Copper(II)- and palladium(II)-modified molecular sieve, a reusable catalyst for the Suzuki-Miyaura-coupling

Article abstract of DOI:10.1016/j.apcata.2014.07.002

Copper-palladium bimetallic catalysts supported on 4 ? molecular sieve were prepared with two step impregnation (TSI) and co-impregnation (CI). These materials were studied to determine the importance of the preparation method, the interaction between the Cu-Pd particles, the activity, selectivity and stability in Suzuki-Miyaura reaction. The key-role of the preparation method in the composition of the metal species on the surface of the support was pointed out with catalyst characterisation methods (ICP, BET, XRD, TEM, EDS and TPR). The relationship between the properties of surface material and the catalytic performance and stability were examined too. The two metals (Cu, Pd) were brought to close interaction on the surface of the support. A correlation between these interactions and improvement of stability were found and proven with surface analysis methods. We found that a Cu-Pd alloy with atom ratio 1:1 is responsible for the catalytic activity. Our catalyst TSI was active and selective during six catalytic cycles in Suzuki coupling, whilst the activity of the catalyst prepared with CI dropped after the sixth use. This loss was due to the absence of Cu-Pd 1:1 alloy and the growth of the particle size.

Full text of DOI:10.1016/j.apcata.2014.07.002

Accepted Manuscript
Title: Copper(II)- and palladium(II)-modified molecular sieve,
a reusable catalyst for the Suzuki-Miyaura-coupling
´
Author: Anna Fodor Zoltan Hell Laurence Pirault-Roy
PII:
S0926-860X(14)00421-9
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.07.002
APCATA 14895
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Applied Catalysis A: General
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Revised date:
Accepted date:
11-4-2014
18-6-2014
2-7-2014
Please cite this article as: A. Fodor, Z. Hell, L. Pirault-Roy, Copper(II)-
and palladium(II)-modified molecular sieve, reusable catalyst for
a
the Suzuki-Miyaura-coupling, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.07.002
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Copper(II)- and palladium(II)-modified molecular sieve, a reusable catalyst
for the Suzuki-Miyaura-coupling
Anna Fodor1,2, Zoltán Hell1*, Laurence Pirault-Roy2
1 Department of Organic Chemistry and Technology, Budapest University of Technology and
Economics, M egyetem rkp. 3. H-1111 Budapest, Hungary
2 Institute of Chemistry of Poitiers: materials and natural resources (IC2MP) UMR-CNRS
7285, University of Poitiers, 4, rue Michel Brunet (Bât B27) 86073 Poitiers Cedex, France
(*) corresponding author: zhell@mail.bme.hu
Keywords
Heterogeneous catalysis, Cu-Pd bimetallic catalysis, Suzuki reaction, recycling,
stability
Abstract
Copper-palladium bimetallic catalysts supported on 4Å molecular sieve were prepared
with two step impregnation (TSI) and co-impregnation (CI). These materials were studied to
determine the importance of the preparation method, the interaction between the Cu-Pd
particles, the activity, selectivity and stability in Suzuki-Miyaura reaction. The key-role of the
preparation method in the composition of the metal species on the surface of the support was
pointed out with catalyst characterization methods (ICP, BET, XRD, TEM, EDS and TPR).
The relationship between the properties of surface material and the catalytic performance and
stability were examined too. The two metals (Cu, Pd) were brought to close interaction on the
surface of the support. A correlation between these interactions and improvement of stability
were found and proven with surface analysis methods. We found that a Cu-Pd alloy with atom
ratio 1:1 is responsible for the catalytic activity. Our catalyst TSI was active and selective
during 6 catalytic cycles in Suzuki coupling, while the activity of the catalyst prepared with
CI dropped after the sixth use. This loss was due to the absence of Cu-Pd 1:1 alloy and the
growth of the particle size.
Introduction
Today, catalysis is at the centre of major social concerns about energy, environment and
sustainable development. In ca. 80 to 85% of all catalytic processes, heterogeneous catalysis
Page 1 of 27

is employed [1]. Thus preparation, optimization, fine tuning of the heterogeneous catalysts are
in the centre of catalytic research.
The formation of novel carbon-carbon, carbon-nitrogen bond has an important role in both
theoretical and in synthetic organic chemistry as well. Over the last few decades, the
formation of carbon–carbon bonds by the transition-metal-catalyzed cross-coupling of carbon
nucleophiles with carbon electrophiles has evolved into a key synthetic approach for the
construction of complex organic molecules [2]. This reaction type enables the selective
connection of even highly functionalized substrates at positions defined by two leaving
groups of opposite polarity. Numerous cross-coupling procedures have been developed, for
example Suzuki [3], Stille [4], Sonogashira [5-6], Heck [7], and Negishi [8] reactions. The
importance of cross coupling reactions was confirmed in 2010 when the Nobel Prize in
Chemistry was awarded jointly to Richard F. Heck, Ei-ichi Negishi and Akira Suzuki ‘for
palladium-catalyzed cross couplings in organic synthesis’. A coupling reaction is therefore
chosen for a given application on the basis of the availability, stability, and price of the
required substrates and catalyst, as well as the efficiency, selectivity, and convenience of the
reaction protocol [9]. The palladium-catalyzed cross-coupling reaction between organoboron
compounds and organic halides or triflates is called as Suzuki coupling. It provides a powerful
and general methodology for the formation of carbon–carbon bonds. Several examples of
homogeneous [10-11] and heterogeneous [12-13] catalytic pathways are reported in the
literature. However it remains in the centre of the catalytic research as Suzuki reaction
provides a facile, selective way to a wide range of product. Mild reaction conditions are
applied and the reaction partners are easily available molecules. Moreover, the inorganic by-
product of the reaction is non-toxic and easily removable from the reaction mixture thereby
making the Suzuki coupling suitable not only for laboratories but also for industrial processes
[3].
Recently our research group studied monometallic catalysis with the use of metals
supported on 4Å molecular sieve (4A MS) in organic synthesis. A method for the dehydration
of oximes into nitriles using copper supported on 4A MS was developed [14]. CuII-4A MS
proved to be a very efficient catalyst which could be used in the reaction at least 10 times
without any loss of activity. This catalyst was used also for the selective conversion of nitriles
into amides [15]. Furthermore, with this catalyst we successfully carried out the so-called A3-
reaction – coupling of aldehydes, alkynes and amines yielding propargylamines [16].
Continuing this work we examined the possibility of the development a bimetallic catalyst on
4A MS support. The two metals we chose were copper and palladium. Our objective was to
tailor a catalyst containing noble-metal, which is not expensive as the loading of palladium is
Page 2 of 27

