Differences in the selective reduction mechanism of 4-nitroacetophenone catalysed by rutile- And anatase-supported ruthenium catalysts

Article abstract of DOI:10.1039/c9cy02260k

Ru/TiO₂ catalysts exhibit excellent catalytic performance for selective reduction of 4-nitroacetophenone to 4-aminoacetophenone at normal temperature and atmospheric hydrogen pressure. Moreover, 99.9% selectivity to 4-aminoacetophenone can be obtained over 2.7 wt% Ru/TiO₂(anatase) catalyst even in a relatively wide temperature (55-115 °C) and time (1-12 h) range. Its excellent catalytic performance is derived from the activation of H₂ on the Ru nanoparticles at atmospheric pressure and the strong interaction of nitro groups with the support surface. Additionally, Ru nanoparticles supported on different crystalline TiO₂ phases (anatase and rutile) result in different reaction pathways for 4-nitroacetophenone. Since the Ti-Ti distance on the rutile surface is smaller than that on the anatase surface, the hydroxylamine species adsorbed on the Ti atoms of rutile are more susceptible to the coupling reaction. Therefore, Ru/TiO₂(rutile) causes a series of intermediates to accumulate during the conversion process, while Ru/TiO₂(anatase) allows the highly selective conversion of 4-nitroacetophenone to 4-aminophenone. In addition, Ru/TiO₂(anatase) can achieve chemoselective reduction of nitroaromatics to the corresponding anilines in the presence of -CN, -CHO, and -COOH, especially nitroaromatics containing CC and CC, indicating the excellent applicability.

Full text of DOI:10.1039/c9cy02260k

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ISSN 2044-4761
PAPER
Chaoqiu Chen, Yong Qin et al.
Highly dispersed Pt nanoparticles supported on carbon
nanotubes produced by atomic layer deposition for hydrogen
generation from hydrolysis of ammonia borane
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Page 1 of 13
CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
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DOI: 10.1039/C9CY02260K
ARTICLE
Differences in Selective Reduction Mechanism of 4-
Nitroacetophenone Catalysed by Rutile and Anatase Supported
Ruthenium Catalysts
Received 00th January 20xx,
Accepted 00th January 20xx
ab
ab
ab
ab
a
ab
DOI: 10.1039/x0xx00000x
Jin Zhang, Linjuan Pei, Jie Wang, Pengqi Zhu,* Xianmo Gu* and Zhanfeng Zheng*
Ru/TiO
aminoacetophenone at the normal temperature and atmospheric hydrogen. Moreover, 99.9% selectivity to 4-
aminoacetophenone can be obtained over the 2.7 wt% Ru/TiO (anatase) catalyst even in a relatively wide temperature
55-115 °C) and time range (1-12 h). Its excellent catalytic performance is derived from the activation of H on the Ru
nanoparticles at atmospheric pressure and the strong interaction of nitro groups with the support surface. Additionally, Ru
nanoparticles supported on different crystalline TiO (anatase and rutile) result in different reaction pathways for 4-
2
catalyst exhibits excellent catalytic performance for selective reduction of 4-nitroacetophenone to 4-
2
(
2
2
nitroacetophenone. Since the Ti-Ti distance on the rutile surface is smaller than that on the anatase surface, the
hydroxylamine species adsorbed on the Ti atom of rutile are more susceptible to the coupling reaction. Therefore, the
Ru/TiO
allows the highly selective conversion of 4-nitroacetophenone to 4-aminophenone. In addition, Ru/TiO
achieve chemoselective reduction of nitroaromatics to the corresponding anilines in the presence of -CN, -CHO, -COOH,
2
(rutile) causes a series of intermediates accumulation during the conversion process, while the Ru/TiO
2
(anatase)
2
(anatase) can
especially
nitroaromatics
containing
C=C
and
C≡C,
indicating
the
excellent
applicability.
Rh/SiO
2
catalyst was nitro > carbonyl > phenyl.9, 12 When the 4-
Introduction
Aromatic amines are significant intermediates to dyes, pesticides,
pharmacecutials and other fine chemicals, which are generally
synthesized by the reduction of nitroaromatics. However, it is
quite challenging to reduce the nitro group when other reducible
groups are present in the same molecule. For instance, the
selective reduction of nitro group in 4-nitroacetophenone is a key
nitroacetophenone was completely transformed, the selectivity to
-aminoacetophenone decreased continuously with the prolonging
4
of reaction time and/or the increase of temperature. Subsequently,
Yin et al. found that the 100% selectivity to 3-aminoacetophenone
was achieved at complete conversion of 3-nitroacetophenone over
1
-3
1
3
the 0.3 wt% Pt/TiO
discovered that the reduction efficiency of nitroacetophenone for
Rh/TiO was 16 times higher than that for Rh/SiO , indicating that
TiO as support can effectively improve the reaction activity of the
2
even at high temperature. In addition, they
4
-7
8
2
2
step to obtain 4-aminoacetophenone. Nevertheless, the traditional
noble metals such as Pt, Rh and Ru catalysts can simultaneously
reduce both nitro groups and carbonyl groups to generate
2
1
3
catalyst. Consequently, the choice of support was crucial to
prepare the catalyst. Selecting suitable supports can not only
effectively promote the maximum utilization of noble metals, but
also transform nonselective catalysts into highly chemoselective
catalysts. Corma et al. found that Ru, which was not chemoselective
for the hydrogenation of 3-nitrostyrene, can be transformed into
9
, 10
undesired byproducts.
It is a challenge to design catalysts with
selective reduction of the nitro groups while maintaining carbonyl
groups unchanged. Hawkins et al. studied the selective
hydrogenation of 4-nitroacetophenone with different Pt, Pd and Rh
catalysts. 97% selectivity toward 4-aminoacetophenone could be
obtained from the hydrogenation of 4-nitroacetophenone over the
1
4
selective catalyst through supporting it on TiO
2
.
