Alkylation reactions of potassium carboxylates supported on alumina: Comparison between ball-milling, impregnation and dispersion procedures

Article abstract of DOI:10.1039/b007703h

Potassium carboxylate alkylation by n-bromooctane on neutral alumina in "dry media" under microwave irradiation was studied for different methods of preparation of salt-support powder. Mechanical milling and impregnation techniques led to equivalent yields, much higher than those obtained through dispersion. The first method avoids using solvents and subsequent drying of powders. Crystallographic and granulometric studies of the resulting powders were performed to ascertain the results.

Full text of DOI:10.1039/b007703h

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Alkylation reactions of potassium carboxylates supported on
alumina: comparison between ball-milling, impregnation and
dispersion procedures
a
b
a
c
a
2
Michel Gasgnier,*† Henri Szwarc, Alain Petit, Jean Nahmias and André Loupy
a
Laboratoire des Réactions Sélectives sur Supports, ICMO, UMR 8615 CNRS, Bâtiment 410,
Université Paris-Sud, 91405 Orsay, France
Laboratoire de Chimie Physique, UMR 8611 CNRS, Bâtiment 490, Université Paris-Sud,
b
c
9
1405 Orsay, France
Laboratoire de Génie Chimique, Institut Universitaire de Technologie, Bâtiment 607,
Université Paris-Sud, 91405 Orsay, France
Received (in Cambridge, UK) 22nd September 2000, Accepted 27th April 2001
First published as an Advance Article on the web 23rd May 2001
Potassium carboxylate alkylation by n-bromooctane on neutral alumina in “dry media” under microwave irradiation
was studied for different methods of preparation of salt–support powder. Mechanical milling and impregnation
techniques led to equivalent yields, much higher than those obtained through dispersion. The first method avoids
using solvents and subsequent drying of powders. Crystallographic and granulometric studies of the resulting
powders were performed to ascertain the results.
Introduction
(mechanochemistry), described as a powerful pulverizing and
13
mixing procedure, was recently advocated as a promising
technique that could be superimposed on various phenomena.
Examples include the amorphization of particles, or the modi-
fication of oxidation degrees, or the phase transition of mineral
materials (reduction of silver oxide into silver metal, formation
of boron nitride and lead dioxide high pressure phases among
others). Another important result relates the change of the
particle surface area and the size reduction observed for hard
1
Since the first experiments reported by Keinan and Mazur
who studied Nef “dry media” reactions on silica gel, solid sup-
ports have been more and more advocated in organic synthesis.
They avoid the use of solvents, something which may be advan-
tageous from experimental and environmental points of view.
They constitute safe, economical, efficient and eco-friendly
2
,3
conditions in so-called “green chemistry”.
Thus, many
anionic alkylations have been performed by using alumina,
silica, or clays (such as montmorillonites) as mineral supports.
Many studies have then recommended alumina as a very effi-
cient support to carry out organic syntheses. Initially, two
different techniques were considered, carrying out reactions
either by dispersion of the nucleophilic salts onto a mineral
support, or by impregnation of the support via a saturated solu-
tion and subsequent solvent removal under reduced pressure.
14
and soft materials. Some examples of organic syntheses carried
out by means of vibratory mills include: the solid-phase syn-
15
thesis of a racemic binaphthol, the mechanohydrogenation of
4
–7
16
naphthalene and biphenyl with alumina and hydrogen, the
synthesis of fullerene C , dimer C , achieved by solid-state
6
0
120
17
mechanochemical reaction of C60 with KCN or CH COOK,
3
and the mechanochemical arylation and alkylation of C60
18
under solvent-free conditions.
8
More generally, though with a very few exceptions, the latter
technique (impregnation) is shown to be far more efficient.
9
More recently, these “dry media” reactions were considerably
improved by using microwave (MW) exposure rather than
classical heating, thanks to the strong specific interactions of
Experimental
Preparation of carboxylate salts
10,11
hard-to-heat solid conductors with the electromagnetic field.
