Mild, selective and switchable transfer reduction of nitroarenes catalyzed by supported gold nanoparticles

Article abstract of DOI:10.1039/c3cy00533j

A highly versatile and flexible gold-based catalytic system has been developed for the controlled and selective transfer reduction of nitroarene using 2-propanol as a convenient hydrogen source under mild conditions. Depending on the specific reaction conditions, multiple products including azoxyarenes, symmetric or asymmetric azoarenes and anilines can be obtained respectively via a controlled reduction of the nitro aromatics with good to excellent yields in the presence of a reusable mesostructured ceria-supported gold (Au/meso-CeO₂) catalyst. The overall operational simplicity, high chemoselectivity, functional-group tolerance and reusability of the catalyst make this approach an attractive and reliable tool for organic and process chemists. The Royal Society of Chemistry.

Full text of DOI:10.1039/c3cy00533j

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Mild, Selective and Switchable Transfer Reduction of Nitroarenes
Catalyzed by Supported Gold Nanoparticles†
Xiang Liu, Sen Ye, Hai-Qian Li, Yong-Mei Liu, Yong Cao,* and Kang-Nian Fan
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A highly versatile and flexible goldꢀbased catalytic system has been developed for controlled and
selective transfer nitroarene reduction using 2ꢀpropanol as a convenient hydrogen source under mild
conditions. Depending on the specific reaction conditions, multiple products including azoxyarenes,
symmetric or asymmetric azoarenes, and anilines, can be respectively obtained via a controlled reduction
10 of nitro aromatics with good to excellent yields in presence of a reusable mesostructured ceriaꢀsupported
gold (Au/mesoꢀCeO2) catalyst. The overall operational simplicity, high chemoselectivity, functionalꢀ
group tolerance, and reusability of the catalyst make this approach an attractive and reliable tool for
organic and process chemists.
have emerged as powerful catalysts that can promote a wide
range of chemical transformations under green and mild condiꢀ
tions.7 For example, Corma et al. have discovered an excellent
50 activity of TiO2ꢀsupported gold NPs for highly selective reductiꢀ
on of substituted nitroarenes to corresponding anilines with
H2.4n,8 We have recently developed an alternative Auꢀcatalyzed
transfer hydrogenation (TH) strategy which can facilitate more
efficient chemoselective nitro group reduction in the presence of
55 other sensitive functional groups under very mild conditions.9
From our continuing studies on Au catalysis, we report herein
that a single Auꢀbased catalyst system enables highly selective
staged reduction of a range of nitroarenes using 2ꢀpropanol as the
hydrogen source. Our results have shown that the Au catalyst
60 comprising gold NPs deposited on mesostructured ceria (Au/
mesoꢀCeO2) is a highly versatile system for targeted synthesis of
azoxyꢀ, azoꢀ, or aniline compounds in good to excellent yields
starting from nitroarenes. This flexible nitro reduction approach
using a robust and recyclable Au catalyst has several advantages
65 over those reported previously. For instance, this catalyst does
not require any kind of poisonous salts, and provides excellent
yields achievable under mild conditions.
Introduction
15 The reduction of nitroarenes is an important process as the
resultant Nꢀcontaining products, such as aniline, azoxy, and azo
compounds, are versatile intermediates and precursors in the
preparation of dyes, pharmaceuticals, pigments, agrochemicals
and polymers.1 Despite a large number of procedures have been
20 developed for the reductive transformation of nitroarenes,2ꢀ4 still
the development of catalytic methodologies that afford high
chemoꢀ and regioꢀselectivity under mild conditions represents an
important challenge. Even more challenging is the selective
conversion of one nitroarene compound into more than one target
25 product by controlled reduction with the use of a single catalyst
system. Such a process would be of particular value to synthetic
chemistry due to its stepꢀeconomical potential and costꢀ
efficiency.5 Compared with the great progress made in the selecꢀ
tive reduction of nitro compounds to anilines, there are scarcely
30 available reports of nitroarenes undergoing a controlled reduction
to a range of different target products with an appreciable level of
functional group selectivity.6 Sakai et al. have recently shown
that highly selective conversion of nitroarenes into azoxyꢀ, azoꢀ,
hydrazoꢀ, or aniline compounds could be realized by using a
35 complex reducing system comprising trivalent indium salts
coupled with hydrosilane as hydrogen source.6b Very lately, Kim
et al. have described that the reduction of nitroarenes can be
finely tuned to obtain three different products by using a simple
Ru nanoparticleꢀethanol combination. This protocol, however,
40 showed only moderate activity and selectivity for the generation
of azoxyꢀ and azoꢀ compounds.6a Therefore, finding a more
benign and economical heterogeneous protocol which is chemoꢀ
selective for the reduction of nitroarenes into both amines and the
corresponding compounds of the reductive coupling under mild
45 and general reaction conditions is highly desirable.
