Pillared Clays and Clay Minerals

Pillared Clays and Clay Minerals

Chapter 10.5 Pillared Clays and Clay Minerals M.A. Vicentea, A. Gilb and F. Bergayac a Departamento de Quı´mica Inorga´nica, Universidad de Salamanc...

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Chapter 10.5

Pillared Clays and Clay Minerals M.A. Vicentea, A. Gilb and F. Bergayac a

Departamento de Quı´mica Inorga´nica, Universidad de Salamanca, Salamanca, Spain Departamento de Quı´mica Aplicada, Universidad Pu´blica de Navarra, Pamplona, Spain c CRMD, CNRS-Universite´ d’Orle´ans, Orle´ans Cedex 2, France b

Chapter Outline 10.5.1. Pillaring Concept 524 10.5.2. Pillared Clay Minerals and Catalysis 524 10.5.3. IUPAC Definition of Pillaring and Pillared Clay Minerals 525 10.5.4. Host Clay Minerals 526 10.5.5. Pillaring Species 528 10.5.5.1. (Al13)7 þPillaring Agent 528 10.5.5.2. Other Pillaring Agents 531 10.5.5.3. Mixed Al–M and M–M0 Pillaring Agents 532 10.5.5.4. Pillaring Agents with More than Two Cations 533 10.5.5.5. New Pillaring Agents 533

10.5.6. Pillaring Methods 534 10.5.6.1. Pillaring in Dilute Dispersions 534 10.5.6.2. Pillaring in Concentrated Medium 536 10.5.7. Main PILC Characteristics for Several Applications 539 10.5.8. Intercalant Stability Before and After Pillaring 540 10.5.9. Linkage Between Pillars and Clay Mineral Layers 541 10.5.10. Mathematical Modelling 542 10.5.11. Conclusions 544 References 544

Developments in Clay Science, Vol. 5A. http://dx.doi.org/10.1016/B978-0-08-098258-8.00017-1 © 2013 Elsevier Ltd. All rights reserved.

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10.5.1 PILLARING CONCEPT The term ‘pillaring’ is often associated with the formation and preparation of catalytically active microporous materials. Thus, many reviews devoted to pillared interlayered clays (PILC1) have been published in catalysis journals (Pinnavaia, 1984; Burch, 1988; Figueras, 1988; Fripiat, 1997; Gil et al., 2000a, 2008) although other summaries have appeared in journals on porous materials (Kloprogge, 1998; Gil et al., 2000b; Ding et al., 2001) and as chapters or subchapters in books on clay minerals (Bergaya, 1990; Bergaya et al., 2006; Gil et al., 2010). Most of these publications deal with the history, synthesis, properties and applications of the PILC. The pillaring concept has been extended to other layered materials (Vaughan, 1988a; Mitchell, 1990) such as anionic clay minerals or layered double hydroxides (LDH) (see Chapter 14.1) and/or manganese oxides (Wong and Cheng, 1993). However, this chapter is an attempt to summarize the vast volume of literature limited to cationic clay minerals that has accumulated over the last 40 years.

10.5.2 PILLARED CLAY MINERALS AND CATALYSIS That clays can function as catalysts had been known for a very long time (Robertson, 1986). For example, Gurvich (1915) used palygorskite (attapulgite) and montmorillonites (Mts) to polymerize pinene. The most widely known application of clay catalysis is the French Houdry cracking process developed in the 1930s (Houdry et al., 1938). By the 1960s, clays had been replaced by synthetic zeolites (Breck, 1980) because the latter show better activity and selectivity for cracking. However, PILC, which may be considered as two-dimensional zeolite-like materials, began to compete with zeolites in the 1970s. Intensive research into the synthesis of suitable cracking catalysts for heavy petroleum was stimulated by the ‘oil shock’ of 1973. This is because zeolite catalysts have a limited micropore size and are rapidly poisoned by metals in the heavy oil fractions. Since the first announcement of the commercial availability of PILC (Vaughan et al., 1979), their use in petroleum cracking alone has exceeded that of other catalysts (Adams, 1987). PILC have also found applications in environmental protection and as molecular sieves, selective adsorbents (Molinard and Vansant, 1995; Karamanis et al., 1997; Tahani et al., 1999; Pires et al, 2008), thermal insulators, and electrochemical and optical devices (Mitchell, 1990).

1. The term PILC is not strictly correct. Since it is the clay mineral that is pillared, the term PILCM (for pillared interlayered clay mineral) should be used. However, analogous to using clay for clay mineral (see Chapter 1), PILC will continue to be used on the basis of past usage.

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Research into the synthesis of PILC builds on the pioneering work by Barrer and MacLeod (1955) who obtained microporous materials by replacing the interlayer exchangeable cations in smectites with tetraalkylammonium ions (Barrer, 1986). The synthesis of the first organic pillared clay minerals with an interlayer distance of 0.5–0.6 nm and a permanent porosity was independently reported by Mortland and Berkheiser (1976) and Shabtai et al. (1976). However, organic pillared clay minerals of this type are thermally unstable at high temperatures (>250  C) when interlayer collapse occurs, as in pristine (unmodified) clay minerals. However, below the decomposition temperature, organic pillared clays can usefully serve as molecular sieves, selective adsorbents and catalysts. Clay minerals intercalated with inorganic species retain their micro- and meso-porosity after heating at  300  C. Although polymeric (hydr)oxy metal cations have been used as pillaring agents for clay minerals since the late 1960s (Sawhney, 1968), the paper by Brindley and Sempels (1977) is regarded as the first account of the preparation of an inorganic pillared clay mineral. The procedure consists of exchanging the Naþ ions in the pristine smectite with oligomeric (hydr)oxy aluminium cations, which are then converted by heating into aluminium oxides. By propping the smectite layers permanently apart, these aluminium oxide species act as pillars in the interlayer space. Here we give the recent IUPAC2 definition, followed by a brief summary of the fundamental principles of pillaring, which includes a description of the clay minerals used, the pillaring species, the properties of the PILC products and the possible modes of bonding between pillar and clay mineral surface. The numerous applications of PILC are not described here. For their industrial and environmental applications, see Chapters 4.3 and 5.1 in Volume 1B.

10.5.3 IUPAC DEFINITION OF PILLARING AND PILLARED CLAY MINERALS Bergaya (1990) suggested that three criteria must be met for successful pillaring: (i) a preliminary step of quasi-reversible intercalation of various species (charged and/or uncharged, organic or inorganic) should occur causing up to a fivefold increase in basal (d001) spacing of the clay mineral by X-ray diffraction (XRD); (ii) after heating at high temperatures, this spacing may decrease slightly but must not collapse, and its final value must be maintained; and, the last and fundamental criterion, (iii) the product must have an accessible porosity, that is, the intercalated species must not fill the entire interlayer space as in chlorite. However, subsequent research into the pillaring of various layered materials, for several applications, led Bergaya (1994) to modify these conditions slightly. The three new criteria for pillaring are (i) intercalation, generally by exchanging the interlayer inorganic cations with cationic pillars, increasing 2. IUPAC: International Union of Pure Applied Chemistry.

