Highly efficient Au/TiO2 catalyst for one-pot conversion of nitrobenzene to p-aminophenol in water media

Article abstract of DOI:10.1002/cjoc.201300875

Au/TiO₂ catalyst is firstly reported to be efficient in the hydrogenation of nitrobenzene to produce p-aminophenol with a high PAP selectivity of 81% and overall yield more than 63%. The catalyst is also quite stable and can be reused for at least 4 times with only slight decrease in activity.

Full text of DOI:10.1002/cjoc.201300875

FULL PAPER
DOI: 10.1002/cjoc.201300875
Highly Efficient Au/TiO2 Catalyst for One-pot Conversion of
Nitrobenzene to p-Aminophenol in Water Media
Luyao Zou, Yuanyuan Cui, and Weilin Dai*
Department of Chemistry and Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials, Fudan
University, Shanghai 200433, China
Au/TiO2 catalyst is firstly reported to be efficient in the hydrogenation of nitrobenzene to produce
p-aminophenol with a high PAP selectivity of 81% and overall yield more than 63%. The catalyst is also quite stable
and can be reused for at least 4 times with only slight decrease in activity.
Keywords nitrobenzene, p-aminophenol, Au/TiO2 catalyst, hydrogenation
Introduction
importation of phase-transfer agents may also improve
the catalytic performance. Various solid acids have also
been applied for this conversion for cleaner synthesis of
PAP.[15-21] However, for metal-solid acid bifunctional
catalysts, the high selectivity of PAP is more difficult to
achieve. Most works could obtain a typically 50%－
80% selectivity while the conversion of NB varies, but
all made the overall yield of PAP less than 50%. Only
Figueras et al.[20] reported the highest selectivity over
90% using the mechanical mixture of supported Pt and
sulphated zirconia, but their reaction conditions varied
and it was difficult for comparison. Recently, Liu
et al.[22] reported a PAP yield of more than 47% using
Pt/C catalyst and carbon-based solid acid.
The unique catalytic application of Au had not been
realized until Haruta’s work.[23] Compared with Pd
group metals, Au is more inert in dissociating H－H
bond, whereas more selective to certain functional
groups in hydrogenation.[24] When catalyzing hydro-
genation of α,β-unsaturated ketones, dinitroaromatic
compounds and chlorinated aromatic compounds, unlike
Pd group metals, Au will not cause the problems of hy-
drogenation of C＝C dibond,[25] complete hydrogena-
tion to diamine,[26] and dechlorination,[27] respectively.
Chemoselective hydrogenation thus could be realized.
Corma[28] and Bokhoven[29] proved that PHA is the main
The Pd group metal catalysts[1-4] are widely used in
the hydrogenation of nitroaromatics. The nitro group,
through hydrogenation, is reduced to amine group,
forming a series of corresponding aromatic amines of
various utility. Compared with traditional Fe-concen-
trated HCl system, catalytic hydrogenation avoids com-
plicated post-treatment and large amount of acidic
sewage, thus is much more molecular economic and
green.
Para-aminophenol (PAP) is an important raw mate-
rial in synthesizing paracetamol,[5] the effective compo-
nent in non-aspirin cold drug such as Tylenol®. The
synthesis of PAP can be achieved by two hydrogenation
pathways, the one is from paranitrophenol (PNP),[6-8]
while the other is from nitrobenzene (NB) combined
with a Bamberger rearrangement process.[9] For the lat-
ter one, a Brønsted acid is required for the rearrange-
ment of N-phenylhydroxylamine (PHA), the intermedi-
ate of NB hydrogenation to aniline (AN) (see Scheme
1)[10]. Henke and Vaughen[11] showed that NB pathway
is feasible using Pt/C catalyst in inorganic acid solution.
