Phosphorus-Doped and Lattice-Defective Carbon as Metal-like Catalyst for the Selective Hydrogenation of Nitroarenes

Article abstract of DOI:10.1002/cctc.201700904

We report carbon can be activated as metal-like hydrogenation catalyst for the selective hydrogenation of nitroarenes. Using DFT calculations we demonstrated the combination of P dopant and lattice defect in carbon can cause significant electron delocaliz

Full text of DOI:10.1002/cctc.201700904

Accepted Article
Title: Phosphorus-doped and lattice-defective carbon as metal-like
catalyst for selective hydrogenation of nitroarenes
Authors: Ruijie Gao, Lun Pan, Jinhui Lu, Jisheng Xu, Xiangwen Zhang,
Li Wang, and Ji-Jun Zou
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ChemCatChem
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Phosphorus-doped and lattice-defective carbon as metal-like
catalyst for selective hydrogenation of nitroarenes
Ruijie Gao,[a],[b],† Lun Pan,[a],[b],† Jinhui Lu, Jisheng Xu,
[a]
[a],[b]
Xiangwen Zhang,[a],[b] Li Wang[a],[b] and Ji-
Jun Zou*,[a],[b]
Abstract: We report carbon can be activated as metal-like
hydrogenation catalyst for selective hydrogenation of nitroarenes.
Using DFT calculations we demonstrated the combination of P-
dopant and lattice defect in carbon can cause significant electron
delocalization and change the band structure as metal-like one, and
catalyst for selective hydrogenation of nitro aromatics.
Nevertheless, there still are significant interests in inexpensive
metal-free catalysts for hydrogenation.
Carbon-based catalysts are attracting intensive attention
because they are cheap, stable and environment-friendly[14-19].
However, carbon is inert in hydrogenation because its
2
thus both H and nitro group are easily activated for selective
hydrogenation. Then we fabricated this carbon catalyst with tunable
concentration of P-dopant and lattice defect by polymerization and
carbonization of phytic acid, and found the concentration of lattice
defect is closely related to that of P-dopants. The synthesized
catalyst exhibits superior catalytic activity, perfect selectivity and
stability in the hydrogenation of nitroarenes, outperforming the
reported metal-free, metal-oxide and nickel catalysts. Importantly,
the hydrogenation activity is linearly dependent on the P-doping
and/or defect concentration, perfectly agreeing with the DFT
calculation. This work is expected to provide a cheap way for large-
scale production of anilines using metal-like carbon catalyst.
2
interaction with adsorbed species (such as H and nitro group) is
very weak due to the uniform charge distribution. Doping non-
metal heteroatom (N, P, S, B, at el.) in carbon matrix can cause
the charge delocalization and thus modulate its electronic
structure into metal-like[20-25], which helps to afford remarkable
catalytic activity in a similar way of metal-based catalysts.
Moreover, the heteroatom doping often brings up new defects in
nearby sites due to the different coordination, bond length and
atomic size between the dopant and carbon atoms[20,24-31], which
leads to local charge enrichment around the defect sites. In this
case, these sites can transfer more charge carriers to adsorbed
2
H and nitro group and thus facilitate the hydrogenation reaction.
Actually N-doped graphene has been reported to show metal-
like electronic structure[ and efficiently catalyze the reduction
32]
Introduction
of 4-nitrophenol[33]
.
