Palladium Supported on Carbon Nanoglobules as a Promising Catalyst for Selective Hydrogenation of Nitroarenes

Article abstract of DOI:10.1007/s10562-019-02974-6

The catalysts 1?wt% palladium supported on carbon nanoglobules (CNGs) were shown to be highly active in the liquid-phase hydrogenation of various nitroarenes and provided nearly 100% selectivity to aromatic amines at complete conversion under mild conditions (323?K, 0.5?MPa, 1?h). The catalytic activity (in terms of turnover frequency and substrate conversion) and selectivity depend on the kind of CNGs support, catalyst preparation method and the reaction conditions (solvent nature). The Pd/CNGs catalyst can be repeatedly used while maintaining the same catalytic performance. The excellent performances of Pd/CNGs catalysts can be due to the globular morphology of the supports as well as the absence of micropores and pronounced surface defects. Graphic Abstract: [Figure not available: see fulltext.]

Full text of DOI:10.1007/s10562-019-02974-6

Catalysis Letters
https://doi.org/10.1007/s10562-019-02974-6
Palladium Supported on Carbon Nanoglobules as a Promising Catalyst
for Selective Hydrogenation of Nitroarenes
R. M. Mironenko1,2 · O. B. Belskaya1,3 · L. N. Stepanova1,2 · T. I. Gulyaeva1 · M. V. Trenikhin1,3 · V. A. Likholobov4
Received: 12 August 2019 / Accepted: 20 September 2019
© Springer Science+Business Media, LLC, part of Springer Nature 2019
Abstract
The catalysts 1 wt% palladium supported on carbon nanoglobules (CNGs) were shown to be highly active in the liquid-phase
hydrogenation of various nitroarenes and provided nearly 100% selectivity to aromatic amines at complete conversion under
mild conditions (323 K, 0.5 MPa, 1 h). The catalytic activity (in terms of turnover frequency and substrate conversion) and
selectivity depend on the kind of CNGs support, catalyst preparation method and the reaction conditions (solvent nature).
The Pd/CNGs catalyst can be repeatedly used while maintaining the same catalytic performance. The excellent performances
of Pd/CNGs catalysts can be due to the globular morphology of the supports as well as the absence of micropores and pro-
nounced surface defects.
Graphic Abstract
Keywords Catalytic hydrogenation · Carbon nanoglobules · Palladium catalysts · Nitroarenes · Aromatic amines
1 Introduction
Catalytic hydrogenation of aromatic nitro compounds over
supported metal nanoparticles (NPs) has recently attracted
huge attention [1–5]. This is due not only to the importance
of the resulting aromatic amines, which are key intermedi-
ates in the production of polymers, pharmaceuticals, dyes
and pesticides [6, 7], but also because the conventional non-
catalytic reduction methods with the use of metallic iron,
zinc, tin in acid media, sulfdes or inorganic hydrides (e.g.,
NaBH4) exhibit serious drawbacks [1–3, 6, 7]. Hydrogena-
tion with H2 gas in the presence of supported metal catalysts
is considered preferable especially from an environmental
standpoint.
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s10562-019-02974-6) contains
supplementary material, which is available to authorized users.
* R. M. Mironenko
mironenko@ihcp.ru
1
Center of New Chemical Technologies of the Federal
Research Center Boreskov Institute of Catalysis of Siberian
Branch of the Russian Academy of Sciences, 54
Neftezavodskaya Street, Omsk, Russia 644040
2
Dostoevsky Omsk State University, 55a Mira Ave, Omsk,
Russia 644077
3
Omsk State Technical University, 11 Mira Ave, Omsk,
Meanwhile, the development of highly efcient catalysts
for the chemoselective hydrogenation of aromatic nitro com-
pounds to corresponding anilines is still a challenging task.
Russia 644050
4
Federal Research Center Boreskov Institute of Catalysis, 5
Lavrentieva Ave, Novosibirsk, Russia 630090
Vol.:(0123456789)
1 3