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the d001 value by at least 0.7 nm, yielding a structure with properties resembling those of zeolites in their applications; (ii) a free height, which is not always correctly evaluated by porosity measurements (Bergaya et al., 1993a; Bergaya, 1995); and (iii) basal spacing and free height that do not change when the material is heated to at least 200  C and in some cases up to 700–800  C, under anhydrous or hydrothermal atmosphere, or when the pH is varied, placed in acidic or basic solutions. The latest technical report of the IUPAC (Schoonheydt et al., 1999) gives a similar definition of pillared clays (enlarged to pillared layered solids): . . . pillaring is a process by which a layered compound is transformed in a thermally stable micro- and/or mesoporous material with retention of the layer structure. The report adds that a pillaring agent is any compound that maintains the distance between adjacent layers upon removal of the solvent and induces an experimentally observable pore structure between the layers called interlayer region accessible to molecules at least as large as N2. Thus, a pillared material must fulfil at least three criteria: (i) chemical and thermal stability, without further specifying the chemical or temperature conditions; (ii) a certain layer ordering that enables at least a d001 value (basal spacing) to be determined, although a rational series of basal reflections is not necessary; and (iii) accessibility of the interlayer space to molecules at least as large as N2. Thus, N2 adsorption must be carried out, and analysed via micropore size pore distribution; however, no order in the pillars and, consequently, in the pores is required. It should be added that various abbreviations were used to denote pillared materials, in particular pillared clay minerals. The most widely used are PILC, but pillared layered structures (PLS) and cross-linked smectites (CLS) were also used (Lahav et al., 1978; Pinnavaia et al., 1985; Sterte and Shabtai, 1987) when chemical bonding between the pillars and the clay mineral layers was proposed. Other abbreviations that occur in the literature are expanded layered structure (ELS) and molecularly engineered layered structure (MELS). The technical report of the IUPAC (Schoonheydt et al., 1999) accepted, with some restrictions, ELS for denoting pillared compounds and MELS for pillared derivatives. The IUPAC report also recommends the term ‘pillared LDH’, for LDHs, rather than pillared anionic clays (see Chapter 14.1). The number of layered materials that can be pillared, including clay minerals and LDHs (Vaccari, 1998), is very large. This chapter is limited to the phyllosilicates. The main factors influencing the intercalating/pillaring process are directly related to the nature of the clay mineral host, the pillar guest, intercalated species and experimental conditions.

10.5.4 HOST CLAY MINERALS The nature of the clay mineral host, or parent material, is very important in the pillaring process since it determines layer composition and charge as well

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as the site of isomorphous substitution in the octahedral and/or tetrahedral sheet. The dimension and shape of the clay mineral layers and the arrangements of particles and aggregates have also influence. These arrangements are related to the initial concentration of the aqueous clay mineral dispersion. Single layers and particles can occur in very dilute dispersions, while aggregates are formed in concentrated dispersions. Natural swelling clay minerals, mainly Mt, usually referred to by the general term bentonite, were mostly used as hosts because the first step in pillaring is the intercalation of the pillaring agent by cation exchange. Later, both other natural (hectorite, beidellite, saponite, stevensite) and synthetic (saponite and Laponite) smectites were considered. Interest in the study of saponite (mostly synthetic ones) has increased remarkably in the last few years (Stievano et al., 2006; Nikolopoulou et al., 2009; Vicente et al., 2009) because of the tetrahedral origin of its charge, which leads to enhanced acidic properties and also allows the study of the pillaring mechanism (see Section 10.5.9). At the same time, the interest in the pillaring of stevensite has increased in recent days (Benhammou et al., 2007, 2011), Laponite, and hectorite (Aceman et al., 2000; Zhou et al., 2010; Lin et al., 2011). Pillaring was extended to clay minerals other than smectites, and even to some minerals that are non-swellable a priori, sometimes requiring certain pretreatments. Thus, vermiculite (Suquet et al., 1991; Campos et al., 2007, 2008; Chmielarz et al., 2008; Chen et al., 2010), phlogopite (del Rey-PerezCaballero and Poncelet, 2000; Chmielarz et al., 2007), talc (Urabe et al., 1991), micas (Sakurai et al., 1990; Shimizu et al., 2006; Baumgartner et al., 2009; Ooka et al., 2009), rectorite, which is a regularly interstratified pyrophyllite-beidellite (Brody and Johnson, 1990; Occelli, 1991; Guan and Pinnavaia, 1994; Xiao et al., 2007; Zhang et al., 2010; Wei et al., 2011; Xu et al., 2011; Zhou et al., 2011), and other interstratified clay mineral smectites (Jie et al., 1986; Sterte, 1990) featured as mineral hosts. The nature of the exchangeable cation initially present is another important factor in the pillaring process. In the majority of cases, Naþ-exchanged smectites were used because they are dispersible in water, which facilitates intercalation of the pillaring agents by cation exchange. In some instances, the clay minerals are purified, acid-activated or organically modified prior to intercalating the pillaring agents. The pioneering works on pillaring of acid-activated clays were carried out in the early 1990s (Mokaya et al., 1991; Mokaya, 1992). Since impurities associated with naturally occurring clays may give rise to inhomogeneous particle size distributions, and since prior purification of such clays is laborious and time consuming, synthetic clay minerals of high purity were used as hosts (Bergaoui et al., 1995; Kloprogge, 1998; Shimizu et al., 2006; Trujillano et al., 2009).

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10.5.5 PILLARING SPECIES Bergaya (1990) set out the metal ions that were studied as pillaring agents in the form of a Periodic Table of Elements. An updated version of this is shown in Table 10.5.1. The vacant squares in this table denote the number of elements that have yet to be investigated. Although the hydrolysis behaviour of metal cations (Baes and Mesmer, 1976) is generally understood, that of aluminium ions is more fully known. For this reason, Al-pillared clay minerals were the first to be prepared and extensively documented.

10.5.5.1 (Al13)7þ-Pillaring Agent 10.5.5.1.1 Hydrolysis products The composition of the Al-pillaring solutions strongly depends on the preparation conditions. The degree of hydrolysis or basicity (OHbase/Alsalt ratio) is an important factor controlling solution pH and, hence, the nature of the Al species. The controlled hydrolysis of Al salts gives rise to a number of species, such as [Al(H2O)6]3þ monomers and several polymers. The cation  IV VI 7þ Al Al12 O4 ðOHÞ24 ðH2 OÞ12 denoted as Al13 (Fig. 10.5.1) is assumed to be the pillaring species in aluminium salt solutions. Its pseudo-spherical structure was deduced from X-ray crystallography using a well-crystallized Al13 sulphate (Johansson, 1960, 1962). The geometrical (MIVAlVI 12 O40) structure had already been deduced from XRD patterns by Keggin (1934), whose name is attached to this structure. However, the first Keggin structure was ammonium 12-molybdophosphate, an anionic metal– oxygen cluster synthesized by Berzelius in 1826 (Ohler and Bell, 2002). 27 Al-NMR spectroscopy was helpful in determining the Al environment in Al Keggin ions both in solution and in the solid state (Akitt et al., 1972; Akitt TABLE 10.5.1 The Elements that were Used as Pillars or Co-pillars Shown as the Periodic Table Be