Results and discussion
Synthesis and characterization
70
Mesostructured ceria was synthesized through a surfactant
assisted precipitation method, with the use of cetyltrimethyl
11
ammonium bromide (CTAB) as templating agent.10,
The
nitrogen adsorption–desorption isotherms of mesoꢀCeO2 are of a
typical type IV with a clear H1ꢀtype hysteresis loop, which is
75 characteristic of highly ordered mesoporous materials (see Figure
S1 in ESI†). When gold nanoparticles were deposited onto the
mesoꢀCeO2, Xꢀray photoelectron spectroscopy (XPS) of the Au
4f7/2 core level showed a sole contribution from metallic Au0 at a
Over the past few years, supported gold nanoparticles (NPs)
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Figure 1. XPS of Au/mesoꢀCeO2: a) before reaction; b) after three runs.
Note that the metallic state of Au practically undergoes no change after
the three successive runs.
5
binding energy of 83.8 eV (Figure 1a). Almost identical Xꢀray
diffraction (XRD) patterns were obtained for Au/mesoꢀCeO2 and
mesoꢀCeO2 (Figure 2), indicating that the crystalline structure and
the average size of the crystalline domain of the support were
well maintained in the Auꢀcontaining samples. No diffraction
25
Figure 3. HRTEM images of Au/mesoꢀCeO2: a) before reaction; b) after
three runs. The white circles indicate crystals showing the Au (111)
planes. No aggregated gold particles were observed on the same sample
after three times reuse.
10 lines of metal gold were observed in Au/mesoꢀCeO2, which
indicated that the gold particles were highly dispersed and the
sizes were very small (<5 nm). High resolution transmission
electron microscopy (HRTEM) experiments have been carried
out to observe a possible structure of the metallic Au particles
15 (Figure 3a). According to the image, the average size of the
mesoꢀCeO2 NPs was 4.8 nm, which confirmed the observations
from the XRD data. The low Au loading and poor contrast
between ceria and gold particles prevented to obtain the gold
particle size distributions, but the existence of small Au particles
30 can be confirmed by a careful lattice fringe analysis of the
observed images (a detailed description for the relevant structural
characterization has been provided in Table S3).
Catalytic performance
At the start of our work, the transfer reduction of nitrobenzene
35 (1a) was chosen as the model reaction for the optimization of
catalytic activity and selectivity. Initially, the TH reactions were
carried out with various heterogeneous catalysts in the presence
of aqueous 2ꢀpropanol with 0.5 equiv. KOH at 30 oC. Azoxybenꢀ
zene (2a), having high value as key intermediates for prodrugs
40 and liquid crystals,1e,1f was obtained as the main reductive
product. As can be seen in Table S1 (see ESI†), the ceria
supported gold catalysts, in particular Au/mesoꢀCeO2 showed
high activities for the transformation (Table S1, entries 1, 2).
Most remarkably, the reaction with Au/mesoꢀCeO2 gave 2a in
45 full conversion with 99% selectivity (Table 1, entry 3). In this
case, a noteworthy productivity with the average TOF and the
TON of up to 20 h−1 and 100 was realized. It should be
underlined that these values are nearly one order of magnitude
greater than those in previously reported heterogeneous protocols
50 such as a Ru/Cꢀethanol system for 2a synthesis directly from
o
nitroarenes (TOF: 1.2 h−1, TON: 22, reaction at 60 C).6a In the
absence of the catalyst, however, the desired selfꢀcoupling
product 2a was not produced. No formation of 2a was observed
in the presence of just mesoꢀCeO2 (Table S1, entry 9). In the case
55 of other supported gold catalysts, such as Au/TiO2, Au/Al2O3,
20
Figure 2. XRD diffractograms: a) mesoꢀCeO2, b) Au/ mesoꢀCeO2, and c)
Au/mesoꢀCeO2 after three runs.