Potassium acetate and benzoate (purchased from Acros)
were used without further purification. Potassium stearate and
phenylacetate were prepared as usual by dissolving 3.25 g
potassium hydroxide (50 mmol containing 15% water) in 50
ml methanol inside an Erlenmeyer flask with stirring. By sub-
sequent addition of carboxylic acid (50 mmol), 14.22 and 6.81 g
of stearic and phenylacetic potassium salts were obtained,
respectively, after methanol removal and drying to give soft
white anhydrous powders. However, due to their hygroscopic
character, these materials must be carefully stored. As a matter
of fact, X-ray diffraction patterns (XDP) and IR spectra reveal
that potassium acetate and phenylacetate quickly absorb water
The aim of the present work is to compare the behavior of
nucleophilic salts–alumina species with respect to alkylation
under MW heating using different modes of preparation:
namely dispersion, impregnation, and low-energy ball-milling.
Our reference reaction model was the nucleophilic alkyl-
ation reaction of carboxylate anionic species with n-
bromooctane, reaction (1). The effects of mechanical activation
are now well known in inorganic chemistry. Ball-milling
12
(
1)
19
after exposure to ambient atmosphere. Neutral alumina from
Merck (activity I, 0.063–0.200 mm, 70–230 mesh ASTM),
characterized by small hard grains, was used for these experi-
ments.
†
8
Present address: Laboratoire de Chimie Inorganique, ICMO, UMR
613, Bâtiment 420, Université Paris-Sud, 91405 Orsay, France.
DOI: 10.1039/b007703h
J. Chem. Soc., Perkin Trans. 2, 2001, 1233–1238
This journal is © The Royal Society of Chemistry 2001
1233

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Table 1 GC analyses of n-octyl esters RCOOn-C H
Table 2 n-Octyl ester yields [reaction (1)] after 10 min 180 W MW
8
17
Ϫ
ϩ
irradiation for different RCOO K –alumina preparation techniques
a
b
b
R
T/ЊC
tR/min
Standard
tR/min
R
Procedures
MW final T/ЊC
Yield (%)
CH3
85
250
190
170
7.6
7.0
3.3
5.8
n-Undecane
4.5
4.4
4.7
5.1
n-C H
Bis(2-ethylhexyl)phthalate
Di-n-butyl phthalate
n-Octyl benzoate
CH3
D
185
203
200
185
186
155
185
156
197
182
170
179
184
200
172
169
168
204
206
49
80
60
75
1
7
35
Ph
PhCH2
I (via H O)
2
I (via MeOH)
M (8 h)
M (20 h)
D
I (via MeOH)
M (8 h)
M (20 h)
D
a
b
Isothermal oven temperature. Retention time.
a
79
n-C H
35
32
35
22
1
7
Preparation of salt–alumina mixtures
a
30
For salt–alumina preparations, three procedures were tested.
Ph
0
34
7
11
34
67
76
65
60
I (via H O)
2
i) Dispersion mode (D). This consists of a simple contact
between potassium carboxylate (1 g) and alumina (4 g). The
two powders are shaken together vigorously for 5 min in a
stoppered flask, with no further treatment.
I (via MeOH)
M (8 h)
M (20 h)
D
I (via H O)
I (via MeOH)
M (4 h)
a
PhCH
2
2
ii) Impregnation mode (I). This involves dissolving at room
temperature potassium carboxylate (1 g) either in methanol
or in water (20 mL), with magnetic stirring, then addition of
alumina (4 g). The solvent is subsequently removed under
reduced pressure and the supported reagent dried under
vacuum.
a
M (8 h)
68
a
Prolongated exposure to MW does not give improved yields.
Ϫ2
(
3 × 10 Torr) at ambient temperature. Scanning electron
microscopy (SEM) was used to observe the particles. The
crystalline unit-cells were determined by X-ray diffraction
iii) Mechanical milling mode (M). This involves a mixture of
potassium salt (1 g) and alumina (4 g) put inside a cylindrical
steel container and shaken together with 20 metallic balls
(
Guinier-de Wolff Camera II, Enraf-Nonius apparatus, fitted
with a quartz monochromator).
(
about 2 g each). After 4 to 48 h, it was observed that the balls
and the lower part of the cylinder inner walls were coated with a
thick and hard layer of the inorganic materials, which indicates
that the compactness of the powders had been increased.