B

Mg

Al

Si

Ga

Ge

Y La

Ti

V

Cr

Zr

Nb

Mo

Mn

Ta Ce

Fe

Co

Ru

Rh

Os Pr

Nd U

Ni

Cu

Zn

P

Sn Bi

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O3

O3 O1

O4

O4

O3 O4

O3 O4

O3

O4

O1

O4

O3

O1 O1

O1 O3

O3

O4 O4

O4

FIGURE 10.5.1 The structure of Al13 (Keggin cation).

and Farthing, 1978; Pinnavaia et al., 1984). The structure of Al13 was also analysed by other methods, including small-angle X-ray scattering (SAXS), Raman spectroscopy and light scattering. Scanning electron microscopy (SEM) images of Al13 crystals show a tetrahedral symmetrical structure of the tridecamers (Furrer et al., 1992). Using 27Al-Nuclear Magnetic Resonance (NMR) spectroscopy, Parker and Kiricsi (1995) reported a facile, reproducible method and the optimal experimental conditions for the preparation of thermally stable Al Keggin cations (Table 10.5.2).

10.5.5.1.2 Procedures for Obtaining Al13 Al13 precursors can be obtained by three main methods. i. The widely used method is hydrolysis of aqueous Al salt solutions, often 1 mol/dm3, with NaOH solution or solid Na2CO3. Al13 is the dominant species in AlCl3 solutions of 101 to 103 mol/dm3 at an Al/OH ratio of 2.0–2.2 (Bottero et al., 1980, 1982). Al salts with anions other than chloride (Akitt and Farthing, 1981a) and M2CO3 where M is not Naþ were also used (Akitt and Farthing, 1978). ii. The second method involves dissolution of Al metal (Akitt and Farthing, 1981b) in HCl or in aqueous AlCl3 solution, which is acidic because of the dissociation of water molecules induced by the high polarizing power of the small Al3þ cation. The second method has the advantage of excluding interfering ions and is preferred for the industrial preparation of commercial aluminium chlorohydrates3 (Jones, 1988).

3. Chlorhydrol (abbreviated as ACH) from Chemical Reheis Company. Locron L from Hoechst.

TABLE 10.5.2 Experimental Conditions Used in the Pillaring Method Using Concentrated Dispersions or Solid Powders Molina et al. Reference (1992)

Del Sanchez Riego Schoonheydt and Frini Storaro Moreno et al. Schoonheydt Schoonheydt and Leeman Montes et al. et al. Fetter et al. et al. (1994) et al. (1994) et al. (1993) (1992) (1998) (1997) (1996) (1997) (1997)

Clay

Mt

Mt

Ht, Sap

Ht, Sap, Lap Sap

Mt

Mt, I–Sm

Mt

Mt

Mt, Sap

Mt, Lap Mt Sap, Bd

Sap

Mt

OH/Al

1.6

2

2.4

1 and 2

1 and 2

2

2



2

1.9







2.4

[Al]f (M)

0.1

0.1

0.07

0.8

0.5



Al–Cu –

2.5 M*

0.1 M, 2 M*

*







Clay/ water

40% in 10% DB in DB

1% in DB

Different Cc

P

P and 10%%

P and 1% 33% in DB

10, 15, 20, P or 50% in 50% in 40, 50% suspension acetone acetone

P in DB P in DB

Al meq/g clay

25, 50, 20, – 75, 100 40, 60, 70



23

9

30, 60 –

15

24 h

1 night under reflux

24 h

48 h

7 days 7 min in 12 h microwave

W until Cl– free

5W

4W

5D

4W

Time of 24 h, exchange 48 h

1, 3, 7 48 h days

W/D

1D

2D

D until Cl– free

11.4

Storaro et al. (1998)

Salerno and Mendioroz (2002)

Vicente and Aouad Lambert et al. (2003) (2005)

30

5, 10, 30



18

24 h

2h

0h

0h

D until Cl– 5D free

5D

W until Cl– Filter-press 4 W free

Bd, beidellite; Ht, hectorite; I, illite; Lap, laponite; Mt, montmorillonite; Sap, saponite; Sm, smectite; Cc, concentrations; D, dialysis; DB, dialysis bags; P, powder; W, washing. *Chlorhydrol® was used as source of aluminium in references Fetter et al. (1997), Moreno et al. (1997) and Storaro et al. (1998).

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iii. The third method involves electrolysis of AlCl3 solution, yielding pure Al13 with an OH/Al ratio of 2.4 (Akitt and Farthing, 1981c).

10.5.5.1.3 Factors Influencing Al13 Formation Although the method of preparation is clearly of prime importance, many other factors influence the formation of pure Al13 in solution, such as the nature and initial concentration of the reagents used, the degree of hydrolysis (OH/Al mole ratio), the rate of adding the reactants, the temperature, and the ageing time and ageing temperature of the hydrolyzed solutions. 10.5.5.1.4 Other Al Oligomers Although [Al13O4(OH)24(H2O)12]7þ is undoubtedly the most studied Al-based polycation, other oligomers have been reported. Vicente and Lambert (2003) adapted an old procedure to prepare [Al13(OH)24(H2O)24]15þ for intercalation in saponite. The same authors reported the use as intercalating agents of the oligomers [Al13(m3–OH)6(m2–OH)12(heidi)6(H2O)6]3þ, containing organic ligands corresponding to N-(2-hydroxyethyl)-iminodiacetate anions, denoted as heidi (Vicente and Lambert, 1999; d’Espinose de la Caillerie et al., 2002). The decomposition kinetics of Al13 indicates the formation of defective Al13 dimers (Al24O72), which become the main active intercalating pillared species after lengthy thermal ageing. Under some particular conditions (NH3), Al Keggin cations can be stabilized by self-condensation leading to the formation of Al26 (Vaughan, 1988b). The wide variety of oligomeric compounds formed from Al (but also from Ga and In) were recently reviewed (Mensinger et al., 2012).