In this work, we reported that inert carbon can be activated to
metal-like catalyst for selective hydrogenation of nitroarenes via
P-doping and defect engineering. Phosphorus (P) possesses the
same number of valence electrons as nitrogen, but has a larger
atomic radius and higher electron-donating ability. So P-doping
delivers more metal-like electronic structure and produces more
Aniline and its derivatives are important chemical immediate for
the synthesis of pharmaceuticals, agrochemicals, polymers and
other fine chemicals. Heterogeneous selective hydrogenation of
nitro group by hydrogen is a promising method to produce amino
group. Generally molecular H
first and crucial step in the hydrogenation reaction. Noble metals
especially palladium and platinum) and their alloys have been
utilized to activate H molecules under moderate conditions and
facilitate subsequent hydrogenation of –NO group by
dissociated H atoms because they are abundant of delocalized
electrons and can afford suitable interaction with H and nitro
2
activation and dissociation is the
2
defect sites, both of which can activate H and nitro group
(
effectively. First we used density functional theory (DFT)
calculations to predict that the combination of P dopants and
lattice defects makes carbon quite active and selective in
hydrogenation. We then designed a facile method to fabricate
the carbon catalyst with tunable P-doping and defect
concentration, and achieved high hydrogenation activity of
nitroarenes that surpasses the reported metal-free, metal oxide
and nickel catalysts.
2
2
2
group[1-9], but the high-cost and rare reserves limit their large-
scale application. Recently, non-noble metal-oxide based
catalysts, for instance nanoscale cobalt[10] and iron oxide[11]
supported on nitrogen-doped carbon matrix, oxygen-deficient
tungsten oxide[12] and iron oxide[13] are reported as potential
Experimental Section
[
[
a]
b]
R. Gao, Dr. L. Pan, J. Lu, J. Xu, Prof. X. Zhang, Prof. L. Wang and
Prof. J.-J. Zou
Key Laboratory for Green Chemical Technology of the Ministry of
Education
School of Chemical Engineering and Technology Tianjin University
Tianjin 300072, China
E-mail: jj_zou@tju.edu.cn
Density Functional Theory Calculations. All density functional theory
calculations were carried out with Vienna Ab-initio Simulation
Package[46,47] (VASP). The exchange correlation functional employed
was the Perdew-Burke-Ernzerhof (PBE) functional of generalized
gradient approximation[48,49]
. To locate transition states (TS), the
R. Gao, Dr. L. Pan, J. Xu, Prof. X. Zhang, Prof. L. Wang and Prof.
J.-J. Zou
Climbing Images Nudged Elastic Band (CI-NEB) algorithm[50] was used.
Each transition state was confirmed by vibrational frequency analysis.
Two-layer slab of graphite was used for modeling carbon materials. The
vacuum region was 20 Å. The kinetic energy cutoff of 500 eV was
employed. The k-point was set to 3 × 3 × 1 and that of static method was
Collaborative Innovative Center of Chemical Science and
Engineering (Tianjin)
Tianjin 300072, China
[†
]
These authors contributed equally to this work.
Supporting information for this article is given via a link at the end of
the document.((Please delete this text if not appropriate))
6 × 6 × 1. The formation energy is defined as the energy difference per
unit cell between the nanocrystal and isolated atoms[51]. The adsorption
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ChemCatChem
10.1002/cctc.201700904
FULL PAPER
energies, Eads, are calculated by using the following equation:
determined by Barrett-Joyner-Halenda (BJH). Before performing the
measurement, all the samples were outgassed under vacuum at 200 °C
for 12 h until the pressure was less than 0.66 Pa. FT-IR spectra were
recorded on a BioRad FTS 6000 spectrometer.
Eads = Eadsorbate+surface – (Eadsorbate + Esurface)
where Eadsorbate+surface is total energy of surface covered with adsorbates,
adsorbate is the energy of adsorbate, and Esurface is the energy of clean
E
Catalytic Hydrogenation of Nitroarenes. The hydrogenation was
conducted in 50 mL autoclave equipped with a PTFE lining, using 0.5
mmol nitroarenes, 5 mL solvent and 50 mg catalyst. The autoclave was
surface. Atomic information of DFT models are given in the Supporting
Information.
