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Clay Minerals (1980) 15, 337 349. THE APPLICATION OF ELECTRON SPIN RESONANCE SPECTROSCOPY TO STUDIES OF CLAY MINERALS: II. INTERLAMELLAR COMPLEXES--STRUCTURE, DYNAMICS AND REACTIONS PETER L. HALL Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham BI5 2TT, England. (Received 26 July 1979; revised 19 May 1980) A BSTRACT: A reviewis givenof the application of ESR spectroscopy to the study of hydrated transition metal ions and nitroxide spin probes in the interlamellar region of smectites and vermiculites. These investigations have not only provided information regarding the structure and mobility of the intracrystalline water-cationlayers but have also demonstrated the reactivity and catalytic properties of certain transition metal exchange forms of smectites. Several novel coordination complexesand redox reactions between the exchange ions and a variety of simple organic molecules have been characterized. The application of ESR spectroscopy to studies of paramagnetic ions and defect centres occurring in aluminosilicate lattices or on external clay particle surfaces was discussed in Part I of this review (Hall, 1980). Apart from these investigations, considerable attention has also been focussed on the characterization by ESR of the nature of the interlamellar water-cation layers in hydrated smectites and vermiculites. These studies, which are reviewed here, are based on the fact that when paramagnetic (transition metal) exchange cations are present in these clays, their ESR spectra serve as sensitive probes of both structural and dynamic properties of the local environment of the ions. Structural information may be derived from the symmetry and orientation-dependence of the ESR spectra. Information regarding the orientation of the symmetry axes of the complex ions can be obtained by working with preferentially-orientated films aligned either parallel or perpendicular to the magnetic field (Angel & Hall, 1973; Clementz et al., 1973). ESR spectra can also probe the dynamic properties of ions or radicals in solution-like environments, yielding information such as rotational correlation times or diffusion coefficients. In general, the elucidation of such information requires the presence of a species having anisotropic g-values (and preferably also anisotropic hyperfine splittings). If the paramagnetic ion is in a quasi-crystalline environment, i.e. is effectively immobile on the time-scale of observation of the ESR experimenL the direction-dependence of the spectrum will be seen in oriented samples. In contrast, if the ion is in a solution-like environment of low viscosity, and rapidly tumbling with a correlation time considerably shorter than the observation time, complete averaging of the anisotropic components of the spectrum will occur, giving effective spectral parameters g - l(gx+gy+gz) and A = 13(A~ + Ay + A~). In intermediate cases, where the mobility is on a time-scale compar- able with the observation time, only a partial averaging of the spectrum will occur. In 0009-8558/80/1200-0337502.00 1980The Mineralogical Society
Transcript
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Clay Minerals (1980) 15, 337 349.

THE A P P L I C A T I O N OF E L E C T R O N SPIN R E S O N A N C E SPECTROSCOPY TO STUDIES OF CLAY MINERALS:

II. I N T E R L A M E L L A R C O M P L E X E S - - S T R U C T U R E , D Y N A M I C S AND REACTIONS

P E T E R L. H A L L

Department of Chemistry, University of Birmingham, P.O. Box 363, Birmingham BI5 2TT, England.

(Received 26 July 1979; revised 19 May 1980)

A BSTRACT: A review is given of the application of ESR spectroscopy to the study of hydrated transition metal ions and nitroxide spin probes in the interlamellar region of smectites and vermiculites. These investigations have not only provided information regarding the structure and mobility of the intracrystalline water-cation layers but have also demonstrated the reactivity and catalytic properties of certain transition metal exchange forms of smectites. Several novel coordination complexes and redox reactions between the exchange ions and a variety of simple organic molecules have been characterized.

The application of ESR spectroscopy to studies of paramagnetic ions and defect centres occurring in aluminosilicate lattices or on external clay particle surfaces was discussed in Part I of this review (Hall, 1980). Apart from these investigations, considerable attention has also been focussed on the characterization by ESR of the nature of the interlamellar water-cation layers in hydrated smectites and vermiculites. These studies, which are reviewed here, are based on the fact that when paramagnetic (transition metal) exchange cations are present in these clays, their ESR spectra serve as sensitive probes of both structural and dynamic properties of the local environment of the ions.

Structural information may be derived from the symmetry and orientation-dependence of the ESR spectra. Information regarding the orientation of the symmetry axes of the complex ions can be obtained by working with preferentially-orientated films aligned either parallel or perpendicular to the magnetic field (Angel & Hall, 1973; Clementz et al., 1973).

