The heterogeneous catalytic heck reaction in an ionic liquid

Article abstract of DOI:10.1134/S0036024408130141

The C-C cross coupling reaction between bromobenzene and styrene in the presence of Pd deposited on mesoporous acetylene soot (Pd/C) occurred in a tetraalkylammonium ionic liquid (IL) as a truly heterogeneous catalytic process. The active palladium form was not transferred into the reaction solution. Deposited palladium activation was observed when the Pd/C catalyst was preliminarily heated in the IL in the presence of dibutylamine as a base. Pleiades Publishing, Ltd., 2008.

Full text of DOI:10.1134/S0036024408130141

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2008, Vol. 82, No. 13, pp. 2238–2242. © Pleiades Publishing, Ltd., 2008.
CHEMICAL KINETICS
AND CATALYSIS
The Heterogeneous Catalytic Heck Reaction in an Ionic Liquid
L. A. Aslanova, P. M. Valetskiib, V. V. Volkovc, V. N. Zakharova, Yu. A. Kabachiib,
S. Yu. Kochevb, B. V. Romanovskiia, and A. V. Yatsenkoa
a Faculty of Chemistry, Moscow State University, Moscow, 119992 Russia
b Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
ul. Vavilova 28, Moscow, GSP-1, 117813 Russia
c Shubnikov Institute of Crystallography, Russian Academy of Sciences,
Leninskii pr. 59, Moscow, 117333 Russia
e-mail: bvromanovsky@mail.ru
Received February 8, 2008
Abstract—The C–C cross coupling reaction between bromobenzene and styrene in the presence of Pd depos-
ited on mesoporous acetylene soot (Pd/C) occurred in a tetraalkylammonium ionic liquid (IL) as a truly heter-
ogeneous catalytic process. The active palladium form was not transferred into the reaction solution. Deposited
palladium activation was observed when the Pd/C catalyst was preliminarily heated in the IL in the presence of
dibutylamine as a base.
DOI: 10.1134/S0036024408130141
INTRODUCTION
uids (ILs) have obvious advantages. Ionic liquids are
organic salts with melting points below 100°C and,
which is of fundamental importance, zero vapor pres-
sure up to the decomposition temperature (400–
500°C). When heterogeneous catalytic reactions are
performed in ILs, reaction products are fairly easily iso-
lated from the reaction mixture by extraction, and, if it
is necessary to separate a solid catalyst, by simple fil-
tration. All these advantages allow ILs to be considered
“green” solvents with a fairly broad spectrum of appli-
cations in organic syntheses [4]. Lastly, the number of
chemical reactions that can be performed in ILs is much
larger than that in perfluorocarbons, which are close to
ILs in their ecological characteristics, and supercritical
fluids [5].
However, note that, in spite of obvious advantages
of ionic solvents for heterogeneous catalytic arylation
of unsaturated compounds according to the Heck reac-
tion, the number of works on the problem is very small
compared with those concerned with the homogeneous
catalysis of this reaction. The reason for this is twofold.
On the one hand, the physical and physicochemical
properties of ILs and, as a consequence, the reactivity
of substrates and catalysts dissolved in them depend
substantially on the composition and structure of their
cationic and anionic components. The selection of var-
ious combinations of cations and anions, whose num-
ber is at present several hundreds and increases rapidly
[5], allows ILs with the required solvent characteristics,
including melting point, polarity, product and reagent
solubilities, etc., to be designed to answer the require-
ments of particular chemical problems.
The arylation of unsaturated compounds with
haloaryles by C–C cross coupling (the Heck reaction) is
basic to many practically important processes of fine
organic synthesis, such as the preparation of biologi-
cally active substances, vitamins, and pharmaceuticals.
Palladium salts or complexes in homogeneous media
are largely used as reaction catalysts. The greatest
progress was achieved thanks to the use of phosphine
ligands, which stabilized palladium-based catalytic
systems.
