Palladium nanoparticles embedded in mesoporous carbons as efficient, green and reusable catalysts for mild hydrogenations of nitroarenes

Article abstract of DOI:10.1039/d0ra05713d

The reduction of nitroarenes is the most efficient route for the preparation of aromatic primary amines. These reductions are generally performed in the presence of heterogeneous transition metal catalysts, which are rather efficient but long and tedious to prepare. In addition, they contain very expensive metals that are in most cases difficult to reuse. Therefore, the development of efficient, easily accessible and reusable Pd catalysts obtained rapidly from safe and non-toxic starting materials was implemented in this report. Two bottom-up synthesis methods were used, the first consisted in the impregnation of a micro/mesoporous carbon support with a Pd salt solution, followed by thermal reduction (at 300, 450 or 600 °C) while the second involved a direct synthesis based on the co-assembly and pyrolysis (600 °C) of a mixture of a phenolic precursor, glyoxal, a surfactant and a Pd salt. The obtained composites possess Pd nanoparticles (NPs) of tunable sizes (ranging from 1-2 to 7.0 nm) and homogeneously distributed in the carbon framework (pores/walls). It turned out that they were successfully used for mild and environment-friendly hydrogenations of nitroarenes at room temperature under H₂(1 atm) in EtOH in the presence of only 5 mequiv. of supported Pd. The determinations of the optimal characteristics of the catalysts constituted a second objective of this study. It was found that the activity of the catalysts was strongly dependent on the Pd NPs sizes,i.e., catalysts bearing small Pd NPs (1.2 nm obtained at 300 °C and 3.4 nm obtained at 450 °C) exhibited an excellent activity, while those containing larger Pd NPs (6.4 nm and 7.0 nm obtained at 600 °C, either by indirect or direct methods) were not active. Moreover, the possibility to reuse the catalysts was shown to be dependent on the surface chemistry of the Pd NPs: the smallest Pd NPs are prone to oxidation by air and their surface was gradually covered by a PdO shell decreasing their activity during reuse. A good compromise between intrinsic catalytic activity (i.e. during first use) and possibility of reuse was found in the catalyst made by impregnation followed by reduction at 450 °C since the hydrogenation could be performed in only 2 h in EtOH or even in water. The catalyst was quantitatively recovered after reaction by filtration, used at least 7 times with no loss of efficiency. Advantageously, almost Pd-free primary aromatic amines were obtained since the Pd leaching was very low (<0.1% of the introduced amount). Compared to numerous reports from the literature, the catalysts described here were both easily accessible from eco-friendly precursors and very active for hydrogenations under mild and “green” reaction conditions.

Full text of DOI:10.1039/d0ra05713d

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Palladium nanoparticles embedded in mesoporous
carbons as efficient, green and reusable catalysts
for mild hydrogenations of nitroarenes†
Cite this: RSC Adv., 2020, 10, 36741
Mohamed Enneiymy,ab Philippe Fioux,ab Claude Le Drian,ab Camelia Matei
ab
ab
*
*
Ghimbeu
and Jean-Michel Becht
The reduction of nitroarenes is the most efficient route for the preparation of aromatic primary amines.
These reductions are generally performed in the presence of heterogeneous transition metal catalysts,
which are rather efficient but long and tedious to prepare. In addition, they contain very expensive
metals that are in most cases difficult to reuse. Therefore, the development of efficient, easily accessible
and reusable Pd catalysts obtained rapidly from safe and non-toxic starting materials was implemented
in this report. Two bottom-up synthesis methods were used, the first consisted in the impregnation of
a micro/mesoporous carbon support with a Pd salt solution, followed by thermal reduction (at 300, 450
or 600 ꢀC) while the second involved a direct synthesis based on the co-assembly and pyrolysis (600 ꢀC)
of a mixture of a phenolic precursor, glyoxal, a surfactant and a Pd salt. The obtained composites
possess Pd nanoparticles (NPs) of tunable sizes (ranging from 1–2 to 7.0 nm) and homogeneously
distributed in the carbon framework (pores/walls). It turned out that they were successfully used for mild
and environment-friendly hydrogenations of nitroarenes at room temperature under H2 (1 atm) in EtOH
in the presence of only 5 mequiv. of supported Pd. The determinations of the optimal characteristics of
the catalysts constituted a second objective of this study. It was found that the activity of the catalysts
was strongly dependent on the Pd NPs sizes, i.e., catalysts bearing small Pd NPs (1.2 nm obtained at
300 ꢀC and 3.4 nm obtained at 450 ꢀC) exhibited an excellent activity, while those containing larger Pd
NPs (6.4 nm and 7.0 nm obtained at 600 ꢀC, either by indirect or direct methods) were not active.
Moreover, the possibility to reuse the catalysts was shown to be dependent on the surface chemistry of
the Pd NPs: the smallest Pd NPs are prone to oxidation by air and their surface was gradually covered by
a PdO shell decreasing their activity during reuse. A good compromise between intrinsic catalytic activity
(i.e. during first use) and possibility of reuse was found in the catalyst made by impregnation followed by
reduction at 450 ꢀC since the hydrogenation could be performed in only 2 h in EtOH or even in water.
The catalyst was quantitatively recovered after reaction by filtration, used at least 7 times with no loss of
efficiency. Advantageously, almost Pd-free primary aromatic amines were obtained since the Pd leaching
was very low (<0.1% of the introduced amount). Compared to numerous reports from the literature, the
catalysts described here were both easily accessible from eco-friendly precursors and very active for
hydrogenations under mild and “green” reaction conditions.