2
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Table 1 Catalytic results of the transfer hydrogenation of nitrobenzene
20 and the yields of azoxyarenes are also lower. Notably, mꢀ
substituted nitrobenzenes had lower reaction rates and yields than
pꢀsubstituted nitrobenzenes (Table 2, entries 2ꢀ5). Furthermore,
the reaction of nitroarenes with electronꢀdonating group can
proceed faster with higher yields for 2 compared with nitroarenes
25 with electronꢀwithdrawing group (Table 2, entries 2ꢀ10). The reꢀ
action tolerates the presence of halogens (Table 2, entries 4ꢀ7),
but a prolonged reaction time was needed for obtaining a high
yield of the desired azoxyarenes. Of particular note is that the
reducible function groups such as nitrile, ketone as well as ester
30 moieties remained intact during the reduction process (Table 2,
entries 8ꢀ10), demonstrating the high versatility of the present
methodology for azoxy compounds synthesis.
To find out if our catalyst system might also be applicable for
the direct synthesis of more valuable azoarenes, the effect of
35 reaction parameters on the product distribution was investigated.
With the Au/mesoꢀCeO2 catalyst, we first studied the effect of
base additives on the reduction of 1a at 30 oC. During exploration
of different bases, we found that KOH was the most suitable
additive, which showed the highest conversion and a high
40 selectivity toward 2a formation (Table S2, entries 2ꢀ4, 6). The
reaction was also affected by the amount of KOH added and a 0.5
equiv. was enough to give the optimal yield (Table S2, entries 1,
5ꢀ8). The major product, however, was still 2a under all the
reaction conditions examined. Intriguingly, by increasing the
45 amount of water in aqueous 2ꢀpropanol to 2.5 mL, we observed
that appreciable amounts of azobenzene (3a) (ca. 12%) was
produced as the main byꢀproduct (Table 1, entry 7).12 We then
hypothesized that, if the reaction was performed in a suitable
aqueous 2ꢀpropanol solution, the reaction might be directed
50 toward the selective formation of 3a. We verified this by
performing the reaction with different amount of water added
(Table 1, entries 1ꢀ3, 7ꢀ9). To our delight, the crude mixture
showed 67% 3a selectivity in the presence of 5.0 mL water
(Table 1, entry 8). For further optimization toward the selective
55 synthesis of 3a, subsequent studies were focused on the effect of
the reaction temperature. Gratifyingly, 3a was obtained in high
selectivity of 95% when the temperature was slightly increased
up to 40 oC (Table 1, entry 10).
over Au/mesoꢀCeO2.a
Sel. (%)b
Base
H2O
T
t
(h)
7
5
5
5
5
5
5
5
5
5
5
Conv.
(%)b
100
100
100
100
ꢀ
Entry
(equiv.) (mL) (oC)
2a
78
95
>99
98
ꢀ
3a
18
4
<1
2
4a
4
1
ꢀ
ꢀ
ꢀ
ꢀ
3
1
5
2
97
98
1
2
3
KOH(0.5)
ꢀ
30
30
30
30
30
30
30
30
30
40
60
80
KOH(0.5) 0.5
KOH(0.5) 1.0
KOH(0.5) 1.0
KOH(0.5) 1.0
KOH(0.5) 1.0
KOH(0.5) 2.5
KOH(0.5) 5.0
KOH(0.5) 7.5
4c
5d
6e
7
ꢀ
ꢀ
ꢀ
ꢀ
100
100
100
100
38
85
32
30
3
ꢀ
ꢀ
12
67
65
95
3
8
9
10 KOH(0.5) 5.0
11
12
ꢀ
ꢀ
5.0
5.0
3
>99
2
a Reaction conditions: Reaction conditions: 1 mmol nitrobenzene, 1 mol%
b
Au, 5.0 mL 2ꢀpropanol, 1atm N2. Conversion and selectivity based on 1a
consumption. Determined by GC using nꢀdodecane as the internal standard.
c
e
Results for the third run. d 1 mol% Pd/mesoꢀCeO2. 1 mol% Ru/mesoꢀ
CeO2 .
