Gold supported on titania for specific monohydrogenation of dinitroaromatics in the liquid phase

Article abstract of DOI:10.1039/c4gc00869c

Liquid-phase selective monohydrogenation of various substituted dinitroaromatics to the corresponding valuable nitroanilines was investigated on gold-based catalysts. Special attention was paid to the effect of Au particle size on this monoreduction reaction. Interestingly, TiO₂ supported gold catalysts containing a relatively larger mean Au particle size (>5 nm) showed far superior chemoselectivity for specific mono-hydrogenation of dinitroaromatics, with the highest performance attainable for the catalyst bearing Au particles of ca. 7.5 nm. Results in the intermolecular competitive hydrogenation showed that the intrinsic higher accumulation rates of the desired nitroanilines associated with the catalyst possessing larger Au particles were responsible for the high chemoselectivity observed. the Partner Organisations 2014.

Full text of DOI:10.1039/c4gc00869c

Green Chemistry
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PAPER
Gold supported on titania for specific
monohydrogenation of dinitroaromatics
in the liquid phase†
Cite this: Green Chem., 2014, 16,
162
4
Shuang-Shuang Liu, Xiang Liu, Lei Yu, Yong-Mei Liu,* He-Yong He and Yong Cao*
Liquid-phase selective monohydrogenation of various substituted dinitroaromatics to the corresponding
valuable nitroanilines was investigated on gold-based catalysts. Special attention was paid to the effect of
2
Au particle size on this monoreduction reaction. Interestingly, TiO supported gold catalysts containing a
relatively larger mean Au particle size (>5 nm) showed far superior chemoselectivity for specific mono-
hydrogenation of dinitroaromatics, with the highest performance attainable for the catalyst bearing Au
particles of ca. 7.5 nm. Results in the intermolecular competitive hydrogenation showed that the intrinsic
higher accumulation rates of the desired nitroanilines associated with the catalyst possessing larger Au
particles were responsible for the high chemoselectivity observed.
Received 13th May 2014,
Accepted 25th June 2014
DOI: 10.1039/c4gc00869c
www.rsc.org/greenchem
a new, efficient and non-pyrophoric catalytic system capable of
highly selective monohydrogenation of dinitroaromatics under
Introduction
Nitroanilines represent an attractive target for organic syn- mild, practical and convenient conditions is highly desired.
thesis and are important intermediates for dyes, pharmaceuti- In recent years, catalysts based on supported gold nano-
cals and agrochemicals because of their inherently reactive particles (NPs) have attracted considerable attention as new
1
nature as highly versatile synthons. Despite many known promising nitro hydrogenation materials owing to their high
methods, there is ongoing interest in the development of con- intrinsic selectivity toward the formation of the desired amino
9
venient and general protocols for the synthesis of these com- compounds. This was prompted by Corma’s discovery of
pounds. Hence, in recent years, novel approaches such as exceptionally high selectivity of TiO supported gold NPs for
2
palladium-catalyzed one-pot conversion of nitro compounds the hydrogenation of nitroaromatics in the presence of other
2
3
10
with silanes and ammonolysis of nitrobenzenes have been reducible functional groups. We have recently contributed to
developed. In spite of all these achievements, the selective the field of Au-catalyzed nitro reductions by discovering that
monoreduction of readily available dinitroaromatics represents the use of CO/H O or ammonium formate as a reducing agent
2
the most straightforward and efficient route to this class of can facilitate more efficient chemoselective nitro reduction
4
11
compounds. Known reductions of dinitroaromatics make use under much milder conditions. Whereas currently much
5
of stoichiometric amounts of sulfide reagents or iron powder.
attention has been focused on new improved protocols for
Recently, some homogeneous as well as heterogeneous cata- selective nitro reduction, reports on the use of Au for the