The high cost of palladium phosphine complexes
and the impossibility of their regeneration, however,
restrain wide use of the Heck reaction on an industrial
scale. At the same time, its exceptional attractiveness as
a labile and multipurpose tool for chemical syntheses
stimulates intense search for alternatives to homoge-
neous catalysts. One of such alternatives is the use of
deposited Pd catalysts, which sometimes accelerate the
Heck reaction no less effectively than homogeneous
systems [1], but are much more efficient technologi-
cally, because they allow catalysts to be easily sepa-
rated from reaction products.
As was mentioned in review [2], the mechanism of
the Heck reaction in phosphine-free systems, such as
heterogeneous catalytic Pd-containing systems, is such
that the reaction can only occur in polar solvents of the
type of dimethylformamide, N-methylpyrrolidone, etc.
At the same time, these and similar traditional molecu-
lar solvents do not meet modern requirements of “green
chemistry.” They are mostly toxic, and their regenera-
tion is energy-consuming and requires more expendi-
On the other hand, the high sensitivity of the Heck
tures than their production [3]. In this respect, ionic liq- reaction to the properties of the medium requires cata-
2238

THE HETEROGENEOUS CATALYTIC HECK REACTION IN AN IONIC LIQUID
2239
lytic systems, such as ILs, to be constructed carefully in
The Preparation of Palladium Deposited
on a Carbon Carrier
each particular case [2]. For instance, C–C cross cou-
pling in the presence of deposited Pd occurs in the tet-
rabutylammonium IL at a high rate, whereas the reac-
tion does not occur at all in the IL based on imidazole
[6]. With such a large number of factors influencing the
process, we can hardly predict the response of the sys-
tem to changes in internal and external parameters,
which makes the solution to the problem “the Heck
reaction in an ionic liquid” highly unstable even when
parameter variations are small.
The initial material for the preparation of a mesopo-
rous carbon carrier was acetylene soot AS100. To
hydrophilize its surface, it was treated with a mixture of
H2SO4 and HNO3 at 90°C. Such a modified carrier with
a specific surface area of 260 m2/g consisted of
mechanically strong fractal agglomerates whose mean
size was 0.5 µm. The agglomerates were in turn formed
from primary nanoparticles with a size of about 30 nm.
The surface of the carrier was amorphous carbon with
inclusions of structurally ordered graphite-like
domains.
Pd/C catalysts were prepared by the reduction of
Pd(II) hydroxide preliminarily deposited on the surface
of the carrier from an alkaline solution under ultrasonic
stimulation conditions. AS100 soot (1.0 g), water (28
ml), and NaHCO3 (227 mg) were loaded into a flask
placed in an ultrasonic water bath. The mixture was
heated at 80°C for 40 min under intense stirring with
ultrasonic stimulation. Next, Na2PdCl4 (0.510 mmol)
and water (22 ml) were added to the hot suspension
during 2 min, the mixture was heated for 1.5 h more,
and an aqueous solution of formaldehyde was added to
it. The mixture was heated for 1 h under ultrasonic
stimulation conditions and for 30 min with ultrasound
switched off. The product was then filtered, washed on
a filter six times with 15 ml portions of demineralized
water and once with doubly distilled water, dried in a
vacuum (12 torr) at 80°C, and then heated in a vacuum
(10–3 torr) at 250°C for two hours. The content of pal-
ladium in the Pd/C catalyst sample was 5 wt %.
The purpose of this work was to solve a compara-
tively narrow problem. We wished to determine the spe-
cial features of C–C cross coupling between bromoben-
zene and styrene in the presence of nanodisperse Pd
deposited on a carbon carrier (mesoporous hydro-
philized soot) in a tetraalkylammonium IL, tributylhex-
ylammonium bis-triflylamide, as a solvent.
EXPERIMENTAL
The Preparation of Tributylhexylammonium
bis-Triflylamide
Tributylhexylammonium
bis-triflylamide
[Bu3HexN][NTf2], where NTf2 is the anion of the com-
position (CF3SO2)2N–, was selected as a solvent for the
Heck reaction because of an optimum combination of
its characteristics, namely, a low melting point
(+19°C), high solubility of the initial reagents (bro-
mobenzene and styrene) and bases (di- and tributyl-
amine), moderate solubility of the main (stilbene) and
side (biphenyl) products, and immiscibility with hydro-
carbons (cyclohexane and n-decane) and water. In addi-
tion, the viscosity of this IL, fairly high at room temper-
ature, which substantially complicates filtration, can be
decreased substantially by the addition of small
amounts of acetone.
