Catalytic properties of a heated ammonium-saturated dioctahedral smectite

Article abstract of DOI:10.1135/cccc20001527

A series of acid catalysts was prepared by heating of NH₄-saturated montmorillonite at 200-600 °C for 24 h. Their catalytic activity was tested in acetylation of 3,4,5-trimethoxybenzaldehyde with acetic anhydride. This reaction is sufficiently

Full text of DOI:10.1135/cccc20001527

Dioctahedral Smectite
1527
CATALYTIC PROPERTIES OF A HEATED AMMONIUM-SATURATED
DIOCTAHEDRAL SMECTITE
1
2,
ꢀuboš JANKOVIČ and Peter KOMADEL *
Institute of Inorganic Chemistry, Slovak Academy of Sciences, 842 36 Bratislava, Slovak Republic;
1
2
e-mail: uachljan@savba.sk, uachkomp@savba.sk
Received May 4, 2000
Accepted August 16, 2000
Dedicated to Professor +udovít Treindl on the occasion of his 70th birthday in the recognition of his
outstanding contributions to physical chemistry.
A series of acid catalysts was prepared by h eatin g of NH4-saturated m on tm orillon ite at
200–600 °C for 24 h . Th eir catalytic activity was tested in acetylation of 3,4,5-trim eth oxy-
ben zaldeh yde with acetic an h ydride. Th is reaction is sufficien tly sen sitive to m odification of
th e catalyst an d th us suitable for testin g catalytic activity of m odified m on tm orillon ites.
Most of th e prepared catalysts were able to catalyse th e test reaction an d produce diacetate
in h igh er th an 50% yields. Th e m ost active catalyst was obtain ed after h eatin g at 300 °C. It
was sligh tly less effective th an com m ercially available acid-activated K10 catalyst.
Key w ord s: Protection ; Aldeh ydes; Acetylation ; Th erm al m odification ; NH4-m on tm orillon ite;
Sm ectites; Alum in osilicates; Heterogen eous catalysis.
Most catalysts used by organ ic ch em ists are based on th e n aturally occur-
rin g sm ectites, th e m ain m in erals of ben ton ite ores. Sm ectites are swellin g
clay m in erals; th e best kn own of th em is m on tm orillon ite. Th is m in eral h as
an alum in osilicate structure, wh ich can be com pared with a m ultilayer h am
san dwich . Th e “bread” layers are th e alum in osilicate layers wh ere two ex-
tern al tetrah edral (m ain ly silica) sh eets surroun d th e in tern al octah edral
(m ain ly alum in a) sh eet in a tetrah edral-octah edral-tetrah edral (TOT) struc-
ture. Th e “h am ” is in terlayers with cation s an d water. Mon tm orillon ites are
m in erals in wh ich isom orph ous substitution of som e of th e octah edral alu-
m in ium (III) atom s with iron (III) an d/or m agn esium (II) atom s h as taken
place. Partial substitution of tetrah edral Si(IV) with Al(III) is also com m on .
Th e Mg(II) an d Al(III) in th e octah edral an d tetrah edral sh eets, respectively,
cause th e n egative ch arge of th e layers. In th e n aturally occurrin g m on t-
m orillon ites, th is ch arge is balan ced by h ydrated in terlam ellar cation s, such
as Na+ or Ca2+. Th ese cation s are exch an geable an d can be substituted with
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1528
Jankovič, Komadel:
proton s or various in organ ic or organ ic cation s. Som e cation s also occupy
broken edge sites. An im portan t property of sm ectites is swellin g in th e
presen ce of eith er water or a h ost of organ ic m olecules, wh en th e
in terlam ellar distan ce between th e layers in creases to accom m odate th e
guest m olecules1,2
.
Th e first in dustrial application of clays as catalysts date back to th e begin -
n in g of th e 20th cen tury, wh ile acid-treated sm ectites h ave been kn own for
m ore th an 50 years as catalysts for oil crackin g3. Sin ce th e 1960’s th ey h ave
been partly replaced with zeolites, m ain ly due to a better zeolite activity
an d selectivity4,5. However, solid acid catalysts, in particular acid-treated
m on tm orillon ites, can be used to catalyse a wide ran ge of im portan t in dus-
trial processes, such as isom erisation , liquid refin in g an d Friedel–Crafts
alkylation , playin g an in creasin g role in th e m ove towards establish in g en -
viron m en tally frien dly tech n ologies6.
