = difference of broadened profile width of the experimental sample (W b ) and standard profile width of the reference sample (W s ), θ = angle of diffraction. It may be noted that for layered materials such as LDHs, crystallite size is defined as disc diameter along the a-b plane (40). A typical diffractogram obtained for an LDH consists of sharp basal (00l) reflections at low values of 2θ corresponding to successive orders of the interlayer spacing. In addition, relatively weak non-basal reflections at higher values of 2θ are present. In certain cases, the non-basal reflections are quite broad, which may be attributed to turbostratic disordering of the hydroxide layers. Generally, the patterns are indexed on the basis of a hexagonal unit cell. The interlayer spacing of the LDH is equivalent to 1/n of the c parameter, where n is the layer repeat of the unit cell, which depends upon the stacking sequence of the layers. Subtracting the hydroxide layer (brucite) thickness of approximately 4.8 Å from the interlayer spacing gives the gallery height (Figure 2). The gallery height depends on the size and orientation of the charge balancing anion. For example, the theoretical value of basal (interlayer) space for Mg-Al-CO 3 is expected to be 7.6 Å, considering the thickness of brucite like layer 4.8 Å and the gallery height 2.8 Å (108). The M 2+ /M 3+ ratio of the LDH may be inferred from a parameter of the unit cell, which is determined using the relationship a=2d 110 where d 110 is the d spacing of the 110 reflection. 18

19 For Mg-Al LDH as a result of the smaller ionic radius of Al 3+ (0.50Å) compared to Mg 2+ (0.65Å), the a parameter decreases as the aluminium content of the LDH increases. The measurement of d 110 for determining the M 2+ /M 3+ ratio of the LDH, along with elemental analysis techniques, enables an approximate chemical formula for the LDH to be deduced (109). Structural features and changes in structure by variation in synthesis procedures have also been explained by XRD techniques ( ). Figure 2. Schematic representation of the interlayer structure of LDH FT-IR study FT-IR spectroscopic study leads to a great deal of information, e.g., the presence of various groups, structure, hydrogen bonding etc. The interaction of IR radiation with molecule results to variation in the dipole moment and therefore, the vibrational energy level of the molecule, which are quantized. The activation of the molecule leads to different mode of vibrational motions such as stretching (symmetric and asymmetric), bending and promotes the molecule in higher vibrational energy level. The energy of the stretching vibrational motion is higher than that of the bending vibration and the asymmetrical stretching motion is higher in energy 19

20 than the symmetrical stretching. The transition in vibrational energy levels of the molecule gives IR spectrum, which is the plot between the wave number and the % transmittance or absorbance. FT-IR is a useful technique for confirming the presence of the intercalated anion in LDH (8). Presence of anions is confirmed by their specific IR peaks. For example the peak at (ν 3 ), (ν 2 ) and (ν 4 ) cm -1 are indicative of CO 2-3 anion (38,100,108). When the symmetry of CO 2-3 is lowered, vibration can be noted at 1050 cm -1 together with a shoulder at 1400 cm -1 or a double band at cm -1. This symmetry lowering indicates the orientation change of interlayer carbonate at high charge density. The incorporation of a carboxylic acid anion into an LDH is identified by strong anti-symmetric and symmetric carboxylate stretching bands at approximately 1560 and 1400 cm -1 respectively (112). Alternatively, intercalation of the undissociated acid form is confirmed by the strong absorption of the carbonyl stretch of the acid at approximately 1700 cm -1. Thus IR spectroscopy is an important tool for determining the structure of LDH (38) Textural properties (N 2 adsorption-desorption isotherm study) The surface area is most often determined by using gas adsorption method. The adsorption isotherm is the relationship between the amount of the gas adsorbed and the relative pressure at constant temperature. The first step in interpretation of a physisorption isotherm is to observe the shape of isotherm, which tells the qualitative nature of the surface coverage and the way of pore filling. Surface area is most often determined by BET-method (Brunauer- Emmett-Teller method) (113) from physisorption isotherm data using BET equation as follows: 20