Moreover, part of the powder was often deposited all along the
cylinder inner walls, and even on the screw top (i.e., 8 cm above
the upper level of the mixture); a dust cloud could be observed
on opening the container. This indicates that very small light
particles were formed during the course of milling. For potas-
sium stearate, the grains are only flattened at the bottom of the
container. We must also stress that, even after shaking for a
long time, almost no temperature increase of the containers
could be observed, and that the 20 balls induce an apparently
negligible pressure.
Results
Alkylation reactions
The ester yields obtained for the three methods, D, I and M, are
listed in Table 2. The reactions were performed under the same
experimental conditions (10 min 180 W microwave exposure).
No general rule can be drawn as to what affects the relative
yields for the three methods. For method I, it appears that
impregnation from water leads to yields improved by 10 to 25%
relative to those obtained via methanol (except for potassium
stearate, which is insoluble in water). This could be due to the
part played by small amounts of water in the reaction. The role
of water has been demonstrated for some substitution reactions
22
Alkylation reaction procedure
performed in “dry media”, or under solid–liquid phase
23
transfer catalysis. Traces of water are shown to induce some
Subsequently, using the three types of preparation for the
reactant, the alkylation reactions were performed by means of a
monomode microwave (Prolabo Synthewave 402, operating at
Ϫ
ϩ
dissociation of the nucleophilic salts as RCOO K and thus to
diminish lattice energies.
Concerning mode M, after ball-milling for 8 h, yields exceed
by 5 to 25% those obtained after 4 h. Mode D leads to yields
nearly equivalent to those by the I and M methods for potas-
sium stearate and phenylacetate, a lower yield (10 to 30%) for
potassium acetate, and a nil yield for potassium benzoate.
Finally, the I and M methods lead to higher, almost equivalent
yields.
Three complementary experiments with potassium phenyl-
acetate were also performed to optimize procedure M. The salt
and alumina were either shaken separately or not shaken at all.
Whatever the treatment of the salt powder was, the alkylation
procedure led to yields close to those given in Table 2 (60%)
after the alumina powder had been shaken alone. However,
this yield is somewhat lower for non-shaken alumina (54%). So,
it appears that, after grinding, alumina enhances slightly the
alkylation yield.
11,20
2
.45 GHz).
In all cases, 5 g of powder and 1.5 molar equiva-
lents of n-bromooctane were firmly homogenized inside a glass
20
or quartz test-tube for each treatment. The mixtures were then
exposed to MW for 10 min (180–300 W). The temperature
versus time variation was detected by means of an IR pyrometer
20,21
and monitored by a computer.
room temperature, the mixtures were eluted with diethyl ether
50 mL) by filtering through sintered-glass. The filtrates were
After cooling down to
(
analyzed by gas chromatography (GC) with an internal
standard using a Carlo Erba apparatus [Vega series 2 GC
6
000 flame ionization, capillary column CPSil 5 CB (25 m),
gas carrier: Ar (70 kPa), injector-detector temperature: 280 ЊC].
For every salt, oven temperatures, retention times, and internal
standards are indicated in Table 1.
Particle analysis techniques
The alkylation reaction was tentatively performed by using α-
Al O (corindon) instead of η-Al O . After mixing with the salt,
Grain size analyses were carried out by means of a Mastersizer
micro (Malvern Cy) granulometer. Surface area measurements
were performed with a Sorptomatic 1900 apparatus (Carlo
Erba Cy). The results were obtained after nitrogen adsorption
according to the Brunauer–Emmett–Teller (BET) method. The
powders were previously maintained under a low vacuum
2
3
2
3
then ball-milling for 48 h, the powder was reacted for 10 min
under MW at 175 ЊC. The final yield was equal to 8%. The
presence of corindon (or possibly of another alumina form, or
aluminium trihydroxide) would seem to have a noticeable effect
on the final yield.
