Highly selective hydrogenation of halogenated nitroarenes over Ru/CN nanocomposites by: In situ pyrolysis

Article abstract of DOI:10.1039/d0nj02165b

A highly chemoselective and recyclable ruthenium catalyst for the hydrogenation of halogenated nitroarenes has been prepared via the simple in situ calcination of a mixture of melamine, glucose and ruthenium trichloride. Superfine Ru particles (2.3 ± 0.3 nm) were obtained and highly dispersed in the nitrogen-doped carbon matrix. The Ru/CN catalyst smoothly transforms a variety of halogenated nitroarenes to the corresponding haloanilines with high intrinsic activity (e.g. TOF = 1333 h-1 for p-chloronitrobenzene) and selectivity of more than 99.6percent. Furthermore, through an analysis of the products in the reaction process, it was concluded that there are two parallel reaction pathways (a direct pathway and an indirect pathway) for the hydrogenation of aromatic nitro compounds over the Ru/CN catalyst, and the direct pathway was proved to be dominant in catalyzing the intermediates. This journal is

Full text of DOI:10.1039/d0nj02165b

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Highly selective hydrogenation of halogenated
nitroarenes over Ru/CN nanocomposites by in situ
pyrolysis†
Cite this:DOI: 10.1039/d0nj02165b
a
a
Shengnan Yue,
Xionggang Lua and Chunlei Zhang*b
Xueguang Wang, *a Shaoting Li,a Yao Sheng,
Xiujing Zou,a
A highly chemoselective and recyclable ruthenium catalyst for the hydrogenation of halogenated
nitroarenes has been prepared via the simple in situ calcination of a mixture of melamine, glucose and
ruthenium trichloride. Superfine Ru particles (2.3 Æ 0.3 nm) were obtained and highly dispersed in the
nitrogen-doped carbon matrix. The Ru/CN catalyst smoothly transforms a variety of halogenated
nitroarenes to the corresponding haloanilines with high intrinsic activity (e.g. TOF = 1333 hÀ1 for
p-chloronitrobenzene) and selectivity of more than 99.6%. Furthermore, through an analysis of the
products in the reaction process, it was concluded that there are two parallel reaction pathways
(a direct pathway and an indirect pathway) for the hydrogenation of aromatic nitro compounds over the
Ru/CN catalyst, and the direct pathway was proved to be dominant in catalyzing the intermediates.
Received 29th April 2020,
Accepted 16th June 2020
DOI: 10.1039/d0nj02165b
rsc.li/njc
catalysts can be simply divided into noble metal group and non-
noble metal group catalysts. The noble metals with high catalytic
1. Introduction
As basic chemical raw materials and organic intermediates, activity and mild reaction conditions are mainly platinum,20,21
aromatic amine and its derivatives play crucial roles in medicines, palladium,2,22,23 rhodium,24 gold,16 silver,25 and ruthenium.26
dyes, pesticides, and everyday chemical industries.1–5 The main And the non-noble metals include nickel,27 copper,28 cobalt,29
method of manufacturing aromatic amines is the reduction of the iron,30 molybdenum,31 and their oxides and alloys32,33 which
corresponding aromatic nitro compounds,6–8 including chemical are low cost but require harsh reaction conditions such as high
reduction,9 electrolytic reduction,10 and catalytic hydrogenation temperature and high pressure.18,34 Currently, supported noble
reduction.11,12 However, from the perspectives of environmental metal catalysts like platinum and palladium have been widely
protection and economic benefits, the traditional chemical reduction applied for the reduction of nitroarenes with H2 as the reduc-
method applying Fe, FeS, Zn, or Na2S as the reductants13,14 has been tant because of their excellent activity. Nevertheless, in the
gradually eliminated due to the production of plenty of hazardous preparation of halogenated aromatic amines, the highly active
wastewater and the low yields. Meanwhile, the development of metals platinum and palladium have a serious drawback – the
electrolytic reduction has also been limited by its high cost.15,16 superior hydrolytic activity of the carbon-halogen bond usually
By contrast, catalytic hydrogenation has become the preferred leads to dehalogenation, which produces many by-products
option and has been widely utilized in preparing aromatic amine and reduces the quality of the product. Ruthenium has been
compounds owing to its high selectivity and less waste as well as demonstrated to exhibit much higher selectivity in hydrogena-
easy post-treatment.17–19
tion of the halogenated nitroarenes,35 and its lower price and
The catalyst is one of the most crucial parameters determining lower dehalogenation trend compared to Pt and Pd mean it
the activity and selectivity of the hydrogenation reaction, and has great potential in the chemoselective hydrogenation of
halogenated nitroarenes.