Subsequent works improved the catalytic performance
by optimizing reaction agents and conditions, pushing
the selectivity of PAP up to 87% in sulfuric acid.[12]
Meanwhile, corresponding kinetics was also carefully
investigated, showing that hydrogenation rate is com-
petitive against rearrangement rate, which limits the
PAP selectivity even in concentrated acids.[13,14] More-
over, since NB is not soluble in water, the reaction hap-
pens in a four-phase system, i.e. gas, solid catalyst,
aqueous and organic phases.[13] Although this property
makes the separation of reagent and product easier (PAP
and its sulphate are water soluble), it hinders the mass
transfer process between molecules, and therefore the
Scheme 1 NB hydrogenation pathways
H2N
OH
NO2
NHOH
p-aminophenol [PAP]
2H2
H2N
nitrobenzene
[NB]
N-phenylhydroxylamine
[PHA]
aniline [AN]
*
E-mail: wldai@fudan.edu.cn
Received December 10, 2013; accepted January 22, 2014; published online February 17, 2014.
Chin. J. Chem. 2014, 32, 257—262
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Zou, Cui & Dai
FULL PAPER
intermediate of NB hydrogenation on Au/TiO2, and its
hydrogenation to AN is the rate-determining step.[28] To
our best knowledge, however, Au has not been reported
in the direct conversion of NB to PAP. Whether Au
could also have activity under strong acidic environ-
ment and could raise the selectivity in the conversion of
NB to PAP, is a question yet to be answered.
fNB (ANB )
Conv.(NB)＝(1－
Sel.(PAP)＝
)×100%
fi (A )
i
∑
i
fPAP (APAP
)
×100%
fi (A )
i
∑
i
Here we demonstrate that Au is also capable of
catalysing NB hydrogenation in sulfuric acid, with a
high PAP selectivity of 81% in pure aqueous media
without any phase-transfer agents, and with much lower
acid concentration requirement than reported Pd or Pt
catalysts. Among various investigated supports (TiO2
(P25), ZrO2[30], CeO2[31]) and acids (H2SO4, TsOH,
H3PW12O40), P25 and H2SO4 showed the best match
with Au.
fAN (AAN )
Sel.(AN)＝
×100%
fi (A )
i
∑
i
where Ai represents the peak area of each component,
and ci＝fi(Ai) is the mole concentration calculated from
the peak area according to the standard curves. Assume
that all Au nanoparticles are semi-sphere on the support
and half area of surface Au atoms is exposed, TOF is
calculated according to the following equations:
Experimental
number of surface Au atoms
number of all Au atoms
3Mr ×α
D×ρ×NA
disp＝
＝
Catalyst preparation
Gold was supported on commercial TiO2 (Degussa).
A traditional deposition-precipitation (DP) method first
introduced by Haruta[23] was applied. First, 0.2 mol•L−1
NaOH was used to adjust the pH of 100 mL HAuCl4
(0.51 mmol•L−1) precursor to 7.2, then a certain amount
of P25 was dispersed in the solution under continuous
magnetic stirring to get the 1 wt% loading. The disper-
sion was heated to 333 K in complete darkness for 2 h
with stirring, then it was cooled to room temperature
and filtered. The as-prepared catalyst was washed with
deionized water until no Cl− was detected in the filtrate
by AgNO3, then dried in air at 353 K overnight and cal-
cined in air at 573 K for 2 h with a heating rate of 2
K•min−1.
m
[catal]
N[surf Au atoms]
＝
LNA idisp
M[Au]
N[NB converted]
TOF＝
tN[surf Au atoms]
where D is the average diameter of Au nanoparticles,
a＝1.15×1019/m2 is the surface atomic concentration
constant of Au, ρ is the density of Au, L is Au loading,
NA is Avogadro constant.
Characterization
Other supports such as ZrO2, CeO2, SO24－ /ZrO2 and
ZSM-5 were prepared according to previous re-
ports.[30,31] Briefly, ZrO2 was prepared by refluxing
Zr(OH)2 precipitated from ZrOCl2 in ammonia at 373 K
for 40 h before calcinations. CeO2 was prepared by
hydrothermal method. SO 24－ /ZrO2 was prepared by
impregnating refluxed Zr(OH)2 in dilute H2SO4 solution
before calcination.