10.5.5.2 Other Pillaring Agents Since Al and Ga have very similar chemical properties, the Ga13 ion appears to be isostructural with Al13 (Bellaloui et al., 1990; Bradley et al., 1990a,b; Bradley and Kydd, 1991; Tang et al., 1993). Using a Ga:Al ratio of 1:12, the polycation is  IV VI 7þ Ga Al12 O4 ðOHÞ24 ðH2 OÞ12 where Ga3þ occupies the central tetrahedral position. The solution chemistry of other elements (M), such as Zr, Cr and Fe, which are potentially capable of acting as pillaring agents, was reviewed in a special volume of Catalysis Today, edited by Burch (1988). References related to other elements such as Be, Mg, Ti, Nb, Ta, Mo, Ni, Cu, B, Si, and Bi pillaring species were mentioned in the review by Bergaya (1990). During the last 20 years, a large volume of literature has appeared on the above elements as well as new ones such as Sn and V. (NH4)2SnCl6 was used as Sn precursor (Palinko et al., 1993), while VOCl3 was used as V precursor, being refluxed with a Hþ-Mt in benzene (Choudary et al., 1990). Looking for

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a control of the magnetic properties of the pillared solids, Stievano et al. (2006) used for intercalation of saponite the well-defined iron polycation [Fe8(m3–O)2(m2–OH2)12(tacn)6]8þ, where ‘tacn’ denoted the ligand 1,4,7triazacyclononane. The structure of the Al Keggin cation, which is the oligomer most used for preparing PILC, was established by instrumental techniques, particularly NMR spectroscopy, but that of other metal intercalants is less understood. Some of these, such as Ga13 and GaAl12, are assumed to be similar in structure to Al13 (Bradley et al., 1990a,b). The dimeric to hexameric formed species of Cr3þ are easily monitored by visible spectroscopy (Stu¨nzi and Marty, 1983). An octameric species was proposed for the polymerization of Ti4þ (Einaga, 1979, 1981) and a tetrameric one for that of Zr4þ (Baes and Mesmer, 1976), while in other cases, the structures of the polymeric species have yet to be determined. There are also numerous patents from many countries, some of which are mentioned by Burch (1988). Since the patent literature is not easily accessible, this is not referred to in this chapter.

10.5.5.3 Mixed Al–M and M–M0 Pillaring Agents The intercalation/pillaring of clay minerals with solutions containing two cations has been investigated since the 1980s. One of the pillaring cation is usually Al. The second inorganic cation is then added in various molar fractions to improve the thermal, adsorptive and catalytic properties of the pillared products (Gil et al., 2000a). Incorporation of Zr, Ga, transition-metal elements such as Cr, Fe, Cu, Mo and Ru, and lanthanides into Al-pillaring solutions has been reported. Such mixed pillaring agents were widely used during the past three decades as the following references indicate. l

l

l l

l

mixed Al–Zr: Occelli (1986), Occelli and Finseth (1986), Moreno et al. (1999), and Timofeeva et al. (2011); mixed Al–Ga: Vieira Coelho and Poncelet (1990, 1991), Bradley et al. (1990a,b,c, 1992), Gonza´lez et al. (1991, 1992), Bradley and Kydd (1991, 1993), Tang et al. (1993, 1995), Bagshaw and Cooney (1995), Pesquera et al. (1995), Hernando et al. (1996a,b), Benito et al. (1999), and Vicente et al. (2009); mixed Al–Ge: Lee et al. (2001); mixed Al–Cr: Carrado et al. (1986a,b), Skoularikis et al. (1988), Zhao et al. (1995), Storaro et al. (1997), Toranzo et al. (1997), and Mata et al. (2007); mixed Al–Fe: Barrault et al. (1988, 1992), Lee et al. (1989), Bergaya and Barrault (1990), Rightor et al. (1991), Bergaya et al. (1991, 1993b), Zhao et al. (1993a), Bakas et al. (1994), Lenarda et al. (1994), Storaro et al. (1995, 1996), Ladavos et al. (1996), Mandalia et al. (1998), Belver et al. (2004a,b), Yuan et al. (2006, 2008), Aouad et al., 2010, and Galeano et al. (2010, 2011, 2012);

Chapter l

l l l

l l

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mixed Al–Cu: Frini et al. (1997), Barrault et al. (1998), Abdelaoui et al. (1999), and Galeano et al. (2010); mixed Al–Mo: Gil and Montes (1997); mixed Al–Ru: Lenarda et al. (1994), and Storaro et al. (1995); mixed Al–La: Sterte (1991), Trillo et al. (1991, 1993a,b), and Zhao et al. (1993b); mixed Al–Ce: Pires et al. (1998); mixed Al–Si: Gil et al. (2005).

Sterte (1991) prepared mixed solutions of Al not only with La and Ce, but also with Pr and Nd. In contrast to La and Ce, the two last lanthanides failed to give a pillared product. Dominguez et al. (1998) compared the experimental results with theoretical calculations using mixed solutions of Al–Ga, Al–La and Al–Ce. The rapid growth in the use of mixed pillared solutions is due to demand for improvement in specific properties. In particular, the heat stability of AlM-PILC is greater than that of the classical Al-pillared clay minerals. The use of Al-free mixed Fe–Cr (Akcay, 2004) and Fe–Zr (Heylen and Vansant, 1997) pillaring solutions has also been reported. Again, the aim here is to produce pillared clay minerals with improved properties for specific applications.

10.5.5.4 Pillaring Agents with More than Two Cations The use of mixed pillaring solutions containing more than two metal cations was also reported: l

l l

for Al–La–Ce by Gonza´lez et al. (1992), Mendioroz et al. (1993), and Booij et al. (1996a,b); for Cr–Fe–Zr by Jamis et al. (1995a, 1995b); for Al–Fe–Cu by Galeano et al. (2010).

10.5.5.5 New Pillaring Agents The literature also mentions the use of metal complexes as pillaring agents. Many of these complexes involved the chlorides of transition metals such as niobium, tantalum and molybdenum (Christiano et al., 1985; Christiano and Pinnavaia, 1986), and tin (Palinko et al., 1993). Pillared clay minerals obtained from inorganic–organic pillaring agents were also described. Coordination compounds, organometallic complexes, surfactants, and/or polymers were the source of the organic carbon in the pillaring agents (Gil et al., 2000a). The early papers by Loeppert et al. (1979) and Pinnavaia (1983) on the preparation of clay minerals pillared with organometallic complexes indicated that their thermal stability is not high because the CdC bonds decompose at about 400  C. PILC can swell in water and the intercalated complexes remain

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exchangeable. However, organometallic precursors based on Si complexes (Endo et al., 1980, 1981) are converted into silica pillars when heated at 500  C. Tetraethylorthosilicate was often used in the Al–Si pillaring process (Wada and Wada, 1980; Sterte and Shabtai, 1987; Figueras, 1988; Zhao et al., 1992; Fetter et al., 1995; Gil et al., 1995). Other precursors based on trimetallic iron complexes (Yamanaka et al., 1984; Doff et al., 1988; Martin-Luengo et al., 1989; Maes and Vansant, 1995) are converted into iron oxide pillars when heated at 350  C. The same structure is adopted by trimetallic acetate complexes of Mn3þ and Cr3þ, and also a very similar structure by a Ti3þ complex, all of them being used for pillaring of clay minerals (Kijima et al., 1991; Mishra and Parida, 1997, 1998). Heating clay minerals intercalated with organic–inorganic binuclear oxobridged Fe complexes (Dick and Weiss, 1998) produced zeolite-like channels in the products that showed interesting properties depending on the ligands used. Direct intercalation of metal oxide sol precursors in the presence of silica and/or alumina or other aqueous oxide sols led to basal spacings of more than twice the thickness of the host clay mineral layer (see references in Pinnavaia, 1992; Cool and Vansant, 1998). Some cases are more complex. For example, intercalated mixed organometallic complexes were converted into oxide sol particles on calcination (Yamanaka and Takahama, 1993). This led to pillars formed by clusters of SiO2 particles covered by TiO2 on their surface. However, by intercalating the sols of the two oxides simultaneously with an organic structure-directing agent and burning the latter off, mixed SiO2 and TiO2 pillars were obtained. The intercalation of imogolite into Mt yielded a tubular silicate–layered silicate nanocomposite (Pinnavaia and Johnson, 1986). Co-intercalation of Al (and/or La) pillaring species and a cationic surfactant as precursors Srinivasan and Fogler (1990a,b) produced a cheap organic–inorganic clay mineral with better adsorptive properties than classical activated carbon. Co-intercalation of Al13 cations and non-ionic surfactants (Michot and Pinnavaia, 1991; Michot et al., 1992, 1993) yielded pillared clay minerals with enhanced capacity for adsorbing toxic pollutants.

