Amino-Modified Silica-Supported Copper-Palladium Alloy. Synthesis and Use in Selective Hydrogenation of Disubstituted Nitroarenes in a Flow Micro Reactor

Article abstract of DOI:10.1134/S1070428019010019

A copper-palladium catalyst supported on amino-modified silica has been synthesized by chemical reduction. It has been found that submicron particles of a copper-palladium alloy are formed on the silica surface. Unlike commercially available palladium catalysts (Pd/Al₂O₃, Pd/C, Pd/BaSO₄), the synthesized copper-palladium catalyst makes it possible to selectively reduce the nitro group in 3-nitrobenzaldehyde and 1-chloro-4-nitrobenzene.

Full text of DOI:10.1134/S1070428019010019

ISSN 1070-4280, Russian Journal of Organic Chemistry, 2019, Vol. 55, No. 1, pp. 1–6. © Pleiades Publishing, Ltd., 2019.
Russian Text © A.T. Nurmukhametova, R.N. Belov, E.D. Sultanova, V.V. Vorob’ev, Yu.N. Osin, V.A. Burilov, I.S. Antipin, 2019, published in Zhurnal
Organicheskoi Khimii, 2019, Vol. 55, No. 1, pp. 9–16.
Dedicated to Full Member of the Russian Academy of Sciences
A.I. Konovalov on his 85th anniversary
Amino-Modified Silica-Supported Copper–Palladium Alloy.
Synthesis and Use in Selective Hydrogenation of Disubstituted
Nitroarenes in a Flow Micro Reactor
A. T. Nurmukhametovaa, R. N. Belova, E. D. Sultanovaa, V. V. Vorob’eva, Yu. N. Osina,
V. A. Burilova,* and I. S. Antipina
a Kazan (Volga Region) Federal University, ul. Kremlevskaya 18, Kazan, 420008 Tatarstan, Russia
*e-mail: ultrav@bk.ru
Received October 31, 2018; revised November 6, 2018; accepted November 10, 2018
Abstract—A copper–palladium catalyst supported on amino-modified silica has been synthesized by chemical
reduction. It has been found that submicron particles of a copper–palladium alloy are formed on the silica
surface. Unlike commercially available palladium catalysts (Pd/Al2O3, Pd/C, Pd/BaSO4), the synthesized
copper–palladium catalyst makes it possible to selectively reduce the nitro group in 3-nitrobenzaldehyde and
1-chloro-4-nitrobenzene.
Keywords: flow chemistry, bimetallic catalysts, selective hydrogenation, copper–palladium alloy.
DOI: 10.1134/S1070428019010019
INTRODUCTION
for furfuryl alcohol in the reduction of furfural [7],
selectively obtain ethylene by the hydrodechlorination
of 1,2-dichloroethane [8], selectively obtain chloro-
anilines from the corresponding nitro derivatives [9],
etc. Furthermore, “dilution” of high-cost palladium
with cheap d-metals provides a way of reducing the
cost of catalyst without substantial loss of catalytic
activity and selectivity. Proper estimation of the
second metal effect on the catalytic properties of
palladium catalysts requires parallel reactions to be
carried out under completely identical conditions,
which is very difficult to achieve using conventional
reactors. In this respect, flow micro reactors offer
undoubted advantages; in particular, built-in hydrogen
generator and the possibility of accurate temperature,
pressure, and flow rate control make them convenient
tools for comparative screening of catalytic activity
and optimization of reaction conditions, as well as for
combinatorial chemistry [10]. In addition, due to
effective stirring, fast heat and mass transfer, and
reverse pressure control technology which makes it
possible to raise boiling point of the solvent, flow
micro reactors provide significant increase of the reac-
tion rate [11–13].
Development of catalysts for selective hydrogena-
tion of polyfunctional molecules is a quite challenging
and practically important problem; the use of such
catalysts could give rise to new synthetic routes to
target compounds [1]. Traditionally, palladium is one
of the most widely used catalytic metals, and it is
successfully utilized to reduce functional groups and
remove protecting groups by hydrogenation [2, 3].
However, the catalytic activity of the most common
catalyst, Pd/C, is so high that the hydrogenation over
Pd/C often leads to exhaustive reduction of all func-
tional groups present in the molecule. This issue is
solved by using catalyst poisons [4] or specific sup-
ports [5]. In recent time, d-metals were added to
palladium catalysts to control their activity and selec-
tivity. The bimetallic catalysts thus obtained often
showed properties different from the properties of the
corresponding monometallic analogs due to electronic
modification of palladium atoms as a result of forma-
tion of heteronuclear metal–metal bonds [6]. For
example, introduction of copper to palladium catalyst
made it possible to significantly increase the selectivity
1

NURMUKHAMETOVA et al.
