Dependence of the Heck coupling in aqueous microemulsion by supported palladium acetate on the surfactant and on the hydrophobicity of the support

Article abstract of DOI:10.1016/j.molcata.2010.03.012

Heck coupling of aqueous microemulsions of bromobenzene and styrene by palladium acetate entrapped within hydrophobicitized silica sol-gel is associated with the formation of supported Pd(0) nanoparticles that catalyze the process. The efficiency of the coupling strongly depends on the hydrophobicity of the sol-gel matrix. In a series of experiments in which the support was prepared by co-condensation of tetramethoxysilane [Si(OMe)₄] and ca. 5% of either alkyl or aryl alkoxysilanes [R_n(OR′)_4-n] the activity of the immobilized catalyst increased along with the increasing length of the hydrophobic alkyl chain R in the alkylalkoxysilane as well as with the increase of n. Hydrophobic aryl groups are superior to alkyl ones but electron-withdrawing substituents on the aryl moieties lower the reactivity. The rate of the coupling process depends also on the nature of the surfactant used to solubilize the reactants. The order of rates is anionic < non-ionic < cationic surfactants. The metallic nanoparticles can be recycled and reused for at least 4-6 further catalytic runs without any loss of catalytic activity.

Full text of DOI:10.1016/j.molcata.2010.03.012

Journal of Molecular Catalysis A: Chemical 323 (2010) 65–69
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Dependence of the Heck coupling in aqueous microemulsion by supported
palladium acetate on the surfactant and on the hydrophobicity of the support
Alina Rozin-Ben Barucha, Dmitry Tsvelikhovskya, Michael Schwarzeb, Reinhard Schomäckerb,
Monzer Fanunc, Jochanan Bluma,∗
a Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem 91904, Israel
b Institut für Chemie, Technische Universität Berlin, Strasse des 17. Juni 124, D-10623 Berlin, Germany
c Faculty of Science and Technology, Department of Colloids and Surface Reasearch, Al-Quds University, 51000 East Jerusalem, Palestinian Authority
a r t i c l e i n f o
a b s t r a c t
Article history:
Heck coupling of aqueous microemulsions of bromobenzene and styrene by palladium acetate entrapped
within hydrophobicitized silica sol–gel is associated with the formation of supported Pd(0) nanoparticles
that catalyze the process. The efficiency of the coupling strongly depends on the hydrophobicity of the
sol–gel matrix. In a series of experiments in which the support was prepared by co-condensation of
tetramethoxysilane [Si(OMe)4] and ca. 5% of either alkyl or aryl alkoxysilanes [Rn(ORꢀ)4−n] the activity of
the immobilized catalyst increased along with the increasing length of the hydrophobic alkyl chain R in
the alkylalkoxysilane as well as with the increase of n. Hydrophobic aryl groups are superior to alkyl ones
but electron-withdrawing substituents on the aryl moieties lower the reactivity. The rate of the coupling
process depends also on the nature of the surfactant used to solubilize the reactants. The order of rates
is anionic < non-ionic < cationic surfactants. The metallic nanoparticles can be recycled and reused for at
least 4–6 further catalytic runs without any loss of catalytic activity.
Received 6 December 2009
Received in revised form 10 February 2010
Accepted 7 March 2010
Available online 12 March 2010
Dedicated to the memory of Professor Dr.
Herbert Schumann.
Keywords:
Heck coupling
Microemulsion
Palladium
Silica sol–gel
Sustainable chemistry
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
been investigated and understood [8,20]. We have now investi-
gated the influence of two such factors on the Heck coupling: the
Public concern about the large amounts of harmful solvents
used in organic synthesis and catalysis led to substantial efforts to
replace the traditional media by water. Since many organic reagents
are insoluble in aqueous media a variety of methods have been
investigated for the solubilization of lipophilic substrates [1]. Our
contribution to this field was the development of a unique three
phase emulsion/sol–gel system for transport and catalysis (EST). In
this system the catalyst is entrapped within the ceramic matrix and
the substrates are in the state of an emulsion [2], thus, enabling
facile recovery and recycling of the catalyst. Replacement of the
emulsion by a microemulsion [3] converts the reaction medium
into a highly active system for catalytic hydrogenation of alkenes,
arenes, carbonyl- and nitro-compounds, for hydroformylation and
for cross-coupling reactions [4–7]. Among the C–C couplings that
have been performed in water and gained much interest in the
industry are the Heck and Suzuki reactions. (For some critical dis-
cussions see Refs. [8–19].) While the mechanistic features of these
processes in organic solvents have been studied in depth some of
the factors that affect the reactions in aqueous media have not yet
effect of the hydrophobicity of the sol–gel matrix in which the cat-
alyst is entrapped, and the nature of the surfactant used to prepare
the microemulsions.