10.5.6 PILLARING METHODS 10.5.6.1 Pillaring in Dilute Dispersions The classical pillaring method involves two successive steps. The first is the intercalation of the pillaring agent. This is done by slowly adding a dilute solution of the precursor to an already prepared dilute clay mineral dispersion (Fig. 10.5.2). The property of the intercalated clay mineral is dependent on the nature of the metallic cation, hydrolysis conditions, and type of clay mineral. Ageing time, with or without stirring, temperature, mode of washing (filtration and/or dialysis) and drying (at room temperature, at low temperature in

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Pillaring methods in dilute

Ex situ method

In situ method

Pillaring solution

NaOH

M Cl3

Added simultaneously

Dispersion (2%)

Dispersion (2%)

Pillaring methods in concentrated medium

Clay slurry in dialysis bag

Pillaring solution

Pillaring solution

Clay powder + pillaring oligomer powder in dialysis bag

Distilled water

FIGURE 10.5.2 Comparison of pillaring methods.

an oven or freeze-drying) are also important parameters (Pinnavaia et al., 1984). The second step consists of heating the intercalated clay mineral. The heating temperature in air or in a closed reactor with flowing gas, as well as the duration and rate of heating, have a strong influence on the properties of the resultant PILC.

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For the Al Keggin cation, the charge (7þ) in solution changes by hydrolysis and after intercalation and heating. The final charge of the pillars is variable, but appears to be about 4 þ. Since each of the parameters mentioned above exerts some influence, data comparison is difficult. Various PILC are commonly characterized by their basal spacing (measured by XRD), specific surface area (SSA) and porosity (see Section 10.5.7). Variants of the classical method have been described, but all these procedures still consume large amounts of liquid. One procedure involved the in situ hydrolysis of the exchangeable cations in the interlayer space (Yamanaka and Brindley, 1978; Brindley and Kao, 1980) and, therefore, required pre-exchange of the clay mineral with a suitable cation, but it did not yield a satisfactory product. Another variant involved adding a dilute solution of both the metal salt and the base simultaneously to a dilute clay mineral dispersion (Fig. 10.5.2). Here, the pillaring species is presumed to form directly in situ in the clay mineral dispersion. The rate of addition of the two solutions needed to be carefully controlled so that the final pH did not vary much with every added drop of the two constituents. This required vigorous stirring of the clay mineral dispersion, which must remain clear during the whole course of addition, showing no sign of precipitation. This latter in situ method is less time consuming than the former procedures because the intercalant does not have to be prepared separately and no pre-exchange of the starting clay mineral is required. During intercalation at ambient temperature, ultrasonic treatment of Ca2þMt with chlorhydrol for 20 min yielded PILC with improved textural properties and higher stability (Katdare et al., 1997, 2000). Microwave irradiation can be used as an adequate, rapid procedure to reduce the time of intercalation with respect to the traditional method (Fetter et al., 1996). However, not all the pillared species are affected by this treatment (de Andre´s et al., 1999).

10.5.6.2 Pillaring in Concentrated Medium Although pillared clay minerals have attracted much attention over the last 40 years, particularly from industry, they have not yet been used as commercial catalysts. One reason is that the pillaring procedure developed in the laboratory is difficult to extend to an industrial scale. The process is time consuming and needs a large volume of liquid and repeated separations of huge amounts of clay mineral dispersions by centrifugation or filtration. To produce pillared clay minerals at an industrial scale, the above procedure must be simplified. In particular, the volume of all reagents needs to be greatly reduced. Since about 20 years ago, methods using concentrated reagents and/or dialysis bags have been considered (Table 10.5.2) (Aouad et al., 2005). Vaughan (1988b) first reported the pillaring of clay minerals by adding the mineral as a powder to the pillaring solution to yield a slurry of up to 40%.

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At this high concentration, the clay mineral does not delaminate or gel. Pillaring can also be achieved by placing a concentrated clay mineral dispersion contained in a dialysis bag in a dilute pillaring solution prepared ex situ (Fig. 10.5.2; Schoonheydt and Leeman, 1992; Frini et al., 1997; Sanchez and Montes, 1998). Using a range of concentrated clay mineral dispersions, Schoonheydt et al. (1993) found other unidentified Al species besides the Keggin ion. Frini et al. (1997) compared the results obtained from a classical method with dilute dispersion with the methods using clay powder and also a concentrated clay mineral slurry in a dialysis bag. After repeated washing and centrifuging of the intercalated clay mineral dispersions and heating of the sediment, it was found that the PILC prepared by the last two methods were easier to grind than the product obtained by the classical method. Washing the concentrated dispersions or pastes by dialysis facilitates the recovery of the intercalated clay minerals (Molina et al., 1992; Del Riego et al., 1994; Schoonheydt et al., 1994) and the number of washing cycles is reduced. However, all these procedures still use dilute pillaring solutions and large amounts of liquid. Several attempts have been made to increase the amount of PILC, even to the pilot scale. Amounts of 1 kg of Al- and Al/Fe-PILC per batch were produced in a small pilot installation at the National Technical University of Athens (NTUA) using 2 mass% clay mineral dispersions and relatively dilute Al(Fe) solutions (Kaloidas et al., 1995). Production of 1.5 kg/run was achieved at the same NTUA installation and at a new one in TOLSA company (Madrid), by adding the dry clay mineral powder to a dilute commercial pillaring solution (chlorhydrol) and washing the dispersion in a filter press (Moreno et al., 1997). PILC prepared at the laboratory scale (g), at the batch level (100 g), and at the pilot scale showed similar catalytic activities. Valverde et al. (2003) prepared Ti-PILC at pilot scale, 1 kg/batch, using the following ratios: HCl/Ti 2.5 mol/mol, Ti/clay 15 mol/kg, H2O/clay 0.75 kg/g. The main differences compared with laboratory-scale conditions were in the volumes of both the pillaring solution and the clay dispersion. A scheme of the pilot plant is shown in Fig. 10.5.3. Samples prepared at pilot scale showed SSA and micropore volumes smaller than those of the samples prepared at laboratory scale, especially after calcination at relatively high temperatures. However, these differences did not affect significantly the catalytic behaviour in the selective catalytic reduction of NO by propylene. Commercially available concentrated Al-pillaring solutions (50% m/m chlorhydrol) were used with concentrated 50% m/m aqueous or acetone clay mineral dispersions (Storaro et al., 1996). The dispersions in acetone were less sticky and easier to handle than their aqueous counterparts. Up to 500 g of PILC was obtained. Storaro et al. (1998) made further improvements by mixing solid chlorhydrol with various clay minerals in acetone. A similar procedure was used by Salerno and Mendioroz (2002) by mixing a dispersion of a

