Layered Double Hydroxides: Present and Future :Lớp đôi hydroxit: Hiện tại và tương lai

In recent years several reviews have appeared on these LDHs, dealing with their general chemistry and properties [3], structure and pillared derivatives [4], analogues with interlayer or

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P RESENT AND F UTURE

V ICENTE R IVES EDITOR

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Chapter 1 Layered Double Hydroxides: Synthesis and Post-Synthesis Modification 1

A de Roy, C Forano and J P Besse

Chapter 2 Crystal Structure and X-ray Identification of Layered Double Hydroxides 41

V A Drits and A S Bookin

Chapter 3 Computer Modelling of Layered Double Hydroxides 101

S P Newman, H C Greenwell, P V Coveney and W Jones

Chapter 4 Study of Layered Double Hydroxides by Thermal Methods 127

Vicente Rives

Chapter 5 Infrared and Raman Spectroscopic Studies of Layered Double

Hydroxides (LDHs) 153

J T Kloprogge and R L Frost

Chapter 6 Solid-State NMR and EPR Studies of Hydrotalcities 217

Part II: Applications

Chapter 9 Layered Double Hydroxides in Water Decontamination 285

María Ángeles Ulibarri and María del Carmen Hermosín

Chapter 10 Applications of Hydrotalcite-Type Anionic Clays

(Layered Double Hydroxides) in Catalysis 323

Francesco Basile and Angelo Vaccari

Chapter 11 Hydrogenation Catalysis by Mixed Oxides Prepared from LDHs 367

A Monzón, E Romeo and A J Marchi

Chapter 12 Layered Double Hydroxides and their Intercalation Compounds in

Photo-chemistry and in Medicinal Chemistry 435

Umberto Costantino and Morena Nocchetti

Chapter 13 Environmental Chemistry of Iron(II)-Iron(III) LDHs (Green Rusts) 469

Hans Christian Bruun Hansen

Layered Double Hydroxides (LDH), hydrotalcite-like (HTl), hydrotalcite-type (HTt), anionic clays, , are the commonest names applied to a wide family of layered materials, despite none of these names fully corresponds to the actual situation Also known in some occasions as layered hydroxycarbonates, whichever the name given, these materials are not so extended in nature as the well known cationic clays, but are very easy to prepare and they are not generally expensive

The first natural mineral belonging to this family of materials was discovered in Sweden in

1842, is known as hydrotalcite, and was given the general formula Mg6Al2(OH)16CO3·4H2O The first studies on the synthesis, stability, solubility and structure determination date back to

1930 and were mostly carried out by Feitknecht [1,2]

Essentially, the structure can be described as a cadmium iodide-type layered hydroxide (e.g., Mg(OH)2, brucite) where a partial Mg2+/Al3+ substitution has taken place (thus the name

“layered double hydroxide”), balancing of the electric charge being achieved by location of anions in the interlayer space (carbonate in most of the samples found in nature, so the name

“layered hydroxycarbonates”), where they co-exist with water molecules Nowadays, solids with this structure, but containing more than two different (a divalent and a trivalent one) cations in the brucite-like layers, are also known

What makes interesting to these materials is the fact that the nature of the layer cations can

be changed among a wide possible selection (almost exclusively restricted by size and charge), and the nature of the interlayer anion can be also (almost freely) selected, among organic or inorganic, simple or complex anions, polyoxometalates, simple anionic coordination compounds, etc An additional feature that makes them similar to cationic clays is the fact that they can be also pillared, although doubts still exist in the literature about the thermal stability

of the structures formed Also as cationic clays, the interlayer species can be rather easily exchanged, thus increasing their applications and opening new synthetic routes to prepare derivatives

A unique property these solids exhibit, so making them different from cationic clays, is that after thermal decomposition under mild conditions, they are able to recover the layered structure, this property representing, again, a new synthetic route for analogues

The possibilities all these properties open are surprisingly wide, and so the applications of these materials are widening almost every day The principal areas of interest include their use

as catalysts and catalyst supports, adsorbents, anion scavengers, anion exchangers, polymer stabilizers, antacids, antipeptins and stabilizers The restricted interlayer space also represents a

Vicente Rives viii

sort of “nanoreactor” to perform chemical reactions in a constrained region, which may even modify well known properties of molecules (e.g., photochemical properties)

In recent years several reviews have appeared on these LDHs, dealing with their general chemistry and properties [3], structure and pillared derivatives [4], analogues with interlayer organic anions [5] or with intercalated anionic coordination compounds or oxometalates [6] with different nuclearity degree With respect to their applications, Cavani, Trifirò and Vaccari published some years ago a very outstanding review [7] which has somewhat become a guide

to learn about the catalytic properties of these solids and their derivatives A general comparison of cationic and anionic clays has been also reported [8] Also, special issues of one

of the leading journals on clays have been devoted to different properties of these fascinating materials [9-11], and special sessions dealt to these materials in regional and world-wide international conferences

Some of these reviews are rather recent, and so we have intended to avoid any sort of repetition or overlapping with their content, unless the scientific production in its particular area has provided a large number of papers, worthwhile to be reviewed and summarised Most of these studies have insisted or are dedicated to particular areas of interest of LDHs,

i e., synthesis, structure, particular applications, etc However, the aim of this book is to present,

in an unified form, an updating of current knowledge about LDHs, from different points of view,

i e., paying attention to their synthesis, their properties and, finally, their applications We have chosen a rather “academic” way to cope with this subject, and we have tried to present the current knowledge about their structures and properties giving an account on the sort of information which may be known from application of specific, but well known and easily available, characterisation techniques Altogether, we hope this represents an updated and comprehensive description of LDHS from almost every point of view

So, the first section comprises a total of eight chapters devoted to the synthesis and physicochemical characterisation of these materials Besse and his coworkers describe the structure of these compounds in relation to their synthesis, their preparative methods, and also providing a developing strategy for post synthesis modification

Drits and Bookin perform a detailed study on the structural features of LDHs, namely, the isomorphous substitutions in the brucite-like layers, regularities in anion locations, order-disorder phenomena in the layers and in the interlayer, different LDH polytypes, stacking faults, etc

Jones and his coworkers report computer simulations to probe the interlayer structure and dynamics of LDHs, due to the lack of detailed structural information available for these materials, especially when containing organic interlayer anions