5
Au/ZrO2, Au/Fe2O3, and Au/ZnO, as well as Pd, Pt, and Ru
supported on mesoꢀCeO2, inferior activity toward 2a formaꢀtion
was observed (entries 3ꢀ8 in Table S1 and entries 5 and 6 in
Table 1).
With these findings in hand, the synthesis of azoxyarene deriꢀ
10 vatives from a variety of nitroarenes was examined. As illustrated
in Table 2, the Au/mesoꢀCeO2 was highly efficient for the
reductive coupling of various nitroarenes bearing structurally
different functional substituents, giving the corresponding azoxy
compounds in good to excellent yields. It is noted that both steric
15 and electronic properties of the substituent affect the reductive
coupling reaction significantly. In general, nitroarenes containing
a substituent on the aromatic ring require a longer reaction time,
Table 2 Synthesis of azoxyarenes from different nitroarenes.a
Table 3 Synthesis of azoarenes from different nitroarenes.a
Entry
1
2
3
4
5
6
7
8
R
H
pꢀCH3
mꢀCH3
pꢀCl
mꢀCl
pꢀBr
pꢀF
t (h)
5
5
7
8
Conv. (%)b Yield (%)b
60
100
100
100
97
>99 (94)
90 (84)
84 (80)
80 (73)
75 (68)
81 (75)
75 (69)
70 (63)
89 (81)
86 (79)
Entry
1
2
3
4
5
6
7
8
R
H
pꢀCH3
mꢀCH3
pꢀCl
mꢀCl
pꢀBr
pꢀF
T (oC) t (h)
Conv. (%)b Yield (%)b
40
35
35
40
40
40
40
40
40
40
5
6
8
10
10
10
10
12
5
100
100
100
100
100
95
95 (88)
93 (87)
95 (90)
98 (91)
95 (89)
92 (87)
90 (83)
99 (95)
97 (91)
94 (86)
8
93
97
10
10
10
6
92
90
pꢀCN
pꢀCOMe
pꢀCOOMe
95
9
10
96
98
pꢀCN
pꢀCOMe
pꢀCOOMe
100
100
97
10
9
10
9
a
Reaction conditions: 1 mmol nitroarene, 1 mol% Au, 0.5
equiv. KOH, 5.0 mL 2ꢀpropanol, 1.0 mL water, 30 0C, 1atm N2.
b Conversion and yields were determined by GC using nꢀdodecane
as the internal standard; values in parenthesis are the yields of the
isolated products.
a
Reaction conditions: 1 mmol nitroarene, 1 mol% Au, 0.5 equiv. KOH,
5.0 mL 2ꢀpropanol, 5.0 mL water, 1atm N2. b Conversion and yields were
determined by GC using nꢀdodecane as the internal standard; values in
parenthesis are the yields of the isolated products.
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Scheme 1. Direct synthesis of asymmetrical azobenzenes from different
nitroarenes.
Having established the optimal catalytic system and reaction
conditions for formation of azobenzene from nitrobenzene, other
nitroarenes were investigated. As shown in Table 3, a range of
nitroarenes were selectively transformed into the corresponding
azoarenes. Again, the reaction activity is highly dependent on the
electronic and steric properties of the substituent. Generally, the
nitroarenes containing an electronꢀdonating substituent on the
benzene ring is more active than the nitroarenes containing an
electronꢀwithdrawing substituent (Table 3, entries 2ꢀ10). For
10 example, a higher reaction temperature and longer reaction time
is necessary for the pꢀ or mꢀchlorineꢀcontaining nitrobenzene
compared with the pꢀ of mꢀCH3ꢀcontaining nitrobenzene (Table
3, entries 2ꢀ5). Furthermore, 4ꢀnitrobenzonitrile, 4ꢀnitroacetopheꢀ
none and 4ꢀnitrobenzoic acid methyl ester can also be converted
15 to the desired azo compounds, without reduction of C=O or C≡
N groups. In contrast to the previously reported catalytic systems,
such as Au/ZrO2 photocatalyst,3c wormꢀlike nanoꢀPd,3b ultraꢀthin
Pt nanoꢀ wires,3a or Ru/C6a, the present Auꢀbased protocol
represents the most mild and efficient system for convenient and
20 controlled synthesis of azoarenes directly from nitroarenes to
date.