Procedures for Physicochemical Studies
The content of palladium in the samples was deter-
mined by X-ray spectral fluorescence analysis on a Carl
Zeiss JenaVRA-30 spectrometer. The characteristics of
the structure of the carbon carrier and metal nanoparti-
cles incorporated into it were obtained by electron dif-
fraction on an EMR-102 instrument at an accelerating
voltage of 75 keV. The specific surface area and the vol-
ume of pores and their mean size were determined
using low-temperature adsorption of nitrogen accord-
ing to Brunauer, Emmett, and Teller on a Micrometrit-
ics ASAP 2000N sorption meter. The morphology of
deposited palladium metal particles was controlled by
high-resolution transmission electron microscopy on a
Tecani G2 30 S-TWIN microscope at an accelerating
voltage of 300 keV and by small-angle X-ray scattering
on AMUR-K and HECUS System-3 units.
[Bu3HexN][NTf2] was synthesized as follows.
Tributylamine dried over sodium metal and distilled at
a reduced pressure (75–77°C, 3 torr) was quaternized
with hexyl bromide (a 20% excess) in boiling acetoni-
trile for 18 h. After reaction termination, the solvent
was distilled off at a reduced pressure, and the residue
was two times washed with hexane to remove unre-
acted hexyl bromide; traces of organic solvents were
removed by heating to 80°C at 3 torr.
Tributylhexylamine obtained this way (30 g) was
dissolved in distilled water (100 g). A stoichiometric
amount of lithium triflylamide was added to the solu-
tion. An oily layer of the “raw” IL immediately formed
at the bottom of the reaction vessel. This layer was sep-
arated from the solution, washed with a 0.5% solution
Catalytic Experiments
The reaction of bromobenzene with styrene was
of lithium triflylamide and two times with distilled performed in a static system at 140°C and atmospheric
water. The IL obtained, tributylhexylammonium bis- pressure. Unless otherwise stated, experiments were
triflylamide, was dried by heating at 2–3 torr until gas performed as follows. IL (10 ml), bromobenzene
release ceased.
(10 mmol), styrene (15 mmol), base (sodium acetate or
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2240
ASLANOV et al.
dibutyl- or tributylamine) (20 mmol), and Pd/C catalyst on the one hand and, on the other, makes the whole
in the amount corresponding to 0.03–0.4 mol % Pd volume of mesopores accessible to not very large cat-
based on initial bromobenzene were placed into a alytic reaction substrates. This allows carbon carrier
Schlenck flask. A flow of dry argon was bubbled mesopores with ultradisperse palladium metal parti-
through the mixture for 15 min. The reactor was then cles in them to be treated as peculiar nanoreactors
closed and immersed into an oil bath preliminarily functionally similar to stabilized Pd metal sols cata-
heated to 140°C, while the mixture was vigorously lyzing the Heck reaction [7].
stirred. Samples (1 ml) were periodically taken for
analyses from the reaction mixture containing unre-
acted initial substances and reaction products.
The Kinetics of the Heck Reaction
between Bromobenzene and Styrene
The kinetic curves shown in Fig. 1 are the experi-
mental dependences of the conversion of bromoben-
zene and the yields of stilbene and biphenyl on reaction
duration obtained for various initial reaction mixture
compositions and various experimental conditions.
First note that the use of sodium acetate (NaOAc) and
tributylamine (Bu3N) as reagents binding HBr in the IL
gives closely similar results (Figs. 1a and 1b). These
bases only differ in the yields of the side product. In the
presence of NaOAc, biphenyl is formed in very small
amounts, within errors in gas-liquid chromatography
measurements.