It is well-known
(rutile and
For the rutile surface, there was a flat profile where
that there are two commonly crystalline forms of TiO
2
1
1
5
wt% Pd/CaCO
et al. studied the selective hydrogenation of 3-nitroacetophenone
and 4-nitroacetophenone with Rh/SiO catalyst. It was found that
3
/Pb catalyst in a low hydrogen pressure. Jackson
1
5, 16
anatase).
the 2-fold-coordinated bridging O anions were bound to the 6-fold-
coordinated Ti cations and projected out of the surface plane,
however, anatase surface has a sawtooth profile in which the 2-
fold-coordinated O anions were bound to the 5-fold-coordinated Ti
2
the hydrogenation reaction activity of the functional group on the
1
7, 18
cations.
Therefore, there were obvious differences between
a. State Key Laboratory of Coal Converison, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, China. E-mail: zfzheng@sxicc.ac.cn；
guxm@sxicc.ac.cn; zhupengqi15@mails.ucas.ac.cn.
rutile and anatase surface which have great influence on the
19-22
catalytic performance.
It is necessary to study the relationship
b.
Center of Materials Science and Optoelectronics Engineering, University of
between the catalytic performance of noble metals supported on
Chinese Academy of Sciences, Beijing 100049, China.
Electronic Supplementary Information (ESI):Catalyst characterisation, catalytic
performances and in situ FT-IR spectra. See DOI:10.1039/x0xx00000x
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CP al et aa lsy es i ds oS c ni eo nt c ae d &j uTs et c mh na or gl oi ng sy
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ARTICLE
TiO
2
of differently crystalline forms and the chemoselective The ruthenium catalyst supported on titanium dioxide was
View Article Online
DOI: 10.1039/C9CY02260K
reduction of nitro group.
3
prepared by the impregnation method. RuCl with the
In the past century, various heterogeneous catalytic required amounts was dissolved in 60 mL ultrapure water to
systems have been widely used in the selective reduction of obtain aqueous RuCl solution. Then nano-titanium dioxide
For the reduction process of (2.0 g) was added and stirred vigorously for 2 h at 25 °C. Water
nitroaromatics, the generally accepted reduction process was was evaporated under 80 °C until dryness and then the NaOH
3
2
3-25
the nitroaromatics.
2
6
proposed by Haber in 1898. In this reaction scheme, Haber solution (30 mL) was added to adjust the pH of the solution to
proposed two different routes (direct route and indirect 13.2. The final suspension was stirred for 24 h at 25 °C. Next,
route). In the direct path, nitro groups passed through nitroso the mixture was separated by centrifugation, washed by a
and hydroxylamine intermediates to further generate large amount of water to neutral, and then dried at 60 °C in a
anilines.27 And the indirect route included the condensation of vacuum oven for 12 h. Finally, the dried solids were reduced
2
nitroso with hydroxylamine to obtain azoxy compound, which with H for 2 h at 200 °C in a tubular furnace. The Ru
was reduced to azo, hydrazone and aniline compound in a nanoparticle was prepared by the same method without
2
8
series of continuous steps. However, Haber's mechanism was adding TiO
2
. The anatase and rutile TiO
2
supported Ru catalysts
not applicable to all nitroaromatics reduction systems. Visentin were designated as Ru/TiO
proposed a new possible mechanism for reduction of
2
(A) and Ru/TiO (R), respectively.
2
nitroaromatics to anilines according to the calorimetric, FTIR-
ATR (attenuated total reflectance), and gas-uptake signals with
Pd/C as catalyst.29 In this case, the direct route from nitro
compound to hydroxylamine was included in the reaction
Characterisation
The powder X-ray diffraction (XRD) patterns were performed
on MiniFlex II diffractometer, which were irradiated with Cu
Kα1 (λ = 1.5418 Å). High resolution TEM (HRTEM) images were
acquired at accelerating voltage 200 kV on a JEM-2100F
transmission electron microscope. Raman spectra were
measured on a Jobin Yvon HR-800 spectrometer equipped
with a 514 nm laser. X-ray photoelectron spectra were
recorded by thermal ESCALAB 250 spectrometer (Al Kα source,
hν = 1486.6 eV). The detailed Ru XPS analysis data of
3
0
scheme: nitro → hydroxylamine → aniline.
In this work, we studied the effect of ruthenium
supported on rutile and anatase TiO on the catalytic
2
performance of 4-nitroacetophenone at normal temperature
and atmospheric hydrogen. Further studies showed that the
crystalline form of TiO had a great influence on the reaction,
2
which was reflected in the transformation route of 4-
nitroacetophenone. In situ FT-IR datas and the experiments
proved that the reduction of 4-nitroacetophenone by
Ru/TiO
2 2
(A) and Ru/TiO (R) catalysts as see in supporting
information (Table S1).
Ru/TiO
reaction path. The reduction of 4-nitroacetophenone by
Ru/TiO (A) catalyst followed the direct hydrogenation route
2
(A) catalyst was in accordance with the Visentin’s
Pulsed hydrogen adsorption and temperature programmed
desorption (TPD)
2
which the 4-aminoacetophenone was the only product from 100 mg catalysts were pretreated for 3 hours in N
2
flow (25 mL
the beginning to the end of the reaction. However, the min ) at 300 °C, and then cooled to 55 °C. After the
Ru/TiO (R) causes a series of intermediates accumulation pretreatment, H was injected into the sample by pulse
during the conversion process and 4-aminoacetophenone was injection for 10-20 min until the adsorption was saturated.
obtained through the condensation route of hydroxylamine After adsorption by pulsed H , the system was purged with N
(A) could selectively flow (25 mL min ) until the signal was stable. The temperature
-1
2
2
2
2
-1
species. It was worth noting that Ru/TiO
2
-1
-1
reduce nitro groups when nitroaromatics contained other was raised from 30 to 900 °C (5 °C min ) with 25 mL min of
competitive reducible groups, proving its excellent velocity of argon flow.
applicability.