1
234
J. Chem. Soc., Perkin Trans. 2, 2001, 1233–1238

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Table 3 Granulometry, surface areas and c (BET) parameter for the
different samples
Surface
Granulometry area BET/
Alkylation
yield (%)
2
Ϫ1
Samples
Al O
D (v, 0.5)/mm m g
c
92
19
159
86
132
126
0
102 ± 2
103 ± 2
—
—
0
2
3
Al O (M)
2
3
AcOK
AcOK (M)
0
0
0
—
(
(
AcOK ϩ Al O ) (D) 97
80
40
22
48 ± 1 49
30 ± 1 75
54 ± 4 80
2
3
AcOK ϩ Al O ) (M)
7
103
2
3
AcOK–Al O (I)
2
3
(
via H O)
2
Fig. 1 Thermal evolution of ball-milled potassium carboxylates–
alumina when submitted to MW exposure (P = 180 W).
Fig. 3 Granulometry of alumina before and after ball-milling.
Ϫ
ϩ
important ionic dissociation of RCOO K due to alumina
interaction.
Fig. 2 Thermal evolution of potassium acetate–alumina (1 : 4 w/w)
when submitted to MW exposure (P = 300 W) according to different
preparation procedures (D = dispersion, I = impregnation, M = ball-
milling).
Granulometry, geometric and surface areas
Powders of alumina, potassium acetate and of potassium
acetate–alumina mixture were studied with respect to their
grain sizes [measured as D (v, 0.5) (medium diameter particles)],
and their surface areas were measured according to the BET
method (nitrogen adsorption). In the case of a pure AcOK
powder, the lack of adsorption of nitrogen is due either to the
very low surface area, or to the fact that a part of the particles
are readily saturated by water. The results reported in Table 3
allow us to correlate these values with the alkylation yields.
1) For granulometry measurements, alumina powder (before
and after ball-milling) is readily suspended in water. To avoid
potassium acetate solubilization in water and the formation of
aggregates, pure acetate and alumina–acetate mixtures were
dipped into a benzine gas–oil mixture (ratio 50 : 50). Grain size
changes after treatments M and D are shown on Fig. 3 (alu-
mina), Fig. 4 (potassium acetate) and Fig. 5 (potassium acetate–
alumina). Note that, after treatment M: a) the formation of
new populations with smaller diameters is obvious (Figs. 3 to
5); b) the medium grain diameter is thus reduced by a ratio of
about 2 (acetate) and 5 (alumina) (Table 3); c) the amount of
submicronic grains is noteworthy (Figs. 3 and 5).
Thermal behaviors of the different preparations when submitted
to MW exposure
The profiles of temperature raising of samples when submitted
12b
to MW exposure are correlated with their polarization: the
more polar the medium, the more important the temperature
increase.
In Fig. 1, the thermal behaviors of various carboxylates, ball-
milled on alumina, are compared.
With potassium acetate and phenylacetate, these variations
are characterized by a thermal runaway that reveals a high
degree of ionic dissociation of these compounds when ball-
milled on alumina. For the stearate and benzoate salts, the
temperature tends towards a plateau (160 ЊC). Thus polarity
effects increase according to the following sequence: acetate >
phenylacetate > benzoate ≈ stearate.
This order is the same as for the reactivity (Table 2) when
one considers the yields of the reaction with n-bromooctane.
Ϫ
ϩ
Different CH COO K –alumina preparations are compared in
3
Fig. 2.
Ball-milled and impregnated reactants exhibit obviously
the same behaviors, with a thermal runaway after short
MW exposures (5–6 minutes). The temperature in dispersed
samples remains moderate and tends towards a plateau. Thus,
the ionic dissociation of RCOO K , and consequently its
polarity increase, is in the order: impregnation ≈ ball-milling >
dispersion.
However, it would be difficult to wash off the AcOK–
alumina mixture without inducing unwanted reactions. Indeed,
only water can be used to dissolve the carboxylate, and this
could lead either to the formation of alumina hydroxide at the
grain surfaces, or to that of the AcOK hydrated phase after
Ϫ
ϩ
19
drying.
2) BET measurements were performed to determine the
surface areas and the c parameters. Unfortunately, it was not
possible to calculate the t-plot values. Indeed, the standard
curve for a non-porous oxide was not recorded under the same
This sequence is the same as that reported for reactivity
(
Table 2). These thermal behaviors are therefore strictly con-
nected to the observed reactivities as involving a more or less
J. Chem. Soc., Perkin Trans. 2, 2001, 1233–1238
1235

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Fig. 4 Granulometry of potassium acetate before and after ball-
milling.