ESR spectra can also probe the dynamic properties of ions or radicals in solution-like environments, yielding information such as rotational correlation times or diffusion coefficients. In general, the elucidation of such information requires the presence of a species having anisotropic g-values (and preferably also anisotropic hyperfine splittings). I f the paramagnetic ion is in a quasi-crystalline environment, i.e. is effectively immobile on the time-scale of observation of the ESR experimenL the direction-dependence of the spectrum will be seen in oriented samples. In contrast, if the ion is in a solution-like environment of low viscosity, and rapidly tumbling with a correlation time considerably shorter than the observation time, complete averaging of the anisotropic components of the spectrum will occur, giving effective spectral parameters g - l ( g x + g y + g z ) and A = 13(A~ + Ay + A~). In intermediate cases, where the mobility is on a time-scale compar- able with the observation time, only a partial averaging of the spectrum will occur. In

0009-8558/80/1200-0337502.00 �9 1980 The Mineralogical Society

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338 P. L. H a l l

these cases the detailed shape of the spectrum, such as the relative heights and widths of the component lines, will depend on the time-scale of the molecular motions, and a suitable model enables the latter to be determined. Theories of ESR lineshapes and linewidths or rotating molecules have been proposed by Kivelson (1960) and Freed & Fraenkel (1963) among others.

A particularly useful class of paramagnetic molecules giving anisotropic ESR spectra are the nitroxide free radicals. These species exhibit anisotropic triplet HFS due to the 14N nucleus ( I = 1), and have been exploited as spin probes, particularly in studies of biological membranes. Specific models for these systems have been developed which permit the time-constants of the molecular motions to be calculated (Sachs & Latorre, 1974; Mason & Freed, 1974).

In addition to information regarding cation-water association in expanding lattice clays, ESR studies on transition ion-exchanged clays have contributed to a greater understanding of the interactions of organic molecules with clays. Polar organic mole- cules, particularly arenes, can function as Lewis bases, donating electrons into vacant d-orbitals on the transition ions, and thus forming coordination complexes in which the organic species displace one or more water molecules from the cation hydration shells. The structure and orientation of such complexes may be deduced by ESR. Perhaps more significantly, it has been shown that for a variety of reasons (steric effects, surface acidity, etc.) the properties of hydrated ions in the interlamellar spaces of smectites and vermicu- lites differ significantly from the same ions in homogeneous solution. In many instances the exchange cations on clays have been found to form previously unknown complexes, and to promote novel redox reactions involving the formation of organic radicals. These in turn initiate catalytic modifications of the adsorbed molecules such as ring cleavage or polymerization.

In the following section, studies of the orientation and mobility of hydrated exchange cations and spin probes are reviewed. The final section covers published work relating to organic-transition ion complexes and reactions in the interlamellar environment. The reader is also referred to earlier papers (McBride, 1976d; Pinnavaia, 1976) which briefly review some aspects of this work.

T H E C O N F O R M A T I O N A N D M O B I L I T Y OF H Y D R A T E D C A T I O N S A N D SPIN P R O B E S IN T H E I N T E R L A M E L L A R S P A CE OF

E X P A N D I N G L A T T I C E C L A Y S

Cu 2 +

Clementz et al. (1973) investigated the ESR spectra of a range of Cu2+-exchanged smectites and vermiculites at differing levels of hydration. Under air-dried conditions the clays contained a monolayer of interlamellar water. The ESR spectrum of Cu 2 +-hectorite (d00~ = 12.4 ,~) exhibited an axially symmetric powder spectrum having gll = 2.34 and g• =2.08, with four-line hyperfine splitting (HFS) on the gi/component, of magnitude A~. = 165 x l0 -4 cm-1. The orientation dependence of the spectra of well-ordered films indicated that the symmetry axis lay approximately perpendicular to the silicate layers. The spectrum was consistent with an axially elongated tetragonal symmetry, most probably a planar Cu(H20)42+ complex, in which the copper ion is also coordinated to two silicate oxygens along the z-axis at a distance of ~2.8 A, compared with a cation-

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E S R o f clays." Part H

~'~ Silicate Layer

zl OH, / / , J / ~ ~ / ) " / ~ H O--~Cud----- OHz - ~ 0 ~/g~" ~

z ,,~O"H ~ O.H2 OH 2

[/ Silicate Layer ~ / H2 I Silicote Layer / A B

339

FIG. 1. Stereochemistry of hydrated Cu(ll) ion in expanding silicates. A. One-layer; B. Two-layer hydrate (Clementz et al., 1973).

water molecule distance of ~ 2.0 A (Fig. la). Air-dried Cu2+-exchanged saponite, mont- morillonite and vermiculite exhibited similar ESR spectra. In the case of an adsorbed bilayer of water on Cu 2+ vermiculite (d001 = 14.2 A) the ESR data could be attributed to a Jahn-Teller distorted Cu(H20)62+ complex in which the symmetry axis lay close to 45 ~ to the silicate sheets (Fig. lb). When more than two layers of adsorbed water were present on Cu 2+ hectorite, a purely isotropic signal having g = 2.19 was observed, which implies either rapid tumbling of the Cu(H20)62+ ions or, alternatively, a dynamic rearrangement of Jahn-Teller states (i.e. the distortions, rather than the actual molecules, are rapidly interchanging).