2. Experimental
2.1. Instruments
1H and 13C NMR spectra were recorded on a Bruker DRX-400
instrument in CDCl3. Mass-spectral measurements were performed
with a Hewlet-Packard model 4989A equipment with an HP model
5890 series II gas chromatograph. ICP-MS analyses were carried out
with a Perkin Elmer model ELAN DRC II instrument. A Micrometrics
ASAP 2020 instrument was used for BET-N2 surface area and pore
diameter measurements of the sol–gel matrices. Determination of
the micelle size by dynamic scattering experiments was performed
with the aid of a 1 W Nd:YAG laser (Compass 150, Coherent, USA)
and an ALV 5000/E autocorrelator (ALV, Germany). Gas chromato-
graphic measurements were made with a Hewlet-Packard model
Agilent 4890D equipped with a 15 m long capillary column packed
with bonded and crosslinked (5% phenyl)methyl polysiloxane [HP-
5]. Transmission electron microscopy was done with a scanning
transmission electron microscope (STEM) Tecnai G2 F20 (FEI Com-
∗
Corresponding author. Tel.: +972 2 6585329; fax: +972 2 6513832.
E-mail address: jblum@chem.ch.huji.ac.il (J. Blum).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.03.012

66
A.R.-B. Baruch et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 65–69
pany, USA) operated at 200 kV and equipped with EDAX-EDS for
identification of elemental compositions.
separated into two phases by addition of NaCl (2 g). The solid used
catalyst as well as the aqueous layer of the filtrate was extracted
with ether (2 × 40 ml), and the extracts were analyzed for palla-
dium by ICP. The extracts never contained more than 0.01 mg Pd.
The combined organic layers were dried (MgSO4) concentrated and
analyzed by both GC and 1H NMR and compared with authentic
samples of E- and Z-stilbene (3 and 4, respectively). The used sol–gel
entrapped catalyst which proved by TEM and XPS studies to contain
Pd(0) nanoparticles was washed with water (2 × 10 ml), in order to
remove remains of the surfactant, dried at 80 ◦C/0.5 Torr for 1 h and
then washed and sonicated for 10 min with CH2Cl2 (3 × 30 ml) and
redried to obtain constant weight. The dry catalyst was ready for
use in the next run.
2.2. Chemicals
Bromobenzene, styrene, 1,1-diphenylethylene, l-proline, E-
and Z-stilbene, (tetramethoxy)silane [TMOS], (tetraethoxy)silane
[TEOS], (4-chlorophenyltriethoxy)-silane and palladium acetate
were purchased from the Sigma–Aldrich company. Propyl-,
octadecyl-, 1-naphthyl- and (4-methylphenyltrimethoxy)silane,
methyl-, ethyl-, iso-butyl (octyltriethoxy)silane [OTEOS] and
(phenyltriethoxy)silane, dimethyl- and (diphenyldiethoxy)silane,
trimethyl- and (triphenylethoxy)silane were obtained from ABCR
(Glest, Inc.). The surfactants were obtained from the follow-
ing sources: sodium dodecyl sulfate (SDS – from Riedel de
Haën),
(Marlipal 24/70 – from the Sasol Co.), dodecanol polyoxyethyleneg-
lycol ether (23-EO) (Brij-35 from Caledon Laboratories,
C
12–C14 alcohols polyoxyethyleneglycol ethers (7-EO)
3. Results and discussion
–
3.1. Effect of the hydrophobicity of the sol–gel support
Ontario), polyoxyethanyl-1,2,3-propanetriol-1-octadecanoate-2-
hexadecanoate (EO-20), ethoxylated ␣-mono-di-glyceride (EMDG)
– from BASF Corp., Gurnee, Illinois, sucrose laurate (L-1695 –
from Mitsubishi-Kasai Food Corp., Mie, Japan), polyoxyethanyl
When a microemulsion of bromobenzene, styrene, sodium
dodecyl sulfate and n-propanol was placed in a pressure ves-
sel together with sol–gel material (prepared from prehydrolyzed
tetramethoxylsilane, Pd(OAc)2 and a solution of aqueous K2CO3),
and the mixture was heated with stirring for 24 h at 80 ◦C, no cou-
pling reaction took place. However, after replacement of ca. 5% of
the TMOS by (octyltriethoxy)silane a quantitative yield of stilbenes
(99% of the E- and 1% of the Z-isomer) was obtained already after
90 min at 66 ◦C. This observation led us to systematically study the
effect of the hydrophobicity of the sol–gel support of the catalyst
on the Heck coupling shown in Scheme 1.