2 2
purged with N and H (25 bar) respectively for three times, then
Catalyst Synthesis. The PV-carbon-series catalysts were synthesized
as follows (Scheme 1): phytic acid aqueous solution (50 w/w%, 10 g)
was placed in 100 mL glass beaker and then heated under 180 °C for 16
h for polymerization, then the black polymer (solid) was carbonized at
pressurized to 20 bar, and heated to 120 °C for defined time. After the
reaction, the autoclave was cooled to room temperature, followed by
centrifugation and analysis of
a sample by GC-MS (Agilent 5975
equipped with HP-5 capillary column) and GC (Agilent 7820 equipped
with FID detector and AT-SE-54 capillary column). The activation energy
is calculated according to the Arrhenius equation
7
00, 800, 900 and 1000 °C under an Ar flow for 1 h respectively. The
obtained catalyst was labeled as PV-x with x denoting the carbonization
temperature. In addition, two other PV-900 catalysts with different
carbonization time were also synthesized (PV-900-2 for 2 h and PV-900-
a
In k = In A - E /RT
where k is the reaction rate constant, A is the pre-exponential factor, E
a
is
2
for 3 h).
the activation energy, T is the reaction temperature and R is ideal gas
constant.
Scheme 1. Schematic Illustration of Synthesis of PV-carbon
Catalysts.
Figure 1. (a) Charge density distribution (red and blue colors
represent electron accumulation and electron depletion,
respectively) and (b) simulated band structure of carbon, P-
carbon, V-carbon and PV-carbon (dotted line at 0 eV represents
the fermi level).
Characterization. Surface morphologies of catalysts were examined by
SEM using a Hitachi S-4800 instrument. TEM images were obtained
using a Tecnai G2 F20 transmission electron microscope at 200 kV, with
elemental compositions being analyzed by energy dispersive X-ray
spectrometer (EDXS). The crystal structures were recorded using a
RigaKu D/max-2500 X-ray diffractometer (XRD) equipped with a Cu Kα
irradiation source. The element analysis was determined by Vario EL
CUBE equipped with METTLER x86 instrument. Elemental composition
and bonding information were analyzed using an X-ray photoelectron
Results and Discussion
DFT design of catalyst. First we evaluated the effect of P
dopants and defects on the electronic structure of carbon using
four models including P-doped carbon (P-carbon), carbon with
vacancy (V-carbon), carbon with both P dopant and vacancy at
β site (PV-carbon), and regular carbon as a reference. It is noted
that the formation energy per atom of PV-carbon (7.65 eV) is
lower than that of P-carbon (7.74 eV), signifying that carbon
defects are easy to form simultaneously when P atoms are
embedded in carbon lattice. The presence of P dopants or/and
defects in carbon lattice cause structural distortion. Unlike pure
carbon, the electron distribution in these catalysts is non-uniform
spectroscopy operated at
a pass energy of 187.85 eV (Physical
Electronics PHI 1600 ESCA XPS system using a monochromated Al Kα
X-ray source) and the C 1s peak at 284.6 eV was taken as internal
standard. Raman spectra were recorded using a Raman spectrometer
(DXR Microscope) and a green semiconductor laser (532 nm) as
excitation source. Nitrogen adsorption-desorption isotherms were
analyzed with ASAP 2020 physisorption Analyzer at 77 K. The specific
surface areas were calculated using the conventional Brunauer-Emmett-
Teller (BET) methods. Pore size distribution in mesopore range was
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ChemCatChem
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and gathered on carbon atoms nearby P dopants and carbon
vacancies, which makes these sites election-rich and notably
carbon atoms around defective site are more electronegative
than those around P dopants (Figure 1a, Figure S1). The
electron delocalization changes the band structure into metal-
like one (Figure 1b). Compared with pure carbon the fermi level
of P-carbon uplifts near the conduction band. V-carbon shows a
more evident upshift with the fermi level partly crossing the
conduction band. Finally, the fermi level of PV-carbon almost
completely locates within the conduction band just like metal
does, suggesting a gradual transition towards metal-like electron
enrichment from P-carbon to V-carbon and finally to PV-carbon.