The same authors (Clementz et aL, 1973) also utilized Cu 2§ exchange ions as a spectroscopic probe to study the expansion properties of reduced-charge montmoril- lonites. Charge reduction was achieved by thermal diffusion of lithium into vacant octahedral sites. Cu 2§ ions in collapsed layers were coordinated to four water molecules in a plane, as in Fig. la; expanded layers exhibited the isotropic (rapidly reorientating) Cu(H20)62+ ESR spectrum. Progressive charge reduction caused an increased tendency for layers to remain collapsed even after prolonged equilibration at 100~ relative humidity.

Further studies of the interaction of Cu 2 + ions with montmorillonite were reported by McBride & Mortland (1974). By heating Ca2+-montmorillonite at temperatures of up to 270~ evidence for migration of Cu 2+ ions into both hexagonal holes and vacant octahedral sites was obtained, causing a reduction in ESR signal intensity. Resolvation caused a partial restoration of the isotropic and anisotropic ESR signals, which was more pronounced on resolvation with ethanol in comparison with water.

The above studies were made on smectites saturated with Cu 2 + ions. These systems will expand up to 001 spacings of ~ 19 20 A in water, corresponding to several interlamellar molecular layers. Nevertheless, apart from the monolayer (d001 = 12-4 A), no well- defined basal spacing is obtained at higher values of P/Po, the layer expansions becoming inhomogeneous (Pinnavaia, 1976). In contrast, by doping Cu 2§ ions at the 5~ level into smectites saturated with other divalent cations such as Mg 2+, well-defined two-layer hydrates were obtained having 001 spacings in the range 14-7-15.0 A. The ESR spectra of these systems differed from that of the homoionic two-layer Cu2+-vermiculite, having axially symmetric spectra with the unique axis lying in this case perpendicular to the

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340 P. L. H a l l

silicate sheets (McBride et al., 1975a). Doping of 5% Cu 2+ into Mg 2+ vermiculite, however, produced essentially the same ESR spectra as that of the pure Cu 2 +-vermiculite, i.e. with the symmetry axis at close to 45 ~ to the silicate sheets (McBride, 1976a). These differences in the orientation of the hydrated ions between smectites and vermiculite can be attributed to the effect of the higher, and more greatly localized, surface charge in the latter mineral.

Apart from the influence of the nature and distribution of surface charge on the stereochemistry of the hydrated ions, it has been shown that equilibration of Cu 2+- exchanged clays with a range of non-aqueous solvents results in a wide variety of orientations of the Cu2+-solvent complexes as revealed by the ESR spectra of orientated films (Berkheiser & Mortland, 1975).

M n 2 +

McBride et al. (1975b) also investigated the properties of hydrated Mn ~+ exchange ions on montmorillonite, hectorite, nontronite and vermiculite. In most cases, the usual six-line HFS spectrum attributable to the Mn(H20)62+ ion was observed. It was found that the linewidths of the components were broader in fully hydrated clays than in bulk solution, as earlier reported for Mn 2+ ions on montmorillonite by Furuhata & Kuwata (1969). The linewidths increased progressively with decreasing levels of hydration, and were in general anisotropic. There are two principal contributions to Mn 2+ ESR line-

120

- - 8 0

c ~ 6 0

ON

I00 |C

,U

OH

40

20 I I I

20 40 60

O -

I I 80 I00

Average Mn Mn interiomc distance (~,)

F I G . 2. Dependence of average Mn 2 + ESR linewidth on interionic distance. Open points are for MnC12 in methanol solution. Solid points are nontronite (N), Upton (U) and Chambers ( C )

montmorillonite and hectorite (H), all fully hydrated (McBride et al., 1975b).

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ESR of clays: Part H 341

(1

Gein=l.25 xlO 4

/~ tOOGo~ss

I ~ Goin:2.5xlO I'*" "" 3

FIG. 3. ESR spectra of (a) Mn C12 in methanol, and of 5~ Mn2+-doped Mg 2+ hectorite films (powder samples). (b) Fully hydrated, (c) Air-dried and (d) Dehydrated at 200~ (McBride et aL,

1975b).

widths: (i) dipolar relaxation mechanisms between neighbouring Mn 2 + ions, i.e. spin-spin relaxation, and (ii) relaxation processes due to ion-solvent molecule collisions. In the case of homoionic Mn 2 + clays the dipolar interactions dominate the linewidths, and provide information regarding the mean Mn 2+ - M n 2+ distances in different minerals (Fig. 2). The anisotropy of the linewidths suggests that spin-spin relaxation between the Mn 2+ ions and lattice-substituted Fe 3+ ions also make a signficant contribution to the line- widths. In hectorite (containing very little structural Fe 3+) dipolar broadening was minimized by doping an MgZ+-hectorite with approximately 5~ Mn 2+. In this sample, the increase in linewidth with decreasing hydration level gave a direct measure of the increase in orientational correlation time of the hydrated ions due to the decreased frequency of ion-water molecular collisions. Thus the ESR data are consistent with a reduction in the mobility of the hydrated Mn 2+ ions as the interlayer separation dec- reases. In the fully hydrated Mg2+/Mn 2+ hectorite (d001 ~21 A), the linewidths indicated a relaxation time which was only ~ 30~ longer than for Mn 2 + in bulk solution, indicating a liquid-like interlamellar environment. Dehydration at 200~ resulted in a crystalline rather than solution-like ESR spectrum, attributable to the migration of Mn 2+ ions into the hexagonal silicate cavities. The data are illustrated in Fig. 3.