Received 30th June 2020
Accepted 22nd September 2020
DOI: 10.1039/d0ra05713d
rsc.li/rsc-advances
supercapacitors3 and more recently as supports for embedding
metal nanoparticles.4,5 In particular, the development of
1 Introduction
heterogeneous reusable Pd catalysts has raised huge interest
since the price of this precious metal has considerably
increased: >28.000 V kgÀ1 in October 2018, >43.000 V kgÀ1 one
year later and >60.000 V kgÀ1 in March 2020. In this respect,
mesoporous carbons have recently emerged as promising
supports for the development of Pd catalysts for ne chemistry,
allowing the recovery and the reuse of these catalysts containing
this scarce and expensive precious metal.6 For example, our
group has recently developed eco-friendly one-pot syntheses of
heterogeneous reusable Pd-containing mesoporous carbons
(Pd@MC) from green and easily available carbon precursors,
Mesoporous carbon materials possessing high surface area,
uniform and tunable pore size/architecture, pore inter-
connectivity, and good electronic conductivity nd numerous
applications1 as absorbers for water/air decontamination and
for H2 storage,2 as electrode materials for batteries and
a
´
Universite de Haute-Alsace, CNRS, IS2M, UMR 7361, F-68100 Mulhouse, France
b
´
Universite de Strasbourg, France. E-mail: jean-michel.becht@uha.fr
† Electronic supplementary information (ESI) available: XPS wide scan of IM1,
IM2, IM3 and DM; 1H and 13C-NMR spectra of compounds 1a–j. See DOI:
10.1039/d0ra05713d
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tannins6 or phloroglucinol7 and glyoxal, a porogen template hollow magnetic mesoporous spheres recovered by a silica
(Pluronic F-127) and a Pd salt. Later, inspired by the previous shell,25 nitrogen-doped nanobers,26 porous organic polymers,27
results, magnetic Pd–Fe@MC and Pd–Co@MC catalysts have poly(vinyl)chloride28 have been reported and allowed efficient
been further developed by simple addition of a cobalt7a or an hydrogenations of nitroarenes.
iron8 salt during their preparation. These magnetic and non-
However, in most cases the preparations of the heteroge-
magnetic catalysts have been successfully used in ne chem- neous catalysts remained long and difficult requiring for
istry for aryl–aryl bond forming reactions.9 For example, some example the use of toxic and sensitive starting materials.
mesoporous carbons containing Pd NPs showed an excellent Recently, some of us have developed easily accessible reusable
activity for Suzuki–Miyaura couplings in green solvents phytochemical resins containing Pd NPs for mild hydrogena-
(propane-1,2-diol6 or even water7b) and were reused several tions of nitroarenes.22 Herein, we describe the preparation,
times with no signicant loss of efficiency.6,7 Noteworthily, our characterization (using TEM, XRD, XPS and gas adsorption) and
group has recently disclosed a Pd@MC catalyst showing an reactivity of other promising families of easily accessible
outstanding activity for Suzuki–Miyaura reactions in water in “green” reusable catalysts, Pd-containing mesoporous carbons
the presence of only ppm amounts of supported Pd affording (Pd@MC), which were obtained via two synthetic approaches.
almost Pd-free products7b without particular purication An indirect method was based on impregnation of a meso-
steps.10 Compared to most of the heterogeneous reusable Pd porous carbon support with a Pd salt solution followed by
catalysts described in the literature for C–C bond forming thermal reduction whereas a more direct route involves co-
reactions,11 a major advantage of the Pd@MC, Pd–Fe@MC or assembly and pyrolysis of green phenolic-resin precursors
Pd–Co@MC catalysts lies in their short and efficient “green” containing a Pd salt. The synthesis method used, the type of
preparations from non-toxic precursors, their high activity and support, the Pd-precursor, the thermal annealing conditions,
excellent reusability.12 A major objective now consists in are well known factors impacting both size and dispersion of
studying the possible use of these catalysts for other crucial the Pd-particles and consequently the catalytic perfor-
reactions in ne chemistry.
mances.29,6 Therefore, this study had two major objectives: (1)
Recently the development of highly efficient heterogeneous optimize the reaction conditions to perform mild hydrogena-
reusable catalysts for the reduction of nitroarenes to the cor- tions of nitroarenes under a 1 atm H2 pressure in EtOH or water;
responding aromatic primary amines has attracted tremendous (2) determine the relationships between the properties of the
attention. Indeed this reaction has found widespread applica- Pd@MC catalysts nely controlled during their synthesis by two
tions on industrial scales since it allows efficient preparations different methods, and their catalytic activities. To the best of
of versatile aromatic primary amines, which are building blocks our knowledge the inuence of both the Pd NPs sizes and their
for active pharmaceutical ingredients (API), agrochemicals, surface chemistry on their catalytic activity for hydrogenations
pigments, dyes or polymers.13 In addition, from an environ- of nitroarenes has not yet been studied.