21 P/P o na (1 - P/P o ) = 1 nm a. c + c - 1 nm a. c P P o Where P/Po = relative pressure, n a = volume of gas adsorbed, n m a = volume of gas adsorbed in monolayer formation, c = constant, indicates the shape of isotherm and order of magnitude of the adsorbentadsorbate interaction. The samples are degassed under vacuum at 150 ºC for 4 hours, prior to measurement, to evacuate the physisorbed moisture. Nitrogen (at 77.4 K) is generally used as a most suitable adsorbate. The area of a rough surface is called as external surface area and the area of the pore walls is an internal surface area. The accessible surface is generally that of the internal pores within the crystallites and the external surface between the crystallites. Corresponding, the measured pores are those inside of and between the crystallites. Within most common organic and inorganic LDHs, the interlayers are full of the anions as well as water, and thus only the external surface of the crystallites contributes to the accessible surface area. Surface area values normally range from m 2 /g (8,114) and only the pores between the crystallites compose the pore space. An important exception provide by pillared LDHs that may posses micropores in the interlayers whose sizes vary with the size and charge of the interlayer pillars (115). It assumes that pillared anions are evenly distributed in the interlayer and estimate the size of the interpillar pores. When LDH is calcined, the decomposition of anions, the dehydroxylation of brucite like layers, and the formation of newly born oxide crystallites bring about formation of new pores 21

22 or enlarge old pores to macropores. Specific surface area and pore size distribution were found to initially increase with calcination temperature of 200ºC and then decreased with increase in calcination temperature to 600ºC (116) Thermal analysis Thermo gravimetric analysis (TGA) and differential thermal analysis (DTA) are performed using simultaneous DSC/TGA to study the decomposition pattern, stepwise and total weight loss in the LDH samples. Samples are scanned in given temperature range ( o C) with certain heating rate under flow of an inert gas (N 2 ). The LDH compounds have extensively studied for their thermal behavior and effect of different factors on their thermal behavior. Structural changes takes place during high temperature in LDH has been explained by insitu heating techniques such as in-situ XRD, in-situ X-ray absorption spectroscopy (XAS), TGA/DTA and in-situ IR techniques ( ). LDHs have recently attracted much attention in the development of new environmental friendly catalysts. After calcination up to o C, LDH decomposes to a high surface area mixed oxide, having strong Lewis basic features (119). As a result of this, calcined LDHs have shown to be suitable catalysts for several vapour-phase base-catalyzed condensation reactions ( ). Structure and activity of these compounds depend on the applied treatment to a large extent. Therefore, insight in the changes in LDH during activation as well as knowledge in the resulting structure after heat treatment is vital. Their calcined products are always area of interest as they are good catalysts and also have other applications ( ). LDH with different cation combination have variation in their thermal decomposition and several studies are reported in literature on thermal decomposition of LDH with different cation combinations ( ). Effect of interlayer anions on thermal decomposition of LDH 22

23 has also been studied and significant difference in both thermal decomposition temperature and pattern was found ( ) Elemental analysis Atomic absorption spectrometry (AAS) and inductivity coupled plasma (ICP) are widely used for elemental analysis, providing the weight percentage of each metal and hence the all important metal atomic ratio in the LDH, which may be well different from that in the initial preparation solution. Combining the results of metal and CHN analysis gives a nominal chemical formula of the LDH as well as indication of the degree of any CO 2-3 and organic ions in particular, the degree of intercalation and uptake of undissociated acid molecules into the interlayer. The total water content of LDH sample also can be analyzed by Karl Fischer titrator or calculated on the basis of weight loss observed during TG analysis (38) Anion exchange property LDHs have a unique property of anion exchange due to their structural positive charge (129). The only structural change brought about by anion exchange is a variation in the interlayer distance, which is in turn dependent on the size of the incoming anion. The extensive application of LDHs is expected due to their anion exchange capacity toward inorganic and organic anions. LDHs can uptake anions from a solution by three different mechanisms: (i) Adsorption (ii) intercalation by anion exchange and (iii) intercalation by reconstruction of a calcined precursor. A pure adsorption process can take place in the surface of LDHs containing interlayer anions that present strong electrostatic interaction with the layers ( ). The sorption of anions in LDHs by anion exchange (intercalation) occurs when the anions in the precursor material are intercalated by weak electrostatic interactions with the layers. The exchange degree depends on the tendency of the substituting and the interlayer 23