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342 P. L. Hall

Similar results, suggesting an overall reduction in the mobility of hydrated Mn 2+ ions in comparison with bulk solution, have been reported for clays in which the water molecules coordinated to Mn 2+ were replaced by organic molecules such as pyridine (Pafamov et al., 1971; Tarasevich & Ovcharenko, 1973).

McBride (1976b) investigated the hydration properties and mobility of Cu 2+ ions in mixed Na +, Cu2+-montmorillonite. The nature of the ESR spectra were similar to that of pure Cu2+-montmorillonite (Clementz et al., 1973). Evidence for inhomogeneity of the exchange sites was obtained from adsorption isotherms, a large preference for Cu 2+ over Na + being observed at high levels of Na +. McBride & Mortland (1975) have reported a study of the segregation between inorganic and organic cations in mixed Cu2+-tetralkyl - ammonium montmorillonites. In the mixed-ion systems the Cu 2+ ions were less fully hydrated and in general less accessible to undergo complexation with volatile arenes such as anisole (Fenn et al., 1973).

V O 2+

The behaviour of hydrated vanadyl ions on hectorite has also been studied by ESR (Pinnavaia et al., 1974; McBride 1979a). When fully hydrated, vanadyl-exchanged hector- ite exhibits the blue colour characteristic of the axially symmetric VO(H20)52+ complex. On drying a tan colour develops, which is readily converted to a bright yellow colour on subsequent exposure to a moist atmosphere. The latter can be attributed to the oxidation of vanadium from the tetravalent to the pentavalent state. At high VO 2+ concentrations, the ESR spectra of fully hydrated hectorite are very similar to that of the hydrated vanadyl ion in solution, but showing an approximately 50~o reduction in rotational mobility. Under these conditions the behaviour of VO 2+ and Mn 2+ ions is quite similar. At lower levels of VO 2+ doping an anisotropic ESR spectrum indicative of an immobile ion is observed; this may be assigned to the hydrolysis product VO(OH)2(H20)3 whose formation is favoured under conditions of low surface acidity. In this respect, initially adsorbed vanadyl ions behave in a manner not dissimilar to that of AP + on smectite surfaces where hydrolysis takes place at higher pH values. Reactions of vanadyl ions on hectorite with volatile organic molecules are discussed in the next section.

Nitroxide spin probes

Extensive ESR work has been undertaken on the dynamical behaviour ofnitroxide spin probes such as TEMPO +, the cationic form of 4-amino-2,2,6,6-tetramethylpiperdine-N- oxide, in the interlamellar region of smectites. (McBride 1976c, 1976d, 1977a, 1977b). Rotational correlation times were derived from the theory of Sachs & Latorre (1974). When TEMPO + was present at up to 10~o of the CEC level on hectorite exchanged with a range of monovalent and divalent cations, rotational correlation times, zc, ranging from 1 • 10 -9 to 5 x 10 9 s were obtained, in comparison with a value Ofrc = 5 x 10 -~1 s for TEMPO + in bulk aqueous solution. Thus the mobility of the probe in the interlamellar environment was reduced by a factor of between 20 and 100. The correlation times were also somewhat dependent on the nature of the solvent used to equilibrate the clays. Also, in some cases a broad background resonance was observed attributable to a fraction of the TEMPO + molecules being immobilized on the ESR time-scale (ze> 10 -8 s). This effect was more pronounced for solvents of lower polarity.

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E S R o f clays: Part H 343

Studies on orientated films showed that the spectra were somewhat anisotropic, even for fully hydrated hectorites. This implies that the molecular tumbling is non-random due to interactions between the probe molecules and the silicate surfaces, the N -O bond axis tending to be aligned preferentially in a direction perpendicular to the plane of the silicate sheet. McBride (1979b) also showed that some non-heterogeneity occurs in the adsorp- tion of the spin probes on hectorite. There is a tendency for the probes to be concentrated in certain layers even at low adsorption levels, due to the high affinity of the probes for the hydrated clay surface in comparison with the strength of the probe-solvent interactions.

Fig. 4 illustrates the ESR spectra of orientated films of TEMPO +-doped Na +-hectorite solvated with water, methanol and ethanol. For the fully hydrated Na+-hectorite, McBride (1977b) related the calculated value of zc to an effective diffusion coefficient of D = 5 • 10 -6 cmZs - l using the relationship D = 12/4Zc, where/is the mean jump distance, i.e. the diameter of the probe molecule. This value is in quite good agreement with values for the water self-diffusion coefficient in montmorillonite obtained from high resolution neutron scattering experiments (Hall et al., 1979), and with the value of the most rapid correlation time for water or proton mobility in Na+-vermiculite derived from N M R measurements (Hougardy et al., 1976).

