Bergaya F. Handbook of Clay Science

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Chapter 10.1: Conventional Applications540

Handbook of Clay Science

Edited by F. Bergaya, B.K.G. Theng and G. Lagaly

Developments in Clay Science, Vol. 1

r 2006 Published by Elsevier Ltd.

541

Chapter 10.2

CLAY MINERALS AS CATALYSTS

J.M. ADAMS

AND R.W. MCCABE

School of Engineering, University of Exeter, Harrison Building, North Park Road,

Exeter, Devon , EX4 4QF, UK

Center for Materials Science, University of Central Lancashire, Preston PR1 2HE,

United Kingdom

In most petrochemical processing, such as cracking, solid catalysts were used for

decades. Originally, the catalysts were based on acid-treated clays (Franz et al., 19 59 )

but, for many years now, framework silicates (zeolites) proved more effective and

selective.

Established manufacturing processes for other organic chemicals are, however,

still largely based on stirred tank reactors and out-dated methods, employing volatile

solvents, stoichiometric reagents and hazardous chemicals. The processes are often

inefﬁcient and produce large volumes of waste (commonly 10–50 times product).

Heterogenisation is one of the most important of the ne w clean technologies, which

are being developed to reduce waste in liquid phase organic reactions. If the catalyst

or reagent stays in a separate, solid phase then a water quench phase (the source of

much of the waste) is unnecessary. Additional beneﬁts of this approach include

avoiding solvent (a heterogeneous catalyst or reagent can act as a solid solvent) and

enhancing selectivity (due to pore constraints).

In order to exploit this exciting, greener technology in organic chemical manu-

facturing, we need to produce highly effective catalysts in ﬂexible physical forms to

suit a multitude of chemical process technologies. Zeolites have attractions but are

limited in the size of molecule they can accommodate in their limited-size pores.

Mesoporous silicas, on the other hand, have suitable pore sizes, but lack chemical

activity.

Clay minerals have a different and interesting set of properties. They are very

effective catalysts for a wide variety of organic reactions, often displaying highly

sought product-, regio- or shape-selectivity. In addition, however, ‘‘soft’’ dimensional

DOI: 10.1016/S1572-4352(05)01017-2

constraints are often displayed. They have significant potential in organic chemical

processing. Their potential was not realised, we believe, because:

(1) The variability of natural clays, the wide variety of clay miner al types and the

range of possible treatments meant that a reliable set of data on which predictive

capability can be based was not generated: there have been too many restricted

studies which do not link together.

(2) Not enough attention was given to the engineering of the clays into catalyst

particles, pellets, membranes, etc. so that they can survive the rigorous condi-

tions in a range of reactor types. The changes in interlayer spacing, which occur

with exposure to reactants/solvents/products makes this particularly challenging,

affecting the mechanical integrity of any catal yst ‘‘artefact’’.

Here, we conﬁrm the range of catalytic options provided by clay minerals. We

hope that the phenomenal range of activity displayed will encourage dedicated study

to overcome the issues above.

Previous reviews in the area included those by Theng (1974, 1982), Solomon and

Hawthorne (1983), Laszlo (1986a), Adams (1987) and McCabe (1996).

Early work with clays concentrate either on acid treatment or on cation exchange

to increase Brønsted or Lewis acidity. However, in the last few years, the reaction

portfolio widened so that clay minerals are now recognised (McCabe, 1996) as ef-

fective for a range of other types of reaction including redox reactions, reactions

involving reactive intermediates and cycloadd itions (such as Diels–Alder reactions).

The high surface area of clays also means that they can act as effective supports for

(usually inorganic) reagents bringing the beneﬁts of heterogeneous catalysis to sev-

eral important reactions.

The beneﬁt of previous reviews allows us to focus on material most relevant to

green/clean technologies. This chapter focuses on smectites and their derivatives,

which display by far the highest activity levels and range of reactions catalysed. After

a short discussion of the origins of the activity of these clay minerals, the autho rs

summarized the performance in different types of reaction. Finally, they covered the

more important recent developments and commented on current utilisation of these

clays in industrial processes and prospects for the future.

