Bergaya F. Handbook of Clay Science

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in terms of different oxygen sites connecting the octahedral and tetrahedral sheets.

All the oxygens on the nontronite stable edge faces are saturated, wher eas the con-

necting oxygens on all hectorite edge faces and nontronite broken edges are coor-

dinatively unsaturated. This difference in reactivity of these particle faces suggests

that the rate-limiting step of the dissolution process is breaking the bonds of con-

necting O atoms (Bickmore et al., 2001).

Mg-rich clay minerals, such as trioctahedral smectites (hectorites, saponites, etc.)

are much less stable in inorganic acids than their Al-rich counterparts. Therefore, the

activation of the former minerals requires low temperatures and acid concentrations.

For example, a hectorite can be completely dissolved in 0.25 M HC l at 30 1C for 8 h

whereas Wyoming montmorillonite requires 6 M acid at 95 1C for >30 h (Madejova

et al., 1998).

dissolves slightly faster than Mg

from hectorite layers at low acid con-

centrations (Komadel et al., 1996b). Thus, protons are preferentially attracted to

sites close to Li

(in the octahedral sheet) that are more negative compared to sites

adjacent to Mg

. This difference disappears at high acid concentrations when the

reaction rates are high. Similarly, octahedrally coordinated Mg

are preferentially

released by HCl in comparison with Fe

and Al

(Christidis et al., 1997; Gates et

al., 2002). The effect of acid anion on dissolution of hectorite is complex and remains

uncertain (Komadel et al., 1996b; Van Rompaey et al., 2002).

Two saponites and a ferrous saponite (grifﬁthite) were treated by up to 2.5% HCl

at 25 1C for periods up to 48 h. Most of the octahedral sheets of the minerals are

dissolved. This is indicated by the high removal of Mg

, the changes in the IR

spectra and the thermal gravimetric analysis-differential thermal analysis (TGA-

DTA) curves of the activated saponites. Destruction of the saponite structure gives

rise to free silica, and the speciﬁc surface area of the saponites is doubled even after

mild acid activation (Vicente et al., 1996b; Sua

rez Barrios et al., 2001). The speciﬁc

surface areas of grifﬁthite samples increase greatly after activation, with values up to

10 times higher than the surface area of the untreated sample. The creation of

microporosity has a sub stantial inﬂuence on surface area. Likewise, the free silica has

a very important contribution to the surface area of leached sampl es (Vicente et al.,

1995b). The high surface area (197 m

/g) of a natural saponite is related to the very

small particle size because of its sedimentary origin. Treatment of the sedimentary

saponites at room temperature by 0.62 wt% HCl for times up to 48 h or by 1.25 wt%

HCl solutions for times up to 6 h gives rise to a partial dissolution of the saponite

structures. A mixture of unaltered saponite and free silica is obtained. The latter

treatment (1.2 5 wt% HCl for over 6 h) leads to an almost total dissolution of the clay

mineral structure. The solid products consist mainly of some delaminated saponite

layers, free silica and insoluble impurities. The surface area is 462 m

/g and the

number of acid centres is 0.98 mmol H

/g (Prieto et al., 1999).

A new short-time synthesis route for preparation of mesoporous materials

(Folded Sheet Materials, FSMs) have been developed from HCl-leached saponite

samples. The acid treatment is performed under stirring for 24 h at 25 1C and 100 1C

7.1.3. Acid Dissolution of Smectites 271

in 6–10 and 3–7 M HCl aqueous solutions, respectively. The leached silicate powd ers

are washed, suspended in hexadecyl trimethylammonium bromide solution as a

structure-directing agent, with stirring at pH ¼ 12.3 for 3 h at 70 1C and afterwards

at pH ¼ 8.5 for an additional 3 h at room temperature, and ﬁnally calcined at 550 1C

to obtain the mesoporous materials. As could be expected, the intensity of the

treatment increases with acid concentra tion and temperature. General improvement

of the mesoporous FSM structure is obtained when a ﬁltration step is added to the

synthesis route after the dissolution at pH ¼ 12.3, by removing all dissolved silicates

and thus, preventing the formation of amorphous silica. The material synthesized

after acid leaching by 8 M HCl at ambient temperature has the most condensed

structure, the highest unit-ce ll dimensions, surfa ce area and pore volume, a nd the

narrowest pore-size distribution (Linssen et al., 2002). The properties of the FMS

prepared from differently HCl-treated saponite samples are reported in Table 7.1.2.

