Bergaya F. Handbook of Clay Science

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including Brønsted and Lewis acids ( Houk and Strozi er, 1973), radical cations

(Belville et al., 1981), use of aqueous solvents (Laszlo and Lucchetti, 1984a) and

solventless reactions on K10 (Avalos et al., 1998a, 1998b). The latter reactions also

show Michael addition products.

The most important (apparently) commercial Diels-Alder reaction catalysed by

clays is the cyclo-dimerisation of oleic acids (Den Otter, 1970a, 1970b, 1970c; Newton,

1984). The reaction involves high temperatures to effect the preliminary dehydrogen-

ation of the oleic acid. Thus it is possible that there is no catalysis of the Diels–Alder

step. Many other reactions, clearly of Diels–Alder type were, however, been shown to

be catalysed by smectite-based catalysts (Table 10.2.5).

Non-acid-activated transition metal-exchanged clay minerals give better control

of stereoselectivity than do acid-activated catalysts (Martin, 1986; Adams et al.,

1987a, 1994). Rates of reaction are much greater with M

-exchanged clay minerals

than with M

-clay minerals, but the former produce a much greater proportion of

endo-isomers.

The greater the layer charge, the smaller the interlayer spacing and the lower the

rate of reaction. The endo/exo ratio also diminishes, suggesting that the less bulky

Table 10.2.5. Clay-catalysed Diels–Alder reactions

Substrate Product Clay Reference

Butadiene Vinyl

cyclohexene

-montmorillonite Downing et al. (1978)

Butadiene, isoprene Dimers TM-exchanged clays Adams and Clapp

(1986), Adams et al.

(1987b)

1,3-Hexadiene Dimer Fe

-K10 Laszlo and Lucchetti

(1984b)

Methyl vinyl ketone or

acrolein with cyclo-

pentadiene, furan or

2,5-dimethylfuran Adducts Cation-exchanged clay,

hydroxy-chromium

pillared clays

Laszlo and Lucchetti

(1984c, 1984d), Laszlo

and Moison (1989),

Martin (1986), McCabe

(1996)

6-Methylpyran-2(H)-

ones with

1,4–naphthoquinone or

N-phenylmaleimide

Adducts Montmorillonite K10,

ﬁltrol-24 (best),

bentonite or pyrophyllite

(poor) clays or AlCl

ZnCl

and FeCl

montmorrilonite or

bentonite

Kamath and Samant

(1999) Kamath et al.

(2000)

Chapter 10.2: Clay Minerals as Catalysts558

transition state (which leads to the exo-isomer) is formed preferentially when the

interlayer space is reduced (Adams et al., 1994).

A small number of other cycloaddi tion reactions were also demonstrated. For

example, stilbazolium cations adsorbed on clays undergo efﬁcient and selective

photodimerisation to the cyclobutane dimer (Takagi et al., 1989). Cis– trans iso-

merisation of the double bond is suppressed between the clay mineral layers.

F. Clays as Supports

In the 1990s, there was a surge in the use of supported reagents and catalysts as the

use of heterogeneous systems simpliﬁes the puriﬁcat ion of reaction products. The

beneﬁts of this approach include low cost, high reactivity and the ease with which the

reactions are carried out. Some examples are given below of the use of ‘‘Clayfen’ ’

and ‘‘Claycop’’ (Table 10.2.6)(Cornelis and Laszlo, 1986; Clark et al., 1994),

iron(III) and copper(II) nitrates supported on acid-treated clay (K10 from Su

Chemie). They all depend on the fact that the reagents are sources of nitrosonium

ions.

Palladium chloride and tetraphenylphosphonium bromide intercalated clay min-

eral were used in the Heck reaction (coupling of aryl halides and styrenes to give

stilbenes) (Varma et al., 1999f) and for Suzuki coupling (coupling of aryl halides with

arylboronic acids to give biphenyls) (Varma and Naicker, 1999a, 1999b). The Heck

coupling vinylation of halobenzenes with methyl acrylate was achieved with clay-

supported palladium catalysts in N-methylpyrrolidone in the presence of triethyla-

mine and/or sodium carbonate (Zhao et al., 2000). Significant amounts of palladium

leach out into the solvent and these dissolved Pd species essentially catalyse the

reaction; however, almost all the palladium species in the solution are re-deposited

onto the surface of the supports after the reaction was completed (at 100% con-

version of iodobenzene).

Alcohols were acylated efﬁciently using anhydrides/acetic acid and [N,N

-ethylene

bis(salicylideneaminato)] manganese(III) chloride supported on a clay (Choudary

et al., 2001).