24 anions to be intercalated, which is determined by the charge density of each anion: the higher the anion charge density, the stronger the electrostatic interaction with the layers ( ). The sorption of anions from aqueous solutions by structural reconstruction of a calcined LDH is based on a very interesting property of these materials, the so-called memory effect. The memory effect takes place when a LDH, calcined at temperature high enough to eliminate most of the interlayer anion, is reconstructed in pure water which gives rise to the intercalation of hydroxyl anions while the intercalation of other anions can occur if they are present in solution. Upon thermal activation, LDHs undergo dehydroxylation and decarbonation, which increase their exchange capacities (129) Molecular modeling Molecular modeling, known as molecular dynamics simulation, has been also widely applied to LDH system in recent years. By choosing a set of suitable force fields and molecular dynamics simulation can improve the understanding of the properties of the target system ( ) such as the anion arrangement in the interlayer ( ), hydration state (136) and ionic absorption and diffusion (137,141). Thermodynamic properties of LDH along with anion exchange properties using molecular modeling have been studied (142). Influence of the composition on the electro-negativity and on the oxygen charge distribution in binary LDH has been determined leading to good prediction of basicity and different basic sites (143). 1.6 Applications of Layered double hydroxide Layered double hydroxide have wide range of industrial applications (Figure 3) due to their anion-exchange, adsorption capacity, the mobility of their interlayer anions and water molecules and the stability and homogeneity of the materials formed by thermal decomposition (136). These materials are receiving considerable attention in recent years 24

25 because of their applications as anion-exchange ( ), adsorption materials for gas molecules ( ), carriers for drugs ( ), antacids in medicine ( ), catalysts, catalysts precursors and supports of catalysts ( ), polymer stabilizers ( ), optical hosts ( ), and ceramic precursors ( ). Different applications of layered double hydroxide are briefly discussed as below: Catalyst -Hydrogenation -Polymerization Catalyst support -Ziegler-Natta -CeO 2 -Steam reforming LDHs Industry -Flame retardant -Molecular sieve -Ion exchanger Medicine -Antiacid -Antipeptin -Stabilizer Adsorbent -Halogen scavenger -PVC stabilizer -Wasterwater Figure 3. Major industrial applications of LDHs In catalysis LDH s have aroused considerable interest because the diversity of their chemical composition and therefore these has many practical applications. One of the important use is as catalysts, catalyst support and catalyst precursor ( ,162). Until 1970, catalysis 25

26 research and research involving LDHs compounds followed separate parallel routes, when first patent was taken by BASF that specifically claimed that the hydrotalcite structure was an optimum precursor for the hydrogenation catalysts (163). LDHs have attracted an increasing attention in solid base catalysis as heterogeneous catalysts resulting in high activity, selectivity, stability and re-usability (164). LDHs have been used as catalysts and catalyst support as such as well as after calcination. (i) LDHs (as such) as catalysts The LDHs have been studied extensively as efficient solid base catalysts for a wide number of reactions such as oxidation and epoxidation of various olefins ( ), oxidation of thiols using air as oxidant (166). Mg-Al hydrotalcites were examined as catalysts for the epoxidation of olefins, styrene and N-oxidation of pyridines using hydrogen peroxide (167,169). Different bivalent system like Zn-Al, Ni-Al other than Mg-Al LDH have also been studied as solid base catalysts and for the epoxidation of olefins like bicycloalkenes using hydrogen peroxide as oxidant (165). Modified hydrotalcite such as novel chiral sulphonatosalen-manganese (III)-pillared Mg-Al hydrotalcite has also been reported as efficient catalyst for the asymmetric epoxidation of styrene and cyclic alkenes (167). The synthesized LDHs having Ru in the Brucite layer showed high catalytic activity for oxidation of allylic and benzylic alcohols in the presence of molecular oxygen ( ). Mo-containing HTlcs were proved to be active catalysts for selective olefin oxidation ( ). Also Ga-Mghydrotalcite and Fe-Mg-hydrotalcite (Mg/Fe=3), shows very high activity in the benzylation of toluene and benzene and other aromatic compounds (118, ). Copper containing hydrotalcites (Cu-HTLcs) were good catalyst in phenol hydroxylation with hydrogen peroxide ( ). 26