␣-tocopher-6-yl sebacate (EO-13) (PTS
– as 15% in H2O),
and dodecyltrimethylammonium bromide (DTAB) as well as
cetyltrimethylammonium bromide (CTAB) – were purchased from
the Sigma–Aldrich company.
2.3. Preparation of the catalyst: entrapment of Pd(OAc)2 within
hydrophobicitized sol–gel
Tetramethoxysilane (TMOS, 3.6 ml, 24.2 mmol) in MeOH (2.4 ml,
59.3 mmol) was hydrolyzed with triply distilled water (TDW,
2.0 ml, 111 mmol) by magnetically stirring the mixture at 20 ◦C
for 10 min. Separately, the hydrophobic alkoxysilane monomer
RnSi(ORꢀ)4−n where R = alkyl or aryl; Rꢀ = Me, Et; n = 1–3 (6.68 mmol)
was hydrolyzed similarly by stirring with the respective carbinol
(MeOH or EtOH, 73 mmol) and TDW (22 mmol). The solutions of the
two hydrolyzed silicon compounds were mixed and stirred mag-
netically at room temperature for 10 min and added to a solution of
Pd(OAc)2 (30 mg, 0.134 mmol) in 4 ml of CH2Cl2. Magnetic stirring
was continued as long as possible. Gelation was usually completed
within 24 h at room temperature. The resulting gel was dried at
80 ◦C at 0.5 Torr for 24 h, then washed with warm CH2Cl2 (3 × 10 ml)
and redried to obtain constant weight. The washing as well as the
immobilized catalyst was subjected to ICP analyses and divided into
two portions for the catalytic processes.
Some representative results are listed in Table 1. In all exper-
iments the main product (>97%) was the E-stilbene accompanied
by traces of the Z-isomer (usually <1%). In none of the experi-
ments has any 1,1-diphenylethene derivative been traced. The
coupling processes proved to take place only above a threshold
temperature below which hardly any reaction could be observed
during the 90 min of the experiments. This temperature which
extended over a very narrow limit of degrees was found to
depend in the first place on the hydrophobicity of the sol–gel
matrices in which the Pd(OAc)2 pre-catalyst was entrapped.
Table 1 indicates that in the series of experiments of alkyl func-
tionalized sol–gel the threshold temperature decreases along
with the increase of the hydrophobic-aliphatic chain length R
(see entries 1 and 4–8). Multiple functionalization of the sol–gel
(i.e., using monomers RnSi(OX)4−n where n = 2 or 3 for the co-
condensation with TMOS) further increases the hydrophobicity
and decreases the threshold temperature (cf. entries 1–3). Com-
parison of the effect of aliphatic and aromatic hydrophobic
groups, R, indicates the higher efficiency of the aromatic ones.
