Bergaya F. Handbook of Clay Science

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However, NMR spectroscopy provided evidence that these ions do not migrate into

vacant octahedral positions but occupy very distorted sites at the bottom of the

ditrigonal cavities (Luca et al., 1989; Alvero et al., 1994; Theng et al., 1997). At high

water vapour pressure, the Li

ions can return to their original positions in the

interlayer space.

Pillared Clays

Pillaring of smectites by intercalation of aluminium oligomers and subsequent heat-

ing at high temperatur es, makes a large fraction of the clay interlayer surface (up to

400–500 m

/g) available for adsorption of extraneous molecules (see Chapter 7.5).

Al and

Si MAS-NMR spectroscopy provides useful information on the mech-

anism of pillaring. In pillared hectorite and laponite, the layers retain their structural

integrity even after heating at 350 1C, while in pillared beidellite the intercalated

polyoxyaluminium cations react with the tetrahedral sheet of the mineral, forming

links through the inversion of some aluminium tetrahedra (Plee et al., 1985; Schultz

et al., 1987). A simila r structural arrangement (i.e., inversion of tetrahedra) was

reported by Pinnavaia et al. (1985) and Tennakoon et al. (1986a) for pillared ﬂuoro-

hectorite and hectorite.

Clay– Organic Interactions

NMR studies on clay–organic complexes were carried out to elucidate the orien-

tation and mobility (rotational or translational) of the sorbed organic species. These

studies are restricted to synthetic or natural clay minerals with low iron content

(notably hectorite).

Pratum (1992) used solid-state

C MAS- NMR spectroscopy to assess the mo-

bility of tetramethylammonium (TMA) and hexadecyl trimethylammonium

(HDTMA) ions in montmorillonite. These organic cations show rapid motions

with average of

H–

C dipolar interactions. This motion appears to be isotropic

for TMA and anisotropic in the case of HDTMA.

Adsorption by 2:1 Clay Minerals

Multinuclear NMR spectr oscopy was used to investigate the interactions of hex-

1-ene with Na

- and Al

-exchanged laponites (Tennakoon et al., 1986b), and of

triethylphosphate (TEP) with Mg

-, Al

-, or tetrabutylammonium (Bu

exchanged smectites (O’Brien et al., 1991).

The intercalation of macrocyclic compounds (crown ethers and cryptands) into

montmorillonite and hectorite results in the formation of interlayer 1:1 and 2:1

ligand/cation complexes. In Na

-hectorite, the chemical shift of the

Na-NMR

signal varies according to the sodium–macrocycle interaction (Aranda and Ruiz-

Hitzky, 1992; Casal et al., 1994). The

C CP-MAS spectra of complexes of Na

-, or Ba

-hectorite with polyethylene oxide (PEO) consist of a single signal at

about 70 ppm, assigned to the gauche conformation of the methylene groups in PEO.

The

Na MAS-NMR spectrum of the PEO/Na

-hectorite complex shows a single

12.7.1. NMR Spectroscopy of Clay Minerals 927

peak at 10.8 ppm, indicating that the Na

ions are directly coordinated to the

oxyethylene units of the polymer (Aranda and Ruiz-Hitzky, 1992).

The arrangement of molecules in the interlayer space may be determined using

samples (clay aggregates) that can be oriented in the magnetic ﬁeld. One interesting

study of this kind is that of benzene intercalated into Ag

-exchanged hectorite

(Resing et al., 1980 )(Fig. 12.7.5). The

C-NMR spectrum, recorded at ambient

temperature, shows a single line whose position depends on the orientation of the

benzene–hectorite complex (upper left in Fig. 12.7.5). When B

is perpendicular to

the sample (y ¼ 0), the observed shift at 182 ppm corresponds to an orientation in

which the plane of the benzene ring is parallel to the magnetic ﬁe ld. From the small

linewidth of the signal, it is inferred that the benzene molecules execute some fast

motion (right in Fig. 12.7.5). When mobility decreases, linewidth increases and for

complexes disposed parallel to the magnetic ﬁeld (y ¼ 901), a two-dimensional an-

isotropy pattern is detected around the central peak at 251 K (Fig. 12.7.5). This

pattern is exactly what is expected for benzene standing on its edge between the

silicate layers. Accordingly, the most realistic model is one in which benzene mol-

ecules are subject to two different rotations: (i) about the hexagonal molecular axis

and (ii) about the normal to the silicate layers. The latter motion is quenched at 77 K

(Fig. 12.7. 5 ; y ¼ 0).