Considering these facts, together with the ESR data for transition ions on clays discussed above, it is clear that the mobility of ions or water molecules in the interlamellar sheets of expanding silicates are lower than in bulk solution, the extent of the reduction depending on the degree of dehydration. Nevertheless, the interlayer water sheets are clearly solution-like rather than quasi-crystalline except under severely dehydrating

FIG. 4. ESR spectra of TEMPO+-doped Na+-hectorite films, solvated in excess (a) H20, (b) CH3OH, (c) CaHsOH. Orientations of films with respect to the magnetic field are designated by II

orJ. (McBride, 1976d).

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344 P. L. Hal l

conditions, when the cations become dehydrated and migrate into the hexagonal silicate cavities. For Cu 2+ smectites, a transition between a 'rigid' and a 'tumbling' aquocation appears to take place between the two- and three-layer hydrates, suggesting a value of "~c ~ l0 9 S in this case, although it is not clear whether the averaging is due to molecular tumbling or a dynamic Jahn-Teller effect. The NMR measurements of Hougardy et al.

(1976) suggest a considerably longer correlation time for the rotation of the hydrated Na + ions in vermiculite, though this might be accounted for by the greater surface charge and lower interplanar spacing in this system. Recent neutron scattering data indicate that hydrated Ca 2+ and Mg 2+ ions in montmorillonite and vermiculite are immobile on the time-scale of the neutron method (10-~1-10-~3 s) (Hall et al., 1979) which is in agreement with both the ESR and NMR data.

T R A N S I T I O N M E T A L - O R G A N I C COMPLEXES AND REACTIONS IN THE I N T E R L A M E L L A R SPACE OF E X P A N D I N G L A T T I C E CLAYS

Mortland & Pinnavaia (1971) characterized two types of complex formed between benzene and Cu 2 + exchange ions on montmorillonite by IR spectroscopy: a green (Type I) and a red-brown (Type II) complex, which could be readily interconverted by altering the degree of hydration of the clay. The IR spectra indicated that in the Type I complex, which was the prefererred form at higher levels of hydration, the benzene ring was planar and retained its aromaticity. In contrast, the vibrational frequencies of the Type II complex were significantly shifted from the values characteristic of liquid (or physically adsorbed) benzene. These shifts suggested that in the Type II complex a distortion of the benzene ring occurred, consistent with direct edge-bonding of Cu 2+ ions. Pinnavaia & Mortland (1971) investigated the adsorption of a range of methyl-substituted benzenes by CuZ+-montmorillonite, and showed that these formed Type I complexes only.

Rupert (1973) showed that the Type II copper-benzene complex on montmorillonite exhibited an ESR spectrum in which a loss of intensity of the Cu 2+ resonance was accompanied by the appearance of a sharp single-line resonance near g = 2-0, attribu- table to a free radical. Type II complexes were also obtained with other unsubstituted arenes (biphenyl, naphthalene and anthracene), and these gave similar ESR spectra. Rupert suggested that under strongly dehydrating conditions, the d 9 C u 2+ ions could accept an electron to form diamagnetic Cu + cations together with arene cation radicals. Cu 2 +-exchanged hectorite and nontronite supported the formation of similar complexes.

Fenn et al. (1973) studied the adsorption of anisole (CH3OC6Hs) and some related aromatic ethers by Cu2+-hectorite. Anisole proved to be the first substituted arene which would also form both types of complex as characterized by ESR and IR spectroscopy. As for the unsubstituted arenes, formation of the Type II complex was favoured at lower levels of hydration. In the case of anisole, however, an interesting feature of the results was that ageing of specimens over a period of some days resulted in the formation of both Type I and Type II complexes of the dimer (4-4'-dimethoxybiphenyl). The dimer could be extracted in methanol, and was identified by IR, NMR and mass spectrometry. Butyl phenyl ether also formed a Type II complex, though biphenyl ether and benzyl methyl ether formed Type I complexes only. The ESR spectra of some of these complexes are illustrated in Fig. 5.

Pinnavaia et al. (1974) found that electron transfer complexes could be formed with exchange cations other than Cu 2+. Free-radical spectra, together with a reduction in the

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E S R o f clays: Part H 345

- - H---~ 5 0 0 Gauss

I I g=2.002a -B

= 2 . 0 0 2 a g = 2 . 0 0 2 6

E . m S i g : 2 " 0 0 2 4 J

cj = 2 .002a

�9 2 + FIG. 5. ESR spectra of (a) air dried Cu -hectorite, (b) blue, type II amide complex, (c)green, type II 4-4'-dimethoxybiphenyl complex, (d) purple, type I1 butyl phenyl ether complex, (e) brown, type I benzyl methyl ether complex, (f) green, type I phenyl ether complex (Fenn et al., 1973).