(See also Ref. [21] for some analogous catalyses that take place
within mesoporous silicates.) We assume that this phenomenon
is associated with the ability of aromatic substrates to form
␲-interactions with the ceramic matrices which are stronger
than the van-der-Waals forces that bind aliphatic groups to the
support. Hence, the application of (diphenyldiethoxy)silane and
(triphenylethoxy)silane used to prepare the supports in entries
10 and 11, respectively, gave the best results among the couplings
studied. Entries 9, 12 and 13 reveal that the efficiencies of the
immobilized catalyst depend also on the electronic nature of
the hydrophobic groups R. Thus, a decrease in the threshold
temperature (and an increase in the reactivity) was observed
when the hydrophobicitization was introduced by the monomers
(4-chlorophenyltriethoxy)silane < (phenyltriethoxy)silane < (4-
tolyltrimethoxy)silane.
For comparative experiments, Pd(OAc)2 was also entrapped
within hydrophilic sol–gel free of the hydrophobic components.
2.4. Preparation of the microemulsions
A mixture of bromobenzene (1) and styrene (2) (1.34 mmol
of each) was stirred magnetically for 20 min at 23 ◦C with
TDW (89.3 wt.%), the respective surfactant (3.3 wt.%) and the co-
surfactant n-PrOH (6.6 wt.%). The resulting microemulsions were
visually clear.
2.5. General procedure for the Heck coupling reactions
To a magnetically stirred mixture of the above immobilized
Pd(OAc)2 (that contained 0.067 mmol palladium) was added the
freshly prepared microemulsion of 1 and 2 (1.34 mmol of each of
them) together with K2CO3 (0.28 g, 2 mmol). The reaction flask was
then introduced into a thermostat preheated to the required tem-
perature. After the needed length of time, the mixture was cooled
to 20 ◦C and the sol–gel material was filtered off. The filtrate was
As shown previously [6] the used catalyst within octylated
sol–gel is recyclable and can be applied at least in 4–6 consecu-

A.R.-B. Baruch et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 65–69
67
Scheme 1.
Table 1
Dependence of the formation of stilbenes on the hydrophobicity of the silica sol–gel support of the palladium acetate catalyst.a.
Entry
R in Scheme 1
n in Scheme 1
Yield of stilbenes (%) at various temperatures (◦C)b,c
50
55
60
63
66
70
3
75
1
2
3
4
5
6
7
8
9
10
11
12
13
14
CH3
CH3
CH3
C2H5
1
2
3
1
1
1
1
1
1
2
3
1
1
1
–
–
–
–
–
–
–
–
–
5
–
–
–
–
5
–
3
41
–
–
1
1
9
27
q
–
14
q
2
1
–
q
qd
7
q
32
q
35
q
C3H7
(CH3)2CHCH2
CH3(CH2)7
CH3(CH2)17
C6H5
C6H5
16
4
q
qe
q
6g
26
qg
nd
nd
3
f
3
f
13
14
nd
nd
nd
f
C6H5
f
4-C6H4CH3
59
nd
10
q
nd
34
4-C6H4Clf
30
q
92h
f
1-C10H7
a
Reaction conditions: microemulsion of 1.34 mmol of 1 and 1.34 mmol of 2, 5 mmol of SDS, 4.8 mmol of PrOH, 39 ml of H2O, 2 mmol K2CO3, 0.067 mmol of Pd(OAc)2
entrapped within sol–gel material from 24 mmol hydrolyzed TMOS and 7 mmol RnSi(OX)4−n where X is either Me or Et; reaction time 90 min.
b
The data refer to the first run.
c
q = 96–100%; the missing percentage refers to unreacted starting materials. nd: not determined.
d
78 ◦C.
Second run.
e
f
1-C10H7: 1-naphthyl. One milliliter of CH2Cl2 was added.
g
57 ◦C.
h
After 3 h at 90 ◦C.
tive runs. We now find that the catalyst entrapped within the other
hydrophobicitized sol–gels behave likewise.
nearly constant. At higher conversion the catalyst is no longer sat-
urated and the reaction rate decreases. In Fig. 2 the rate/conversion
profile for the second run of the stilbene formation is shown. The
rates were calculated from an Eley–Rideal rate law fitted to the
experimental conversions shown in Fig. 1.