One of the outstanding properties of LDHs is their ability to recover their layered structure even after being calcined at moderate temperatures The effect of using different atmosphere conditions during decomposition, discriminating steps associated to dehydration, dehydroxylation, structure collapsing and formation of crystalline phases, as well as the effect

of the nature of the interlayer anion on the final solids, are the aim of the following chapter Spectroscopic techniques have been also applied to characterise LDHs The main results reported in the literature on these solids, obtained by application of vibrational spectroscopic techniques (Infrared and Raman) are reviewed by Kloprogge and Frost, and Rocha reports on the application of solid state resonance techniques (mainly MAS-NMR, but also, although in a lesser extent, EPR) to characterize the solids in order to obtain a complete description of these systems, for different spectroscopically active nuclei studied so far

reader with a deep knowledge of the LDHs literature may feel some applications are lacking, but we have again tried to include those applications for which a systematic and rather pedagogic analysis can be carried out

So, the first chapter in this section is dedicated by Ulibarri and Hermosín to the study of the application of LDHs and calcined LDHs to decontamination processes, mostly making us of two important features: the ability of LDHs to exchange their interlayer anions, and the ability

of calcined LDHs of recovering the layered structure when put in contact with solutions of anions

Then, two chapters by Vaccari and Basile, and by Monzón and coworkers, deal with the catalytic applications of LDHs and calcined LDHs; although these applications have been the aim of reviews published in the last decade, the increasing number of papers in the literature on this subject makes worthwhile to include an account of this (probably the most outstanding) application of LDHs Emphasis is made on present and potential applications, related to the wide number of composition and preparation variables, as well as on upgrading from 2- to 3-dimensions by pillaring and/or intercalation processes

Costantino and coworkers provide a chapter which includes two types of applications: photochemical properties/applications, from the ability of LDHs to organize photoactive species in the interlayer and/or on the surface of the microcrystals, with applications in non-linear optics, energy storage and conversion Applications of LDHs in Medicine are also reviewed; this is still a rather unexplored area, but we should not forget that some of very common drugs contain hydrotalcite in their formulation

Finally, Hansen reports a detailed study of a material known as “green rust”, an LDH containing Fe(II) and Fe(III), which usually occur as transient phases on corrosion of iron, and plays an plays an important role in transitions between anoxic and oxic soils conditions, and overall may help to understand the role of iron in Nature

Promising applications appear every day in the literature, and probably unforeseen applications will turn up shortly Their versatility, wide range of composition and very especially their low cost will probably favour in the near future an extended and widened interest in Layered Double Hydroxides

Vicente Rives

x

Last, but not the least, it has been a pleasure for me to act as Editor of this multi-author book I apologize for the subjects not included, and for the potential authors who were not invited to participate; this was only my exclusive fault But the undoubted success of the book will be a shared one with a bunch of excellent scientists, but, overall, good friends

Thank you

[1] W Feitknecht and G Fischer, Helv Chim Acta 18 (1935) 555

[2] W Feitknecht, Helv Chim Acta 25 (1942) 131

[3] F Trifirò and A Vaccari, in Comprehensive Supramolecular Chemistry (Eds J L

Atwood, J E D Davies, D D MacNicol, F Vögtle, J.-M Lehn, G Albert, T Bein), Pergamon, Oxford, vol 7 (1996) pp 251-291

[4] A De Roy, C Forano, K El Malki, J.-P Besse, in Synthesis of Microporous Materials

(Eds M L Occelli and H E Robson), Van Nostrand Reinhold, New York, vol 2 (1992) pp 108-169

[5] S P Newman and W Jones, New J Chem (1998) 105

[6] V Rives and M A Ulibarri, Coord Chem Rev 181 (1999) 61

[7] F Cavani, F Trifirò and A Vaccari, Catal Today 11 (1991) 173

[8] A Vaccari, Appl Clay Sci 14 (1999) 161

[9] A Vaccari (guest editor), Appl Clay Sci 10 (1995) pp 1-186

[10] D Tichit and A Vaccari (guest editors), Appl Clay Sci 13 (1998) pp 311-511

[11] F Basile, M Campanati, E Serwicka and A Vaccari (guest editors), Appl Clay Sci

18 (2001) pp 1-110

P OST -S YNTHESIS M ODIFICATION

A de Roy, C Forano and J P Besse*

Laboratoire des Matériaux Inorganiques Université Blaise Pascal (Clermont-Fd) – UMR 6002 F-63177 AUBIERE CEDEX, FRANCE

*E-mail : jpbesse@chimtp.univ-bpclermont.fr

The term of Lamellar Double Hydroxides (LDHs)1 is used to designate synthetic or natural lamellar hydroxides with two kinds of metallic cations in the main layers and interlayer domains containing anionic species This wide family of compounds is also referred to as

anionic clays, by comparison with the more usual cationic clays whose interlamellar domains contain cationic species LDHs are also reported as hydrotalcite-like compounds by reference

to one of the polytypes of the corresponding [Mg-Al] based mineral More seldom are they named pyroaurite-like compounds, lamellar hydroxides of transition metals, mixed metallic hydroxides, double layer hydroxides, or hybrid layer structures

Such minerals are reported since the beginning of this century2 and the preparation of synthetic phases is generally based on the controlled precipitation of aqueous solutions containing the metallic cations and began with the early work of Feitknecht.3

Since the end of the sixties, an increasing interest is being given to LDHs in the fields of structural characterisation, preparation of new compounds and new preparative methods, anionic exchange properties, electrochemical and magnetic properties, heterogeneous catalysis, pharmaceutical applications, etc Several review papers and references therein give current trends on this subject.4-6

The aim of this paper is to picture a general overview on LDHs, but we shall mainly point out some particular features about the structure of these compounds (in relation to their synthesis), their preparative methods and give a tentative development strategy for post synthesis modification illustrated by some examples

A de Roy, C Forano and J.P Besse

2

The LDH structure is based on M(OH)6 octahedral units sharing edges in order to build M(OH)2 brucite-like layers These octahedral units contain both divalent and trivalent metallic cations ; the main layers are therefore positively charged, and the charge density is proportional

to the trivalent metal ratio x = MIII/(MII+MIII) The whole structure is constituted by the stacking of such layers, intercalating charge-balancing anionic species and water molecules as shown in Figure 1, where the heavy general chemical formula, shortened as [MII-MIII-X] is also given

Figure 1.-Schematic view of the LDH structure and general formula Reprinted from A de Roy, C Forano, K

El Malki and J.P Besse, Anionic Clays: Trends in Pillaring Chemistry, in Expanded Clays and Other Microporous Solids, edited by M.L Occelli and H.E Robson (Van Nostrand Reinhold, New York 1992), vol II, Chap 7 pp 108-169, reproduced with permission from the authors