For most preparations obtained at 25 1C, the Brunauer Emmett and Teller (BET)

speciﬁc surface areas are much higher and more sensitive to the acid concentra tions

than for the materials prepared at 100 1C. On the other hand, the material obtained

after leaching with 5 M HCl at 100 1C has the highest pore volume.

7.1.4. FINAL SOLID REACTION PRODUCT

For industrial uses, complete decomposition of the parent mineral is rarely

wanted. The bleaching earths contain substantial amounts of parent smectite,

exhibiting an increased surface area and porosity. The ﬁnal reaction product of

various acid-treated clay minerals is always the same. For several silicat es, such as

hectorite (Komadel et al., 1996b), saponite (Vicente et al., 1995b), dioctahedral

smectites (Komadel et al., 1990; Tka

et al., 1994; He et al., 2002), illite–smectite

(Schmidt et al., 1990), sepiolite (Vicente et al., 1995a) and palygorskite (Sua

rez

Barrios et al., 1995) this product consists of amorphous, porous, protonated and

hydrated silica with a three-dimensional cross-linked structure (Komadel, 1999).

Table 7.1.2. Effect of acid concentration on BET surface areas and pore volumes (PV) of

mesoporous FSM materials prepared from saponite samples leached by HC1 at 25 and 100 1C

Acid leaching at 25 1C

HC1 (mol.dm

)678910

BET

/g) 643 726 900 785 478

PV (cm

/g) 0.44 0.54 0.61 0.54 0.37

Acid leaching at 100 1C

HCl (mol.dm

)345

BET

/g) 521 539 575

PV (cm

/g) 0.50 0.45 0.98

[Data from Linssen et al. (2002)]

Chapter 7.1: Acid Activation of Clay Minerals272

Only SiO

remained after leaching of phlogopite powder in nitric acid. At high

temperatures, the reaction rate is fast and the resulting speciﬁc surface area of the

porous silica product increases at each activation temperature. The porous silica

maintains its original platy particle shape. The

Si MAS-NMR spectra of the

products, however, reveal that the layered type structure of SiO

tetrahedra in the

original phlogopite is converted into a framework type. A porous silica product with

micropores of 0.7 nm, mesopores of 4 nm, and a maximum speciﬁc surface area

of 532 m

/g is obtained. The pore size of the products, ranging from 0.7 to 4 nm,

reaches 6 nm with increased leaching time (Okada et al., 2002).

7.1.5. OPTIMUM ACTIVATION CONDITIONS

Acid activation of be ntonites with HCl results in a substantial increase of the

surface area of the raw materials. As mentioned above, activation is generally char-

acterized by destruction of the original smectite structure, removal of octahedral

cations, uptake of OH

–

and formation of an amorphous Si-rich phase. Mg-rich

montmorillonites are more easily activated than other forms because Mg is the most

readily removed element.

Optimum conditions for activation can be achieved by combining different acid

strengths and react ion times. The activated materials are suitable for the bleaching of

rapeseed oil through removal of b-carotene. The optimum bleaching capacity is not

associated with maximum surface area (Christidis et al., 1997). The difference in the

bleaching efﬁciency of cottonseed oil is related to differences in physical and chem-

ical properties of the activated clay minerals. The oil acid value is not affected by the

bleaching procedure but a slight shift in the absorption maximum of the bleached

cottonseed oil is observed. A progressive decrease in CEC values is found when

-montmorillonite is treated with sulphuric acid solutions. Elemental analysis

shows that with moderate activation only 25–30% of the octahedral cations are

removed. At the same time, the total surface area and the clay acidity increase.

Moderate activation of the clay is most effective in bleaching cottonseed oil, result-

ing in the best colour index and the lowest peroxide value. A linear dependence of the

bleaching efﬁciency on the clay surface area and acidity is observed (Falaras et al.,

1999).