3.2. Involvement of Pd nanoparticles in the reactions
An alternative explanation based on active palladium species
leaching into the aqueous medium seems rather unlikely since ICP
analysis found the entire palladium stays within the sol–gel mate-
The dependence of the yields of the coupling reactions on the
hydrophobicity of the sol–gel matrices is notable. Since TEM anal-
ysis reveals that in all the experiments listed in Table 1 palladium
nanoparticles are formed [22,23] we suggest that these particles
interact with the differently hydrophobicitized silicates. Thus, dif-
ferently modified catalysts may have been generated. We have
shown that much of the sol–gel entrapped Pd(0) nanoparticles are
obtained already during the entrapment of the Pd(OAc)2 within
the hydrophobic sol–gel material [22], but further active species
are likely to be formed (as well as to be removed through aggrega-
tion) during the coupling process. Support for this assumption was
obtained from some kinetic measurements of the formation of the
stilbene shown in Scheme 1. The composition/time profiles shown
in Fig. 1 clearly indicate an induction period in the first run during
which some additional active catalyst may have been generated. In
the second, and in further runs, this induction period disappears,
probably because at this point the entire pre-catalyst has already
turned into enough active species. After the induction period the
formation of the stilbenes increases in a nearly linear fashion with
time up to 80% conversion and slows down afterwards. This behav-
ior can be explained considering that the rate determining step of
the Heck coupling is the oxidative addition of the aryl halide to
the palladium. During the linear course of the reaction the catalyst
is completely saturated by the aryl halide and the reaction rate is
Fig. 1. Conversion/time profiles for the formation of stilbenes from 1 and 2 in water
by Pd(OAc)2 within octylated sol–gel under EST conditions in the first (ꢀ) and second
(ꢁ) runs at 65 ◦C. The solid line was obtained from the Eley–Rideal rate law fit.

68
A.R.-B. Baruch et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 65–69
Table 3
Sizes of some surfactant micelles determined by dynamic light scattering (DLS).a.
Entry
Surfactant
dmicelle (nm)
1
2
3
4
SDS
Marlipal 24/70
Brij-35
2.1
12.8
7.9
CTAB
5.4
a
The setup for the determination of the micelle sizes consisted of a 1 W Nd:YAG
laser and an ALV 5000/E autocorrelator. The analysis of the recorded correction
functions was done by the comulant method.
10, respectively) initiated the formation of stilbene. Yet much bet-
ter results were obtained by the application of hydrophobicitized
sol–gel supports. Thus, e.g., in the presence of phenylated sol–gel,
SDS, Marlipal 24/70 and CTAB yielded already after 1.5 h at 57 ◦C,
6, 14 and 28% of the desired products, respectively (Table 2, entries
3, 6 and 12). Since the EST process relies on the ability of the sur-
factant to transfer the substrates onto the immobilized catalyst,
it is obvious that the catalytic process is affected by the adsorp-
tion capacity of the sol–gel matrix. Because of electronic reasons
the anionic SDS is actually repelled by the negatively charged non-
hydrophobicitized TMOS polymer. This effect is less pronounced
by the non-ionic Marlipal 24/70 and actually reversed when the
cationic surfactants are applied [21,25,26]. Working with a micro-
porous sol–gel material that enables the recycling of the catalyst,
one should also take into consideration that EST conditions are
limited to surfactant micelles (and substrates) that are capable of
penetrating freely the pores of the ceramic material. The initial
average pore diameters of the octylated and phenylated sol–gel
supports were determined by BET measurements [6] to be 31.5
and 28 Å, respectively. Thus, while the surfactant micelles with rel-
atively small sizes shown in Table 3 can be applied in the Heck
coupling under EST conditions, larger ones proved useless. While
Marlipal 24/70, e.g., is quite effective with conversions of 14–15%,
the closely related, but more voluminous ethoxylated ␣-mono-di-
glyceride (EMDG) micelles, do not lead to any stilbenes even if
the reaction temperature is raised to 90 ◦C (cf. a similar size effect
of the substrates on the disproportionation of hydroaromatics by
sol–gel entrapped RhCl3·R4N [27]). Also, the commercially available
polyoxyethanyl ␣-tocopher-6-yl sebacate (PTS) surfactant which
promotes the Heck and other coupling reactions at room temper-
ature in sol–gel free systems [8], cannot be applied to our process
under the EST conditions. The sizes of the active surfactant micelles
(without the substrates) have been determined by dynamic light
scattering (DLS). The fact that their diameters increase in the fol-
lowing order: SDS < CTAB < Brii 35 < Marlipal 24/70 reveals that the
size of the micelles is not the only factor that has an effect on the
yields disclosed in Table 2.