The performance of a catalyst for the hydrogenation of
halogenated nitroarenes depends not only on the kind of active
metal but also on the properties of the support. It is generally
accepted that the characteristics of the catalyst support, such as
the strength, specific surface area, pore structure, and surface
properties can remarkably affect the dispersion, morphology,
and metal–support interaction of the active phase.36 In addition,
the size and shape of the active phase particles also play a
a State Key Laboratory of Advanced Special Steel, School of Materials Science and
Engineering, Shanghai University, Shanghai 200444, China.
E-mail: wxg228@shu.edu.cn
b Education Ministry Key Lab of Resource Chemistry, Shanghai Key Laboratory of
Rare Earth Functional Materials, College of Chemistry and Materials Science,
Shanghai Normal University, Shanghai 200234, China.
E-mail: zhangchunlei@shnu.edu.cn
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj02165b
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crucial role in the catalytic reduction.37 These factors are inter-
related and closely related to the performance of the catalyst in
halogenated nitroarene reduction. Therefore, choosing an
appropriate support in the preparation of an effective catalyst
is of great significance. Many studies have shown that activated
carbon materials could be successfully applied as supports for
noble metal catalysts in the fine chemical industry due to their
excellent thermal stability, strong acid and alkali resistance, and
large specific surface area.38 However, the active metal nano-
particles tend to be exuded from the support in the catalytic
process because of the weak interaction between the metal
nanoparticles and the carbon surface, which leads to inferior
recyclability.39 In order to satisfy the high standards of catalytic
requirements, the introduction of heteroatoms has been further
developed. Compared with pure mesoporous carbon, the intro-
duction of heteroatoms can improve the physical characteristics
of the support and simultaneously change the electronic char-
acteristics of the active site.40,41 The physical structure, surface
properties, electronic structure of the active phase, and metal–
support interaction of the catalyst are regulated by improving the
preparation method and doping elements, and these factors
promote the mass transfer rate of hydrogen and hydrogen
dissociation adsorption performance which are conducive to
achieving high catalytic activity and selectivity.42
In order to solve the dehalogenation of chemoselective
catalytic hydrogenation and take both activity and cost into
account, in this work we investigated a highly dispersed Ru
nano-catalyst supported on a carrier of melamine and glucose
by a simple one-step method. The reaction model selected was
p-chloronitrobenzene (p-CNB) and the hydrogenation perfor-
mances of catalysts with different carriers and preparation
methods were systematically studied, and so the influence of
carriers on active metals was analyzed. In addition, a mecha-
nism for chemoselective hydrogenation of halogenated nitro-
benzene was further probed.
Scheme 1 The preparation and application of the Ru/CN catalyst.
The grey solids were transferred into a drying oven for pro-
longed drying at 45 1C for 12 h, and then the solids were
crushed and calcined in a tube furnace under the protection of
high-purity nitrogen. The pyrolysis temperature was increased
from ambient temperature to 350 1C, held for 2 h, and then
increased to 800 1C at a heating rate of 5 1C minÀ1, and finally
cooled down naturally to room temperature. In addition, to
further test the superiority of the Ru/CN catalyst for the hydro-
genation of halogenated nitroarenes, an Ru/C catalyst without
the addition of nitrogen was obtained in a similar way.
Scheme 1 reveals the preparation process and application of
the catalyst.