7.1.6. ACID-ACTIVATION AND PORE STRUCTURES

Acid activation of a Ca

-montmorillonite by treatment with sulphuri c-acid so-

lutions and subsequent pillaring (intercalation of oligomeric Al (hydr)oxides,

‘Keggin ions’, and calcination at temperatures to 500 1C) produce new materials for

bleaching of cottonseed oil. These materials have bleaching pro perties dependent on

the extent of activation of the clay mineral prior to pillaring. The pillared

7.1.5. Optimum Activation Conditions 273

acid-activated montmorillonites possess a higher bleaching efﬁciency compared with

the pillared non-activated clay minerals usually called ‘pillared interlayered clays’ or

‘pillared interlayered clay’ (PILC) (see Chapt er 7.5). Mild activation of the mont-

morillonite followed by pillaring produces materials wi th the best fractional degree

of bleaching (Falaras, 2000b).

Detailed study of preparation and characterisation of Al PILC mate rials derived

from an acid-treated montmorillonite shows that careful selection of the level of acid

treatment is necessary to optimise the surface area, pore volume, surface acidity and

thermal stability of the ﬁnal pillared clay. The optimum level of acid treatment

corresponds to the removal of between 19 and 35% of the octahedral cations.

However, these values depend on the clay mineral. The pillared acid activated clay

minerals have signiﬁcantly higher pore volume and acidity than conventional PILC,

but similar basal spacings, surface areas and thermal stability. Higher acidity is

mainly due to an increase in intrinsic Brønsted acid sites arising from acid treatment

before pillaring procedure. The higher acidity of the pillared acid activated clay

mineral is reﬂected in better catalytic activity for the acid-catalysed reactions as

compared with PILC ( Mokaya and Jones, 1995).

Falaras et al. (2000a) have investigated the effect of acid activation of montmo-

rillonite before pillaring and of calcination temperature on the efﬁciency of

Al-pillared acid-activated clay-modiﬁed electrodes. The electrochemical behaviour

of PILC has been compared with that of two pillared acid-activated mont-

morillonites. The pillared acid-activated montmorillonite-modiﬁed electrodes pre-

sent better electro-activity than the modiﬁed electrodes of the conventional PILC

for cationic and anionic redox active species. The same conditions as mentioned

before (mild acid activation and calcination up to 500 1C) lead to modiﬁed materials

that efﬁciently concentrate the cationic species. Lower calcination temperatures

reverse the electrode activity. For anionic redox-active species the best electro-

activity is observed for pillared acid-activated montmorillonite ﬁlms corresponding

to a medium acid activation. In that case, dependence of the electro-chemical

response on the pH is conﬁrmed. The mechanism responsible for the observed cat-

ionic electro-activity was investigated. The behaviour of the Al-pillared acid-

activated montmorillonite-modiﬁed electrodes is attributed to the speciﬁc structure,

the increase in surface area due to mesopores and the acidity of the clay ﬁlms

(Falaras et al., 2000a).

Porous clay heterostructures with enhanced acidity may be prepared from suit-

ably acid-activated montmorillonites. The high acidity arises from Brønsted acid

sites (Pichowicz and Mokaya, 2001). The ion-exchange properties of Na

-mont-

morillonite and its derivative PILC were investigated in electrolyte solutions con-

taining KCl. The pH curves of the two clay materials indicate anion and cation

uptake as a function of the solution pH. In highly acidic medium, the surface area

and micropore vo lume decrease signiﬁcantly, and a considerable amount of Al

leached from the pillars. The alumina pillars are attacked during this treatment,

leading to partial destruction or dissolution of the pillars (Ahenach et al., 1998).

Chapter 7.1: Acid Activation of Clay Minerals274

7.1.7. ACID ACTIV ATION OF OTHER CLAY MINERALS

A. Kaolinite and Metakaolinite

Dissolution rates of natural kaolinites of different origin, and of halloysite and illitic

clays in sulphuric and hydrochloric acids were determined by measuring the release

rate of aluminium. The dissolution rate of kaolinite in 0.5 M sulphuric acid at 25 1 C

is approximately three times higher than in hydrochloric acid of equivalent proton

concentration. The dissolution in 5 M sulphuric acid was eight times faster when the

solid phase is periodically separated from the acid solution, washed by distilled water

and dried. The aluminium release rate decreases as the amo unt of clay-size micas in

kaolinitic clays increases. The rate is also signiﬁcantly inﬂuenced by the crystallinity

of the clay mineral (Hradil et al., 2002).