mental point of view, by reduction, these highly toxic and
pollutant nitroarenes (oen obtained as by-products in
synthesis or present as contaminants) are transformed in less
hazardous aromatic primary amines of possible use.14 Two
2 Results and discussion
Preparation and characterization of Pd@MC catalysts IM1–
IM3 and DM
major routes for the reduction of nitroarenes are generally used,
either in the presence of sodium borohydride (NaBH4) and Inspired by a previous work from our group,7b catalysts IM1–
a heterogeneous Ru-,15 Au-,16 Cu- (ref. 17) or Pd-catalyst (ref. 18) IM3 were prepared via an indirect method (IM) consisting in
(rst route) or in the presence of H2 and a heterogeneous Rh-,19 a two-step bottom-up approach involving rstly the preparation
Pt- 20 or Pd-catalyst (second route).21,22 However, the rst route of the mesoporous carbon support followed by the introduction
suffers from a lack of chemoselectivity and above all from the of the Pd NPs in the carbon network (Fig. 1). For this purpose,
formation of salts as by-products which should be avoided on self-assembly of the phloroglucinol–glyoxal resin in the pres-
an industrial scale. For example in 2019, the group of Yang has ence of Pluronic F-127 surfactant in EtOH followed by thermal
developed efficient Pd NPs graed on conjugated macrocyclic treatment (TT) at 600 ꢀC under argon afforded the porous
polymers for the reduction of nitroarenes using NaBH4.23 carbon. This material was then impregnated with a H2PdCl4
Despite the good activity of this catalyst, it presents two draw- solution followed by thermal reduction under Ar/H2 mixture at
backs: the Pd catalyst was obtained using a seven-step synthesis three different temperatures: 300 ꢀC, 450 ꢀC and 600 ꢀC,
and an excess of NaBH4 was required for the reduction of respectively for Pd@MC catalysts IM1, IM2 and IM3. This
nitroarenes. The second route to aromatic primary amines via allowed to obtain catalysts having distinct Pd NPs sizes and
transition metal-catalyzed hydrogenation is much more conve- distributions in the carbon matrix. Besides, a Pd@MC catalyst
nient and atom-economical since no by-products are generated. obtained by a direct method (DM) was also prepared using the
However, in some cases harsh conditions for the reduction of same approach as described above but with H2PdCl4 being
nitroarenes (high H2 pressure) are required.24 Recently consid- directly introduced at the outset of the synthesis (Fig. 1) and the
erable improvement has been achieved with the development of obtained assembly polymer/surfactant/Pd salt thermally treated
several heterogeneous catalysts for mild hydrogenations under under Ar at 600 ꢀC to obtain catalyst DM.6 The advantage of this
H2 at atmospheric pressure and room temperature. In this direct synthesis was that both carbon and Pd NPs were formed
respect, heterogeneous catalysts bearing Pd NPs supported on in a one-pot reaction with no need of a supplementary
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face-centered-cubic (fcc) Pd crystal structure according to the
COD database (COD 9009820). The peak around 22ꢀ was
attributed to the (002) reection plane of graphite and its large
width was due to imperfect crystalline order, in line with what is
observed for other disordered-like porous carbon materials. By
comparison of the patterns of IM1–IM3 materials prepared at
different temperatures, it can be seen that the pattern of IM1
obtained at 300 ꢀC showed very broad Pd peaks probably due to
the presence of ultra-small Pd NPs. This result was in accor-
dance with literature reports showing that a decrease of the Pd
NPs sizes resulted in a decrease of the crystallinity of the lattice
of Pd and an increase of the wideness of the XRD peaks.30 In
addition, when the temperature during the thermal reduction
under Ar/H2 mixture increased from 300 ꢀC (IM1) to 600 ꢀC
(IM3), a considerable increase in peak intensity was observed
indicating an increase of both size and crystallinity index of the
Pd NPs.
Fig.
1 Two-step synthesis (indirect method, IM) and one-step
synthesis (direct method, DM) of Pd@MC catalysts IM1–IM3 and DM.
The average sizes of the Pd NPs were determined by the
Scherrer equation applied to the peak (022) and found to be
4.0 nm for the crystallites present in IM2 and 7.4 nm for those in
IM3. Since the X-ray diffraction pattern of IM1 presented only
overlapped peaks, the size of the Pd crystallites could not be
determined by this method. By comparison of the patterns of
the two materials prepared at the same temperature, i.e. 600 ꢀC,
but using either the two-step indirect method (IM3) or the direct
method (DM), it could be observed that the peaks were more
intense and narrower for catalyst DM. This corresponded to
a computed higher crystallite size of 9.5 nm for DM (Table 1). To
get more insights on the morphologies of the catalysts, scan-
ning transmission electron microscopy (STEM) for IM1 (Fig. 3a),
and transmission electron microscopy (TEM) for IM2 (Fig. 3b),
IM3 (Fig. 3c) and DM (Fig. 3d) were performed. The catalysts
STEM and TEM images showed homogeneously distributed Pd
NPs on carbon framework for all catalysts. The Pd NPs size
distribution histograms showed that the NPs presented
a narrowly distributed size around 1.2 nm, 3.4 nm, 6.2 nm and
7.0 nm respectively for IM1, IM2, IM3 and DM (inserts Fig. 3). It
is noteworthy that, compared to the other catalysts, IM1 con-
tained ultra-small Pd NPs and, consequently, a very high density
of NPs population. The Pd NPs sizes determined by TEM were
found in accordance, even if somewhat smaller, with those
obtained by XRD (Table 1).31 Therefore, it turned out that the Pd
NPs sizes are both impacted by the annealing temperature and
by the synthetic route: the Pd NPs sizes, as determined by TEM,
increased from 1.2 nm to 6.2 nm when increasing the temper-
ature from 300 ꢀC to 600 ꢀC for IM1, IM2 and IM3. In the case of
Fig. 2 X-Ray diffraction patterns of Pd@MC catalysts IM1, IM2, IM3
and DM.
impregnation/reduction step since the Pd salt was reduced in
situ during the pyrolysis treatment by a carboreduction reac-
tion.6 Noteworthily, this direct route afforded Pd NPs of
different sizes and localizations in the carbon network
compared to the two-step synthesis, giving the opportunity to
determine the inuence of catalyst structure on its activity. It
should be emphasized that all preparations of Pd-containing
mesoporous carbons performed here were rapid and
environmentally-benign involving only non-toxic precursors.