The Au loading of the catalysts was measured by
inductively coupled plasma atomic emission spectros-
copy (ICP-AES) using a Hitachi P-4010 ICP spec-
trometer. Samples were first dispersed in aqua regia in
order to dissolve all Au, and then sent for analysis. XRD
patterns of the samples were recorded on a Bruker D8
advance diffractometer with Cu Kα radiation (λ＝0.154
nm), operated at 40 mA and 40 kV. The scanning rate
was 4 (°)•min−1 from 20° to 85°. N2 adsorption profiles
of the samples were performed in Micromeritics Tristar
ASAP 3000 at 77 K. Specific surface areas of the sam-
ples were calculated by using Brunauer-Emmett-Teller
(BET) method. TEM images of the catalysts were taken
by a JEOL 2011 electron microscope operating at 200
kV. Samples were first dispersed in ethanol and depos-
ited onto a carbon-coated copper grid before being
moved into the TEM vacuum chamber. The average size
and size distribution of the Au nanoparticles were de-
termined by measuring more than 150 Au particles on
the images. XPS analysis was performed using a Perkin
Elmer PHI 5000C ESCA system equipped with a hemi-
spherical electron energy analyzer. The Mg Kα (hν＝
1253.6 eV) anode was operated at 14 kV and 20 mA.
Catalytic activity measurement
Reaction was performed in an autoclave with ap-
proximately 90 mL volume. Typically, 0.3 g NB, 50 mg
catalyst, 10 mL sulfuric acid, 1 MPa H2, 4 h and stirring
rate of 800 r/min were chosen. Acid concentration and
reaction temperature were investigated. Products were
analyzed by HPLC with methanol-water solution (V/V
4∶6). The composition of the buffer is 4 mmol•L−1
NaH2PO4 and 8 mmol•L−1 Na2HPO4. Peaks other than
NB, AN and PAP were weak enough to be ignored, and
PHA was not detectable. The performance of the cata-
lyst was measured by the conversion of NB and selec-
tivity of PAP and AN by using the typical calculating
method:
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Chin. J. Chem. 2014, 32, 257—262

Au/TiO2 Catalyst for One-pot Conversion of Nitrobenzene to p-Aminophenol
The energy scale was internally calibrated by setting the
C1s peak at 284.6 eV.
TOF calculation shows that the difference is moderate
between the catalysts when the reaction rate is normal-
ized to per surface Au unit, discounting the effect of
catalyst loading. This may imply that the catalytic activ-
ity is not so sensitive to the particle size when the mean
particle size is below 5 nm. Nevertheless, 1% Au/TiO2
still owns the best dispersion and economic efficiency,
therefore 1% was chosen as the typical Au loading
amount.
Results and Discussion
Comparing various support, we found that Au/ZrO2
was less active than Au/TiO2 in our experiment, and
CeO2 was not stable enough even in dilute H2SO4. For
solid acids, mechanical mixture was not able to carry
out the rearrangement reaction, and Au was also diffi-
cult to be supported because of the strongly acidic sur-
face. Therefore, Au/TiO2 was eventually chosen as the
ideal catalyst.
Anatase
Rutile
BET analysis shows that the surface areas before
(51.0 m2•g−1) and after (50.2 m2•g−1) 1% Au loading
have no significant change, indicating that the surface
structure of the support is not effected by Au nanoparti-
cles. The pH condition and calcination temperature were
both optimized to obtain well-dispersed Au nanoparti-
cles of best catalytic performance. It is consistent with
other reported works that pH determines the loading and
dispersion of Au(OH)3 species and further Au particle
size on the surface of TiO2.[32]
d
c
b
a
20
30
40
50
60
70
80
2θ /(o)
A series of Au/TiO2 catalysts with Au loading rang-
ing from 1% to 3% were prepared. Physicochemical
properties of the catalysts and their reaction perform-
ance are summarized in Table 1. XRD patterns (Figure 1)
show no distinguishable difference between catalysts of
different Au loading. All of the patterns correspond to
anatase-rutile mixed phase of TiO2, without diffraction
peaks of Au detected which is due to the low loading of
Au and its ultra-small particle size. XPS characteriza-
tion (Figure 2) identifies the surface chemical state of
Au species. Although calcined in air, there was almost
completely metallic Au found on the surface. Catalysts
of different Au loading showed only the difference in
XPS peak areas, while the binding energy remained
nearly the same. This finding means the uniformity of
surface state remains while Au loading varies. The exact
Au loading was determined by ICP as 0.89%, 1.55%
and 1.84%, respectively. The Au content deviated from
the theoretical values at high Au loading, indicating the
difficulty in loading large amount of Au onto the sup-
port. From the TEM images (see Figure 3), we found
that 1% Au/TiO2 gives the smallest average particle size.