In situ diffuse reflection Fourier transform infrared (FT-IR)
measurement
Experimental section
Materials
Catalyst powder was placed in an in situ infrared cell. The
reactor has several gas inlets for the introduction of Ar or
High purity nitroaromatics, anhydrous ruthenium trichloride
(
(
H
2
/Ar and one outlet for the exhaust. Gas flow was controlled
ruthenium content 45-55%) and nano-titanium dioxide by mass flow controllers. In situ FT-IR measurements were
anatase, 99.8%, 10-20 nm) were purchased from Aladdin carried out on Bruker Tensor II infrared spectrometer with a
-1
Reagent Network. Nano-titanium (IV) oxide (rutile, 0.9-1.6 µm, resolution of 4 cm . The sample was heated to 100 °C at 5 °C
9.99%) was purchased from Alfa Aesar Reagent Network. min with a flow rate of 30 mL min in Ar atmosphere, kept 2
-1
-1
9
Sodium hydroxide (AR) was purchased from Tianjin Kermel hours at 100 °C, and then cooled to 55 °C. 4-
Chemical Reagent Network. Methanol (99.5%) was purchased nitroacetophenone was introduced into the catalyst sample,
from Sinopharm Chemical Reagent Co. Ltd.
and then Ar was used to sweep the sample pool again until a
stable spectral signal was obtained. The airbag containing H
2
was introduced into the in situ tank slowly through the intake
port and the infrared spectra were recorded.
Preparation of Catalysts
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DOI: 10.1039/C9CY02260K
Fig. 1 (a) XPS profiles of Ru species in the Ru/TiO
TiO (A) catalysts. (c) XPS profiles of Ti species in the Ru/TiO
Ru/TiO (A) and TiO (A) catalysts.
2
(A) and Ru/TiO
2
(R) catalysts. (b) XPS profiles of Ti species in the Ru/TiO
2
(A) and
(R),
2
2
(R) and TiO
2
(R) catalysts. (d) Raman spectra of Ru/TiO (R), TiO
2
2
2
2
observed in Ru/TiO
due to the high
dispersion of Ru on the supports. Fig. S2a and Fig. S2b showed
the transmission electron microscopic images (TEM) of
2 2
(A) and Ru/TiO (R) catalysts. This may be
Catalytic performance evaluation
0 mg of catalyst, 0.1 mmol of 4-nitroacetophenone, and 2 mL
1
of solvent were added to a 10 mL glass reaction flask. Then
atmospheric hydrogen was introduced into the reaction bottle
to exhaust the air in the reaction bottle. The bottle must be
sealed to guarantee atmospheric hydrogen. The vial was
placed in an oil bath to react at the specific temperature. After
the reaction, qualitative and quantitative analysis of reactants
and products were carried out by high-performance liquid
chromatography (HPLC) (Agilent 1260-C18 column). The
conversion rate was calculated based on the amount of
nitroaromatics.
Ru/TiO
2
(A) and Ru/TiO
2
(R) catalysts, the Ru nanoparticles were
. The lattice fringe
uniformly distributed on the surface of TiO
2
spacing of 0.214 nm corresponds to the interplanar distance of
Ru metal (002) planes (Fig. S2c and Fig. S2d). In addition, Fig.
S2e and Fig. S2f showed that the average particle size of Ru
nanoparticles were 1.6 nm and 2.4 nm in Ru/TiO
Ru/TiO (R) catalysts, respectively. The X-ray photoelectron
spectroscopy (XPS) analysis of Ru/TiO (A) catalysts were
provided in Fig. 1a. The binding energies of Ru 3d5/2 and Ru
3/2 at 280.4 and 284.2 eV, respectively, could be attributed
2
(A) and
2
2
3
d
3
1
to the zero valence state of Ru. And the binding energies at
64.4 and 458.7 eV were assigned to Ti 2p1/2 and Ti 2p3/2 of
supports. In order Ti4+, respectively (Fig. 1b). The spin-orbital splitting was 5.7 eV,
to have a clear understanding of the crystal structure of which was greatly consistent with the expected value of Ti4+
Ru/TiO (A) and Ru/TiO
(R) catalysts, the structural oxidation state in titanium dioxide nanocomposites.32 By
characteristics were characterised by different microscopic comparing the Ru/TiO (A) catalyst with the pure TiO (A), we
Results and discussion
4
The interaction between Ru metal and TiO
2
2
2
2
2
and spectral methods. Fig S1 illustrated the X-ray diffraction detected the other two peaks at 463.6 and 458.1 eV which
XRD) patterns of Ru/TiO (A) and Ru/TiO
(R) catalysts. It can be could be classified as Ti 2p1/2 and Ti 2p3/2 of Ti3+ state.33 This
seen that the loading of Ru nanoparticles does not affect the indicated that electrons transfer occurs between Ru and
crystal structure of TiO and no Ru diffraction peaks are TiO (A), resulting in electron deficiency on the surface of the
(
2
2
2
2
3
+
34
2
Ru and the formation of Ti on the surface of the TiO (A).
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And preceding literature also involve in the electrons transfer of the reaction and remained at 99.9% until the reaction was
View Article Online
DOI: 10.1039/C9CY02260K
from Ru to Ti, this may be because the electron of Ru atoms completed. On the contrary, when
3
5
cannot withstand the large attraction from Ti atoms.
had different ability to attract the nitroacetophenone to 4-aminoacetophenon.a
Different crystal of TiO
electrons. By comparing the Ru/TiO
catalyst, the binding energies of Ru of Ru/TiO
were higher for 0.1 eV than that of the Ru/TiO
Compared with TiO
(R), the binding energies of Ti4+ of
Ru/TiO (R) catalysts were moved to the lower binding energy
by 0.2 eV, and we also detected a large amount of Ti (Fig.
c). Therefore, compared with TiO (A), TiO (R) got more
Table 1 Catalytic performance of Ru catalyst for conversion of 4-
2
2
(R) with the Ru/TiO
(R) catalysts
(A) catalyst.