R. M. Mironenko et al.
By now, a great number of supported catalysts containing
Pt, Pd, Ru, Rh, Ni, Cu, Ag, Au NPs have been examined in
the hydrogenation of various substituted nitroarenes [1–12].
Despite the progress made, the procedure for the preparation
of some hydrogenation catalysts requires the use of scarce
and expensive reagents, and also includes many stages.
Additionally, and most importantly, the hydrogenation of
nitroarenes in the presence of many of the proposed cata-
lysts often requires the use of harsh conditions to achieve
high substrate conversion and high yield of aromatic amines
[8–12].
by the variation of the production conditions [12, 24]. This
makes it possible to tune the structure and properties of
CNGs-supported Pd NPs and thus achieve improved per-
formances of Pd/CNGs catalysts [24].
In the present work, we continued to study 1% Pd/CNGs
catalysts in the hydrogenation of 1a, revealing how the
catalytic activity (in terms of turnover frequency (TOF)
and substrate conversion) and selectivity depend on the
physicochemical characteristics of CNGs, catalyst prepara-
tion method and the nature of solvent used as the reaction
medium. We also found that these catalysts exhibit excellent
performances in the hydrogenation not only of 1a, but also
of other nitroarenes, providing nearly 100% selectivity to
aromatic amines under mild reaction conditions.
In the synthesis of supported catalysts, the support plays
a crucial role in defning the structure and electronic state
of supported metal NPs and, ultimately, the activity and
selectivity of the resulting catalyst. Recently, carbon mate-
rials of diferent origin and structure (active carbons, carbon
nanotubes, carbon nanofbers) are widely used as supports
in the catalyst preparation [13–19]. It is well-known that the
carbon-supported palladium catalysts are extensively applied
for the liquid-phase hydrogenation of various substrates and
perhaps are the most common heterogeneous catalysts uti-
lized in organic synthesis [20–24]. It is possible to regulate
and improve the catalytic properties of Pd/C, including by
varying the nature (structure and physicochemical prop-
erties) of the carbon support as well as the conditions of
catalyst preparation. These considerations, although they
are obvious and important for the production of efcient
Pd/C catalysts, are not always taken into account. There are
few reviews that discuss in detail the synthesis issues of
Pd/C catalysts [24–28]. However, so far it has not always
been possible to obtain a catalyst with desired properties,
and, as a rule, the development of an optimal procedure for
the catalyst synthesis is required. In addition, a reduction
in the content of expensive metal in a catalyst while main-
taining the catalytic performances at a high level is still a
topical task. For instance, commercial Pd/C catalysts used
for the hydrogenation of aromatic nitro compounds com-
monly contain 5–20 wt% of palladium [5, 22]. However, a
much smaller Pd loading in the catalyst may be sufcient to
achieve excellent catalytic performances.
2 Experimental
2.1 Materials and Reagents
Three samples of CNGs obtained at the Department of
Experimental Technologies, (Center of New Chemical Tech-
nologies, Boreskov Institute of Catalysis) were used as the
supports. These CNGs samples have already been used in
our previous study [12], and here, for brevity, we denote
them as C-1, C-2 and C-3. Before the experiments, the sup-
ports were dried overnight in air at 393 K.
Palladium(II) chloride, PdCl2 (98%, Aurat, Rus-
sia), was used for the catalyst preparation without fur-
ther purifcation. Palladium(II) acetate, [Pd(OAc)2]3, and
tris(dibenzylideneacetone)dipalladium(0), [Pd2(dba)3],
were synthesized by known procedures (see Supplemen-
tary Materials for more information). Solvents and most of
nitroarenes were purchased from commercial suppliers and
used as received (Tables S1 and S2). 1-(2,3-Difuoro-6-nitro-
phenoxy)acetone was synthesized and supplied by courtesy
of Dr. G.L. Rusinov (I.Ya. Postovskiy Institute of Organic
Synthesis, Yekaterinburg, Russia). The reduction of cata-
lysts and the catalytic hydrogenation were performed using
hydrogen of high-purity grade (99.99%, NCCP, Russia).
We recently reported [12] that 1–2% palladium catalysts
supported on carbon nanoglobules (CNGs) are highly active
in the hydrogenation of ethyl 4-nitrobenzoate (1a) under
mild conditions and provide selective formation of ethyl
4-aminobenzoate (2a). CNGs (carbon blacks) we used to
prepare the catalysts are produced on a large scale, which
makes them quite afordable [29, 30]. However, it is note-
worthy that despite several advantages of CNGs for use as
a catalyst support [19, 24, 29], numerous works concerning
the catalytic application of various carbons pay unreasonably
little attention to this carbon material [15, 19]. One should
note that the structure, porosity and surface acid–base prop-
erties of CNGs can be gently and independently regulated
2.2 Preparation of Catalysts
Various palladium precursors (see Supplementary Materi-
als) and diferent techniques were used for the preparation of
palladium catalysts. The catalysts denoted as 1% Pd(CCs)/
CNGs were prepared by incipient wetness impregnation of
the CNGs supports with aqueous solutions of palladium(II)
chloride complexes (CCs) formed by dissolving PdCl2 in
water containing equimolar amount of HCl. After deposi-
tion of Pd CCs on the supports, the samples were dried in
air at room temperature and then at 393 K. Before catalytic
1 3

Palladium Supported on Carbon Nanoglobules as a Promising Catalyst for Selective Hydrogenation…
experiments, the catalysts were reduced in fowing hydrogen
at 573 K for 2 h.
samples were completely dissolved in a mixture of concen-
trated HNO3 and HClO4. The Pd content measurement error
was 0.05 wt%.
The second preparation technique used in the present
study includes the adsorption of polynuclear hydroxy com-
plexes (PHCs) of Pd(II) onto carbon supports, as described
in detail elsewhere [24, 31]. A further reduction of sup-
ported Pd PHCs was performed in sodium formate solution
at 363 K for 0.5 h. The catalysts obtained by this method are
denoted hereinafter as 1% Pd(PHCs)/CNGs.
2.4 Procedure of Catalytic Hydrogenation
Typically, the liquid-phase hydrogenation of nitroarenes in
the presence of palladium catalysts was performed in a Mini-
clave drive (Büchi AG) periodic reactor at 323 K, 0.5 MPa
under vigorous stirring at 1000 rpm for 1 h. The ratio of the
amount of substrate to that of catalyst was kept constant
at 10 g g−1 in all catalytic experiments. The reaction was
controlled by measuring the amount of hydrogen consumed.
The results of these measurements as well as palladium dis-
persions derived from CO chemisorption data were used for
the calculation of TOF [12]. After completion of the reaction
and cooling, the reaction solutions were separated from the
catalysts by fltration.
The catalysts denoted as 1% Pd(Ac)/CNGs and 1%
Pd(dba)/CNGs were prepared by incipient wetness impreg-
nation of the CNGs supports with solutions of [Pd(OAc)2]3
and [Pd2(dba)3] in trichloromethane, as described earlier
[32, 33]. After deposition of the precursors, the samples
were dried in air at room temperature and then at 393 K.
Before catalytic experiments, the catalysts were reduced
with hydrogen at 573 K for 2 h.
2.3 Characterization of Catalysts
2.5 Analysis of the Reaction Products
The reducibility of Pd CCs deposited on the carbon sup-
ports was examined by H2-TPR using an AutoChem II 2920
(Micromeritics) chemisorption analyzer equipped with a
thermal conductivity detector (TCD). The H2-TPR experi-
ments were carried out using a mixture of 10 vol% H2 and
argon (the fow rate 30 cm3 min−1) in a temperature range
of 303–573 K at a heating rate of 10 K min−1. The catalysts
prepared from Pd PHCs and reduced with sodium formate
were exposed in a 10 vol% H2–Ar medium under H2-TPR
conditions in a temperature range of 303–393 K at a heating
rate of 10 K min−1 with subsequent cooling in fowing argon.
The dispersion of Pd in the reduced catalysts was esti-
mated by pulse chemisorption of CO (> 99.999 vol% purity)
using an AutoChem II 2920 (Micromeritics) instrument, as
described in [12]. The palladium dispersion was calculated
assuming a stoichiometry Pd:CO = 2 [34, 35]. The mean
volume-surface diameter (in nm) of Pd NPs was obtained
from the Pd dispersion: d=1.11/D [36].
Gas chromatography–mass spectrometry (GC–MS) used for
the identifcation of the reaction products was performed
on an Agilent 5973 N/6890 N instrument equipped with an
HP-5MS column. MS measurements were carried out using
electron impact ionization at 70 eV. Identifcation was per-
formed by comparison of mass spectra with those given in
the NIST/EPA/NIH Mass Spectral Library [37] and in the
published literature.
The quantitative composition of the reaction solutions
was determined by GC on an Hewlett Packard 5890 Series
II instrument equipped with an HP-1 capillary column and
fame ionization detector. The substrate conversion and
selectivity for each product were calculated as follows:
moles of reacted nitroarene
Nitroarene conversion (%) =
× 100
moles of initial nitroarene
moles of product obtained
moles of reacted nitroarene
Transmission electron microscopy (TEM) images of the
reduced catalysts were taken by a JEM-2100 (JEOL) instru-
ment with a lattice resolution of 0.145 nm at an accelerating
voltage of 200 kV. The samples were prepared by dispersing
the catalysts in ethanol and spraying the suspension onto a
copper grid coated with a perforated carbon flm. The energy
dispersive X-ray (EDX) analysis was carried out with an
INCA 250 (Oxford Instruments) spectrometer. The mean
volume-surface diameter of Pd NPs was determined by
measuring the size of more than 100 NPs. Image process-
ing was performed using the DigitalMicrograph™ (Gatan)
program package.
Product selectivity (%) =
× 100
3 Results and Discussion
3.1 Characterization of Catalysts
The structure, morphology and surface chemistry of the
CNGs supports have been discussed in our previous papers
[12, 24]. Briefy, the C-1 and C-2 samples obtained by oxi-
dative pyrolysis of aromatic hydrocarbons (furnace black
process) are the nanostructured carbons consisting of the
3D aggregated (accreted) globules with an average diam-
eter of 22 and 31 nm, respectively. C-3 was produced by
Palladium content in the catalysts was measured by
atomic absorption spectrometry on an AA-6300 (Shi-
madzu) spectrometer. Before measurements, the catalyst
1 3