First the crystalline structures of catalysts IM1–IM3 and DM
were studied by X-ray diffraction (XRD, Fig. 2). The patterns of
IM2, IM3 and DM showed the presence of several peaks posi-
tioned at 40.0ꢀ, 46.6ꢀ, 68.1ꢀ, 82.0ꢀ and 86.5ꢀ, corresponding
respectively to the (111), (200), (220), (311) and (222) planes of
Table 1 Characteristics of Pd catalysts IM1–IM3 and DM
Catalyst
Pd NPs sizea (nm)
Pd crystallites sizeb (nm)
Pdc (wt%)
Pdd (wt%)
Pd2+d (wt%)
IM1
IM2
IM3
DM
1.2
3.4
6.2
7.0
—
2.3
2.3
2.3
2.2
1.48
2.40
3.15
0.90
25.82
10.83
5.62
4.0
7.4
9.5
34.43
a
Determined by TEM. b Determined by XRD. c Determined aer mineralization of the catalyst and UV-visible spectroscopy. d Determined by XPS.
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were more complex since the formations of both carbon
framework and Pd NPs occurred in the same step. In solution,
the carbon precursors, the surfactant and the Pd salt were
homogeneously mixed at molecular level. The phenolic resin
(phloroglucinol–glyoxal) was then formed and co-assembled
with the Pluronic F-127 template via H-bonding. The disper-
sion of the Pd salt in this assembly was most likely impacted by
its interactions with the precursor oxygen groups (–OH,
–COOH).32 Citric acid used herein may also favor the interaction
with the Pd and the formation of stable Pd-complexes, which
may limit the particle size growth during the thermal anneal-
ing.29 At annealing temperatures lower than 400 ꢀC, the
decomposition of phenolic resin and surfactant proceeds33 and
PdO was probably generated from PdCl2 due to the presence of
oxygen-containing functional groups. The Pd NPs formed by
subsequent carboreduction reaction would likely be more
embedded in the carbon framework that in the case of catalysts
obtained by the indirect route where they would rather be
placed at the surface (vide infra), changing therefore their
accessibility. Hence, the Pd NPs formation and growth during
preparation may explain the size variations observed between
catalysts IM1–IM3 and DM.
Fig. 3 (a) STEM image and NPs size distribution histogram of IM1. TEM
images and NPs size distribution histograms of (b) IM2, (c) IM3 and (d)
DM.
X-ray photoelectron spectroscopy (XPS) analysis was then
carried out to determine the surface chemistry of IM1–IM3 and
DM catalysts (Fig. 4 and ESI, Fig. S1†). The wide scan showed
the existence of 3 peaks positioned around 284 eV, 335 eV and
532 eV, attributed respectively to C1s, Pd3d and O1s (see ESI†
part). The deconvolution of C1s peak (Fig. 4a) showed an
intense major peak at 284.40 eV corresponding to the Csp2
aromatic carbon and four others peaks located at 284.9 eV,
286.0 eV, 287.3 eV and 288.7 eV, respectively, attributed to
carbon–oxygen functional groups such as (C–C]O) (C–OR),
(C]O) and (–COOR). The presence of such groups ensured
a hydrophilic behavior of carbon, benecial for metal salt
impregnation and subsequently, uniform dispersion of the
nanoparticles.
the two-step synthesis, it was therefore easy to control the sizes
of Pd NPs by adjusting the temperature. Differences of Pd NPs
size (6.2 vs. 7.0 nm) were also observed between the indirect and
the direct route. In the indirect method, the carbon framework
was impregnated with a H2PdCl4 solution, therefore the
porosity and the oxygen-functional groups (–OH, –COOH–,
–COOR, .) which were present on the carbon surface (ESI,
Fig. S2†) were playing an important role in the diffusion and
interaction of the salt solution and further on the formation of
the Pd particles.1,2 A porous carbon structure, possessing an
important developed specic surface area presents several
active sites which can serve as anchoring points for the NPs.
Moreover, the presence of a signicative amount of oxygen
functionalities confers a hydrophilic character to the carbon
surface which may enhance the adsorption of the Pd salt solu-
tion, and further induces the uniform dispersion of the parti-
cles. Therefore, the impregnation of the carbon support with
the Pd salt was favorable to the formation of small and
dispersed particles. In addition, the thermal annealing under
Ar/H2 mixture is of great importance in the formation of the Pd
NPs. During this step, the thermal decomposition of Pd salt
precursor occurred, leading probably to the formation of PdO in
a rst step, due to the interactions between Pd and oxygen-
containing functional groups of the carbon framework. In
a second step, PdO is further reduced to Pd NPs in the presence
ꢀ
of H2. A low temperature (300 C) is sufficient to obtain small
ꢀ
and well dispersed Pd NPs. Higher temperatures (450–600 C),
triggered the formation of larger Pd NPs with a broader size
distribution. This phenomenon, named Ostwald ripening, due
to the greater thermodynamic stability of larger NPs, led to
a decrease of the total number of NPs and to an increase of their
average size. In the case of the direct route, the mechanisms Fig. 4 XPS high resolution deconvoluted spectra of the (a) C1s peak of
IM1, (b) O1s peak of IM1, and (c) Pd3d spectra of IM1–IM3 and DM.
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during the formation of the carbon framework yielding more
conned Pd NPs (both pores and walls).