27 (ii) LDHs (after calcination) as catalysts After calcination up to C, LDH decomposes to a high surface area mixed oxide, having strong Lewis basic features (119). Calcined LDHs have shown to be suitable catalysts for several vapour-phase base-catalyzed condensation reactions and have emerged as an important area for catalysis research ( ). Hydrotalcite-derived mixed oxides obtained after calcination are used as efficient catalysts because of their high surface area, phase purity, basic surface properties, and structural stability (8)Hydrotalcite-derived oxides catalyze higher alcohol synthesis from CO/H 2 (178), oxidation of mercaptans (179) and water gas shift reactions (180). Also, Mg-Al mixed oxides prepared by oxidative decomposition of hydrotalcite precursors have been used in base-catalyzed aldol condensations (181), alkylation (182), Knoevenagel condensations (183) and double-bond isomerization (184). When heat-treated LDHs are brought into contact with water at room temperature, restoration of the layered structure with mainly OH - ions in the interlayer is achieved (185) yielding a highly active base catalyst, suitable for liquid-phase aldol condensation reactions (65, 174, 186). (iii) As catalysts support Calcination derivatives of LDHs i.e. layered double oxides (LDOs) are often used as catalyst supports. Due to the flexibility of compositions (nature and ratios of cations) and the good cationic dispersal, the basic strength of LDOs as supports can be finely tuned. As a consequence, the activity and selectivity of loaded active component can be adjusted or controlled, at least to some degree to meet the requirement for the specific catalytic reactions. The various metals such as Ru, Rh, Gd and Pd can be loaded by immersing LDOs in the solution containing salts or complexes of the metal of interest. These catalysts have been investigated for various reactions, including hydrogenation and hydrodesulfurization (HDS) 27

28 ( ), aromatization reactions (189) synthesis and application of synthesis gas (190), CO oxidation (191) and some other reactions (189, ). Synthesis gas can be converted to common chemicals, such as methanol and hydrocarbons over LDOs derived catalysts such as Zn-Cr and Cu-Zn-Al-LDOs (9,194, ) Basicity of LDHs The basic property of these materials was initially envisaged by Nakatsuka et al. (198), Reichle (184) and Laylock et al. (199) for catalytic polymerization and aldol condensation reactions. Whereas for the hydrated material, the active base sites are mainly structural hydroxyl anions, however, strong Lewis basic O 2 -Mn + pairs are present in completely waterfree calcined materials. The basicity is affected by the calcination procedure, typically at C temperature, and by structural and compositional parameters ( ). Combination of different elements, and elements ratios in the brucite-like layer and selection of different anionic species can tune up the basicity of the LDHs (154,165). Cations like Zn or Ni give less basicity than Mg; less basic catalysts are also obtained from Cl 2 or SO 4-2 precursors than from CO 2-3 or OH - containing materials (184). The basicity also depends on the Mg/Al ratio (201). Corma et al. report that the total number of basic sites increases by decreasing the Mg/Al ratio, but the portion of strong basic sites decreases (202). The correlation of the LDH basic properties with the Mg/Al ratio, however, is not always straightforward. Attempts have been made to quantify the basicity of the calcined samples. Reichle (184) and Schaper (203) reported data on isotopic H D exchange with acetone, toluene, cyclohexane, and on olefins, and calculated base strength of Mg-Al-CO 2-3 (used for isomerisation of olefins) calcined at 450 C in air with Mg:Al ratio of 3, which was found between pka 35 and 45. Corma et al (202) carried out Knoevenagel condensations of benzaldehyde with activated methylenic compounds and found that most of the basic sites were in the range 10.7 < pka < 13.3 with 28

29 exceptional pka values up to Similar results were obtained by Tichit et al. with pka values of about 11 (201). These values were obtained from condensation reactions, thus in contact with water produced by the reaction itself and the values comparable to that of piperidine (pka=11.1) in aqueous solution. Nonaqueous titration techniques were adopted for the determination of the Lewis Basicity and a range of pka values between 15 and 35 was obtained (182,204). LDHs may also exhibit acid properties after anion exchange with inorganic polyacids. One of the most reliable model substrates to check the acid base properties of the calcined and intercalated LDHs is acetonylacetone. This 1,4-diketone model compound may undergo a base-catalyzed intramolecular cyclization leading to 3-methyl-2- cyclopentenone, as well as an acid-catalyzed cyclization resulting in a furan (205) As gas adsorbent Various nitrogen oxides (NOx), generally produced in the combustion of fossil fuels, are the major pollutants in air that cause some environmental problems like photochemical smog and acid rain, as well as human diseases such as asthma (206). SO x are one of the most hazardous atmospheric pollutants since they contribute directly to acid rain formation produced in the FCC process (fluid catalytic cracking) after cracking reactions during regeneration of catalyst (207). The combustion of fossil fuels such as coal or natural gas releases large volumes of CO 2, the major global warming gas to the environment resulting into pollution problem. Therefore, the removal and recovery of these gases from gas mixture is very important for environment point of view ( ). LDHs have property for the adsorption of gas molecules like SO X (207), NO X (125), and for CO 2 ( ). In general a model NOx storage/reduction catalyst comprises three major components: a high-surface-area support material (e.g., Al 2 O 3 ), a NOx storage component containing alkali or alkaline earth metals (e.g., Ca, Sr, Ba, K, or Na), and a noble metal (e.g., Pt, Rh, or Pd) as 29