Table 10.2.6. Stoichiometric oxidation reactions with iron(III) or copper(II) nitrates sup-

ported on K10

Reaction Reference

Alkyl secondary alcohols oxidised to ketones Cornelis and Laszlo (1980)

Aromatic primary alcohols oxidised to aldehydes Cornelis and Laszlo (1980)

Thiols oxidised to disulphides Cornelis et al. (1983)

Thioketones oxidised to ketones Chalais et al. (1985)

Hydrazides converted to azides Laszlo and Polla (1984)

10.2.3. Summary of Catalytic Activity 559

Palladium(II) chloride/copper(II) chloride complex (Wacker catalyst) heterogenised

on montmorillonite catalyses low-temperature oxidation of carbon monoxide, car-

bonylation in the synthesis of ibuprofen or naproxen, oxidative carbonylation for the

synthesis of diphenyl carbonate and the functionalisation of methane (Park et al.,

2000). There appears to be no significant change in the reaction mechanisms when the

catalyst is transferred from solution to the solid surface.

Chirally modiﬁed hectorites (Laponite RD) are prepared by ion exchange with (R)-

and (S)-N-alkyl-phenylethyl ammonium salts (Torok et al., 2000). Enantiodifferen-

tiation is observed on using these chiral nanoreactors for a transition metal-catalysed

hydrogenation reaction. A chiral rhodium complex intercalated into hectorite con-

taining guest molecules shows drastic changes in catalytic properties for asymmetric

hydrogenation depending on the orientation of the guests (Sento et al., 1998).

Cubooctahedral palladium particles deposited on montmorillonite from colloidal

dispersion give particles ranging in size from 1.5 to 6.2 nm in mean diameter (Veisz

et al., 2002). These catalysts hydrogenate styrene to ethylbenzene under mild con-

ditions. Good correlation between the turnover frequencies and the defect site den-

sities suggest that hydrogenation occurs on the defect sites and that terrace sites have

only minimal catalytic activity or are inactive.

Anhydrous iron(III) chloride supported on K10 acts as a Lewis acid catalyst for

the benzylation of arenes with benzyl chloride (Natekar and Samant, 1995; Pai et al.,

2000); the latter authors also report efﬁcient reaction with aluminium(III) chloride

and zinc(II) chloride on the same support. Anhydrous lanthanide salts supported on

K10 act as good acylating agents for arenes with acid chlorides and anhydrides

(Baudry-Barbier et al., 1999) and as arene alkylating agents (Baudry-Barbier et al.,

1998). In both cases fairly activated substrates were used.

Zinc bromide supported on mesoporous silica or acid-activated montmorillonite

is a fast, efﬁcient, selective and reusable catalyst for the para-bromination of ac-

tivated and moderately deactivated aromatic substrates (Clark et al., 1997).

Pillared clay minerals are being examined as catalysts for removal of NO

from

exhaust fumes (Serwicka, 2001). Zirconium aquahydroxycation pillared clay minerals

prepared from untreated or HCl-pretreated montmorillonite are promoted by ad-

sorption of manganese and they show catalytic activity in the speciﬁc catalytic re-

duction of NO

(Grzybek et al., 2001). Thermally stable Al- and Zr-PILC loaded with

copper and cobalt cations and silver nanoparticles are examined for their speciﬁcity for

the catalytic reduction of NO

by propane, propylene and decane in excess of oxygen.

Formation of adsorbed-NO

, nitroxyl-hydrocarbon C

and isocyanate NCO

intermediates is observed by ESR and IR spectroscopy (Konin et al., 2001).

G. Organo-Clays as Phase Transfer Agents

Quaternary ammonium organo-clays were used as phase transfer catalysts for nu-

cleophilic substitution (Cornelis and Laszlo, 1982; Lin and Pinnavia, 1991) reactions,

including the formation of cyanides, thiocyanides and alcohols from benzyl bromides

Chapter 10.2: Clay Minerals as Catalysts560

and the corresponding aqueous sodium salts (Varma et al., 1999d), and azides from

benzyl and alkyl bromides or a–tosyloxyketones in organic solvents (Varma and

Kumar, 1998; Varma and Naicker, 1998; Varma et al., 1999c) with sodium azide in

water. 18-Crown-6-ether supported on a clay was also used as a phase transfer catalyst

for the conversion of alkyl bromides to alkyl azides (Varma et al., 1999e).

Benzoic anhydride can be prepared from benzoyl chloride and sodium benzoate

using clay-supported quaternary ammonium salts as phase transfer catalyst (Yadav

and Naik, 2000).

10.2.4. SIGNIFICANT RECENT DEVELOPMENTS

In line with the strong trends towards green chemistry over the last decade or so,

clays were investigated as supports for many toxic reagents or as replacements for

stoichiometric reagents that produce large volumes of pollutant by-products. An-

other trend in the same vein is the upsurge of ‘‘solventless’’ reactions initiated by

microwave or ultrasound irradiation.