LDHs exhibit a high charge density on the main layers For example, a x = 1/3 trivalent metal ratio corresponds to one charge for 50 Å2 on each side of the layer, leading to one charge for 25 Å2 in the interlamellar domains

Trivalent metal ratio

Most of LDH systems accommodate a relatively wide range of trivalent ratios, but it is not reported that it could vary from 0 to 1 without main structural changes While larger ranges are sometimes given, the most reliable limits, are based for example on a clear evolution of lattice parameters and correspond approximately to 0.2 ≤ x ≤ 0.4 (Figure 2) Some authors describe

Figure 2.-Comparison of trivalent metal ratio x scale and divalent vs Trivalent R scale, and divalent vs trivalent R scale, and corresponding limits for LDH compositions

The upper limit of trivalent ratio is generally attributed to electrostatic repulsion between neighbouring trivalent metals in the layers, which is unavoidable if x > 1/3, and also to repulsion between the charge-balancing anionic interlamellar species The lower limit could correspond to a too high main distance between these interlamellar anions leading to a collapse

of the interlamellar domains The structure of the α-variety of divalent metals hydroxides with neutral M(OH)2 sheets and interlamellar domains containing salts or basic salts and water molecules seems to be close to a LDH structure with x = 0, but, as far as we know, compared to regular LDH structures there is a solution of continuity in the values of x

The metallic cations are arranged in the layers on an hexagonal framework of parameter a0 For particular values of x, superstructures can be expected by ordering of divalent and trivalent cations In hexagonal symmetry, the solutions are given by the relation 1/x = (a/a0)2, where a is

any metal-metal distance in the hexagonal framework The first solutions are x = 1/3, 1/4, 1/7, 1/9, 1/12, 1/13, … The survey of experimental data show that the existence of such superstructures seems clearly evidenced only in a few cases In other cases, the use of a stoichiometric formula is only a simplified formalization and, in fact, the studied compounds are essentially nonstoichiometric, with a random distribution of metallic cations in the layers

On the contrary, a particular value can be systematically observed in some systems, for example x = 1/3 in [Zn-Cr] based LDHs Such particular values are also often reported in minerals, mainly with x = 1/3 and x = 1/4 (Figure 2) These structural properties are discussed

in more detail in another chapter of this book

Metal cations in the layers

The divalent and trivalent metal cations found in LDHs belong mainly to the third and fourth periods of the periodic classification of the elements :

- divalent cations: Mg, Mn, Fe, Co, Ni, Cu, Zn,

- trivalent cations: Al, Mn, Fe, Co, Ni, Cr, Ga

A de Roy, C Forano and J.P Besse

Structure of the layers

In LDHs, the octahedral environment of metallic cations is far from being a regular polyhedron The octahedra are strongly flattened along the stacking direction, lowering the symmetry from Oh to D3d, as illustrated in Figure 3 for a [Zn-Al] based LDH The higher is the mean metal ionic radius, the more flattened are the octahedra with a lowering of the layer

thickness h and an increase of the distance a between metals – which is the same as between

OH groups on same side of the layer

Figure 3.-Flattening of the M(OH)6 octahedron in a [Zn-Al] LDH

This evolution is illustrated in Table 1 with results from X-ray Rietveld structure refinement, for two [Zn-Al] and [Zn-Cr] based LDHs The comparison of calculated and observed M-OH distances show that a simple geometrical model based on ionic radii is unable

to provide quantitative predictions It also appears clearly that the hydoxyl groups on each side

of the main layer do not build a really close-packing layout with such high OH-OH distances (≈ 3.1 Å)

If the radius of one of the metallic cations becomes too high, the octahedral coordination is lost by opening of one side of the octahedron on the interlamellar domain leading to additional coordination with one interlamellar water molecule The symmetry around the metal is lowered from D3d to C3v Such a behaviour is observed in minerals from the hydrocalumite group For [Ca-Al] based layers, three different short range distances are observed around calcium: 3 Ca-OH at 2.375 Å, 3 Ca-OH at 2.455 Å, and 1 Ca-OH2 at 2.497 Å

Interlamellar 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 weak bondings occur between these interlamellar ions or molecules and the host structure A great variety of anionic species can therefore be located between the layers during the formation of the lamellar structure, or by further anionic exchange

These anions can be:

- halides : fluoride, chloride,…

- oxo-anions : carbonate, nitrate, sulphate, bromate, …

- oxo and polyoxo-metallates : chromate, dichromate, (Mo7O24)6-, (V10O28)6-, …

- anionic complexes : ferro and ferricyanide, (PdCl4)2-, …

- organic anions : carboxylates, phosphonates, alkyl sulphates, …

In relation to the size, charge and layout of these interlamellar species, the basal spacing of the layers is dramatically modified as shown in Figure 4, which gives a selection of inorganic and organic species The most remarkable features are:

− the large gap between brucite and the smallest basal spacings in LDHs, corresponding to the intercalation of an interlamellar monolayer,

− the small distance range for a series of small anions - hydroxyl, fluoride, carbonate, chloride, attributed to a “levelling” effect of water molecules,

− he clear separation between inorganic anions intercalated LDHs with basal spacings lower than 15 Å, even for species such as the decavanadate, and hybrid LDHs intercalating organic anions and displaying distances larger than 15 Å

A de Roy, C Forano and J.P Besse

6

Interlamellar structure

The structure of interlamellar domains is more difficult to characterize than the main layers With small anionic species, such as halides and carbonates, and up to sulphate-containing LDHs with a basal spacing of 11Å, a regular stacking of the layers is observed in the X-ray diffractograms With bulky anions, the stacking of the layers displays in most cases no more long-range ordering (turbostratic effect) and the diffractograms show only lines relative to the basal spacing and to the structure of the main layers

As an example, we give here the results of X-ray Rietveld structure refinement on a [Zn-Cr-Cl] LDH (Table 2) The same R-3m space group was used for the first structural resolution on a monocrystalline LDH mineral.7 This space group is very “low cost” with only one refinable atomic position parameter for the main layers, and one more for the interlamellar domain, where chloride and oxygen of water molecules are randomly distributed on a high multiplicity position around the C3 axis As reported in Figure 5, the hydroxyl groups are facing one another between two succesive layers and the layout of interlamellar species is in agreement with hydrogen-bonding onto the main layers

Figure 4.-Evolution of basal spacing with intercalated anions

Figure 5.-Disposition of interlamellar species in a [Zn-Cr-Cl] LDH

This 3R rhombohedral stacking is also reported with other interlamellar halides or with carbonates For LDHs intercalating anions of tetrahedral shape such as sulphate, the structures are described in hexagonal space groups (P63/mmc and P63/mcm) leading to a 2H stacking of the layers.8