10.2.1. ORIGIN OF ACTIVITY

A. Low Dimensionality

Laszlo (1987) made the point elegantly that not only is the acti vity of clay minerals a

consequence of their high surface area and chemical nature: there are two other

closely related factors:

(1) Local concentration effects—the act of adsorption on the solid surface increases

the reactant concentrations.

Chapter 10.2: Clay Minerals as Catalysts542

(2) Low dimensionality (o3) of the clay minerals means that molecules on the surface

are more likely to meet (drunkards walk) than they would be in three dimensions.

Both factors lead to enhanced collision frequency and, hence, reactivity.

B. Structural Characteristics Imparting Activity

Most of the reactions catalysed by smectites make use of the acidic nature of acid-

treated or cation-exchanged clay minerals. Both Lewis and Brønsted activity are

common, the form er deriving from aluminium or iron species located at crystal edges

(see Chapters 5 and 7.3) (Theng, 1974). The Brønsted activity, however, results either

from free acid (in some acid-activated clays) or from the dissociation of interlayer

water molecules coordinated to polarising interlayer cations (M

). It was shown

(see, e.g., Mortlan d and Raman, 1968) that acidity in the latter case increases at low

water contents and is enhan ced when using interlayer cations of high charge and

small radius such as Al

,Fe

or Cr

½MðOH

Þn

mþ

þ B !½Mð OH

n1

OH

ðm1Þþ

þ BH

where B is water or an organic species in the interlayer space. H

or the pro-

tonated base are available for further reaction. Weiss (1981) suggested that interlayer

concentrations could be as high as 10 M in some cases.

For acid-activated clays (see Chapter 7.1) a significant amount of the activity

derives from Al

or Fe

ions which have been liberated from the octahedral sheet

and relocated to crystal edges or into the interlayer region (Fahn and Fenderl, 1983).

In this case, also, there is an enhancement of activity deriving from increased surface

area (up to 300 m

1

, Su

d-Chemie, 1961).

Increases in Lewis acidity at room temperature were made by depositing Lewis

acids such as ZnCl

(clayzic) and AlCl

onto clay mineral surfaces (Clark et al., 1994).

Activity is enhanced by synergistic effects with the clay, and the Lewis acidity of the

supported salt is often greater than would be expected (e.g., clayzic>clay/AlCl

Redox reactivity can be derived from either Fe

or Fe

in the octahedral sheet

(Solomon et al., 1968), from exchange of redox-active cations such as Cu

,Fe

into the interlayer region (Caine, 1999) or by depositing anhydrous metal

nitrates on the clay support (e.g., claycop and clayfen, Laszlo (1987)).

C. Shape/Size Select ivity

Smectites ideally consist of regular, parallel layers. The reality is somewhat different

(see Fig. 10.2.1 from Cebula et al., 1979). Nonetheless, if we consider the energy

required to separate the ‘‘parallel’’ layers, it depends upon electrostatic and van der

Waals components (Lee et al., 1987). For a given set of reaction conditions (reactant

concentrations, solvent, temperature, etc.) the interlayer spacing will be set by the

10.2.1. Origin of Activity 543

balance between energy gained by intercalation (usually coordination around the

interlayer cation) and that required to separate the layers (Fig. 10.2.2), thus giving, in

effect, a soft constr aint to reaction geometry. In principle, these clay minerals can

accommodate larger molecules between their layers than do relatively rigid frame-

work silicates (zeolites).

If catalytic properties of clays and zeolites are compared, the activity of acid-

treated or cation-exchanged clays are relatively poor at temperatures above 150 1C, a

result of dehydration and collapse of the clay layers. To overcome this problem, the

so-called pillared clays were produced. These materials use inorganic cations to prop

the layers apart (Vaughan et al., 1979; Pinnavaia, 1986; Poncelet and Schutz, 1986)

and have the ad vantage that the pillar itself may be catalytically active (see, e.g., Suib

et al., 1986). Pillaring also confers some degree of size/shape selectivity (e.g., Kikuchi

and Matsuda, 1988).

Fig. 10.2.2. For intercalation to occur, the energy gain on intercalation X that required to

separate layers.

Fig. 10.2.1. Schematic structure of a clay: water system.