Table 4 Synthesis of anilines from different nitroarenes.a
5
Entry
1
2
3
4
5
6
7
8
R
H
pꢀCH3
mꢀCH3
pꢀCl
mꢀCl
pꢀBr
pꢀF
T (oC)
80
80
80
90
80
80
80
80
t (h)
3
5
5
8
Conv. (%)b Yield (%)b
>99
>99
92
98
99
91
99
98
96
94
95
99
95
>99
98
7
10
10
8
6
10
96
94
96
pꢀCN
pꢀCOMe
pꢀCOOMe
9
10
60
60
>99
95
a
Reaction conditions: 1 mmol nitroarene, 1 mol% Au, 5.0 mL 2ꢀ
propanol, 5.0 mL water, 1 atm N2. b Conversion and yields were determined
by GC using nꢀdodecane as the internal standard.
55
o
higher reaction temperature (80 C) without introducing any base
Encouraged by the excellent yields of the reductive selfꢀ
coupling of nitroarenes, an effort was initiated to explore the feaꢀ
sibility of using the Au/mesoꢀCeO2ꢀmediated protocol to produce
25 asymmetrical azoarenes by the transfer hydrogenative crossꢀ
coupling of two different nitroarenes. To this end, we have set out
to prepare three representative asymmetrical azoarenes with poteꢀ
ntial applications in dyes,13 liquid crystals,14 or optical storage
media,15 respectively. In a preliminary experiment, the reaction
30 was carried with an equimolar mixture of 1a and 4ꢀchloronitroꢀ
benzene. A mixture of azoarenes, namely 3a and 4ꢀchloroazobenꢀ
zene was obtained in 61% and 33% yields, respectively, based on
1a conversion (Scheme S1 in ESI†). Formation of 4ꢀchloroazoꢀ
benzene corresponds to the cross coupling of Nꢀphenylhydroxylꢀ
35 amine, the possible and most reasonable pathway according to the
literature,8a with 4ꢀchloronitrosobenzene. Meanwhile, the more
reactive 1a favoured the preferential formation of selfꢀcoupling
product of 3a. To optimize the reaction further, the reaction was
carried out in the presence of two equivalents of 4ꢀchloronitrobeꢀ
40 nzene. In this case, the selectivity towards asymmetric 4ꢀchloroꢀ
azobenzene increased remarkably, and a high yield up to 87%
was achieved. By using this newly established procedure, three
asymmetrical azoarenes with industrial interest were successfully
synthesized with moderate to good yields (see Scheme 1).
additives, the desired aniline (4a) could be obtained with excelꢀ
lent yield (Table 1, entries 11, 12). The scope of this new Auꢀ
based reduction protocol was established by using a wide range
60 of nitroarenes with various reducible functional groups such as
halogens, nitrile, ketone, or ester and in many cases the reaction
was completed within 3ꢀ10 h with high yields (Table 4).
The stability and reusability of the catalyst were tested in the
reduction of 1a to 2a. It was found that the activity of the catalyst
65 can be maintained even after 3 times of reuse, and the selectivity
for 2a was still up to 98% (Table 1, entry 4). Inductively coupled
plasma atomic emission spectral (ICPꢀAES) analysis results
showed that there was no leaching of gold during the reaction,
verifying the inherent stability of the Au/mesoꢀCeO2 catalyst.
70 XRD, HRTEM and XPS results confirmed no change in the
dispersion of the Au NPs or metallic state of Au before and after
reuse, which was in good agreement with an excellent activity
retention of this catalyst (see Figures 1ꢀ3 and Table S3 in ESI†).