30 the catalytic redox component ( ). These storage/reduction catalysts are efficient for removing NOx under lean-rich cycles in the absence of SO 2. Nevertheless, a problem is that SO 2 generated in trace amounts in the exhaust dramatically deactivates the storage/reduction catalysts (212). However, LDH has been reported as efficient adsorbent for NO x. In a mixed oxide derived from an Mg-Al LDH compound, Al 2 O 3 acts as the support; MgO acts as a NOx storage component; and M (M II and/or M III ) can act as the active component for promoting storage of NOx. In fact, Cu-containing calcined Mg-Al LDHs have been found to be active and selective catalysts for NOx removal ( ). The studies ( ) show that these LDHs derived catalysts perform well for NOx storage/reduction, especially at low temperatures. Taking into account the fact that cobalt species are active components for NOx removal and decomposition ( ). SOx production and removal mechanisms are different from those of energy power plants; for this purpose the characteristics of the sorbent should be different. Then in a first step, the sorbent, currently called an additive since it is added in small amounts to the total catalyst inventory, must have the property to oxidize SO 2 to SO 3 and produce a metal sulphate. Finally, the metal sulphide can be hydrolyzed by steam in the stripper to form the original metal oxide. Worldwide research groups have studied several materials for this i.e. MgO, Al 2 O 3, and Mg-Al spinel were evaluated as possible additives. However, their performance was limited, since MgO forms very stable MgSO 4 compounds, restricting the additive regeneration; besides it has a low density and attrition index. Al 2 O 3 showed a low SOx removal capacity because the Al 2 (SO 4 ) 3 formed is very unstable at the regenerator temperature so it releases the sulfate species as produced in the regenerator. Eventually, MgAl 2 O 4 spinels were also used, but these had low SOx removal capacity and sulfate reduction which causes the solid s deactivation ( ). In recent years, basic mixed 30

31 oxides obtained from hydrotalcite-like (HT) compounds have shown good SOx activities to reduce these emissions from several sources including fluid catalytic cracking ( ). The removal and recovery of CO 2 from hot gas stream is becoming increasingly significant in the field of eco-friendly energy production. It is true that the most common adsorbents i.e. zeolite 5A, zeolite 13X, carbon molecular sieve, alumina, calcium oxide suffer from low CO 2 adsorption capacity at elevated temperatures and are thus limited to operation at lower temperatures. Studies of CO 2 adsorption on LDHs suggest that these materials have the attributes of a good adsorbent such as high adsorption selectivity and capacity, adequate adsorption/desorption kinetics at operating conditions and mechanical strength of adsorbent particles after cyclic exposure to high-pressure streams (209, ). However, the adsorption capacity of these materials is significantly influenced by their structural, textural and thermal behavior, which is further determined by the synthetic parameters. In general, higher x values results higher adsorption capacity (226). But an optimum amount of Al content is found necessary for maximum adsorption (209). Modification with alkali salts such as K 2 CO 3 increased both the adsorption capacity and the stability of the LDHs (142, 227). Addition of rare earth elements has also enhanced the adsorption capacity (228). The amount of CO 2 adsorption is affected not only by the interlayer spacing but also by the larger charge and the ionic form, i.e., the anion intercalated in layer also plays important role in adsorption but incorporation of large anion with high charge density depends upon the Al-Mg ratio. The optimum space and the charge density for CO 2 adsorption by LDH in the Mg 2+ -Al 3+ - Fe(CN) 4-6 system were found at the Al 3+ substitution of 0.37 ( ). The layer charge and the void space have to be taken into account for interpretation of maximum adsorption (226). Depending upon the conditions of pretreatment of the adsorbent some variation in adsorption parameters is expected ( ). The effect of different temperatures on CO 2 adsorption 31