The three kaolin polytypes—kaolinite, dickite and nacrite—can be identiﬁed with

great certainty by examining the hydroxyl-stretching region of the IR spectra of the

samples before and after treatment with hydroﬂuoric acid (HF). SEM observations

suggest that the rate of dissolution of kaolinite is largely dependent on particle size. In

general, dickite and nacrite tend to occur in the coarser clay fractions, and for this reason

the ﬁner grained kaolinite is preferentially dissolved by the HF treatment. However, in

the Keokuk kaolinite, which occurs in exceptionally large particles, it is still possible to

concentrate a dickite fraction by HF treatment. This suggests that in some cases kao-

linite may be more susceptible to HF dissolution for reasons other than particle size.

Kaolinite and dickite components of disordered kaolinite dissolve at the same rate in

HF, supporting the idea that disordered kaolinite consists of an intimate association of

randomly stacked dickite-and kaolinite-like components (Fraser et al., 2002).

Proton adsorption or desorption may be computed from potentiometric titration data

at pH 2–12 using surface complexation models. The surface behaviour is explained in

terms of the formation of four active sites. The pH of zero proton charge is close to 5.5.

The positive charge that develops below pH 5.5 is due to proton adsorption on alumin-

ium sites of the octahedral sheet. The external hydroxyls of the octahedral sheet are the

ﬁrst to be protonated, whereas the second protonation may take place either at the inner

hydroxyls or at the edge aluminol groups. Above pH 5.5, the kaolinite surface undergoes

two successive deprotonations, the ﬁrst occurs at pH about 5.5 and the second at pH

about 9 (Huertas et al., 1998). The dissolution mechanism of kaolinite is mainly con-

trolled by aluminol surface sites (external and internal structural hydroxyls, and aluminol

at the particle edges) under both acidic and alkaline conditions (Huertas et al., 1999).

Devidal et al. (1997) described dissolution and cryst allization rates of kaolinite at

both acidic and alkaline pH, over the full range of chemic al afﬁnity and aqueous

and Si

concentrations. Kaolinite dissolution rates were obtained based on

the release of Al

and Si

at steady state conditions. In general, dissolution rate

increases with temperature and decreases with pH (Cama et al., 2002).

Kaolinite dissolution rates at pH 2–4 and temperatures of 25–70 1Cwereobtained

using stirred and non-stirred ﬂow-through reactors. The rates increase with increasing

7.1.7. Acid Activation of Other Clay Minerals 275

stirring speed and the stirring effect is reversible. The effect of stirring speed on

kaolinite dissolution rate is higher at 25 1Cthanat50and701C and at pH 4 than at

pH 2 and 3. Stirring induces spalling or abrasion of kaolinite giving rise to the for-

mation of ﬁne particles. As a result the ratio of reactive surface area to speciﬁc surface

area increases, and the dissolution rate of kaolinite is enhanced. A balance between the

production and dissolution of the ﬁne particles explains the reversibility as well as the

temperature and the pH dependence of the stirring effect (Metz and Ganor, 2001).

Wieland and Stumm (1992) investigated the acid/b ase properties of the terminal

–

groups and ion exchange reactions occurring at the kaolinite surface and in-

terpret the dissolution of kaolinite in terms of the surface complexation model. A

three-site model, incorporating solid-solution equilibria at aluminol groups of the

edge and basal surfaces and at negatively charged groups of the siloxane surfaces,

accounts for the protonation of kaolinite in acidic solutions. The dissolution kinetics

of kaolinite at 25 1C was studied as a function of solution pH. The dissolution of

kaolinite is non-stoichiometric in the pH range 2–6.5 with a preferential release

of Si

. Stoichiometry of the dissolution reaction is observed in the presence of

oxalate as Al

-complexing liga nd. The detachment of Al

from the layer structure

of kaolinite surface and its readsorption on distinct surface sites occur simultane-

ously during the dissolution process. This accounts for the experimentally observed

non-stoichiometry of the process. Although oxalate and salicylate form surface

complexes only with Al

sites, they promote the release of both Al

and Si

during dissolution. The detachment of Al

is considered as the rate-limiting step.

The proton-promoted dissolution of kao linite occurs at the edge surface at pH o6.5

and the basal surface at pHo4. The pH-dependence of the dissoluti on rate reﬂects

sequential protonation of terminal OH

–

groups on both surfaces.