Clay catalysts h ave been sh own to con tain both Brøn sted an d Lewis acid
sites, with th e Brøn sted sites m ain ly associated with th e in terlam ellar re-
gion an d th e Lewis sites m ain ly associated with edge sites. Th e acidity of
sm ectites is in fluen ced by th e quan tity of water between th e layers1. If th e
catalyst is h eated to about 100 °C to rem ove m ost of th e in terlam ellar wa-
ter, th e Brøn sted acidity in creases m arkedly to th at of stron g acids. Heatin g
to h igh er tem peratures (200–300 °C) causes collapse of th e in terlayer struc-
ture as th e water is driven out, gen eratin g a decrease in Brøn sted but an in -
crease in Lewis acidity. Furth er h eatin g (>450 °C) results even tually in
com plete deh ydroxylation of th e alum in osilicate lattice7.
A differen t kin d of acidity is due to proton s as exch an geable cation s. In
con trast to oth er cation s, it is n ot easy to prepare th e H-form of a sm ectite
via ion exch an ge with an acid or a resin in th e H-form 8. H-sm ectites are un -
stable m aterials, proton s attack th e clay m in eral structure an d a part of th e
structural cation s gets tran sferred in to th e ion -exch an ge sites. Th is effect
occurs even un der relatively m ild con dition s. For exam ple, van Olph en
foun d th at th e con version of Wyom in g m on tm orillon ite with 0.01 M HCl
at room tem perature led to th e 60% occupan cy of th e exch an ge sites by Al
(ref.9). A way to overcom e th is acid leach in g problem is to prepare th e NH4-
form via ion exch an ge, followed by h eat treatm en t at 550 °C (ref.10). Th e
NH4+ ion decom poses, NH3 evaporates an d H-saturated sm ectite is prepared.
Th e effect of h eatin g of H+-exch an ged clay m in erals depen ds on th e type
of th e layer silicate an d on th e preparation m eth od of th e H-form . In th e
case of m on tm orillon ites, proton s can m igrate in to vacan cies in th e octah e-
dral sh eet, wh ere th ey associate with lattice oxygen s10,11. In gen eral, th ese
proton s are n ot accessible to catalysis, because th e free diam eter of th e h ex-
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Dioctahedral Smectite
1529
agon al h ole in th e tetrah edral sh eet is on ly 0.26 n m (ref.12), th us proton -
ation of reagen ts bein g very lim ited. After calcin ation , th e acid sites left will
be situated at edges, but th eir n um ber will be substan tially decreased.
High er stability was observed for m aterials prepared via calcin ation of
NH4-form s th an for acid-treated or H-resin -exch an ged catalysts8,10
.
A solid surface m ay in teract with organ ic m olecules also as a fun ction of
its h ydroph obicity. A weak in teraction precludes an y catalytic step, wh ile
too stron g bon din g of organ ic products to th e m in eral surface m akes
desorption too slow or even im possible. Th e h ydroph ilic/h ydroph obic
ch aracter of th e surface m ay be m an ipulated by an appropriate ch oice of
th e exch an geable cation s13.
Th e acid-treatm en t con dition s correspon din g to m axim um catalytic ac-
tivity depen d on th e precise reaction bein g catalysed14,15. Reaction s be-
tween polar m olecules require clays acid-treated un der m ild con dition s or
for sh ort tim es, capitalisin g on th e large n um ber of acid sites available on
th e in tern al surface. In con trast, th e surface accessible to n on -polar m ole-
cules con sists on ly of th e extern al face an d edge areas of th e clay platelets.
Th ese reaction s require m ore severely acid-treated clays an d th e catalytic
activity depen ds on fin e tun in g of th e ch an ges in th e surface area an d de-
crease in th e cation exch an ge capacity. However, in a swellin g m edium , op-
tim um catalytic activity m ay be ach ieved n ot on ly th rough acid treatm en t,
but sim ply by ion exch an ge with acid cation s such as Al3+ ion s; on th e con -
trary, th is last treatm en t h as a n egligible effect in a n on -swellin g m edium 16
.
