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Kloprogge, J. Theo (2006) Spectroscopic studies of synthetic and natural beidellites: A review. Applied Clay Science 31:pp. 165-179 Copyright 2006 Elsevier Accessed from: https://eprints.qut.edu.au/secure/00003640/01/SPECTROSCOPIC_STUDIES_OF_SYNTHETIC_AND_NATURAL_BEIDELLITES.pdf

SPECTROSCOPIC STUDIES OF SYNTHETIC AND NATURAL

BEIDELLITES: A REVIEW

J. Theo Kloprogge

Inorganic Materials Research Program, Queensland University of Technology, GPO

Box 2434, Brisbane, Qld 4001, Australia

Correspondence: phone +61 7 3864 2184, fax +61 7 3864 1804, E-mail

t.kloprogge@qut.edu.au

Abstract

This paper represents an overview of the spectroscopic studies of both synthetic and

naturally occurring beidellites performed as part of my research over the past 16

years. It shows that detailed information on the local structure of beidellite and

changes in this local structure upon heating can be obtained by combining a range of

spectroscopic techniques such as mid- infrared, near-infrared, infrared emission,

Raman, nuclear magnetic resonance and X-ray photoelectron spectroscopy.

Key words : Beidellite, hydrothermal synthesis, infrared spectroscopy, Raman

spectroscopy, solid state Nuclear Magnetic Resonance spectroscopy, X-ray

photoelectron spectroscopy

Introduction

Clays and more specifically smectites have played an important role in our

society for many decades. Of special interest are modified smectites such as

organoclays (Lagaly, 1984; Breen et al., 1997; Frost and Parker, 1997; Komadel,

1999) and pillared clays with their specific properties in areas such as adsorption,

heterogeneous catalysis and molecular sieving (Ballantine, 1986; Burch, 1988;

Vaughan, 1988; Kloprogge, 1998; Vaccari, 1998; Ding et al., 2001). For example, the

use of clay minerals as heterogeneous catalysts asks for very high phase purity and

well-controlled chemistry. Therefore, synthesis of smectites has become an important

issue due to the high chemical purity and the ability to adjust the synthetic smectite

composition and structure. From this point of view beidellite is the most interesting of

the smectites due to its relatively high acidity and chemical simplicity.

Beidellite is a dioctahedral smectite named after Beidell, Colorado, U.S.A..

Weir and Green-Kelly (1962) and Ross and Hendricks (1945) defined it as the

aluminium endmember of the dioctahedral smectites with a general formula of

(0.5Ca, Na, K)xAl4(Si8-xAlx)O20(OH)4.nH2O based on the material from the Black

Jack Mine, Idaho. Other locations of natural beidellite include Unterrrupsroth

(Germany) and the so-called Ascan beidellite (Besson and Tchoubar, 1972; Tispursky

and Drits, 1984; Nadeau et al., 1985).

Synthesis of beidellite has been described following a variety of methods. A

detailed review on the synthesis of clay minerals including beidellite has been

published in 1999 (Kloprogge et al., 1999b).The earliest reports date back to halfway

the nineteen thirties with the work of Ewell and Insley (1935) who hydrothermally

treated aluminosilicate glasses at temperatures up to 390°C. Granquist and Pollack

(1967) and Granquist et al. (1972) reported on the hydrothermal synthesis of a

randomly interstratified dioctahedral smectite with only aluminium as the octahedral

cation and partial substitution of Al for Si (i.e. beidellite) from ion-exchanged Na-

silicate solutions and gibbsite. The first well defined beidellite crystals were

synthesised by Roy and Sand (1956) from a mixture of oxides at 400°C and 1 kbar for

2 hours. Low temperature hydrothermal experiments at temperatures ranging from

130 to 175°C at pressures to 8.8 bars were tried by De Kimpe (1976). Only in the case

of synthesis at 175°C and a pH of around 8 beidellite was formed. In all other

experiments the resulting materials were either amorphic or contained kaolinite or

zeolites.

In the second half of the sixties and early seventies the first patents were filed

on the synthesis of beidellitic materials aimed at catalytic applications (Capell and

Granquist, 1966; Granquist, 1966; Jaffe, 1974). Plee and coworkers (Plee et al., 1984;

Plee and Fripiat, 1987) and Schutz et al. (1985; 1987) were the first who described the

synthesis of beidellite followed by pillaring with the Al13 Keggin complex in order to

obtain permanent two-dimensional porosity between the clay layers.

This paper aims at giving an overview of a range of spectroscopic methods

used to characterise synthetic and natural beidellites as a part of my ongoing research

during the past 15 years. In addition to the more standard techniques such as Infrared

spectroscopy, examples will be given of information that can be obtained from

techniques such as Infrared Emission Spectroscopy, solid-state NMR and X-ray

photoelectron spectroscopy.

Experimental

Beidellite synthesis

Beidellite with composition Nax(AlVI)4Si8-x(AlIV)xO20(OH)4 have been synthesized

and described by Kloprogge and coworkers (Kloprogge et al., 1990b; 1990c; 1993).

