Catalytic hydrogenation of nitrophenols and nitrotoluenes over a palladium/graphene nanocomposite

Article abstract of DOI:10.1039/c4cy00048j

We report a stable palladium/graphene (Pd/G) nanocomposite with differing Pd content for use in the catalytic hydrogenation of nitrophenols and nitrotoluenes. Various microscopic and spectroscopic techniques were employed to characterize the as-prepared catalysts. Catalytic hydrogenation reactions of nitrophenols were conducted in aqueous solution by adding NaBH₄, while the nitrotoluene hydrogenation was carried out in methanol in the presence of H₂ because of the poor solubility in water. The Pd/G hybrids exhibited much higher activity and higher stability than the commercial Pd/C. Due to the presence of a large excess of NaBH₄ compared to p-nitrophenol, the kinetic data can be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. The resulting high catalytic activity can be attributed to the graphene sheets' strong dispersion effect for Pd nanoparticles and good adsorption ability for nitrobenzene derivatives via π-π stacking interactions. A plausible mechanism is proposed. Considering inductive and conjugation effects that may affect the reactions, the reactivity of nitrophenols in this study is expected to follow the order m-NP > o-NP > p-NP > 2,4-DNP > 2,4,6-TNP, which is in good agreement with the experimental results. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cy00048j

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Catalytic hydrogenation of nitrophenols and
nitrotoluenes over a palladium/graphene
nanocomposite
Cite this: Catal. Sci. Technol., 2014,
4, 1742
a
b
b
a
Jingwen Sun,ab Yongsheng Fu, Guangyu He, Xiaoqiang Sun and Xin Wang
*
*
*
We report a stable palladium/graphene (Pd/G) nanocomposite with differing Pd content for use in the
catalytic hydrogenation of nitrophenols and nitrotoluenes. Various microscopic and spectroscopic
techniques were employed to characterize the as-prepared catalysts. Catalytic hydrogenation reactions
of nitrophenols were conducted in aqueous solution by adding NaBH4, while the nitrotoluene hydro-
genation was carried out in methanol in the presence of H2 because of the poor solubility in water. The
Pd/G hybrids exhibited much higher activity and higher stability than the commercial Pd/C. Due to the
presence of a large excess of NaBH4 compared to p-nitrophenol, the kinetic data can be explained by
the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. The resulting high catalytic
activity can be attributed to the graphene sheets’ strong dispersion effect for Pd nanoparticles and good
adsorption ability for nitrobenzene derivatives via π–π stacking interactions. A plausible mechanism is
proposed. Considering inductive and conjugation effects that may affect the reactions, the reactivity of
nitrophenols in this study is expected to follow the order m-NP > o-NP > p-NP > 2,4-DNP > 2,4,6-TNP,
which is in good agreement with the experimental results.
Received 14th January 2014,
Accepted 3rd February 2014
DOI: 10.1039/c4cy00048j
www.rsc.org/catalysis
urgent necessity to preserve the environmental quality.
Therefore, the development of high performance catalysts
Introduction
Aminophenols are extensively used in various industries,
such as dyes, pesticides, medicine, surfactants and cosmetic
products.1–3 As a prominent example, p-aminophenol is the
most promising intermediate in the industrial synthesis of
analgesics and antipyretics, like acetanilide and paracetamol.4–6
It is generally known that nitrophenols are very useful starting
materials for producing aminophenols and other chemicals,7–9
though they are considered as one of the top 114 organic pol-
lutants listed by the United State Environmental Protection
Agency (USEPA).10–12 Furthermore, nitrophenols can also be
produced as byproducts in industrial processes, for example,
the production of nitrobenzene through benzene nitration with
nitric and sulfuric acid is usually carried out in continuous
mode, forming nitrophenols as the main byproducts, namely,
di- and tri-nitrophenols (DNP and TNP).13 The multi-nitro
compounds are more toxic, carcinogenic, mutagenic, and
teratogenic,11–13 and elimination of these compounds is an
for the reduction of nitrophenols to aminophenols is not
only valuable for industrial processes, but also very impor-
tant for the environment.