intensity of the ESR spectra of the transition ion, were observed in the cases of VO 2+- and Fe3+-exchanged hectorite exposed to benzene, toluene, anisole and thiophene. The electron-transfer reaction in all cases could be summarized by the equation Ar+Mn+--*[Ar] + +M (n-l)+ and depended only on the presence of a transition metal which could be fairly readily reduced. The Type II complexes (as defined by their characteristic IR spectra) could not, however, be directly identified with the monomeric cation radicals, but were more probably coupled, or associated, radical cations. In the case of toluene no Type II infrared spectrum was observed, but radical cations were produced which subsequently initiated polymerization of the arene, indicated by the presence of additional IR bands in aged samples not attributable to the monomer, and the production of a waxy material by methanol or acetone extraction. Anisole complexes on VO 2+- and Fe3+-hectorite led to dimerization, as on the Cu 2+ clay. A further interesting point to emerge from these studies was that complex formation with Fe3+-exchanged clays could only be achieved using freshly cation-exchanged clays. No reactions took place with Fe 3 +-smectite films aged in the laboratory for a period of weeks. This could be explained by the slow conversion of hydrated Fe 3+ ions to relatively inert polymeric oxyhydroxides (Pinnavaia et al., 1974).

Van de Poel et al. (1973) also investigated the Cu 2 +-benzene-montmorillonite system at

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346 P. L. Hail

a range of temperatures and conditions of hydration. Workers in the same group also first reported the formation of Type II complexes of the heterocyclic molecule thiophene on Cu2+-montmorillonite (Cloos et al., 1973). Stoessel et al. (1977) showed that complex formation initiated the polymerization of benzene to polyparaphenylene on Cu 2 +-mont- morillonite. Mass spectrometric studies of the higher molecular weight products formed by a range of arenes on Cu 2+ and Fe 3+ smectites have been reported (Tricker et al., 1975; Mortland & Halloran, 1976). The latter authors comment that the catalytic modification of simple aromatic molecules, especially by ferric-ion exchanged clays, form a feasible mechanism for the abiotic synthesis of macromolecules in soils and sediments.

Other redox reaction involving free radical formation on smectite surfaces include the well-known benzidine blue reaction, which has recently been re-examined (McBride, 1979c). It was demonstrated that both structurally substituted Fe 3+ ions and oxygen molecules can play a role in the formation of the blue semiquinone radical, though only a small proportion of structural Fe 3 + is involved. Fe 3 + exchange cations were only effective in promoting the reaction when freshly prepared. This result again indictes that Fe 3+ on smectites can only function as an electron acceptor when present as a hydrated cation prior to hydrolysis to hydroxides or oxyhydroxides.

Nagai et al. (1974) obtained the ESR spectra of orientated films of Cu 2+ montmoril- lonite following the adsorption of molecules such as pyridine, glycine and/~-alanine. Observed changes in the ESR spectra could be attributed to complex formation, the complexes being axially symmetric with the unique axis orientated perpendicular to the silicate layers. Nagai (1975) also investigated the nature and orientation of free radicals produced in the interlamellar region of montmorillonite by y-irradiation of stearic acid and its salts.

ESR spectroscopy has also been utilized in the characterization of the binding to smectites of a range of transition metal-organic complexes, including the 2-2'-dipyridyl complexes of Cu 2 + and Fe 2 § (Traynor et al., 1978), the ortho-phenanthroline complexes of Cu 2+ and Fe 2+ (Berkheiser & Mortland, 1977), and tris-ethylenediamine copper (II) (Velghe et aL, 1977; Burba & McAtee, 1977). All of these complexes show some tendency to be unstable in the interlamellar region of smectites, disproportionating to produce the his complexes plus free ligand molecules. Other organometallic complexes, such as diethylenetriamine and tetraethylenepentamine complexes of Cu(II) and Ni(II), lose axial water molecules, the smectite surfaces functioning as weak ligands to the planar com- plexes (Schoonheydt et aL, 1979). Two interesting recent developments are the produc- tion of heat-stable, expanded smectites and vermiculites with high nitrogen surface areas by thermal decomposition of intersalation complexes with bipyridyls and phenanthro- lines (Leoppert et al., 1979), and also the possibility that heavy-metal complexes of a similar nature (e.g. with ruthenium) may exhibit interesting properties, e.g. the ability to catalyse the hydrolysis of water, though this has not yet been achieved when the com- plexes are adsorbed on layer silicates (Traynor et al., 1978).

REFERENCES

ANGEL B.R. & HALL P.L. (1973) Electron spin resonance studies of kaolins. Proc. Int. Clay Conf. Madrid, 47-60. BERKI~ISER V.E. & MORTLAND M.M. (1975) Variability in exchange ion position in a smectite: dependence on

interlayer solvent. Clays Clay Miner. 23~ 404-410. B~RKHEISER V.E. & MORTLANO M.M. (1977) Hectorite complexes with copper (II) and iron (II)-I, 10-phenan-

throline chelates. Clays Clay Miner, 25, 105-112.