R. M. Mironenko et al.
non-oxidative pyrolysis of methane (thermal black pro-
cess) and has rather large predominantly single or slightly
aggregated globules which are mainly 100–200 nm in diam-
eter. This material exhibits the lowest BET surface area
(10 m2 g−1) compared to the C-1 and C-2 samples (103 and
415 m2 g−1, respectively). The total pore volume is of 0.04,
0.91 and 1.00 cm3 g−1 for C-3, C-1 and C-2, respectively.
The micropore volume for all the samples does not exceed
0.015 cm3 g−1. According to infrared spectroscopy, the
CNGs samples difer to some extent in the intensity of the
absorption bands corresponding to C–O stretching vibrations
of various surface oxygen functionalities, such as quinones
(1550–1650 cm−1), ethers, lactones (1200–1300 cm−1), and
hydroxyls (1000–1200 cm−1).
S3), this indicates a complete reduction of supported Pd
PHCs with sodium formate. The hydrogen release observed
at 320–370 K corresponds to the decomposition of palla-
dium β-hydride [38, 39] formed due to the interaction of
palladium with hydrogen at low (room) temperature, when
H2-TPR test was started.
According to TEM measurements and CO chemisorption
data, the size of Pd NPs in the catalysts may depend on the
kind of CNGs as well as the palladium precursor composi-
tion (Fig. 1, Table 1). For instance, Pd NPs formed from pal-
ladium CCs and supported on C-3 are highly dispersed and
their size distribution is very narrow (Fig. 1c), whereas Pd
NPs in the 1% Pd(CCs)/C-1 and 1% Pd(CCs)/C-2 catalysts
are larger in size with a wider range of distribution (Fig. 1a,
b). The size of Pd NPs in the Pd/C-2 catalyst is mostly in
the range of 2–3 nm, but in the case of Pd/C-1 sample, the
fraction of particles larger than 4 nm is close to 50%. Mean-
while, the kind of CNGs has little efect on the dispersion
of Pd NPs obtained from Pd PHCs, and all of the Pd(PHCs)
CNGs samples have close average sizes of Pd NPs with a
distribution mainly within the range of 2–5 nm (Fig. 1d–f).
Apparently, this is due to the fact that Pd PHCs are not sensi-
tive to the size of the nanoglobules and the structure of their
aggregates, since, unlike Pd CCs, they do not form surface
complexes [25, 31].
Therefore, mainly the structure and surface area are
substantially diferent for the chosen CNGs supports, and
it can be expected that the palladium catalysts will difer
in structure of supported Pd NPs as well as their catalytic
properties, depending on the nature of interaction between
Pd NPs and CNGs. At the same time, this Pd–CNGs interac-
tion and therefore the size and catalytic properties of Pd NPs
may also be afected by the composition of the palladium
precursor used for the catalyst preparation [24]. Depending
on the composition of the palladium precursor and the con-
ditions of its deposition, the localization of Pd NPs on the
carbon surface may be very diferent. In the present study,
the following palladium compounds, which are quite aford-
able and can be easily synthesized, were chosen: Pd CCs,
Pd PHCs, [Pd(OAc)2]3, and [Pd2(dba)3]. These palladium
precursors are very diferent in composition, and various
conditions are required for their deposition on CNGs and for
the formation of Pd NPs. The Pd CCs and Pd PHCs com-
plexes are highly soluble in water and will be adsorbed pre-
dominantly on hydrophilic sites of the carbon surface (e.g.,
oxygen functional groups). In contrast, the [Pd(OAc)2]3 and
[Pd2(dba)3] complexes bearing organic ligands are highly
soluble in organic solvents and therefore will be localized
on hydrophobic sites of the carbon surface.
As can be seen, for the samples based on C-3, the use of
Pd PHCs as the palladium precursor leads to the formation
of larger Pd NPs, as compared to the Pd(CCs)/C-3 catalyst
(Fig. 1f vs. 1c, Table 1). Besides, unlike the catalysts pre-
pared from Pd CCs, the distribution of PHCs-derived Pd NPs
is uneven over the globules (Fig. S4). The C-3-supported
catalyst prepared using palladium acetate as the precursor
has a wide bimodal size distribution of Pd NPs (Fig. 1i).
In some cases, the Pd NPs in this sample are coated with
a layer of amorphous carbon. The 1% Pd(dba)/C-3 catalyst
contains spherical Pd NPs, many of which are encapsulated
with amorphous carbon (Fig. 1k). Possibly, such encapsula-
tion of Pd NPs is due to the interaction between palladium
surrounded by organic ligands and carbon support [40, 41].
This may explain the absence of CO chemisorption for the
Pd(dba)/C-3 sample (Table 1, entry 11). Certainly, the for-
mation of carbon overlayer on the metal surface causes a dis-
crepancy between the Pd NPs size values determined from
TEM and CO chemisorption [24, 41].
The reduction profles of the catalysts prepared from Pd
CCs exhibit a single peak corresponding to the hydrogen
consumption due to the reduction of supported palladium
precursor (Fig. S2). The Pd chloride precursor supported on
C-3 is reduced within a very narrow temperature range with
a maximum at 371 K, whereas the peaks for the catalysts
based on the C-1 and C-2 supports are wider and shifted
towards higher temperatures. Apparently, diferences in the
structure of aggregates of globules and the surface area value
for the supports lead to the fact that the clusters of supported
palladium CCs have diferent size and localization place
[31] and therefore are reduced under diferent temperature
conditions.
3.2 Screening of Pd/CNGs Catalysts
in the Hydrogenation of Ethyl 4‑nitrobenzoate
The liquid-phase hydrogenation of 1a is currently of great
interest as an alternative way to produce 2a which is an
important local anesthetic used in medicine [42, 43]. We
have previously found [12] that the hydrogenation of 1a in
the presence of Pd/CNGs catalysts proceeds through the
Since the hydrogen consumption was not observed when
studying the Pd(PHCs)/CNGs catalysts by H2-TPR (Fig.
1 3