In this work, an Ru/CN-imp catalyst as a comparison was
prepared by the impregnation method. Firstly, the CN carrier
was synthesized according to the previous process (Scheme 1)
without the introduction of ruthenium. The CN carrier (0.55 g)
was dispersed in deionized water (20 mL) under stirring at
45 1C. Then ruthenium chloride aqueous solution (3.25 mL, Ru
0.0037 g mLÀ1) was slowly dropped into the solution while the
mixture was evaporated under stirring at 45 1C. Finally,
the black powder was directly reduced at 200 1C (2 1C minÀ1
)
for 3 h in a 30 vol% H2/N2 flow.
2. Experimental
2.1 Materials
2.3 Catalyst characterization
Melamine (C.P.), glucose monohydrate (A.R.), anhydrous ethanol
(A.R.), RuCl3ÁxH2O, and halogenated nitro compounds were
purchased from Sinopharm Chemical Reagent Co. Ltd. The
commercial 5% RuC catalyst, azoxybenzene, and N-phenylhydroxyl-
amine were purchased from Aladdin Reagent Company. Deionized
water was used as the solvent for all solutions.
The Ru content in the catalyst was analyzed by inductively
coupled plasma atomic emission spectrometry (ICP-OES). The
analysis of C and N elements in the catalyst was undertaken
with an organic elemental analyzer (Vario Micro cube). Nitrogen
absorption–desorption isotherms, BET specific surface areas,
and pore size distribution curves were measured on a Micro-
meritics ASAP 2020 Sorptometer at À196 1C. The samples were
degassed at 250 1C for 6 h before analysis. The specific surface
2.2 Catalyst preparation
The Ru/CN catalyst was prepared by a simple one-pot pyrolysis areas (SBET) were determined using the Brunauer–Emmett–Teller
process, as shown below. Firstly, the glucose (1 g) was dissolved (BET) method in a P/P0 range from 0.05 to 0.25. Pore size
in deionized water (40 mL) in a water bath at 45 1C. Then the distribution curves were calculated using the desorption branch
melamine (20 g) was dispersed in the above solution with of the isotherms and the Barrett–Joyner–Halenda (BJH) method,
stirring for about 30 min until the suspension was well mixed. and pore sizes (Dp) were obtained from the peak positions
Finally, the ruthenium chloride aqueous solution (3.25 mL, Ru of the distribution curves. Pore volume (Vp) was taken at the
0.0037 g mLÀ1) was added and the mixture was continuously P/P0 = 0.990 single point. A field emission scanning electron
stirred at 45 1C until the mixture had dried to grey solids. microscope (SEM, FEI Nova Nano SEM 450) was applied to
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observe the catalyst morphology. X-ray diffraction (XRD) char- then analyzed by gas chromatography-mass spectrometry
acterization was recorded on a Rigaku D/MAX-2200 diffracto- (GC-MS, Shimadzu GCMS-QP 2010 Plus) and gas chromatography
meter with Cu-Ka radiation (l = 0.1542 nm) at a voltage of 40 kV (GC, GC-9790) with a flame ionization detector and a capillary
and a current of 40 mA. Raman spectroscopy (InVia Reflex) with column (CP-SIL 5 CB, 30 m  0.32 mm  0.5 mm).
an Ar+ ion laser (514.4 nm) as the excitation source was used to
determine the degree of graphitization and defect level of the
3. Results and discussion
3.1 Characterization of the catalyst
prepared materials. The size and distribution of the samples
were studied with a field emission transmission electron
microscope (TEM, JEOL JEM-2010F) operated at 200 kV. The
samples were prepared by dropping an appropriate amount of
the solution obtained after grinding and ultrasonically shaking
the powder in ethanol onto copper mesh or microgrid
(300 mesh) and then drying. The average particle sizes were
calculated based on the average values of 200 particles in the
TEM diagrams. The chemical composition and state of
the sample surfaces were detected by an X-ray photoelectron
spectrometer (XPS, ESCALAB 250Xi) with Al Ka radiation (hn =
1486.6 eV). The binding energy (BE) was calibrated against the
C 1s signal (284.6 eV) of contaminant carbon.