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E S R o f clays: Par t H 347

BURBA J.L. & MCATEE J.L. (1977) The orientation and interaction of ethylenediamine copper (It) with montmorillonite. Clays Clay Miner. 25, 113-118.

CLEMENTZ D.M., PINNAVAIA T.J. & MORTLAND M.M. (1973) Stereochemistry of hydrated copper (II) ions on interlamellar surfaces of layer silicate: an electron spin resonance study. J. Phys. Chem. 77, 196-200.

CLoos P., VAN DE POEL D. & CAMERLYNCK J. (1973) Thiophene complexes on montmorillonite saturated with different cations. Nature Phys. Sei. 243, 54-55.

FENN D., MORTLAND M.M. & PINNAVA1A T.J. (1973) The chemisorption of anisole on Cu (II) hectorite. Clays Clay Miner. 21,315 322.

FREED J.H. & FRAENKEL G.K. (1963) Theory of linewidths in ESR spectra. J. Chem. Phys. 39, 326-348. FURAHATA A. & KUZWATA K. (1969) Electron spin resonance spectra of manganese (I1) and copper (It)

adsorbed on clay minerals and silica-alumina mixtures. Nendo Kagaku 9, 19 27. HALL P.L. (1980) The application of electron spin resonance spectroscopy to studies of clay minerals: I.

Isomorphous substitutions and external surface properties. Clay Miner 15, 321-335. HArL P.L., Ross D.K., TUCK J.J. & HAVES M.H.B. (1979) Neutron scattering studies of the dynamics of

interlamellar water in montmorillonite and vermiculite. Proc. Int. Clay Conf. Oxford, 121 130. HOUGAROY J., STONE W.E.E. & FRIPIAT J.J. (1976) NMR study of adsorbed water. I. Molecular orientation and

protonic motions in the two-layer hydrate of Na + vermiculite. J. Chem. Phys. 64, 384~3851. KIVELSON D. (1960) Theory of ESR linewidths of free radicals. J. Chem. Phys. 33, 1094-1106. LEOPP~RT R.H. & MORTLAND M.M. & PINNAVAIA T.J. (1979) Synthesis and properties of heat-stable expanded

smectite and vermiculite. Clays Clay Miner. 27, 201208. MCBRtDE M.B. (1976a) Hydration structure of exchangeable Cu 2+ in vermiculite and smectite. Clays Clay

Miner. 24, 211-212. McBRIDE M.B. (1976b) Exchange and hydration properties of Cu 2+ on mixed Na+-Cu 2+ smectites. Soil Sei,

Soc. Am. J. 40, 452-456. McBRIDE M.B. (1976c) Nitroxide spin probes on smectite surfaces: temperature and solvation effects on the

mobility of exchange cations. J. Phys. Chem. 80, 186203. McBRIDE M.B. (1976d) Use of nitroxide spin probes in ESR studies of adsorbed molecules on solvated layer

silicates. Am. Chem. Soc. Syrnp. Set. 34, 123 140. MCBRIDE M.B. (1977a) Mobility and orientation of charged molecules at silicate surfaces. Clay Miner. 12,

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ESR studies. Clays Clay Miner. 25, 6 13. MCBRIDE M.B. (1979a) Mobility and reactions ofVO 2+ on hydrated smectite surfaces. Clays Clay Miner. 27,

91-96. MCBRIDE M.B. (1979b) Cationic spin probes on hectorite surfaces: demixing and mobility as a function of

adsorption level. Clays Clay Miner. 27, 97 104. MCBRIDE M.B. (1979c) Reactivity of adsorbed and structural iron in hectorite as indicated by the Oxidation of

benzidine. Clays Clay Miner. 27, 224-230. McBRIDE M.B. & MORTLAND M.M. (1974) Copper (II) interactions with montmorillonite: evidence from

physical methods. Soil Sci. Soc. Am. Proc. 38, 408-415. McBRIDE M.B. & MORTLAND M.M. (1975). Surface properties of mixed Cu (II)-tetraalkyl-ammonium mont-

morillonites. Clay Miner. 10, 357-368. McBRIDE M.B., PINNAVAIA T.J. & MORTLAND M.M. (1975a). Electron spin resonance studies of cation

orientation in restricted water layers on phyllosilicate (smectite) surfaces. J. Phys. Chem. 79, 2430-2435. McBRIDE M.B., PINNAVAIA T.J. & MORTLAND M.M. (1975b) Electron spin relaxation and the mobility of

manganese (II) exchange ions in smectites. Am. Miner. 60, 66-72. MASON R.P. & FREED J.H. (1974) Estimating rotational correlation times from lifetime broadening of nitroxide

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complexes adsorbed on interlamellar surfaces of montmorillonite. Chem. Phys. Letters 26, 517-520.

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PAFAMOV N.N., SILCHENHO V.A., TARASEVICH Y.I., TELICHKUN V.P. & BRATASHEVSKI! Y.A. (1971) State of Cu 2+ and Mn 2+ exchange cations in montmorillonite saturated by acetonitile and pyridine studied by EPR. Ukr. Khim. ,gh. 37, 672.

