Clay Minerals (1968) 7, 399.
REACTIONS
CATALYSED
BY M I N E R A L S
W, THE R E A C T I O N OF L E U C O DYES A N D U N S A T U R A T E D
ORGANIC
COMPOUNDS WITH CLAY MINERALS
D. H. S O L O M O N , B. C. L O F T AND J E A N D. S W I F T
Division of Applied Mineralogy, C.S.I.R.O., Melbourne, Australia
(Received 6 May 1968)
ABSTRACT: The mechanism of colour reactions of clay minerals with amines,
diphenylpicrylhydrazyl, tetracyanoethylene, and leuco bases are elucidated, and
the results used as a guide to the reaction mechanism of polymerization reactions
catalysed by clay surfaces.
INTRODUCTION
In the preceding paper (Solomon, Loft & Swift, 1968), a model was developed to
explain the manner in which clay minerals oxidize benzidine. This paper is concerned with the examination of a number of other reactions in which the minerals
produce coloured products from organic compounds or bring about polymer
formation. The slzecific aims of the paper are to develop further a theory of the
manner in which organic molecules interact with the mineral and to seek a
mechanistic relationship between colour producing reactions and polymerizations.
RESULTS
AND DISCUSSION
Examination of a large number of colour producing reactions has shown that whilst
they follow the general pattern reported in the preceding paper for the benzidine
oxidation, in some cases significant deviations from the benzidine model occur.
These deviations can be related to the ease with which the molecule can approach
the active sites on the mineral surface and to the ability of the organic molecule
to undergo reaction at the planar surface or crystal edge.
The reactions studied will be discussed in terms of the manner in which they
differ from the reaction of benzidine.
The structural formulae of leuco crystal violet, crystal violet and DPPH are
given in the Appendix.
400
D. H. Solomon, B. C. Loft and Jean D. Swift
A. The influence of the mineral structure on the product of reaction
Reaction of aromatic amines with clay minerals occurs at transition metal sites
in the silicate layer and at aluminium atoms exposed at crystal edges, and consequently, these reactions are analogous to those of benzidine. The colours developed
with the amines were p-anisidine and p-phenetidine-violet; o-phenetidine-red,
o-nitroaniline-orange, p-nitroaniline-yellow, p-phenylenediamine-dark blue and
o-tolidine-bright green.
Identification of the products of reaction from many of the organic amine/
mineral complexes is difficult because of the ease with which the initial oxidation
product is further oxidized by traces of oxygen. However our results on the reaction
product from dimethylaniline/mineral complexes (Table 1) together with those
of Kruger & Oberlies (1941) show that the product formed is dependent on the
mineral. Thus kaolinite converts dimethylaniline into crystal violet whereas montmoritlonite gives a different product, possibly a tetramethyl-benzidine derivative.
TABLE1. Reaction of dimethylaniline (0-04Min benzene) with minerals
Mineral
Calcium oxidized montmorillonite*
Calcium reduced montmorillonite
Calcium polyphosphate-treated oxidized montmorillonite
Clacium polyphosphate-treated reduced montmorillonite
Oxidized kaolinitet
Oxidized attapulgate
Talc
Polyphosphate-treated oxidizedkaolinite
Polyphosphate-treated oxidizedattapulgate
Colour after 10 min
Bright green
Very slight green
Bright green
None
Yellow green
Green
None
None
Light green
*After seven days the colour was violet. The product had absorption bands at 361 and 578tz in
ethanol/1 ~ HCI.
rAfter two days the colour was violet. The product had an absorption band at 586/z in ethanol/l
HC1. Crystal violet had an absorption band at 586t~in this solvent.
With kaolinite, crystal edges are the main reaction sites and a sequence in
which one of the N-methyl groups is split off from the dimethylaniline, and then
condenses with other dimethylaniline molecules to form crystal violet appears likely
to occur. This mechanism is supported by the results of Sato (1965) who has shown
that conventional Lewis acids, such as aluminium chloride, convert dimethylaniline
to crystal violet in the presence of oxygen.