A de Roy, C Forano and J.P Besse

8

A Nomenclature for LDHs

A LDH phase is mainly described by its chemical formula, the basal spacing of the layers and the symmetry of the stacking sequence We have shown that the heavy general formula given in Figure 1 could be shortened as [MII-MIII-X] We propose now an extended symbolic notation n[MII-MIII-X]d

ss constituted of three parts :

- the qualitative chemical data between brackets, with divalent-trivalent anion symbols, in this order and separated by hyphens,

- on the left side, quantitative chemical data: trivalent metal ratio x and hydration state n

based on a M(OH)2 formula,

- on the right side, structural data: basal spacing d of the layers in Å and their stacking sequence ss (3R, 2H, …)

A great adaptability can be expected from this notation by the use of all or part of the full symbol, for example in a 0.33[Zn-Cr-Cl] phase, the [Zn-Cr] symbol refers to the framework regardless to the nature of the interlamellar anion.4

Coprecipitation Method

This is the most common preparative method of LDHs It is based on the slow addition of a mixed solution of divalent and trivalent metals salts in adequate proportions into a reactor containing water A second solution (alkaline solution) is added in the reactor in order to maintain the pH at a selected value leading to the coprecipitation of the two metallic salts A schematic experimental device is given in Figure 6 - all of the sub-systems are not required for every experiment

The metal cations in the obtained LDH phase are obviously issued from the metallic salts solution, but the origin of interlamellar anions has to be discussed If these anions are the counter-anions of the metallic salts they come from the same solution If the preparation is performed at very high pH values, the interlamellar anion can be the hydroxyl anion coming from the alkaline solution When the alkaline solution is a sodium or potassium carbonate solution, the intercalated anion is the carbonate because of its high selectivity for LDHs interlamellar domains Moreover, when the preparation is performed at relatively high pHs, one have to work under CO2-free conditions in order to avoid carbonate contamination Another way to intercalate a given anion is to prepare a solution of this anion in the reactor prior to the beginning of the coprecipitation

So, we see that there is often competition between several anionic species, and the control

of experimental conditions can lead to the selective intercalation of one of them as the primary interlamellar anion Secondary interlamellar anions can replace them by further treatments

such as anionic exchange (vide infra)

A de Roy, C Forano and J.P Besse 10

Figure 6.-Experimental device for the preparation of LDHs by the coprecipitation method

Experimental Parameters

Depending on the precipitation conditions, one can generally obtain well crystallized LDH phases or quasi amorphous materials Some of these experimental parameters are obvious, like:

- temperature in the reactor,

- pH of the reaction medium,

- concentration of metallic salts solution,

- concentration of alkaline solution,

- flow rate of reactants,

- aging of the precipitate,

other parameters are less obvious, such as :

- accumulation of electrolytes in the reaction medium,

- hydro-dynamics of the dilution of reactive species, related to the stirring mechanism, geometry of the reactor including reactants injection pipes,

- complexation state of the metallic cations which, depending on the previous history of the metallic salts solutions, can give rise to a great number of different M(OH2)u(OH)vXwcharged monomers and also to oligomers

In order to obtain well organised phases, the operating conditions have to be optimized for each system In all cases, synthetic LDHs are obtained as microcrystalline platelets generally

3R 0.33 3Rcrystallized or amorphous materials are obtained at pH values higher than 5.0 An X-ray diffraction study of LDH prepared from pH = 4.5 to 10 (Figure 7) shows an improvement in the crystallinity when the pH is lowered It should be notice that, in this case, the optimal pH of 4.5

is lower than the pH of precipitation of Zn(OH)2, and therefore the coprecipitation does not occur in these conditions The reaction must probably proceed via the precipitation of Cr(OH)3, and then the reaction of Cr(OH)3 with Zn2+ cations in solution The chemical composition of the final product diverges from the initial M2+ and M3+ ratio

Figure 7.-X-ray powder diffractograms of 0.33[Zn-Cr-Cl]3R prepared at pH=10.0 (a) and pH=4.5 (b) Reprinted from A de Roy, C Forano, K El Malki and J.P Besse, Anionic Clays: Trends in Pillaring Chemistry,

in Expanded Clays and Other Microporous Solids, edited by M.L Occelli and H.E Robson (Van Nostrand Reinhold, New York 1992), vol II, Chap 7 pp 108-169, reproduced with permission from the authors The effect of the pH upon the formation of 0.33[Cu-Cr-Cl]3R is similar The best crystallized phase is obtained at the lowest pH value of 5.5 Below this pH, an additional unidentified phase appears It must be noticed that, in some cases, the pH does not have a direct observable effect

on the diffraction pattern The typical example comes from the 0.33[Ni-Cr-Cl] phase This phase

A de Roy, C Forano and J.P Besse 12

displays the same powder X-ray diffraction patterns characteristic of a quasi-amorphous material, whatever the pH of precipitation from 5.5 to 11.5 However, under hydrothermal treatment, the only phase that crystallizes is the LDH prepared at pH = 11.5

However, no clear relationship seems to exist between the individual precipitation pH of metals and the precipitation pH of mixed hydroxides (Figure 8) It seems reasonable to think that, for chromium-based LDH compounds, the intermediate pH range from 6.0 to 10.0 does not favor the formation of a well-ordered phase, probably because of the less reactivity of Cr3+ions, which form a great number of oligomeric complexes in this domain On the other hand, real coprecipitation conditions are not respected for 0.33[Zn-Cr-Cl]3R, 0.33[Cu-Cr-Cl]3R, and sometimes [Mg-Al-Cl] 3R

The coprecipitation method was extensively used to prepare new LDH in recent years From the point of view of the chemical composition, we will review what is possible to put in

A very wide range of compositions may be obtained by synthesis; for instance, even

Figure 9.-Association of divalent and trivalent metallic cations in LDHs ( : monovalent; : tetravalent) Adapted from[4]

Which anions ?