PINNAVAIA T.J. (1976) Orientation and mobility of hydrated metal ions in layer lattice silicates. Am. Chem. Soe. Symp. Ser. 34, 94~108.

PINNAVAIA T.J., HALL P.L., CADY S.S. & MORTLAND M.M. (1974) Aromatic radical cation formation on the intracrystal surface of transition metal layer lattice silicate. J. Phys. Chem. 78, 994 999.

PINNAVAIA T.J. & MORTLAND M.M. (1971) Interlametlar metal complexes on layer silicates. I. Copper (II) complexes on montmorillonite. J. Phys. Chem. 75, 395%3962.

RUPERT J.P. (1973) ESR spectra of interlamellar Cu (II)-arene complexes on montmorillonite. J. Phys. Chem. 77, 784~790.

SACHS F. & LATORRE J. (1974) Cytoplasmic solvent structure of single barnacle muscle cells studied by electron spin resonance. J. Biophys 14, 316-326.

SCHOONHEVDT R.A., VELGHE F., BAERTZ R. & UVTTERHOEVEN, J.B. (1979) Complexes of diethylenetriamine (dien) and tetraethylenepentamine (tetren) with Cu (II) and Ni (II) on hectorite. Clays Clay Miner. 27, 269-278.

STOESSEL F., GUTH L.J. & WEY R. (1977) Polymerization of benzene to polyparaphenylene on copper (2+)-montmorillonite. Clay Miner. 12, 255 259.

TARASEVICH Y.I. & OVCHARENKO F.D. (1973) On the mechanism of interaction between nitrogenous organic substances and montmorillonite surface. Proc. Int. Clay Conf. Madrid, 627 636.

TRAYNOR M.F., MORTLANO M.M. & PINNAVAIA T.J. (1978) Ion exchange and intersalation reactors ofhectorite with tris-bipyridyl metal complexes. Clays Clay Miner. 26, 318 326.

TRICKER M.M., TENNAKOON D.T.B., THOMAS J.M. & GRAHAM S.H. (1975) Novel reactions of hydrocarbon complexes of metal substituted sheet silicates: thermal dimerization of trans-stilbene. Nature Phys. Sei. 253, l l ~ l l l

VAN DE POEL, D., CLOOS P., HELSEN J. & JANINI E. (1973) Comportement particulier du benzene adsorb6 sur la montmorillonite cuivrique. Bull. Grpe Fr. Argiles 25, 115 126.

VELGHE F., SCHOONHEYDT R.A., UYTHERHOEVEN J.B., PEIGNEUR P. & LUNSFORD J.H. (1977) Spectroscopic characterization and thermal stability of copper (II) ethylenediamine complexes on solid surfaces. II. Montmorillonite. J. Phys. Chem. 81, 11821194.

RI~SUMI~: On passe en revue l'application de la spectroscopie RPE fi l'6tude des ions de m6taux de transition hydrat6s et des sondes paramagn&iques oxynitr6es, situ6s dans l'espace interlamel- laire des smectites et des vermiculites. Ces recherches apportent non seulement des informations concernant la structure et la mobilit6 des couches eau-cations intracristallines, mais 6galement sur la r6activit6 et les propri6t6s catalytiques de certaines formes de smectites 6chang6es par des m6taux de transition. On a pu caract6riser plusieurs complexes nouveaux de coordination ainsi que des r6actions redox entre les ions 6changeables et des mol6cules organiques varibs.

KURZREFERAT: Es wurde ein Uberblick gegeben fiber die Anwendung der ESR-Spektrosko- pie zur Untersuchung yon Obergangsmetall-Ionen und Nitroxydspin-Proben im interlamellaren Bereich von Smektiten und Vermiculiten. Diese Untersuchungen brachten nicht nur Kenntnisse bezfiglich der Struktur und Mobilit/it der interkristallinen Wasserkationen-Schichten, sondern haben auch die Reaktivit/it und katalytischen Eigenschaften bestimmter Austauschformen der Ubergangsmetalle von Smektiten gezeigt. Mehrere ungew6hnliche Koordinationskomplexe und Redox-Reaktionen zwischen Austauschionen und einer Vielzahl einfacher organischer Molek/ile konnten charakterisiert werden.

RESUMEN: Se facilita una resefia de la aplicaci6n de la espectroscopia de resonancia del espin de los electrones al estudio de los iones de metales de transici6n hidratados y a las sondas del espin de los nitr6xidos en la regi6n interlamelar de esmectitas y vermiculitas. Estas investigaciones no s61o

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ESR of clays." Part II

ban proporcionado informaci6n sobre la estructura y la movilidad de las capas intracristalinas de cationes del agua, sino que han demostrado adem~is la reactividad y las propiedades cataliticas de ciertas formas de intercambio de metales de transici6n de las esmectitas. Se han caracterizado varios complejos de coordinaci6n y reacciones de reducci6n-oxidaci6n entre los iones de intercambio y una variedad de mol~culas orgfinicas sencillas.

349


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