The actual Ru contents of the catalysts in Table 1 were measured
by ICP-OES. The contents of Ru in the Ru/CN (2.40 wt%) and
Ru/CN-imp (2.36 wt%) catalysts were similar, and the Ru content
of the Ru/C catalyst (3.01 wt%) was higher because of the lack of
N. In addition, the nitrogen contents in the CN carrier, Ru/CN,
and Ru/CN-imp catalysts were 20.1 wt%, 19.8 wt% and 19.7 wt%,
respectively. The nitrogen adsorption–desorption isotherms and
the BJH pore size distributions of different catalysts are pre-
sented in Fig. 1. Similar to CN, the Ru/CN and Ru/CN-imp
catalysts exhibited the apparent hysteresis loops of type IV
adsorption isotherms (Fig. 1a), indicating typical mesoporous
materials.43 This result demonstrated that the doping with Ru
did not change the pore structure of CN. Moreover, Table 1 also
shows that the BET specific surface area, pore volume, and BJH
2.4 Catalytic hydrogenation
The hydrogenation of nitroarenes was conducted in a 100 mL
stainless steel high-pressure tank reactor with an internal
diameter of 45 mm with magnetic stirring. Before the series
of experiments, we examined the effect of stirring rates on the
catalytic activity for the hydrogenation of nitro aromatics under
various reaction conditions including reaction temperature,
substrate concentration and hydrogen pressure for the hydro-
genation of p-CNB as a model compound over the Ru/CN
catalyst. The results revealed that the catalytic activity showed
little change when the stirring rate used was higher than
600 rpm; that is, the influencing factors of mass transfer of
the reactants were eliminated in the reaction process.
Table 1 Characterization results of different catalysts
Sample
Ru (wt%) N (wt%) SBET (m2 gÀ1
)
Vp (cm3 gÀ1
)
Dp (nm)
CN
Ru/C
Ru/CN
—
3.01
2.40
20.1
—
19.8
19.7
414
118
384
375
0.97
0.01
0.81
0.65
3.8
2.0
3.7
3.5
Ru/CN-imp 2.36
The following reaction conditions were adopted to compare
the activity of different catalysts: 2 mmol of p-CNB dissolved
in 10 mL of anhydrous ethanol at 80 1C, 2 MPa (H2), and the
appropriate amount of catalyst ([p-CNB]/[metal] = 1700 (mol molÀ1)).
Before the reaction, the reactor was rinsed with 1 MPa H2
3 times to expel air from the reactor. In addition, when the
temperature reached 80 1C, stirring of the reaction started at
800 rpm until the reaction stopped. Afterwards, the catalyst
activity was calculated by the number of converted p-CNB moles
per mole Ru per hour (TOF).
The recyclability of the Ru/CN catalyst was investigated
using p-CNB as a model. In the cyclic reaction, 10 mmol of
p-CNB was dissolved in 30 mL of anhydrous ethanol and 50 mg
of Ru/CN catalyst was added into the reactor, and the reaction
was performed for 1 h at 80 1C with 2 MPa (H2). After each
reaction, the catalyst was recovered by filtration and washing
with anhydrous ethanol three times and vacuum drying at
room temperature. The loss of catalyst was inevitable during
the washing process, and because of this, the ratio of p-CNB to
Ru/CN was kept unchanged by adding fresh catalyst for each
cycle test.
Fig. 1 (a) Nitrogen adsorption–desorption isotherms and (b) BJH pore
size distributions of the Ru/C, CN, Ru/CN and Ru/CN-imp catalysts;
(c) SEM images of Ru/C and (d) Ru/CN.