A very wide range of anions are reported in the literature :

- inorganic anions (halides and oxyanions such as CO32-, NO3-, SO42-, OH-, CrO42-, WO42-,

S2O32-, etc.) ;

- isopolyanions (V10O286-, Mo7O246-, etc.) and heteropolyanions (PMo12O403-, PW12O403-, etc.) ;

- complex anions (Fe(CN)63-, Fe(CN)64-, etc.), and organometallic complexes ;

- organic anions (carboxylates and dicarboxylates, benzene carboxylates, alkylsulfates, chloro-cinnamate, etc.) ,and

- layered compounds, such as the mineral chlorite (Mg,Fe,Al)6[(Si,Al)4O10](OH)8

The major problem in obtaining pure LDH phases arises from contamination by easily intercalated carbonate anions, which must be prevented by using very strict CO2-free conditions On the other hand, the pH range where the anion is stable must overlap the pH domain of formation of the layered double hydroxide For example, Keggin anions PW12O403-and SiW12O404- are not stable at a pH greater than 5.0, preventing direct precipitation of [Zn-Al] LDH, usually obtained at pH ≈ 9.0

A de Roy, C Forano and J.P Besse 14

Chemical analysis of LDH usually confirms that all metal cations are precipitated and, provided a suitable pH of precipitation is used, the initial MII/MIII ratio is always retained A difficult problem to deal with when one wants to determine the chemical composition of an LDH is to know whether all of the metal cations have been precipitated as LDH, and often, microscopic chemical analysis has not been performed on the samples However, for a well-ordered LDH, the deviation from the initial MII/MIII ratio is low and the prepared LDH must be considered as highly pure One method often neglected to characterize the domain of composition where the LDH exists as a pure phase is to measure the variation of the cell

parameters (a and c for hexagonal unit cell) with the composition Most of the mixed double

hydroxides can be prepared with a variable MII/MIII ratio Table 3 gives the domain of composition of this series, with comparable results reported in the literature

Table 3.- Chemical composition range of various LDH

M II -M III -X pH formation M II /M III (R) range

- 10.0 9.0 9.0

1.0 ≤ R ≤ 5.0 1.0 ≤ R ≤ 3.0 1.0 ≤ R ≤ 3.0 1.0 ≤ R ≤ 2.0 1.6 ≤ R ≤ 2.3 1.7 ≤ R ≤ 2.3 1.0 ≤ R ≤ 3.0

R ≈ 2.0 2.0 ≤ R ≤ 3.0 2.7 ≤ R ≤ 5.6 1.0 ≤ R ≤ 3.0 1.8 ≤ R ≤ 4.0 1.0 ≤ R ≤ 3.0

We can see, here again, the determining effect of the pH value For instance, PXRD study

of the [Zn-Al-Cl]3R samples series shows that, at a neutral pH, only the LDH phase crystallizes, the X-ray pattern being even better when Zn2+/Al3+ ≈ 3 At pH = 10.0 for a M2+/M3+ ratio ≥ 3, [Zn-Al-Cl] coexists with Zn(OH)2, while for a ratio ≤ 1 the excess of Al3+ ions crystallize as bayerite, Al(OH)3; the best crystalline phase is obtained for Zn2+/Al3+=3, whatever the pH In the case of pure [Ni-Cr-Cl]3R and [Ni-Cr-CO3]3R phases, LDH with ratio comprised from 1.0 to 3.0 and 1.0 to 2.0, respectively, have been obtained only after hydrothermal treatment

In many cases, optimisation of the pH value of the coprecipitation, and the aging time, does not lead to well crystallized LDH phases, and thermal treatment often gives good results for improving the crystallinity of the amorphous xerogel or the badly crystallized materials

• Temperature of coprecipitation Most of the precipitations are carried out at room temperature, and sometimes near reflux conditions are used to favour the crystallization, but no significant effect of the reaction temperature has been reported Hydrothermal treatment after precipitation is often more efficient

• Hydrothermal treatment In most of the cases, hydrothermal treatment in the presence of water vapour strongly improves the crystallinity of the LDH, provided the temperature of decomposition of the LDH is not exceeded Two typical experiments are usually performed The most accessible one consists of heating a closed stainless steel reactor containing an aqueous suspension of the LDH precursor at a temperature below the critical point under autogenous pressure Another method consists of heating the sample in a gold or silver sealed tube under a high pressure of the order of 1500 bars Synthetic takovite [Ni-Al-CO3]3R was prepared by treating quasi-amorphous precursors at 200°C under 1500 bars for 10 days [Ni-Cr-X]3R LDH with X = Cl-, CO32-, SO42-, and Ni/Cr = 1.0, 1.5, and 2.0, were obtained in an amorphous state by coprecipitation of the mixed nitrate salts at constant pH = 13.0, and thermally treated at 300 °C and 1500 bars pressure for 18 h, in order to obtain materials with sharp diffraction lines

Addition rate or aging

The addition rate or ageing are two determining factors that affect the crystallinity of the mixed double hydroxides Even if people agree on the fact that ageing or slow addition rate must be performed in order to prepare a well-crystallized phase, only a few systematic studies have been reported on the influence of these parameters on the chemical composition, morphology, or crystallinity With an automatic titration device, it is possible to impose a slow addition rate of about 1 ml/h 48 h ageing is often necessary to obtain a high crystallinity But the conditions of ageing must be adapted to the nature of the LDH to be obtained; [MII-MIII-NO3] will need a longer ageing time than carbonate LDH

A de Roy, C Forano and J.P Besse 16

The urea method

Urea has a series of properties that makes its use as an agent for precipitation from

"homogeneous" solution very attractive and it has long been used in gravimetric analysis to precipitate several metal ions as hydroxides or as insoluble salts when in the presence of a suitable anion.15 Urea is a very weak Brönsted base (pKb = 13.8), highly soluble in water, and its hydrolysis rate may be easily controlled by controlling the temperature of the reaction According to Shaw and Bordeaux,16 the mechanism of hydrolysis consists of the formation of ammonium cyanate, as the rate determining step, and the fast hydrolysis of the cyanate to ammonium carbonate, i.e.:

CO(NH2)2 → NH4CON

NH4CNO + 2H2O → (NH4)2CO3The rate constant increases by about 200 times when the temperature is increased from 60

to 100°C The hydrolysis of ammonium to ammonia and carbonate to hydrogen carbonate gives

a pH of about 9, depending on the temperature This pH is suitable for precipitating a great number of metal hydroxides

After some preliminary positive tests, a wider investigation was undertaken to find the optimal conditions to produce, with a simple procedure, LDH microcrystals of uniform size, well crystallised, and with the required stoichiometry The following couples of cations, as chlorides, were considered : Mg(II)-Al(III) ; Zn(II)-Al(III) ; Ni(II)-Al(III) In the case of Zn(II),

a weighed amount of ZnO was dissolved in a stoichiometric amount of 6 mol/dm3 HCl solution.17