And such alteration of electronic structure upon P-doping and
adsorption of dissociated H atoms on C atoms around defective
site is very strong (with binding energy of 2.73 eV), which may
hinder H desorption for subsequent hydrogenation although it
has a fairly good capability for H
combining P-dopant and defect, H
2
dissociation. In the case of
is very easily activated and
2
dissociated on PV-carbon with the lowest barrier (0.54 eV)
−
owing to abundant charge (0.241 e ) transferring to H
2
. Different
from the case of V-carbon, one dissociated H atom is adsorbed
on P atom that is not so electron-rich compared with nearby
carbon atoms. So the adsorption of dissociated H atoms can
easily take part in subsequent hydrogenation. These results
predict PV-carbon as the most promising hydrogenation catalyst
among the investigated models.
defect-formation is expected to benefit H
activation.
2
and nitro group
Adsorption of nitro group is also a prerequisite for the
hydrogenation[54,55]. Nitro group is electron-withdrawing and thus
will have strong interaction with electron-rich catalyst surface like
PV-carbon. Hence we calculated the adsorption of 4-
chloronitrobenzene (CNB) on these catalysts (Figure 2c-e,
Figure S2). CNB is weakly adsorbed on P-carbon with
adsorption energy of only −0.02 eV, and N-O bond is almost
unchanged compared with free CNB, along with only 0.019 e−
transferring from P-carbon to CNB. On V-carbon, due to the
Then we computed the activation and dissociation of H
molecule on these catalyst models (Figure 2a, b). On P-carbon,
is almost inactivated (with only 0.020 e− transferring from
catalyst to H in TS) and the H-H bond dissociation is kinetically
unfeasible (with a high barrier of 1.63 eV) and endothermic by
.35 eV, indicting P-carbon is unfavorable for hydrogenation. On
2
H
2
2
0
−
enhanced charge transfer (0.225 e ) the adsorption is improved
considerably as indicated by the increased adsorption energy (-
0
.28 eV) and lengthened N-O bond. On PV-carbon, CNB is
adsorbed very strongly via two oxygen atoms of nitro group
interacting with P and C atoms around defected site with the
highest adsorption energy (−0.40 eV), owing to most charge
−
transfer (0.416 e ). These results suggest that metal-like PV-
2
carbon can activate both H and nitro group easily, which will
contribute a lot for catalyzing hydrogenation. The different
adsorption modes were also considered, and the adsorption
energy through the benzene ring and –Cl are +0.54 and +0.06
eV (Figure S3), indicting that the ring and –Cl adsorption modes
are instable and the adsorption via nitro group is most favorable,
which predicts high selectivity for hydrogenation of nitro group.
In addition, the P the C atoms nearby the defected sites are
believed to be the hydrogenation active sites according to the
calculation.
Catalysts Synthesis and Structure. Since the DFT
calculations predict PV-carbon as a promising catalyst candidate
for the hydrogenation, we then synthesized this catalyst via a
simple polymerization-carbonization route. All the synthesized
catalysts possess particle-like morphology as witnessed in the
scanning electron microscopy images (Figure S4). X-ray
Figure 2. (a) Free energies of initial (IS), transition (TS), and
final states (FS) for the dissociation of H molecule on P-carbon,
2
V-carbon and PV-carbon (H adsorption sites highlighted by
purple circles); (b) Differential charge density of carbon surface
in TS (The yellow color represents charge depletion, with iso-
diffraction diagram of carbonized catalysts show a broad peak at
55,56]
2
4.0° indexed to graphitic carbon[
, with the peak becoming
−
3
surface value of 0.02 eÅ . Number of electrons (N) represents
the electron transferred from H to catalyst, and minus (-) is
electron transferring from surface to H ); Adsorption of 4-
more intensive with increasing carbonization temperature
Figure 3a). And prolonging carbonization time can enhance the
2
(
2
crystallization of resulted catalysts, as evidenced by a stronger
XRD peak. The energy dispersive X-ray mapping confirms
homogeneous distribution of P atoms in carbon (Figure 3b). The
porosity of the synthesized PV-carbon is evaluated by nitrogen
adsorption/desorption (Figure S5). At a higher carbonization
temperature, like 900 °C, the catalyst affords a high specific
surface area. However, prolonging carbonization time makes
chloronitrobenzene based on (c) P-carbon, (d) V-carbon and (e)
PV-carbon with corresponding adsorption energies and bond
length (Å) labeled in figures.