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Reaction area

2 4 1 8

5

10

7 3

11

6 10

11 9 Pillaring solution area

Filtration area FIGURE 10.5.3 Pilot plant. (1) osmosis plant; (2) osmosized water tank; (3) pillaring solution tank; (4) synthesis tank; (5) slurry stream; (6) basket centrifuge; (7) filtrated stream; (8) filtered stream recirculation; (9) waste stream; (10) centrifuge pump; and (11) peristaltic pump. Reprinted with permission from Valverde et al. (2003). Copyright (2003) American Chemical Society.

raw clay in acetone with Locron, a commercial Al hydroxychloride containing 47% Keggin cations. The method proposed by Fetter et al. (1997) consisted of mixing a highly concentrated chlorhydrol solution (50% m/m, 2.5 mol/dm3) with a concentrated 50% m/m clay mineral dispersion and microwave irradiation for the intercalation step. Vicente and Lambert (2003) reported a new synthetic route of pillaring by placing 1 g clay in powder form in a dialysis tube with 10 cm3 of very concentrated solutions, 40 times smaller in volume than in the classical pillaring procedure with Al poly(hydroxo cations). This Al compound was obtained by reacting alumina with a chloride salt, but its nature was not elucidated. The mixture was immediately washed five times by dialysis, centrifuged and heated. This method using the highest clay content with a minimum time period and water was then described by Aouad et al. (2005), and it offers the potential for extension to an industrial scale. Here, 1 g of raw clay powder and solid Al13 were dialyzed five times against 5 cm3 of water (Fig. 10.5.2). Such a truly solid–solid reaction was not achieved before. The dialysis step seemed crucial for pre-pillaring of the clay mineral. Galeano et al. (2012) reported the preparation of Al- and Al/Fe-PILC from initial metal salt solutions up to 2.23 mol/dm3, hydrolyzed with 1.2 mol/dm3 NaOH, using an OH/metal ratio of 1.6 (final metal ion concentration of 0.628 mol/dm3) and ethanolic or aqueous clay mineral dispersions with clay

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mineral contents up to 25.0 m/v%. The use of ethanol made the oligomer intercalation less susceptible against the clay mineral concentration. The use of concentrated conditions slightly decreased the basal spacing and broadened the reflections but increased the amount of bound oligomers. The solids were excellent catalysts at semi-batch lab scale in wet peroxide oxidation for the removal of natural organic matter from raw surface water. In the last years, new approaches were considered for Al-pillaring in concentrated medium, such as the synthesis of Keggin cations from concentrated metal salt solutions (Aouad et al., 2006; Chen et al., 2007; Guo et al., 2009; Olaya et al., 2009a,c; Sanabria et al., 2009b) and the preparation of a solid precursor by sulphate precipitation followed by nitrate metathesis (Shi et al., 2007). The use of concentrated dispersions and microwave or ultrasound irradiation during the intercalation step also gave promising results (Sun et al., 2006; Olaya et al., 2009a,b,c; Sanabria et al., 2009a; Yapar et al., 2009), although it implied the consumption of an extra amount of energy. It was noticed that metal salt concentrations >0.2 mol/dm3 and/or OH/metal mole ratios <1.5 did not impede the formation of Al13 Keggin ions (Chen et al., 2007; Feng et al., 2007). This is an important finding, considering that the use of highly concentrated metal salt solutions requires lower OH/metal ratios during hydrolysis to avoid precipitation of the metal hydroxides.

10.5.7 MAIN PILC CHARACTERISTICS FOR SEVERAL APPLICATIONS The first common characteristic of PILC is the distance (Dd) separating contiguous layers after heating the intercalated clay minerals. Thus, PILC have a permanent porosity and a higher accessible SSA than the pristine clay, which are important properties. The basal spacing of smectite intercalated with Al Keggin cations is about 1.8 nm, corresponding to Dd of 0.9 nm (Fig. 10.5.4). If a smaller Dd value is observed, pillaring is not considered as achieved. A basal spacing of about 2.7 nm was reported for Al-PILC (Singh and Kodama, 1988) and for Ti-PILC (Sterte, 1986). However, Singh and Kodama showed by Fourier transform analysis of the XRD basal reflections that a regular interstratified structure of 0.96 and 1.8 nm occurred. Pillaring occurred between every two layers of Mt. Still higher interlayer distances, up to 7 nm, were observed by Mandalia et al. (1998) for a series of Al/FePILC. However, a TEM image analysis performed on one of these samples (Clinard et al., 2003) showed that this value refers to a correlation distance for Fe particles present between clay mineral particles, each of which is composed of four to five layers. In this case, pillaring occurs between particles rather than between individual layers. This PILC is also characterized by an inter-pillar distance (Fig. 10.5.4), which is more difficult to determine. The increase in the accessible porosity leads to higher SSA values of PILC than of the pristine (parent) clay minerals. A smectite with completely

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d001=1.86 nm

Porosity FIGURE 10.5.4 Structure of an Al-PILC.

exfoliated layers has an SSA of about 800 m2/g. The highest SSA recorded for PILC was 600 m2/g with a total pore volume of 0.6 cm3/g (Vaughan et al., 1979). This porosity is due to pores of several sizes, although micropores (<2 nm) are usually obtained by Al pillaring. In some instances, such as the mixed Al/Fe-PILC, mesopores (2–50 nm) were observed (Mandalia et al., 1998). Macropores (>50 nm) were observed with pillared delaminated Laponites (Butruille and Pinnavaia, 1996). The acidity of PILC is important for catalytic applications. This property arises from both the clay mineral and the pillar. By comparing a number of pillared bentonites, Zonghui and Guida (1985) found that Ti-PILC is the most acidic. The nature, number, and strength of acid sites in PILC can be controlled by competitive ion exchange during the intercalation step, or by ion exchange of the residual cations after the pillaring step or by steaming of the PILC (Figueras et al., 1990; Tichit et al., 1991).

10.5.8 INTERCALANT STABILITY BEFORE AND AFTER PILLARING The stability of the intercalant species before and after pillaring is of primary importance for the use of PILC in several applications. The stability of Al7þ 13 sulphate hydrate was investigated by Kloprogge et al. (1992), who showed that the adsorbed excess water is lost at 80  C. On further heating, the oligomer gradually decomposes, losing its 12 water molecules and 24 hydroxyl groups and, finally, becoming X-ray amorphous at 360  C. Above 360  C and till 950  C, Al oxide is formed.