relatively low. To improve the advantageous properties of this catalyst with low palladium
content an inexpensive metal namely copper was introduced which has also good catalytic
properties as described above. The aim was to bring these two metals in close interaction
therefore to obtain better performance and stability of the bimetallic catalysts compared to the
monometallic analogues.
Metal catalysis has a dominant role in the modern organic chemistry. In particular, cross-
coupling reactions allow bond formations, which have previously been impossible to perform
[9,17]. Buchwald et al. reported that reactions with FeCl3 may in certain cases be significantly
affected by trace quantities of other metals, particularly copper [18]. Research interest in
bimetallic catalysts is mainly due to their tuneable chemical/physical properties by a number
of parameters like composition and morphostructure [19]. Compared to monometallic
catalysts, bimetallic or multimetallic catalysts may show an increased selectivity or better
resistance to deactivation. In the literature numerous bimetallic catalysts have been shown to
exhibit unique properties which are different from those of their monometallic counterparts
[20-24]. One of the most widespread methods for the preparation of supported metal catalysts
is wet impregnation (WI). Due to its easy operation and inexpensiveness, it has been widely
used for catalyst synthesis. WI requires the support to be put into a precursor solution to
achieve the loading of active components. For bimetallic catalysts requiring more than one
metal precursor, either co-impregnation (CI) or sequential impregnation (TSI) can be used.
Our catalysts were prepared both with TSI and CI. The surface properties of the prepared
catalysts were analyzed using different methods (ICP, TEM, EDS, TPR, XRD, and BET) so
that we were able to compare their properties. Besides the characterisation of the catalysts
application in organic synthesis were carried out too. Surface study of the used catalysts was
also performed.
Experimental
1. Reagents and materials
Sodium tetrachloropalladate(II) (98%) and phenylboronic acid (97%) were purchased
from Sigma-Aldrich Co. LLC. Molecular sieve 0.4 nm beads, copper(II) chloride (•99%) for
synthesis, iodobenzene (•99%) for synthesis and all further organic compounds were
purchased from Merck Ltd.
2. Catalyst preparation
Preparation of monometallic catalysts with wet impregnation
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To prepare the monometallic catalyst 1g of fine grounded 4Å molecular sieve (4A) was
stirred with the corresponding metal precursor (1 mmol of copper(II) chloride or 0.2 mmol of
sodium tetrachloropalladate(II)) in 100 ml of deionized water at room temperature for 12h.
Then the solid was filtered off, washed with deionized water, and dried at 120°C for 5 hours.
The targeted metal content of the catalyst thus obtained is 6.3 wt% copper (Cu-4A) and 2.1 wt
% palladium (Pd-4A).
Preparation of bimetallic catalysts with successive and co-impregnation
During successive impregnation (TSI) the monometallic catalyst (Cu-4A) was stirred with
sodium tetrachloropalladate(II) as described above.
In the co-impregnation method (CI), the two metal salts were added simultaneously to the
suspension of 4A in deionized water. The procedure was the same as described above. The
targeted Pd/Cu molar ratio was 0,2/1 in both cases.
3. Catalyst Characterisation
The metal content on the support was determined by ICP-OES analysis carried out on
Perkin-Elmer Optima 2000DV.
Surface area of 4A and the impregnated supports were measured to investigate their
changes after metal deposition. After preparing each material, they were dried in anaerobic
chamber for 12 h. Specific surface areas of samples were measured by nitrogen adsorption
and desorption at • 196°C with BET surface analyzer (Micromeritics Model TRISTAR 3000)
using BET-BJH method.
X-ray powder diffraction (XRD) patterns of samples were obtained with a PANanalytical,
Empyrean diffractometer using CuKα radiation from 10° to 70° (in 2θ) with 0,017° steps. The
identity of samples were checked by analyzing their XRD patterns with PAN-ICSD
(PANalytical version of the ICDD)
Dispersion of Cu and Pd particles on the surface of 4A were identified by TEM/EDS
(JEM-2100 LaB6 Transmission Electron Microscope). EDS mapping was performed to
visualize elemental and spatial information of Cu and Pd on the 4A surface.
The distribution and reducibility of Cu and Pd particles were investigated via TPR
experiment. Each catalyst – both mono- and bimetallic – after drying at 120°C for 5 hours
were introduced into a quartz glass reactor for the TPR analysis and examined in the
temperature range of 30–600°C with ramping of 5°C/min. After purging with argon 1% H2/Ar
pulses were passed through the system bed at the flow rate of 30 ml/min. The H2 consumption
was determined by TCD.
4. Typical procedure for Suzuki-Miyaura-coupling
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GC-MS measurements were performed on an Agilent 6890 N-GC-5973 N-MSD
chromatograph, using a 30 m × 0.25 mm Restek, Rtx-5SILMS column with a film layer of
0.25 μm. The initial temperature of column was 45°C for 1 min, followed by programming at
10°C/min. up to 310°C and a final period at 310°C (isothermal) for 17 min. The temperature
of the injector was 250°C. The carrier gas was He and the operation mode was splitless.
1H NMR spectra were made on BRUKER Avance-300 instrument using TMS as an
internal standard in CDCl3.
The catalyst was treated at 120°C for 1 hour before the reaction. Phenylboronic acid (1.5
mmol), iodobenzene (1 mmol), potassium carbonate (3 mmol) and the pretreated catalyst (0.1
g; 2,26 mol% Pd and 9,86 mol% Cu for TSI and 1,88 mol% Pd and 8,98 mol% Cu for CI)
were stirred in 5 ml refluxing ethanol for 1 hour. Then the solid was filtered out, and washed
with ethanol. The filtrate was evaporated. The residue was extracted three times with diethyl
ether and water. The organic phase was dried over anhydrous sodium sulphate, filtered and
1
the solvent was evaporated. The product was subjected to either GC-MS analysis and/or H
NMR. The filtered catalyst can be recycled after washing with water and acetone and drying
at 120°C for 1h.
Biphenyl was isolated as white powder. 1H NMR (300 MHz, CDCl3)
δ (ppm): 7.34-
7.37 (d, 2H); 7.41-7.46 (dd, 4H); 7.58-7.61 (d, 4H). GC: Rt: 12.101 min. MS (m/z): 154
(M+), 128, 115, 102, 76. p-Terphenyl was isolated as white powder. GC: Rt: 22.488
min. MS (m/z): 230 (M+), 202, 152, 115, 77. 4-Chloro-3-trifluoromethilbiphenyl was
isolated as transparent liquid. 1H NMR (300 MHz, CDCl3)
δ (ppm): 7.37-7.40 (1H, d);
7.43-7.46 (2H, t); 7.52-7.56 (3H, d); 7.63-7.65 (1H, dd); 7.87 (1H, d). GC: Rt: 13.802
min. MS (m/z): 256 (M+), 237, 221, 201, 152, 110, 75.
Hot filtration test: This test was developed to verify that no leaching of active phases
occured in the reactants’ solution that can lead to a homogeneous catalysis in addition to
heterogeneous one. Thus, the catalyst was treated at 120°C for 1 hour before the reaction.
Phenylboronic acid (1.5 mmol), iodobenzene (1 mmol), potassium carbonate (3 mmol) and
the pretreated catalyst (0.1 g) were stirred in 5 ml refluxing ethanol for 30 minutes. Then the
solid was filtered out. Potassium carbonate (3 mmol) was added again and the boiling was
continued for another 30 min. The reaction mixture was worked up as described before.
Results and discussion
Four catalysts were prepared with wet impregnation: Cu-4A, Pd-4A as monometallic
catalysts; Cu-Pd-4A-TSI and Cu-Pd-4A-CI as bimetallic catalysts. The catalysts were
characterized by ICP, BET, XRD, TEM, EDS and TPR.
Page 5 of 27