There are several synthesis routes for the reduction of
nitrophenols to aminophenols, including metal/acid reduc-
tion, electrolytic reduction and catalytic hydrogenation.14,15 It
is known that the metal/acid system requires a strong acidic
medium and exhibits poor selectivity. The electrolytic reduc-
tion needs an acidic or alkaline catholyte, resulting in low
yields. Among all the previously mentioned methods, only cat-
alytic hydrogenation is a promising process as it can achieve
high conversion, has little impact on the environment and
does not generate acid effluents. However, noble metal-based
catalysts are still most frequently used in this system, such
as palladium-,16,17 platinum-18,19 and aurum-based systems,20
which may lead to several unfavorable points such as high
cost and limited resources of noble metals.
It is well known that a catalyst support can improve specific
properties such as mechanical strength, stability, activity and
selectivity of catalysts, and especially it can dilute noble metals in
a large volume. Widely used supports include silica, alumina,9,21
zeolites,22 TiO2,23 polymers24 and carbon materials.25–27 In the
past few decades, carbon nanomaterials, such as carbon nano-
tubes and carbon nanofibers, have found numerous promising
applications in catalytic hydrogenation. Graphene, as a new
a Key Laboratory of Soft Chemistry and Functional Materials, (Nanjing University
of Science and Technology), Ministry of Education, Nanjing, 210094, China.
E-mail: wxin48@163.com, fuyongsheng0925@163.com; Fax: +86 25 8431 5054;
Tel: +86 25 84305667
b Jiangsu Province Key Laboratory of Fine Petrochemical Engineering, Changzhou
University, Changzhou 213164, China. E-mail: xqsun@cczu.edu.cn
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member of the carbon nanomaterials, exhibits many unique
properties, such as high surface area, extraordinary electronic
transport properties, superior mechanical stiffness and flexibil-
ity. These characteristics make graphene highly desirable for
application as a 2D support to load a catalyst.
Pd-based catalysts are known to be capable of selectively
hydrogenating nitro-compounds to their corresponding ani-
line. The combination of graphene sheets and Pd nano-
particles may provide a brand new avenue in utilizing the
two-dimensional planar carbon material since it can not only
serve as an effective platform for fast transportation of reac-
tants to the catalyst layers, but also result in improved effects
of the individual components. Recently, Pd–graphene nano-
hybrids have been reported to act as catalysts for the Suzuki–
Miyaura cross-coupling reactions.28
Characterization
Transmission electron microscopy (TEM) images were taken
using a JEOL JEM-2100 microscope operating at 200 kV,
by depositing a drop of dispersed sample onto a 300 mesh
Cu grid coated with a carbon layer. Powder X-ray diffraction
(XRD) analyses were performed on a Bruker D8 Advanced
diffractometer with Cu Kα radiation and the scanning angle
ranged from 5° to 70° of 2θ. Raman spectra of the samples
were collected with a Renishaw Reflex Raman Microprobe.
X-ray photoelectron spectroscopy (XPS) measurements were
carried out on an RBD upgraded PHI-5000C ESCA system
(Perkin Elmer) with Mg Kα radiation (hν = 1253.6 eV).
Catalytic hydrogenation of nitrophenols and nitrotoluenes
We have developed a soft chemistry method to directly
fabricate metal–graphene nanocomposites, which have been
used as catalysts for formic acid and methanol oxidation.19,29,30
Since Pd-based catalysts can be used as catalysts for reduction
reactions as well, in this study, a Pd/graphene nanocomposite
was prepared and used for the hydrogenation of nitrophenols
and nitrotoluenes. Herein, we report a stable palladium/
graphene nanocomposite for use in the catalytic hydrogenation
of nitrophenols and nitrotoluenes. It was found that the π–π
stacking interactions are the dominant driving force for the
binding between the nitrophenol derivatives and reduced
graphene oxide sheets, leading to enhanced adsorption ability
of graphene, which helps in promoting the reduction reaction
as the reactants can easily gain access to the Pd nanoparticles.
The negatively charged hydrogen in the Pd–hydrogen struc-
ture can readily attack the positively charged nitrogen in the
nitro group. The reactivity of nitrophenols follows the order
m-NP > o-NP > p-NP > 2,4-DNP > 2,4,6-TNP.