Palladium Supported on Carbon Nanoglobules as a Promising Catalyst for Selective Hydrogenation…
Fig. 1 TEM images at diferent magnifcations and the corresponding
histograms of the Pd NPs diameter distribution for the catalysts pre-
pared using Pd CCs (a, b, c), Pd PHCs (d, e, f), [Pd(OAc)2]3 (g, h,
i) and [Pd2(dba)3] (j, k) as palladium precursors, and C-1 (a, d, g, j),
C-2 (b, e, h) and C-3 (c, f, i, k) as the supports
formation of intermediate ethyl 4-(hydroxyamino)benzo-
ate (3a), diethyl 4,4′-azoxydibenzoate (4a) and diethyl
4,4′-azodibenzoate (5a) (Scheme 1), i.e. according to the
mechanism proposed by Haber [1–3, 44]. Therefore, the
selectivity to target amine 2a can be high if the mentioned
intermediates are quickly converted and thus do not accu-
mulate in the reaction mixture.
and [Pd2(dba)3] as the precursors are much less active, and
the selectivity to 2a during the reaction does not reach 100%
due to accumulation of intermediate by-products 3a, 4a and
5a (Table 1, entries 10 and 11). The least active catalyst
prepared from palladium acetate provides only 75% conver-
sion of 1a and nearly 67% selectivity to 2a. Apparently, the
encapsulation of Pd NPs with carbon is the reason for the
low activity of the 1% Pd(Ac)/C-3 and 1% Pd(dba)/C-3 cata-
lysts, since the carbon layer makes the Pd sites inaccessible
to the reactant molecules.
The 1% Pd/CNGs catalysts were screened in the hydro-
genation of 1a under identical reaction conditions: aqueous
ethanol medium, temperature of 323 K, hydrogen pressure
of 0.5 MPa, reaction time of 1 h. It was found that the cata-
lysts prepared by deposition of Pd CCs and especially Pd
PHCs on the C-3 support demonstrate high activity (in terms
of TOF) providing complete conversion of 1a and nearly
100% selectivity to target amine 2a (Table 1, entries 8 and
9). The C-3-supported catalysts prepared using [Pd(OAc)2]3
Compared to the catalysts based on C-3, the C-1 and
C-2-supported ones are less active in the hydrogenation
of 1a. However, most of them also allow achieving 100%
conversion of 1a for 1 h (Table 1, entries 1–7). Their cata-
lytic performances depend on the composition of Pd pre-
cursor and conditions of the catalyst preparation. Thus,
1 3