V-carbon, the energy barrier for H-H bond cleavage deceases
greatly (0.99 eV) attributed to the significantly enhanced charge
−
2
−1
transfer from catalyst to H
2
in TS (0.146 e ). However, the
their specific surface areas (726.8, 578.6 and 117.3 m ·g ) and
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ChemCatChem
10.1002/cctc.201700904
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pore volumes (0.440, 0.345 and 0.098 cm ·g −1 for PV-900, PV-
00-2 and PV-900-3, respectively) decrease significantly (Table
S1).
3
embedded in the carbon lattice. The fitted C 1s spectra show
that obtained catalysts contain two different carbon components
(C-C, C-P) centered at 284.5 eV and 285.5 eV (Figure S6),
attributed to graphitic C and C bonding with P respectively,
further supporting P dopants are incorporated in the carbon
lattice. FT-IR spectrum spectra also confirm the formation of P-C
bond during carbonization (Figure S7), which shows several
9
XPS spectra were carried out to obtain further information
on the chemical composition of these catalysts. Two distinct
peaks are observed in the fitted P 2p spectra of synthesized PV-
carbon catalysts with an electron-binding energy of 132.5 eV
and 134.4 eV (Figure 3c), respectively assigned to phosphorus
in the carbon matrix (P-C) and P-containing functional
groups[34,35] (P-O). The former confirms P atoms are successfully
−
1
weak peaks at 663~750 cm referring to the vibration of P-C
bonds[
.
34]
1
Figure 3. (a) XRD patterns of synthesized PV-carbon catalysts; (b) TEM image of PV-900 and the corresponding EDX mapping for C,
P elements; (c) The fitted P 2p XPS spectra and (d) Raman spectra of PV-carbon catalysts; (e) Linear correlation between P-doping
and defect concentration (P-doping concentration is obtained via peak fitting of the P 2p spectra, see detail in Table S3).
1
From chemical element analysis (Table S2) and XPS peak
survey (Figure S8), the P concentration in bulk is very close to
that on surface, indicating that the as-prepared catalysts have
homogeneous distributions of P atoms in carbon as witnessed in
EDX mapping. Although the overall P concentration decreases
with the increase of carbonization temperature, the fitted P 2p
spectra show more P atoms are incorporated in the carbon
lattice (Figure 3c, Table S3). Actually, the P-doping atom
concentration in carbon matrix changes in the order of PV-900
(Figure 3d). Besides the G band appearance at 1595 cm−1
referred to the graphitic phase[35,37], a strong D band at 1350
−
1
cm is also observed that is attributed to the disordered
structure due to the presence of lattice defects[38]. So the ratio of
these two peak intensity, namely I
relative concentration of defect sites. The
D
/I
G
, is used to index the
/I value
I
D G
increasesuntil the carbonization temperature reaches 900 °C
and then decreases at 1000 °C, and it keeps almost unchanged
when prolonging the carbonization time. Notably, it is linearly on
the P-doping concentration in carbon lattice (Figure 3e),
confirming the defect structure is a consequence of P doping, as
predicted by DFT calculations.
(
(
2.24%) > PV-1000 (1.44%) > PV-800 (0.80%) > PV-700
0.70%), and at the optimal carbonization temperature, i.e.
900 °C, high amount of P atoms are incorporated in carbon.