Intercalation of Kaolin Minerals

Intercalation complexes of kaolin minerals were the subject of numerous NMR

studies. Thompson (1985) used

Si and

C-NMR spectroscopy to study the dis-

position and bonding of formamide, hydrazine, dimethylsulphoxide (DMSO) and

pyridine-N-oxide (PNO) intercalated into ka olinite. The

Si-NMR spectra of the

untreated kaolinite show two

Si resonances (at 91.9 and  91.5 ppm), which

converge into a single signal in the intercalate (interlayer co mplex). In the

C-NMR

spectra, the

C resonances of the intercalated molecules are shifted downﬁeld as a

result of increased hydrogen bonding. The

C-NMR spectrum of the kaolin-

ite–DMSO complex shows two equally intense methyl carbon signals (Fig. 12.7.6a).

The interpretation of this doublet, however, was controversial. Hayas hi (1997) and

Hayashi and Akiba (1994) proposed that be low 300 K, all interlayer DMSO mol-

ecules are equivalent. One of the methyl groups of DMSO is keyed into the di-

trigonal holes of the silicate sheet, and these groups can rotate freely about their C

axis. At low temperatures (about 160 K) this axis is blocked but at high temper-

atures, the methyl groups initiate a wobbling motion whose amplitude increases with

temperature. Above 320 K, the keyed methyl groups are released from the ditrigonal

holes, and the intercalated DMSO molecules undergo an isotropic rotations (Fig.

12.7.6b).

Kaolinite-polymer nanocomposites also attracted considerable attention. Sugahara

et al. (1990) obtained a kaolinite–polyacrylamide composite by intercalating acryl-

amide (basal spacing ¼ 1.13 nm), heating the clay–monomer complex at increasing

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy928

temperatures to induce polymerization, and following the process with

CP/MAS-NMR spectroscopy. Polymerization of ethylene glycol (EG) did not occur

with the kaolinite-EG intercalate (basal spacing ¼ 0.94 nm). However, complexes of

kaolinite with poly(ethylene glycol) (PEG) can be obtained by direct displacement of

intercalated DMSO with PEG 3400 and PEG 1000 from their melts at 150–200 1C

(Tunney and Detellier, 1996).

Fig. 12.7.5.

C-NMR spectra of benzene sorbed on an oriented aggregate of Ag-hectorite

taken at various temperatures and for several orientations of the platelet director N in the

magnetic ﬁeld B

. The lower-bottom left ﬁgure shows the model deduced for the average

orientation of the benzene molecules between the clay layers. The lower-bottom right ﬁgure is

the calculated anisotropy patterns for T ¼ 251 K when B

is disposed parallel to the layers.

From Resing et al. (1980).

12.7.1. NMR Spectroscopy of Clay Minerals 929

12.7.2. REACTIVITY OF CLAY MINERALS

A. Alkaline Activat ion of Kaolinites

Sanz et al. (1988) investigated the thermal decomposition of kaolinite by

Si and

MAS-NMR spectroscopy. They show that kaolinite is completely dehydroxylated at

Fig. 12.7.6. (a)

C CP/MAS-NMR spectra of kaolinite–DMSO intercalate at different

temperatures. (b) Schematic diagram showing the disposition and motion of DMSO in the

interlayer space of kaolinite. From Hayashi (1997).

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy930

750 1C, forming tetra- and penta-coordinated aluminium (Fig. 12.7.7b). The partial

breaking of the tetrahedral sheet takes place at 850 1C and the segregation of amorphous

silica occurs at 980 1C(Fig. 12.7.7a). The exothermic peak at 980 1C in the DTA pattern

is due to transformation of Al

into the more stable Al

. Above 980 1C, characteristic

peaks at 42 and 60 ppm of mullite 3:2 intensify, while the peak due to octahedrally

coordinated Al narrows (Fig. 12.7.7b). Similar results have also been reported by

Lambert et al. (1989), Rocha and Klinowski (1990) and Massiotetal.(1995).

NMR spectroscopy shows that alkali treatment of kaolinites that were heated

between 400 and 1000 1C causes dissolution of the mineral with the formation of

Al(OH)



and SiO

4

species (Madani et al., 1990). These species constitute the base

for zeolite formation (Fig. 12.7.8b). The nature of the zeolites (hydroxysodalite,

Na–A, and tetragonal P

phillipsite) varies with thermal activation of the starting

silicates. Prolonged leaching of the samples favours formation of the more stable P

zeolite at the expense of hydroxysodalite and Na–A zeolites (Fig. 12.7.8a). As the Si/

Al ratio of the P

zeolite is >1, its formation requires the release of aluminium from

the zeolites initially formed (Si/Al ¼ 1). This process is accompanied by the appear-

ance of ﬁve signals in the

Si MAS-NMR spectra, associated with Si

,Si

Al, Si

SiAl

, and Al

environments (Fig. 12.7.8a).