(a)
2
(b)
10 μm
2 μm
(c)
Cu Kα1
5 μm
Pd Lα1
5 μm
5 μm
Fig. 1. (a) SEM photographs of CuPd/SiO2NH2 with different magnifications and (b) multilayer EDS map with separate copper and
palladium distribution maps.
The present work was aimed at synthesizing cop-
which leads to contamination of the products and
shortens the catalyst lifetime. To solve this problem,
it was proposed by our research team to use amino-
modified silica as a support [14], taking into account
that modification of silica surface by donor amino
groups provides more efficient metal binding to the
support [15]. The same approach was utilized in
the present work. The bimetallic copper–palladium
catalyst was synthesized by the reduction of a mixture
of CuSO4 ·5H2O and Pd(OAc)2 with 20 equiv of
NaBH4 in the presence of amino-modified silica in
aqueous ethanol. The obtained material (hereinafter
CuPd/SiO2NH2) was analyzed by scanning electron
per–palladium catalysts supported on amino-modified
amorphous silicon dioxide, studying their morphology,
and comparing their activity and selectivity in the cata-
lytic hydrogenation of some difunctional compounds
in a flow micro reactor with those of commercially
available palladium catalysts.
RESULTS AND DISCUSSION
It is known that one of the key problems of flow
chemistry is washout of the active metal [12] with
continuous stream of solvent containing reactants,
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AMINO-MODIFIED SILICA-SUPPORTED COPPER–PALLADIUM ALLOY.
3
I, pulse s–1 eV–1
microscopy (SEM) and energy-dispersive X-ray spec-
Si
troscopy (EDS).
The SEM microphotographs of CuPd/SiO2NH2
(Fig. 1a, b) show that particles with a size of 100–
200 nm are present on the silica surface. These
particles form aggregates with different sizes. The
multilayer EDS map (Fig. 1c) shows uniform distribu-
tion of copper and palladium in the same region,
indicating formation of doped bimetallic nanoparticles.
The elemental composition of CuPd/SiO2NH2 was also
determined by EDS; it was found that the catalyst
contains copper and palladium atoms in addition to
silicon, oxygen, nitrogen, and carbon constituting the
amino-modified silica (Fig. 2).
30
20
10
0
O
N
Cu
Pd
C
Cu
8
Pd
Cu
0
2
4
6
E, keV
Fig. 2. EDS spectrum of CuPd/SiO2NH2.
The catalyst structure was also studied by X-ray
powder diffraction (Fig. 3). The X-ray powder pattern
of CuPd/SiO2NH2 showed an intense diffraction peak
at 2θ = 41.76° located between the corresponding dif-
fraction peaks of Pd (111) (2θ = 40.5°) and Cu (111)
(2θ = 43.2°). This peak can be assigned to the Pd–Cu
alloy phase, namely to reflection from the (111) plane
of the crystal. According to the JCPDS card no. 48-
1551, these data completely conform to face-centered
cubic crystals of Pd/Cu alloy (space group Fm3m).
Apart from the Pd/Cu (111) peak (2θ = 41.76°),
additional diffraction peaks were observed at 2θ =
48.35, 71.16, 86.25, and 91.22° due to reflections from
the (200), (220), (311), and (222) planes of Pd/Cu
crystal [16].
41.76
48.35
71.16
86.25
91.22
0
20
40
60
80
100
2θ, deg
The degree of oxidation of metals was estimated by
X-ray photoelectron spectroscopy (XPS). The XPS
spectrum of a sample of CuPd/SiO2NH2 in the copper
region (Fig. 4a) contained main peaks 2p3/2 932.6 eV
and 2p1/2 952.7 eV, as well as a satellite peak at
942.9 eV. According to published data, Cu0 gives 2p3/2
and 2p1/2 peaks at 933 eV and 953 eV, respectively;
2p3/2 peak of CuI and CuII appears at 932.5 eV, and the
halfwidth of the 2p3/2 peak is 1.9 eV for CuI and 3.4 eV
for CuII [17]. In our case, the 2p3/2 halfwidth was
3.0±0.1 eV which better matches CuII. The presence of
a satellite peak at 942.9 eV is also typical of Cu(II).