Thus the reaction of dimethylaniline with minerals serves to illustrate that
reaction of an organic compound with a number of clay minerals does not necessarily give the same product even though similar colours may form. Different
products are most likely to arise from minerals in which the active sites are at
crystal edges and those (e.g. montmorillonite) in which planar surface sites are
involved.
Reactions catalysed by minerals. V
401
B. Reactions influenced by the exchangeable cations
The stable free-radical diphenyl-picryl-hydrazyl (DPPH), forms intensely
coloured solutions in organic solvents. These solutions react readily with aluminosilicates with the loss of the characteristic colour. The loss of colour by reaction
with the mineral offers a m a j o r advantage over the colour-producing reactions in
assigning relative activities to a series of minerals; it is m u c h simpler experimentally
to measure a loss of the colour from solution than to measure the generation of
colour on, or between, solid surfaces. In addition, this reaction is readily studied
by electron paramagnetic resonance spectroscopy.
The reaction of D P P H with clay minerals illustrates clearly two aspects of
major importance not found with benzidine. These are the catalytic influence of
certain exchange cations on the reaction, and the inhibition of the reaction by
solvents which are strong electron donors (see Section C below).
The results recorded for the reaction of benzene solutions of D P P H with calcium
montmorillonites (Table 2) and with other alumino-silicates, show conclusively
that reaction occurs at the Lewis acid sites at the crystal edges. Thus magnesium
silicates, w h i c h do not have Lewis acidity, are not active whereas the aluminosilicates exhibit activity which is proportional to the strength and n u m b e r of the
Lewis acid sites (Table 5). However, in marked contrast to benzidine, the reaction
of D P P H with minerals proceeds much more readily when cobalt is present in the
TABLE 2. Reaction of ~, ~-diphenyl-fl picryl hydrazyl (0"005M in benzene) with minerals
Mineral
Kaolinite
Attapulgite
Calcium oxidized montmorillonite
Calcium reduced montmorillonite
Talc
Vermiculite
Polyphosphate-treated kaolinite
Polyphosphate-treated attapulgite
Calcium polyphosphate-treated oxidized montmorillonite
Calcium polyphosphate-treated reduced montmorillonite
Cobalt kaolinite
Cobalt oxidized montmorillonite
Cobalt reduced montmorillonite
Cobalt polyphosphate-treated oxidized montmorillonite
Cobalt polyphosphate-treated reduced montmorillonite
Cobalt vermiculite
Colour rating* of solution
after 5 mint
2
0
5
5
10
10
I0
10
10
10
1
2
5
8
10
4
*A scale of 0-10 was used to describe the intensity of the DPPH colour in the solution where 0
was assigned to no purple colour and 10 to the original solution.
tThe ESR radical signal of DPPH decreased during the reaction; when the colour of the DPPH was
discharged, no radical signal was detected either in the solution or on the clay.
402
D. H. Solomon, B. C. Loft and Jean D. Swift
exchange sites provided that the clay is in the oxidized form (Table 2). A striking
example of the enhanced activity of the cobalt clays is found with vermiculite
Whereas sodium vermiculite does not react with DPPH, cobalt vermiculite
decolourizes solutions of D P P H rapidly. Further evidence comes from the montmorillonite clays; the activity of calcium montmorillonite is destroyed by polyphosphate treatment (i.e. edges are the only active sites) whereas cobalt montmorillonite shows only a reduction in activity (i.e. planar surface sites still available).
These results suggest that D P P H molecule cannot approach sufficiently close to
the silicate layer to undergo electron transfer and that the cobalt in the exchange
site acts as an electron-bridge for the transfer reaction. Other large a n d / o r very
hydrophobic molecules have given similar results with cobalt clays (see e.g. the
reaction of styrene and leuco dyes). Molecules such as benzidine can approach the
layer and undergo transfer reactions and hence the exchange cation has little
influence on these reactions.