A. Supported Acids

Very high acidities can be achieved by supporting superacids and heteropoly acids on

clay mineral surfaces. For example, triﬂuoromethanesulphonic acid supported on a

montmorillonite gives a superacid sulphonic clay with a Hammett acidity function,

o–12.75. The clay mineral activity has been evaluated in the solvolysis of pro-

pylene oxide with a series of alcohols, yielding high conversion (88.9 – 99.8%) and

selectivity (27.6 – 80.8%) to monomer formation (Salmon et al., 1997).

The esteriﬁcat ion of lactic acid with iso-propanol and maleic acid with ethanol

catalysed by ion-exchange resins and 20% dodecatungstophosphoric acid supported

on K10 at reﬂux were compared (Yadav and Kulkarni, 2000; Yadav and Thathagar,

2002). The ion-exchange resins produce somewhat better results.

Heteropoly acids, such as 12-tungstophosphoric acid and 12-molybdophosphoric

acid, on HF-treated montmorillonites or mineral acid-activated clay minerals are

very effective in the synthesis of methyl tert-butyl ether from metha nol/tert-butanol

(Knifton and Edwards, 1999) and for the formation of acetone and phenol from the

cleavage of cumene hydroperoxide (Knifton and Sanderson, 1997), using continu-

ous, plug-ﬂow, reactor systems. Dodecatungstophosphoric acid supported on K10 is

found to be a very efﬁci ent catalyst in comparison with several others for alkylation

of hydroquinone with alkylating agents such as methyl tert-butyl ether (MTBE) and

tert-butanol (Yadav and Doshi, 2000).

Palladium (1 wt%) supported on alumina pillared acid-activated montmorillonite or

sulphated zirconium pillared sodium montmorillonite exhibits much higher activities

and isomerisation selectivities for hydroisomerisation of hexanes and cyclohexanes

than those obtained on a conventional pillared montmorillonite (Issaadi et al., 2001).

10.2.4. Significant Recent Developments 561

The sulphated zirconium pillared sodium montmorillonite catalyst is two to three times

more active than the other catalysts and the reaction is thought to go through a pro-

tonated alkane intermediate.

B. Reactions Initiated by Microwave, Ultras ound or Gamma Irradiation

A large number of the more traditional clay-catalysed or stoichiometric clay-sup-

ported reagent reactions were reported, usually under solvent-free conditions using

microwave irradiation of the dry mixture followed by solvent extraction. Improved

yields (often>90%), much shortened reaction time (seconds to minutes), occasional

improved selectivity and speciﬁcity for polar substrates are found. Varma (2002) and

Loupy (1999) reviewed this growing ﬁeld recently and a selection of the types of

compounds examined is listed in Table 10.2.7 .

The Beckman rearrangement of ketoximes to 2 amides on K10 (Gutierrez et al.,

1989) and the pinacol to pinacolone rearrangement on Al

-exchanged K10 (Bosch

et al., 1995) are readily promoted by microwave irradiation. Several heterocyclic

systems were synthesised on acid- activated clays under microwave irradiation, in-

cluding the following (Table 10.2.8):

1. The rates of formation of azides from benzyl and alkyl bromides or a–tosyl-

oxyketones in organic solvents (Varma and Kumar, 1998; Varma and Naicker,

1998; Varma et al., 1999c) and sodium azide in water are enhanced by ultrasound

irradiation.

2. The rate of glucosylation of butanol and dodecanol in the presence of KSF/O as

catalyst is increased by application of ultrasound (Brochette et al., 1997). In the

case of dodecanol high yields of 1,6-polyglucose are obtained. Radiolysis of car-

boxylic acids adsorbed on clay minerals shows mainly decarboxylation in contrast

to condensation/dimerisation reactions in free solution (Negron-Mendoza and

Ramos-Bernal, 1998).

C. Clay Nanocomposites

The catalytic properties of clay minerals were used to form a variety of clay-polymer

nanocomposites with novel material properties (Carrado, 2000; see Chapter 10.3).

Examples include: polybenzoxazine-montmorillonite ( Shi et al., 2002) and formal-

dehyde with ethylenediamine on montmorillonite (Stackhouse et al., 2001).

10.2.5. INDUSTRIAL SITUATION

Acid-treated clay minerals continue to be used industrially (see Chapter 10.1). The

Claycop and Clayfen type of supported reagents became ‘‘standard practice’’ in

many laboratory situations and related catalysts, the so-called Envirocats (environ-

mentally friendly catalysts) made some headway, marketed by Contract Chemicals.

Chapter 10.2: Clay Minerals as Catalysts562

Table 10.2.7. Microwave-initiated, clay-catalysed functional group modiﬁcations

Substrate Co-substrate Product Clay Reference

Aldehyde Amine Imine K10 or Envirocat

EPZG

Dewan et al.