This beidellite was synthesized from gels prepared according to the method of

Hamilton and Henderson (1968). In such a synthesis 50 mg of the stoichiometric Na-

Al-Si gel together with 70 µl water or NaOH solution was placed in a gold capsule

followed by hydrothermal treatment in a Tuttle- type, externally heated, cold-seal

pressure vessel using argon as the pressure medium (Tuttle, 1949). Beidellite BSK3

was synthesized by Yanagisawa et al. (1996) according to the method of Luth and

Ingamells ( 1965). They used a gel prepared from Al(NO3)3.9H2O, NaNO3 and Ludox

colloidal silica. Dried gel powders mixed with various amounts of H2O were treated

in an autoclave at 350°C under autogenous pressure with agitation by turning the

autoclave 20 times/minute in an oven. For this study we used a sample treated for 3

days at 350°C.

Natural beidellite

The natural beidellite was obtained from the CMS Clays Source Repository as sample

CMS Source Clay SBCa-1.

Scanning Electron Microscopy

The morphology of the beidellite was investigated with a Cambridge S150 Scanning

Electron Microscope operating at 15 keV. The sample was coated with carbon prior to

the analysis.

X-ray diffraction (XRD)

The XRD analyses were carried out on a Philips wide angle PW 1050/25 vertical

goniometer equipped with a graphite diffracted beam monochromator. The d-values

and intensity measurements were improved by application of an in-house developed

computer aided divergence slit system enabling constant sampling area irradiation (20

mm long) at any angle of incidence. The goniometer radius was enlarged to 204 mm.

The radiation applied was CuKa from a long fine focus Cu tube operating at 35 kV

and 40 mV. The samples were measured at 50 % relative humidity in stepscan mode

with steps of 0.02° 2? and a counting time of 2s.

Mid-infrared spectroscopy (IR)

All samples were oven dried to remove any adsorbed H2O and stored in a desiccator.

Each sample (1mg) was finely ground for 1 minute, combined with 250 mg oven

dried spectroscopic grade KBr (refractive index of 1.559) and pressed into a disc

using 8 tonnes of pressure for 5 minutes under vacuum. The spectrum of each sample

was recorded in triplicate by accumulating 64 scans at 4 cm-1 resolution between 400

cm-1 and 4000 cm-1 using the Perkin-Elmer 1600 series Fourier transform infrared

spectrometer equipped with a LITA detector. Data interpretation and manipulation

was carried out using the Spectracalc software package GRAMS (Galactic Industries

Corporation, NH, USA) and Microsoft Excel.

Near-infrared spectroscopy (NIR)

The NIR spectroscopy analyses were performed on a Perkin Elmer System 2000 NIR-

spectrometer. For the samples 32 scans were accumulated at a spectral resolution of

16 cm-1 using a mirror velocity of 0.2 cm-1/s to get an acceptable signal/noise ratio.

Fourier transform Infrared Emission Spectroscopy (IES)

FTIR emission spectroscopy was carried out on a Digilab FTS-60A spectrometer,

which was modified by replacing the IR source with an emission cell. A description

of the cell and principles of the emission experiment have been published elsewhere

(Vassallo et al., 1992; Frost et al., 1995). Approximately 0.2 mg of the beidellite

sample was spread as a thin layer on a 6 mm diameter platinum surface and held in an

inert atmosphere within a nitrogen-purged cell during heating. The infrared emission

cell consists of a modified atomic absorption graphite rod furnace, which is driven by

a thyristor-controlled AC power supply capable of delivering up to 150 amps at 12

volts. A platinum disk acts as a hot plate to heat the basic aluminum sulphate sample

and is placed on the graphite rod. An insulated 125-µm type R thermocouple was

embedded inside the platinum plate in such a way that the thermocouple junction was

<0.2 mm below the surface of the platinum. Temperature control of ± 2°C at the

operating temperature of the beidellite sample was achieved by using a Eurotherm

Model 808 proportional temperature controller, coupled to the thermocouple.

The design of the IES facility is based on an off axis paraboloidal mirror with a focal

length of 25 mm mounted above the heater capturing the infrared radiation and

directing the radiation into the spectrometer. The assembly of the heating block, and

platinum hot plate is located such that the surface of the platinum is slightly above the

focal point of the off axis paraboloidal mirror. By this means the geometry is such

that approximately 3 mm diameter area is sampled by the spectrometer. The

spectrometer was modified by the removal of the source assembly and mounting a

gold coated mirror, which was drilled through the centre to allow the passage of the

laser beam. The mirror was mounted at 45°, which enables the IR radiation to be

directed into the FTIR spectrometer. In the normal course of events, 3 sets of spectra

are obtained: 1) the black body radiation over the temperature range selected at the

various temperatures, 2) the platinum plate radiation is obtained at the same

temperatures and 3) the spectra from the Pt plate covered with the sample. Normally

only one set of black body and Pt radiation is required. The emittance spectrum at a

particular temperature was calculated by subtraction of the single beam spectrum of

the Pt backplate from that of the Pt + sample, and the result ratioed to the single beam

spectrum of an approximate blackbody (graphite). This spectral manipulation is

carried out after all the spectral data have been collected.