Catalytic hydrogenation reactions of nitrophenols were
conducted at room temperature (25 °C) in the presence of
Pd/G catalysts with differing Pd contents. Typically, aqueous
solutions of p-nitrophenol (0.3 mM) and NaBH4 (0.1 M) were
freshly prepared. 4 mg of Pd0.05/G was dispersed in 40 mL of
p-nitrophenol (p-NP) solution under ultrasound irradiation for
several minutes, and then 3 mL of NaBH4 solution was injected
into the mixture under continuous stirring. To evaluate the
reaction progress, 3 mL aliquots were taken out of the reaction
mixture at specified time intervals and monitored by UV-vis
spectroscopy (SHIMADZU TCC-240A). After the reaction, the
catalysts can be easily recovered by filtration, and can be
reused five times in succession without any treatment. The
Pd0.10/G and Pd0.20/G nanocomposites and the commercial
Pd/C (5 wt% Sigma-Aldrich 205699) also participated in the
hydrogenation, respectively. In addition, the hydrogenation of
o-nitrophenol (o-NP), m-nitrophenol (m-NP), 2,4-dinitrophenol
(2,4-DNP) and 2,4,6-trinitrophenol ( 2,4,6-TNP) were carried
out under the same conditions.
Unlike for the nitrophenols, the nitrotoluene hydrogenation
was carried out in methanol in the presence of H2 because of
the poor solubility in water. The HPLC analyses of the reaction
products were performed on a SHIMADZU LC-20A.
Experimental
Synthesis of Pd/graphene catalyst
Graphite oxide (GO) was synthesized directly from graphite
powder according to the Hummers method.31 Palladium/
graphene (Pd/G) hybrids with differing Pd contents (5, 10,
20 wt%) were prepared through a hydrothermal method. In a
typical procedure, 80 mg of GO was dispersed in a mixture of
ethyleneglycol (40 mL) and aqueous solution (20 mL) under
ultrasound irradiation for 1 h. Then 40 μL of Pd(NO3)2 was
added to the above GO dispersion with magnetic stirring
at room temperature for 30 min. The resulting mixture was
transferred into a 100 mL Teflon-lined stainless steel auto-
clave and sealed tightly. The autoclave was then heated to
120 °C and kept there for 12 h. After the solvothermal treat-
ment, the reaction was quenched and cooled to room temper-
ature. The product, denoted as Pd0.05/G, was centrifuged,
washed, and finally vacuum freeze-dried overnight. For
comparison, 10 wt% and 20 wt% Pd supported on graphene
were also prepared under the same experimental conditions,
and denoted as Pd0.10/G and Pd0.20/G, respectively.
Results and discussion
Structure and morphology of Pd/graphene
nanocomposite catalyst
Fig. 1a & b show the typical TEM images of the commercial
Pd0.05/C and the Pd0.05/G catalysts. It can be seen that there is a
more uniform dispersion of Pd nanoparticles on the graphene
sheets than on the commercial carbon, thus Pd0.05/G is
expected to exhibit a higher efficiency for the catalytic hydroge-
nation of nitrophenols and nitrotoluenes. The lattice spacing
was measured to be 0.223 nm (Fig. 1c), implying that the
growth of the Pd nanoparticles occurred preferentially on the
(111) planes32 and the average diameter of Pd is about 6.6 nm
(Fig. 1d). With increasing loading content, the Pd nanoparticles
tend to aggregate as shown in Fig. 1e & f and the aggregation
of Pd nanoparticles may be a fatal factor leading to a decrease
in catalytic activity.
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particles of the hybrid can be calculated from the (220) peak
by the Scherrer equation:34
0.9
1 cos
2
d 
(1)
where d is the average particle size (nm), λ is the wavelength
of the X-ray used (0.15406 nm), β1/2 is the width of the dif-
fraction peak at half height in radians and θ is the angle at
the position of the peak maximum. The calculated average
particle size of Pd supported on graphene is 5.8 nm, which is
consistent with the TEM results.
Raman spectroscopy has been widely used as a powerful
tool for characterizing the defect quantity in graphitic mate-
rials. Fig. 2b shows the Raman spectra of Pd/G and GO. The
D- and G-bands were witnessed at approximately 1359 and
1595 cm−1, which may be assigned to the breathing modes of
the κ-point phonons of A1g symmetry and the E2g phonons of
sp2 carbon atoms, respectively.35 Compared to the spectrum of
GO, significant peak shifts of the D- and G-bands of Pd/G were
observed from 1356 to 1347 cm−1 and 1598 to 1594 cm−1,
respectively, indicating that GO has been reduced.36 Meanwhile,
an increasing D/G intensity ratio was perceived, which is attrib-
uted to a lower degree of crystallinity in graphitic materials. Fur-
ther, the 2D band (2757 cm−1) and combination bands (D + G)
were also detected, further indicating the reduction of GO.