After the reaction, the resulting mixture was separated from
the catalyst by filtration, diluted with anhydrous ethanol, and
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Fig. 2 (a) The wide-angle XRD patterns and (b) Raman spectra of the CN,
Ru/C, Ru/CN and Ru/CN-imp catalysts.
pore size obviously increased with the addition of nitrogen in the
order Ru/CN (384 m2 gÀ1, 0.81 cm3 gÀ1, 3.7 nm) 4 Ru/CN-imp
(375 m2
g g ,
À1, 0.65 cm3 À1, 3.5 nm) 4 Ru/C (118 m2 gÀ1
0.01 cm3 gÀ1, 2.0 nm), but decreased slightly with the introduc-
tion of Ru, which might be related to the Ru loading in the
pores. The catalyst structures observed from the SEM images in
Fig. 1c and d could further verify the above results. Compared
Fig. 3 TEM images and particle size distributions of (a) Ru/C, (b) Ru/CN-imp,
(c) Ru/CN, and (d) HRTEM image of the Ru/CN catalyst.
with the Ru/C catalyst, a loose and porous lamellar structure calculated from about 200 particles. As shown in Fig. 3a–c,
was more evident in the Ru/CN catalyst. the particle size order was as follows: Ru/C (B5.3 nm) 4
Fig. 2a displays the XRD patterns of CN and several catalysts. Ru/CN-imp (B3.8 nm) 4 Ru/CN (B2.3 nm). It was found
It is well known that the XRD peak of amorphous carbon is that the Ru particles of the catalysts doped with nitrogen were
generally around 251. After nitrogen doping, the weak and much smaller and more homogenously-distributed than those
broad diffraction peak of the graphitic (002) (JCPDS 41-1487) without nitrogen, which was consistent with the XRD results.
shifted to a slightly higher value from 24.41 to 26.41 due to This phenomenon was attributed to the interactions between
lattice distortion, which also demonstrated that the nitrogen the N and Ru limiting the overgrowth and aggregation of
atoms had been effectually doped into the graphitic system.44 particles. The Ru particle size of the Ru/CN catalyst prepared
Compared with the Ru/C catalyst, which possessed an obvious by the one-pot pyrolysis method was smaller than that of
diffraction peak of Ru (101) (JCPDS 06-0663) around 2y = 44.11, the impregnated Ru/CN-imp catalyst due to the stronger inter-
the peaks of Ru in the Ru/CN and Ru/CN-imp catalysts were action between N and Ru. Besides, the HRTEM images in
not obvious. This result indicated that the Ru particles of the Fig. 3d indicated that the particles in the Ru/CN catalyst
N-doped catalysts were tiny and well dispersed, which could be exhibited a lattice distance of 0.198 nm, attributed to the
attributed to the fact that N doping enhanced the interaction (101) crystal faces of the hcp Ru crystallites.
between Ru and the carrier, and effectively reduced the agglom-
eration of Ru clusters.45
XPS analysis was performed to identify the composition and
electronic structure of the catalysts, and the C, N, O, and Ru
Fig. 2b shows the Raman spectra, which reflected the elements of the Ru/CN catalyst were detected in the XPS full
chemical structures of the catalysts. The D peak (1350 cmÀ1
spectrum (Fig. S1, ESI†). Fig. 4 exhibits the XPS spectra of N and
)
is the feature of disorder-induction that corresponds to the Ru of the prepared catalysts. As shown in Fig. 4a, the N 1s peaks
impurity content and defect level in graphene, while the G peak of the CN and Ru/CN catalysts could be divided into four peaks
(1580 cmÀ1) which represents the complete structure of at 397.7 eV (398.0 eV), 399.0 eV (399.3 eV), 400.3 eV (400.5 eV),
sp2 graphite is related to the degree of graphitization.46 Hence and 402.6 eV, representing pyridinic-N, pyrrolic-N, graphitic-N
ID/IG can be used as an indication of the amount of disorder and pyridinic N-oxide, respectively,44 and pyridinic-N and
and defects in graphene. Compared with the Ru/C catalyst graphitic-N were in the majority. These types of N indicated
(ID/IG = 1.08), the ID/IG ratios of the CN carrier (ID/IG = 1.26), the successful introduction of nitrogen-containing functional
Ru/CN catalyst (ID/IG = 1.25), and Ru/CN-imp catalyst (ID/IG = 1.28) groups into the carbon grid of graphene.47 Moreover, it is worth
were significantly larger after nitrogen doping under the same mentioning that the N 1s peaks of the Ru/CN shifted to slightly
calcination temperature in an inert atmosphere, which indi- higher binding energy after the Ru mixing. Fig. 4b displays the
cated an increase in structural defects, such as sheet edges, due peaks of Ru 3p of the Ru/C and Ru/CN catalysts at 462.8 eV
to the incorporation of nitrogen.