Chromatographic and NMR Spectral Analyses
The reaction mixture was analyzed by gas-liquid
chromatography. A sample was extracted with cyclo-
hexane containing 1 vol % n-decane as an internal ref-
erence, and the filtrate was analyzed. Analyses were
performed on a CHROM-5 chromatograph equipped
with a katharometer and a 2 m metallic column 3 mm
in diameter packed with Chromaton N-Super (0.6–
1.2 mm) with 3 wt % SP-2000 phase under the condi-
tions of two-step temperature programming, from 100
to 150°C (heating rate 10 K/min) and from 150 to
250°C (heating rate 12 K/min). The instrument was cal-
ibrated against solutions of bromobenzene, styrene,
biphenyl, and stilbene in cyclohexane with an internal
reference.
The degree of initial reagent (bromobenzene and
styrene) conversions was determined by comparing
their contents in the initial mixture and in the samples
taken during the catalytic reaction. The yields of reac-
tion products, stilbene and diphenyl, were calculated as
ratios between their contents in the samples and the the-
oretically possible amount equivalent to the content of
bromobenzene in the initial mixture.
Conversely, when we pass from tributylamine to
less basic dibutylamine (Bu2NH), the kinetics of the
reaction changes substantially (Fig. 1c). The reaction
turnover frequency increases more than twofold, and
the number of reaction turnovers during 10 h also
increases two times. It follows that the effectiveness of
binding hydrogen bromide does not depend on reagent
basicity; that is, it is determined by the kinetic rather
than thermodynamic factor. At the same time, these
results are substantially different from those obtained
by Köhler et al. [8]. In that work, the Heck reaction was
performed in molecular solvents (N-methylpyrrolidone
and N,N-dimethylacetamide), and the number of reac-
tion turnovers for Pd deposited on activated carbon was
3 times larger in the presence of NaOAc than in the
presence of Bu3N as a base.
The reaction mixture was analyzed for palladium by
the MALDI-TOF method. 1H NMR spectra were used
to qualitatively analyze the initial and regenerated IL;
the spectra were recorded on an MSL-400 Bruker
instrument at a 400.13 MHz frequency.
Such discrepant results can hardly be explained by
differences in the properties of the carbon carriers, acet-
ylene soot in this work and activated carbon in [8]. To
interpret these results, it must be borne in mind that the
reaction system was heterophase in both experiments.
It consisted of the liquid phase containing dissolved
reagents and products and the solid with the catalyti-
cally active component immobilized in its narrow
pores.
RESULTS AND DISCUSSION
The Morphology of Pd Metal in Carbon Carrier
Mesopores
According to the transmission electron microscopy
and small-angle X-ray scattering data, reduced palla-
dium in the samples 0.5% Pd/C, 5% Pd/C, an 20% Pd/C
formed nanosized crystals with the unit cell parameter
0.389 nm, which corresponded to massive Pd metal.
The mean size of Pd particles in all the three samples
was ~4–5 nm, and the size of 90% of metal particles did
not exceed 10 nm. Note that the metal particle size and
carrier pore diameter distribution curves were closely
similar, which was evidence that Pd nanocrystals pre-
dominantly formed within carbon carrier mesopores.
As distinct from molecular liquids, ionic liquids are
characterized by the presence of nanosized structured
regions, which has very serious consequences for diffu-
sion processes in these media. Moreover, it was shown
in [9] that polar and nonpolar domains were structurally
separated in nanostructured regions of ILs with non-
symmetric alkyl substituents. As a consequence, ILs
show dual behavior, they behave as low-polarity media
Such a sterically hindered formation of the active with respect to nonpolar molecules, whereas the trans-
metal phase determines its nanostructured character port of polar and ionic particles of similar sizes is sub-
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THE HETEROGENEOUS CATALYTIC HECK REACTION IN AN IONIC LIQUID
2241
Conversion, %
40
(a)
Conversion, %
40
(b)
1
1
2
3
20
20
2
3
0
2
4
6
8
10
12
τ, h
0
2
4
6
8
10
12
τ, h
Conversion, %
100
(d)
Conversion, %
100
(c)
1
2
80
60
40
20
1
2
80
60
40
20
3
3
0
3
6
9
12
τ, h
0
3
6
9
12
15
τ, h
Fig. 1. Dependences of (1) bromobenzene conversion, (2) stilbene yield, and (3) biphenyl yield on reaction duration. Reaction con-
ditions: 0.1 mol % Pd, base (a) sodium acetate, (b) tributylamine, (c) dibutylamine, and (d) dibutylamine with preliminary catalyst
activation in the IL in the presence of dibutylamine.