Fig. 5 Granulometry of ball-milled and dispersed potassium acetate–
alumina systems.
Fig. 6 Scanning electron microscopy (SEM) imaging of alumina
powder: a) before ball-milling; b) after ball-milling.
energy recorded for this material. So far, we cannot make any
conclusions about the activation modes for the different pro-
cedures, particularly as the possible presence of water could
play a non-negligible role (as a catalyst) during and after the
treatments D, I and M.
experimental conditions as those used for our samples.
Adsorption/desorption isotherm curves show an hysteresis and
can be classified as type IV. The BET plot is linear with the
origin at 0, or very close to 0. Such curves allow us to qualify
the materials as mesoporous.
24
The results given in Table 3 clearly show the following. a)
Surface area of alumina is weakly affected by ball-milling; this
could be due to the increase in the external surface (see Figs. 6a
and 6b) and a decrease in the mesoporosity observed after pres-
sure treatment. b) Potassium acetate does not adsorb nitrogen;
this is related either to the presence of water inside numerous
pores, or to the low surface area of the particles. c) Surface
areas decrease from dispersed to ball-milled and finally to
impregnated samples, whereas the c parameter, relative to that
for alumina, drops by a factor of 2 after treatments D and I,
and by a factor of 3 after treatment M (the c parameter is
exponentially related to the surface energy).
As we could not perform microcalorimetry experiments, it is
difficult to draw conclusions about the activation processes and
consequent alkylation yield changes. In fact, these data seem to
be related to the parameter changes as reported in Table 3, to
the particle aspects as seen in Figs. 6 to 9, and to MW heating
Particle imaging
In order to check the above hypothesis, SEM imaging was
carried out.
a) Alumina. SEM imaging shows that the initial powder is
made of polygonal crystallites of various sizes (Fig. 6a). After
grinding, these crystallites are smaller, thinner, and exhibit
large apparent surfaces (Fig. 6b). Such a configuration can
enhance interfacial contacts with grains of another very fine
powder.
b) Potassium acetate. For a non-milled potassium acetate
powder, the initial small crystallites appear as spherolites or
rounded shapeless volumes with sizes higher than 300 µm.
After grinding, these spheres and volumes are smaller (50–200
µm), many grains exhibit polygonal shapes, and others are
characterized by very small sizes (less than 5 µm) (Fig. 7a
and b).
19
(
Fig. 1). For instance, the decrease of surface area after treat-
ments M and I could be linked either to site coverage (sticking
of small potassium salt particles on alumina grains), or to
micropore filling (therefore a loss of microporosity). The high
alkylation yields after ball-milling could be due to the large
amount of very small crystallites and to the larger apparent
surface of the aggregates. This could balance the lower surface
c) Potassium acetate–alumina. When alumina and potassium
acetate powders are ball-milled together, SEM imaging shows
large shapeless aggregates, with many submicronic particles
sticking to their surfaces (Fig. 8). This can be understood in
1
236
J. Chem. Soc., Perkin Trans. 2, 2001, 1233–1238

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Fig. 7 Scanning electron microscopy (SEM) imaging of potassium
acetate powder: a) before ball-milling; b) after ball-milling.
Fig. 9 Scanning electron microscopy (SEM) imaging of potassium
benzoate powder: a) before ball-milling; b) after ball-milling.
Crystallographic data
a) Alumina. As revealed by XDP, commercial alumina
25
(
η-Al O , fcc, a = 0.795 nm) is poorly crystallized. Indeed,
2 3
the patterns exhibit only the presence of a faint and diffuse
halo, corresponding to the (440) diffraction line. During ball-
milling (ambient atmosphere) and MW exposure (maximum
temperature: 130 ЊC), no phase transition was induced.
b) Potassium salts. Crystallographic data concerning
potassium salts are somewhat intricate. It has been previously
established that commercial potassium acetate is a mixture of
hydrated and unhydrated salts. Moreover, under MW exposure,
19
a phase transition occurs near 130 ЊC. It has been demon-
strated that potassium phenylacetate is very sensitive to water;
the dry powder can absorb one molecule of water per potas-
sium phenylacetate molecule in less than 30 min. The two
other salts are less contaminated by water. IR spectra support
Fig. 8 Scanning electron microscopy (SEM) imaging of ball-milled
alumina–potassium acetate mixture powders.