1. Catalysts Characterization
The ICP-OES analysis showed that the results were in accordance with the expected
values. A slightly higher metal content in the case of successive impregnation was observed
comparing to co-impregnation (Table 1). Each monometallic catalyst was also subjected to
the measurement to determine the real metal content on the support.
According to BET measurements the surface area of the support 4Å MS changed from
800 m2/g to 310 m2/g in the first case (TSI) and to 230 m2/g in the second case (CI) (Table 1).
This can be explained by the difference in the preparation method. In case of TSI it is a two-
step process whilst CI takes place in one step. The production in several steps allows the
sintering of the particles leading to more free sites and thus higher surface area.
The pore volume of the molecular sieve decreased from 0.3 cm3/g to 0.204 cm3/g in case
of TSI and to 0.142 cm3/g when the catalyst was prepared with CI for same reasons as above.
Nitrogen molecules cannot fit the pores of the support. The hysteresis at high P/P0
indicates pores with the following diameters: 26 Å (TSI) and 25 Å (CI) using BJH
measurements.
According to the IUPAC classification of adsorption isotherms for gas-solid equilibria
[25] the catalysts belong to Type IV. Characteristic features of the Type IV isotherm are its
hysteresis loop, which is associated with capillary condensation taking place in mesopores,
and the limiting uptake over a range of high P/P° [25]. Types IV characterize mesoporous
adsorbents with strong and weak affinities respectively. For lower temperatures they show
adsorption hysteresis [26].
XRD analysis was performed to identify the crystallized phases. Good correlation was
observed between the diffractogram of 4A and the diffractogram described in the PAN-ICSD
database. (98-000-9326 Zeolit A, Na exchanged, dehydrate) The diffractograms of the
bimetallic catalyst samples consisted of mostly the support’s signals (Figure 1. a). Besides the
4A molecular sieve’s peaks the signs of copper-palladium crystals were found, after zooming
to the value of 2θ between 40°-70°. In case of TSI copper-palladium species were found with
ratios: 1/1; 0.6/0.4. (98-010-3082; 98-016-6154) For CI two different copper-palladium solid
solutions were present: Cu-Pd 3/1; 0.6/0.4. (98-010-3084, 98-016-6153) (Figure 1. b).
In both catalysts two different morphologies of the support can be seen: amorphous and
stick. On the surface of both catalysts besides the well dispersed parts, agglomerated particles
were also found (Figure 2). Crystalline particles according to EDS analysis were defined to be
copper or palladium or copper-palladium co-crystals. At the amorphous phases copper and
palladium were present together (see supplementary data). As the result of the TEM analysis
Page 6 of 27