(462.6 eV) and 485.1 eV (484.8 eV) with a slight shift to lower
The microstructure of the catalysts was characterized by binding energy after nitrogen doping, corresponding to Ru0 3p3/2
TEM, as shown in Fig. 3, and the average particle sizes were and Ru0 3p1/2
.
Both the shift to higher binding energy of N
48
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Fig. 4 XPS spectra of the prepared catalysts: (a) N 1s and (b) Ru 3p.
Fig. 5 Reusable properties of the Ru/CN catalyst for hydrogenation of
p-CNB to p-CAN. Reaction conditions: 10 mmol of p-CNB, 30 mL
of anhydrous ethanol, 50 mg of Ru/CN catalyst, 2 MPa H2, 80 1C, 1 h,
800 rmp.
and the shift to lower binding energy of Ru clearly demon-
strated that the interaction between N and Ru led to electron
transfer from N to Ru.
3.2 Catalytic reaction
The p-CNB was used as a typical sample to estimate the catalyst temperature and pressure on the performance of the Ru/CN
performance on the selective reduction of halogenated nitroar- catalyst, revealing that the effect of temperature on activity was
enes, and the reaction results of different catalysts are shown in obviously greater than that of pressure.
Table 2. It was clearly demonstrated that the CN carrier was
After the catalyst comparison experiments, cyclic hydro-
inactive, and all of the Ru catalysts obtained by in situ pyrolysis genation experiments on p-CNB were performed to evaluate
exhibited a certain conversion of p-CNB and p-CAN selectivity, the recyclability of the Ru/CN catalyst. The reaction results of
higher than that of the commercial 5% RuC catalysts. Com- p-CNB hydrogenation to p-CAN in Fig. 5 were almost identical
pared with Ru/C, Ru/CN showed much higher p-CNB conver- either in activity or selectivity after 8 cycles. This result revealed
sion (13.2% vs. 1.7%) and TOF (1333 hÀ1 vs. 171 hÀ1). It could that the Ru/CN catalyst had superior stability. As we all know,
be concluded that the N doping not only improved the pore leaching and growth of active metal are the major problems for
structure and Ru particle dispersion, as shown in Table 1 and the deactivation of a catalyst. Nevertheless, the active metal Ru
Fig. 1–3, but also promoted the hydrogenation of p-CNB on the was not detected in the liquid after the reaction, and the Ru
catalyst surface, leading to high catalytic activity. The higher content of the spent Ru/CN catalyst by ICP analysis was 2.38 wt%,
p-CAN selectivity of the Ru/CN catalyst might be caused by the similar to that of the fresh catalyst (2.40 wt%), showing almost no
combination of factors including larger surface area and pore size, loss of Ru. In addition, the pore structure of the spent Ru/CN
and the interactions between N and Ru, which contributed to catalyst in Fig. 6a and b remained nearly unchanged with an
deep hydrogenation of the hydroxyaniline intermediate to amine average pore size of 3.8 nm and pore volume of 0.79 cm3 gÀ1
,
(note: no dehalogenated aniline was detected over Ru/CN). The compared with that of the fresh catalyst in Table 1. This result
preparation method also strongly affected the hydrogenation of indicated that the catalyst possessed good structural stability and
p-CNB. The Ru/CN catalyst obtained by one-pot pyrolysis exhibited the pore structure was not destroyed during the reaction. The
obviously higher catalytic activity and selectivity of p-CAN than the particle size distribution of Ru (Fig. 6c) with an average particle
Ru/CN-imp catalyst likely due to different interactions between size of 2.3 nm and the chemical state of Ru (Fig. 6d) were similar
N and Ru particles. Fig. S2 (ESI†) shows the influence of reaction to those of the fresh catalyst in Fig. 3 and 4, further indicating the
superb physical and chemical stability of the Ru/CN catalyst for
the hydrogenation of halogenated nitroarenes.