The porosity of catalysts IM1–IM3 and DM was studied by
nitrogen adsorption/desorption isotherms (Fig. 5a). The
isotherms exhibited two different parts, i.e. type I in the low
pressure domain P/P0 < 0.1, showing the presence of micro-
pores, and type IV recognized by the presence of a hysteresis of
type H1, proving the presence of uniform mesopores. The
specic surface areas (SSA) were calculated via the Brunauer–
Emmett–Teller (BET) equation and the pores size distributions
(Fig. 5b) were determined by the DFT method with slit-like pore
geometry (Table 2). The materials surface area increased from
734 m2 gÀ1 for IM1 to 824 m2 gÀ1 for IM3, which could be
related to the size of the Pd NPs and their location in the carbon
frameworks, since the carbon framework was identical for IM1–
IM3 and the amount of Pd was ca. 2.3% for all catalysts (Table
2). In catalyst IM1, the Pd NPs size was very small (ca. 1.4 nm)
and some of the NPs maybe localized in the carbon micropores
(pores having sizes < 2.0 nm) leading to some pore blocking and
to a decrease of the SSA, which is closely related to the micro-
pores. The smaller microporous volume (Vmicro) along with the
low content of Pd of the surface of catalyst IM1 conrmed this
hypothesis. For catalyst IM2, the Pd NPs were larger (ca. 3.4 nm)
than for IM1 (ca. 1.4 nm), resulting in only scarce Pd NPs able to
block the small pores. Catalyst IM3 contained still larger Pd NPs
(ca. 6.2 nm) unable to be embedded in small pores resulting
thus in a catalyst exhibiting larger SSA. The Pd NPs were
therefore located either in the surface of the carbon or in the
mesopores having larger sizes (ca. 5.4 nm). These results were in
accordance with the Pd content of the surfaces determined by
XPS analysis (vide supra). The total pore volumes of IM1, IM2
Fig. 5 (a) Nitrogen adsorption desorption isotherms for IM1–IM3 and
DM and (b) their pores size distribution.
The Pd3d high resolution spectra of IM1, IM2, IM3 and DM
catalysts (Fig. 4c) have been deconvoluted in four peaks (two
doublets) at 335.4–340.7 eV for metallic Pd (Pdꢀ) and 337.6–
342.9 eV for PdO (Pd2+). These doublets correspond to Pd3d5/2
–
Pd3d3/2 spin–orbit splitting with a splitting of 5.3 eV and a ratio
equal to 1.5. The percentage of PdO at the surface of the Pd NPs
was found to vary in the following order DM > IM1 > IM2 > IM3 as
seen in Table 1. The increase of the oxidation of Pd NPs in the IM
series was related to the Pd NPs sizes: a decrease of their size
resulted in a higher reactivity towards oxygen present in air in
accordance with a previous report.34 However, catalyst DM
prepared via the direct method exhibited large Pd NPs and con-
tained the highest amount of PdO at the surface of the carbon.
This result can be associated to the fact that H2 reduction step
was not performed and the carbon framework presented signif-
icant amount of oxygen-containing functional groups (surface
percentage of O: 10.53) in the vicinity of the Pd NPs.
The Pd-content of catalysts IM1–IM3 and DM was then
compared using either a titration of Pd aer complete mineral-
ization35 of the catalysts or XPS (Table 1). It is noteworthy that
catalysts IM1–IM3 were prepared using the same amount of Pd
precursor, corresponding to an expected Pd-content of ca. 2.3%.
The Pd-content obtained aer complete mineralization followed
by titration was indeed determined to be ca. 2.3% for catalysts
IM1–IM3 and DM. The Pd-contents of catalysts IM1–IM3 and DM
found by XPS were respectively 1.48%, 2.40%, 3.15% and 0.9%
(Table 1). To understand this trend, it should be recalled that XPS
analyses only the surface of materials (max. 10 nm in depth). The
increase of the Pd-content of the surface observed between IM1
and IM3 was therefore tentatively explained by the greater
proportion of the Pd NPs located on the surface when their size is
larger. The surface of catalyst DM has the lowest amount of Pd
(0.9%), which is more related to the incorporation of Pd directly
and IM3 were composed mainly of mesopores (ꢁ0.44 cm3 gÀ1
)
and also non-negligible microporous volumes (ꢁ0.27 cm3 gÀ1).
The large SSA and pore volume should be benecial to increase
the surface area of Pd NPs and therefore favour their reactivity.
Finally, the comparison of IM3 with DM (Fig. 5b and Table 2)
showed no signicant differences in SSA and microporous
volume, but at decrease in the mesoporous volume and size (ca.
4.9 nm). The Pd NPs sizes were larger for catalyst DM and
mostly embedded in the carbon matrix as already showed by
XPS analysis.
Activity of catalysts IM1–IM3 and DM for hydrogenations of
nitroarenes
The relationship between the characteristics and preparation
methods of Pd catalysts and their catalytic activity for the
Table 2 Textural and structural properties of Pd@MC catalysts synthesized under different conditionsa
Catalyst
BET (m2 gÀ1
)
VT (cm3 gÀ1
)
Vmicro (cm3 gÀ1
)
Vmeso (cm3 gÀ1
)
Mean pore size (nm)
IM1
IM2
IM3
DM
734
800
824
841
0.70
0.72
0.73
0.62
0.25
0.28
0.29
0.30
0.45
0.44
0.44
0.32
5.6
5.6
5.4
4.9
a
VT: total pore volume, Vmicro: microporous volume determined by the Dubinin–Radushkevich method, Vmeso: mesoporous volume.
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Table 3 Activity of catalysts IM1–IM3 and DM for the hydrogenation of 4-nitroacetophenone
Entrya
Catalyst
Solvent
Pd (mequiv.)
Reaction time (h)
Yieldb
1
2
3
4
5
6
7
8
IM1
IM1
IM1
IM2
IM2
IM2
IM3
DM
IM1
IM1
IM1
Pd/C
Pd Encat NP30
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
EtOH
iPrOH
Propane-1,2-diol
H2O
EtOH
EtOH
5
1
5
5
1
5
5
5
5
5
5
5
5
2
2
1
2
2
1
2
2
2
2
2
2
2
99
35
64
98
35
72
n.r.c
n.r.c
52
9
10
11
12
13
20
n.r.c
60
26
a
b
Reactions performed using 4-nitroacetophenone (2 mmol), the indicated amount of supported Pd in a solvent (10 mL). Isolated yields aer
c
ltration of the catalyst, concentration and drying of the reaction product under vacuum. The starting material was recovered unchanged.
hydrogenation of nitroarenes was determined. For this purpose, (entries 12 and 13) conrming therefore the high activity of IM2.