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The required thermal stability of PILC is generally difficult to define, because it is application dependent. Of course, heating of the intercalated clay mineral is supposed to stabilize the structure of PILC. Except for Zr-PILC, however, both intercalated and pillared clay minerals are chemically modified when stored in air (Chevalier et al., 1992). According to Schoonheydt et al. (1993), the increase in basal spacing alone is not a sufficient criterion for thermal stability; the line width of the (001) reflection must also be taken into account. The most important question is whether the structure of the intercalated oligomers is maintained after heating, or whether the intercalant species are transformed into oxide pillars as proposed by Vaughan et al. (1979). The available data are scattered and contradictory. If the intercalant is transformed into classical oxides, why does the basal spacing remain almost unchanged? Many NMR studies showed that the Al13 structure is maintained till 500  C, except for pillared beidellite (Ple´e et al., 1985). This might be because the water of hydration of the intercalant is lost by heating and dehydroxylation occurs at 300–400  C (Schutz et al., 1987). At higher temperatures, oxide pillars with a g-alumina structure are presumed to form in the interlayer space (Tennakoon et al., 1986). An NMR and microcalorimetry study (Occelli et al., 2000) of Al-pillared Mt showed that heating clay minerals with intercalated Keggin ions at 500  C led to the formation of blocks of 13 Al ions containing IV- and VI-, as well as V-coordinated Al3þ ions. The Al13 structure was no longer maintained at this temperature, but g-alumina was also not detected by XRD. Moreover, a thermo-XRD study (Aceman et al., 2000) of Al-pillared smectites showed that the intercalated Keggin ions first dissociated into smaller hydroxy- and/or chloro-Al oligomers, which increased in size after water addition (thorough washing or dialysis). At 600  C, these Al oxocations condensed with the silicate sheet, preventing further swelling in water. However, the exact nature of the obtained alumina clusters that form the pillars in Al13-PILC still remains a subject of discussion.

10.5.9 LINKAGE BETWEEN PILLARS AND CLAY MINERAL LAYERS Many hypotheses have been proposed about the manner in which the pillars are linked to the interlayer surface of the clay mineral, but the precise mechanism is not well understood. In fact, there is no generally accepted mechanism that can account for the stable linkage that exists between pillar and clay mineral surface, especially at high temperatures. When Al13 is heated, protons and water are simultaneously released, and condensation takes place between the OH groups of the oligomeric ions and lattice hydroxyl groups of the silicate layer. One suggestion is that the Al oxide pillar is linked via oxygen to cations of the octahedral sheet (Tennakoon et al., 1986), giving rise to a rigid cross-linked structure and loss

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Smectite layer

O

O

O

O

Si

Si

O

O Si

Si O

O

Al

Al

O O

O O Al

O O

Cross-linking Si sites

O O

FIGURE 10.5.5 Scheme of cross-linking between a hydroxy-SiAl oligocation and the tetrahedral layer of fluorhectorite. Adapted from Sterte and Shabtai (1987).

of expansion. This hypothesis, however, was not widely accepted because it is difficult to visualize how such a link can form with the tetrahedral sheet acting as a barrier. Cross-linking between pillars and silica tetrahedra that become inverted after calcination was proposed by Sterte and Shabtai (1987) for Si–Al-pillared fluorohectorites and confirmed by Fripiat (1988) on the basis of NMR studies (Fig. 10.5.5). Tetrahedral inversion is influenced by the origin of the layer charge as in the case of beidellite or synthetic saponite where the (substituted) Al tetrahedra invert after calcination (Ple´e et al., 1985; Kloprogge et al., 1994; Lambert et al., 1994; Lambert and Poncelet, 1997). This inversion would also depend on the composition of the octahedral sheet. For example, the presence of fluoride ions in fluorohectorite stabilized the SidO bond, promoting coupling between inverted Si tetrahedra and the Al13 pillars on calcination (Pinnavaia et al., 1985). In the case of synthetic saponite, a covalent Sid(OH)dAlpillar bond was suggested. It was assumed that protons released during calcination attacked siloxane groups on the surface forming silanol groups (Sterte and Shabtai, 1987; Jones, 1988; Bukka et al., 1992; Li et al., 1993; Kloprogge, 1998; Aceman et al., 1999; Occelli et al., 2000). To our knowledge, no papers have been published proposing cross-linking structures in the case of intercalants based on elements different from Al.

10.5.10 MATHEMATICAL MODELLING Properties of pillared clays have rarely been analysed by mathematical modelling. Sahimi and co-workers (Yi et al., 1995, 1996, 1998; Ghassemzadeh et al., 2000) considered the tetrahedral sheets of the clay minerals as the (100) face of an FCC cubic structure, and represented the pillars by rigid chains consisting of Lennard–Jones-type spheres separated by their size

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FIGURE 10.5.6 Schematic representation of model pillared clay minerals.

parameter and intercalated vertically, either uniformly or randomly, between the layers (Fig. 10.5.6). Sahimi and co-workers applied a dynamic Monte Carlo method for studying diffusion, adsorption and reactivity of pillared clays (Sahimi, 1990; Ghassemzadeh et al., 2000). The pore space was represented by parallel clay mineral layers represented by horizontal parallelepipeds, interconnected by orthogonal parallelepipeds (pillars) and long needle-like objects (interlayer molecules). Diffusion was visualized as a random walk process. Adsorption was related to a probability proportional to the Boltzmann factor, and the reactivity was dependent on the average distance of reactive molecules to reactive sites. Effective diffusivity depended on the geometry of the pore space, the size of the molecules, their intermolecular interaction and their adsorption on the surface of the pillars. Percolation resulted from irreversible adsorption when the size of the molecules was comparable to the effective size of the pores. Extensive molecular dynamics simulation of diffusion in these solids was applied to solvation force, molecular density distribution function, and effective diffusivity, considering various spatial distributions of the pillars, at several molecular densities and porosities (Yi et al., 1995). Grand canonical-ensemble Monte Carlo and molecular dynamics simulations were used to study the dependence of the adsorption isotherms and selfdiffusivities on temperature, pillar configuration, porosity, and pressure. It was found that clustering and spatial distribution of the pillars did not influence the adsorption isotherms at moderate and high porosities (Yi et al., 1996). Molecular dynamics simulation of the interconnectivity of the pores and the adsorbent– adsorbate interactions applied to the adsorption and diffusion of several gases showed that the spatial distribution of pillars affected the total loading but not the adsorption selectivity (Yi et al., 1998; Ghassemzadeh et al., 2000). Grand canonical Monte Carlo simulation was also applied to the adsorption of simple inert gases in PILC at various temperatures (Cracknell et al., 1993), assuming that the pillars form a hexagonal close-packed lattice. A first-order phase transition below a critical pillar density was predicted, in agreement with experimental data. Cao and Wang (2001, 2002) predicted the ideal PILC’s properties for storage of methane, after simulating its

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behaviour by the same method, assuming a uniform distribution of pillars between two solid walls, and using various pore widths, porosities and temperatures, even supercritical temperatures (Cao et al., 2002). In the last few years, new adsorption data have been collected (Gil et al., 2007; 2009) and new models proposed (Peng et al., 2007; Stoecker et al., 2008; Almarza et al., 2009).