(1995), Varma and

Dahiya (1997a)

Creatine Aromatic

aldehyde

Z-arylidene

creatinines

Clay Villemin and

Martin (1995)

Diethyl or dimethyl

malonate

Aldehydes gem-bis(Alkoxy-

carbonyl)alkenes

K10 Abdallahelayoubi

and Texierboullet

(1995)

Ketone Amine Enamine K10 or Envirocat

EPZG

Varma and

Dahiya (1997a,

1997b), Varma

et al. (1997),

Rechsteiner et al.

(1993)

Dialkyl sulphide n.r. Sulphoxide Clayfen Varma and

Dahiya (1998a)

Alcohol n.r. Aldehyde or

ketone

Clayfen Varma and

Dahiya (1997b)

Alcohol Hydrogen

peroxide

Aldehyde or

ketone

Claycop Varma and

Dahiya (1998b)

Aldehyde 11 Amine 21 Amine K10 then

NaBH

/K10/

water

Varma and

Dahiya (1998c)

Aromatic Aldehyde n.r. Nitrile NH

OH/K10 Varma et al.

(1999a)

Semicarbazones or

hydrazones

n.r. Ketone (NH

)

K10



Varma and

Meshram (1997)

Thioketones or

dithioketals or

dithioacetals

n.r. Ketone or

aldehyde

Clayfen or clayan

(NH

/K10)

Varma and

Kumar (1999a),

Varma and Saini

(1997a)

Aldoximes Tetrachloro-

pyridine

Nitrile Acid clay Lingaiah and

Narender (2002)

Aldehyde bisulﬁte

addition products

n.r. Aldehydes Montmorillonite

KSF

Mitra et al. (1999)

n.r. ¼ not required.



Also promoted by ultrasound irradiation.

10.2.5. Industrial Situation 563

There was some work in the 1970s and 1980s by Laporte aimed at pelletising clays

with attapulgite in a similar manner to zeolites, with some success. In addition, some

work was carried out in adapting some of the laboratory-based chemistry outlined

above to reﬁnery-based situations using acid-activated and PILC minerals. Examples

include the direct addition of carboxylic acids to alkenes (Gregory et al., 1983;

Ballantine et al., 1984b) and Friedel-Crafts alkylation with C3–C5 alkenes reducing

the mutagenicity of reﬁnery stre ams and coal tars containing polynuclear aromatic

compounds (PACs) (Roy et al., 1999). However, smectite- (and other clay-) derived

catalysts are rare in industry.

10.2.6. CONCLUSIONS

Generating activity from clay-based catalysts is not really the problem. The issues

that remain are those set out at the beginning of this chapter, i.e.:

(1) There remains too much ‘‘stamp collecting’’. Fundamental work needs to be

done on a set of carefully selected reactions using extremely well-characterised

materials, to generate predictive capability.

(2) Not enough attention was paid to engineering of clays into catalyst particles ,

pellets, membranes, etc. that can survive industrial processing conditions.

Table 10.2.8. Microwave-initiated, clay-catalysed formation of heterocyclic compounds

Substrate Co-substrate Product Clay Reference

a, b-Dibromoester 11 Amine Aziridine Bentonite Saoudi

et al.(1996)

Orthoesters o-Phenylene-

diamine

Benzimidazoles KSF Villemin et al.

(1996)

Ketone+amine Then a

2–hydroxy-

benzaldehyde

Isoﬂav-3-ene K10

then+NH

OAc

Varma and

Dahiya

(1998d)

o–Hydroxydibenzoyl-methanes n.r. Flavone K10 Varma et al.

(1998)

-Aminochalcones n.r. Tetrahydroquinolones K10 Varma and

Saini (1997b)

Arylmethyl

ketone+[hydroxyl(tosyloxy)iodo]-

benzene

Thioamide Thiazoles K10 Varma et al.

(1999b)

Arylmethyl

ketone+[hydroxyl(tosyloxy)iodo]-

benzene

Ethylenethiourea Fused imidazothiazoles K10 Varma et al.

(1999b)

2-Amino pyridines, pyrimidines or

pyrazines

Aldehydes

then+isocyanide

Imidazo[1,2a]pyridine,

pyrimidine or pyrazine

Clay Varma and

Kumar

(1999b)

N-(carbotriﬂuoromethyl)-ortho-

arylenediamines

n.r. 2-Triﬂuoromethyl-

arylimidazoles

K10 Bougrin et al.

(2001)

n.r. ¼ not required.

Chapter 10.2: Clay Minerals as Catalysts564

Both matters need addressing if engineers and other industrialists are to have

conﬁdence in clays as solutions to their real problems. A focussed, integrated project,

concentrating on these issues could well be profitable.

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