R. M. Mironenko et al.
Table 1 Mean size of Pd NPs in the 1% Pd/CNGs catalysts and their catalytic properties in the hydrogenation of 1a
Entry
Catalyst
Mean size of Pd NPs (nm)a
TOF
Conversion of Selectivity (%)d
(mol H2 mol Pd−s 1 min−1 b
)
1a (%)c
2a
3a
4a
5a
1
2
3
4
5
6
7
8
1% Pd(CCs)/C-1
1% Pd(PHCs)/C-1
1% Pd(Ac)/C-1
1% Pd(dba)/C-1
1% Pd(CCs)/C-2
1% Pd(PHCs)/C-2
1% Pd(Ac)/C-2
1% Pd(CCs)/C-3
1% Pd(PHCs)/C-3
1% Pd(Ac)/C-3
1% Pd(dba)/C-3
3.3
4.6
5.0
9.3
3.3
5.0
3.7
2.8
<30
153
<30
<30
<30
328
<30
253
398
> 99.9
> 99.9
99.3
> 99.9
96.1
> 99.9
> 99.9
> 99.9
> 99.9
75.2
96.2
> 99.9
87.8
98.0
86.4
> 99.9
88.9
> 99.9
> 99.9
66.7
1.2
0
8.5
0
5.4
0
4.9
0
1.6
0
1.5
0
6.6
0
3.8
0
1.0
0
2.2
2.0
1.6
0
2.4
0
0
9
10
11
4.3
4.7
0
23.4
12.0
0
8.7
8.1
<30
<30
1.2
2.1
No chemisorption
> 99.9
77.8
Reaction conditions: temperature of 323 K, pressure of 0.5 MPa, 1 h, 50 cm3 of aqueous ethanol, 500 mg of 1a, 50 mg of catalyst
aAccording to CO chemisorption
bTOF values were calculated using Pd dispersions derived from CO chemisorption data. For the least active catalysts, it was not able to deter-
mine the reaction rate and TOF, since a mass fow meter used in the present study does not allow measuring H2 consumption at too low fow rate
(<3 cm3 min−1) due to automatic valve closing
cSubstrate conversion was determined by GC after 1 h of the reaction
dSelectivity for the products was determined by GC; the identifcation was carried out by GC–MS, see Figs. S5–S9
the hydrogenation over the catalysts prepared from Pd
PHCs always proceeds with a high rate, resulting in com-
plete conversion of 1a to the desired amine 2a with nearly
100% selectivity (Table 1, entries 2 and 6). At the same
time, hydrogenation over the catalysts obtained from Pd
CCs, [Pd(OAc)2]3 and [Pd2(dba)3] proceeds slowly, and the
selectivity to 2a is <100% (Table 1, entries 1, 3–5, 7). For
example, in the case of the least active C-2-supported cata-
lyst obtained using Pd CCs as the precursor, the conversion
of 1a does not reach 100% after 1 h of the reaction (Table 1,
entry 5), and the selectivity to 2a is decreased (ca. 86%)
because intermediate 3a, 4a and 5a accumulate in the reac-
tion mixture.
C-2-supported catalysts suspended in the reaction solution
have an internal porosity and their surface has a pronounced
roughness, while the suspension of C-3-based catalysts con-
sists of spherical particles with a smooth surface. As a result
of this diference, the adsorption–desorption processes on
the Pd/C-3 catalysts proceed faster than on the Pd/C-1 and
Pd/C-2 catalysts. This can contribute to the high hydrogena-
tion activity and selectivity of the catalysts supported on the
C-3 support.
According to published data, the hydrogenation of 1a
can be performed in the presence of various carbon-sup-
ported catalysts containing Ni [10, 45, 46], Pt [47, 48] or Pd
[49–53]. The advantage of Pd/C catalysts over nickel and
platinum ones is that they allow achieving complete conver-
sion of the substrate and high yield of amine 2a under mild
reaction conditions (temperature of 293–333 K, pressure of
0.1–1.3 MPa) for a short time. For instance, Visentin et al.
[51] showed that almost 100% yield of 2a can be achieved
during the hydrogenation of 1a in the presence of 1% Pd
supported on activated carbon in a methanol solution at
323 K, 1.3 MPa for about 2 h. According to other work [53],
the use of the Pd catalyst supported on N-modifed ordered
mesoporous CMK-3 carbon made it possible to achieve
almost 100% yield of 2a by the liquid-phase hydrogenation
of 1a at 333 K, 1 MPa for 4 h. The CNGs-supported 1%
palladium catalysts studied in the present work exhibit high
activity in the liquid-phase hydrogenation of 1a to target
amine 2a under mild conditions (323 K, 0.5 MPa, 1 h) and,
as can be seen, are not inferior and even superior to the
When comparing the catalytic properties of the CCs-
derived catalysts, the increased hydrogenation activity of the
C-3-based catalysts in comparison with catalysts supported
on C-1 and C-2 may be related with the high dispersion of
supported Pd NPs (Fig. 1c vs. a and b; Table 1, entries 1,
5, 8), which ensures the accessibility of Pd to molecules of
reactants. Besides, the C-3 support has such structure and
properties that are apparently favorable to achieve excel-
lent catalytic performances in the hydrogenation of 1a.
This support consists predominantly of single or slightly
aggregated CNGs, whereas C-1 and C-2 are constructed of
aggregates containing more than a hundred accreted CNGs.
In the reaction solution, the catalysts based on these car-
bon materials form a suspension, the least particle size of
which is determined by the size of the corresponding aggre-
gates. Therefore, the particles (i.e., aggregates) of C-1- and
1 3

Palladium Supported on Carbon Nanoglobules as a Promising Catalyst for Selective Hydrogenation…
According to published data, such solvents as methanol
[51], ethanol [11, 54], 2-propanol [50, 52], ethyl acetate
[53], tetrahydrofuran (THF) [55] and toluene [49] can be
efectively used for the liquid-phase hydrogenation of 1a
over palladium catalysts. These solvents have a good dis-
solving capacity for substrate 1a. However, they may difer
in their infuence on both the rate of hydrogenation and the
yield of products due to the diference in ability of formation
of adducts with carbon surface or palladium sites.
In the present study, we used the solvents which can be
diferentiate by polarity into three groups: (1) polar protic
solvents, such as methanol, aqueous ethanol (96%) and
2-propanol; (2) polar aprotic solvents, such as ethyl acetate,
THF and N,N-dimethylformamide (DMF); (3) nonpolar
solvents, such as 1,4-dioxane, benzene and cyclohexane
(see also Table S3). To investigate the solvent efect, the
hydrogenation of 1a was performed under identical reac-
tion conditions: temperature of 323 K, pressure of 0.5 MPa,
reaction time of 1 h. It was found that when using aliphatic
alcohols and cyclohexane as the solvents, the substrate 1a
is completely hydrogenated with nearly 100% selectivity to
2a (Table 2, entries 1–3, 9). The highest TOF values are
observed during hydrogenation in aqueous ethanol and espe-
cially cyclohexane media. Under the same conditions, the
hydrogenation of 1a in ethyl acetate, THF (polar aprotic sol-
vents) and such nonpolar solvents as benzene and 1,4-diox-
ane proceeds very slowly and does not ensure a complete
conversion of 1a (Table 2, entries 4, 5, 7, 8); the selectivity
to 2a does not always reach 100% (THF and 1,4-dioxane)
due to accumulation of intermediate products 3a and 4a.
When using DMF as the solvent, no conversion of 1a is
observed (Table 2, entry 6).
Scheme 1 Reaction network for hydrogenation of 1a over Pd/CNGs
catalysts [12]
The dramatic infuence of solvents can be clearly seen
when plotting the substrate conversion (or another catalytic
response) against some parameter that characterizes the
polarity of the solvent [56–60]. For this purpose, the empiri-
cal solvatochromic parameter ETN proposed by Reichardt [61,
62] can be used (see Table S3). Plots of the 1a conversion
after 1 h of the reaction as a function of ETN for each group
of solvents are shown in Fig. 2. As can be seen, when using
nonpolar and polar aprotic solvents, the conversion of 1a
decreases with increasing solvent polarity, whereas in alco-
hols, which are polar protic solvents, the conversion of 1a
always reaches 100%.
Pd/C catalysts previously proposed for this reaction. The
catalytic activity (in terms of TOF and substrate conversion)
and selectivity depend on the kind of CNGs and the catalyst
preparation method. Although most Pd/CNGs catalysts pro-
vide complete conversion of 1a with the selective formation
of 2a, the catalysts based on the C-2 and C-3 supports and
prepared using Pd PHCs as the precursor are superior to oth-
ers apparently due to the optimal combination of the support
structure and texture, size and uniformity of Pd NPs, as well
as the nature of localization place of Pd sites.
3.3 Efect of the Solvent Nature on the Catalyst
Activity and Selectivity in the Hydrogenation
of Ethyl 4‑nitrobenzoate
The observed infuence of solvent nature on the catalytic
performance can be explained by diferences in the inter-
actions between molecules of solvent, reactants and the
catalyst surface. Thus, the interaction between substrate
and nonpolar cyclohexane is obviously weak. Besides, the
adsorption of cyclohexane molecules on Pd active sites is
also weak. As a result, the hydrogenation of 1a is not inhib-
ited in cyclohexane, and the substrate is completely con-
verted to 2a with a high rate. With increasing polarity in the
To study the infuence of solvent nature on the catalytic pro-
cess, we chose the 1% Pd(PHCs)/C-2 catalyst which is one
of the most active and highly selective in hydrogenation of
1a.
1 3