The influence of transition metal cations in the exchange sites on the reaction
of leuco dyes with minerals is intermediate between that observed with benzidine
and DPPH. With the leuco dyes, transition metal cations increase the rate of
oxidation but they have no significant influence on the final colour (leveloped. This
suggests that the large leuco dye molecule has some difficulty in approaching the
oxidation sites, but that given time access to the oxidant is possible.
C. Reactions influenced by solvents
In the preceding paper it was shown that solvent influences the pH of the
mineral surface, and hence the colour formed, when benzidine is reacted with a
mineral. However, in the benzidine reaction alcohol and benzene gave similar
results provided water was carefully excluded from the system. In reactions reported
here the use of alcohol in place of benzene resulted in inhibition of the reaction.
Thus D P P H does not react with dried sodium or calcium montmorillonite or with
other alumino-silicates when alcohol is used as the solvent; with benzene as solvent
reaction is rapid. These observations are readily explained by the solvent competing
with the D P P H for the Lewis acid sites. Thus alcohol being a stronger Lewis base
than D P P H is preferentially adsorbed at the crystal edges and in a sense is comparable in its effect to the polyphosphate. Proof of the ability of alcohol to compete
with DPPH for the Lewis acid sites is given by experiments in which alcohol was
added to c l a y / D P P H complexes formed from benzene; the alcohol displaced the
DPPH.
The desorption of D P P H by alcohol is also useful to demonstrate differences
in the ability of minerals to physically adsorb and to chemically react with organic
compounds. Thus, although both kaolinite and attapulgite decolourize benzene
solutions of DPPH, alcohol desorbs unchanged D P P H from attapulgite, which
decolourizes mainly by physical adsorption, but not appreciably from kaolinite
with which D P P H forms a yellow product by a rapid chemical reaction.
The solvent is also important in the reactions of leuco dyes with minerals. The
general characteristics of these reactions are similar to those noted for benzidine
Reactions catalysed by minerals. V
403
in benzene. Thus transition metals in the silicate layers need to be in the oxidized
form to convert the leuco base to the dyestuff and the crystal edge of the mineral is
a further site for reaction. The relative importance of the planar surface and edge
sites depends on the mineral; with montmorillonite the edge contribution is small
whereas with kaolinite and attapulgite the edge accounts for a significant amount
of oxidation.
Solvents which are strong electron donors can compete successfully with the
leuco base for the mineral surface and if present in sufficient quantity, the reaction
is inhibited (Table 3). Small amounts of polar solvents, or the water present on
TABLE3. The influence of solvent on the reaction of leuco-malachite green (0"001u) with oxidized
montmorillonite
Solvent for reaction
Benzene
Benzene
Ethanol
Benzene/ethanol 80 ~/20 ~
Benzene saturated with
water*
Colour after 10 min
Dark green
Dark green
None
None
Solventa d d e d
Ethanol 10
Water 1
--
Very pale blue
Colour after 5 rain
Blue-green
Light blue-green
--
*Undried clay was used for this experiment.
the undried clay, slow down the reaction by restricting the ease with which the
leuco base can approach the surface. The dried clay gives the colours expected
in strongly acidic media and the addition of water shifts the colour towards the
blue (Table 3). This is as expected for an increase in the pH as a result of adding
water. On the other hand, where water is present before the dye is added competition
takes place between the water and leuco dye molecules and as a consequence, the
amount of dye adsorbed and oxidized is less than with the dried clay.
The equation for the oxidation of leuco bases t9 dyes requires two moles of
oxidant per mole of dye and possibly proceeds as below. This mechanism is
similar to that proposed by Swain & Hedberg (1950) for the ceric ion oxidation of
leuco dyes. Thus where the Lewis basicity of the solvent is greater than that of the
organic reactant, the solvent can compete successfully for the edge site and minimize
or prevent reaction. On the other hand with benzidine which is a stronger Lewis
base than most solvents, adsorption and oxidation take place.