B. Heterogeneous Catalysis

NMR spectroscopy is an important technique for studying ‘in situ’ transformations

of adsorbed species at solid surfaces (heterogeneous catalysis). For example,

Tennakoon et al. (1983) used

C-NMR to follow the catalytic conversion of 2-

methyl-propene (isobutene) into t-butanol in the interlayer space of a synthetic Al

hectorite. The

C-NMR spectrum of the end product shows two peaks at 35 and

75 ppm corresponding to the two types of carbon atoms in t-butanol , (CH

)

–COH.

On the other hand, the intercalation of methanol into Al

-hectorite, followed by

adsorption of isobutene, gives rise to methyl-t-butyl ether as shown by the presence

of three peaks in the

C-NMR spectrum.

Carrado et al. (1990) investigated the cleavage of the oxygen–methyl bond in

m-methylanisole, guiacol, 4-hydroxy-3-methoxytoluene, and 4-phenoxy-3-methoxy-

toluene adsorbed on clay and pillared clay surfaces. All four molecules have some

similarity to the monomeric unit of lignin and can serve as model compounds in the

formation of coal. NMR spectroscopy shows that the mobility of these compounds

is markedly reduced on adsorption.

C. Grafting of Organic Species

The grafting of organic molecules to minera l substrates through the formation of

true covalent bonds is well documented in the literature (Iler, 1979; Sherrington,

1980; Rosset, 1985; Ruiz-Hitzky, 1988). Most of these reactions take place through

Si–OH groups present at the edges of mineral particles with the formation of

12.7.2. Reactivity of Clay Minerals 931

Fig. 12.7.7. (a)

Si and (b)

Al MAS–NMR spectra of kaolinite samples heated at dif-

ferent temperatures. For comparison, the spectrum of a pure 3:2 mullite is given. Variation of

the

Si chemical shift versus the temperature of treatment is also given. From Sanz et al.

(1988).

Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy932

Si–O–Si– or –Si–O–C–bonds, although reactions of organic molecules with –Al–OH

of gibbsite or kaolinite were also reported.

The content of reactive Si–OH groups in ﬁbrous clays (sepiolite and palygorskite)

is much larger than in plate-like clays. The former minerals can therefore serve as

suitable substrates for direct grafting reactions. Aznar and Ruiz-Hitzky (1988) and

Fig. 12.7.8. The upper ﬁgures correspond to

Si and

Al MAS-NMR spectra of kaolinite

samples heated at 850 1C, after different NaOH treatments. (a) The lower ﬁgures correspond to

and

Al NMR spectra of the liquid phase for the same treatments (b) From Madani et al. (1990).

12.7.2. Reactivity of Clay Minerals 933

Aznar et al. (1992) studied the grafting of phenyl–organosilanes (Si–C

,Si–(C

)

and Si–(CH

)

–C

) on sepiolite. In general, the process leads to a simultaneous

extraction of Mg, implying an alteration of the mineral substrate. The

Si-NMR

spectrum of the amorphous silica produced shows new signals at  110, 102, and

92 ppm, attributed to Si surrounded by (4Si), (3Si, 1OH), and (2Si, 2OH) (Q

and Q

in Lippmaa’s notation). The

C CP-MAS spectra of the phenyl derivatives

give information on the nature of the grafted groups in the solid substrate (Aznar et

al., 1992). The preparation of sulpho or nitro derivatives, with SO

HorNO

radicals attached to the aromatic rings of the grafted species, was reported by Aznar

and Ruiz-Hitzky (1988). These derivatives exhibit properties that make them useful for

some industrial applications.

Grafting reactions in the interlayer space of kaolinite were reported by Tunney

and Detellier (1993). The kaolinite was ﬁrst expanded by intercalation of DMSO or

N-methylformamide (NMF), and then refluxed with alcohols (e.g., EG, propane-

diol). This produces organo derivatives in which the alcohol molecules are graft ed to

the interlayer surface of the mineral throu gh Al–O–C bonds.

12.7.3. CONCLUDING REMARKS

The applic ation of multinuclear high-resolution NMR spectroscopy over the last

two decades provided valuable information on the struc tures of phyllos ilicates, such

as crystallographic sites, cation coordination, silicate network distortion, and cation

distributions. However, conventional low-resolution NMR spectroscopy is still use-

ful for investigating the surface orientation of adsorbed molecules and their mobility.

In situ reactions between co-adsor bed molecules or between adsorbed species and

clay mineral surfaces can now be studied by high-resolution NMR (MAS and

CP-MAS) techniques. In particular, the formation of pillared clays was extensively

studied by multinuclear NMR spectroscopy. The same can be said for zeolite for-

mation by alkali treatment of clay minerals. These developments have important

implications for the industrial applications of clays and clay minerals.

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Chapter 12.7: Nuclear Magnetic Resonance Spectroscopy936