However, the intensity of the satellite peak of
CuPd/SiO2NH2 is not high, whereas the intensity of
satellite peaks in samples containing Cu(II) is usually
no less than 40% of the 2p3/2 peak intensity [18]. Thus,
the catalyst contains both CuO and Cu2O, but the
presence of Cu(0) cannot be excluded on the basis of
the available data since Cu(0) signals are completely
overlapped by the signals of copper oxides. Taking
into account the ease of oxidation of Cu0 and the depth
of the XPS analysis which is limited to 5–10 nm, we
Fig. 3. X-Ray powder pattern of CuPd/SiO2NH2.
presume that copper atoms in the alloy are oxidized
on the particle surface. The XPS spectrum of
CuPd/SiO2NH2 in the palladium region (Fig. 4b)
showed main peaks at 335.4 and 340.8 eV, which
correspond to 3d5/2 and 3d3/2 of Pd(0), respectively
[19]. The obtained data suggests that the catalyst
CuPd/SiO2NH2 contains particles of Cu/Pd alloy
coated by copper(I) and copper(II) oxides.
The influence of copper on the selectivity and
catalytic activity of palladium particles was estimated
in the hydrogenation of nitroarenes with molecular
hydrogen in a flow reactor. The catalysts were packed
in 30-mm steel cartridges which were mounted in
a ThalesNano H-Cube Pro™ flow reactor equipped
with a built-in hydrogen generator, an automated
sampler, and a six-position switch allowing successive
comparative screening of 6 different catalysts. Among
commercially available palladium catalysts, we
RUSSIAN JOURNAL OF ORGANIC CHEMISTRY Vol. 55 No. 1 2019

NURMUKHAMETOVA et al.
4
(a)
(b)
Cu2p
Pd3d
920
930
940
950
960
970
325
330
335
340
345
350
355
Bond energy, eV
Bond energy, eV
Fig. 4. X-Ray photoelectron spectra of CuPd/SiO2NH2 in the (a) copper and (b) palladium regions.
selected 5% Pd/Al2O3 (THS 01118), 5% Pd/BaSO4
(THS 03112), and 5% Pd/C (THS 02131) manufac-
tured by ThalesNano. The reaction conditions were
optimized using 4-nitroaniline as model substrate; the
optimal conditions ensured 100% conversion with all
catalysts (Table 1). Tetrahydrofuran was used as
solvent since it equally well dissolves the initial nitro
compounds and hydrogenation products. The progress
of the model reaction was monitored by GC/MS.
and Pd/BaSO4; and CuPd/SiO2NH2 ensured selective
reduction of the nitro group without involving the
halogen. The lower activity of CuPd/SiO2NH2 and
hence its higher selectivity are related to the presence
of catalytically inactive copper atoms in the alloy. This
is consistent with the available published data, accord-
ing to which mixed palladium alloys with d-metals
show increased selectivity [8] in the hydrogenation of
difunctional aromatic amines.
In summary, a copper–palladium catalyst supported
on amino-modified silica has been synthesized by
chemical reduction with sodium tetrahydridoborate.
Using a combination of SEM, EDS, XPD, and XPS
methods, we have shown that the synthesized catalyst
contains submicron copper–palladium alloy particles
on the amino-modified silica surface. Introduction of
catalytically inactive copper atoms into the crystal
lattice of palladium considerably changes the catalytic
activity and selectivity. Unlike commercially available
palladium catalysts, the silica-supported copper–palla-
dium catalyst ensures selective reduction of the nitro
group in 3-nitrobenzaldehyde and 1-chloro-4-nitro-
benzene in a flow hydrogenation reactor.
Having found optimal conditions for the hydro-
genation of 4-nitroaniline, we compared the catalytic
activities and selectivities of palladium-containing
catalysts in the hydrogenation of 3-nitrobenzaldehyde
in which both nitro and carbonyl group can be reduced.
It was found that the catalyst and support nature
directly affect the catalytic activity. The most active
was Pd/Al2O3 which caused exhaustive reduction of
both nitro and carbonyl group and even the aromatic
ring (Table 1). The next in activity was Pd/C which
reduced the carbonyl group to hydroxymethyl; in
addition, a small amount of m-toluidine was formed.