The emission spectra were collected at intervals of 50°C over the range 200 - 750 °C.

The time between scans (while the temperature was raised to the next hold point) was

± 100 seconds. It was considered that this was sufficient time for the heating block

and the powdered sample to reach temperature equilibrium. The spectra were

acquired by coaddition of 64 scans for the whole temperature range (approximate

scanning time 45 seconds), with a nominal resolution of 4 cm-1. Good quality spectra

can be obtained provided the sample thickness is not too large. If too large a sample is

used then the spectra become difficult to interpret because of self absorption.

Spectral manipulation

Spectral manipulation such as baseline adjustment, smoothing and

normalisation were performed using the Spectracalc software package GRAMS

(Galactic Industries Corporation, NH, USA). Band component analysis was carried

out using the peakfit software package by Jandel Scientific. Lorentz-Gauss cross

product functions were used throughout and peak fitting was carried out until the

squared correlation coefficients with r2 greater than 0.995 were obtained.

Solid-state Magic-Angle Nuclear Magnetic Resonance Spectroscopy (MAS-NMR)

27Al, 29Si and 23Na MAS-NMR spectra were recorded on a Bruker WM500 (11.7

Tesla) at the Department of Physical Chemistry, Faculty of Science, University of

Nijmegen, The Netherlands, at 130.321, 99.346 and 132.258 MHz, respectively.

Chemical shifts are reported in ppm relative [Al(H2O)6]3+ for 27Al, tetramethylsilane

(Me4Si) for 29Si and NaCl for 23Na. The samples were spun at a frequency of

approximately 10.5 kHz for for Al and Si NMR and 3 kHz for Na NMR. Standard 256

Free Induction Decays (FIDs) were accumulated at a repetition time of 1 s.

X-ray Photoelectron Spectroscopy (XPS)

The minerals were analyzed in freshly powdered form in order to prevent surface

oxidation changes. Prior to the analysis the samples were out gassed under vacuum

for 72 hours. The XPS analyses were performed on a Kratos AXIS Ultra with a

monochromatic Al X-ray source at 150 W at the Brisbane Surface Analysis Facility,

University of Queensland, Australia. Each analysis started with a survey scan from 0

to 1200 eV with a dwell time of 100 milliseconds, pass energy of 160 eV at steps of 1

eV with 1 sweep. For the high resolution analysis the number of sweeps was

increased, the pass energy was lowered to 20 eV at steps of 100 meV and the dwell

time was changed to 250 milliseconds.

Results and discussion

Synthetic beidellite

Fig. 1 shows an SEM image of Na-beidellite synthesised at 350°C and 1 kbar for 7

days. In general the beidellite is present as small flakes up to 30 µm with less than

10% amorphous material. At 350 and 400°C beidellite was the predominant

crystalline product (Fig 2a). At pressures above 2 kbar quartz was formed as an

additional crystalline product, while higher temperatures resulted in the formation of

paragonite and metastable cristobalite. Below 250°C kaolinite was formed as the only

crystalline product (Kloprogge et al., 1990c; 1993). The crystallisation of beidellite at

400°C and 1 kbar proceeded quite rapidly in the first 5 days and then slowed down

until a maximum amount of approximately 95 vol.% crystallisation was reached (Fig

2b). XRD data agreed well with results published by Brindley and Brown (1984) and

Weir and Green-Kelly (1962) with a basal spacing of 12.4 Å expanding to 16.5 upon

expansion with ethylene glycol. Saturation with Li and glycerol followed by heating

overnight at 300°C (Hoffmann-Klemen and Greene-Kelly test) resulted in a basal

spacing of 17.7 Å confirming that the smectite formed is indeed beidellite (Kloprogge

et al., 1990a). From a catalytic perspective H+-smectites are of interest. These

catalysts are normally prepared by heat or acid treatment of ammonium-exchanged

smectites. However, it is also possible to synthesise directly ammonium smectites.

Ammonium-beidellite was successfully synthesised by adding glycine to the oxidic

gel similar to that used for the Na-beidellite but without the sodium present followed

by hydrothermal treatment. Similar experiments with ammonium salts and urea were

not successful (Kloprogge and Vogels, 1995).

27Al and 29Si Solid-State NMR of synthetic beidellite

Solid-state NMR of smectites is a useful tool to get a better understanding of possible

substitutions in the octahedral and tetrahedral sheets. The presence of impurities,

especially iron, in the crystal structure can have a distrastrous effect on the quality of

the spectra. However, NMR of synthetic beidellite with no impurities and known

composition will not show these drawbacks. Fig. 3 shows the 27Al and 29Si spectra of

a Na-beidellite with composition Na0.6Al4.6Si7.3O20(OH)4 (Kloprogge et al., 1990d).