Fig. 2. Rate/conversion profile for the formation of stilbenes for the second run at
65 ◦C.
rial. No active palladium species could be traced in the filtered
reaction mixture after the second run.
It is notable, that the coupling rate of 1 and 2 in toluene at 100 ◦C,
by immobilized bis-palladium l-prolinate [22], which does not gen-
erate detectable Pd(0) nanoparticles, is hardly affected by changes
in the hydrophobicity of the ceramic support. In fact, the coupling
of 1 and 2 by the prolinate catalyst [24] in aromatic solvents (e.g.,
toluene) gives the best results in a hydrophilic sol–gel prepared
from TMOS alone.
3.3. Effect of the structure of the surfactant
Apart from the effect of the hydrophobicity of the sol–gel sup-
port on the Heck coupling, we investigated the dependence of the
reaction on the nature of the surfactant. When the anionic SDS,
used in our initial experiments, was replaced by either the non-
ionic polyethoxylated C12–C14 alcohol (Marlipal 24/70) or by the
cationic decyl- or cetyltrimethylammonium bromide, the coupling
was accelerated. While the attempts to perform the coupling of 1
and 2 with an SDS based microemulsion of the reactants in the
presence of hydrophilic sol–gel at 80 ◦C for 24 h, gave negative
results (Table 2, entry 1), the replacement of this surfactant by the
non-ionic or by the cationic ammonium bromides (entries 4, 9 and
Table 2
Dependence of the coupling of bromobenzene and styrene under the EST conditions
on the surfactants.a.
Entry
Surfactantb
Lipophilic
function
Reaction
Yield after
1.5 h (%)
temperature (◦C)
1
2
3
4
5
6
7
8
9
SDS
SDS
SDS
Marlipal 24/70
Marlipal 24/70
Marlipal 24/70
Brij-35
L1695
DTAB
CTAB
CTAB
CTAB
None
CH3(CH2)7
C6H5
None
CH3(CH2)7
C6H5
C6H5
C6H5
None
None
80
63
57
80
63
57
57
57
80
80
63
57
0c
4
6
4. Conclusions
2
Palladium acetate-catalyzed Heck coupling of bromobenzene
and styrene under microemulsion/sol–gel transport conditions in
aqueous media depends both on the hydrophobicity of the sol–gel
material, as well as on the electronic nature of the surfactant
micelles. Hydrophobicitation of the ceramic support by phenyl
moieties is superior to that by aliphatic groups. The Pd(OAc)2
decomposes during its immobilization and during the catalytic
coupling into sol–gel stabilized Pd(0) nanoparticles. The latter are
assumed to react with the various hydrophobicitizing additives.
Cationic and non-ionic surfactants are more efficient than anionic
ones. The surfactant micelles must be small enough to be able to
freely penetrate the pores of the sol–gel within which the catalyst
is entrapped.
15
14
8
5
12
9
10
11
12
CH3(CH2)7
C6H5
18
28
a
Reaction conditions: microemulsion of 1.34 mmol of each of 1 and 2, 1.42 g of
surfactant, 2.84 g PrOH, 38.5 g TDW, 15 mg Pd(OAc)2 entrapped within sol–gel from
24.2 mmol of TMOS and 6.68 mmol of the hydrophobic monomer RnSi(ORꢀ)4−n; heat-
ing with stirring for 1.5 h.
For the structures and sources of the surfactants see Section 2. The inactive
surfactants EMDG and PTS have not been incorporated in the table.
b
c
After 24 h.

A.R.-B. Baruch et al. / Journal of Molecular Catalysis A: Chemical 323 (2010) 65–69
69
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