4-nitroacetophenone was used as a model substrate in the The study was then pursued in conditions of Table 3, entry 1
presence of a Pd catalyst in EtOH at rt under a 1 atm H2 (Table (catalyst IM1) or entry 4 (catalyst IM2).
3). It turned out that performing the reaction in the presence of
The possibility to reuse catalysts IM1 and IM2 was then
IM1 (5 mequiv. of supported Pd) afforded almost quantitatively determined. For this purpose, aer reaction the catalyst IM1 or
the corresponding amine 1a in only 2 h (entry 1). Lowering the IM2 was ltered off using a 0.1 mm membrane, washed with
amount of supported Pd or the reaction time gave 1a in lower EtOH and reused (Fig. 6). Very interestingly, IM2 could be used
yields (entries 2 and 3). Comparable results were obtained using seven times with no signicant decrease in the yield whereas
catalyst IM2 (entries 4–6) showing that the textural and struc- a loss of efficiency was observed for catalyst IM1 aer the fourth
tural properties of IM1 and IM2 do not affect their reactivity use. This difference in activity during the reuse of catalysts IM1
during their rst use. The hydrogenation was then performedꢀin and IM2 was explained by the increase, during the successive
the presence of IM2 and heating the reaction mixture at 50 C uses, of the PdO layer around the ultra-small, and therefore air-
(otherwise conditions of Table 1, entry 4): no improvement was sensitive, Pd NPs. XPS analyses (Fig. 4 and ESI, Fig. S3†) show
achieved since 1a was obtained in almost quantitative yield but that the percentage of PdO at the surface of these NPs increased
required also 2 h of reaction. However, no reaction was observed from 25.82% to 41.45% aer the 4th use of IM1. Then TEM
using either catalyst IM3 or DM (entries 7 and 8). Noteworthily,
the only signicant difference between IM1–IM2 on one side
and IM3 on the other was the Pd NPs sizes. Therefore, it
appeared that smaller NPs were active, whereas larger NPs were
inactive leaving the starting material unchanged. In the case of
catalyst DM no reaction was observed, which could be explained
by the size of the Pd NPs. Moreover the Pd NPs are less acces-
sible there than in catalyst IM3. Finally, changing the nature of
the solvent was unsuccessful since low yields were observed in
iPrOH or propane-1,2-diol and no reaction was observed in H2O
(entries 9–11). It can be pointed out that, compared to
numerous catalysts and reducing agents described,15–18 both
IM1 and IM2 allowed a chemospecic reduction of the nitro
group (the acetyl remaining unchanged) and avoided the
necessity of high H2 pressure. Interestingly, performing the
hydrogenation in the presence of a commercially available Pd/C
Fig. 6 Reuses of catalysts IM1 (conditions of Table 3, entry 1) and IM2
or of Pd Encat NP30 catalyst afforded 1a in much lower yields
(conditions of Table 3, entry 4).
36746 | RSC Adv., 2020, 10, 36741–36750
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compromise between Pd NPs sizes, catalytic activity and
reusability.
The homogeneous/heterogeneous nature of the catalysis was
then determined by performing a hot ltration test. For this
purpose the hydrogenation was carried out in conditions of
Table 3, entry 4. Aer 15 min of reaction (yield at that point:
40%), catalyst IM2 was ltered off and the ltrate was stirred
under H2 (1 atm) for another 105 min time during which no
increase in the yield was observed. This suggested that the
hydrogenation might occur via a heterogeneous pathway. The
amount of Pd present in the reaction mixture aer reaction was
then determined: aer ltration of catalyst IM2, the crude
mixture was completely mineralized using
a procedure
described in the literature.35 It could be established that the
amount of Pd leached in the reaction medium had an extremely
low value of less than 0.1% of the initial amount. Then the
synthetic potential of catalyst IM2 was ascertained under the
best reaction conditions (Scheme 1): IM2 was used successfully
for the hydrogenations of versatile nitroarenes bearing either
electron-donating or electron-withdrawing groups, or even for
the reduction of 3-nitropyridine yielding 1j.
Performing organic reactions in water raises currently huge
interest for both environmental and economic viewpoints.7b
Therefore, the possibility to perform hydrogenations of nitro-
arenes in water was determined. For this purpose, 4-nitro-
acetophenone was reacted using catalyst IM2 under the
optimized conditions (Table 3, entry 4), but replacing EtOH with
water. Unfortunately, only the starting material was recovered
unchanged. This lack of reactivity could be explained by the very
poor solubility of 4-nitroacetophenone in water. Nitrobenzene
was then chosen as starting material and, in the same condi-
tions, aniline 1e was almost quantitatively formed. Other
nitroarenes were then hydrogenated in H2O and afforded
almost quantitatively the expected primary aromatic amines
(Scheme 2).
Scheme 1 Hydrogenations of nitroarenes in EtOH. aReactions per-
formed using a nitroarene (2 mmol), catalyst IM2 (5 mequiv. of sup-
ported Pd) in EtOH (10 mL) at rt for 2 h; bReactions performed using
a nitroarene (2 mmol), catalyst IM2 (5 mequiv. of supported Pd) in
EtOH (10 mL) at rt for 4 h.
Finally, the activity of Pd@MC catalyst IM2 was compared to
that of representative heterogeneous reusable Pd catalysts
described in the literature for the reduction of nitroarenes using
either 1 atm of H2 or NaBH4 as reducing agent (Table 4). The
results pointed out that IM2 presented similar TOFs compared
to other catalysts at rt in the presence of 1 atm of H2 (entries 1–
5). However, all other previous reported catalysts required
longer, much more fastidious preparations and in some cases
even a higher amount of supported Pd (entries 4 and 5).