stantially decelerated in them. This is likely the main
factor in the kinetics of liquid-phase heterogeneous
catalysis in ionic solvents. Indeed, the viscosity of ionic
liquids is 1–2 orders of magnitude larger than that of
molecular liquids. Accordingly, the effective diffusion
coefficients of dissolved molecules are 1–2 orders of
magnitude smaller in them. For this reason, when a liq-
uid-phase heterogeneous catalytic reaction is per-
formed in an ionic solvent, it is quite possible that the
diffusion transfer of reagents and products in catalyst
pores filled by the IL rather than chemical transforma-
tion becomes the limiting reaction stage.
It was shown in [10] that the imidazole IL as a
medium for performing the Heck reaction in the pres-
ence of palladium deposited on SiO2 caused the transfer
into solution of larger amounts of active Pd compared
with molecular dimethylformamide. Accordingly, this
increased the rate of the reaction. At the same time,
when similar heterogeneous catalysts such as Pd/C,
Pd/SiO2, Pd/Al2O3, and Pd/Mg(OH)2 · Al(OH)3 and the
tetrabutylammonium IL are used, palladium is trans-
ferred into the liquid phase in small amounts, and such
solubilized palladium does not catalyze the Heck reac-
tion [11]. Clearly, the heterogeneous or homogeneous
character of the Heck reaction is determined in each
particular case by the mechanism and conditions of
deposited Pd activation. The generally accepted activa-
tion mechanism includes the oxidative addition of aryl
Clearly, a key question is that of whether the reac-
tion occurs on the surface of the catalyst by the classic
heterogeneous catalysis scheme or it is quasi-homoge-
neous, that is, occurs in the volume of the solvent. This halide to Pd(0) with the formation of a soluble form of
question has repeatedly been discussed in the literature, Pd(II). The interaction of palladium with this reagent is
considered the only method for Pd activation, whereas
other reaction mixture components, that is, the polar
solvent and the base, are assumed to play a secondary
role of stabilizing agents [8, 12].
The results obtained in this work, however, show
that the nature of the IL solvent substantially influences
the activation of the Pd/C catalyst of the Heck reaction.
The kinetic curves shown in Figs. 1c and 1d were
obtained using two different regimes for performing the
process. In the first case (Fig. 1c), the reaction was per-
formed by the introduction of all reaction mixture com-
ponents into the tetraalkylammonium IL solvent,
including bromobenzene, styrene, dibutylamine, and
but no unambiguous answer to it was given [1]. More-
over, the experimental data obtained for different heter-
ogeneous catalysts and different solvents are fairly dis-
crepant. For instance, it was convincingly shown in [8]
that the rate of the Heck reaction in molecular solvents
was determined by the concentration of active Pd-con-
taining particles in the liquid phase; these particles
were formed in the interaction of initial aryl halide with
deposited palladium and were transferred into solution
but again became deposited on the surface of the carrier
after reaction termination. In other words, the solid
Pd/C catalyst in this reaction is only a source of palla-
dium, but is inactive itself.
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ASLANOV et al.
the Pd/C catalyst. The kinetic curves obtained in this
experiment are not typical of heterogeneous catalytic
reactions. They contain a distinct inflection point, and
the rate of the reaction shows no tendency to decrease
even at deep reagent transformation degrees and almost
coincides with the initial reaction rate. Such a shape of
kinetic curves is obvious evidence of reaction accelera-
tion with time, that is, of catalytic component activation
in interactions with the reaction medium.
In the second experiment (Fig. 1d), the Pd/C catalyst
was heated at 140°C for several hours in the IL solvent
in the presence of only Bu2NH. After this, bromoben-
zene and styrene were introduced and the reaction was
performed in the usual way. After such a preliminary
activation in the presence of amine, the initial rate
increased more than twofold, the kinetic curves did not
contain inflection points, and the effectiveness of the
catalytically active particle was constant as follows
from the shape of the kinetic curves. Note that the effect
of preliminary activation in the presence of Bu2NH is
the stronger the larger the excess of amine over palla-
dium. In a control experiment, when the Pd/C catalyst
was preliminarily heated in the IL in the absence of
amine, no activation of the catalytic system was
observed.