Surface characteristics and dissolution behaviour of well-crystallized kaolinite KGa-1b

and poorly crystallized kaolinite KGa-2 were compared. Particles of KGa-1b generally

have hexagonal micro-morphology and crystallographically controlled micro-topo-

graphic features. Particles of KGa-2 are also hexagonal, but their micro-morphology is

more rounded and their basal-plane surfaces are more irregular with fewer clearly cry-

stallographically controlled features. KGa-1b particles are larger in diameter and thicker

than those of KGa-2. Dissolution experiments were conducted in oxalic acid and in-

organic acids at pH 3. Dissolution in oxalic acid is approximately twice as fast for KGa-2

as for KGa-1b, while it is similar in HNO

. The comparable dissolution rates for these

two sedimentary kaolinites suggestthatthefundamentalstructure of kaolinite has a

larger inﬂuence on dissolution kinetics than speciﬁc surface details. For dissolution in

–3

Moxalate,Al

-oxalates complexes are observed almost exclusively in agreement

with the results of equilibrium speciation calculations. For dissolution in HNO

, un-

complexed Al

species are identiﬁed (Sutheimer et al., 1999).

Pyridine adsorption data indicate that the strong acid sites on activated kaolinite

are of the Lewis type (Tabak and Afsin, 2001). This also applies to the adsorption of

. Acid activation increases the protonated species on a kaolinite surface at the

expense of coordinately bound NH

. The presence of NH

ions on an activated

Chapter 7.1: Acid Activation of Clay Minerals276

sample is not a proof of the presence of protonic acid sites alone, since the added

proton may come from the residual water in the interlayer space. Progressive

dehydration of the surface results in a strong increase in chemisorbed NH

A natural kaolinitic clay is metakaolinized at 550 1C and activated separately with

, HNO

and HClO

of varying concentrations (Sabu et al., 1999). Silica/

alumina molar ratio, surface area and the number of strong acid sites increase when

the concentration of acid used for activation increases. Metakaolinite activated with

4 M HNO

has the highest surface area and surface acidity.

B. Sepiolite and Palygorskite

After dissolution of sepiolite samples in HCl, the free silica produced has little

inﬂuence on the properties of the mildly acid treated solids. However, the inﬂuence

of this silica becomes very important when solids are obtained by more intense acid

treatments (Vicente et al., 1995a). As the amount of Fe

and Al

extracted from

sepiolite by acid treatment increases, the speciﬁc surface area of the mineral increases

from 195 to 306 m

/g and the original microporous structure becomes mesoporous.

The CEC of sepiolite may be fully eliminated by acid treatment, during which the

mineral structure is progressively transformed into amorphous silica-alumina

(De

ny et al., 1999). The BET surface area of the original sepiolite increases from

148 to 263 m

/g, after which it decreases. Approximately 16% of the total volume is

in the micropores. Acid activation restricts particle deformation during thermal

treatment. The micropore volume increases by 20% and the BET surface area

reaches values >500 m

/g for the acid-trea ted samples (Balci, 1999).

When sepiolite and palygorskite are activated by HCl, there is progressive dis-

solution of the octahedral layers. Silica contents increase and octahedral cations

decrease with the intensity of the acid attack. In both cases, ﬁbrous free silica is

obtained. Sepiolite decomposes more rapidly than palygorskite because its octahe-

dral sheet contains more Mg

and the structural microchannels are larger. The

removal of the cations and disaggregation of the particles as well as the increase in

the micropore volume cause an increase in speciﬁc surfa ce area (Myriam et al., 1998).

A substantial increase in the speciﬁc surface area is also observed for HCl-treated

palygorskite. The free silica obtained has the ﬁbrous morphology of natural paly-

gorskite and no microporosity is detected (Sua

rez Barrios et al., 1995).

7.1.8. CATALYTIC PROPERTIES OF ACID ACTIVATED CLAY MINERALS

A. Smectites

Acid-activated clays are well established as both solid acid catalysts and catalyst

supports. Measurements of heats of adsorption of ammonia were carried out with a

K10 catalyst in the acid and Na

-forms. This is performed in a ﬂow calorimeter

7.1.8. Catalytic Properties of Acid Activated Clay Minerals 277

linked to a thermal conductivity detector. Ammonia is used as a probe because it

interacts with acid sites. The irreversibly adsorbed ammonia is mobile on the

catalyst. Molar heats of adsorption as surface coverage increases are recorded. The

results show that the sites ﬁrst covered do not necessarily correspond to those with

the highest heats of adsorption. This illustrates that the ﬂow technique provides

important information on the relative accessibilities of acid sites as well as on their

strengths (Brown and Groszek, 2000).