we can conclude that the two metals are present on the support together, single metallic
particles were found only in a few cases.
In case of TSI palladium and copper-palladium alloys were found on the support in
different atomic ratio: Cu : Pd = 1:3; 1:1; 2:1. In the majority of the mixed phase, copper and
palladium are in 1:1 atomic ratio. The particle size distribution study gave an average particle
size of 2.5 nm (Figure 3. a).
With co-impregnation crystals of copper and copper-palladium co-crystals were found in
different ratio: Cu : Pd= 7:3; 3:2; 3:1; 2:1; 2:3 according to the EDS analysis. The majority of
the particles had a composition of copper : palladium 2:1. An average particle size of 2.2 nm
was obtained (Figure 3. b).
On TSI sample the majority of the bimetallic particles consisted of Cu:Pd 1:1 (equal metal
content) while on CI the ratio of the metals was 2:1 (copper-rich alloy). The average particle
size was smaller in case of CI however the difference in this parameter was not significant
between the two samples (2.5 nm Cu-Pd-4A-TSI; 2.2 nm Cu-Pd-4A-CI). From the TEM
results we can conclude that the catalysts prepared with two methods have nearly same
average particle size whereas the distribution of metals is different.
To have more insight into the metal-metal interaction, TPR measurements were
performed. H2-TPR profiles are shown in 35 as hydrogen uptake as a function of heating
temperature for the monometallic catalysts and for the bimetallic catalysts as well. In each
case extra peaks occurred and the peaks shifted compared to the monometallic catalysts. The
catalysts prepared with two different methods showed different TPR profiles proving that the
preparation has a very important effect. From the TPR results we had another evidence that
the two metals are very close to each other on the support.
Cu-4A
According to the literature [27] the reduction of isolated Cu2+ species occurs in the two
following steps:
1
Cu2+ + H2 → Cu+ + H +
2
1
Cu+ + H2 → Cu0 + H +
2
While the reduction of copper oxide species occurs in a one-step mechanism:
CuO + H2 → Cu0 + H2O
The first reduction peak is between 155-270°C with a maximum at around 200°C, while the
second is between 360-600°C with a maximum of 525°C. According to the reduction steps
described in the literature [28-29] the peak at 200°C might be assigned to the reduction of
Page 7 of 27

several highly dispersed copper oxide species, which include isolated copper ions, weak
magnetic associates, and small two- and three-dimensional clusters. The high temperature
peak might be attributed to the reduction of the supported Cu+ ions (arising from partial
reduction of isolated Cu+) to Cu0. In our case the hydrogen consumption between 155-270°C
compared to the amount of copper on the support gives a H2:Cu molar ratio of 0.18. This
value is remarkably lower than the expected 0.5. The deviation can suggest three things: (i) A
part of the metal is already in oxidation state +1. (ii) A part of the metal is already completely
reduced to Cu0. (iii) A part of our metal is not accessible for reduction. The hydrogen
consumption between 360-600°C compared to the copper loading of the support gives a
H2:Cu molar ratio of 0.6, which is higher than the expected 0.5 value. It confirms the initial
presence of the Cu+ particles as seen from the first reduction peak although determination of
the exact Cu+ and Cu2+ ratio cannot be made with this TPR analysis. As the Cu+ species were
identified the next question was in which form they are. They can be present as Cu+ ions or
oxide form (Cu2O). In the literature it is reported that the reduction of Cu2O takes place at
high temperature, around 377°C [30]. At this temperature we have a wide reduction peak,
which may imply the reduction of Cu2O as well. Based on the area of both reduction peaks, it
is concluded that copper in Cu-4A exists both as Cu+ (Cu2O) and Cu2+.
Pd-4A
The reduction of isolated Pd2+ species occurs in the following step:
Pd2+ + H2 → Pd0 + 2H+
The reduction of palladium oxide species occurs also in a one-step mechanism [31] (there are
assumptions for two-step mechanism as well, but no evidence was found [32]):
PdO + H2 → 2Pd + H2O
It is important to consider that palladium absorbs hydrogen to form Pd hydride even at low
hydrogen pressure and room temperatures [33]. Therefore the amount of hydrogen consumed
during the experiment cannot determine the reduction state of palladium. However useful
information can be gained from the temperature of the reduction [34].
The only peak starts at room temperature and ends at 165°C. In the literature [21] the
peaks at 100°C and 400°C are assigned to the reduction of palladium(II) to palladium(0) in
two different environments. The reduction at lower temperature corresponds to solvated Pd2+
weakly interacted with the molecular sieve support. Higher temperature reduction peak refer
to the reduction of palladium atoms in oxide form, bound to the support. Reduction at higher
temperature did not occur during our experiment.
Cu-Pd-4A-TSI
Page 8 of 27

The profile corresponding to Cu-Pd-4A-TSI is shown in Figure 4. Two peaks at 50°C and
130°C are overlapped. In addition, a broad peak of hydrogen consumption was observed
between 240-500°C. The first peak is due to the reduction of palladium. As the hydrogen
consumption (area under the first peak between 30-180°C) is higher than the theoretical
consumption for the palladium reduction it is assumed that a part of the copper is reduced as
well at low temperature either to Cu+ or to Cu0. It is well reported in the literature that the
reduction peak at low temperature in case of copper-palladium bimetallic catalyst can refer to
the co-reduction of the two metals [35]. Canner et al. [36] proposed that the enhanced
reduction of copper species in the presence of Pd on the catalyst surface could be attributed to
hydrogen spillover from metal Pd to copper species. A negative peak at 60°C refers to the
presence of Pd-β-hydride. Its absence can refer to either well dispersed or partly alloyed Pd
species with Cu [37]. It is interesting to observe that the reduction temperature of supported
copper species in Cu-Pd-4A-TSI was diminished from 200°C (as seen in case of Cu-4A) due
to the presence of palladium. This modification indicates a close interaction between copper
and palladium species. The second, broad peak between 280-450°C can refer to the reduction
of Cu+ to Cu0. This second reduction step is also decreased compared to the monometallic
copper catalyst (Figure 4) indicating a redox cooperation between Pd and Cu.
Cu-Pd-4A-CI
An alliance of two or three peaks can also been observed with the maxima of 67°C; 87°C
and 130°C. These values are similar to the peaks seen in case of Cu-Pd-4A-TSI. This peak
(50°C-160°C) belongs to the reduction of palladium and also copper, as explained before. It
has to be added that in this case even higher amount of copper was reduced. Compared to the
H2-TPR profile of Cu-Pd-4A-TSI a new peak appeared at 253°C and the last peak shifted to
320°C. These reduction peaks may belong to reduction of copper in different interaction with
the support and with palladium (corresponding to different Pd-Cu ratio particles). All in all,
the shifting of reduction peaks indicated that the presence of Pd facilitated the reduction of
copper.
Summing up characterization results indicated that the two metals are present on the
support in close interaction to each other. If TPR indicated that Pd and Cu are in close
interaction, no information about the form of interaction can be deduced from the results: pure
Pd and pure Cu particles side by side on the support, bimetallic core-shell particles alloys, etc.
But due to the results of XRD and EDS characterization, we prove that this interaction
corresponds to Pd-Cu alloy particles with different ratio for both samples. So the differences
between the two catalysts produced in different ways were evidenced. Thus the different
activity of the two catalysts was suggested and tested afterwards.
Page 9 of 27