Th e first step in th e activation procedure, wh ich is required for both n at-
ural an d syn th etic clay m in erals, is to con vert th e clay in to th e desired ion
form . In th e case of m etal ion s, th is can be don e with relative ease by ion
exch an ge usin g a con cen trated solution of a suitable salt. Th e excess salt is
rem oved by repeated wash in gs. As an altern ative, on e can use ion -exch an ge
resin s in suitable ion form s.
Th ere is still a great dem an d for acid catalysts un der m ild con dition s an d
in en viron m en tally frien dly processes. Mon tm orillon ites, a class of in ex-
pen sive an d n on -corrosive solid acids, h ave been used as efficien t catalysts
for a variety of organ ic reaction s17. Th e reaction s catalysed by m on tm o-
rillon ites are usually carried out un der m ild con dition s with h igh yields
an d h igh selectivities an d th e workup of th ese reaction s is very sim ple; on ly
filtration to rem ove th e catalyst an d evaporation of th e solven t are re-
quired. Mon tm orillon ite catalysts are easily recovered an d reused18
.
Som e of frequen tly used organ ic reactan ts con tain in g reactive groups,
such as aldeh ydes, keton es, etc., require protection of th ese groups to avoid
un desirable side reaction s. Protection of aldeh yde with acetic an h ydride19
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1530
Jankovič, Komadel:
in th e presen ce of an acid catalyst is a suitable reaction for testin g catalytic
activity of m odified sm ectites. Th e aim of th is work was to prepare catalysts
of differen t acidity from a NH4-saturated m on tm orillon ite by h eatin g at
various tem peratures an d to test catalytic activity of th ese m aterials for th e
con version of 3,4,5-trim eth oxyben zaldeh yde in acetic an h ydride in to di-
acetate.
EXPERIMENTAL
Mon tm orillon ite separated from ben ton ite from deposit Ivančice (Czech Republic) was used
for th e experim en ts. Ben ton ite was groun d in an agate m ortar an d <2 µm fraction was sepa-
rated by sedim en tation in order to rem ove m ost of th e im purities. In th e course of th is pro-
cess, th e sam ple was repeatedly saturated with a calcium ch loride solution to obtain
a
h om oion ic form of m on tm orillon ite. Th e am m on ium form was prepared from th e Ca-form
via m ultiple overn igh t saturation with 1 M NH4Cl solution . Excess salt was rem oved after-
wards by wash in g with distilled water un til a n egative test for ch lorides with silver n itrate
solution ; th e NH4-saturated m on tm orillon ite was dried at 60 °C an d groun d to pass a
0.2 m m sieve. Th e catalysts were prepared by h eatin g th e NH4-form at 200–600 °C for 24 h
(in steps of 100 °C).
Com m ercially available acid m on tm orillon ite-based catalysts K10 an d KSF were obtain ed
from th e m an ufacturer (Südch em ie Moosburg, Germ an y). Solven ts an d ch em icals were ob-
tain ed from Merck (reagen t grade) an d were used as supplied.
All catalytic tests were perform ed in sm all glass flasks usin g 10 m g sm ectite im m ersed in
10 m l of a 0.1 M solution of 3,4,5-trim eth oxyben zaldeh yde in acetic an h ydride. Th e reaction
m ixture was kept un der stirrin g at 20 °C for up to 24 h . 10 µl portion s of th e liquid ph ase
were rem oved from th e flasks with a syrin ge, diluted to 1 m l with aceton itrile an d an alysed
usin g HPLC ch rom atograph y. Blan k experim en ts (with out catalyst) were perform ed sim ulta-
n eously. Th ree parallel catalytic experim en ts were carried out for each catalyst.
An alytical Meth ods
Th e prepared catalysts were groun d to pass a 0.2 m m sieve an d ch aracterised by FTIR spec-
troscopy, X-ray diffraction an d th erm al an alysis.
Fourier Transform Infrared (FTIR) Spectroscopy
Th e spectra were obtain ed on a FTIR spectrom eter Nicolet Magn a 750 equipped with a DTGS
detector. For each sam ple, 128 scan s were recorded in th e 4 000–400 cm –1 spectral ran ge in
th e tran sm ittan ce m ode with a resolution of 4 cm –1. Th e KBr pressed disk tech n ique (1 m g
of sam ple an d 200 m g of KBr) was used.
X-Ray Diffraction (XRD)
Powder specim en s were an alysed by XRD usin g a Ph ilips PW 1710 diffractom eter (Ni-filtered
CuKα radiation ).