Meanwhile PV-900-2 and PV-900-3 under longer carbonization
time show similar P-doping concentration with PV-900 (Figure
Selective Hydrogenation of Nitroarenes. The synthesized
PV-carbon catalysts were applied in the hydrogenation of
nitrobenzene to test the catalytic activity (Table 1 and Table S5).
As expected, PV-900 having the highest concentration of P
doping and lattice defect shows excellent selectivity (>99.0) to
aniline at 98% conversion of nitrobenzene. There is no any
by-product in both low and high conversion reactions as
detected by GC-MS analysis (Figure S9). At the same time,
3
c, Table S3). Correspondently the same result can be obtained
from the fitted C 1s spectra, showing the highest and similar C-P
content for PV-900, PV-900-2 and PV-900-3 (Figure S6, Table
S4).
Raman spectra show there are many defects formed as
accompanied with the doping of P atoms in carbon matrix
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Table 1. Hydrogenation of Nitrobenzene: Comparison of Various
Catalysts.
the turnover frequency (TOF) based on the molar of catalyst
also reaches the highest level. Other catalysts synthesized at
higher or lower temperatures show relatively lower activity. As
comparison, graphite oxide (GO), reduced graphite oxide (rGO)
and the precursors of PV-carbons (including phytic acid (PA)
and polymerized PA) without pyrolysis show no catalytic activity
under the similar reaction condition. PV-900-2 and PV-900-3
show relatively less activity than PV-900, because of the partial
disappearance of porous structure and reduced specific surface
area (Figure S5, Table S1).
Entry
Catalyst
PV-900
PV-700
PV-800
PV-1000
GO
Con.(%)
98.0/49.3[a]
9.6/4.9[a]
14.2/7.1[a]
52.0/25.4[a]
--
Sel.(%)
>99.0
>99.0
>99.0
>99.0
--
TOF×10-3 (h-1)
12.0/11.8[a]
1.2/1.2[a]
1.7/1.7[a]
1
2
3
4
5
6
7
Different solvents such as water, THF and cyclohexane
were tested with PV-900 as catalyst, and an activity
improvement with the polarity of solvent was observed (Table S5,
6.4/6.1[a]
--
--
--
entries 1-4). It has been shown that the polarity of solvent
rGO
--
--
58]
employed has
especially the hydrogenation of nitrobenzene
a
significant effect on many reactions[
,
PA
<1.0
--
[
4,10]
. One important
polymerized
PA
8
9
<1.0
--
--
factor is the interaction between nitro group and polar solvent via
OH‧‧‧‧‧‧N and OH‧‧‧‧‧‧O bond, which accelerates the polarization
of N=O and N-O and weakens the bond strength[59-60]. The other
is the easy adsorption and dissociated of hydrogen on the
catalyst surface in the polar solvent[61]. Meanwhile, higher
reaction temperature can accelerate the reaction (Table S5,
entry 5), but at a lower temperature like 110 °C the reaction is
retarded with very low yield (Table S5, entries 6 and 10). The
82.0/42.1[a]
48.0/24.5[a]
10.0/10.1[a]
5.9/5.9[a]
PV-900-2
PV-900-3
>99.0
>99.0
10
Reaction conditions: 0.5 mmol nitrobenzene, 50 mg catalyst, 5 mL
ethanol, 20 bar H
, 120 °C , 10 h. [a]Reaction time: 5 h.
2
Table 2. Hydrogenation of nitrobenzene: comparison of synthesized
and reported catalysts.
increase of
considerable impromvement in the reaction because more H
molecules are dissolved and adsorbed on the catalyst surface
Table S5, entries 1 and 8), but further increasing the pressure
2
H pressure from 10 bar to 20 bar shows
2
Yield
%)
TOF
(h )
Entry
Substrate
Condition
-1
Ref.
(
PV-900,
(
7.0×10-2
This
work
137 mg
100
cannot change the reaction any more probaly because the
catalyst surface is already saturated by sufficient hydrogen
19 min, R.T.