2. Suzuki-Miyaura-Coupling
Catalyst selection
As a Suzuki model reaction, we selected the reaction of phenylboronic acid with
iodobenzene in the presence of potassium carbonate as base in ethanol as solvent (Figure 5).
To select the most suitable catalyst to perform the reaction the monometallic catalysts and
the bimetallic catalysts were examined. In each reaction 0.1 g catalyst/1.0 mmol
iodobenzene/1.5 mmol phenylboronic acid ratio were used.
The results obtained for the different catalysts are summarized in Table 2. The use of Cu-
4A is not convenient in the coupling reaction, the GC-MS analysis showed only 6% of
biphenyl in the reaction mixture near the starting iodobenzene (38%) and triphenyl boroxine
(49%), which is a known byproduct of the reaction. Pd-4A seemed to be a good catalyst for
the reaction as good yield (93%) for product formation was obtained. The activity of the
catalyst decreased significantly after the second use where biphenyl was provided by 66%
yield. On the contrary, Cu-Pd-4A catalysts prepared by either method remained efficient
during two cycles resulting in biphenyl production with quantitative yield. Time resolved
reaction profile observed for the Suzuki coupling in the presence of Cu-Pd-4A-TSI is
presented as supplementary data.
Hot filtration test
Before performing all catalytic tests we wanted to rule out the possibility of
homogeneous or semi-heterogeneous catalysis. The hot filtration test is a common method to
prove or disprove heterogeneous catalysis in a reaction. During the test the catalyst is filtered
out and the reaction is continued without the presence of the catalytic material. If the reaction
is only semi-heterogeneous – the metal is dissolved in the reaction mixture and after the
reaction it precipitates to the surface of the support – the reaction continues even with the
absence of the catalyst leading to the same yield as with the full time presence of the catalyst.
In our case the reaction was stopped at 30 min (which corresponds to an approximate
conversion of 73%), the catalyst was filtered out along with the base. Catalyst filtration was
performed at the reaction temperature (78°C) in order to avoid readsorption of leached metals
on the catalyst surface during cooling of the reaction mixture. After the addition of 3 mmol
potassium carbonate base, the reaction was continued until 1 hour of reaction time. The
reaction mixture was worked up as described before and the product biphenyl was obtained
with 71% yield. Thus the hot filtration test was negative as the reaction completely stopped
Page 10 of 27

after the filtration. This way, we can conclude that only heterogeneous catalysis occurs during
our experiments.
Recycling
The catalyst could be easily separated from the reaction mixture by filtration. No tedious
procedures and treatments were required before the reuse of the catalyst. As the base in the
reaction was potassium carbonate, the catalyst should only be washed with distilled water to
remove inorganic residues then with acetone to eliminate organic materials. Before the new
cycle the catalyst was dried at 120°C for one hour. The performance was checked after 6
cycles to obtain more information about catalyst activity. The TSI catalyst still had excellent
activity (>99%, Table 3) in contrary with CI whose activity dropped yielding only 80% of
biphenyl. The different characteristics found by surface analysis – different particle size,
particle dispersion, distribution, interaction between the metals, surface area, etc. – can
explain the different stability of the catalyst. Based on our results described above one can
conclude that Cu-Pd-4A-TSI is an active and stable catalyst of the Suzuki-Miyaura coupling
of phenyl boronic acids with aryl halides.
Suzuki-Miyaura reaction with further aryl halides
Table 4 summarizes the results obtained with further aryl halides in the presence of Pd-
Cu-4A-TSI in Suzuki reaction. The well reported aryl halide reactivity order (I>Br>>Cl) was
observed with our catalyst, too. Nevertheless, aryl bromides gave the corresponding products
also with good yield (Table 4 entry 1, 2, 4). Remarkable selectivity between halides were
shown (Table 4, entry 3, 4) as in case of a starting material with different halide groups only
the more reactive part of the molecule reacted resulting bromine or chloride substituted
biphenyls. Thus, there is the possibility to prepare halogen-substituted biaryl derivatives. Aryl
chlorides failed to react even during increased reaction time. (Table 4 entry 5, 6)
3. Characterization of the recovered catalysts
The used samples - Cu-Pd-4A-TSI (excellent activity: 99% yield) and Cu-Pd-4A-CI (good
activity: 80%) - were subjected to catalyst characterization method to better understand the
reason of continuous activity in case of TSI and the drop of activity of CI. Our aim was to
find the catalytic species responsible for the activity and stability of the material. As the
properties of the fresh catalysts were already different, the difference in stability was
expectable. The features of the fresh catalysts were compared to the used catalysts’ ones as
much as the properties of the used materials prepared with different impregnation methods
were confronted. The samples were subjected to XRD and TEM analysis. ICP analysis was
Page 11 of 27