LDHs with two divalent or trivalent metal cations

The flexibility of the LDH structure is clearly demonstrated by the fact that pure compounds with two divalent metals may be obtained by coprecipitation method So, Ni/Mg/Al LDH with various Ni/Mg ratio are synthesized and are precursors of high-surface-area Ni/Mg/Al mixed oxides with many catalytic applications.18 Also LDH are obtained with two trivalent cations, for example Mg(Al, Y).19

Other Preparative Methods and Comparison

Induced hydrolysis

The induced hydrolysis20 is a two step method The trivalent metal hydroxide is first precipitated by an alkaline solution The second step consist in the slow addition of this precipitate on a solution of the divalent metal salt at a constant pH, inducing a controlled release of trivalent metal species and formation of the LDH phase

MIIO + xMIIIXm-3/m + (n+1) H2O → MII

1-xMIIIx(OH)2Xm-x/m · nH2O + x MIIXm-2/mThe nature of the reagents leads us to name this synthesis the "salt-oxide method"

The [Zn-Cr-Cl] system

The "salt-oxide method" is a simple solid-liquid reaction Experimentally, this method consists of adding small volumes of a 1M solution of chromium chloride at constant periods of time to a 3-5 wt % ZnO aqueous suspension under vigorous stirring at a given temperature pH recording during the addition of the trivalent salt (Figure 10) allows one to follow the reaction progress Drops in pH at each addition of the acid salt, followed by a return to the initial pH value of the buffering zinc oxide, are well shown The reaction is completed when an excess of CrCl3aq. no longer reacts

This step is evidenced by the equivalent point on the potentiometric curve, and the pH value remains constant at about 4.0 after a further addition of CrCl3aq. X-ray diffraction studies

of small fractions of the suspension taken off at various titration points show the disappearance

of ZnO and the development of the diffraction lines of the LDH phase

A quantitative X-ray analysis allows to calculate the stoichiometric coefficient of the reaction when plotting the molar ratio ZnO/CrCl3 versus diffraction line intensity ratios :

The corresponding equation of the reaction is given by:

3ZnO + CrCl3 + (n+3) H2O → Zn2Cr(OH)6Cl·nH2O + ZnCl2The chemical analysis of the [Zn-Cr-Cl] phase so obtained confirms the definite ratio Zn/Cr = 2, previously proposed by Boehm, Steinle, and Vieweger.21 Attempts to vary the Zn/Cr ratio in this way were unsuccessful The relatively low weight fraction of ZnO (2-5 wt % range) and the slow addition of the CrCl3 solution or long aging time in contact with the mother liquor greatly improve the crystallinity of the double hydroxide (Figure 11)

A de Roy, C Forano and J.P Besse 18

Figure 10.-Stepwise potentiometric titration of a MIIO suspension by a MIIICl3 solution Reprinted from A de Roy, C Forano, K El Malki and J.P Besse, Anionic Clays: Trends in Pillaring Chemistry, in Expanded Clays and Other Microporous Solids, edited by M.L Occelli and H.E Robson (Van Nostrand Reinhold, New York 1992), vol II, Chap 7 pp 108-169, reproduced with permission from the authors

From the point of view of the mechanism, the reaction seems to proceed first via an acidic hydrolysis of ZnO during the addition of CrCl3aq and then a coprecipitation of the mixed Zn/Cr double hydroxide, the formation of which is favoured at a pH range of 4 to 7 Following this procedure, the pH value always varies during the addition, and may affect the homogeneity of the product The experimental device was modified in order to fix the pH at a constant value during the experiment This can be done by regulating the addition of the solution of CrCl3 This method, which operates at constant reactivity of of CrCl3, does not allow us to prepare pure 0.33[Zn-Cr-Cl]3R LDH, and small amounts of ZnO still remain in the final product

Figure 11.-X-ray powder diffraction patterns (Cu K ) for 0.33[Zn-Al-Cr]3R, 0.33[Zn-Cr-Cl]3R and 0.33[Cu-Cr-Cl]3R Reprinted from A de Roy, C Forano, K El Malki and J.P Besse, Anionic Clays: Trends in Pillaring Chemistry, in Expanded Clays and Other Microporous Solids, edited by M.L Occelli and H.E Robson (Van Nostrand Reinhold, New York 1992), vol II, Chap 7 pp 108-169, reproduced with permission from the authors

pH drops from 6.8 after the first addition of CrCl3, and then remains constant at ca 4.5 The disappearance of CuO in the diffractograms (Figure 11) occurs at a Cu/Cr ratio near 2, and the phase is characterized by the chemical formula : Cu2Cr(OH)6Cl·nH2O.23

• Others attempts to prepare new LDH phases or LDH with definite stoichiometry by this method were unsuccessful With Zn/Fe/Cl, Cu/Al/Cl or Cu/Fe/Cl systems, mainly hydroxichloride phases are formed, respectively Zn5-xFex(OH)8 Cl2+x.H2O [22], a phase with the structure of the simonkolleite, Zn5(OH)8Cl2.H2O,24 and Cu2(OH)3Cl, the paratacamite.25With the Mg/Cr/Cl system, a new metastable phase of composition MgxCr1-x(OH)3-3xOx/2 21 is prepared, which displays the structural properties of the bayerite β-Al(OH)3 LDH phases based on divalent metals Ni ([Ni-Cr-Cl], [Ni-Al-Cl], [Ni-Fe-Cl]) or Co ([Co-Fe-Cl]) were not prepared by this way because of the unreactivity of the respective oxides NiO and CoO

Comparison of Preparative Methods

While the three methods - coprecipitation, induced hydrolysis and salt-oxide, seem to be quite different, the mechanism for the construction of the LDH structure is probably the same; this can also be extended to the reconstruction method described later In all cases the aim is to obtain in the reactive medium each constituent of the future LDH structure in the most appropriate concentration and association state The “best method” does not exist; depending

on the studied system and the final use of the product, one or another method has to be chosen

The physicochemical properties and the reactivity of the LDHs are determined not only by the chemical nature of the overall structure, but also largely by the tight interactions between the host matrix and the guest species

Subsequent treatments can modify as prepared LDHs The most obvious is anionic exchange, but moderate thermal treatments and even washing and drying processes can also give rise to new materials

A de Roy, C Forano and J.P Besse 20

Anionic exchange

The lamellar structure of LDH, based on a stacking of positive layers trapping anionic species in the interlayer domains, is highly favourable to anion diffusion, and LDH is one of the principal classes of inorganic ion exchangers.26 This property has been mainly used in order to prepare new LDH phases by anionic exchange reactions The reaction can be described by the equilibrium:

