
The following summary may interest people who want to experiment with the
conversion of styrenes to nitrostyrenes[13], the oxidation of alcohols to
carbonyl compounds [14, 16] or other reactions supported on K10 clay [15].

Check out ref. 16 for the conversion of phenyl acetic acids to
benzaldehydes or conversion of phenethylamine to phenylacetaldehyde.

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Reactions assisted by clays and other lamellar solids - a survey. [1]

ACIDITY OF CLAYS AND TYPES OF CLAY CATALYSTS

The vast majority of clay catalysts used by organic chemists are based on
the naturally occurring smectite clay, montmorillonite (also known as
bentonite). This clay has an aluminosilicate structure which can be
compared to a multi-decker ham sandwich. The bread layers represent the
extended aluminosilicate sheets where two external tetrahedral silica
groups surround internal octahedral alumina groups in a tetrahedral
octahedral tetrahedral ('TOT') structure. The ham represents an
interlamellar water layer containing dissolved cations.

The montmorillonites are a group of TOT-type clays in which isomorphous
substitution of some of the octahedral aluminium(III) atoms by
magnesium(II) or iron(II) atoms has taken place, with the result that the
sheet retains a residual negative charge. In the naturally occurring form,
this charge is balanced by the introduction into the water layer of
interlamellar cations such as Na+ or Ca2+, some cations also occupying
broken edge sites. Different deposits of montmorillonites can be found in
which there are between 25 and 125 mequiv. of exchangeable cation per 100 g
of clay. Such smectite clays have the added property of swelling in the
presence of either water or a host of organic molecules, when the
interlamellar distance between the sheets increases to accommodate the
guest molecules.

Natural montmorillonite clays have almost no catalytic activity, but it is
relatively easy to convert them into useful catalysts by either (a) acid
treatment or (b) cation exchange with polyvalent ions such as AI3+ or Cr3+.
These cations can polarise their coordinated water molecules to yield
protons in the interlamellar zone (equation 1)

[Al(OH2)6]3+  -->  [Al(OH2)5(OH)]2+   +  H+    (equation 1)

However, even mild acid ion-exchange of the natural clays can cause partial
leaching of aluminium from the octahedral layer, resulting in de-lamination
of the aluminosilicate sheets, providing a less crystalline
aluminium-exchanged clay. Treatment of natural montmorillonites with strong
mineral acids causes considerable de-lamination of the structure, producing
an increase in surface area, particularly at the sheet edges, as well as
adsorption of quantities of acid onto both external and internal surfaces.
The tetrahedrally substituted smectite clay, beidellite, can also be used
to prepare acid catalysts with high acidity by cation exchange.

Clay catalysts have been shown to contain both Bronsted and Lewis acid
sites, with the Bronsted sites mainly associated with the interlamellar
region and the Lewis sites mainly associated with edge sites. The acidity
of the ion-exchanged clays is very much influenced by the quantity of water
between the sheets. If the clay is heated (to around 100 C) so as to remove
most of the interlamellar water until only 'one layer' of water remains, at
about 5% total water level, the Bronsted acidity increases markedly to that
of a very strong acid indeed. Heating to a higher temperature (at around
200-300°C) results in collapse of the clay interlayer structure as the
water is driven out, resulting in a decrease in Bronsted acidity but an
increase in Lewis acidity. Further heating (to around 450°C and above)
results eventually in complete dehydroxylation of the aluminosilicate
lattice, producing a completely amorphous solid that retains Lewis acidity.

The swelling properties of montmorillonite and beidellites can be employed
to manufacture 'pillared clays'. Large polyatomic inorganic ions can be
intercalated into the swelled clay and then modified by heat treatment, so
as to produce inorganic pillars that hold the roof up at a larger than
normal interlamellar distance. Pillared clays can be prepared that have
pore sizes larger than most zeolites, as well as having increased
thermostability over normal layered clays, and much activity has been
concentrated in incorporating transition metal elements into pillared clays
to provide useful inorganic catalysts.

However, organic chemists with synthesis in mind have so far confined their
interest to the swellable montmorillonite clays, and almost all of their
clay catalysts have been either (a) acid-treated clays such as K-l0, or
ion-exchanged clays such as Al3+, Cr3+ or H+ exchanged Wyoming or Texas
bentonites. Further details of clay structures are given in Chapter 1.

The acid-treated and cation exchanged clays can be simply regarded as solid
acids and act as heterogeneous catalysts, with all of the advantages of
easy removal of the catalyst from the products. Acid-treated clays, because
of their increased surface area and swelling properties, have also been
widely used as solid supports for inorganic reagents such as potassium
permanganate, thallium(III) nitrate and both copper(II) and iron(III)
nitrates.

The ion-exchanged clays have mostly Bronsted acidity in the interlamellar
zone and so are characterized by promoting acid-catalysed reactions often
of a bimolecular type between protonated and neighbouring unprotonated
reactants.