X-ray photoelectron spectroscopy (XPS) is the most widely
used technique to reveal information about the surface ele-
mental composition and their oxidation states for composites
due to its relative simplicity in use and data interpretation.
The C 1s core-level spectra of GO and Pd0.05/G are depicted
in Fig. 3a & b, respectively. For graphite oxide, the peak at
284.5 eV is associated with C–C bonds, while the interlaced
peak centered at the binding energies of 286.1, 287.7 and
Fig. 1 (a,b) Typical TEM images of the Pd0.05/C and Pd0.05/G catalyst,
(c) high-resolution TEM image of the Pd0.05/G catalyst, (d) particle size
distribution for the Pd0.05/G catalyst and (e,f) typical TEM images of the
Pd0.10/G and Pd0.20/G catalysts.
Fig. 2a shows the X-ray diffraction (XRD) patterns of the
as-prepared Pd0.05/G nanocomposite, reduced graphene oxide
(RGO) and graphite oxide (GO). It is clear that the Pd0.05/G
nanocomposite exhibits an XRD pattern with major peaks at
2θ values of 40.02°, 46.59° and 68.08°, which can be indexed
to the (111), (200) and (220) planes of the face-centered cubic
(fcc) structure of Pd (JCPDS 46-1043), respectively.33 Mean-
while, no typical diffraction peak belonging to RGO (002) or
GO (001) was observed in the Pd0.05/G nanocomposite. The
absence of these peaks may be ascribed to the disruption
and exfoliation of graphene oxide in the composites during
the hydrothermal reaction. In addition, the average size of Pd
Fig. 3 (a,b) C 1s core-level XPS spectra of GO and the Pd0.05/G catalyst,
(c,d) Pd 3d core-level XPS spectrum of commercial Pd0.05/C and the pre-
pared Pd0.05/G catalyst.
Fig. 2 (a) XRD patterns of Pd0.05/G, RGO and GO, (b) Raman spectra of
Pd0.05/G and GO.
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289.1 eV can be attributed to the C–OH, C–O–C and OH–CO
functional groups, respectively.37 For Pd0.05/G, the intensity of
oxygen-containing groups was obviously reduced during the
solvothermal reaction, while the peak at 284.5 eV, correspond-
ing to the C–C bond, became predominant. The splitting pat-
tern of the Pd 3d band of commercial Pd0.05/C and Pd0.05/G
consist of two doublets (Fig. 3c & d): the intensive doublet
(335.6 and 340.8 eV) may be assigned to metallic Pd and the
other doublet (337.8 and 343.2 eV) belongs to the +2 oxidation
state of Pd.38 The existence of Pd2+ in the catalyst may be due
to the reduction of Pd2+ not being entirely completed during
the solvothermal reaction in the presence of ethylene glycol
and the naked metal Pd atoms can be easily oxidized to the
form of Pd oxide under ambient conditions.39,40 The XPS
results also allow us to determine the percentage of the respec-
tive oxidation states: 79.7% as Pd(0) and 20.3% as Pd(+2) in
Pd/G, while only 60.9% as Pd(0) in Pd/C. The higher the reduc-
tion degree of Pd, the higher the catalytic activity for the hydro-
genation of nitrophenols.
the peak at 400 nm sharply decreased and a typical absorption
of p-aminophenol (p-AP) at 300 nm increased significantly,
and as a result, the initially light yellow solution consequently
underwent fading to become colourless.39,41
Fig. 4b shows the absorbance versus wavelength plots at
various times for the reduction reaction of p-NP to p-AP in
the presence of Pd0.05/G. Fig. 4c shows the relationship of
C/C0 versus reaction time during the course of the reduction of
p-NP in the presence of various catalysts, where C and C0 are
the p-NP concentrations at times t and 0, respectively. It can be
seen that the catalytic activities are in the following order:
Pd0.05/G > Pd0.10/G > Pd0.05/C > Pd0.20/G. It should be noted
that the activity decreases with increasing Pd content in the
catalyst. As shown in Fig. 1e & f, the aggregation of Pd nano-
particles occurred for Pd0.10/G and Pd0.20/G. High Pd loading
may result not only in high costs for preparation of catalysts,
but also in the increase in particle size due to particle aggrega-
tion, leading to lower catalytic activities. As shown in Fig. 4d,
the catalyst can be easily separated by a simple centrifugation
technique facilitating catalyst recovery and reuse, and the con-
version rate of p-NP can be maintained at over 85% after five
cycles, exhibiting higher activity and higher stability than the
commercial Pd/C with the same Pd content.