[MII-MIII-X] + Y → [MII-MIII-Y] + X This thermodynamic system is bivariant in isothermal and isopressure conditions It can be completely described by two extensive parameters which resume in a unique X’1 = f(X1) relation if the total concentration of the anion in the liquid phase is retained constant (X’1 is the molar fraction of the anion to be intercalated in the LDH, and X1 its molar fraction in the liquid solution) In 1983, Miyata reported ion-exchange isotherms at 25°C of [Mg-Al-X/Y] for a series of monovalent and divalent anions All isotherms display a sigmoid shape arising from a mixed continuous composition range of the anions in the LDH The PXRD studies that we performed during exchange of various systems [MII-MIII-X/Y], and more particularly of [Zn-Cr-Cl/Y] and [Zn-Al-Cl/Y] (Y = F-, Br-, I-), did not evidence any continuous variation of the basal spacing with increase of the molar fraction in the liquid phase of the anion to be intercalated Non-miscibility of the different anions in LDH and the short range of coexistence

of two LDH phases were observed

Thermodynamically, exchange in LDH depends mainly on the electrostatic interactions between positively charged hydroxylated sheets and the exchanging anions and, to a lower extent, on the free energy involved in the changes of hydration.27 Another important remark was that the equilibrium constant increases when the ionic radius of the bare anion decreases Exchange is therefore favoured for in-going anions with a high charge density From calculations of the equilibrium constant of various exchange reactions, Miyata12 gave a comparative list of ion selectivities for monovalent anions: OH- > F- > Cl- > Br- > NO3- > I- and divalent anions: CO32- > C10H4N2O8S2- > SO42- For [Cu-Al-X/Y] systems, Yamaoka et al.28determined an equivalent selectivity sequence for divalent oxoanions: HPO42-, HAsO42-, > CrO42- > SO42- > MoO42- Moreover, it appears that the selectivities of divalent anions are higher than those of monovalent anions According to these results, nitrate- and chloride-containing LDHs appear to be among the best precursors for exchange reactions Such anions can easily be replaced by more selective anions, such as Fe(CN)62-, Fe(CN)63-, Mo(CN)84-, or IrCl62- Organic anions with long chains can also be directly intercalated by exchange reactions on LDH precursors with inorganic anions, for example, n-CmH2m+1SO4- (n

= 8, 12, 14, 16, 18) on 0.33[ZnCr-Cl]3R21 and 5,10,15,20-tetra(4-sulphonatophenylporphin) on [Mg-Al-Cl]3R.29 In this way, a large variety of organic anion-containing LDHs have been prepared.30,31

From a kinetic point of view, the rate-determining step of the reaction is the diffusion of the in-going anions within the interlayer, provided the "infinite solution conditions" are respected The diffusion of big anions inside the interlayer can be prevented by a too small basal spacing

of the precursor Exchange reactions via organic-anion-pillared precursors are then used Intercalation of bulk polyoxometalate anions, such as Mo7O246-, V10O286-, or H2W12O406-,was

more easily Higher pH values (10.0-12.0) strongly favour intercalation of carbonate, and a

CO2-free atmosphere must be used if carbonation is to be prevented

Therefore, this method of synthesis remains an easy pathway to prepare synthetic anionic clays, and we succeeded in preparing a large number of LDH [Zn-Al-X]3R, 0.33[Zn-Cr-X]3R, 0.33[Cu-Cr-X]3R, and [Ni-Cr-X]3R, where X can be a halide or an oxoanion Total exchange was observed for anions, except for ClO3-, IO3-, ReO4- and ClO4-.36,37

Washing and drying process

After preparation, the LDH precipitate has to be separated from the reactive medium, washed and dried The following example corresponding to 0.33[Zn-Cr-SO4] phases (Figure 13) shows that the operating conditions can strongly modify the obtained LDH

In the 3D diagram where the basal spacing d, the temperature of thermal treatment t and the

relative humidity (RH) at room temperature %RH are reported, five structural varieties could

be identified:

- a 2H “11Å” phase which intercalate the sulphate groups and also alkaline cations (sodium

or potassium) surrounded by water molecules,

- a 3R “10.9Å” phase without alkaline cations obtained by washing of the previous one,

- a 2H “8.9Å” phase corresponding to a partial dehydration of interlamellar domains,

- a 3R “8.3Å” phase corresponding to severe dehydration at room temperature

All of these phases can be reversibly obtained This points out that the washing and drying processes can be considered as post-synthesis treatments In this example, the “as-prepared” phase in the reactive medium is always the 2H "11 Å" phase.38

A de Roy, C Forano and J.P Besse 22

Figure 12.-Schematic representation of polyoxometalate exchange via terephthalate compound in an LDH

Grafting of anions onto LDH layers

The last 3R “7.1 Å” phase in Figure 13 diagram corresponds to an irreversible transformation of the structure with no more rehydration or further anionic exchange capacity Such transformations are related to moderate thermal treatments of several LDHs intercalating tetrahedral oxoanions such as sulphates, selenates, phosphates, chromates and dichromates, etc The low basal basal spacing is incompatible with the presence of “free” (XO4)2- anions The only consistent hypothesis is the grafting39 of the anions onto the LDH layers with elimination of water molecules, leading to neutral layers The new phase has to be considered as a layered oxy-hydroxy-salt and belongs no longer to the LDHs family This behaviour was also recently evidenced by XAFS.40

Figure 13.-Structural transformations of a 0.33[Zn-Cr-SO4] LDH related to the washing process, the relative humidity of atmosphere (%RH), and temperature, t °C

Examples: Modification of oxoanions intercalated LDHs

The structure of the 0.670.32[MII-MIII-X]8.922H interlamellar domains is described as an ordered arrangement of alternatively inverse interlayer SO4 tetrahedra, retaining their C3 axis perpendicular to the layer, with one oxygen pointing to a metallic cation of a brucite-like layer and the three other facing three OH groups of the opposite layer The short hydrogen bonds lengths of, respectively, 2.93 Å and 2.71 Å, reveal strong interactions between layers and sulphate anions and lead to the 8.92 Å observed basal spacing

Sulphate and chromate anions have the same size, with anionic radii of 2.42 Å and 2.44 Å, respectively.23 The dichromate anion is a simple condensation of two CrO4 groups witha Cr-O-Cr angle close to 126° (in K2Cr2O7) and displays, with such a stereochemistry a modelled hindrance of roughly 5.5 x 2.8 Å, but flexible geometry of the Cr-O-Cr bonding can be

expected The similar d values obtained for the Cr2O7 and XO4 LDH phases must account for a similar orientation of the anions, Cr2O7 lying flat and parallel to the layers This can ensure optimal interactions of all polyhedra oxygen atoms with the OH groups