The nature of the exchangeable cations substantially affects the acidity of clay

catalysts due to possible formation of strong acid sites resulting from their polarising

power. The high catalytic activity of A1

-exchanged montmorillonites was attrib-

uted to the enhanced polarization of water molecules in the primary coordination

sphere of the Al

cation, giving rise to strong Brønsted acidity (Varma, 2002;

Jankovic

and Komadel , 2003b).

-saturated montmorillonites of considerable catalytic activity can be prepared

by means of thermal decomposition of ammonium clays (Jankovic

and Komadel ,

2000, 2003a); however, a more typical way is through acid activation with a mineral

acid. Acid-activated clays are of interest in their role as high surface area supports

for environmentally benign catalysts. Commercial products are normally treated

with a ﬁxed amount of acid, sufﬁcient to remove the required number of octahedral

cations to optimise the surface area and Brønsted acidity for a particular application.

Consequently, systematic appraisals of how the extent of acid decomposition of the

parent mineral contributes to the catalytic activity are not very frequent.

Several catalytic studies illustrate the application of commercial acid-activated

montmorillonites, K-catalysts. Flessner et al., (2001) investigated the surface acidity

of a series K-catalysts using a wide range of complementary experimental tech-

niques. The different methods applied allow a rather complete characterization of

the surface acidity. The strength and abundance (density) of Brønsted acid sites are

correlated with the trend in iso-butene conversion.

The catalytic acti vity of acid-treated montmorillonite towards Brønsted acid cat-

alysed reactions is highly dependent on the degree of acid treatment. Two contrast-

ing model reactions have been used.

The ﬁrst, involving highly polar reactants, is the acid-catalysed addition of

3,4-dihydropyran to methanol. The dihydropyran mo lecule is protonated to give a

stabilized carbocat ion that reacts with methanol to form tetrahydropyranyl ether as

the only product.

The second reaction, involving a non-polar, hydrophobic reactant is the acid-

catalysed rearrangement of alpha-pinene to camphene. The optimum treatment

conditions for an acid-treated clay catalyst also depend upon the type of reaction

being catalysed (Rhodes and Brown, 1994).

Acid treatment of Ca

-montmorillonite increases signiﬁcantly its effectiveness as

a support for ZnCl

Friedel–Crafts alkylation catalysts; optimum treatment condi-

tions are established and there is evidence for a synergistic interaction between

adsorbed salt and acid-activated clay (Rhodes et al., 1991). Maximum activity is

Chapter 7.1: Acid Activation of Clay Minerals278

associated with long acid treatment times. Structural characterization by XRD,

MA-NMR spectroscopy, and elemental analysis suggest there is little residual clay

mineral in the most active supports (Rhodes and Brown, 1992).

A series of progressively acid-treated montmorillonites and a range of porous

silicas were tested for their effectiveness as supports for ZnCl

alkylation catalysts.

The highest catalytic activities are associated with signiﬁcant pores of 1 0–12 nm in

diameter (Rhodes and Brown, 1993). Supports exhibiting pore diameters below this

range produce catalysts with very low activities. The fall in catalytic activity

associated with larger pore diameter supports is less dramatic.

Different clay minerals with different contents of octahedral cations, such as

magnesium- or aluminium-rich montmorillonites, a ferruginous smectite, an iron-rich

beidellite and a hectorite, have been leached with H

or HCl. The acid concen-

trations and treatment temperatures are selected to control the extent of activation.

The elemental composition of the starting materials makes no signiﬁcant contribution

to the catalytic activity of 2,3-dihydropyran with methanol reaction to yield the

tetrahydropyranyl ether. But the composition plays a key role in controlling the

conditions required for the optimization of catalytic activity. The Brønsted acidity and

catalytic activity of the resulting materials are highest for the samples prepared with

the mild acid treatments but decreases as more octahedral cations are removed. The

acid sites formed in the acid-treated montmorillonite are strong enough to produce

tetrahydropyranyl ether in 80% yield. However, the acid-treated hectorite shows no

catalytic activity. FTIR spectroscopy is as sensitive to structural acid attack as

Si MAS NMR. The octahedral depletion correlates well with the acidity (determined

from thermal desorption of cyclohexylamine) and the catalytic activity for the chosen

test reaction (Breen et al., 1995a, 1995b 1997b; Komadel et al., 1997).