N-doped
0
.25 mmol
1
4-nitrophenol
graphene,
6.3×10-2
100
[33]
(Table S5, entries 9 and 10).
137 mg
2
1 min, R.T.
PV-900,
We further compared PV-900 with recent reported carbon
catalysts under the same reaction conditions, and found PV-900
exhibits a better activity than N-doped graphene[34] and carbonyl
group modified carbon[39] in the reduction of 4-nitrophenol by
This
work
100 mg
100
6.0×10-4
20 h, 100 °C
Carbonyl group
modified carbon,
100 mg
0
.1 mmol
2
3
Nitrobenzene
8
0
4.0×10-4
1.2×10-2
1.0×10-2
[39]
4 2
NaBH and nitrobenzene by H , respectively (Table 2). PV-900
24 h, 100 °C
catalyst also achieves a higher activity compared with reported
[12]
and nickel[12]
PV-900,
nanoscale Fe
O
2 3
/N-doped carbon[11], W18
O
49
This
work
50 mg
98
98
catalysts in the hydrogenation of nitrobenzene under similar
reaction conditions (Table 2).
The kinetic profile in hydrogenation over PV-900 reveals the
concentration of nitrobenzene and aniline change linearly with
the reaction time (Figure 4a and Figure S10), suggesting the
hydrogenation reaction on PV-carbon catalysts follow pseudo-
0.5 mmol
Nitrobenzene
10 h, 120 °C
Fe-phen/C-800,
4
2 mg
5 h, 120 °C
49, 0.8 g
[11]
1
W
18
O
7.2×10-3
6.5×10-3
1
0.5
[12]
[12]
20 g
5 h, 120 °C
Nickel, 0.8 g
5 h, 120 °C
Nitrobenzene
36.8
zero-order kinetics, different from those on metallic catalysts
which generally follow pseudo-first-order kinetics[40-44]. As proved
ch of nitro group[4,33] is presented after adsorption, well
demonstrating the strong adsorption of -NO on PV-900,
explains the perfect selectivity towards aniline, and agrees with
the DFT calculation. The Arrhenius plot gives activation energy
by DFT calculations (Figure 2e), the adsorption of nitrobenzene
is very easy and fast on catalyst surface, so the reaction rate is
independent on the concentration of substrate, which will lead to
pseudo-zero-kinetic reaction. Additional experiment at higher
concentration of nitrobenzene (1.0 mmol) was performed (Table
2
4
1 kJ mol-1 for hydrogenation of nitrobenzene over PV-900
(
Figure S11), relatively lower than 47 kJ mol-1 for nickel
catalyst[41], atributed to the suitable adsorption of disscoated H
atoms and NO group on PV-carbon compared with the very
strong adsorption on nickl surface. However, the activity of PV-
S5, entry 11), and TOF is calculated to be 11.8 ×10-3 h-1,
comparative with that at 0.5 mmol nitrobenzene, suggesting the
reaction conversion is independent on the concentration of
reactant. The adsorption of 4-chloronitrobenzen on PV-900 was
further characterized by FT-IR spectra (Figure 4b). A strong
characteristic peak at 1530 cm−1 assigned to asymmetrical stret-
2
9
00 is still not comparable with noble metal lilke Pd nanoparticle
catalyst[ which shows an extremely low activation energy of
42]
-
1
18.2 kJ mol .
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ChemCatChem
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Table 3. Hydrogenation of Substituted Nitroarenes Using PV-900
as Catalyst.