Chapter 10.2: Clay Minerals as Catalysts544

D. Catalysis of Inorganic Reactions

Clay minerals are well-known catalysts for organic reactions, but recently they were

shown to catalyse the equation of [Cr(H

]

to [Cr(H

Cl]

and

[Cr(H

]

(Caine et al., 1999). Normally, these reactions are quite slow (weeks),

as the d

chromium (III) cations have a half t

sub-shell and are substitutionally

inert. However, intact (not acid activated) clays, along with alumina and zeolites,

catalyse these reactions in a matter of minutes or hours. The catalytic reaction

appears to be dissoc iative and aided by conjugate base formation, i.e., S

1CB (Mao,

2003). Thus, unusually, the clay minerals appear to be acting as base catalysts by

exchanging protons from solution. Similar results are being found wi th other chro-

mium (III) halide complexes and with even more inert Co(III) complexes (d

-ﬁlled t

sub-shell). Improved yields of sodium or potassium tris-oxalatochromate(III) (3–4

fold) and tris-ethylenediaminechromium(III) chloride (1.5-fold) complexes were

found using powdered Surrey bentonite (Mao, 2003).

E. Clays as Supports for Reagents

Clays provide high surface area on which to support reactive compounds. They can

also provide (see above) a variety of types of active sites, which can be co-active with

the supported reagent. A surge of interest in the area in the 1990s used ‘‘claycop’’

and ‘‘clayfen’’ for selective oxidations (e.g., Corne lis and Laszlo, 1986 ). Very high

acidities can be achieved by supporting superacids and heteropolyacids on clays

(e.g., Knifton and Edwards, 1999).

10.2.2. PREPARATION/ACTIVATION OF CLAY CATALYSTS

To achieve the highest activity, the smectite component needs to be separated

from the impurities in the mineral ore, usually by sedimentation/size fractionation

(see Chapter 4).

Cation exchange of smectite is usually carried out by exposing the clay to an

appropriate salt solution of 0.5–1 M for 24 h. The clay can then be centrifuged and

re-suspended repetitively to remove excess exchangeable cation before drying at

40–50 1C and ﬁne grinding (see Chapt er 4).

Commercial acid treatment is carried out using concentrated hydrochloric, sul-

phuric or phosphoric acids. The concentration of the acid and the treatment time

varies between producers and products, as does the removal of excess acid or other-

wise by washing (see Chapters 7.1 and 10.1). There is, therefore a great variety of

products available (e.g., Fahn, 1979, and commercial literature such as that from

d Chemie).

Ion exchange of montmorillonites with cations such as: Al

,Fe

,Cu

,Ni

,Co

and Na

can modify the catalytic activities of the clay catalyst.

10.2.2. Preparation/Activation of Clay Catalysts 545

For example, the rearrangement of a-pinene to camphene and the rearrang e-

ment of camphene hydrochloride to isobornyl chloride were used to determine the

catalyst’s Brønsted and Lewis acid strengths, respectively (Brown and Rhodes,

1997). Thermal activation of the catalyst is also important with maximum

Brønsted acidity developing at app roximately 150 1 C and maximum Lewis acidity

at 250–300 1C. Significantly, Al

-exchanged clay minerals demonstrate relatively

low surface Lewis acidity. Metal oxide pillared clay (PILC) possesses several in-

teresting properties, such as large surface area, high pore volume and tunable pore

size (from micropore to mesopore), high thermal stability, strong surface acidity

and catalytic active substrates/metal oxide pillars. These unique characteristics

make PILC an attractive material in catalytic reactions (Kloprogge, 1998; Ding

et al., 2001; Vansant and Cool, 2001). The activity and porosity of the PILC are

highly dependent on the method of preparation (Salerno et al., 2001; Sapag and

Mendioroz, 2001).

The surface acidity of alumina- and double-pillared montmorillonite and sapo-

nite was studi ed by ammonia TPD, iso-propanol conversion and n-butene skeletal

isomerisation catalysis and by FTIR spectroscopy of the surface hydroxy -groups,

and of adsorbed acetonitrile and pivalonitrile (Trombetta et al., 2000). The alu-

mina pillars in montmorillonite carry stronger Lewis sites than those of pillared

saponite. Stronger Brønsted sites occur on the montmorillonite layers and the

pillared montmorillonite is more active in converting iso-propanol. However, it is

too active in converting n-butene, giving rise to faster coking and more extensive

cracking; thus, the pillared saponite was more selective for converting n-butene

into iso-butene.