The substitution of 0.7Al for Si in the tetrahedral sheet is clearly visible in Fig 3a with

a small peak at 69.9 ppm, while the aluminium in the octahedral sheet is visible as a

much stronger peak at 3.9 ppm. The difference in band width and the sideband

structure made it impossible to directly calculate the substitution. The 29Si NMR

spectrum however could be used to calculate the amount of tetrahedral substitution as

the ratios of the three bands associated with Si(0Al) at 92.7 ppm, Si(1Al) at 88.4 ppm

and Si(2Al) at 83.3 ppm is related to the tetrahedral Al/Si ratio according to the

formula:

210

)212 3/4(2SiSiSiSiSiSi

++−+

Where Si0, Si1 and Si2 represent the intensities of the NMR signals Si(0Al), Si(1Al)

and Si(2Al) (Sanz and Serratosa, 1984). The result was a substitution of about 0.76,

which was, within instrumental accuracy, considered to be similar to the chemical

analysis of 0.7. The peak positions for both the 27Al and 29Si NMR spectra agree

well with those reported by Diddams et al. (1984) for synthetic beidellite and by

Nadeau et al. (1985) for natural beidellite from Unterrupsroth. The results presented

by Plee et al. (1985) for their 29Si spectra are however consistently 3 ppm shifted to

higher values.

Infrared spectroscopy of beidellite

The near- infrared (NIR) spectral region has been defined to extend from

roughly 14285 to 4000 cm-1 (Rossman, 1988; Kloprogge et al., 2000b). The only

vibrations in the NIR region between 4000 and 8000 cm-1 are those associated with

hydrogen atoms associated with hydroxyl groups in the case of micas and with

hydroxyl groups and water in the case of clay minerals. Rossman (1988) reported

average band positions for OH-groups and H2O in the NIR region around 4500 cm-1

as being due to M-OH motions, 5200 cm-1 as the H2O combination mode (bend +

stretch) and around 7100 cm-1 as the first OH stretch overtone. This means that NIR

spectroscopy is a very suitable technique to study phyllosilicates.

In order to come to a detailed interpretation of the NIR spectra of the synthetic

micas and clays it is necessary to have a more detailed understanding of the associated

bands in the mid- infrared spectra of these minerals. Fig. 4 shows the mid- infrared low

frequency region between 400 and 1900 cm-1 and the OH-stretching region between

2900 and 4000 cm-1. Table 1 reports the band centres for the structural OH-bending

and OH-stretching vibrations shown in Fig. 4.

Kloprogge et al. (2000a) have recently reported the effect of increasing layer

charge in synthetic beidellites on their mid- infrared spectra. They showed that an

increase in sodium and aluminium content together with a decrease in silicon content

in the starting gel resulted in an increase in the layer charge of the beidellite

(Kloprogge et al., 1993). However, they found that the resulting clay layer charge

could deviate substantially from the layer charge calculated from the chemical

composition of the initial gel. In this study the difference between starting gel and

product for the low charge beidellite was only small and within approximately 15 %.

For the high layer charge beidellites synthesised from Na1.5 gel the layer charge varied

between 1.68 and 1.87 and for the Na2.0 the layer charge was 1.89 for the beidellite

and 2.0 for the paragonite.

The mid- infrared spectra of the lower charge beidellites (0.6 and 0.7) are very

similar in the Si-O region around 1000 cm-1. The high charge beidellite spectra are

much more similar to the paragonite spectrum. In the low frequency region the OH-

libration modes associated with the octahedral Al-OH groups in the beidellite

structures are observed around, 878-882, 913-920 and 935-965 cm-1. Similar bands

were reported by Farmer (1974) around 915-950 cm-1 for the Al-OH in-plane

librational modes in all dioctahedral species and around 880 cm-1 for the out-of-plane

librational mode in pyrophyllite. Only the last band shows a clear shift towards higher

wavenumbers with increasing layer charge and has been interpreted as the in-plane

vibration (Kloprogge et al., 2000a). Small amounts of other silica species such as

amorphous material or quartz are indicated by the presence of a minor band around

810 cm-1 (Madejova et al., 2000a)..

For beidellite the OH-stretching vibrations are affected by the configuration

and charge distribution in the neighbouring tetrahedral layers. Vedder and McDonald

(1963) have shown that in dioctahedral clay minerals such as beidellite the OH-

transition moment is tilted by a small angle out of the 001 plane. A consequence of

this orientation is that the proton of the OH-group lies closely to two of the six oxygen

atoms at the apices of the tetrahedra forming a hexagonal cavity in the tetrahedral

layer. In the relatively low-charge beidellite structure this results in two OH-stretching

modes, one associated with hydroxyl protons located near charge-bearing oxygen

atoms of the Al substituted hexagonal rings and one associated with neutral oxygen

atoms of non-substituted hexagonal rings. Increasing the layer charge will

consequently be reflected in the intensity ratio between these two OH-stretching

modes. Very dominantly present in the mid- infrared spectra of the beidellites are two

bands due to interlayer water. The interlayer water in the beidellite samples is

characterized by a H2O bending vibration at 1635 cm-1 plus the associated OH-

stretching vibration around 3450 cm-1. This band is most likely associated with water

molecules in the outer hydration sphere of the interlayer sodium cation (Bishop et al.,