R. M. Mironenko et al.
Table 2 Hydrogenation of
1a over 1% Pd(PHCs)/C-2 in
various solvents
Entry
Solvent
TOF (mol H2 mol
Conversion of Selectivity (%)
Pd−s 1 min−1
)
1a (%)
2a
3a
4a
5a
1
2
3
4
5
6
7
8
9
MeOH
EtOH (aq.)
i-PrOH
THF
EtOAc
413
491
216
<30
<30
<30
<30
<30
556
> 99.9
> 99.9
> 99.9
96.8
26.0
0
54.1
60.0
> 99.9
> 99.9
> 99.9
98.3
95.6
> 99.9
–
86.8
> 99.9
> 99.9
0
0
1.7
3.5
0
–
13.2
0
0
0
0
0.9
0
–
0
0
0
0
0
0
0
0
–
0
0
0
DMF
1,4-Dioxane
Benzene
Cyclohexane
0
Reaction conditions: temperature of 323 K, pressure of 0.5 MPa, 1 h, 100 cm3 of solvent, 1.000 g of 1a,
100 mg of catalyst
Takagi et al. [59] reported a lack of conversion of benzyl
alcohol when trying to hydrogenate it in a solution of DMF
over ruthenium catalyst.
Like aprotic solvents, the protic ones (i.e., alcohols)
are able to strongly interact with the substrate molecules
and metal surface [58]. However, the excellent perfor-
mance observed during the hydrogenation of 1a in alcohols
(Table 2, entries 1–3) is apparently not due to the polar-
ity of these solvents, but rather are related to their higher
hydrogen-bond donating ability which, unlike nonpolar and
polar aprotic solvents, has nonzero values (cf. α-parameters
in Table S3). Therefore, since the protic solvents easily form
hydrogen bonds with the intermediates and product, a strong
interaction between them facilitates the desorption of the
intermediates and product from the catalyst surface and,
Fig. 2 Conversion of 1a after 1 h of hydrogenation versus solvent
thus, promotes the catalytic hydrogenation [63]. Further-
more, the protic solvents can interact with the nitro groups
of adsorbed substrate via the formation of hydrogen bonds,
which favors the polarization of N=O bonds and makes them
easy to be attacked by chemisorbed hydrogen [60, 63]. For
these reasons, the higher TOF value in aqueous ethanol com-
pared to that in other alcohols (Table 2, entries 1–3) may be
due to the presence of water which has a higher hydrogen-
bond donating ability than alcohols [62]. It was previously
shown [63] that adding even 5% of water to ethanol leads
to a signifcant acceleration of the hydrogenation of nitro
compound.
polarity EN. Reaction conditions: temperature of 323 K, pressure
T
of 0.5 MPa, 1 h, 100 cm3 of solvent, 1.000 g of 1a, 100 mg of 1%
Pd(PHCs)/C-2 catalyst
series cyclohexane<benzene<1,4-dioxane, the mentioned
interactions are enhanced and, thus, hamper the substrate
conversion.
The molecule of polar aprotic solvent can donate lone pair
electrons from the oxygen atoms. The adsorbed solvent mol-
ecules block the active sites due to the interaction between
suitable atomic orbital of Pd and lone pair electrons of sol-
vent molecule (donor–acceptor interaction). As a result, the
hydrogenation activity (TOF) is lowered due to competitive
adsorption between reactants and solvent on the metal sur-
face. At the same time, the interaction between polar aprotic
solvents and the substrate (solvation) hampers the adsorption
of the substrate onto the metal surface [58]. Apparently, in
the case of DMF, the solvent–substrate and solvent–catalyst
interactions are so strong that the hydrogenation of 1a in
DMF is completely suppressed. This result for DMF is con-
sistent with some earlier observations [59, 63]. For example,
Note that the high intrinsic rate (TOF) of hydrogena-
tion using cyclohexane and alcohols as solvents could be
also explained by hydrogen-transfer processes, since these
compounds act as good hydrogen donors and can reduce
nitroarenes in the presence of a catalyst without an external
source of hydrogen [64–68]. However, in the present study,
even traces of by-products that should have been formed
during the catalytic transfer hydrogenation (for example,
carbonyl compounds and the products of their interaction
with the amino group [66–68]) were not detected in the
1 3