32 shows pattern Q (300 ) > Q (20 ) > Q (200 ) where Q is amount of CO 2 adsorbed. The process of heat treatment of the LDHs has two functions; one in forming micropores in the decomposition product of the LDHs which is favorable for adsorption of carbon dioxide and the other has an opposite effect such as causing the decreased d spacing which in turn decreases the amounts of adsorbed carbon dioxide on the surfaces with increasing temperature. In literature carbon dioxide sorption was studied with varying temperature and by increasing number of cycles maximum weight gain was observed at 150 C temperature (235). For wet feed conditions the adsorption capacity was found to be approximately 10 % higher than that under dry feed conditions. The presence of water vapour is favorable for the adsorption of CO 2 onto LDHs at high temperature while water vapor is known to compete with CO 2 in adsorption behavior onto zeolites. Hydrotalcite has been also used as coating material on the commercial zeolites to improve its adsorption quality (236). Reports showed that certain hydrotalcites have the ability to absorb a large amount of CO 2 at elevated temperatures while maintaining stability over time (233, 237). Measurement also suggests rapid and irreversible chemisorption on freshly packed adsorbent followed by reversible and relatively weak adsorption perhaps on the chemisorbed material (233,238). Studies have been reported on CO 2 adsorption ( , 209, 228,239) and on breakthrough CO 2 adsorption (208, ) Anion exchanger LDHs have one important property for mobility of anion and water molecules present in their interlayer and because of this property they can be extensively applied for anionexchange ( ,243). LDHs can uptake anions from a solution by three different mechanisms: (i) adsorption (ii) intercalation by anion exchange and (iii) intercalation by reconstruction of a calcined 32

33 precursor. A pure adsorption process can take place on the surface of LDHs containing interlayer anions that have strong electrostatic interaction with the layers. It includes mainly the adsorption of surfactants ( ) and pesticides ( ). The sorption of anions in LDHs by anion exchange (intercalation) occurs when the anions in the precursor material are intercalated by weak electrostatic interactions with the layers, such as chloride or nitrate. The exchange degree depends on the tendency of the substituting and the interlayer anions to be intercalated, which is determined by the charge density of each anion: the higher the anion charge density, the stronger the electrostatic interaction with the layers. The uptake of anions usually occurs by a combination of two processes, i.e., anion exchange and adsorption. Examples of sorption by this process include radioactive wastes (I 129, I 131, TcO - 4 and ReO4 - ) ( ) and Cr (VI) compounds (chromate and dichromate) ( ). The sorption of anions from aqueous solution by structural reconstruction of a calcined LDH is based on a very interesting property of these materials, the so-called memory effect. The memory effect takes place when a LDH, calcined in a temperature high enough to eliminate most of the interlayer anion, is reconstructed in pure water which gives rise to the intercalation of hydroxyl anions, while the intercalation of other anions can occur if these are present in solution. Works concerning the uptake from aqueous solutions by LDH have been reported such as for surfactants (130,248), trichlorophenol (249), trinitrophenol (230,250), pesticides ( ), arsenate ( ), chromate (139, 247), chloride ( ), fluoride ( ), boron (259), humic substances (75) and bacteria (260). LDHs are attractive in the field of green sustainable chemistry, since they are useful for the removal of harmful oxo anions (such as nitrate ions, etc.) from aqueous solutions. The reported LDHs systems with carbonate and nitrate as intercalated anions for nitrate removal 33

