Bergaya F. Handbook of Clay Science

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past decade that information on methods using concentrated reagents and/or dialysis

bags is available. This is summarized in Table 7.5.2 (Aouad et al., 2005).

Vaughan (1988b) was the ﬁrst to report the successful pillaring of clay minerals by

adding the mineral as a powder to the pillaring solution to give a slurry of up to

40%. At this high concentration, the clay mineral does not delaminate or gellify. Ex-

situ pillaring (Fig. 7.5.2) can also be achieved by placing a concentrated clay sus-

pension, contained in a dialysis bag, in a dilute pillaring solution (Schoonheydt and

Leeman, 1992; Frini et al., 1997; Sanchez and Montes, 1998). Using a range of

concentrated clay suspensions, Schoonheydt et al. (1993) fou nd other unidentiﬁed Al

species besides the Keggin ions.

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

slurry in dialysis bag’. After repeated washing and centrifuging of the intercalated clay

mineral dispersions, and heating the precipitate, the PILC prepared by the two last

methods are easier to grind than the product obtained by the classical method.

The use of concentrated dispersions or pastes, and dialysis, facilitates the recovery of

the intercalated suspensions (Molina et al., 1992; Del Riego et al., 1994; Schoonheydt

et al., 1994). At the same time the number of washing cycles is reduced. However, all

these procedures still use dilute pillaring solutions and large amounts of liquid.

In the pilot scale production (>1 kg per run) at N.T.U.A. (Athens) and Tolsa

(Madrid) pilot, the dry clay mineral powder was added to a dilute commercial pillaring

solution (chlorhydrol), and the dispersion was washed in a ﬁlter-press (Moreno et al.,

1997). PILC prepared at the laboratory scale (g), at a batch level (100 g), and at a pilot

scale shown similar catalytic activities. Commercially available concentrated Al-pilla-

ring solutions (50% w/w chlorhydrol) were used with concentrated 50% w/w aqueous

or acetone clay mineral dispersions (Storaro et al., 1996). The suspensions in acetone

are less sticky and easier to handle than their aqueous counterparts. By this means, up

to 500 g of PILC can be obtained. Storaro et al. (1998) made further improvements by

mixing solid chlorhydrol with different clay minerals in acetone. A similar procedure

was used by Salerno and Mendioroz (2002) by mixing a suspension of raw clay in

acetone with Locron (a commercial Al hydroxychloride containing 47% Keggin cat-

ions). The method proposed by Fetter et al. (1997) consists of mixing a highly con-

centrated chlorhydrol solution (50% w/w, 2.5 M) with a concentrated 50% w/w clay

mineral suspension, and microwave irradiation for the intercalation step.

Recently, Vicente and Lambert (2003) reported a new synthetic route of pillaring

by putting 1 g clay in powder form in a dialysis tube with 10 ml very concentrated

solutions (40 times smaller volume than in classical pillaring) of Al poly(hydroxo

cations). This compound is obtained by reacting alumina with a chloride salt, but its

nature was not elucidated. The mixture is immediately washed ﬁve times by dialysis,

centrifuged, and heated. However, the method at the highest clay content with a

minimum expenditure of time and water is that of Aouad et al. (2005). At the same

time, this method offer s the potential for extension to an industrial scale. Here, 1 g of

raw clay powder and solid Al

is dialyzed ﬁve times against 5 ml of water

Chapter 12: Pillared Clays and Clay Minerals404

Table 7.5.2. Experimental conditions used in the pillaring method using concentrated dispersions, or solid powders

Reference 123456789 10 11121314

Clay Mt Mt Ht, Sap Ht, Sap

Lap

Sap Mt Mt Ilite-

Mt Mt Mt Sap Mt, Lap

Sap, Bd

Mt Sap Mt

OH/Al 1.6 2 2.4 1 and 2 1 and 2 2 2 Not

deﬁned

2 1.9 Not

deﬁned

Not

deﬁned

Not

deﬁned

2.4

[Al]

(M) 0.1 0.1 0.07 0.8 0.5 Not

deﬁned

Al–Cu Chlorhyd.S

50%

Chlorhyd.S

50% 2.5 M

0.068 or

chlorhyd.S

Chlorhyd.P Locron

(Al-

hydr.chlor.)