Although it is well accepted that Au nanoparticles with
small particle size should show high catalytic activity,[33]
Figure 1 XRD patterns of (a) TiO2; (b) 1% Au/TiO2; (c) 2%
Au/TiO2; (d) 3% Au/TiO2.
A series concentration of H2SO4 from 0 to 1.0
mol•L−1 was applied to investigate the effect of acid
concentration on PAP selectivity. Figure 4 shows that if
pure water (i.e. c(H2SO4)＝0) was used, the main prod-
uct was AN and there was no PAP detected, proving the
necessity of acid in the rearrangement. HPLC peaks of
the amount of by-products other than NB, AN and PAP
were too low to be detected. The drop on NB conversion
may be ascribed to the aggregation of Au nanoparticles
on the surface, since we observed the particle size
growth after reaction. To our surprise, selectivity results
were greatly different from those of Pt and Pd catalysts
reported by other works.[10,12,13] We found when
c(H2SO4) was larger than 0.1 mol•L−1, the selectivity of
PAP kept quite stable at about 81% with a fluctuation
within 5%, which is within normal experimental error.
Usually for Pt and Pd catalysts, the maximum selectiv-
ity of PAP could only be achieved at comparatively high
acid concentration (5－10 wt% for example), and both
lower and higher concentration could cause the decrease
Table 1 Physicochemical properties and performance of Au/TiO2 catalysts with Au loading of 1%, 2% and 3%a
Surface Au/Ti ratio
Sel./%
Catalyst Au/TiO2 Au loadingb/wt%
Mean particle size/nm Au 4f7/2 binding energy/eV Conv./%
TOFd/h−1
(×10−2)c
PAP AN
1%
2%
3%
0.89
1.55
1.84
1.35
1.23
2.08
2.5
2.9
4.6
83.0
82.8
82.8
78
41
34
81 19
86 14
914
923
83 17 1137
a Reaction conditions: 0.3 g NB, Au/TiO2＝50 mg (1%), 25 mg (2%), 17 mg (3%), 10 mL H2SO4, 373 K, 1 MPa H2, 4 h and 800 r/min.
b Analyzed by ICP. c Calculated according to XPS peak areas. d Attributed to initial conversion rate of NB molecules per surface Au atom
per second.
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259

Zou, Cui & Dai
FULL PAPER
the comparative hydrogenation of PHA to AN by-
product.
The results of different reaction temperatures on the
reaction are shown in Figure 5. The conversion of NB
increases with the temperature, while the selectivity to
PAP first increases and then drops obviously. The
maximum selectivity of PAP is achieved at 363 K. Be-
cause of the complexity of the four-phase reaction, it is
difficult to determine the mechanism of each intermedi-
ate step. Nevertheless, according to the previous re-
search, the selectivity to PAP is determined by the reac-
tion rate ratio of rearrangement and hydrogenation of
PHA, kPAP/kAN.[20] When T rises, the hydrogenation rate
of both NB and PHA increases faster than the rear-
rangement rate of PHA, resulting in the increase of NB
conversion as well as AN yield, which corresponds well
to the right-hand side of Figure 5. Considering the
left-hand side is against the reasons above, we also as-
sume that there is also a chemical desorption process.
As we know, H2 and NB are adsorbed on the surface of
Au, and Bokhoven et al. also observed PHA adsorption
only on Au-TiO2 interface.[34-36] Therefore, the hydro-
genation of NB and PHA occurred on the surface. On
the other hand, H＋ is in the bulk solution, so PHA may
first desorb from the surface before its rearrangement
(see Figure 6). It can be expected that the desorption
rate also accelerates while the reaction temperature in-
creases. At low temperature, a limited desorption rate
restrains the rearrangement in the bulk, leading to lower
PAP selectivity. To investigate the reusability of the
e
d
c
b
a
80
82
84
86
88
90
Binding energy/eV
Figure 2 XPS spectra of (a) 1% Au/TiO2; (b) 1% Au/TiO2 after
reaction without calcination; (c) 1% Au/ TiO2 after re-calcination;
(d) 2% Au/ TiO2; (e) 3% Au/ TiO2.