1994). The shoulder around 3240 cm-1 was ascribed to interlayer H2O chemically

bound or coordinated to the interlayer cation with an ice-like structure (van der Marel

and Beutelspacher, 1974). Clearly only one Al2OH band can be observed around 3668

cm-1. The small band at 3620 cm-1 is thought to be characteristic for the asymmetric

water OH-stretching vibration associated with the inner hydration shell of the

interlayer cation (Bishop et al., 1994). However, due to the presence of strong water

bands overlapping the 3668 and 3620 cm-1 bands, the positions and intensities of these

bands can deviate significantly and obscure the second Al2OH band. Therefore, no

definite conclusions can be drawn from this region of the mid- infrared spectra.

The NIR spectra of the synthetic beidellites are shown in Fig. 5, while Fig. 6

shows the band component analysis of a low charge beidellite. A detailed list of the

band centers for all samples is given in Table 2. As for the mid- infrared spectra the

NIR spectra of the low charge beidellites are very similar. Band component analysis

indicates the presence of two bands around 4545 (weak) and 4595 (strong) cm-1. This

split disappears in the spectra of the high charge beidellites. The range around 4500

cm-1 is characteristic for the combination bands of the Al-OH-stretching modes and

the Al-OH libration modes around 3660 cm-1 and 880 and 915 cm-1 respectively

(Cariati et al., 1981; 1983a; 1983b; Bishop et al., 1993; Madejova et al., 2000b).

Pyrophyllite shows a strong sharp band around 4615 cm-1 accompanied by two weak

bands at both slightly higher and lower frequencies. The presence of the extra band

around 4548 cm-1 may indicate the presence of a very small amount of beidellite, not

observed in the XRD, corresponding with the band around 854 cm-1 in the mid-

infrared spectrum. Kloprogge et al. (2000b)observed similar bands for both natural

and synthetic pyrophyllites. Clarke et al. (1990) reported a band at 4617 cm-1,which

agrees very well with the band observed in this study at 4615 cm-1, and a weaker band

at 4794 cm-1 with a clear shoulder at 4850 cm-1. This second weaker band with its

shoulder is identical to the two weak bands observed in this study around 4805 and

4844 cm-1.

Three bands can be observed for beidellite around 5140, 5240 and 5260 cm-1.

Isolated water molecules show an absorption band near 5333 cm-1, due to the

combination mode of the stretching and bending modes of the water molecule (Hunt

and Salisbury, 1970). The presence of interlayer water in smectitic clay minerals will

be observed as broad bands close to this position, due to the poorly ordered sites in

the interlayers of a smectitic clay (Clark et al., 1990). In the case of the synthetic

beidellites this means that the first two bands around 5140 and 5240 cm-1 can be

ascribed to the combination modes of interlayer water modes (Bishop et al., 1993).

Similar bands were observed by Sposito et al for montmorillonite around 5130 and

5250 cm-1 (Sposito et al., 1983). Band component analysis indicate that the relatively

broad band around 5250 actually contains two components around 5240 and 5260 cm-

1, which can probably be explained by the fact that a part of the interlayer water is

coordinated to the interlayer sodium cations, while the remaining water molecules are

less tightly bond to the clay structure, in agreement with dehydration studies that

indicate two dehydration stages (Koster van Groos and Guggenheim, 1984a; 1986a;

1987a; Kloprogge et al., 1992a). The strong intensity of these bands are due to the

presence of a relatively large amount of interlayer water. Clearly visible is a decrease

in intensity with increasing layer charge. This increase in layer charge is compensated

by the incorporation of more sodium interlayer cations leaving less interlayer space

for water.

The bands observed in the high wavenumber region can be attributed to the

overtones and combination modes associated with fundamental stretching and

bending modes observed in the mid- infrared region. For the beidellites this region is

relatively complex showing multiple overlapping bands. Two relatively sharp bands

occur around 7100 and 7150 cm-1 plus a much broader band around 6900 cm-1. This

broad band deceases in intensity with increasing layer charge of the beidellite and is

absent in the spectra of the pyrophyllite and paragonite. Therefore, plus the fact that

the free water molecule has an absorbance near 6878 cm-1 (Hunt and Salisbury, 1970;

Sposito et al., 1983), this band must be ascribed to the first overtone of the strongly

hydrogen bonded interlayer water OH-stretching mode (Madejova et al., 2000b). The

7100 cm-1 band is characteristic for the first overtone of the Al-OH stretching modes

of the beidellites. The origin of the second band around 7150 cm-1 is less clear though.