2
(A)
O
O
NO2
NH2
2
2
Entry
Catalyst
Time
Conv.
(%)
Sel.
(%)
2
(
h)
1
1
1
1
2
2
2
2
2
2
1
2
3
4
5
6
1.0 wt% Ru/TiO
2.7 wt% Ru/TiO
4.1 wt% Ru/TiO
7.0 wt% Ru/TiO
2.7 wt% Ru/TiO
2
2
2
2
2
(A)
(A)
(A)
(A)
(A)
65.6
83.2
45.4
41.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
83.5
99.9
99.9
99.9
99.9
-
3
+
1
2
2
electrons from Ru. It was because the concentration of
unsaturated Ti cations of anatase was less and the reduction
rate was slower than that of rutile.
were consistent with density functional theory (DFT)
calculations, which showed that the formation energy of
o
oxygen vacancy (V ) on rutile was lower than that on
3
6, 37
These observations
2.7 wt% Ru/TiO
2
(R)
(A)
(A)
(A)
(A)
(A)
b
7
2.7 wt% Ru/TiO
2.7 wt% Ru/TiO
2.7 wt% Ru/TiO
2.7 wt% Ru/TiO
2.7 wt% Ru/TiO
2
8c
2
2
2
2
d
9
anatase.3
8, 39
Therefore, TiO
(R) was easier to get electrons
2
1
0
12
2
2
2
12
4
+
3+ 40
from Ru and then Ti was reduced to Ti .
e
1
1
Not detected
Not detected
Not detected
99.9
The interaction between Ru and TiO
by Raman spectroscopy (Fig. 1d). Typical Raman bands
2
was further verified
1
1
2
3
2
TiO (A)
-
-
Ru nanoparticle
2.7 wt% Ru/TiO
2.7 wt% Ru/TiO
-1
caused by TiO
attributed to the E
modes of rutile phase, respectively. Deposition of Ru species
2
(R) occur at 469 and 631 cm , which can be
f
1
4
2
(A)
(A)
97.8
g
(plane O-O vibration) and A1g (Ti-O stretch)
g
1
5
2
12
99.9
97.6
4
1
a
Reaction conditions: 10 mg catalyst, 0.1 mmol reactant, 2 mL
(R) resulted in a red shift of about 8 cm-
1
on the surface of TiO
for E mode, which showed the modification of the planar O-O
vibration and the formation of Ru-O-Ti bond. While the similar
phenomenon did not occur in Ru/TiO (A) catalyst. The
observed frequency shift was consistent with the results (HPLC). Conv. = Conversion. Sel. = Selectivity.
2
b
c
d
e
f
methanol, 55 °C, 1 atm H
2
.
75 °C. 95 °C. 115 °C. 1 atm Ar. 25
g
g
mg catalyst, 1 g reactant, 10 mL methanol, 5 atm H
catalyst, 3 g reactant, 10 mL methanol, 10 atm H
was determined by high-performance liquid chromatography
2
.
25 mg
2
. The conversion
2
4
2
reported by Hong Zhong. The obvious red shift indicated that
there was a strong interaction between Ru and TiO (R), which
was beneficial for the electrons transfer from Ru nanoparticles
Ru/TiO
-aminoacetophenone in the early reaction stage. The
conversion of 4-nitroacetophenone reached 99.9% within 80
min, which was faster than that of Ru/TiO (A) catalyst, but the
main product were N-(4-acetylphenyl)- hydroxylamine besides
-aminoacetophenone. The selectivity of N-(4-acetylphenyl)-
2
(R) catalyst was used, there was only a small amount of
2
4
3
+
to TiO
surface of Ru/TiO
The hydrogenation depth and chemoselectivity controlled by
TiO phases. Anatase and rutile crystals were the most
common types of TiO . In order to investigate the influence of
the crystal phase of TiO on the catalytic performance, we
tested the performance of Ru/TiO (A) and Ru/TiO (R) catalysts
under the same experimental conditions. As can be seen from
Table 1, the catalytic performance of Ru/TiO (A) changed
obviously with the increase of Ru loading. The results showed
that the Ru/TiO (A) catalyst with 2.7 wt% Ru loading exhibited
the highest activity for the hydrogenation of 4-
nitroacetophenone with H (1 atm) at 55 °C (Table 1, entries 1-
). 99.9% of 4-nitroacetophenone was consumed in 2 h with
9.9% selectivity for 4-aminoacetophenone (Table 1, entry 5).
(R) catalysts gave 99% conversion
2
(R). Therefore, there were more Ti species on the
2
2
(R).
4
2
hydroxylamine reached the maximum at 60 min (57.4%). With
the continuation of the reaction, the selectivity of N-(4-
acetylphenyl)-hydroxylamine decreased significantly, while the
selectivity of 1,2-bis(4-acetylphenyl)-diazene oxide increased
slightly, and then 1,2-bis(4-acetylphenyl)-diazene oxide
decreased with the increase of 4-aminoacetophenone as the
final reduction product. These results indicated that the
2
2
2
2
2
2
2
reduction of 4-nitroacetophenone over Ru/TiO (R) occurred by
a condensation route presented by Haber while the Ru/TiO
2
(A)
2
43
catalysts did not. Therefore, Ru/TiO
2
(A) had higher selectivity
(R).