Scheme 2 Hydrogenations of nitroarenes in H2O. aReactions per-
formed using a nitroarene (2 mmol), catalyst IM2 (5 mequiv. of sup-
ported Pd) in H2O (5 mL); bReactions performed using a nitroarene (2
mmol), catalyst IM2 (5 mequiv. of supported Pd) in H2O (10 mL).
analysis of catalyst IM2 was performed aer the seventh use. It
turned out that the size of the Pd NPs has only slightly increased
from ca. 3.4 nm to ca. 4 nm (ESI, Fig. S4†). Catalyst IM2 was
therefore chosen to continue this study since it showed a good
Table 4 TOFs for the hydrogenation of 4-nitrophenol to 4-aminophenol in the presence of reported heterogeneous Pd catalysts
Entry
Catalyst
Reducing agent, solvent
Catalyst (mequiv.)
TOF (hÀ1
)
1
2
3
4
IM2 (this work)
H2, EtOH
H2, EtOH
H2, EtOH
H2, EtOH
5
5
5
10
100
200
200
100
Pd@porous organic ligand21
Pd@porous organic polymer27
Pd@hollow mesoporous
spheres25
5
6
Pd@polypyrrole@Fe19
Pd@hypercrosslinked
polymer18
NaBH4, H2O
NaBH4, EtOH/H2O
10
1.8
133
2128
7
Pd@PVP28
NaBH4, H2O
1
1000
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Reusable Pd catalysts in the presence of NaBH4 presented impregnation of the obtained carbon support with an aqueous
generally higher TOFs, required a lower amount of supported solution of a Pd salt followed by a thermal treatment under
Pd (entries 7 and 8) but their major drawbacks lie in the use of a reducing atmosphere. The direct method involves the use of
an excess of reducing agent generating mineral by-products the same precursors as in the rst step of IM but with addition
which could be problematic in the case of large scale of a Pd salt. The whole assembly is then pyrolysed to obtain C/
syntheses. Worth to mention is that non-precious metal cata- Pd composites. The synthesis pathways are illustrated in Fig. 1
lysts have also been reported for the reduction of nitroarenes. and each method will be described below in detail.
For example Fe NPs36 or N-doped graphene supported Fe are
efficient but the reduction of nitroarenes required either an Preparation of catalysts IM1 and IM3 by indirect method (IM)
excess of Fe NPs (3 equiv.) or a high pressure of H2 (30 atm).
Firstly, a solution of phenolic resin precursors (phloroglucinol
Other copper-containing carbon17 or N-doped porous carbon
and glyoxal) containing a surfactant able to create micelles and
containing copper/cobalt37 catalysts have been described but
they present generally a low activity compared to IM2.
to self-assemble with the resin is prepared. Pluronic F-127
surfactant (3.27 g) and phloroglucinol (1.65 g) were dissolved
under stirring in ethanol (80 mL), followed by the addition of
3 Conclusions
citric acid (0.2 g) and glyoxal solution (40% in water, 1.62 mL).
The obtained solution was disposed in several Petri dishes and
le to evaporate under the fume hood at rt for 15 h in order to
form a polymer/surfactant assembly lm which is removed by
mechanical scratching, heated at 80 ꢀC for 24 h and then
pyrolyzed at 600 ꢀC for 2 h under an atmosphere of argon to
obtain the mesoporous carbon (MC). In a second step, a solu-
tion of H2PdCl4 (prepared by mixing PdCl2 (0.066 g) with HCl
(37% wt, 0.5 mL)) was added dropwise to a stirred suspension of
the mesoporous carbon (1.74 g) in ethanol (10 mL). The
suspension was stirred at 60 ꢀC under a fume-hood until
complete evaporation of the solvent. The solid residue was
placed in an oven under a 90/10 Ar/H2 mixture and heated upꢀto
The Pd@MC catalysts IM1–IM3 and DM were prepared by two
different environment-friendly and efficient approaches. The
impregnation (indirect method), performed at three tempera-
tures, allowed to tune the Pd NPs size from 1.2 to 6.4 nm while
the one-pot (direct method) lead to larger (7.0 nm) and more
conned NPs. The catalysts efficiency for nitroarenes reduction
was found to be closely dependent on the Pd NPs size, i.e. the
smallest NPs sizes (IM1 and IM2) being the most efficient,
whereas, the larger NPs (IM3 and DM) were inactive. Moreover,
it could be shown that the Pd NPs surface chemistry played an
important role during the catalyst reuse, namely, the smallest
Pd NPs were oxidized more easily at their surface (PdO) leading
to a decrease of activity during reuse. Therefore, the best
compromise between the Pd NPs size and their stability towards
oxidation was found in catalyst IM2 which allowed hydrogena-
tion reactions of nitroarenes in EtOH in the presence of low
amounts of supported palladium (5 mequiv.) and was used 7
times without decrease of efficiency. Noteworthily, in some
cases the hydrogenation could even be performed using only
water as a solvent. It is important to highlight that the major
advantages of IM2 compared to heterogeneous Pd catalysts
hitherto described in the literature for the hydrogenation of
nitroarenes lie in: (1) its easy preparation avoiding the use of
hazardous reagents or solvents, (2) its activity under very mild
and “green” conditions and (3) its excellent reusability. To the
best of our knowledge, IM2 is to date the only catalyst that
conciliates both a fast and easy preparation from eco-friendly
precursors and an excellent activity and reusability for mild
and “green” hydrogenations. Further studies are underway to
determine the synthetic potential of IM2 for other versatile C–C
couplings and reductions.