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Dioctahedral Smectite
1531
Thermal Analysis
Sim ultan eous TG an d DTA were con ducted from am bien t tem perature up to 1 000 °C, usin g
a T.A.I. SDT 2960 in strum en t (10–20 m g sam ple m ass, 10 °C m in –1 in flowin g air).
HPLC Chromatography
Sam ples were an alysed usin g a Hewlett Packard HP-1090M HPLC apparatus with a Hypersil
colum n (BDS 5 µm /200 × 2.1 m m ) an d a m obile ph ase con sistin g of a m ixture of water an d
aceton itrile. Detection was perform ed with a diode array detector at 210 n m an d th e prod-
ucts were iden tified by th e reten tion tim es of referen ce substan ces. Th e separation of all
an alysed com poun ds is sh own in a represen tative ch rom atogram (Fig. 1). Differen ces in th e
determ in ed con cen tration s of th e reaction products in th ree parallel run s were always
with in ±2% from th e average used in th e figures. Th e yield was determ in ed as th e percen t-
age of th e reactan t con verted to th e reaction product.
RESULTS AND DISCUSION
Th e catalysts used in th is study in clude h eated NH4+ -m on tm orillon ites an d
com m ercial K10 an d KSF, wh ich are acid-treated m on tm orillon ite-based
catalysts. Figure 2 illustrates ch an ges in in ten sity of υ4 deform ation vibra-
tion s of th e NH4+ ion n ear 1 400 cm –1 in th e IR spectra of NH4-saturated
m on tm orillon ite upon h eatin g. Th e υ4 NH4+ deform ation ban d in th e spec-
tra of sm ectites is th e m ost con ven ien t ban d to use for quan tification pur-
poses, i.e., to quan tify th e am oun t of am m on ium 20 or to m easure th e layer
ch arge21,22. Two differen t ban ds were reported to occur in th e IR spectra of
1
3
2
0
2
4
6
8
10
12
τ, min
FIG. 1
HPLC an alysis of th e reaction m ixture of catalysed con version of 3,4,5-trim eth oxy-
ben zaldeh yde (2) to (3,4,5-trim eth oxyph en yl)m eth ylen e diacetate (3) with acetic an h ydride (1)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1532
Jankovič, Komadel:
NH4-saturated sm ectites in th e 1 300–1 500 cm –1 region , wh en recorded in
KBr pellets. Th e ban d n ear 1 400 cm –1 is related to th e NH4Br form ed by re-
placem en t of NH4+ in th e m in eral with K+ from KBr, wh ile th e ban d n ear
1 440 cm –1 is assign ed to vibration s of NH4+ ion s presen t in th e in terlayers
an d h ydrogen -bon ded to oxygen atom s of th e clay m in eral. Th is ban d also
appears in th e DRIFTS spectra obtain ed with out KBr (ref.22). Figure 2 sh ows
th at th e ban d n ear 1 400 cm –1 is m ore in ten se in th is series an d th at its in -
ten sity decreases with th e in creasin g tem perature of h eatin g. Th is is in ac-
cord with th e expected rem oval of NH3 upon h eatin g. Very low absorption
at 1 400 cm –1 in th e spectra of sam ples h eated at ≥500 °C proves th at m ost
of am m on ium ion s were decom posed durin g th e h eat treatm en t. Two com -
pon en ts n ear 1 400 an d 1 440 cm –1 sh ow th at a part of NH4+ ion s substi-
tuted K+ in KBr but an oth er part got trapped in sm ectite h eated at 300 °C
(ref.23). Th is tem perature is too low for NH4+ decom position but h igh
en ough to cause deh ydration an d th us collapse of th e in terlayer spaces,
wh ich causes stabilisation of NH4-sm ectite to ion exch an ge with KBr. Grad-
ual collapse of th e in terlayers was proved by X-ray diffraction (Fig. 3),
sh owin g th at d001 of th e sam ples h eated at 500 an d 600 °C sh ran k close to
9.6 Å, i.e. to th e th ickn ess of th e m on tm orillon ite layers24.