In the control experiment, when the reaction temperature was
raised from 55 to 115 °C and the reaction time was prolonged to 12
h, more than 99% selectivity to 4-aminoacetophenone was still
4
9
for 4-aminoacetophenone than Ru/TiO
2
In contrast, 2.7 wt% Ru/TiO
2
of 4-nitroacetophenone with a selectivity of 83.5% for 4-
aminoacetophenone (Table 1, entry 6). The results showed
that the crystal phase of TiO may affect the reduction
2
pathway of 4-nitroacetophenone. Thus, the kinetic
experiments for the reduction of 4-nitroacetophenone over
2
attained (Table 1, entries 7-10). This showed that Ru/TiO (A)
catalyst could perfectly control the synthesis of 4-
aminoacetophenone without further hydrogenation of carboxyl
group. It was further confirmed by studying the competitive
reaction of nitrobenzene and acetophenone at a molar ratio of 1:1.
We found that only the aniline was detected in the product, which
Ru/TiO
Ru/TiO
2
(A) and Ru/TiO
2
(R) were performed (Fig. 2). When
2
(A) catalyst was used, the selectivity for 4-
aminoacetophenone was closed to 99.9% from the initial stage
showed that Ru/TiO
2
(A) catalyst had excellently selective reduction
of nitro groups (Fig. 3). According
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Fig. 2 Time-conversion plots for 4-nitroacetophenone reduction using (a) Ru/TiO
2
(A) and (b) Ru/TiO
2
(R) as the catalyst. Reaction
conditions were identical with Table 1. The conversion was determined by HPLC.
to the literatures, the activation energy for further hydrogenation conditions were identical with Table 1. The conversion was
of 4-aminoacetophenone was higher than that of nitro group determined by (HPLC).
reduction in the process of selective hydrogenation of 4- selectivity (97.8%) at low pressure (5 atm) (Table 1, entry 14). It is
nitroacetophenone.9
, 12, 44
2
It was worthy noted that the Ru/TiO (A) noted that the selectivity to 4-aminoacetophenone remained
catalyst thermodynamically allowed catalytic hydrogenation of nitro almost the same at 97.6% when the amount of 4-
groups, but was unfavourable for the reduction of carbonyl groups. nitroacetophenone was expanded to 3 g at 10 atm H pressure
Therefore, Ru/TiO (A) catalyst can control the synthesis of 4- (Table 1, entry 15).
aminoacetophenone without further hydrogenation. When the Activation of nitroacetophenone on TiO
catalytic hydrogenation of 4-nitroacetophenone over 2.7 wt% reduction of 4-nitroacetophenone catalysed by Ru/TiO
Ru/TiO (A) was conducted under Ar atmosphere, no 4- infrared spectrum (IR) clearly reflected the adsorption relation of 4-
nitroacetophenone was converted (Table 1, entry 11). These results nitroacetophenone on the surface of the catalysts (Fig. 4). Two
2
2
2
surface. In the selective
catalyst, the
2
2
-1
suggested that the main hydrogen source for 4-nitroacetophenone different absorption bands appeared at 1534 and 1346 cm , which
reduction in our work was H . In addition, TiO (A) or unsupported were the asymmetric stretching vibration (νasym) of the nitro group
2
2
Ru particles did not exhibit any activity for the hydrogenation of 4- adsorbed on the surface of Ti-OH group and the symmetric
nitroacetophenone (Table 1, entries 12 and 13). When the amount stretching vibration (νsym) of the nitro group adsorbed on the
3
+ 45
of 4-nitroacetophenone was expanded to 1 g, we found that surface of Ti . The strength of (νsym) band on Ru/TiO
Ru/TiO (A) catalyst could still catalyse 4-nitroacetophenone to 4- was much stronger than that on Ru/TiO (A) catalyst, because there
(R) surface. Yasuhiro et al. also proved
that the surface Ti atoms were the active site for nitro group
2
(R) catalyst
2
2
3
+
aminoacetophenone with high conversion (99.9%) and high
were more Ti on Ru/TiO
2
3
+
4
6
hydrogenation. Therefore, the initial conversion rate of 4-
nitroacetophenone catalysed by Ru/TiO
Ru/TiO (A). Fig. S2 showed the in situ FT-IR spectra of 4-
nitroacetophenone adsorbed
2
(R) was higher than that of
Fig. 3 Time-conversion plots of the mixture of nitrobenzene and
acetophenone with a 1:1 molar ratio on Ru/TiO (A). Reaction
2
2
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catalysts.
Fig. 4 In situ FT-IR spectra of 4-nitroacetophenone adsorbed on Ru/TiO
Table 2 Reaction pathway study of the catalytic performance using Ru/TiO
2
(A)
and
2
Ru/TiO (R)
View Article Online
DOI: 10.1039/C9CY02260K
2
(R) and Ru/TiO
2
(A) catalysts with various intermediates as
Sel. (%)
Entry
1
Reactant
Catalyst
Conv. (%)
AOB
66.1
62.3
99.9
4.2
2.7
-
AB
11.8
16.9
-
18.4
8.9
HAB
4.3
0.7
-
AN
17.8
20.1
-
NSB
NSB
NSB
PHA
PHA
AOB
AOB
Ru/TiO
Ru/TiO
2
(R)
(R)
99.9
99.9
33.1
99.9
99.9
99.9
43.5
b
2
2
3
4
5
6
Ru/TiO
2
(A)
Ru/TiO
2
(R)
(A)
30.3
-
53.2
57.2
47.1
88.4
39.3
-
Ru/TiO
Ru/TiO
2
2
(R)
(A)
7.5
42.8
7
Ru/TiO
2
-
a
substrates.
a
Reaction conditions were identical with Table 1. b 12 h. The conversion was determined by HPLC. Conv. = Conversion. Sel. =
Selectivity. NSB = Nitrosobenzene, PHA = Hydroxylamine, AOB = Azoxybenzene, AB = Azobenzene, HAB =Hydrazobenzene, AN =
Aniline.