Catalytic activity of montmorillonite in dimerization of oleic acid increases upon mild

activation in HCl. However, the activity of a catalyst with about 50% of octahedral

removed is comparable to that of the untreated clay mineral (C

el et al., 1992).

Acid-activated smectites can convert the alkenes from the thermal decomposition

of high-density polyethylene into light gases and aromatic species. Total conversion

increases with both the extent of acid treatment and the temperature. The proportion

of aromatic products is greatest for catalysts prepared using short acid-treatment

times (Breen et al., 2000).

Acid-activated natural bentonite (and kaolin) can debutylate 2-tert-butylphenol

and shows varying debutylation vs isomerization selectivity. The resulting catalytic

activity of these samples is dependent on the type of acid used. Samples treated with

acetic acid shows relatively low conversions, whereas those treated with hydrochloric

or phosphoric acid are very active (Mahmoud and Saleh, 1999).

B. Metakaolinite

The solid acid-activated metakaolinites are promising as adsorbents and catalyst

supports (Belver et al., 2002). The ability of these products to transform the waste

7.1.8. Catalytic Properties of Acid Activated Clay Minerals 279

gases from the con version of waste plastics into aromatic hydrocarbons was eval-

uated. The amount of adsorbed water and the number of acid sites increase with the

severity of acid treatment. Both Brønsted and Lewis acid sites are present until

425 1C. Pyridine bonded to the Lewis acid sites is more therm ally stabl e than that

associated with Brønsted sites. The materials are all selective to the production of

toluene with respectable, but lesser, amounts of xylenes and trimethylbenzenes

(Breen et al., 2002).

All the activated metakaolinites are active catalysts for the alkylation of benzene

with benzyl chlori de at reﬂux, giving above 75% conversion of the alkylating agent.

Metakaolinite activated with 4 M HNO

gives 87% conversion of benzyl chloride

into diphenylmethane with 100% selectivity within 30 min of reaction time. This may

be correlated with the greater surface acidity of this sample. Extremely efﬁcient solid

catalysts of remarkable acidic properties can be produced by the activation of met-

akaolinite with H

, HNO

and HClO

(Sabu et al., 1999). Metakaolinites were

also prepared by calcination of natural kaolinites at 600–900 1C and are more re-

active than the parent kaolinite after acid activation, with 6 M HCl at 90 1C under

reﬂux conditions. Treatment for 6 h leads to the removal of most of the octahedral

cations and the formation of an amorphous silica phase with high speciﬁc

surface area. Acid treatment for 24 h also removes the octahedral cations, but leads

to the formation of amorphous silica with much lower speciﬁc surface area. The

metakaolinite prepared by calcination at 900 1C shows a lower reactivity than the

materials obtained at lower temperatures (Belver et al., 2002).

C. Modiﬁed Clay Minerals

The hydrophilic aluminosilicate surface of smectites can be rendered hydrophobic by

exchanging the naturally occurring inorganic cations with organic cations. Acid

activation of tetralkylammonium-exchanged smectites produces hybrid catalysts for

the isomerisation of alpha-pinene to camphene. This catalytic activity is attributed to

the enhanced hydrophobicity of the organo-clay. Acid-activated tetramethylammo-

nium-exchanged smectites are the most active and give 60–90% conversion based on

alpha-pinene. The yields are comparable with those obtained with other solid cat-

alysts such as zeolites and PILC. The iron-substituted smectites are more active than

the aluminium counterparts. The dodecyl trimethylammonium- and octadecyl

trimethylammonium- exchanged smectites are generally less active (Breen et al.,

1997a).

SWy-2 and SAz-1 montmorillonites loaded with increasing amounts of the po-

lycation magnaﬂoc 206 were acid-treated with 6 M HCl at 95 1C. The acid-treated

polycation-exchanged smectites are active c atalysts for the isomerisation of alpha-

pinene to camphene and limonene. The conversion by the polycation-exchanged

SAz-1 sampl es is greater than by the unloaded activated counterpart, because the

former material is more hydrophobic. In the case of SWy-2 the yield s in the absence

and presence of polycation are similar suggesting good dispersion of both samples in

Chapter 7.1: Acid Activation of Clay Minerals280