PREPARATION OF CLAY CATALYSTS

Organic chemists can purchase ready-made acid-treated montmorillonite
catalysts from a variety of commercial sources. Sud-Chemie AG, of Munich,
West Germany, produce K-10, KSF and Tonsil acid-treated clays, of which
K-10 can be obtained in Europe from Fluka AG and in America from Aldrich
Chemical Co., and this easy availability has meant that organic chemists
have made most use of this material. Additionally, Filtrol clays are
manufactured by the Filtrol Corporation in Los Angeles, USA, and a series
of clays (Fulmont, Fulbent and Fulcat) are produced by Laporte Industries,
Widnes, England.

Ion-exchanged clays, e.g. Al3+ exchanged montmorillonite, are simply
prepared by suspending the clay in water containing low concentrations (ca.
0.5 M) of salts of the cation to be introduced. The clay is then washed by
decantation or centrifugation to get rid of most of the exchanged sodium
cations and then dialysed until the supernatant is free of sodium ion.

Pillared clays have an important role to play as industrial catalysts, but
to date have not been used much by practising organic chemists for
synthesis purposes and hence their preparation will not be detailed here.

Clay-supported reagents have mainly been prepared by depositing inorganic
materials on clay supports. Potassium permanganate/montmorillonite is
prepared by grinding potassium permanganate and montmorillonite together as
dry powders in approximately equal quantities. Reactions are carried out by
refluxing the resultant solid in dichloromethane with the appropriate
substrate.

Thallium(III) nitrate/K-10 (TTN/K-10) is prepared by stirring K-10
montmorillonite with a solution of thallium(III) nitrate in a mixture of
methanol and trimethyl orthoformate, then evaporating to dryness. The
resultant free-flowing solid catalyst is stable in a well-stoppered bottle
for months.

Claycop [copper(II) nitrate/K-10] and Clayfen [iron(III) nitrate/K-10] are
prepared by adding K-10 montmorillonite to the acetone solvate of the
inorganic nitrates and removing the solvent under vacuum.

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Clay-Supported Copper(II) and Iron(III) Nitrates [ref. 2]

Choice of an Acidic Clay as Support

Supported reagents have the following assets of which points 4-6 were the
most immediately important for our purposes:

1. restricted diffusion of the reaction partners
2. microenvironments of differing polarity, and with acidic or basic sites;
3. enzyme-like pockets to bind the substrate;
4. activation or stabilisation of the reagents;
5. promotion of selective modes of reaction;
6. ease of work-up by immobilisation of by-products, or of toxic chemicals.

K10 clay, an inexpensive acidic industrial catalyst [K10 clay is made by
Sud-Chemie (Munchen); montmorillonite K10 is also available from Fluka],
was chosen as the support after a comparison with other clays (natural or
industrial), sand, silica gel, titanium dioxide, acidic alumina, and
zeolites. These other possible inorganic supports either deactivate the
reagent or present theological properties after impregnation incompatible
with an easy experimental set-up and work-up. [3, 4]

A considerable amount of data dealing with the interaction of layer
silicates with organic molecules, and with clay-activated organic
reactions, has accumulated recently. A general and quite exhaustive
introduction to the subject was published in 1974.[5]  Updated data are
available from recent reviews[6, 7, 8], or from individual papers.
Unfortunately, this new chapter of organic chemistry has yet to be indexed
in a standardised form: it is thus advisable to scan geology and soil
science literature, in addition to chemical publications, when seeking
bibliographic information.

The Reagents

Preparation of Clay-Supported Iron(III) Nitrate (Clayfen)

Whereas these procedures worked out very safely in our hands, nitrates are
potentially dangerous compounds, and appropriate caution is to be applied
in each step. In particular, we urge to avoid confinement conditions, and
we recommend to proceed to scaling-up only after appropriate safety tests.

Clay-Supported Iron(III) Nitrate (Clayfen) [4, 9]

Iron(III) nitrate nonahydrate (22.5 g) is added to acetone (375 mL) in a 1
litre, pear-shaped evaporating flask. The mixture is stirred vigorously for
~ 5 min until complete dissolution of the crystals of hydrated iron(III)
nitrate. The first-formed homogenous rusty brown solution turns after a
short time into a muddy, light brown suspension. K 10 clay (30 g) is added
in small amounts and stirring is continued for another 5 min. The solvent
is then removed from the resulting suspension under reduced pressure
(rotary evaporator) on a water bath at 50 C. After 30 min. the dry solid
crust adhering to the walls of the flask is flaked off and crushed with a
spatula, and rotary evaporator drying is resumed for another 30 min. This
procedure yields K10 clay-supported iron(III) nitrate (Clayfen) as a
yellow, floury powder; yield: ~ 50 g.