34 were Mg-Al, Zn-Al, Ni-Fe, Ni-Fe (HT), Mg-Fe, Co-Fe with chloride anion (7, 9, ). The LDHs with Ni or Fe as skeletal metal atoms had a high selectivity for nitrate ions. Especially, high crystalline sample Ni-Fe (HT) shows a strong selective nitrate removal property called ion-sieve property. From the standpoint of adsorbent design, the concept of an ion sieve effect is beneficial for the development of a novel adsorbent for selective removal of harmful oxo-anions including nitrate ions. The reported study on Mg-Al-Cl and Mg-Al- CO 3 showed that after 24h total anions were adsorbed and for mixed anions solution adsorption order is SO 4 2- > F - > HPO 4 2- > Cl - > B(OH) 4 - > NO 3 (261) LDHs as stabilizing agents For polymers Chlorine-containing polymers such as PVC (poly vinyl chloride) undergo an autocatalytic dehydrochlorination reaction under the influence of elevated temperatures or UV radiation. Since it is necessary to sustain this autocatalytic process, and required stabilizers that irreversibly bond HCl can thus inhibit the degradation. Heavy metal compounds such as cadmium stearate or lead stearate are currently used for this purpose; alternatives are required however in the light of environmental concerns associated with the use of heavy metals. LDHs have been employed to good effect and, indeed, the largest current commercial application of LDH materials is in the polymer industry, mainly to stabilize PVC (269). Kyowa Chemical Industries of Japan were the first to demonstrate that adding Mg/Al LDHs to PVC in combination with other additives such as zinc stearate and tin maleate leads to an enhancement in thermal stability of the resin (270). The role of the LDH in absorbing HCl was confirmed experimentally by Van der Ven et al. (157) who measured the capacity of LDHs having the same M II /M III ratio and different counter-ions to react with HCl gas and 34

35 found a linear correlation between increasing HCl capacity and thermal stability of the LDH in the order SO 4 2- Cl - ~OH - NO 3- CO 3 2- C 17 H 35 COO -. It has been demonstrated (156) that an Mg-Al-CO 3 LDH with Mg/Al=2 has a better stabilizing effect on PVC than LDHs with higher Mg/Al ratios. It has been also shown (271) that LDHs containing interlayer maleate anions have superior heat stabilizing properties as compare to mixtures of carbonatecontaining LDHs and tin maleate For dyes Organic dyes are extensively used in the textile and polymer industries, but suffer from limited UV and/or oxygen and thermal stability. Ways of enhancing their stability continue to attract a great deal of interest and there has been considerable work on intercalation of cationic dyes in zeolites, aluminosilicate clays, and metal (IV) phosphonates (272). Several examples of the intercalation of anionic organic dyes in LDHs have also been reported in the literature ( ). Intercalation of a large anionic azo pigment into an Mg-Al-NO 3 LDH host by ion-exchange has also been reported (275). The UV-visible diffuse reflectance spectra of azo pigment, after heating, indicated that the thermostability was markedly enhanced by intercalation in the LDH host LDHs as flame retardants (FRs) In recent years there has been increasing concern about the environmental hazards associated with halogen-containing flame retardants because of the toxicity of the products formed during a fire or when incinerating or recycling waste plastics. Studies (276) have shown that an LDH with the formula Mg 4 Al 2 (OH) 12 CO 3 3H 2 O has better flame retardant properties and it seems that the layered structure may play a key role in this respect. In literature focus has been on the preparation of modified LDH flame retardants 35

36 with enhanced properties. Like borate-pillared LDH may be prepared by a simple procedure involving reaction of boric acid with an LDH in the carbonate form (277). The LOI (limiting oxygen index value)values for the resins containing carbonate and borate-pillared LDHs are comparable and significantly higher than that of the pure resin, indicating that addition of the LDHs has reduced the flammability of the material. Where, the LOI represents the minimum concentration of oxygen (expressed as percent by volume) in a mixture of oxygen and nitrogen that support flaming combustion of a material initially at room temperature. The LDH-carbonate gives a reduction in smoke density during combustion and the borate-pillared LDH has significantly better smoke suppressant properties. LDH-borate formed a more complete protective layer which has been suggested (278) to be the basis of smoke inhibiting action of flame retardants. It has also found that the flame retardancy of LDHs can be modified by tuning the composition of the layers as well as that of the interlayer galleries. Thus for example, ternary LDHs of the type Zn/Mg/Al-CO 3 have better flame retarding properties than binary Mg/Al-CO 3 LDHs (279) LDHs as heat retention additives in horticultural plastic films Horticultural crops are often grown under cover, in order to protect them from rain damage, low temperatures and pests. In temperate Europe, glass greenhouses are most commonly used, whereas expansion of agriculture in Asia has been mainly associated with the development of low cost plastic films (280). Low density polyethylene (LDPE) is the most commonly used plastic in horticultural films because of its low cost, light weight and high visible light transparency. However, it has the disadvantage of high transmission to radiated heat, leading to poor heat retention and low night-time temperatures. The heat retention properties of LDPE can be greatly improved by the addition of mineral fillers (281). Whilst many inorganic materials have suitable IR absorption bands to meet to varying extents 36