26
these observations. As revealed by XDP, the neat powders
used before treatment are well crystallized. After mixture with
alumina according to procedures I and M, potassium acetate,
phenylacetate and stearate become amorphous. In the case of
potassium benzoate, only the most intense peaks are observed
as faint diffraction lines; they are more intense after pro-
cedure I. It is difficult to determine the unit-cell parameters for
potassium benzoate and phenylacetate.
terms of small potassium acetate crystals sticking onto bigger
alumina ones, thus diminishing surfaces able to adsorb
nitrogen.
d) Potassium benzoate. In the case of potassium benzoate, the
initial powders are made of large aggregates (50 to 250 µm).
After grinding, they appear flattened and smoothed and some
particles have been pulled out and are isolated from one
another (Fig. 9a and b). Such a configuration seems to increase
efficiently the interfacial contact with alumina.
Conclusion
Our experiments have allowed us to point out the behavior of
four potassium carboxylates during alkylation reactions on
J. Chem. Soc., Perkin Trans. 2, 2001, 1233–1238
1237

View Article Online
alumina by n-bromooctane. From our results it appears that
potassium stearate and benzoate give the worst yields, and that
the dispersion procedure is the least suitable to obtain high
reactivities. Potassium acetate and phenylacetate give better
yields, overall, after ball-milling and impregnation procedures.
To explain such phenomena one can put forward various
hypotheses. First, it appears that for potassium acetate and
phenylacetate water may play a role, as a catalyst, during the
reaction. Second, the size and nature of pores and of defects
might also be very important parameters. Particularly, the
process M could induce high-activity defect sites. Isotherm
curves indicate the mesoporous character of the particles.
Unfortunately, SEM observations do not allow us to obtain a
sufficient resolution to observe objects smaller than 100 nm.
Process M might also induce a better contact between the salt
and alumina grains by increasing the external surfaces, and
it involves a crystalline→amorphous transition (except for
potassium benzoate) which should favor salt reactivity. Lastly,
one must add that MW treatments depict yields and salt
reactivities as a function of the three different procedures.
Indeed, thermal runaways are recorded for potassium acetate
and phenylacetate and, after I and M procedures, for potassium
acetate–alumina mixtures.
6 A. Foucaud, G. Bram and A. Loupy, in Preparative Chemistry Using
Supporting Reagents, ed. P. Laszlo, Academic Press, New York,
1
987, ch. 17, pp. 317–341.
7
8
G. W. Kabalka and R. M. Pagni, Tetrahedron, 1997, 53, 7999 and
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(a) J. Barry, G. Bram, G. Decodts, A. Loupy, P. Pigeon and
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9
(
1
0 G. Bram, A. Loupy and D. Villemin, in Solid Supports and Catalysts
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1 A. Loupy, A. Petit, J. Hamelin, F. Texier-Boullet, P. Jacquault and
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2 (a) G. Bram, A. Loupy, M. Majdoub, E. Guttierez and E.
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(
1
1
Procedure M constitutes a significant simplification of the
overall reaction as there is no need for prior impregnation of
salts on solid supports and subsequent (often tedious) drying
under vacuum. Note that processes I and M last equivalent
times (8–20 hours). Moreover, as simple dispersion is not
usually sufficient to promote any activation of carboxylate
salts, ball-milling (low-energy mechanochemistry) is as efficient
as impregnating reagents. Therefore, associated with MW
treatment and solvent elimination, procedure M appears very
economic and non-polluting. It is a new efficient procedure to
obtain satisfactory alkylation yields.
16 L. D. Field, S. Sternhell and H. V. Wilton, Tetrahedron Lett., 1998,
39, 115.
1
7 K. Komatsu, G. W. Wang, Y. Murata, T. Tanaka, K. Fujiwara,
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