Earlier Infrared Emission Spectroscopy work on synthetic beidellites has shown

however that the broad band around 3660 cm-1 actually consists of two overlapping

Al-OH stretching modes around 3615 and 3650 cm-1 (Kloprogge et al., 1999c), while

montmorillonite SAz shows also two bands around 3622 and 3668 cm-1 after heating

to 110°C (Madejova et al., 2000a). This means that even though the two Al-OH

stretching modes cannot always be identified in the mid- infrared spectra, both bands

do show up in the NIR spectra.

In the very high wavenumber region the low charge synthetic beidellites show a

very weak band around 8670 cm-1. For the high-charge beidellite this band is shifted

to 8631 cm-1. Madejova et al. (2000b) described a similar band around 8661 cm-1 plus

a band at 10550 cm-1 for montmorillonite SAz, which they assigned to the second

overtone and combination of variously bound water molecules. Pyrophyllite exhibits a

weak absorption band at 8122 cm-1, close to the value of 8110 cm-1 reported by Clarke

et al. (1990) for natural pyrophyllite. The interpretation for the second band around

10550 cm-1 given by Madejova et al. (2000b) disagrees with the observations of

Clarke et al. (1990) for pyrophyllite, which has no interlayer water, with a similar

band at 10500 cm-1. A better alternative explanation for this weak band is that it is a

combination band of Al-OH in the phyllosilicate structure. No corresponding band

was observed for the synthetic paragonite in the 8000-10000 cm-1 region.

X-ray photoelectron spectroscopy of beidellite

Photo-electron spectroscopy as an analytical tool has only received limited interest in

the field of mineral science. Photo-electron spectroscopy, together with Auger

electron spectroscopy, gives information about the positions of the energy levels in

atoms or molecules. Therefore can be used to not only obtain a chemical analysis but

high resolution photoelectron spectroscopy can also give more insight in the local

environment of an atom in a crystal structure. In this case, where Al monochromatic

X-rays are used, the technique is known as X-ray Photo-electron Spectroscopy (XPS)

or Electron Spectroscopy for Chemical Analysis (ESCA). Fig. 7 gives an example of

a survey scan of natural beidellite SBCa-1. Each of the peaks visible corresponds not

only to a specific atom but also to a specific energy level in that atom. Table 3 gives

an overview of the transitions observed, the corresponding binding energies and the

chemical composition. The high resolution spectra of Al show two peaks at 74.30 and

74.87 eV associated with the tetrahedral and octahedral Al based on the chemical

composition based on the survey spectra. The high resolution spectrum in the region

of F 1s shows the presence of a trace of F in the beidellite structure.

Thermal behaviour of beidellite: Infrared emission spectroscopy and 23Na Solid-State

NMR

Dehydration reactions can provide important information about the nature of

the interlayer. Commonly used techniques are thermal analysis and differential

thermal analysis (TG and DTA), which show that for most smectites dehydration

takes place in two steps associa ted with a weakly bonded, outer hydration shell

around the interlayer cation and with a more strongly bonded inner hydration shell at

temperatures around 140-150°C and 200-210°C, respectively (Koster van Groos and

Guggenheim, 1984b; 1986b; 1987b; 1989). Solid-state NMR has been used in a

number of studies to elucidate the structural environment of exchangeable cations in

zeolites and clays (Chu et al., 1987; Bank and Ellis, 1989; Janssen et al., 1989; Weiss

et al., 1990a; 1990b).

Kloprogge et al. (1992a) used 23Na MAS-NMR to study the dehydration

behaviour of synthetic beidellite. At room temperature in the hydrated state the

spectrum shows a sharp resonance near 0 ppm, comparable to that observed for Na+ in

solution. This indicates that in the hydrated state the sodium cation behaves similar to

that in solution. Upon dehydration the resonance shifts in a linear fashion to 1.56 ppm

at 105°C, while the linewidth decreases by about one third (Fig. 8). The change in the

local environment upon dehydration and the collapse of the interlayer space makes the

resonance become more positive. Although an exact interpretation of the changes is

difficult, it seems that the tetrahedral sheet and especially the Al3+ substituted

tetrahedra have an increasing influence on the Na+ site. In contrast, no changes are

observed in the 29Si and 27Al spectra of the beidellite with increasing temperature.

Fig. 9a shows the infrared emission spectra of synthetic beidellite from 200° to

750°C. The hydroxyl stretching region between 3000 and 4000 cm-1 clearly shows the

dehydration and dehydroxylation reactions. Although some of the interlayer water has

already disappeared from the interlayer at 200°, still a large band remains visible

around 3420 cm-1 (Fig. 9b), while at 450°C this band has split in two much weaker

bands around 3350 and 3525 cm-1. In addition a band becomes visible at 3720 cm-1

(Fig. 9c). The band at 3720 cm-1 has been assigned to silanol groups that are formed

on the crystal edges during the dehydration and dehydroxylation as an intermediate in

the dehydroxylation as this band is not visible at lower temperatures (Kloprogge et al.,

1999c). Further heating results in a decrease of the Al-hydroxyl modes and the

disappearance of the silanol bands. Associated with the dehydroxylation and the

changes in the hydroxyl stretching region are the changes in the OH-bending and -

libration modes. Two bands at 788 and 820 cm-1 decrease in intensity during heating,

while two other bands at 876 and 929 cm-1 completely disappeared between 650 and

700°C, which indicates that these two bands are the Al-OH bending vibrations. The

other two bands are probably the librational modes. A new band is observed at 722

after the dehydroxylation associated with the formation of new Al-O bonds in the

dehydroxylated beidellite structure.