The analysis of the structural data enable two different modes of anion-layer interactions to

be differentiated:

A de Roy, C Forano and J.P Besse 24

One first group of LDHs ([Zn-Cr-SO4], [Zn-Al-SO4], [Cu-Cr-SO4], [Zn-Al-CrO4], [Cu-Cr-CrO4], [Zn-Cr-Cr2O7] and [Cu-Cr-Cr2O7]) display, as prepared, basal spacings near the

way to the SO42- ions in the reference material Among this group, [Zn-Cr-SO4] and [Zn-Al-SO4] appear to be structurally stable under usual storing conditions

Other phases of this group ([Zn-Al-CrO4], [Cu-Cr-CrO4] [Zn-Cr-Cr2O7] and [Cu-Cr-Cr2O7]) undergo a spontaneous interlayer contraction, leading to d values ranging from

7.90 Å to 7.34 Å and are referred to as "aged phases" This is the case for [Cu-Cr-CrO4]7.68; this phase is the result of the ageing in air of [Cu-Cr-CrO4]8.42 Freshly prepared [Cu-Cr-Cr2O7] also undergoes a contraction process (from 8.95 Å to 7.87 Å) to a structurally stable LDH [Zn-Al-CrO4] has also been demonstrated to contract after ageing in its mother solution These phases define a second group of contracted LDHs Two other freshly prepared LDHs, [Zn-Cr-CrO4]8.13 and [Zn-Al-Cr2O7]7.85, show short interlayer spacings and can be included in this second group For all of these contracted phases, intercalated oxo-anions remain exchangeable, by chloride anions for example In these layered structures, the shortening of the interlayer distances is then no longer compatible with "free" oxo-anions lying as described in

the former phases

Further investigations of the LDH contraction properties have been carried out by a study

of the effect of thermal treatment on the LDH structure Moderate heating in air during 24 h was performed at 150°C, temperature at which dehydroxylation has generally not yet occurred in chloride LDHs All the phases undergo a further contraction which then appears irreversible, since anion exchange is no longer possible The new phases still display PXRD patterns typical

of LDHs The basal spacings are now included in a narrower range, from 6.80 Å to 7.20 Å, for all the oxo-anion intercalated LDHs and [Cu-Cr-Cl] For the other chloride phases, [Zn-Cr-Cl] and [Zn-Al-Cl], the shortening is of lower amplitude, leading respectively to basal spacings of 7.43 Å and 7.63 Å

Analogous short basal spacings are found for lamellar zinc and copper basic salts, in which

OH groups of the hydroxylated layer are partially substituted by planar XO3 or tetrahedral XO4 oxo-anions.41,42 On heating, the LDH precursors undergo a permanent pillaring of the

oxo-anions on the hydroxylated layers The experimental d values fit well with calculations

based on a LDH structure where some of the OH groups have been replaced by the grafted

SO42-, CrO42- and Cr2O72- anions as shown in Figure 14 It should be noted that, in such structural configuration, Cr2O72- anions must be grafted via two oxygen atoms

The contracted LDH phases can now be regarded as "pre-grafted" phases in which the anions come nearer to the metallic cations of the layer, probably pointing one apical oxygen atom of the mono or di-tetrahedra towards the triangular apertures of the OH close-packed monolayers, then minimising the interlayer distances

From a mechanistic point of view, the grafting process necessarily occurs simultaneously with partial dehydroxylation and the loss of water molecules, and then leads to the formation of new pillared lamellar structures with neutral layers

For the [Cu-Cr] compounds, the evolution of interlayer distances was studied with the temperature of the thermal treatment, from room temperature up to 150 °C (Figure 15)

[Cu-Cr-Cl] displays a contraction of greater amplitude than those observed in the other host structures This contraction is no longer compatible with free interlayer chloride, but it is too low to consider replacements of OH groups of the main layer by Cl- whose larger size prevents its incorporation among the coplanar hydroxyls It follows that the octahedral

2 7

Figure 14.-Schematic view of LDH phases grafted with XO42-, and X2O72- oxoanions Reprinted from C Forano, A De Roy, D Depège, M Khaldi, F Z El Métoui and J P Besse, Post-synthesis modification of layered double hydroxides, in Synthesis of Porous Materials Zeolites, Clays, and Nanostructures, edited by M

L Occelli and H Kessler (Marcel Dekker, Inc., New York 1996), pp 607-625

A de Roy, C Forano and J.P Besse 26

These structural post-synthesis modifications lead to an interesting improvement of the thermal stability and to an increase of the specific surface area of the calcined phases compared

to the chloride phases.43

Figure 15.-(a) Effect of heating on the basal spacings of [Cu-Cr-X] LDHs (X=Cl, SO4, CrO4, Cr2O7) Reprinted from C Forano, A De Roy, D Depège, M Khaldi, F Z El Métoui and J P Besse, Post-synthesis modification of layered double hydroxides, in Synthesis of Porous Materials Zeolites, Clays, and Nanostructures, edited by M L Occelli and H Kessler (Marcel Dekker, Inc., New York 1996), pp 607-625 (b) PXRD of [Cu-Cr-SO4] at various temperatures

Polycondensation of Silicate in [Zn-Al] and [Zn-Cr] LDHs

The limited size of the oxometalate anions, their low charge density, and the high charge density in the LDHs main sheets, do not allow to obtain consequent interlamellar microporosity.43 Indeed, the basal spacing for the oxopolyanions containing LDH never surpasses 14 Å, while intercalation of Al13 hydroxo-cations in cationic clays, for example, yields a greater interlamellar expansion.44 In order to obtain new LDH microporous materials, high expansions of the LDH sheets are needed, so pillaring of LDHs by silicate isopolyanions has been investigated

Intercalation of Silicate Species in [Zn-Cr] and [Zn-Al] LDH45

Intercalation of silicate species in [Zn-Cr] and [Zn-Al] compounds has been performed either by anion exchange reactions or by coprecipitation Such reactions occur provided the pH

of the solution is greater than 9.0, value above which silicates are soluble; otherwise, mixtures

of amorphous silica and metallic hydroxides, far from the LDH chemical composition, are obtained PXRD patterns of such phases are related to typical LDH diffractograms (Figure 16) According to the method of preparation, either a phase with a low interlayer spacing (typically