Palladium Supported on Carbon Nanoglobules as a Promising Catalyst for Selective Hydrogenation…
reaction mixtures. The result of a separate experiment (with-
out hydrogen gas) shows that a possibility of reduction by
hydrogen transfer from solvent is excluded under the chosen
experimental conditions. Obviously, the reaction conditions
are too mild to allow catalytic transfer hydrogenation to be
sufciently intensive.
In general, the regularities of the solvent efect found
for the Pd/CNGs catalyst turned out to be close to those
described in the literature for carbon-supported palladium
catalysts. However, the absence of microporosity leads to
a greater “manifestation” of the solvent efect due to the
absence of blockage of catalytically active sites located in
the micropores by solvent molecules.
Fig. 3 Stability test for the 1% Pd(PHCs)/C-2 catalyst in the hydro-
3.4 Stability of Pd/CNGs Catalyst
genation of 1a. Reaction conditions: temperature of 323 K, pressure
of 0.5 MPa, 5–8 min each cycle, 5.000 g of 1a, 500 mg of catalyst.
The selectivity to 2a was measured at incomplete conversion of 1a
(70–85%)
in the Hydrogenation of Ethyl 4‑nitrobenzoate
As shown above, the hydrogenation of 1a in cyclohexane
proceeds with a highest rate. However, the aqueous ethanol
was chosen as a solvent for further experiments for the fol-
lowing reasons. Firstly, aqueous ethanol is more afordable
than cyclohexane. Secondly, the use of ethanol is preferable
from an environmental standpoint, because cyclohexane is
recognized as a dangerous substance for the environment
(in contrast to ethanol) [69]. Finally, since the hydrogena-
tion of nitroarenes is always accompanied by the formation
of water, carrying out the reaction in cyclohexane, espe-
cially on a large scale (i.e., at high substrate concentration
in the reaction solution), can lead to undesirable formation
of emulsion.
time, the selectivity to 2a would be the same (about 80%)
for each cycle. The intrinsic reaction rate (TOF), although
slightly decreases, still remains at a high level for all cycles.
This slight decline in activity from cycle to cycle is not due
to deactivation but rather to the inevitable catalyst loss dur-
ing the recycles. Note that the analysis of the spent catalyst
(after ffth cycle) showed a slight decrease in palladium
content (by 0.2 wt%). However, this is apparently due not
to the loss of palladium during the experiment, but to the
accumulation of products that are difcult to remove from
the catalyst surface.
We have previously shown [12] that the 1% Pd(PHCs)/C-3
catalyst can be repeatedly used many times without losing
activity (100% conversion of 1a) and maintaining nearly
100% selectivity to amine 2a. In this paper, to study the
stability of the 1% Pd(PHCs)/C-2 catalyst in the hydrogena-
tion of 1a, the reusability test was performed at incomplete
conversion of the substrate (70–85%) under the same con-
ditions as indicated above (EtOH (aq.), 323 K, 0.5 MPa)
but on an enlarged scale (5.000 g of 1a and 500 mg of the
catalyst). After each cycle, the catalyst was fltered of, thor-
oughly washed with acetone, dried and used in the next run
with a freshly prepared solution of 1a. As can be seen from
Fig. 3, the catalyst can be repeatedly used fve times while
maintaining selectivity to 2a at about 80%. The slight devia-
tions in the selectivity to 2a for diferent cycles are associ-
ated with the fact that the reaction proceeds very fast under
the chosen conditions (just 5–8 min is enough to achieve
the necessary values of substrate conversion and selectivity
to 2a), and thus, using the technique for reaction control
(measuring the amount of hydrogen consumed in a given
time interval), it was not possible to distinctly defne the
moment when the experiment should be discontinued, so
that conversion of 1a would be incomplete and, at the same
Therefore, the 1% Pd(PHCs)/C-2 catalyst exhibits stabil-
ity in hydrogenation of 1a, which may be related with the
absence of microporosity in the support. Thus, catalytically
active sites are not blocked by intermediate condensation
products (although such a process would have to be very
pronounced at incomplete conversion of the substrate), and
the hydrogenation proceeds with high selectivity to 2a.
3.5 Catalytic Hydrogenation of Various Nitroarenes
The 1% Pd(PHCs)/C-2 catalyst, while being not the most
active sample, still showed excellent performance in hydro-
genation of 1a under mild conditions (complete conversion
of 1a and nearly 100% selectivity to 2a) and thus was uti-
lized in hydrogenation of a series of nitroarenes to explore
its general applicability. The choice of substrates subjected
to hydrogenation was due in particular to the high practical
importance of the resulting hydrogenation products [6, 7,
42, 43, 70, 71]. The results summarized in Table 3 show that
most of the substrates are completely converted in just 1 h
giving corresponding amines with nearly 100% selectivity
under mild reaction conditions (323 K, 0.5 MPa). It proves
1 3

R. M. Mironenko et al.
Table 3 Hydrogenation of various nitroarenes over 1% Pd(PHCs)/C-2 catalyst
Entry
Substrate
Producta
TOF (mol H2 mol
Conversion (%)b
> 99.9
Selectivity (%)
> 99.9
Pd−s 1 min−1
1204
)
1
2
481
> 99.9
> 99.9
3
4
498
701
> 99.9
> 99.9
> 99.9
> 99.9
5
6
7
<30
965
216
95.1
> 99.9
> 99.9
> 99.9
> 99.9
> 99.9
8
337
> 99.9
> 99.9
9
615
292
> 99.9
> 99.9
> 99.9
> 99.9
10
11c
154
> 99.9d
90.0
Reaction conditions unless otherwise noted: temperature of 323 K, pressure of 0.5 MPa, 1 h, 100 cm3 of aqueous ethanol, 1.000 g of substrate,
100 mg of catalyst
aCompounds 2b–l were identifed by GC–MS, see Figs. S10–S30
bSubstrate conversion was determined by GC after 1 h of the reaction
cReaction conditions: temperature of 343 K, pressure of 0.5 MPa, 6 h, 100 cm3 of aqueous ethanol, 1.000 g of substrate, 200 mg of catalyst
dConversion of 1l after 6 h of the reaction according to GC
that the 1% Pd(PHCs)/C-2 catalyst possesses high activity
and selectivity for the hydrogenation of structurally diverse
nitroarenes.
by extension of the reaction time. The hydrogenation of
1-(2,3-difluoro-6-nitrophenoxy)acetone (1 l) proceeds
slowly even at higher temperature (343 K) and higher
content of catalyst than in the hydrogenation of other
nitroarenes (Table 3, entry 11). In this case, it takes
6 h to achieve complete conversion of 1l. The reaction
is accompanied by intramolecular cyclization to inter-
mediate 7,8-difluoro-3-methyl-2H-1,4-benzoxazine (3l)
(Scheme 2). Under the specified reaction conditions, the
further hydrogenation of C=N bond in 3l is hampered,
and the selectivity to 7,8-difluoro-3-methyl-3,4-dihy-
dro-2H-1,4-benzoxazine (2l) does not exceed 90%. In the
As can be also seen from Table 3, the intrinsic reaction
rate (in terms of TOF) essentially depends on the struc-
ture of the substrate. Thus, the hydrogenation of nitroben-
zene (1b) to aniline (2b) proceeds with the highest rate
(Table 3, entry 1). Under the same reaction conditions,
4-nitrophenol (1f) is hydrogenated most slowly (TOF
could not be determined), and the substrate conversion
is incomplete after 1 h of the reaction (Table 3, entry
5), although 100% conversion of 1f could be achieved
1 3