Catalytic hydrogenation of nitrophenols
The catalytic reduction of p-NP by NaBH4 was chosen as a
model reaction to evaluate the catalytic activities of the Pd/G
(5, 10 and 20 wt%) catalysts and commercial Pd/C. UV-vis
absorption spectra were recorded over time to monitor the
change in raw material concentration after adsorption equi-
librium. As shown in Fig. 4a, the original adsorption peak of
p-NP was centered at 317 nm and shifted to 400 nm after the
addition of freshly prepared NaBH4 aqueous solution, and the
colour of the solution changed from light yellow to bright yel-
low immediately. This red shift was just due to the formation
of p-nitrophenolate ions in alkaline conditions caused by the
addition of NaBH4.17 After the addition of the Pd0.05/G catalyst,
Since the initial concentration of NaBH4 largely exceeds
that of p-NP (~100 times) and the rate of reduction is inde-
pendent of the concentration of NaBH4. Thus the reduction
rate can be assumed to be independent of NaBH4 and the
hydrogenation process can be described with pseudo-first-
order kinetics40 with respect to the concentration of p-NP:
C
ln
 kt
(2)
C0
where C0 and C (mM) are the concentrations of p-NP at
beginning and at time t, respectively. As shown in Fig. 5, a
linear relationship of ln(C/C0) versus time was obtained, indi-
cating that the reaction followed pseudo-first-order kinetics.
The observed rate constants for Pd0.05/G, Pd0.10/G, Pd0.20/G
and Pd0.05/C were 0.0365 s−1, 0.0196 s−1, 0.0024 s−1 and
0.0155 s−1, respectively.
On the basis of the above observations, we propose a plausi-
ble mechanism of the catalytic reduction of p-NP in the
Fig. 4 (a) UV-vis absorption spectra of p-NP before (i) and after (ii) the
addition of NaBH4, and p-AP (iii), (b) representative time-dependent
UV-vis absorption spectra of the reduction of p-NP over the Pd0.05/G
catalyst in aqueous media at room temperature, (c) hydrogenation of
p-NP over different catalysts, (d) reuse of Pd0.05/G and Pd0.05/C cata-
lysts for p-NP reduction.
Fig. 5 Plot of ln(C/C0) against reaction time for the catalytic reduction
of p-NP with different catalysts.
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Table
1
Pseudo-first-order kinetics study with nitrophenols and
different catalysts
) )
Catalyst Raw material k (min−1 Catalyst Raw material k (min−1
Pd0.05/G p-NP
Pd0.10/G
Pd0.20/G
2.192
1.176
0.144
0.932
Pd0.05/G o-NP
m-NP
2.279
3.562
2.192
0.974
0.531
p-NP
2,4-DNP
2,4,6-TNP
Pd0.05/C
effect of o-NP makes its nitrogen atom more positively
charged, resulting in higher reactivity. For m-NP, the phenox-
ide oxygen is not directly conjugated to the meta-nitro group
and thus the inductive effect alone stabilizes the nitro group
to a smaller extent. Taking into account the structure of
nitrophenols with mono- and multi-nitro groups, mono-
nitrophenol has the smallest steric resistance and 2,4,6-TNP
has the largest one. Based on the discussion above, the reac-
tivity of nitrophenols in this study is expected to follow the
order m-NP > o-NP > p-NP > 2,4-DNP > 2,4,6-TNP, which is
in good agreement with the experimental results (Table 1).
Scheme 1 A plausible mechanism for the reduction of p-nitrophenol
catalyzed by the Pd/G catalyst in the presence of sodium borohydride.
presence of NaBH4 as shown in Scheme 1. In the reaction sys-
tem, the π–π stacking interactions are the dominant driving
force for the binding between the nitrophenols and reduced
graphene oxide sheets, leading to enhanced adsorption abili-
ties of graphene.42,43 Good adsorption helps in promoting the
reduction reaction as the nitrophenol molecules can easily gain
access to the Pd nanoparticles.44 NaBH4 can react with water at
ambient temperature, yielding H2 and the sodium metaborate
(NaBO2) by-product. In the presence of Pd nanoparticles, the
H–H bond in H2 cleaves, and each hydrogen attaches to the
metal nanoparticle surface, forming metal–hydrogen bonds.