performed as well, but its result - due to the potassium contamination of the support – was not
reasonable, so a correction based on EDS measurements was necessary.
As for the result of the hot filtration test we determined that no metal leaching took place
during the catalytic test. This statement was based on data from the literature proving by
experiments that the Suzuki-reaction may be catalyzed even with minute amount of palladium
(ppm level) to obtain the C-C coupled product with good yield [38-40]. Since our reaction
stopped entirely and immediately after the catalyst was filtered out the leaching of the metal
into the reaction mixture can be ruled out.
XRD
Similarly to the fresh catalysts the used samples XRD diffractograms’ consisted of the
signals of the support (Figure 6 a). Examining the area between 40-70° 2θ showed the
presence of copper: palladium 1:1 (98-010-3082) in case of TSI-6x. CI-6x gave a
diffractogram more complex with the signs of copper-rich particle: Cu:Pd 0.6:0.4 (98-016-
6153) and palladium particles (98-064-8679; 98-064-8680) (Figure 6 b).
TEM
The morphology of the support didn’t change in any of the catalysts’ case. Henceforward
amorphous and stick-like phases were present. The composition of the support, therefore the
mass of the support changed. Even if the catalyst was washed with water after every use to
remove the base (K2CO3) and with acetone to remove organic compounds the EDS analysis
showed the presence of potassium on the support (see supplementary data). The ICP
measurement for the used samples gave 3.7 w/w% Cu; 1.1 w/w% Pd for CI-6x and 3.4 w/w%
Cu; 1.1 w/w% Pd for TSI-6x. These values are notably less than the fresh metal content
(Table 1, entry 4-5); however the ratio between copper and palladium didn’t change
significantly. (Cu/Pd ratio was 2.6 for fresh TSI and 2.9 for TSI-6x; while for fresh CI it gave
2.9 and 3.4 for the CI-6x).To determine the ratio between K (the contaminant of the support)
and Cu (the catalytic active metal) EDS analysis was used. 40.7 w/w% K and 59.3 w/w% Cu
may represent the whole recovered TSI catalyst. Correcting the ICP results with these values
our metal content shows to be 5.7 % Cu and 2 % Pd which are close to the fresh values.
Performing the same corrections in case of reused CI samples the metal contents are
transformed into 4% Cu and 1.22% Pd. This still differs from the theoretical value. Two
different explanations can be found. On one hand it can be assumed that these EDS results
don’t represent the whole mass of the support, so we still don’t know the real value. On the
other hand if these are the real values the loss of activity is explained with a further method,
confirming the decrease of the metal content.
Page 12 of 27

From the TEM pictures it can be seen that the metal particles on the surface were
underwent important changes. In both cases two populations of the metal species were found
(Figure 7). Smaller particles with an average of 2.6 nm at Cu-Pd-4A-TSI recovered (TSI-6x)
(see supplementary data) and 2.8 nm at Cu-Pd-4A-CI recovered (CI-6x). In the case of TSI-6x
it means only a growth of 0.1 nm in average particle size compared to fresh catalyst while for
CI-6x it indicates a rise of 0.6 nm. According to EDS analysis these small particles contained
both copper and palladium. Larger particles consisted mainly of palladium. The average
particle size was 17.6 nm in case of TSI-6x and 22.8 nm in case of CI-6x. It represents a
remarkable difference of 5.2 nm between the sizes of the big particles between the two
catalysts.
The EDS analysis showed the presence of palladium-rich particles which were not present
in the case of fresh catalysts. In TSI-6x besides the Pd-rich species the phase with Cu:Pd
atomic ratio of 1:1 were still present - as seen at the fresh catalyst - representing the majority
of the mixed phase. At CI-6x this kind of composition was not observed at all. Particles rich
in Cu was mostly detected along with particles rich in palladium however this latest
constitution was not significant.
One can conclude from the TEM results that in both cases two populations of the metal
particles were present on the surface of the recycled catalysts. In case of CI with decreased
activity after six cycles bigger particles were present, with high percentage of copper content.
While in the case of TSI particles of Cu:Pd with the ratio of 1:1 were present in both fresh and
used samples. We assume that these mixed phase metal particles are responsible for the
excellent catalytic activity of the material in the Suzuki-reaction as much as for its continuous
stability. This observation on the catalytic activity of Pd/Cu (1/1) alloy in Suzuki-Miyaura
coupling has never been mentioned in the literature. Only a few examples for Pd-Cu
bimetallic catalysis are published for this coupling [41-42]. In these reactions mostly
homogeneous catalysts [41] or in an example nanoclusters [42] are used. However in a patent
Pd(50%)/Cu(50%) colloid proved to be the most efficient catalyst compared to others with
different molar ratios. [43] The common values for metal loadings in the literature are 5 mol%
Pd-salt and 10-100 mol% Cu-salt to perform the reaction.
Discussion
A new bimetallic catalyst was developed containing low amount of noble metal palladium
and higher amount of inexpensive copper ( Pd/Cu molar ratio: 0.2/1). Our aim was to evolve
an easily producible, simple and cheap catalyst with the beneficial effect of palladium, doped
with copper to fine-tune its stability and activity. The metals were not reduced, and the
Page 13 of 27