Warnings:

1. Prolonged heating, or use of a bath temperature > 50 C may yield an
   unstable reagent, which decomposes in a vigorous exothermic reaction, with
   emission of a large amount of nitrogen dioxide fumes. This decomposition
   generally takes place within 15 min after the end of an incorrect
   preparation of Clayfen.

2. Preparation of Clayfen includes flaking off of the dry solid crust
   formed in the evaporator. The physical state of the reagent at this step
   is crucial: if this removal is performed while the solid is still a
   little muddy, it will aggregate in spheres that remain wet inside. and
   the resulting reagent will display little or no activity, or will
   decompose as described above. This difficulty will be best avoided by
   strict observance of the experimental conditions described (quantities,
   flask volume and shape).

3. Clayfen, spontaneously, slowly loses nitrous fumes, and should never he
   stored in a closed flask.

Stability of Clay-Supported Iron(III) Nitrate (Clayfen)

Left in air contact at room temperature, Clayfen retains its activity for
only a few hours. It may be stored for a few days and retains its activity
when it is covered with n-pentane immediately after preparation; the
hydrocarbon is evaporated prior to use. It is a good practice to use only
freshly prepared reagent: it loses about 40 % of its reactivity upon
standing exposed to air for 4 h, or under n-pentane for 24 h.

The thermal stability of Clayfen was investigated by differential
calorimetric analysis. Above 59°C, it decomposes with an enthalpy release
of ~ 20 cal/g. This is a first order process, with calculated half-lives of
53 min at 69 C (in boiling n-hexane, for instance) or of 14 min at 80 C
(the temperature of boiling benzene). In our experience the latter is the
upper temperature limit for the practical use of Clayfen.

Clay-Supported Copper(II)Nitrate (Claycop)

This reagent, also based on a salt forming covalent bidentate complexes
upon dehydrations [10,11], shows no loss of reactivity in the applications
described hereafter, even after standing in an open powder box for one
month. In our hands, it never underwent any spontaneous decomposition.

Clay-supported Copper(II) Nitrate (Claycop) [12]

Clay-supported copper(II) nitrate (Claycop) is prepared in a process
similar to the preparation of Clayfen, by adding K10 clay (30 g) to a
solution of copper(II) nitrate trihydrate (20 g) in acetone (375 ml). The
resulting suspension is placed in a rotary vacuum evaporator and the
solvent is eliminated under reduced pressure (water jet aspirator) on a
water bath at 50 C. After 30 min. the dry solid crust adhering to the walls
of the flask is flaked off with a spatula, and vaporation is resumed for
another 30 min in the same conditions, giving Claycop as a light blue,
free-flowing powder; yield: ~ 50 g.

For examples of applications of Clayfen, Claycop and K10 see references
13,14,15.

References:

1.  J A Ballantine in Chp 4. p100-103. In "Bologh, Laszlo, Organic
    Chemistry Using Clays", 1993, Springer-Verlag.
2.  From "Clay-Supported Copper(II) and Iron(III) Nitrates: Novel
    Multi-Purpose Reagents for Organic Synthesis. A Cornelis, P Laszlo.
    Synthesis 1985, 909-918.
3.  Cornelis, Laszlo & Pennetreau, Clay Miner 1983, 18, 437; CA 1984, 100, 102362.
4.  Cornelis, Laszlo & Pennetreau, Bull. Soc. Chim. Belg. 1984, 93, 961
5.  B. Theng, The Chemistry of Clay-Organic Reactions, Wiley NY, 1974.
6.  McKillop & Young, Synthesis 1979, 401, 481.
7.  Theng, Dev. Sedimentol. 1982, 35, 197; CA 1982, 97, 197524;
8.  Lagaly, Phil. Trans. R. Soc. Lond. A 1984, 311, 315
9.  Cornelis & Laszlo, Synthesis 1980, 849.
10. Addison. Prog. Inorg. Chem. 1967, 8, 195.
11. Addison, Logan, Wallwork, Garner. Q. Rev. Chem. Soc. 1971, 25, 289.
12. Bologh, Hermecz, Meszaros, Laszlo. Helv. Chim. Acta 1984, 67, 2270.
13. Selective Nitration of Styrenes with Clayfen and Clayan: A Solvent-free
    Synthesis of beta-Nitrostyrenes. Rajender S. Varma, Kannan P. Naicker
    and Per J. Liesen. Tetrahedron Letters 39 (1998) 3977-3980.
14. Varma, R.S.; Dahiya, R. Tetrahedron Lett. 1997,38. 2043;
15. Sodium Borohydride on Wet Clay: Solvent-free Reductive Amination of
    Carbonyl Compounds Using Microwaves. Rajender S. Varma and Rajender
    Dahiya. Tetrahedron 54, 6293-8 (1998)
16. Varma & Dahiya. Tetr. Lett. 39 (1998), 1307-8.

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