The negatively charged hydrogen in the metal–hydrogen struc-
ture can easily attack the positively charged nitrogen in the
nitro group of nitrophenols, and as a result, the nitro group is
reduced to the nitroso group, followed by the reductive addi-
tion of two hydrogen atoms to form hydroxylamine. Finally, the
hydroxylamine is further reduced to the aniline derivative.
Catalytic hydrogenation of more nitrophenols, such as
o-NP, m-NP, 2,4-DNP and 2,4,6-TNP, was also conducted with
freshly prepared NaBH4 in the presence of Pd0.05/G, and the
results are shown in Fig. 6.
Influence of substitution at phenyl positions on
hydrogenation reactions
Nitrophenols and nitrotoluenes were selected as models to
evaluate the comparative influence of substitution at differ-
ent phenyl positions on the hydrogenation reactions. Because
of the poor solubility of nitrotoluenes in water, the solvent
and reducing agent were replaced by methylalcohol and
hydrogen, respectively. The hydrogenation conversion rates
of nitrobenzenes substituted by –OH or –CH3 at different
positions (ortho-, meta- or para-) were calculated and summa-
rized in Table 2, where it can be found that the conversion
rate of nitrotoluene was nearly three times as much as that of
Because of the conjugation effect of o-NP and p-NP, the
negative charge in the phenoxide ion may be delocalized onto
the nitro group, making the group more stable. Moreover,
the inductive effect may also play a certain role, which is
dependent on the distance between the substituent group
and the group that reacts, so that the inductive effect of
o-NP is stronger than that of p-NP. The stronger inductive
Table 2 Reduction of nitrophenols and nitrotoluenes with hydrogen
in the presence of methylalcohol
Catalyst
Pd0.05/G
R
Product
Conversion (%)
35.72
–OH
40.82
29.55
89.32
–CH3
100.0
80.33
Fig. 6 Plot of C/C0 versus time during the course of reduction of various
nitrophenols with Pd0.05/G.
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nitrophenol substituted at the same site. This phenomenon
can be attributed to the electron donating ability of the
methyl group being weaker than that of the hydroxyl
group, and as a result, the more positively charged nitrogen
can be more easily attached to the negatively charged hydro-
gen from the Pd metal–hydrogen structure. Similar to
nitrophenols, for o-nitrotoluene or p-nitrotoluene, the nega-
tive charge in the toluene-ring may be delocalized into the
nitro group, making the group more stable. Further consider-
ing that the inductive effect of o-nitrotoluene is stronger than
that of p-nitrotoluene, the former is expected to exhibit higher
reactivity. For m-nitrotoluene, the toluene-ring is not directly
conjugated to the meta-nitro group and thus the inductive
effect alone stabilizes the nitro group to a smaller extent.
Thus, the reactivity of nitrotoluenes in this study follows
the order m-nitrotoluene > o-nitrotoluene > p-nitrotoluene,
which is consistent with the experimental results.
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Conclusions
In summary, we have fabricated and characterized a stable
palladium/graphene nanocomposite with differing Pd con-
tent for use in the catalytic hydrogenation of nitrophenols
and nitrotoluenes. Catalytic hydrogenation reactions of nitro-
phenols were carried out in an aqueous solution by adding
NaBH4, the Pd/G hybrids exhibited much higher activity and
higher stability than the commercial Pd/C. It was found that
the π–π stacking interactions may enhance adsorption ability
of graphene, which helps in promoting the reduction reac-
tion as the reactants can easily gain access to the surface of
the Pd particles. Taking into account the inductive and con-
jugation effects, the reactivity of nitrophenols may follow
the order m-NP > o-NP > p-NP > 2,4-DNP > 2,4,6-TNP, which
is consistent with the experimental results. Because of the
poor solubility of nitrotoluenes in water, the hydrogenation
was conducted in methanol in the presence of H2. The
reactivity of nitrotoluenes was found to be m-nitrotoluene >
o-nitrotoluene > p-nitrotoluene.
Acknowledgements
This investigation was supported by NNSF of China (no.
21171094), NSAF (no. U1230125), RFDP (no. 20123219130003),
STPP of Jiangsu (no. BE 2012151), the Fundamental Research
Funds for the Central Universities (no. 30920130122002), PAPD
of Jiangsu and the Jiangsu Province Key Laboratory of Fine Pet-
rochemical Engineering (no. KF1206).
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