Although the precise route and mechanism by which the
75 reduction occurs are not yet fully understood, the transient AuꢀH
species formed by the interaction with the hydrogen donor could
be involved at the initial stage of the reaction.9c A plausible
reaction pathway, consistent with the generally accepted Harber
mechanism proposed for electrochemical hydrogenation of
80 nitrobenzene and its derivatives,16 is depicted in Scheme 2. In
general, the nitro group of the substrate is firstly attacked by the
45 It is noteworthy that this simple Auꢀbased transfer reduction
method could also be applied to prepare anilines via the reduction
of aromatic nitrobenzene. After carefully tuning the reaction
conditions, we found that when the substrate 1a was treated at a
50
4
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CTAB (2.19 g) in 200 mL distilled water at room temperature.
The mixture was then maintained in a sealed glass vessel under
o
45 stirring for 24 h. After thermal aging at 90 C for 3 h, the pale
yellow precipitate was filtered and washed with hot water (80 oC)
to remove the residual CTAB. The resultant powder was dried at
100 oC for 6 h and then calcined at 450 oC for 4 h. The Brunauerꢀ
EmmettꢀTeller (BET) surface area of the asꢀsynthesized mesoꢀ
50 CeO2 was 150 m2ꢁg−1.
Preparation of Au/CeO2 catalysts: Au/CeO2 catalysts were
prepared by a routine depositionꢀprecipitation (DP) method.10 An
appropriate amount of aqueous solutions of chloroauric acid
o
(HAuCl4) was heated to 70 C under vigorous stirring. The pH
55 was adjusted to 9 by dropwise addition of NaOH (0.2 M), and
then 1.0 g of CeO2 powders (Evonic, Adnano 50, specific surface
area: 50 m2ꢁg−1) or mesoꢀCeO2 was dispersed in the solution. The
mixture was aged for 1 h, followed by filtering and washing
several times with deionized water to remove Cl−. The resulting
60 solid product was dried overnight before reduction at 300 oC for 2
h in 5 vol% H2/Ar. The two catalysts were designated as
Au/CeO2ꢀ50 and Au/mesoꢀCeO2, respectively. The gold loading
of the asꢀprepared catalysts was 0.49 wt% Au/CeO2ꢀ50 and 0.5
wt% Au/mesoꢀCeO2 as determined by ICPꢀAES.
Scheme 2. A plausible reaction pathway for the reduction of nitrobenzene.
metalꢀhydride, leading to a direct hydride transfer from the 2ꢀ
propanol to the reactant. At elevated temperatures and baseꢀfree
conditions, the formation of anilines from nitrobenzenes proceeds
smoothly via the intermediate formation of nitrosobenzene/Nꢀ
phenylhydroxylamine molecules (Route a). A switch of the major
reaction path way might occur, provided that suitable amount of
KOH is introduced into the reaction system. Depending on the
10 concentration of the aqueous 2ꢀpropanol and reaction
temperature, either azoxybenzenes or azobenzenes can be
selectively obtained in good to excellent yields (Route b). In the
overall process, the fact that the reduction capability of the AuꢀH
species can be finely tuned by optimising the reaction conditions
15 is essential for controlling the reduction of the nitro group at
different intermediate stages.
5
65
Preparation of Au/ZrO2 catalyst: ZrO2 powders were
prepared by a conventional precipitation method following the
reported procedure.17 Au/ZrO2 catalysts were prepared by a
modified depositionꢀprecipitation (DP) method by mixing ZrO2
powders (2 g) with appropriate amounts of aqueous solutions of
70 chloroauric acid (100 mL, 1 mM), the pH was adjusted to 9.0 by
o
dropwise addition of 0.25 M NH4OH. After 6 h stirring at 25 C
Conclusions
the catalyst was washed five times with deionized water and
separated by filtration. The samples were dried at 110 C in a
o
We have developed a highly efficient and flexible heterogeꢀ
neous goldꢀcatalyzed approach for controlled and chemoselective
20 reduction of nitroarenes into three different target products of
industrial importance. An essential feature of the present Auꢀ
based methodology is that the switchable products, including
azoxyꢀ, azoꢀ and aniline compounds can be selectively obtained
in high yields by simply varying the reaction conditions. We
25 expect this method to be useful in the synthesis of more complex
molecules by optimizing the stepꢀeconomy in synthesis planning.
forced air oven for 1 h, followed by reduction with a stream of 5
75 vol% H2/Ar at 350 oC for 2 h. The BET surface area of the
resultant Au/ZrO2 catalyst was 113 m2ꢁg−1. The concentration of
gold in Au/ZrO2 was 0.8 % Au by weight.