Pillared beidellite

For the preparation of Al-pillared clays a polyoxycation known as Al13,

[AlO4Al12(OH)24(H2O)12]7+, formed by forced hydrolysis of Al3+ by for example

NaOH, Na2CO3 or decomposition of urea (Akitt and Elders, 1985; 1985b; Bertsch,

1987; Kloprogge et al., 1992b; 1992c) is the mostly used pillaring agent. The cage-

like structure (or Keggin structure) of this complex was proposed for the first time by

Johansson (1960) based on X-ray diffraction work of basic aluminum sulphate. This

structure was later confirmed with other techniques like small-angle X-ray scattering

(Rausch and Bale, 1964) and 27Al NMR (see e.g. review (Akitt, 1989).

Based on differences between the 27Al and 29Si MAS NMR spectra of pillared

beidellite and Mg-smectites, Plee et al. (1985) suggested that calcining of pillared

beidellite results in linking the Al13 species to the tetrahedral layer. This process

involves the inversion of certain tetrahedra and the formation of terminal OH groups

(Si-OH-Al, Chourabi and Fripiat, 1984). Pillared beidellite exhibited basal spacings

around 17.5-18.0 Å after calcination at 350°C while the BET surface area increased to

values around 250 to 300 m2/g (Kloprogge et al., 1994). 27Al solid-state NMR was not

capable of distinguishing separate resonances from the tetrahedrally and octahedrally

coordinated Al in the beidellite structure and the Al13 complex, except for a change in

the relative intensities of both resonances. Upon calcination above 350°C however

two new resonances were observed at 54.9 ppm and 29.6 ppm. A similar resonance at

54 ppm has been reported for tetrahedral Al in zeolites (Fyfe et al., 1983; Kentgens et

al., 1983). In pillared beidellite this resonance might be due to a new tetrahedral Al

environment associated with the formation of a covalent bond between the Al-pillar

and the clay layer.

The structural changes of synthetic beidellite and Al13-pillared analogues

during dehydroxylation were studied in situ using infrared emission spectroscopy.

The OH-stretching region is characterised by 2 OH-stretching modes at 3605 cm-1 and

3650 cm-1. The Al13-pillared beidellite shows stronger OH-stretching bands in

comparison to beidellite alone associated with the water and hydroxyl group initially

present in the Al13 structure, while the silanol band, originally visible in the beidellite

spectra, is absent. The OH-stretching mode of the Al13 is visible around 3450 cm-1 and

a very broad band is present around 3200 cm-1 assigned to water molecules in the Al13

pillar. This band quickly disappears upon heating to 300°C while the OH-stretching

band at 3450 cm-1 diminishes in intensity.

Significant differences between the beidellite and its pillared analogue are the

splitting of the single band at 814 cm-1 into two bands at 812 and 830 cm-1 and the

bands associated with Al-OH modes show increased intensities for the pillared

beidellite (Fig. 10). Secondly the bands associated with Al-OH modes show increased

intensities for the pillared beidellite. This second observation agrees well with earlier

work by Kloprogge et al. (1994), who observed an increase in intensity of the Al-OH

bands at 921 and 849 cm-1 for Al13 pillared montmorillonite Swy-1. Increasing the

temperature results in a decrease in intensity of the bands at 945 and 889 cm-1 and

they disappear at approximately 600° and 700°C respectively. At 650-700°C two new

bands become visible at 642 and 730 cm-1 in both the beidellite and its pillared

analogue. No other bands indicating the formation of new bonds between the

aluminum oxide pillar and the clay structure.

These results show that upon calcination the pillars dehydroxylate, thereby

releasing protons which are able to diffuse through the tetrahedral sheet into the

octahedral sheet where they interact with the structural OH-groups and form water. In

addition the formation of Si-OH- groups is observed in the infrared emission spectra

of the Al13-pillared beidellite indicating that these protons can also interact with Si-O-

Al bonds upon calcination and form new Si-OH-Al bonds. A reaction between the

Al13 pillar and the protonated Si-OH-Al linkage will yield Altetrahedral-O-Alpillar

linkages (Kloprogge and Frost, 1999; Kloprogge et al., 1999a).

Conclusions

This review clearly demonstrates that beidellite can be synthesised in the laboratory.

Since its discovery both natural and synthetic beidellite have been thoroughly

characterised by a large variety of techniques, since beidellite has a for smectites

rather simple chemical composition and for heterogeneous catalysis interesting

acidic/catalytic properties. However, synthesis of beidellite at low temperatures and

pressures has been proven to be elusive. From the perspective of heterogenous

catalysis pillared beidellite would be preferred over other pillared clays. Therefore,

from an applied/industrial perspective the development of a low temperature/pressure

synthesis method is still of interest.