3-Aminobenzyl alcohol was selectively formed in the
hydrogenation over Pd/BaSO4, whereas CuPd/SiO2NH2
selectively reduced the nitro group while the carbonyl
group remained intact. Another substrate tested was
4-nitro-1-chlorobenzene which is capable of under-
going catalytic hydrodehalogenation. The relative
activities of the examined catalysts with respect to
4-nitro-1-chlorobenzene were the same as in the
hydrogenation of 3-nitrobenzaldehyde. The reduction
of the nitro group in the presence of Pd/Al2O3 was
accompanied by complete hydrodechlorination; mix-
tures of 4-chloroaniline (major product) and aniline
were formed in the hydrogenation catalyzed by Pd/C
EXPERIMENTAL
Solvents of chemically pure and analytical grades
were purified according to known procedures [20]
before use. Commercially available reagents from
Sigma–Aldrich and Alfa Aesar were used. Amino-
modified silica was prepared as described in [14].
Samples of the catalyst were analyzed with a Carl
Zeiss Merlin auto-emission scanning electron micro-
scope (Germany). To minimize effect on the sample,
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AMINO-MODIFIED SILICA-SUPPORTED COPPER–PALLADIUM ALLOY.
5
Table 1. Catalytic hydrogenation of some nitroarenes with molecular hydrogen over palladium catalysts in a flow reactora
Substrate
Catalyst
Product composition,b %
NH2
H2N
100
100
100
100
NH2
Pd/Al2O3
Pd/C
O2N
Pd/BaSO4
CuPd/SiO2NH2
H2N
CHO
H2N
CH2OH
H2N
Me
H2N
Me
Pd/Al2O3
Pd/C
90
04
10
O2N
CHO
096
100
001
Pd/BaSO4
CuPd/SiO2NH2
99
Cl
H2N
H2N
Cl
Pd/Al2O3
Pd/C
100
O2N
70
89
99
030
011
001
Pd/BaSO4
CuPd/SiO2NH2
a
Reaction conditions: THF, 100°C, flow rate 0.5 mL min–1, p = 10 bar, substrate concentration 0.05 M.
According to the GC/MS data; substrate conversion 100%.
b
the microphotographs were obtained in the secondary
electron mode (primary electron accelerating voltage
5 kV, probe current 300 pA). Elemental analysis was
performed with an Oxford Instruments AZtec X-MAX
energy-dispersive X-ray spectrometer (United
Kingdom) at an accelerating voltage of 20 keV and
a focal length of 9.6 mm.
The X-ray powder patterns were obtained at room
temperature on a Rigaku MiniFlex 600 diffractometer
(Japan) equipped with a D/teX Ultra detector (Cu Kα
radiation, λ = 1.54056 Å; 40 kV; 15 mA; 2θ range
10–80°; scanning with a step of 0.02°; no sample
spinning).
The X-ray photoelectron spectra were recorded
using a SPECS ultrahigh vacuum analytical chamber
equipped with an X-ray source with magnesium and
aluminum anodes and a Specs Phoibos 150 hemi-
spherical electron energy analyzer; the residual pres-
sure in the chamber did not exceed 5×10–10 mbar. All
spectra were processed using CasaXPS package [21].
Background was subtracted by the Shirley method
after calibration against the carbon 1s line (284.8 eV).
Gas chromatographic–mass spectrometric analysis
was performed on a Shimadzu GCMS-QP2010 Ultra
instrument; HP-5MS capillary column, 30 m, film
thickness 0.25 μm; carrier gas helium (purity
99.995%), flow rate 2 mL/min, split ratio 40; injector
temperature 250°C; oven temperature programming
from 70 to 140°C at a rate of 10 deg/min, 2 min at
140°C, 140 to 250°C at a rate of 10 deg/min; a.m.u
range 35–400.
Synthesis of CuPd/SiO2NH2. Amino-modified
silica, 0.5 g, was added to a solution of 0.1 g
(0.39 mmol) of copper(II) sulfate pentahydrate in
30 mL of deionized water, and a solution of 0.05 g
(0.24 mmol) of palladium(II) acetate in 15 mL of
ethanol was added. A solution of 0.3 g (7.89 mmol) of
sodium tetrahydridoborate in 15 mL of deionized
water was then added dropwise with stirring and
cooling to the resulting suspension. The mixture was
stirred for 3 h at 40°C, and the precipitate was
separated by centrifugation, washed in succession with
water (3×40 mL) and ethanol (3×40 mL), each time
being separated by centrifugation, and dried under
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NURMUKHAMETOVA et al.
6
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catalyst was placed in 30-mm steel cartridges with
combined steel reticular and Teflon membranes at the
inlet under reduced pressure (5 mm Hg), and steel
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This study was performed under financial support
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Analytical Microscopy Interdisciplinary Center of the
Kazan (Volga Region) Federal University. The authors
thank the staff of the Synthesis and Analysis of Thin
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measurements on the equipment of the Federal Joint
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The authors declare the absence of conflict of
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