Preparation of Pt/meso-CeO2, Pd/meso-CeO2 and Ru/meso-
CeO2 catalysts: 1 wt% Pt/mesoꢀCeO2, 1 wt% Pd/mesoꢀCeO2 and
80 1 wt% Ru/mesoꢀCeO2 catalysts were prepared by incipientꢀ
wetness impregnation of the support, with aqueous solution of
H2PtCl6ꢁ6H2O, PdCl2, and RuCl3ꢁxH2O precursors of appropriate
concentrations (typically 1.0 mL/g support). After a perfect
mixing of the corresponding slurries, samples were dried under
85 vacuum at room temperature for 12 h and then reduced in 5%
H2/Ar at 400 oC for 2 h.
Experimental
General
Gold catalysts including 1 wt% Au/Al2O3 (Strem catalogue
30 number: 79ꢀ0160), 1 wt% Au/TiO2 (Strem catalogue number: 79ꢀ
0165), and 1 wt% Au/ZnO (Strem catalogue number: 79ꢀ0170)
were supplied by Mintek. 4.5 wt% Au/Fe2O3 (type C, lot no.
Au/Fe2O3 no. 02ꢀ5) was supplied by the World Gold Council
(WGC). Palladium chloride (PdCl2), hexachloroplatonic acid
35 hexahydrate (H2PtCl6ꢁ6H2O), ruthenium chloride hydrate
(RuCl3ꢁxH2O), and chloroauric acid tetrahydrate (HAuCl4ꢁ4H2O),
were supplied by Aldrich and used without further purification.
Catalyst characterization
Elemental analysis: The Au loading of the catalysts was
measured by inductively coupled plasma atomic emission
90 spectroscopy (ICPꢀAES) using a Thermo Electron IRIS Intrepid
II XSP spectrometer. The detection limit for Au is 7 ppb.
BET analysis: The BET specific surface areas of the prepared
catalysts were determined by adsorptionꢀdesorption of nitrogen at
liquid nitrogen temperature, using a Micromeritics TriStar 3000
o
Catalyst preparation
95 equipment. Sample degassing was carried out at 300 C prior to
acquiring the adsorption isotherm.
Preparation of mesostructured ceria materials: MesoꢀCeO2
X-ray diffraction (XRD) analysis: The crystal structures of
mesostructured CeO2 were characterized with powder Xꢀray
diffraction (XRD) on a Bruker D8 Advance Xꢀray diffractometer
100 using the Niꢀfiltered Cu Kα radiation source at 40 kV and 40 mA.
11
40 was prepared by a templateꢀassisted precipitation method.10,
Typically, a NaOH solution (2 g in 300 mL of distilled water)
was added to a stirred solution of Ce(NO3)3ꢁ6H2O (4.34 g) and
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DOI: 10.1039/C3CY00533J
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X-ray photoelectron spectroscopy (XPS): XPS data were
recorded with a Perkin Elmer PHI 5000C system equipped with a
hemispherical electron energy analyzer. The spectrometer was
operated at 15 kV and 20 mA, and a magnesium anode (Mg Kα,
hν = 1253.6 eV) was used. The C 1s line (284.6 eV) was used as
the reference to calibrate the binding energies (BE).
Transmission electron microscopy (TEM): A JEOL 2011
microscope operating at 200 kV equipped with an EDX unit
(Si(Li) detector) was used for the TEM. The samples for electron
10 microscopy were prepared by grinding and subsequent dispersing
the powder in ethanol and applying a drop of very dilute
suspension on carbonꢀcoated grids.
High resolution transmission electron microscopy
(HRTEM): HRTEM images for catalysts were taken with a
15 JEMꢀ2100F electron microscope operating at 200 kV. The size
distribution of the metal nanoclusters was determined by
measuring about 200 random particles on the images.