Acknowledgements

This work had not been possible without the help of many colleagues and students.

The author especially wishes to thank Olaf Schuiling, Ben Jansen, John Geus, Ad van

der Eerden, Arno Kentgens, Sridhar Komarneni, Kazumichi Yanagisawa, Jim

Amonette, Barry Wood, Ray Frost, Loc Duong, Liesel Hickey, Ernst Booij, Roland

Vogels and Robin Fry.

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Table 1. Mid-infrared spectral analysis (in cm-1) of the OH band centres of synthetic beidellite with increasing layer charge (assignment after (Farmer,

1974)).

Sample x Al-OH

lib Al-OH

lib Al-OH

lib Al-OH stretch

Al-OH stretch

B0.6 0.6 beidellite 882 913 934 3668 B0.7-2 0.7 beidellite 882 912 933 3662 B0.7-1 0.7 beidellite 882 915 936 3663 B1.5 1.5 beidellite 880 919 966 3638 B2.0 2.0 beidellite 878 913 965 3634

Table 2. Near-infrared band centres (in cm-1) of synthetic beidellite with increasing layer charge

Sample x

B0.6 0.6 4536 4597 - 5140 5237 5260 6963 7106 7160 B0.7-2 0.7 4553 4596 - 5139 5233 5258 6939 7073 7140 B0.7-1 0.7 4543 4596 - 5144 5241 5259 6999 7098 7157 B1.5 1.5 - 4571 - 5075 5172 5233 6938 7099 - B2.0 2.0 - 4570 - 5158 - 5221 6866 7094 -

Table 3a Element Binding energy (eV) FWHM (eV)

O 1s 530.0 2.903 Si 2p 101.0 2.789 Al 2p 72.5 2.725 Fe 2p 711.0 5.037 Mg 2s 87.0 2.906 Ca 2p 349.5 4.421 K 2p 291.0 4.422 Na 1s 1070.0 2.757

Table 3b

Element Atom % composition O 71.5 O: 20 OH: 4 H2O: 8

Si 15.2 6.78 Al 10.0 Tet: 1.22 Oct: 3.24

Fe3+ 1.4 0.62 Mg 1.0 Oct: 0.24

Int: 0.21 Ca 0.7 0.31 K 0.2 0.09 Na 0.05 0.02

Fig. 1

Fig. 2a

Fig. 2b

Fig. 3a

Fig. 3b

400 450 500 550 600 650 700 750 800 850

Wavenumber (cm-1)

Abs

orba

nce 429 cm-1

472 cm-1

531 cm-1

554 cm- 1

586 cm-1

630 cm-1

709 cm-1785 cm -1

813 cm -1

Fig. 4a

850 950 1050 1150 1250 1350

Wavenumber (cm-1)

Abs

orba

nce

882 cm-1

915 cm-1

936 cm-1

1026 cm -1

1058 cm-1

1117 cm-1

1219 cm-1

Fig. 4b

3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000

Wavenumber (cm-1)

Abs

orba

nce

3240 cm -1

3450 cm-1

3668 cm -1

3620 cm-1

Fig. 4c

4400 4800 5200 5600 6000 6400 6800 7200

Wavenumber/cm-1

Rel

. Abs

orba

nce

B2.0

B1.5

B0.7-2

B0.6

B0.7-1

Fig. 5

0

0.1

0.2

0.3

0.4

0.5

0.6

4400 4800 5200 5600 6000 6400 6800 7200

Wavenumber/cm-1

Abs

orba

nce

4536 cm-1

4597 cm-1

5140 cm-1 5260 cm-1

5237 cm-1

6963 cm-1

7106 cm-1 7160 cm-1

Fig. 6

Fig. 7

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

25 35 45 55 65 75 85 95 105

Temperature (oC)

Che

mic

al s

hift

(ppm

)

20

22

24

26

28

30

32

34

36

FWH

M (

Hz)

Fig. 8

450 950 1450 1950 2450 2950 3450 3950

Wavenumber/cm-1

Rel

ativ

e E

mis

sivi

ty

200°C

750°C

Fig. 9a

3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000

Wavenumber/cm-1

Rel

ativ

e E

mis

sivi

ty

Al-OH stretching modes

interlayer H2O

Synthetic beidellite at 200oC

Fig. 9b

3000 3100 3200 3300 3400 3500 3600 3700 3800 3900 4000

Wavenumber/cm-1

Rel

ativ

e E

mis

sivi

ty

strongly bonded interlayer H2O

Al-OH stretching modes

Si-OH mode

Synthetic beidellite at 450oC

Fig. 9c

450 550 650 750 850 950 1050 1150

Wavenumber/cm-1

Rel

ativ

e E

mis

sivi

ty

750°C

200°C945 cm-1

889 cm-1

812cm-1

627 cm-1

534 cm-1478 cm-1

830 cm-1

642 cm-1 730 cm-1

Fig. 10