Palladium Supported on Carbon Nanoglobules as a Promising Catalyst for Selective Hydrogenation…
In the case of the hydrogenation of ortho-substituted
nitroarenes such as 2-nitroaniline (1h), 3-nitrophthalic
acid (1 k) and 1 l, the rate of hydrogenation is relatively
low (Table 3, entries 7, 10 and 11), which may be due
to the steric hindrances that are characteristic of ortho-
substituted nitrobenzenes.
F
O
F
F
F
O
F
O
N
F
O
H2
H2
Me
–H2O
NO2
Me
N
H
Me
2l
1l
3l
Scheme 2 Reaction pathway of the catalytic hydrogenation of 1l
4 Conclusions
same time, this is a rather high selectivity to 2l, which is
apparently achieved due to the absence of microporosity
in the support, as has been previously shown for the pal-
ladium catalysts supported on mesoporous Sibunit™ [72].
Meanwhile, it is difficult to present a general picture of
the influence of a substituent on the hydrogenation rate.
It is known that the study of substituent electronic effects
is limited (or complicated) for heterogeneous catalytic
systems by the involvement of adsorption phenomena,
the inhomogeneous distribution and nature of active sites
and the contribution of steric effect [73]. Such limita-
tion should be particularly pronounced when using the
catalysts supported on microporous supports. However,
we found the relationships in some particular cases when
studying the Pd/CNGs catalyst. For example, an increase
in TOF for the hydrogenation of substrates in a series
4-nitroaniline (1i) < 1-methoxy-4-nitrobenzene (1g) < 1b
(Table 3, entries 1, 6 and 8) can be explained taking into
account a nucleophilic mechanism for the hydrogena-
tion of nitroarenes [74–76] and the Hammett equation
considering substituent electronic effects [73–76]. In a
nucleophilic attack, the reaction rate is enhanced by elec-
tron-withdrawing substituents. The substituent electron
donating/acceptor character can be evaluated by σ factor
which increases in a series 1i (− 0.66) < 1 g (− 0.27) < 1b
(0) [77]. Therefore, according to the Hammett equa-
tion for nucleophilic reactions, the hydrogenation rate
increases in proportion to the increase in σ constant of
the para-substituent.
In summary, it was found that 1% palladium catalysts sup-
ported on CNGs exhibit high activity and selectivity in the
liquid-phase hydrogenation of various nitroarenes under
mild conditions (323 K, 0.5 MPa, 1 h). On the example
of ethyl 4-nitrobenzoate hydrogenation, it was shown that
the catalytic activity (in terms of TOF and substrate con-
version) and selectivity depend on the physicochemical
characteristics of CNGs, the composition of Pd precursor,
catalyst preparation method and the nature of solvent used
as the reaction medium.
It is possible to highlight the following advantages of
our proposed Pd/CNGs catalysts for their use in the hydro-
genation of nitroarenes: (1) low content of expensive pal-
ladium in the catalyst (1 wt%); (2) CNGs material used as
the catalyst support is produced on a large scale (carbon
black), which makes it quite afordable; (3) Pd/CNGs cata-
lysts can be prepared by simple and well-known methods;
(4) complete conversion of nitroarene and nearly 100%
selectivity to amine can be achieved under mild conditions
of hydrogenation; (5) Pd/CNGs catalyst can be repeat-
edly used while maintaining the same selectivity to target
amine, that makes palladium catalysts of this type attrac-
tive for the hydrogenation of nitroarenes on a large scale.
The above-mentioned advantages of Pd/CNGs catalysts
could arise due to the globular morphology of the chosen
supports as well as the absence of micropores and pro-
nounced surface defects. The results of the present study
clearly show that variation of the physicochemical prop-
erties of CNGs makes it possible to change the formation
conditions and size of supported Pd NPs and thus achieve
improved performances of Pd/CNGs catalysts.
When comparing the hydrogenation of nitrophenols
1e and 1f (Table 3, entries 4 and 5), the conjugation and
inductive effects should be taken into account [78]. Due
to the conjugation effect in the molecules of 1e and 1f,
the negative charge in the nitrophenoxide ion may be
delocalized onto the nitro group, making the group more
stable. On the other hand, the inductive effect depend-
ing on the distance between the substituent group and
the NO2 group also plays a certain role. Obviously, the
inductive effect in ortho-isomer 1e is stronger than that in
para-isomer 1f. The stronger inductive effect in 1e makes
its nitrogen atom more positively charged, resulting in
higher reactivity.
Acknowledgements We thank O. V. Maevskaya, Dr. R. R. Izmailov, I.
V. Muromtsev, Dr. A. B. Arbuzov, E. N. Kudrya, Dr. V. P. Talsi and S.
N. Evdokimov for their help with experiments. Besides, the authors are
grateful to Dr. A. S. Kostyuchenko (Omsk State Technical University,
Omsk, Russia) for the supply of some nitroarenes. The authors also
gratefully acknowledge Dr. G. L. Rusinov (I. Ya. Postovskiy Institute of
Organic Synthesis, Yekaterinburg, Russia) for the supply of 1-(2,3-dif-
luoro-6-nitrophenoxy)acetone.
Funding This work was supported by the Russian Foundation for Basic
Research (Grant Number 16-29-10742).
1 3

R. M. Mironenko et al.
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