on the Ru/TiO
2
(A) and Ru/TiO
2
(R) catalysts, the absorption band at Ru/TiO
2
(A) catalysts. When hydroxylamine was used as a reactant,
-1
47
1
636 cm belonged to hydroxyl of water. In addition, the the conversion of hydroxylamine was 99.9% regardless of which
-1
absorption bands at 1695 and 1653 cm were the C=O stretching catalyst was used, and the products also included aniline (Table 2,
vibration (νC=O) of 4-nitroacetophenone and 4-aminoacetophenone, entries 4 and 5). Therefore, hydroxylamine species were the
-1
respectively. The band at 1605 cm belonged to the skeleton important intermediates both in the reduction of 4-
4
8
vibration peak of the benzene ring. With the introduction of H
2
nitroacetophenone catalysed by Ru/TiO
2
(R) and Ru/TiO
2
(A)
stream into Ru/TiO
2
(A) (Fig. S3a), the absorption bands at 1695 and catalysts. However, when Ru/TiO
2
(R) was used as catalyst, the
-1
-1
1
533 cm decreased and the band at 1653 cm gradually increased. selectivity of aniline only 47.1% for the conversion of
Meanwhile, the characteristic absorption band of 4- hydroxylamine (Table 2, entry 4), and the selectivity of azobenzene
-1
aminoacetophenone appeared at 1597 cm was gradually obvious, compounds was 52.9%, therefore, hydroxylamines easily generate
which was attributed to the NH stretching vibration of 4- anilines through the condensation path in the reduction reaction
aminoacetophenone. The peak at 1497 cm-1 belonged to the NH- catalysed by Ru/TiO
(R) catalysts. When azoxybenzene was used as
(R) could achieve 99.9% conversion of
peaks of azoxybenzene (1477 cm ) and azobenzene (1454 cm ) azoxybenzene, and the products included azobenzene (7.5%),
were found in the in situ FT-IR infrared spectra of Ru/TiO (R) hydrazobenzene (53.2%) and aniline (39.3%) (Table 2, entry 6).
which further verified the accumulation of azobenzene However, the conversion of azoxybenzene catalysed by Ru/TiO (A)
2
4
9
2
5
0
OH vibration peak. Apart from above bands, obvious characteristic reactant, Ru/TiO
2
-1
-1
2
5
1, 52
catalyst,
2
compounds during the catalytic process (Fig. S3b). In contrast, was only 43.5%, and no aniline was detected (Table 2, entry 7).
under the same conditions, no similar in situ FT-IR evolution was Based on the above results, it could be concluded that there were
observed for both pure TiO
To clarify the effect of different crystal phase of TiO
reaction pathway, the reduction of possible intermediates conversion of nitroso species and azoxy compounds catalysed by
nitrosobenzene, phenylhydroxylamine and azoxybenzene) were Ru/TiO (A) catalysts was very slow and the product did not include
conducted using the Ru/TiO (A) and Ru/TiO (R) catalysts under the aniline species. In contrast, when Ru/TiO (R) was used as catalyst,
same conditions (Table 2). When Ru/TiO (R) was used as catalyst, azoxybenzene compounds were important intermediates in the
the conversion of nitrosobenzene was 99.9%. The products included reaction.
2 2
(A) and TiO (R) (Fig. S3b).
no nitroso species and azoxy compounds in the reduction of 4-
2
on the nitroacetophenone catalysed by Ru/TiO (A), because the
2
(
2
2
2
2
2
azobenzene, azoxybenzene, hydrazobenzene and aniline (Table 2,
We propose a reaction pathway to explain this difference, as
entry 1). Therefore, nitroso species seemed to be one of the shown in Fig. S4. The catalytic reduction of 4-nitroacetophenone
intermediates in the reduction of 4-nitroacetophenone catalysed by over Ru/TiO
2
(R) catalyst proceeded via a direct pathway and a
Ru/TiO (R) catalysts. However, when the reaction time was condensation pathway. Firstly, 4-nitroacetophenone quickly
2
prolonged to 12 h, we found that almost no change in the generates hydroxylamine species in the reaction process, and then
selectivity for 4-aminoacetophenone (Table 2, entry 2). Besides, we 4-aminoacetophenone was mainly obtained by the condensation
have not found nitroso species accumulation in the reaction of 4- route from hydroxylamine species. However, when Ru/TiO
2
(A) was
nitroacetophenone catalysed by Ru/TiO (R) (Fig. 2b), therefore, we used as the catalyst, 4-aminoacetophenone was completely
2
have excluded the possibility that nitroso species was the generated from 4-nitroacetophenone at the initial stage of the
intermediate in the reduction of 4-nitroacetophenone catalysed by reaction, and no other by-products were detected (Fig. 2),
Ru/TiO (R) catalysts. When Ru/TiO (A) catalyst was used, the indicating that the reaction pathway over Ru/TiO (A) catalyst was
2 2 2
conversion of nitrosobenzene was only 33.1%, and the product only not in conformity with the Haber’s reaction pathway, but through
included azoxybenzene (Table 2, entry 3), so there were no nitroso the fast pathway (nitro → hydroxylamine → aniline) proposed by
2
9, 53
species in the reduction of 4-nitroacetophenone catalysed by Visentin et al.
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In order to eliminate the influence of catalyst specific surface
area and Ru particle size on the reaction mechanism, we also
prepared Ru/TiO catalysts with different specific surface areas
Table S2). We still detected a series of hydroxylamine species and
View Article Online
DOI: 10.1039/C9CY02260K
2
(
accumulated azoxy compounds from the kinetic experiments for
the
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2
View Article Online
. At the same
improved the saturation adsorption amount of H
DOI: 10.1039/C9CY02260K
time, the hydrogen saturation adsorption capacity of different
catalysts had similar trends with the catalytic activity for
hydrogenation of 4-nitroacetophenone. Excessive
H
2
adsorption on the metal surface also makes it possible for
hydrogen spill from the metal surface to the support surface.