Com parison of DTA curves of Ca- an d NH4-m on tm orillon ites is sh own in
Fig. 4. Both sam ples lost water upon h eatin g. Th e m ain en doth erm ic peak
on th e DTA curve of th e Ca-form , cen tered at 84 °C, con tain s a com pon en t
1
2
3
4
5
6
1 500
1 450
1 400
1 350
1 300
–1
ν, cm
FIG. 2
IR spectra (1 300–1 500 cm –1 region ) of NH4-saturated m on tm orillon ite un h eated (1) an d
h eated at 200–600 °C for 24 h (2–6)
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Dioctahedral Smectite
1533
n ear 170 °C, wh ich is due to rem oval of water m olecules from th e in n er
h ydration sh ells of th e Ca2+ cation s. As expected, th is com pon en t is absen t
from th e curve of NH4-m on tm orillon ite. On th e oth er h an d, loss of am m o-
n ia upon h eatin g is observed n ear 315 °C.
12.0
11.5
11.0
10.5
10.0
9.5
unheated
200
300
400
500
600
t, °C
FIG. 3
Decreasin g d001 values of NH4-saturated m on tm orillon ite un h eated an d h eated at various
tem peratures for 24 h
1
2
∆T
0
200
400
600
800
1 000
t, °C
FIG. 4
DTA curves of NH4- (1) an d Ca- (2) m on tm orillon ites
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1534
Jankovič, Komadel:
Catalytic activity of th e prepared m aterials was tested in th e reaction of
preparation of (3,4,5-trim eth oxyph en yl)m eth ylen e diacetate from 3,4,5-tri-
m eth oxyben zaldeh yde (Sch em e 1). In th e reaction , th e aldeh yde group is
activated by th e acid cen ters presen t on th e catalyst, followed by a n ucleo-
H3CO
H3CO
H3CO
H3CO
H3CO
H3CO
Ac2O
CH O
CH(OAc)2
heated NH4-form
montmorillonite
SCHEME 1
ph ilic attack of th e acetan h ydride m olecule. Th is reaction is con sidered to
be a suitable altern ative for preparation of 1,1-diacetates from aldeh ydes,
with th e advan tage of selectivity, operation al sim plicity, h igh yields, sh ort
reaction tim es an d m in im al en viron m en tal im pact19. Th e advan tages for
th e tests of catalysts in clude h igh stability of reaction products an d th eir
relatively easy an d sim ple an alysis, as well as sen sitivity to th e presen ce of
weak acid sites in th e catalysts. Th erefore, it is a suitable reaction for testin g
catalytic activity of differen tly m odified sm ectites. Th e results of catalytic
tests are sh own in Figs 5 an d 6.
Both un h eated form s are of rath er low catalytic activity. For th e NH4-
form , h eatin g up to 300 °C results in a substan tially in creased catalytic ac-
90
80
70
60
50
40
30
K10 unheated 200
300
400
500
t, °C
600
FIG. 5
Yield of diacetate over am m on ium -saturated m on tm orillon ite h eated at various tem pera-
tures for aldeh yde protection for 24 h
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

Dioctahedral Smectite
1535
tivity, th ough n ot fully approach in g th e activity of a com m ercial
acid-activated m on tm orillon ite catalysts K10 (Fig. 5) an d KSF (sam e as K10,
n ot sh own in Fig. 5). Th e in creased activity is due to th e form ation of pro-
ton s in th e course of decom position of NH4+ cation s. Th e proton s con trib-
ute substan tially to th e catalytic activity of NH4-saturated catalysts h eated
at ≥300 °C. Th ey can also partly attack th e layers of m on tm orillon ite, caus-
in g partial liberation of structural cation s (Al3+, Mg2+, Fe3+) an d th us un -
availability of proton s for catalysis. Th is is sim ilar to autotran sform ation ,
wh ich is kn own to occur in aqueous dispersion s of H-sm ectites8. However,
th e con sequen ce of autotran sform ation is com plete absen ce of stron g acid
sites25, wh ile th e results sh own in Fig. 5 prove h igh catalytic activity (yields
> 50%) for all catalysts prepared at ≥300 °C, th ough th e yield of th e reac-
tion gradually decreases with in creasin g tem perature of th e catalyst prepa-
ration . It is assum ed th at th is decrease is due to a decreased n um ber of
catalytically active sites presen t after h eatin g at h igh er tem peratures.
Th e kin etics of con version of th e arom atic aldeh yde in reaction s
catalysed with K10 an d NH4-saturated m on tm orillon ite h eated at 300 °C
for 24 h are sh own in Fig. 6. Both catalysts are h igh ly effective. Un der th e
con dition s used, th e reaction catalysed with K10 is clearly faster, reach in g
90% yield with in about 7 h , wh ile th e application of th e h eated NH4-clay
results in about 70% con version after 11 h .