50%

Not deﬁned 0.07

Clay/water 40% 10% 1% Different P P P 50% 10, 15, 20 P 50% 50% P P

in DB in DB in DB Cc and and in water 40, 50% or in acetone in acetone in DB in DB

10% 33%in DB in acetone suspension

Al meq/g

clay

25, 50, 75,

100

20, 40, 60,

Not

deﬁned

Not

deﬁned

23 9 30, 60 30 15 11.4 30 5, 10, 30 Not

deﬁned

Time 24 h 1, 3 48 h 24 h 1 night 24 h 48 h 17 7 min 12 h 24 h 2 h 0 h 0 h

of exchange 48 h 7 days under

reﬂux

microwave

W/D 2 D 1 D D until



free

W until



free

5W 4W 5D 4W WuntilCl



free

Filter-press 4 W D until



free

5D 5D

Notes:

1: Molina et al. (1992) Bd: Beidellite

2: Del Riego et al. (1994) Ht: Hectorite

3: Schoonheydt et al. (1994) Il: Illite

4: Schoonheydt et al. (1993) Lap: Laponite

5: Schoonheydt and Leeman (1992) Mt: Montmorillonite

6: Sanchez and Montes (1998) Sap: Saponite

7: Frini et al. (1997) Sm: Smectite

8: Storaro et al. (1996) Cc: Concentrations

9: Fetter et al. (1997) Chlorhyd: Chlorhydrol

10: Moreno et al. (1997) D: Dialysis

11: Storaro et al. (1998) DB: Dialysis bags

12: Salerno and Mendioroz (2002) P: Powder

13: Vicente and Lambert (2003) W: Washing

14: Aouad et al. (2005) S: Solution

7.5.6. Pillaring Methods 405

(Fig. 7.5.2). Such a truly solid–solid reaction was not reached till now. The dialysis

step seems crucial for pre-pillaring. More fundamental research is needed to under-

stand the reactions and mechanisms involved.

7.5.7. MAIN PILC CHARACTERISTICS FOR DIFFERENT APPLICATIONS

The ﬁrst common characteristic of PILC is the distance (Dd) separating contig-

uous layers after heating the intercalated clay miner als. Thus, PILC have a perma-

nent porosity and a higher a ccessible speciﬁc surface area (SSA) than the pristine

clay. The basal spacing of smectit e intercalated with Al Keggin cations is about

1.8 nm, corresponding to Dd of 0.9 nm ( Fig. 7.5.3). If a smaller Dd value is observed,

pillaring is not regarded to were 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 nm and 1.8 nm oc-

curred. Pillaring occurs between each two-layers of montmorillonite. Still higher

interlayer distances (up to 7 nm) were observed by Mandalia et al. (1998) for a series

Porosity

001

= 18.6 nm

Fig. 7.5.3. AlPILC structure.

Chapter 12: Pillared Clays and Clay Minerals406

of Al–Fe PILC. However, a TEM image analysis performed on one of these samples

(Clinard et al., 2003) shows that this value refers to a correlation distance for Fe

particles present between clay mineral particles, each of which is composed of 4–5

layers. In this case, pillaring occurs between particles rather than between individual

layers.

Moreover this interlayer distance, PILC are also characterized by interpillar dis-

tance (as shown in Fig. 7.5.3), which is more difﬁcult to determine (Gil and Gandia,

2003).

The important property is the accessible porosity leading to a higher SSA than the

pristine (parent ) clay mineral. A smectite with completely exfoliated layers has an

SSA of about 800 m

/g. The highest SSA recorded for PILC is 600 m

/g with a total

pore volume of 0.6 cm

/g (Vaughan et al., 1979). This porosity is due to pores of

different size, although micropores (o2 nm) are usually obtained by Al pillaring. In

some instances, such as the mixed Al—Fe PILC, mesopores (2–50 nm) are observed

(Mandalia et al., 1998). Macropores (>50 nm) are also obtained with delaminated

pillared 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. In comparing a number of pillared bentonites,

Zonghui and Guida (1985) found that the Ti PILC are the most acidic (Table 7.5.3). 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 the PILC (Figueras et al., 1990; Tichit et al., 1991).

The structure of the Al Keggin cation, the most used intercalant for PILC, was

established by instrumental techniques, particularly NMR spectroscopy, but that of

other metal intercalants is less understood. Some of these, such as Ga

, are assumed

to be similar in structure to Al

(Bradley and Kydd, 1990a, 1990b). In other cases,

hypothetic structures were proposed for Zr and Si pillars, as reported in some

Table 7.5.3. Comparative acidities of different pillared bentonites as measured by titration

with an ammonia adsorption differential thermal method developed in China

Samples Acidity (mV)

Al-CLB 425–442

Zr-CLB 570

Ti-CLB 620

Fe-CLB 340

Ni-CLB 228

Al–Zr-CLB 390

Al–Fe-CLB 340

Na–B 86

Notes: The unit of acidity is expressed in mV. From Zonghui and Guida (1985). B: bentonite; CLB: cross-

linked bentonite.

7.5.7. Main PILC Characteristics for Different Applications 407

reviews (Figueras, 1988; Kloprogge, 1998; Konta, 2001), while the structures of the

metal intercalant in Cr and Sn PILC have yet to be determined (Bodman et al.,

2002).

7.5.8. INTERCALANT STABILITY BEFORE AND AFTER PILLARING

The stability of the intercalant species before and after pillaring is of primary

importance to using PILC in different applications.