The active sites of H
temperature programmed desorption (TPD). From the H
spectra of Ru/TiO (A), we can see that there were a small H
desorption peak at 37 °C and a large one at 125 °C (Fig. 6b).
These results showed that there were at least two H
chemisorption sites on the catalyst surface, and the peak at 37 °C
2
adsorption were studied by
2
-TPD
2
2
2
5
9
was usually attributed to hydrogen adsorbed on Ru metal. The
low desorption temperature indicated that it was a weak
chemisorption. The broad peak at 125 °C may be related to the
6
0
strong chemisorption between hydrogen and the catalyst. The
weak desorption peak near 300 °C were designated as hydrogen
desorption from metal onto the surface of the support or hydrogen
6
1
adsorbed at the interface between the metal and the support.
In order to investigate the activation of H on the surface
Fig.5 Plausible catalytic actions of hydroxylamine compound of Ru/TiO
(A) catalyst, the adsorption of H on the surface of
2
2
2
conversion on the surface of TiO
reduction of 4-nitroacetophenone over the Ru/TiO
while the Ru/TiO
2
.
catalyst was also measured by in situ FT-IR, which further
(R-2) catalyst, verified the spillover process of H atom on the surface of
2
2
(A-2) allows the highly selective conversion of 4- catalyst. It was found that there was a significant change in the
nitroacetophenone to 4-aminophenone (Fig. S5). Therefore, the peak as H was flowing into the surface of the catalyst. The
2
specific surface area of the catalyst did not affect the conversion peaks at 1889, 1942, 1967 and 1991 cm- belonged to Ru-H
1
6
2
mechanism of 4-nitroacetophenone. We have prepared Ru/TiO
2
(A) stretching vibration peak (νRu-H
)
(Fig. 6c).
With the
catalysts with ruthenium nano-size of 1.8 and 2.1 nm by controlling introduction of H , the intensity of these peaks first increased
2
the reduction time of Ru catalysts under hydrogen. No and then decreased, which indicated that hydrogen atoms
accumulation of intermediates has been detected under the same were formed on Ru and then spilled over onto the support.
-1
experimental conditions (Table S3 and Fig. S6). Furthermore, we And the stretching vibration peaks at 1617 and 1636 cm
obtained the Ru dispersions of Ru/TiO catalysts based on the gradually increased which can be attributed to O-H stretching
pulsed hydrogen adsorption. There was no obvious difference for vibration
the Ru dispersions on the TiO support (Table S4, entries 1 and ).
2
2
2
Therefore, in this work, the specific surface area of the catalyst, Ru
dispersions and the particle size of Ru had no influence on the
reduction mechanism of 4-nitroacetophenone.
For the rutile surface, the distance between two adjacent
surface Ti atoms was 296 pm, and that of the anatase was 379
pm (Fig. 5).54 Since the actual N=N bond length of azobenzene
5
5
compound was 127 pm, in the case of rutile, the reaction
intermediate hydroxylamine species were easily coupled to
3
0, 56, 57
form azoxy compound compared with anatase.
why the subsequent conversion efficiency of 4-
nitroacetophenone catalysed by Ru/TiO (R) was poor than that
of the Ru/TiO (A) catalyst.
Activation of H on Ru and H spillover. For the hydrogenation
This was
2
2
2
of nitroaromatics, the dissociation of hydrogen was an rate-
determining step, thus, effective dissociation of hydrogen by
5
8
the catalyst can greatly improve the activity of the reaction.
In order to confirm this, a series of Ru/TiO (A) with different
Ru loadings were measured by H pulse adsorption at reaction
temperature (Fig. 6a). The saturation adsorption amount of
2
2
Fig. 6 (a) Dependencies of H
nitroacetophenone conversion of different Ru loadings in TiO
A)-supported catalysts. (b) H -TPD profiles of Ru/TiO (A). (c, d)
The in situ FT-IR spectra of H adsorbed on TiO (A) and Ru/TiO
A) catalyst in a flow of H at 55 °C.
chemisorption capacity and 4-
2
2
-1
hydrogen on pure TiO
adsorption amount of hydrogen on 2.7 wt% Ru/TiO
catalyst was 74.7 μmol g , indicating that the loading of metal
2
(A) was 1 μmol g , while the saturation
(A)
(
2
2
2
2
2
2
-1
(
2
Ru greatly
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Table 3 Catalytic reduction of substituted nitroaromatics using the
ARTICLE
Combined with the peaks of νO-H, νTi-H and νRu-H observed during the
View Article Online
a
Ru/TiO
2
(A) catalyst.
DOI: 10.1039/C9CY02260K
adsorption process,
it can be concluded that H
then
spill over to the TiO (A) surface. These spillover hydrogen species
2
2
can dissociate on the Ru surface and
Entry
Reactant
Product
Conv.
%)
Sel.
(%)
(
combined with surface oxygen atoms and titanium atoms formed
O-H and Ti-H bonds, and then reacted with nitro groups which were
adsorbed on TiO (A) support, showing excellently catalytic
2
NO2
NH2
R
R
performance of the catalyst.
2
The generality and recyclability of Ru/TiO (A). The reusability
of the catalyst was an important indicator for testing the
performance of the catalyst. We separated and dried the used
COCH3
NO2
COCH3
NH2
1
2
3
4
5
6
7
8
9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
99.9
98.5
97.1
95.3
Ru/TiO
reaction conditions. The results showed that the reused
Ru/TiO (A) catalyst had no obvious change in activity and
selectivity, proving its excellent stability and repeatability (Fig.
S7).