catalyst was not even calcined before application. The catalysts were prepared with wet
impregnation, which is a simple and cheap procedure. Two samples were made: the metals
were put on the support with TSI and CI. By the use of co-impregnation, we favour alloy
while during successive impregnation copper had enough time to form pure particles before
the addition of Pd. The monometallic and bimetallic catalysts prepared with two different
methods were characterized using ICP, BET, XRD, TPR TEM and EDS. We found that the
metal content of the catalyst was corresponding to the theoretical values. TSI has a higher
surface area compared to CI. From DRX, information was hard to gain due to the
characteristic diffractogram of the support. However the signals of Cu-Pd crystals were found.
From the TEM analysis we concluded the presence of bimetallic alloys. The composition of
these alloys was different. On TSI the Cu and Pd ratio was equal, while on CI copper-rich
crystals were present. The average particle size did not differ notably in the catalysts prepared
with two methods. The TPR analysis revealed the different interaction between the two metals
on the support in case of TSI and CI. Comparing the two TPR profiles the peaks were shifted
and new peaks occurred proving the different properties of the two materials. Based on the
results of catalysis characterisation one can conclude that the two metals were brought to
close interaction with wet impregnation. The applicability of the surface analyzed catalysts
was examined in the Suzuki-Miyaura cross-coupling reaction. The activity of the bimetallic
catalysts was confronted with the monometallic counterparts’ as much as their stability. No
difference after the first application was shown. Both bimetallic catalysts performed with
excellent yield. The difference was not even shown after two cycles, but the sixth usage of the
CI catalyst gave the product only with good, 80% yield, compared to >99% obtained from the
reaction of TSI catalyst. One can conclude that the difference in the characteristics of the two
catalysts has its imprint on the stability during the Suzuki-Miyaura-reaction. We tried to
explain this phenomena with the help of surface analysis of the 6 times used catalysts.
According to their results we concluded that the role of Cu in our reactions was stabilizing
palladium due to Pd:Cu interaction, thus Pd can keep its active form.
In the literature deactivation studies are rare about copper-palladium bimetallic catalysts,
especially in organic reactions. However understanding catalyst deactivation in all kinds of
catalytic reaction attracts great attention and initiates several discussions. The observations
are well summarized for instance in the Handbook of Heterogeneous Catalysis [1] and in the
edition of Applied Catalysis A in 2001 on catalyst deactivation [44]. Gladys [45] discussed
the question of recoverable catalysts from green chemistry’s point of view defining the ideal
recoverable catalyst and concluded that the majority of the academic research papers focused
not on the most important aspect of recoverability (i.e. TOF, TON, conversion profiles) alike
Page 14 of 27

in his chapter of Recoverable and Recyclable Catalysts. Jones [46] stated that during the 40
years of the research of heterogeneous catalysts only two significant commercial processes
utilized supported metal complex catalysis. It is due to the few rigorous studies made on
reaction kinetics, catalyst recycle, catalyst stability and deactivation pathways. From organic
point of view Molnár [47] gave an extensive summary of the recyclability, efficacy and
selectivity of palladium catalysts in C-C coupling reactions containing Suzuki-Miyaura
reaction. Several recyclable catalysts were reported including palladium nanoparticles,
immobilized palladium complexes, palladium complexes under homogeneous conditions and
miscellaneous examples. However morphology of the catalysts and determination of
catalytically active species are rare as much as in the reviewed articles.
To understand better the stability of a supported metallic catalyst we need an insight to
the properties of the metals. Hughes [48] gave the following increasing order of stability for
metals: Ag < Cu < Au < Pd < Fe < Ni < Co < Pt < Rh < Ru < Ir < Os < Re. Therefore it is
not surprising if a copper-palladium catalyst can be more sensitive than other commonly used
metallic catalysts. Theoretical studies have shown that Cu can strongly modify the Pd valence
state by injecting charge into the sp sub-band [49]. It is reported in the literature [50] that the
beneficial effect of copper species on the bimetallic Pd-Cu catalyst are related to the
formation of an active Pd-rich Pd-Cu alloy, which is more reactive than either of the
monometallic (Cu or Pd cluster) species. In our case this Pd-Cu alloy contained equal amount
of the two metals. According to the XRD, TEM and EDS analyses we assume that these
particles are responsible for the catalytic activity and stability. They were present in the fresh
and used TSI catalysts, but they were absent in the used CI catalysts whose catalytic activity
dropped. Wang and Lu [35] reported that for Cu-rich Pd-Cu bimetallic catalysts the effect of
copper species is complicated, it highly depends on the Cu precursor. They named the
increase of the Cu:Pd molar ratio as the reason for the catalyst deactivation in CO oxidation
(in case of CuCl2 copper precursor – similarly to our catalyst). It is in correspondence with
our observations: the rise of Cu-rich Cu:Pd alloy in case of recovered CI, whose activity
dropped in the reaction. In addition to this poisoning and fouling of the catalyst by inorganics
(K2CO3 in our case) and unreacted reagents may take place too. The active metal sites can be
blocked such way. Sintering of the Cu:Pd particles also occurs as evidence by TEM images of
the catalysts. This can be induced by the reaction temperature [51].
Conclusion
Page 15 of 27

A new, simple method was found to perform the Suzuki-Miyaura coupling. A full study was
made including the catalyst preparation, characterization, application in the Suzuki coupling,
reuse of the catalyst, the characterization of the recovered catalyst and the explanation of the
catalytic activity and stability thanks to the surface analysis. The importance of the catalyst
preparation method was proven. The two metals (Cu, Pd) were brought to close interaction on
the surface of the support even with WI method which in certain cases can be unsatisfactory
for merging the metals. We found that a Cu-Pd alloy with atom ratio 1:1 is responsible for the
catalytic activity. Our TSI catalyst was active and selective during 6 catalytic cycles in Suzuki
coupling, while the activity of the catalyst prepared with CI dropped after the sixth use. This
loss was due to the absence of Cu-Pd 1:1 alloy, the presence of Cu-rich Cu-Pd species and the
growth of the particle size. Experiments were carried out with different aryl halides in the
presence of TSI catalyst. Aryl bromides showed good activity and low activity for aryl
chlorides were shown. Selectivity between halides was also observed.
Acknowledgements
The authors would like to thank the French Government for the scholarship that made the
cooperation between the two research groups possible.
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Page 17 of 27