37 criterion, unless the particles of the material have a small size with a narrow distribution, there will be a considerable reduction in visible light transparency (282). It have been reported by certain trials that LDH can be used as additives to improve the heat retention capacity of plastic films (283). The LDHs used in these trials contain carbonate as the interlayer anion, but working on second generation materials is going on which contain mixtures of anions such as sulfate and carbonate having a wider range of IR absorption bands and give films with further enhanced heat retaining properties (284) Biological applications of LDHs Studies on both natural and synthetic clay minerals including LDHs for biological applications are extensively carried out. Recently, considerable attention has been focused on the intercalation of biomolecules into LDHs (285). In addition to pharmaceuticals and enzymes which are discussed in briefly, these include amino acids and peptides ( ), vitamins (288), DNA and other nucleosides (245,289), ATP (290), and polysaccharides such as alginate (291), chitosan (292), and carrageenans (293). These materials possess excellent properties such as low or null toxicity, good biocompatibility, and promise for controlled release, thus give rise to the incessant interest to their development for biological purposes, for example, pharmaceutical, cosmetic, and medical ones Pharmaceutical applications Pharmaceutical applications of LDHs mainly rely on acid buffering effect and anion exchange property ( ). Studies also suggested that the drugs-ldhs nano-hybrids could form the basis for developing a drug release system (293) such as ibuprofen, citrate, salicylate, aspartic and glutamic acids has been reported (151, ). LDH has a rapid onset of action, a high buffering capacity and a long duration of action. In particular, LDH 37

38 binds cytotoxic bile acids. These pharmacochemical properties make hydrotalcite a most suitable antacid (151). As LDHs are biocompatible systems (8), these can be used in pharmaceutical technologies (38, 298) and in medicine as drug supports or matrices. Once encapsulated, the drug could be released at a rate determined by ph. Mg-Al LDH have already found pharmaceutical applications as excipients (299) drug stabilizers (300) and for preparation of aluminium magnesium salts of antipyretic, analgesic and anti-inflammatory drugs (301). The LDH materials may have potential use as molecular containers for storing or transporting unstable chiral biomolecules or pharmaceutical agents. For example, Wei et al. (302) prepared intercalated L-Tyrosine-LDHs systems and was found that intercalation into LDHs can inhibit racemisation of L-tyrosine. Concerning the interaction between antibiotics and LDH, there are some emphasized works like Li et al. (303) reported that phenoxymethylpenicillin has been reversibly intercalated into a LDH, and the resulting system exhibited effective anti-bacterial activity Genetic application LDHs can intercalate many important biomolecules with negative charge such as oligomers, single or double stranded DNA, and simple molecules like nucleotides ( ). Especially, the single or double stranded DNAs have a great deal of application potentials in various fields, expanding from gene therapy to biosensing and even high density information storage. However, DNA strands are very susceptible to degradation and denaturation occurring during manufacture processes and storage. Pioneering works reported (307) they clearly demonstrated that biomolecules such as CMP, AMP, GMP and even DNA are bound by LDHs by anion exchange, yielding heterostructured nanohybrids (Figure 4). 38

39 Figure 4. Various biomolecule LDH hybrids obtained by intercalation reaction (a) the pristine MgAl LDH, (b) CMP LDH hybrid, (c) AMP LDH hybrid, (d) GMP LDH hybrid, and (e) DNA LDH hybrid The intercalated DNA was safely protected against harsh condition including strong alkaline, weak acidic environments, and DNAse attack. It could be also recovered very easily by exposing DNA LDH hybrids to an acidic condition due to the solubility of LDHs in acid, implying promising potential of LDHs in biological applications. Application of DNA LDH hybrids for information storage was proposed by Choy et al. (308). A large amount of information can be incorporated into a short DNA without any concern about forgery. However, the development of a genetic code system was retarded due to the inherent problems of naked DNA strands, including instability, flocculation property, and difficult recovery. Engineered double stranded DNA coding genetic information was intercalated into 39