The stability of Al

sulfate hydrate was investigated by Kloprogge et al. (1992)

who shows that the water adsorbed in excess is lost at 80 1C. On further heating, the

oligomer gradually decomposes losing its 12 water molecules and 24 hydroxyl

groups, becoming ﬁnally X-ray amorphous at 360 1C. Above this temperature, the Al

oxide is formed till 950 1C.

The required thermal stability of the 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). Accor ding to Schoonheydt et al. (1993), the increase in basal

spacing alone is not a sufﬁcient criterion for thermal stability, the line width of the

(0 0 1) reﬂection must also be taken into account.

The most important question is whether the s tructure of the intercalated

oligomers is maintained after heating, or w hether the intercalant species are trans-

formed into oxide pillars, as proposed by Vaughan et al. (1979). The available data

are s cattered and contradictory. If the intercalant is transformed into classical

oxides, why does the basal spacing remain almost unchanged in the PILC? Many

NMR studies showed that the Al

structure is maintained till 500 1C, except for

pillared beidellite (Ple

e et al., 1985). This might be because the water of hydration

of the intercalant is lost by heating, an d dehydroxylation occurs in the 300–400 1C

range (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). A

recent NMR and microcalorimetry study (Occelli et al., 2000) of A l-pillared mont-

morillonite shows that heating the Keggin ion at 500 1C in the intercalated clay

leads to the formation of blocks of 13 Al atoms containing Al (IV and VI) as well

as new Al (V). The Al

structure is no longer maintained at this temperature, but

g-alumina is also not detected by XRD. Moreover, a thermo-XRD study (Aceman

et al., 2000) of Al-pillared smectites shows that the added Keggin ion ﬁrst dis-

sociates into smaller hydroxy- and/or chloro-Al oligomers, which increase in size

after w ater addition (thorough washing or dialysis) in the interlayer space. At

600 1C, these Al oxocations adsorbed by the smectites, condensed with the silicate

sheet, preventing further s welling in water. However, the exact nature of the ob-

tained alumina clusters that form the pillars in Al

-PILC sti ll remains a subject of

discussion.

Chapter 12: Pillared Clays and Clay Minerals408

7.5.9. LINKAGE BETWEEN PILLARS AND CLAY MINERAL LAYERS

Many hypotheses were 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.

When Al

is heated, protons and water are simultaneously released, and con-

densation takes place between the OH groups of the oligomeric ion and lattice

hydroxyls of the silicate layer. One suggestion is that the resultant Al oxide pillar is

linked via oxygen to the cation of the octahedral sheet (Tennakoon et al., 1986),

giving rise to a rigid cross-linked structure and loss 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.

There is no generally accepted mechani sm that can account for the stable linkage

that exists between pillar and clay mineral surface, especially at high temperatures.

Cross-linking between pillars and silica tetrahedra that become inverted after

calcination, was proposed by Fripiat (1988) on the basis of NMR studies. Tet-

rahedral 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; Lambert et al., 1994; Kloprogge et al., 1994; Lambert

and Poncelet, 1997). It would also depend on the composition of the octahedral

sheet. For example, in ﬂuorohectorite the presence of electronegative ﬂuorine ions

stabilizes the Si–O bond, promoting coupling between inverted Si tetrahedra and

the Al

pillars on calcination (Pinnavaia et al., 1985). In case of synthet ic sapo-

nite, a covalent Si–(OH)–Al

pillar

bond was suggested. The silanol groups result

from the attack of the Si–O groups on the surface of the tetrahedral sheet, by the

protons released during calcination (Sterte and Shabtai, 1987; Jones, 1988; Bukka

et al., 1992; Li et al., 1993; Kloprogge, 1998; Aceman et al., 1999; Occelli

et al., 2000).

7.5.10. CONCLUSIONS

A lot of fundamental work was realized in these three past decades on PILC,

particularly with Al-pillared smectites. Surprisingly, all the available data open the

way to numerous questions and to the need of better understanding the hydrolysis of

cations, even for the Al ion as the most studied one.

Many other metal ions can act as pillaring species. For example, a few papers are

published on Ti PILC (Valverde et al., 2002, 2003‘) despite their high thermal sta-

bility and high acidity. The challenge will be to pillar modiﬁed kaolinite or sepiolite

to take proﬁt about its particular texture.

To be of useful interest, these PILC must be used in some industrial or envi-

ronmental applications. For this reason , one has to pay more attention to the

7.5.9. Linkage between Pillars and Clay Mineral Layers 409

fundamental point of view toward the pillaring in concentrated clay dispersions.

Solid–solid state reaction remains a challenge for the future.

In spite of the knowledge actually accumulated on PILC, two main questions

remain (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 that

maintains the stability of the pillared clays minerals.

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