In order to prove the general applicability of Ru/CN catalyst
for the hydrogenation of other halogenated nitroarenes, the
Table 2 Hydrogenation reduction of p-CNB to p-CAN over several
catalystsa
substrates in Table 3 were tested for the hydrogenation reaction.
It is noteworthy to mention that no obvious dehalogenated
product was detected during the hydrogenation of halogenated
nitroarenes to corresponding haloanilines including the multiple
identical or different halogen groups of –F, –Cl or –Br (entries 5–8,
Table 3). That was a significant advantage over many highly
active but dehalogenating catalysts: for instance, the noble
metal catalysts of Pt and Pd. In the presence of electron-
donating groups such as –CH3, –NH2 (entries 9–12, Table 3)
Entry
Cat.
Conv.b (%)
Sel.c (%)
TOFd (hÀ1
)
1
2
3
4
5
CN
Ru/C
Ru/CN
Ru/CN-imp
5% RuC
0
0
0
171
1333
394
1.7
13.2
3.9
1.5
96.7
100
98.4
95.1
152
a
Reaction conditions: 2 mmol of p-CNB, [p-CNB]/[metal] = 1700 (mol/mol),
b
10 mL of anhydrous ethanol, 80 1C, 2 MPa H2, 800 rmp. Conversion at
10 min. Selectivity of p-CAN at 100% conversion of p-CNB. The
number of moles of p-CNB converted per mole of Ru per hour.
c
d
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Table 3 The selective hydrogenation of various halogenated nitroarenes
to corresponding amines over the Ru/CN catalysta
Entry Substrate
1
Time (min) Conv. (%) Sel. (%) TOFb (hÀ1
)
90
100
100
1333
2
3
4
140
150
135
100
100
100
100
100
100
984
930
1060
5
6
120
115
100
100
99.8
100
1194
1208
7
8
9
120
60
100
100
100
100
100
100
1159
1923
956
Fig. 6 Characterization of the spent Ru/CN catalyst: (a) nitrogen adsorp-
tion–desorption isotherms, (b) pore size distribution, (c) TEM image and
particle size distribution, and (d) Ru 3p XPS spectra.
and electron-withdrawing groups including –F, –Cl and –Br
(Table 3) which were more easily reduced they also showed an
outstanding selectivity (499.6%). Remarkably, some substrates
with double or triple bonds (entries 13 and 14, Table 3) and
nitrogen-containing heterocycles (entries 15–17, Table 3) were
successfully reduced to the corresponding haloanilines without
almost any byproducts. Factors for the hydrogenation activity of
halogenated nitroarenes are complex, including the electronic
effect with groups and the conjugated effect of multiple groups.
The sequence of catalytic activity in the halogen groups with
different electronegativities at the same position was as follows:
–F 4 –Cl 4 –Br (entries 1 and 4–6, Table 3), and in general, the
activity of hydrogenation in the para-position of the same group
was greater than that in the ortho-position or meta-position
(entries 1–3, Table 3) which could be ascribed to the halogens
in the para-position making the polarity of NQO stronger.
Moreover, for the unsaturated groups such as carbonyl groups
150
10
11
165
150
100
100
100
100
918
956
12
13
420
210
100
100
99.7
99.7
285
493
(CQO, entry 14, Table 3) which were easily hydrogenated,49 the 14
catalyst still exhibited a high selectivity of nearly 100%.
The mechanism of the hydrogenation reduction of aromatic
150
100
100
957
nitro compounds is rather complicated and, among many
related studies, the two reaction paths of the direct and indirect
reactions proposed by Haber are generally accepted.50 In the
previous reaction, we found that when the reaction was incom-
plete, the selectivity of p-CAN did not reach the highest level,
and the selectivity increased with the conversion. Therefore, the
products were analyzed after the reaction by GCMS, and the
intermediates of hydroxyaniline and azoxybenzene were detected
in the products, as shown in Fig. 7. The content of hydroxyani-
line was much higher than that of azoxybenzene, and their yields
increased first and then decreased as the reaction proceeded.