in PAP selectivity. In our own comparison experiment at
0.1 mol•L−1 H2SO4 using 5% Pd/C purchased from
UChem Co. Ltd., NB was completely converted within
30 min but PAP selectivity was below 25%. This phe-
nomenon indicates that when Au is used in this reaction
system, the hydrogenation rate of PHA intermediate is
much slower than the rearrangement rate. This result is
probably due to the weak dissociation ability of Au to
H－H bond. Although it is a weakness in other situa-
tions, the property here is rather an advantage to weaken
Figure 3 TEM images of (a, b) 1% Au/TiO2; (c, d) 2% Au/ TiO2; (e, f) 3% Au/TiO2 and particle size distribution of Au.
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Chin. J. Chem. 2014, 32, 257—262

Au/TiO2 Catalyst for One-pot Conversion of Nitrobenzene to p-Aminophenol
ing to the exact weight of the catalyst added. Meanwhile,
another sample without calcination was also collected
for comparison. A considerable activity loss of the non-
calcined catalyst was observed at the second run, as NB
conversion dropped to only 35%. The selectivity to PAP
slightly increased to 90%, maybe due to a slower hy-
drogenation rate of PHA. However, after calcination, the
catalyst was recovered to show 80% NB conversion and
84% PAP selectivity (see Figure 7). XPS analysis re-
veals that after the reaction, a small amount of ositive
Au species appear (see Figure 2), whose 4f 7/2 binding
energy locates at 84.5 eV. The small peak disappears
again after the calcinations process. This finding may
imply the significance of calcination in re-activating the
spent catalyst.
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
PAP
NB
AN
0.2
0.0
0.4
0.6
0.8
1.0
-1
.
Concentration/(mol L )
Figure 4 NB conversion and PAP, AN selectivity at different
H2SO4 concentration. Reaction conditions: 0.3 g NB, 50 mg catal.,
10 mL H2SO4, 373 K, 1.0 MPa H2, 4 h and 800 r/min.
After the fourth run, a slight decrease in NB conver-
sion was observed, indicating the good stability of
Au/TiO2 catalyst in the title reaction.
100
90
80
70
60
50
40
30
20
10
0
100
90
80
70
60
50
40
30
20
10
0
PAP
NB
AN
343
353
363
373
383
Reaction temperature/K
Figure 5 NB conversion and PAP, AN selectivity at different
reaction temperature. Reaction conditions: 0.3 g NB, 50 mg catal.,
10 mL 0.1 mol•L−1 H2SO4, 1.0 MPa H2, 4 h and 800 r/min.
Figure 7 Repeat use of 1% Au/TiO2. 2a: Second-run with
non-calcined catalyst. Reaction conditions: m(NB)∶m(catal.)＝
6∶1, 10 mL 0.1 mol•L−1 H2SO4, 1.0 MPa H2, 4 h and 800 r/min.
Conclusions
In summary, we have successfully applied Au/TiO2
catalyst in the hydrogenation of NB to PAP for the first
time. The catalyst shows superior activity in water me-
dia with high NB conversion (78%) and PAP selectivity
(81%), and is quite stable in repeated usage for at least 4
runs. Future research will be focused on the design of an
effective synthesis route with highly dispersed Au spe-
cies on acidic support in order to get multi-functional
catalyst.
Acknowledgement
Figure 6 Probable mechanism of the hydrogenation, desorption
and rearrangement.
Financial support by the Major State Basic Resource
Development Program (Grant No. 2012CB224804),
NSFC (Project Nos. 21373054, 21173052), the Natural
Science Foundation of Shanghai Science and Technol-
ogy Committee (No. 08DZ2270500) and Hui-Chun
Chin and Tsung-Dao Lee Chinese Undergraduate Re-
search Endowment (CURE).
catalyst, after reaction, the catalyst was collected by
centrifugation and washed with methanol, and then
dried and calcined at 573 K for 2 h (ramping rate, 2
K•min−1). Then it was reused for the next run, while the
amount of NB was stoichiometrically adjusted accord-
Chin. J. Chem. 2014, 32, 257—262
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Zou, Cui & Dai
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