Entry
Substrate
Product
Time (h)
Con./Sel. (%)
1
8
99.0/>99.0
2
3
8
9
99.2/>99.0
97.4/>99.0
4
5
6
9
11
8
94.1/>99.0
94.4/>99.0
90.6/>99.0
Figure 4. (a) Kinetic profile of nitrobenzene hydrogenation to
aniline using PV-900 catalyst. Reaction conditions: 120 °C, 3
2
mmol nitrobenzene, 300 mg PV-900 catalyst, 20 bar H , 30 mL
ethanol; (b) FT-IR spectra of PV-900 before and after adsorption
of 4-chloronitrobenzen, respectively; (c) Linear correlation
7
8
9
18
14
15
82.8/>99.0
87.4/>99.0
86.6/>99.0
between TOF and P-doping concentration and I
conditions: 120 °C , 10 h, 0.5 mmol nitrobenzene, 50 mg catalyst,
0 bar hydrogen, 5 mL ethanol; (d) Recycling performance of
D G
/I . Reaction
2
PV-900 catalyst in hydrogenation of nitrobenzene.
It is also noted that TOF is linearly dependent on the P-
doping concentration and I
D G
/I (Figure 4c), in good agreement
with the DFT prediction that the combination of P-dopant and
lattice defect switches carbon into metal-like catalyst for easy H
activation, moderate adsorption of dissociated H atom and easy
2
1
0
1
10
11
98.1/>99.0
94.2/>99.0
activation of nitro group for subsequent hydrogenation. In order
to demonstrate the stability and recyclability of the PV-900, we
reused the catalyst in the reaction up to 10 times. The selectivity
of aniline keeping almost constant in all runs, although the
conversion goes down a little (Figure 4d). XPS and Raman
characterizations further confirm that the composition and
structure of the catalyst are well retained in the recycling test
1
12
12
18
72.1>99.0
88.3/>99.0
(Figure S12, S13, Table S3 and S4). However, the slight loss of
activity can be attributed to the decrease of BET surface in the
reaction (Figure S14, Table S1).
13
Furthermore, we explored the hydrogenation of 13 other
industrially interesting nitroarenes using PV-900 as catalyst and
obtained the corresponding anilines in excellent yield and
selectivity (Table 3). Especially, PV-900 is able to selectively hy-
drogenate nitro-group to corresponding amino-group with full
Reaction conditions: 0.5 mmol substrate, 50 mg PV-900, 5 mL ethanol,
0 bar H , 120 °C .
2
2
conversion and remains other unsaturated functional groups Conclusions
such as –CHO, -COOH and -C≡C unchanged (Table 3, entries 1,
2
and 12). And this catalyst is active and selective for substrates
with either electron-withdrawing or electron-donating groups
Table 3, entries 3-7 and 8-11), suggesting PV-900 serves as a
We reported the activation of carbon to metal-like catalyst for
selective hydrogenation of nitroarenes via P-doping and defect
engineering. Combining P-dopant and defect site in carbon can
induce charge enrichment around the dopant and defect site,
and generate metal-like electronic structure which can benefit
(
versatile catalyst for the selective hydrogenation of substituted
nitroarenes.
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ChemCatChem
10.1002/cctc.201700904
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the H
2
and nitro group activation for hydrogenation reaction. For
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The catalyst (PV-900) shows excellent activity, selective and
stability in hydrogenation of nitroarenes, much better than
recently reported carbon-based, metal-oxide and nickel catalysts.
Moreover, the hydrogenation activity is linearly dependent on the
P-doping and/or defect concentration. This work is expected to
provide a new way for large-scale production of anilines using
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Entry for the Table of Contents (Please choose one layout)
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†
†
Phosphorus-doped and lattice-
defective carbon serves as an efficient
metal-like catalyst for selective
R. Gao , L. Pan , J. Lu, J. Xu, X. Zhang,
L. Wang and J. Zou*
hydrogenation of nitroarenes and
outperforms the reported metal-free,
metal-oxide and nickel catalysts due
to its metal-like electronic properties
Page No. – Page No.
Phosphorus-doped and lattice-
defective carbon as metal-like
catalyst for selective hydrogenation
of nitroarenes
and excellent H
capability.
2 2
and -NO activation
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