Table 1. Metal content and BET surface area of the mono-and bimetallic catalysts
Targeted values
Cu (wt%) Pd (wt%)
Measured values (ICP)
BET surface
area (m²/g )
800
Catalyst
Cu (wt%)
Pd (wt%)
MS-4A
Cu-4A
-
-
-
-
6.3
-
-
360
5.8
-
-
2.1
2.1
2.1
Pd-4A
2.0
2.4
2.0
-
6.3
6.3
310
230
Cu-Pd-4A-TSI
Cu-Pd-4A-CI
6.2
5.7
Table 2. Catalyst selection for the reaction of iodobenzene with phenylboronic acida
Catalyst
Cu-4A
Yieldb
-(6%)e
Conversionc
TOF (h-1)d
62%
n/a
Pd-4A
93%
66%
100%
97%
n/a
Pd-4A (second cycle)
Cu-Pd-4A-TSI
n/a
>99%
>99%
>99%
>99%
100%
100%
100%
100%
23.3
n/a
Cu-Pd-4A-TSI(second cycle)
Cu-Pd-4A-CI
22.6
n/a
Cu-Pd-4A-CI(second cycle)
a: Reaction conditions: phenylboronic acid (1.5 mmol), iodobenzene (1 mmol), potassium carbonate (3 mmol), 0.1 g catalyst (2 mol% Pd
and/or 9 mol% Cu) in EtOH, 78°C, 1h
b: Isolated yield
c: Conversion of iodobenzene
d: At 1 hour
e: According to GC-MS analysis of the reaction mixture
Table 3. Recycling of bimetallic catalystsa
Yield
2nd use
>99%
>99%
Conversion
6th use
TOF (h-1)
6th use
94.3
Catalyst
1st use
>99%
>99%
6th use
>99%
80%
Cu-Pd-4A-TSI
Cu-Pd-4A-CI
100%
96%
109.2
a: Reaction conditions: phenylboronic acid (1.5 mmol), iodobenzene (1 mmol), potassium carbonate (3 mmol), 0.1 g catalyst (2.26 mol% Pd
and 9.76 mol% Cu) in EtOH, 78°C, 1h
Page 18 of 27

Table 4: Suzuki-Miyaura coupling of aril halides with phenylboronic acida
Entry
Aryl halide
Time
(h)
Product
Yield
(%)
Conversion
(%)
TOF
(h-1)
Br
1
1
80
81
18.1
Br
2
1
99b
100
23.3
Br
Br
Br
3
4
5
6
1
1.5
1
70d
93e
2c
93
100
21
16.3
14.1
4.7
I
Cl
Cl
CF3
Br
CF3
Cl
CHO
CHO
7
13
10
0.3
Cl
a: Reaction conditions: phenylboronic acid (1.5 mmol), iodobenzene (1 mmol), potassium carbonate (3 mmol), 0.1 g catalyst (2.26 mol% Pd
and 9.76 mol% Cu) in EtOH, 78°C, 1h
b: 3 mmol of phenylboronic acid was used
c: According to GC-MS analysis of the final product
d No 4-iodobiphenyl was detected, ca. 10% of terphenyl was shown
e: Selectivity=100%
Page 19 of 27

Figure 1. XRD diffractograms a) of the support and the fresh TSI and CI catalysts b) zoom in
to determine metal crystals (◊) Cu-Pd-4A-TSI; (†) Cu-Pd-4A-CI; (§) Zeolit A 98-000-9326;
(&) Cu Pd (0.6/0.4) 98-016-6153; (#) Cu Pd (3/1) 98-010-3084; (¤) Cu Pd (0.6/0.4) 98-016-
6154; (*) Cu Pd (1/1) 98-010-3082
Page 20 of 27

Figure 2. TEM image of the fresh catalysts: a) agglomerated particles in Cu-Pd-4A-TSI; b)
well dispersed particles in Cu-Pd-4A-TSI; c) agglomerated particles in Cu-Pd-4A-CI; d) well
dispersed particles in Cu-Pd-4A-CI
Page 21 of 27

Figure 3. Particle size distribution of a) Cu-Pd-4A-TSI; b) Cu-Pd-4A-CI
Figure 4. TPR profiles of mono-and bimetallic catalysts
Page 22 of 27

Figure 5. The Suzuki-Miyaura coupling of phenylboronic acid with iodobenzene
OH
catalyst
EtOH
B
I
OH
+
K2CO3
Figure 6. XRD diffractogram of a) recovered catalysts; b) zoom in to determine metal
particles in fresh and used catalysts (◊) Cu-Pd-4A-TSI-fresh; (†) Cu-Pd-4A-CI-fresh; (§) TSI-
6x; (&) CI-6x; (#) Zeolit A 98-000-9326; (¤)Pd 98-064-8680; (*) Pd 98-064-8679 (°)Cu Pd
(0.6/0.4) 98-016-6153; (+) Cu Pd (1/1) 98-010-3082
Page 23 of 27

Page 24 of 27

Figure 7. TEM image of the recovered catalysts: a) small particles in TSI-6x; b) big particles
in TSI-6x; c) small particles in CI-6x; d) big particles in CI-6x
Page 25 of 27

Graphical abstract
Page 26 of 27

Highlights
-
-
-
-
-
Cu-Pd/4AMS catalysts were prepared with impregnation [successive (TSI) and co (CI)]
The catalysts characteristics differed confirming the importance of preparation
Tests were made in Suzuki coupling, where activity and stability were investigated
Cu/Pd 1/1 alloy proved to be the reason of activity and stability in the coupling
TSI was active during 6 runs, while the activity of CI dropped
Page 27 of 27