Heating cation-exchanged or acid-treated clay minerals leads to loss of inter-

layer water. For clay minerals exchanged with monovalent cations, heating much

above 100 1C leads to a decrease in interlayer spacing, after which re-expansion is

difﬁcult. For higher charged cations, the greater stability of the primary coordi-

nation sphere means that the layers do not collapse completely, even at 300 1C.

Breen et al. (1987) showed that excessive heating can lead to a reduction in re-

activity. PILCs are much more thermally stable and can be pre-conditioned at

500–600 1C if required.

Since the reactivity of smectites depends so critically on water content ( Mortland

and Raman, 1968) to attain reproducibility in the rate of reactions carried out at low

temperatures, it is essential to equilibrate the clay at a known relative humidity

before reaction (Adams, 1987). Finally, while solvents in clay-catalysed reactions are

often chosen for their reﬂux temperature or for solubility reasons, it is known that

even allowing for these factors, alteration of solvent can lead to extreme variation in

reactivity (e.g., Adams et al., 1983a).

Three main strategies are employed with supported reagents/catalysts (McCabe,

1996): (a) deposition of a large excess of reagent from solution onto the clay; (b)

heterogenisation of small amounts of expensive catalyst by sorption onto the clay;

and (c) covalent bonding of the catalyst to the clay mineral surface.

Chapter 10.2: Clay Minerals as Catalysts546

10.2.3. SUMMARY OF CATALYTIC ACTIVITY

The diverse catalytic activity of clay minerals is derived mainly from four main

sources, Brønsted acidity, Lewis acidity, presence of redox active species and intro-

duction of catalyticall y active (mainly) transition metals or cations. Such activity can

be either a natural property of the clay or can be introduced by cation exchange, acid

activation or deposition of the co-catalyst onto the clay surface.

The expandable layers of clay minerals provide useful two-dimensional hosts,

which facilitate introduction of catalytically active metal complexes by cation ex-

change and ligand exchange methods (Tao and Zou, 2002). Intercalation of metal

complexes into the interlayer space of clay minerals is an excellent method for ho-

mogeneous catalyst heterogenisation. Metal complex catalysts intercalated into clay

minerals display molecular-recognising catalysis, such as shape selectivity, stereo-

selectivity and regio-selectivity. The application of these heterogenised catalysts in

asymmetric hydrogenation, regioselective hydrogenation, regioselective carbonylat-

ion and stereoselective arylation is describe d below. Metal complex catalysts inter-

calated into clay minerals exhibit sufﬁciently high activity and selectivity to make

them competitive to the homogeneous catalysts.

A. Brønsted Acid Catalysis

The inherent Brønsted acidity of cation-exchanged clays can be used to catalyse a

very large number of organic reactions (Table 10.2.1): they are often clean, high

yielding and offer routes to products that are inaccessible or difﬁcult to obtain by

other, more conventional means. The reactivity arises from dissociation of interlayer

water molecules and reaction in the interlayer space often leads to shape selectivity in

terms of reactants or products.

Adams et al. (1983a) produced a set of guidelines concerning Brønsted reactivity

of cation-exchanged clay minerals which covered:



reaction site (interlayer space for unsaturated hydrocarbons; interlayer space and

surface for polar, oxygenated species);



effective interlayer cations (Cr

and Fe

most active and reproducible);



solvent (in liquid phase, when low acidity is satisfactory, solvent should be chosen

for miscibility; when higher acidity is requ ired, a non-polar solvent is more ef-

ﬁcacious); and



type of carbocations involved (below 100 1C, tertiary or allylic; at 150–180 1C,

primary or secondary carbocations could be involved).

Laszlo (1986b) subsequently pointed out that primary carbocat ions are probably

not involved, and that by 150 1C benzyl carbocations probably would be involved,

where relevant.

While interest in clay minerals for coupling amino acids to give polypeptides

continues (Bujda

k and Rode, 1996, 1997, 1999; Bujda

k, et al., 1996; Rode, 1999;

Rode et al., 1999; Porter et al., 2000, 2001), clay minerals were also implicat ed in the

10.2.3. Summary of Catalytic Activity 547