100
1
80
2
60
40
20
0
0
100
200
300
400
500
600
700
800
900
τ, min
FIG. 6
Com parison of con version s of 3,4,5-trim eth oxyben zaldeh yde obtain ed with K10 an d am m o-
n ium -saturated m on tm orillon ite h eated at 300 °C for 24 h
Collect. Czech. Chem. Commun. (Vol. 65) (2000)

1536
Jankovič, Komadel:
CONCLUSIONS
Active acid catalysts can be prepared by thermal decomposition of NH4-saturated
m on tm orillon ite at 200–600 °C for 24 h . Acetylation of 3,4,5-trim eth oxy-
ben zaldeh yde with acetic an h ydride is a suitable reaction for testin g cata-
lytic activity of m odified m on tm orillon ites. Most of th e prepared m aterials
catalysed th e test reaction an d gave product yields > 50%. Th e m ost active
catalyst obtain ed after h eatin g at 300 °C was sligh tly less effective th an
K10, a well-kn own com m ercially available acid-activated m on tm orillon ite.
Financial support of VEGA, the Grant Agency of the Slovak Republic (grant No. 2/7202/00) is
acknowledged. The authors thank Dr M. Ďuriš, from the Hydroanalytical Laboratories of the Water
Research Institute, Bratislava, for HPLC analyses.
REFERENCES
1. Adams J. M.: Appl. Clay Sci. 1987, 2, 309.
2. Laszlo P.: Preparative Chemistry Using Supported Reagents. Academic Press, San Diego 1987.
3. Houndry E., Burt W. F., Pew A. E., Jr., Peters W. A., Jr.: Nat. Pet. News 1938, 48, 570.
4. Decarreau A.: Matériaux Argileux. Societé Francaise de Minéralogie et Cristallographie, Paris 1990.
5. Csicsery S. M.: Zeolites 1984, 4, 202.
6. Varma R. S.: Green Chem. 1999, 43.
7. Vaccari A.: Catal. Today 1998, 41, 53.
8. JanekM., Komadel P.: Geol. Carpathica, Ser. Clays 1993, 43, 59.
9. van Olphen H.: An Introduction to Clay Colloid Chemistry. Wiley, New York1977.
10. Wright A. C., Granquist W. T., Kenedy J. V.: J. Catal. 1972, 25, 65.
11. Davidtz J. C.: J. Catal. 1973, 43, 260.
12. Barrer R. M.: Zeolites and Clay Minerals as Sorbents and Molecular Sieves. Academic Press,
New York1978.
13. Cornelis A., Gerstmans A., Laszlo P., Mathy A., Zieba I.: Catal. Lett. 1990, 6, 103.
14. Rhodes C. N., Brown D. R.: Catal. Lett. 1994, 24, 288.
15. Breen C., Watson R., Madejová J., Komadel P., Klapyta Z.: Langmuir 1997, 13, 6473.
16. Elsen A., Grobet P., Keung M., Leeman H., Schoonheydt R., Toufar H.: Proceedings of
Euroclay ‘95, Leuven 1995.
17. Newman A. C. D.: The Chemistry of Clays and Clay Minerals. Longman, London 1987.
18. Balogh M., Laszlo P.: Organic Chemistry Using Clays. Springer-Verlag, Berlin 1993.
19. Zhang Z. H., Li T. S., Fu Ch. G.: J. Chem. Res., Synop. 1997, 174.
20. Lindgreen H.: Clay Miner. 1994, 29, 527.
21. Petit S., Righi D., Madejová J., Decarreau A.: Clay Miner. 1998, 33, 579.
22. Petit S., Righi D., Madejová J., Decarreau A.: Clay Miner. 1999, 34, 543.
23. Russell J. D., White J. L.: Clays Clay Miner. 1966, 14, 181.
24. White E. W., McKinstry H. A., Bates T. F.: Advances in X-Ray Analysis. Plenum Press, New
York1960.
25. Komadel P., JanekM., Madejová J., Weekes A., Breen C.:
J. Chem. Soc., Faraday Trans.
1997, 93, 4207.
Collect. Czech. Chem. Commun. (Vol. 65) (2000)