60
65
5
2
3
70
75
Catalytic activity test
4
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80
General procedure for azoxy-, azox-, and aniline compounds
20 synthesis via Gold-Catalyzed Reduction of Nitroarenes:
Supported gold catalyst (1.97 mg Au, 0.01 mmol) was placed in a
25 mL twoꢀneck flask, adding nitroarenes (1 mmol), a certain
amount of KOH, 2ꢀpropanol and water. The reaction mixture was
then vigorously stirred (800 rpm with a magnetic stir bar) at the
25 designed temperature under 1 atm of N2 atmosphere for the given
reaction time. After completion of the reaction, the reaction
mixture was filtered and the catalyst was washed thoroughly with
ethanol. Then the filtrate was concentrated and dried under
reduced pressure using a rotatory evaporator. The crude product
30 was purified by column chromatography [silica 200–300;
85
90
o
petroleum ether (60~90 C)/ethyl acetate mixture] to afford the
95
product. All the products were characterized identified by GCꢀ
MS and the spectra obtained were compared with the standard
spectra. The conversion and yields were determined by GCꢀ17A
35 gas chromatograph equipped with a HPꢀFFAP column (30 m ×
0.25 mm) and a flame ionization detector (FID).
Recovery and reuse of Au/meso-CeO2: The catalyst was
collected after filtration washed with acetone for three times and
then with distilled water for several times. The catalyst was then
40 dried at 100 oC for 12 h before used for next reaction.
100
105
110
115
120
5
6
Acknowledgements
7
Financial support by the National Natural Science Foundation of
China (21073042, 21273044), New Century Excellent Talents in
the University of China (NCETꢀ09ꢀ0305), the State Key Basic
45 Research Program of PRC (2009CB623506), the Research Fund
for the Doctoral Program of Higher Education (2012007000011)
and Science & Technology Commission of Shanghai Municipaꢀ
lity (08DZ2270 500) is kindly acknowledged.
8
9
Notes and references
50 Shanghai Key Laboratory of Molecular Catalysis and Innovative
Materials, Department of Chemistry, Fudan University, Shanghai 200433,
P. R. China. Fax: (+86-21) 65643774; E-mail: yongcao@fudan.edu.cn
† Electronic Supplementary Information (ESI) available: experimental
data, TEM, 1H and 13C NMR data. See DOI: 10.1039/b000000x/0
125 10 M. M. Wang, L. He, Y. M. Liu, Y. Cao, H. Y. He and K. N. Fan，
Green Chem., 2011, 13, 602.
55
1
(a) H.ꢀU. Blaser, H. Steiner and M. Studer, ChemCatChem, 2009, 1,
210; (b) S. Nishimura, Handbook of Heterogeneous Catalytic Hydro-
6
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DOI: 10.1039/C3CY00533J
11 B. Campo, G. Santori, C. Petit and M. A. Volpe, Appl. Catal., A,
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12 Note that the solvent plays a vital role in reactants adsorption or
products desorption. The concentration of aqueous 2ꢀpropanol may
favor the adsorption of azoxybenzene on the surface of Au catalyst,
facilitating the azobenzene formation. See: F. Figueras and B. Cop, J.
Mol. Catal. A: Chemical, 2001, 173, 223.
5
13 K. Hunger, Industrial dyes: Chemistry, Properties, Applications,
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10 14 Y. Zhao and T. Ikeka, Smart Light-Responsive Materials: Azoben-
zene-Containing Polymers and Liquid Crystals, Wiley, Chichester,
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15 T. Ikeda and O. Tsutumi, Science, 1995, 268, 1873.
16 F. Haber, Z. Elektrochem. 1898, 4, 506.
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DOI: 10.1039/C3CY00533J
Graphical Abstract
Mild, Selective and Switchable Transfer Reduction of Nitroarenes Catalyzed by
Supported Gold Nanoparticles
5
Xiang Liu, Sen Ye, Hai-Qian Li, Yong-Mei Liu, Yong Cao,* and Kang-Nian Fan
10 A highly efficient mesostructured ceria-supported gold catalyst system has been developed for controlled and
selective transfer reduction of nitroarenes into azoxyarenes, azoarenes, or anilines with good to excellent yields using
2-propanol as a hydrogen resource under mild conditions.
15