D. Reaction in which the mineral is an electrdn donor
In the reactions discussed so far the organic molecule acts as an electron donor
and the clay as electron acceptor. Tetracyanoethylene (TCNE) is an example of
a compound which is very strong electron-acceptor, and this reacts with montmorillonites which contain transition metals in the lower valency state to give
404
D. H. Solomon, B. C. LoJ? and Jean D. Swift
CH3~NjCH3
2 [- Fe+++
n
cH3
Lin mineralI
H--O\H
CH3~N/CH3
+
)N-C,
CH3
~
,'~-c ==~
~
Crystal Violet
From Swain & Hedberg (1950)
~=N
\CH3
I Fe++ 1
+ 2 in mineral
red coloured products (Table 4). Electron spin resonance spectra suggests that the
TCNE accepts an electron from the clay and forms a radical-anion. In this reaction
the clay acts as an electron donor. The fact that oxidized montmorillonites do not
react, and that polyphosphate treatment does not inhibit the reaction indicates
that transition metal atoms in the planar surfaces are the sites of reaction.
The ability of a clay mineral to act as either an oxidant or a reductant depending
on the oxidation level of the transition metals and on the organic molecule, is
particularly important in reactions of the 'natural' clays many of which contain
transition metals in both valency states. This has led previously to misinterpretation
of results and in our experience difficulty was experienced in obtaining reproducible
results. These problems are overcome once it is realized that the transition metal
sites responsible for the colour reactions can be converted to the Oxidized or
reduced form by conventional methods.
E. The reaction o] polymerizable compounds with clay minerals
In recent years, a number of reports have described the catalytic activity of
clay minerals (Solomon & Rosser, 1965; Solomon & Loft, 1968; Blumstein,
1961, 1965; Friedlander & Fink, 1964; Glavati et al., 1963) in promoting the polymerization of unsaturated organic molecules. Considerable interest has been shown
in the mechanisms by which these reactions proceed since the systems have
Reactions catalysed by minerals. V
405
TABLE4. Reaction of tetracyanoethylene (0"016M in benzene) with minerals
Mineral
Calcium oxidized montmorillonite
Calcium reduced montmorillonite
Calcium polyphosphate-treated oxidized montmoriUonite
Calcium polyphosphate-treated reduced montmoriUonite
Reduced kaolinite
Reduced attapulgite
Colour after 10 min.
None
Brick red*
None
Brick red*
Very pale pink
Light red
*ESR spectra of the samples showed a signal at g= 2 which has been attributed to the
radical-anion (Flockhart et al., 1965).
theoretical and industrial significance. Two examples will be discussed here to
indicate the use of the colour producing reactions in assigning mechanisms to the
polymerization.
Hydroxyethyl methacrylate undergoes spontaneous polymerization on some
montmorillonites (Solomon & Loft, 1968) and this reaction has the characteristics
found in the reaction of T C N E with montmorillonite where the mineral transfers
an electron to the organic compounds. Hence it has been suggested that the
hydroxyethyl metacrylate polymerization is initiated by one electron transfer
from the mineral to the organic monomer.
The mechanism of polymerization of styrene by alumino-silicates (Solomon &
Rosser, 1965) is also clarified by the results of colour producing reactions and this
reaction serves also to show the difference in mineral activity in chain reactions
and in molecular reactions. Thus by analogy with the benzidine oxidation, and
the reaction of 1, 1,-diphenylethylene with alumino-silicates, it has been proposed
that the styrene polymerization is initiated by styrene radical-cations formed by
reaction at the Lewis acid edge sites. However, the order of activity of the minerals
in promoting the polymerization (Table 5) is not the same as found for the oxidation
of benzidine even though both the styrene and benzidine give radical-cations. One
difference is that styrene is very hydrophobic and behaves in a similar manner to
D P P H (Tables 2 and 5) and only undergoes transfer reactions with layer sites
when cobalt is present in the exchange site. Another difference is that styrene is
much more difficult to oxidize than benzidine and gives an unstable radical-cation.