ꢀ
ꢀ
300 C (for catalyst IM1), 450 C (for catalyst IM2) and 600 C
(for catalyst IM3) for 3 h.
Preparation of catalyst DM by direct method (DM)
The synthesis procedure is similar but the Pd salt was intro-
duced at the outset. Pluronic F-127 surfactant (3.27 g) and
phloroglucinol (1.65 g) were dissolved under stirring in ethanol
(80 mL), followed by the addition of citric acid (0.2 g), glyoxal
solution (40% in water, 1.62 mL), and H2PdCl4 solution
(prepared by mixing PdCl2 (0.066 g) with HCl (37% wt, 0.5 mL)).
The obtained solution was disposed in several Petri dishes and
le to evaporate under the fume hood at rt for 15 h in order to
form a polymer lm which was removed by mechanical
scratching, heated at 80 ꢀC for 24 h and then pyrolyzed at 600 ꢀC
for 2 h under an atmosphere of argon to obtain catalyst DM.
General procedure for the hydrogenation of nitroarenes in
EtOH
The nitroarene (2 mmol) in EtOH (10 mL) was vigorously stirred
(1000 rpm) in the presence of catalyst IM2 (5 mequiv., 46.3 mg)
under H2 at atmospheric pressure at rt for 2 h (for the prepa-
ration of aromatic amines 1a, 1e–j) or 4 h (for the preparation of
4 Experimental section
The catalysts were prepared using two bottom-up approaches, aromatic amines 1b–d). The catalyst was recovered by ltration
in the aim of obtaining carbon/Pd catalysts having distinct on a membrane (0.1 mm). Catalyst IM2 was then washed twice
properties: the “indirect method” (IM) and a one-pot method, with EtOH (5 mL), ltered and dried under vacuum before
the “direct method” (DM). The indirect method consisted, in reuse. The solvent was evaporated and the aromatic primary
the rst step, in the preparation of a mesoporous carbon amines were dried under vacuum. If necessary, the aromatic
support via self-assembly of a phenolic resin and a template amines can be puried by ash-chromatography on silica gel
followed by pyrolysis and, in the second step, in the with AcOEt and cyclohexane mixtures.
36748 | RSC Adv., 2020, 10, 36741–36750
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Nanotechnology, 2016, 27, 465401; X. Liu and D. Astruc,
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General procedure for the hydrogenation of nitroarenes in
H2O
6 C. Peter, A. Derible, J.-M. Becht, J. Kiener, C. Le Drian,
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7 (a) M. Enneiymy, C. Le Drian, C. Matei Ghimbeu and
J.-M. Becht, RSC Adv., 2018, 8, 1717; (b) M. Enneiymy, C. Le
Drian, C. Matei Ghimbeu and J.-M. Becht, Appl. Organomet.
Chem., 2019, 33, e5104.
The nitroarene (1 mmol) in H2O (5 mL for the preparation of
aromatic amines 1e–h or 10 mL for the preparation of aromatic
amine 1i) was vigorously stirred in the presence of catalyst IM2
(5 mequiv., 23.2 mg) under H2 at atmospheric pressure at rt for
2 h. The catalyst was recovered by ltration on a membrane (0.1
mm). Catalyst IM2 was then washed twice with EtOH (5 mL),
ltered and dried under vacuum before reuse. The solvent was
evaporated and the aromatic primary amines were dried under
vacuum. When required, the aromatic amines can be puried
by ash-chromatography on silica gel with AcOEt and cyclo-
hexane mixtures.
8 C. Peter, A. Derible, J. Parmentier, C. Le Drian and
J.-M. Becht, New J. Chem., 2017, 41, 4931.
´
9 J. Hassan, M. Sevignon, C. Gozzi, E. Schulz and M. Lemaire,
Chem. Rev., 2002, 102, 1359; A. Suzuki, Angew. Chem., Int. Ed.,
2011, 50, 6722; Angew. Chem., 2011, 123, 6854.
10 For the interest of obtaining almost Pd-free products aer
reaction see: H. Ren, C. A. Strulson, G. Humphrey,
R. Xiang, G. Li, D. R. Gauthier and K. M. Maloney, Green
Chem., 2017, 19, 4002.
Conflicts of interest
There are no conicts to declare.
11 L. Yin and J. Liebscher, Chem. Rev., 2007, 107, 133;
N. E. Leadbeater and M. Marco, Chem. Rev., 2002, 102,
3217For heterogeneous reusable magnetic Pd catalysts
see:V. Polshettiwar, R. Luque, A. Fihri, H. Zhu,
M. Bouhrara and J.-M. Basset, Chem. Rev., 2011, 111, 3036;
Q. M. Kainz and O. Reiser, Acc. Chem. Res., 2014, 47, 667.
12 S. Schweizer, J.-M. Becht and C. Le Drian, Adv. Synth. Catal.,
2007, 349, 1150; S. Schweizer, J.-M. Becht and C. Le Drian,
Tetrahedron, 2010, 66, 765; C. Diebold, J.-M. Becht, J. Lu,
P. H. Toy and C. Le Drian, Eur. J. Org. Chem., 2012, 893;
C. Diebold, S. Schweizer, J.-M. Becht and C. Le Drian, Org.
Biomol. Chem., 2010, 8, 4834; C. Diebold, A. Derible,
J.-M. Becht and C. Le Drian, Tetrahedron, 2013, 69, 264.
Acknowledgements
We are grateful to the Fondation pour l’Ecole Nationale
´
Superieure de Chimie de Mulhouse for a doctoral grant and
generous nancial support to M. Enneiymy, to B. Rety for
helpful technical assistance, to Dr L. Vidal for TEM, to Dr J.-M.
Le Meins for the XRD, and to Dr C. Vaulot for adsorption
analyses via IS2M technical platform.
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