15
120
75
100
100
99.6
100
1211
1777
16
17
390
100
100
313
a
Reaction conditions: 2 mmol of the substrate, 10 mg of Ru/CN catalyst,
10 mL of anhydrous ethanol, 80 1C, 2 MPa H2, 800 rmp. b TOF: the TOF value
is calculated based on total metal and initial conversions of less than 15%.
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The two reaction paths were obtained, as shown in Fig. 8, and
the direct path was dominant. The reduction of nitro to nitroso
and further reduction to hydroxylamine happened rapidly, while
the main rate-controlling step of nitro reduction to amino was
the reduction of hydroxylamine to aniline. Furthermore, the
indirect path took place in a series of consecutive steps through
the condensation of nitroso and hydroxyaniline to azoxybenzene
and further hydrogenation to azobenzene, phenylhydrazine,
and aniline.
It was concluded that the synergistic effect between the
mixed N and the active metal Ru not only accelerated the
dissociation of hydrogen on Ru, but also increased the amount
of electron-rich hydrogen. Attributed to the nucleophilic sub-
stitution reaction of the nitro reduction,51 the more electron-
rich hydrogen is conducive to the reduction of the nitro group.
Meanwhile, the electronegative halogen is vulnerable to attack
by hydrogen in an electron-poor state. So that hydrogen in
a more electron-rich state can exactly inhibit the hydrogen
dissolution of an R–C bond, in which case the dehalogenation
is depressed to a certain degree and the chemical selectivity is
significantly improved.
Fig. 7 The yield of each product in the hydrogenation of p-CNB over the
Ru/CN catalyst, including p-CAN, hydroxyaniline and azoxybenzene.
Reaction conditions: 10 mmol of p-CNB, 25 mg of Ru/CN catalyst,
10 mL of anhydrous ethanol, 80 1C, 2 MPa H2, 800 rmp.
4. Conclusions
In this study, highly chemoselective Ru/CN nanocomposites
Therefore, it was speculated that the two reaction paths existed with superfine and uniformly dispersed Ru nanoparticles were
simultaneously in the hydrogenation of halogenated nitroar- successfully synthesized by a simple one-pot pyrolysis method.
enes to corresponding haloanilines over an Ru catalyst. In order The Ru/CN catalyst exhibited good activity and superior selec-
to further verify this, experiments were conducted on the tivity in the hydrogenation of various halogenated nitroarenes
intermediates of hydroxyaniline and azoxybenzene respectively to corresponding haloanilines with almost 100% selectivity
and it was found that both intermediates could be hydroge- of the complete conversion. The interaction between N and
nated, but the reaction rate of hydroxyaniline (100%) was much Ru promoted the production of hydrogen in an electron-rich
faster than that of azoxybenzene (35.4%) under the same state, and thus accelerated the reduction of the nitro group
reaction conditions.
and effectively reduced the dehalogenation. Moreover, it was
Fig. 8 The reaction routes of the Ru/CN catalyst for the hydrogenation of halogenated nitroarenes.
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Paper
NJC
19 J. J. Song, Z. F. Huang, L. Pan, K. Li, X. W. Zhang, L. Wang
and J. J. Zou, Appl. Catal., B, 2018, 227, 386–408.
20 Q. F. Wu, B. Zhang, C. Zhang, X. C. Meng, X. Su, S. Jiang,
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demonstrated that two pathways, direct and indirect, existed
simultaneously in the hydrogenation of halogenated nitroarenes
over the Ru/CN catalyst, and the direct pathway dominated.
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Conflicts of interest
There are no conflicts to declare.
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Acknowledgements
This research was supported by the Shanghai Engineering
Research Center of Green Energy Chemical Engineering (No.
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