However, once the radical-cation is formed, further polymerization by a chain
mechanism is rapid and each styrene radical-cation can result in the conversion
of many molecules into polymer; with benzidine each molecule oxidized requires
a separate site on the mineral. This difference in activity is a function of the number
and strength of the sites available. The relatively few but powerful electron accepting sites of kaolinite make it a very effective catalyst for the polymerization of
c
D. H. Solomon, B. C. Loft and Jean D. Swift
406
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Reactions catalysed by minerals. V
407
styrene but give only pale colours with benzidine. Attapulgite has a large number
of acid sites of various strength; only the strong sites would be expected to oxidize
styrene, whereas all acid sites and also the transition metal sites in the planar
surfaces can convert benzidine to benzidine-blue. Hence, attapulgite is a n effective
catalyst for styrene polymerization and for the oxidizing of benzidine.
SUMMARY
Mechanistically the colour reactions are related to a number of polymerizations
and can be used as a guide to the mechanism by which polymers form on mineral
surfaces.
The reactions discussed in this and the preceding paper are consistent with the
clay minerals possessing electron donor and electron acceptor properties. The
electron donating centres are transition metal atoms in the lower valency state
whereas the electron accepting sites are transition metal atoms in the higher valency
state and aluminium atoms exposed at crystal edges.
The cations in the exchange positions have an important bearing on the reaction
of organic compounds with minerals. In reactions carried out in aqueous suspension,
the influence of the cation is confined to the swelling clays where it affects the
rate of penetration and reaction of the organic compound. In non-polar media,
such as benzene, and with reactions involving large organic molecules, the presence
of a transition metal cation in the exchange site is essential for effective electron
transfer to the planar sites in the silicate lattice.
Solvents influence markedly the 'acidity' of the mineral surface and they also
compete with the organic reactant for the active surface sites, particularly the
Lewis acid edges.
The products of reaction between an organic compound and a mineral can vary
with the mineral used. The different products can be explained in terms of the
action of the oxidation-reduction sites associated with the planar surfaces and of
Lewis acid centres at crystal edges.
EXPERIMENTAL
Materials
All solvents and reagents were carefully purified. The clay minerals were purified
and treated as in the previous paper (Solomon, Loft & Swift, 1968). The montmorillonite used was obtained from Volclay (Wyoming bentonite).
The reactions were carried out in a similar manner to those described in the
previous paper for the reaction of benzidine in benzene with the minerals. The
results are shown in Tables 1-5.
408
D. H. Solomon, B. C. Loft and Jean D. Swift
APPENDIX
Structural formulae of compounds referred to in the text.
~N(CH3)2
(CH3)2N ~ C ~ N ( C H 3 ) 2
Leuco-crystal violet
~-N(CH3)
2
Crystal violet as a singly charged ion in
weakly acidic solution.
Q - - N ~ N O
2
U
~, e-diphenyl-fl-picryl hydrazyl (DPPH).
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BLUMSTEINA. (1965) J. polym. Sci. A, 3, 2653
FRIEDLANDERH.Z, t~ FINK C.R. (1964) ], polym. Sci. B, 2, 475.
FLOCKHARTB.D., NACCACrmC., ScoTt J.A.N, & PiNK R.C. (1965) Chem. Commun. 238.
GLAVATIO,L., POLAKL.S. & SHCHEKINV.]I. (1963) Neftekhimiya, 3, 905.
KR0~ER D. & OBERLIESF. (1941) Chem. Ber. TaB, 1711.
SATOn. (1965) Bull. chem. Soc. Japan, 38, 1719.
SOLOMOND.H. & LOFT B.C. (19'68) J. appl. polym. Sci. 12, 1253.
SOLOMOND.H., LOFT B.C. & SWIFT J.D. (1968) Clay Miner. 7, 389.
SOLOMOND.H. & ROSSER M.J. (1965) J. appl. polym. Sci. 9, 1261.
SWAIN C.G. & HEDBERGK. (1950) d. Am. chem. Soc. 72, 3373.