2
(A) catalyst and reused it five times under the same
2
H3COC
NO2
NH2
H3COC
NH2
NH2
2
To explore the applicability of the Ru/TiO (A) catalyst, a
O2N
2
H N
series of substituted nitroarenes were used to verify its activity
under optimized conditions (Table 3). It was found that these
substituted nitroarenes containing other reducing functional
groups can generate corresponding amino derivatives with
HOOC
NO2
NO2
HOOC
F
NH2
NH2
2
high efficiency and high selectivity over Ru/TiO (A) catalyst.
Both 2-nitroacetophenone and 3-nitroacetophenone could be
selectively reduced to the corresponding aminoacetophenone,
which showed that steric hindrance did not affect the
selectivity of the catalyst. At the same time, 4-
halonitrobenzene containing halogen (F, CI) also had no
obvious dehalogenation phenomenon in the reaction process.
In particular, nitroaromatics with some sensitive -CN, -CHO
and -COOH substituents were also selectively converted into
corresponding amino compounds. Especially, nitrobenzene
derivatives containing C=C and C≡C can also selectively reduce
nitro, but keep the C=C and C≡C unchanged. These results
F
CI
NO2
CI
NH2
NH2
H2NOC
NO2 H NOC
2
HO
NC
NO2
HO
NH2
NH2
showed that Ru/TiO
hydrogenation of different nitroarenes to anilines with
atmospheric H
Proposed mechanism. Ru/TiO
2
(A) catalyst had universal applicability for
2
.
NO2
NO2
NO2
NC
2
(A) exhibited high activity
and selectivity in the reduction of 4-nitroacetophenone to 4-
aminoacetophenone, which may be beneficial from the strong
0b
OHC
OHC
NH2
1
1
1
interaction between TiO
support and Ru metal. Electrons
surface to form
2
were transferred from Ru metal into the TiO
2
3
+
Ti species which were conducive to the polarization of nitro
groups adsorbed on the surface of the support, thus making
the N=O bond more vulnerable to hydrogen attack. Moreover,
1b
NH2
NO2
NH2
Ru had an excellent ability to adsorb H
for H spillover from the Ru nanoparticles onto the surface of
TiO . Therefore, we proposed such a reaction mechanism to
explain this phenomenon (Scheme 1). Firstly, nitro groups
2
, which make it possible
2b
2
a
Reaction conditions were identical with Table 1. b 1.5 h. The
conversion were determined by HPLC. Conv. = Conversion, Sel. =
Selectivity.
O-H) (Fig. 6d).63 Meanwhile, the peaks of 1393 and 1419 cm-1
became more and more clear which belonged to the Ti-H
stretching vibration peaks (νTi-H). However, the vibration peaks of
3
+
2
were adsorbed on the Ti sites on the surface of Ru/TiO (A)
catalysts by symmetric stretching vibration (step 1). Secondly,
hydrogen was dissociated under the activation of Ru metal to
form Ru-H bond (step 2). Thirdly, the hydrogen atoms on the
surface of Ru metal spilled to the TiO (A) support to form O-H
2
and Ti-H bonds (step 3). Subsequently, O-H and Ti-H bonds
reacted with nitro groups which was adsorbed on the surface
(ν
6
4
2
these bonds were not observed in situ FT-IR of pure TiO (A).
of Ru/TiO
2
(A) to produce hydroxylamine intermediates(step 4),
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Journal Name
and hydroxylamine intermediates were adsorbed on the catalyst. Moreover, Ru/TiO
ARTICLE
2
(A) also exhibited excellent
View Article Online
6
5
DOI: 10.1039/C9CY02260K
2 2
surface of TiO (A) through nitrogen atoms . Then the stability and repeatability. In addition, Ru/TiO (A) can achieve
hydrogen
chemoselectivity reduction of nitroaromatics to corresponding
anilines in the presence of -CN, -CHO, -COOH, C=C and C≡C.
The excellent catalytic performance derives from the
synergistic effect between TiO
effectively adsorbed on the surface of TiO
transferred from Ru metal into the TiO surface to form Ti
2
and Ru: nitro groups were
2
; electrons were
3
+
2
species, which were conducive to the polarization of nitro
groups adsorbed on the surface of the support, thus making
the N=O bond more vulnerable to hydrogen attack; hydrogen
was dissociated on the surface of Ru, then hydrogen atoms
spilled onto the surface of TiO to form Ti-H and O-H bond, and
2
further reacted with 4-nitroacetophenone to generate 4-
aminoacetophenone.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We appreciate the financial support from the National Natural
Science Foundation of China (No. 21773284) and the Hundred
Talents Program of the Chinese Academy of Sciences and Shanxi
Scheme 1 Possible mechanism of 4-nitroacetophenone Province.
conversion catalysed by Ru/TiO (A) catalysts.
2
2
dissociated under the site of Ru and spilled onto the TiO (A)
support to form O-H and Ti-H bonds repeatedly (steps 5-6).
Finally, hydroxylamine was further hydrogenated and
Notes and references
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dehydrated
to
produce
4-aminoacetophenone.
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Catalysis Science & Technology
Page 12 of 13
View Article Online
DOI: 10.1039/C9CY02260K
Differences in Selective Reduction Mechanism of 4-Nitroacetophenone
Catalyzed by Rutile and Anatase Supported Ruthenium Catalysts
Jin Zhang,ab Linjuan Pei,ab Jie Wang,ab Pengqi Zhu,*ab Xianmo Gu* and Zhanfeng Zheng*
a
ab
a
State Key Laboratory of Coal Converison, Institute of Coal Chemistry, Chinese Academy of
Sciences, Taiyuan 030001, China.
b
Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of
Sciences, Beijing 100049, China.
*
E-mail: zfzheng@sxicc.ac.cn (Z. F. Zheng); guxm@sxicc.ac.cn (X. M. Gu);
zhupengqi15@mails.ucas.ac.cn (P. Q. Zhu).

Page 13 of 13
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C9CY02260K
Ru nanoparticles supported on different crystalline TiO (anatase and rutile) result in different
2
reaction pathways for 4-nitroacetophenone.