CONCLUSIONS
To summarize, the experimental results obtained in
this work are evidence in favor of the truly heteroge-
neous mechanism of the Heck reaction between bro-
mobenzene and styrene catalyzed by palladium depos-
ited on mesoporous soot. The presence of amine in the
IL at the stage of the preliminary activation of the Pd/C
catalyst is a necessary condition for creating a station-
ary concentration of active centers on which the Heck
reaction occurs. At the same time, the question of the
sequence in which the oxidative addition of aryl halide
to palladium and amine ligand addition to the complex
formed take place requires additional studies.
ACKNOWLEDGMENTS
This work was financially supported by the Federal
Special-Purpose Programs “Studies and Developments
in Priority Directions of the Development of the Sci-
ence and Technology Complex of Russia in 2007–
2012,” lot no. 7 “The Technology for Creating Mem-
branes and Catalytic Systems,” project no. 2007-2-1.3-
28-01-534.
The activation effect is fully retained when the reac-
tion is repeatedly performed in the same reaction sys-
tem, even without the removal of reaction products
from it. This is of importance for the understanding of
the mechanism of Pd/C catalyst activation in the IL.
After a 100% conversion of bromobenzene (see Fig. 1d),
standard amounts of bromobenzene, styrene, and dibu-
tylamine were again introduced into the system, and the
reaction began in the second cycle at the same rate as in
the first one.
REFERENCES
1. D. Astruc, L. Feng, and J. R. Aranzaes, Angew. Chem.,
Int. Ed. Engl. 44, 7852 (2005).
2. I. P. Beletskaya and A. V. Cheprakov, Chem. Rev. 100,
3009 (2000).
3. A. D. Hea and N. M. Jackson, J. Phys. Org. Chem. 15,
52 (2002).
4. V. Parvulescu and J. Hardacre, Chem. Rev. 107, 2615
(2007).
An additional argument in favor of the suggestion
made above according to which both the activation of
palladium and the chemical reaction proper in the tet-
raalkylammonium IL, as distinct from molecular sol-
vents and the imidazole IL, occurred in catalyst meso-
pores rather than in solution volume is the following
observation. The Heck reaction did not occur in the IL
in which Bu2NH and the Pd/C catalyst were heated at
140°C and from which the catalyst was removed by fil-
tration; that is, no Pd-containing particles were present
in the filtrate, which was also substantiated by its mass
spectrometric analysis. These data do not rule out the
possibility of the activation of deposited palladium by
the oxidative addition of aryl halide to it, but this reac-
tion in the tetraalkylammonium IL requires the pres-
ence of the amine component. At the same time, the
occurrence of the Heck reaction, at least in the system
studied in this work, inside catalyst pores is obvious.
5. L. A. Aslanov, M. A. Zakharov, and N. L. Abramycheva,
Ionic Liquids in a Series of Solvents (Mosk. Gos. Univ.,
Moscow, 2005) [in Russian].
6. V. Calo, A. Nacci, A. Monopoli, et al., Organometallics
23, 5154 (2004).
7. S. Klingelhofer, W. Heitz, A. Greiner, et al., J. Am.
Chem. Soc. 119, 10116 (1997).
8. R. G. Heidenreich, J. G. E. Krauter, J. Pietsch, and
K. Kohler, J. Mol. Catal. A: Chem. 182–183, 499 (2002).
9. C. Chiappe, Monatsh. Chem. 138, 1035 (2007).
10. K. Okubo, M. Shirai, and C. Yokoyama, Tetrahedron
Lett. 43, 7115 (2002).
11. B. M. Choudary, S. Madhi, N. S. Chowdari, et al., J. Am.
Chem. Soc. 124, 14127 (2002).
12. S. A. Forsyth, H. Q. N. Gunaratne, C. Hardacre, et al.,
J. Mol. Catal. A: Chem. 231, 61 (2005).
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