10.5.11 CONCLUSIONS Much fundamental work has been carried out in the last four decades on PILC, particularly with Al-pillared smectites. All the available data open the way to numerous questions, such as the need for a better understanding of the hydrolysis of cations, even for the most studied Al3þ cation, and also for many other cations that can act as pillaring species. To be of real interest, PILC must be used in some industrial or environmental applications. For this reason, more attention should be paid to the fundamental point of scaling up the pillaring process by using concentrated clay dispersions. Solid–solid reactions remain a challenge for the future. Several clay minerals have been used in pillaring, mainly, but not only, smectites. The challenge will be to pillar new types of clay minerals such as modified kaolinite so as to profit from its particular texture. In spite of the body of knowledge actually accumulated on PILC, two main questions remain unsolved: (i) the real structure of the intercalated species before and after heating, and (ii) the type of bonds formed between the clay mineral host and the pillar guest, which maintain the stability of the pillared clay minerals.

REFERENCES Abdelaoui, M., Barrault, J., Bouchoule, C., Srasra, N., Bergaya, F., 1999. Oxydation catalytique du phe´nol par H2O2 en pre´sence d’argile ponte´e par des espe`ces mixtes (Al-Cu). J. Chim. Phys. 96, 419–429. Aceman, S., Lahav, N., Yariv, S., 1999. A thermo-FTIR-spectroscopy analysis of Al-pillared smectites differing in source of charge in KBr disks. Thermochim. Acta 340–341, 349–366. Aceman, S., Lahav, N., Yariv, S., 2000. A thermo-XRD-study of Al-pillared smectites differing in source of charge, obtained in dialyzed, non-dialyzed and washed systems. Appl. Clay Sci. 17, 99–126. Adams, J.M., 1987. Synthetic organic chemistry using pillared, cation-exchanged and acid-treated montmorillonite catalysts. A review. Appl. Clay Sci. 2, 309–342. Akcay, M., 2004. The catalytic acylation of alcohols with acetic acid by using Lewis acid character pillared clays. Appl. Catal. A. Gen. 269, 157–160. Akitt, J.W., Farthing, A., 1978. New 27Al NMR studies of the hydrolysis of the aluminium (III) cation. J. Magn. Reson. 32, 345–352. Akitt, J.W., Farthing, A., 1981a. Aluminium-27 nuclear magnetic resonance studies of the hydrolysis of Aluminium (III). Part 4. Hydrolysis using sodium carbonate. J. Chem. Soc. Dalton Trans. 7, 1617–1623.

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Akitt, J.W., Farthing, A., 1981b. Aluminium-27 nuclear magnetic resonance studies of the hydrolysis of aluminium III. Part 5. Slow hydrolyse using aluminium metal. J. Chem. Soc. Dalton Trans. 7, 1624–1628. Akitt, J.W., Farthing, A., 1981c. Aluminium-27 nuclear magnetic resonance studies of the hydrolysis of aluminium III. Part 2. Gel-permeation chromatography. J. Chem. Soc. Dalton Trans. 7, 1606–1608. Akitt, J.W., Greenwood, N.N., Khandelwal, B.L., Lester, G.D., 1972. 27Al nuclear magnetic resonance studies of the hydrolysis and polymerisation of the hexa-aquo-aluminium(III) cation. J. Chem. Soc. Dalton Trans. 5, 604–610. Almarza, N.G., Gallardo, A., Martin, C., Guil, J.M., Lomba, E., 2009. Topological considerations on microporous adsorption processes in simple models for pillared interlayered clays. J. Chem. Phys. 131, 244701. Aouad, A., Mandalia, T., Bergaya, F., 2005. A novel method of Al-pillared montmorillonite preparation for potential industrial up-scaling. Appl. Clay Sci. 19, 175–182. Aouad, A., Pineau, A., Tchoubar, D., Bergaya, F., 2006. Al-pillared montmorillonite obtained in concentrated media. Effect of the anions (nitrate, sulfate and chloride) associated with the Al species. Clays Clay Miner. 54, 626–637. Aouad, A., Anastacio, A.S., Bergaya, F., Stucki, J.W., 2010. A Mo¨ssbauer spectroscopy study of aluminum and iron-pillared clay minerals. Clays Clay Miner. 58 (2), 164–173. Baes Jr., C.F., Mesmer, R.E. (Eds.), 1976. The Hydrolysis of Cations. Wiley, New York. Bagshaw, S.A., Cooney, R.P., 1995. Preparation and characterization of a highly stable pillared clay. Chem. Mater. 7, 1384–1389. Bakas, T., Moukarika, A., Papaefthymiou, V., Ladavos, A., 1994. Redox treatment of an Fe/Al pillared montmorillonite. A Mo¨ssbauer study. Clays Clay Miner. 42, 634–642. Barrault, J., Zivkov, C., Bergaya, F., Gatineau, L., Hassoun, N., Van Damme, H., Mari, D., 1988. Iron-doped pillared laponites as catalysts for the selective conversion of syngas into light alkenes. J. Chem. Soc. Chem. Commun. 21, 1403–1404. Barrault, J., Gatineau, L., Hassoun, N., Bergaya, F., 1992. Selective syngas conversion over mixed Al-Fe pillared Laponite clay. Energy Fuel 6, 760–763. Barrault, J., Bouchoule, C., Echachoui, K., Frini-Srasra, N., Trabelsi, M., Bergaya, F., 1998. Catalytic wet peroxide oxidation (CWPO) of phenol over mixed (Al-Cu)-pillared clays. Appl. Catal. B. 15, 269–274. Barrer, R.M., 1986. Expanded clay minerals: a major class of molecular sieves. J. Incl. Phenom. 4, 109–119. Barrer, R.M., MacLeod, D.M., 1955. Activation of montmorillonite by ion exchange and sorption complexes of tetra-alkylammonium montmorillonites. Trans. Faraday Soc. 51, 1290–1300. Baumgartner, A., Wagner, F.E., Herling, M., Breu, J., 2009. Towards a tunable pore size utilizing oxidative pillaring of the mica ferrous tainiolite. Microporous Mesoporous Mater. 123, 253–259. Bellaloui, A., Plee, D., Meriaudeau, P., 1990. Gallium containing pillared interlayer clays: preparation, characterization and catalytic properties. Appl. Catal. 63, L7–L10. Belver, C., Ban˜ares, M.A., Vicente, M.A., 2004a. Fe-saponite pillared and impregnated catalysts. I. Preparation and characterisation. Appl. Catal. B. 50, 101–112. Belver, C., Vicente, M.A., Martı´nez-Arias, A., Ferna´ndez-Garcı´a, M., 2004b. Fe-saponite pillared and impregnated catalysts. II. Nature of the iron species active for the reduction of NOx with propene. Appl. Catal. B. 50, 227–234. Benhammou, A., Yaacoubi, A., Nibou, L., Tanouti, B., 2007. Chromium(VI) adsorption from aqueous solution onto Moroccan Al-pillared and cationic surfactant stevensite. J. Hazard. Mater. 140, 104–109.

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