lytic systems in combination with different hydrogen donors monoreduction of dinitroaromatics are scarce. In a precedent
6
have been developed. The most ideal reductant for this related to this work, Keane and co-workers have shown that
2 2
purpose is molecular hydrogen (H ), because, theoretically, monoreduction of dinitroaromatics under a gas phase H
7
water is the only by-product. Although there are a few success- atmosphere is possible over Au supported on Al O or TiO ,
2
3
2
e,f
8
ful reports on the selective monoreduction of dinitroaromatics but an elevated temperature (commonly >200 °C) is required.
using H , the applied catalytic metals such as palladium and This is not favourable for the synthesis of complex, thermo-
2
8
ruthenium are flammable when exposed to air, and, in many labile N-containing compounds typical in fine chemistry. A sus-
cases, special handling is required. Thus, the development of tainable and industrially friendly low temperature process for
the highly selective monohydrogenation of dinitroaromatics is
thus of great fundamental as well as practical interest.
With our ongoing interest in selective nitro reduction cata-
lyzed by a heterogeneous catalyst, we herein describe the
unique catalytic behaviour of supported Au catalysts in the
liquid phase mono-hydrogenation of dinitroaromatics. We
Shanghai Key Laboratory of Molecular Catalysis and Innovative Materials,
Department of Chemistry, Fudan University, Shanghai 200433, P. R. China.
E-mail: yongcao@fudan.edu.cn, ymliu@fudan.edu.cn; Fax: (+86-21) 65643774
†
Electronic supplementary information (ESI) available: Experimental data, TEM,
1
13
H and C NMR data. See DOI: 10.1039/c4gc00869c
4162 | Green Chem., 2014, 16, 4162–4169
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demonstrate for the first time that, by fine tuning the particle
size of Au NPs deposited on an inorganic support such as
TiO , it is possible to obtain valuable nitroanilines in high
2
yields via selective monohydrogenation of the corresponding
dinitroaromatics under mild conditions. In contrast to the
trends identified in the traditional Au-based nitro hydrogen-
1
2
ation systems, we have found that catalysts containing a rela-
tively larger mean particle size (>5 nm) showed higher efficacy
in the selective monoreduction of dinitroaromatics than cata-
lysts containing a smaller particle size (<5 nm). Notably, the
use of TiO -supported 7.5 nm gold NPs (Au-7.5/TiO ) turned
2
2
out to be the most effective catalyst for high-yielding mono-
reduction of a diversity of dinitroaromatics to nitroanilines. The
catalytic system presented here, in which fine tuning of the
product composition can be achieved by simply varying the Au
particle size, may contribute to the design of new efficient and Fig. 1 XRD diffractograms: (I) Au-1.8/TiO
2
2
, (II) Au-3.9/TiO , (III) Au-5.8/
TiO
2
, (IV) Au-7.5/TiO
2
, and (V) Au-9.3/TiO
2
.
green catalytic systems for clean synthesis of nitroanilines and
related derivatives under mild and convenient conditions.
to 10 nm. X-ray photoelectron spectroscopy (XPS) was used to
probe the oxidation state(s) of gold deposited on the surface of
the TiO2 support (Fig. 3). All samples showed features at
Results and discussion
Synthesis and characterization
83.8 eV and 87.3 eV in a 4 : 3 peak area ratio, which were
readily assigned to the 4f and Au 4f5/2 peaks, respectively, of
7/2
Two general methods were employed for the synthesis of
0
metallic gold (i.e. Au ). Additional information on the surface
a series of TiO
meters (Au-x/TiO
nm) of metallic Au species): for the sample with a Au size
smaller than 3 nm, a routine NaOH deposition–precipitation
DP) procedure was used (method A), whereas for the sample
containing Au NPs with a size larger than 3 nm, a colloidal
2
supported Au NPs with different mean dia-
electronic states can be obtained from UV-Vis diffuse reflec-
2
, where x denotes the average particle size
14
tance measurements. As depicted in Fig. 4, the Au-x/TiO cat-
2
(
alysts exhibit a band with the peak in the wavelength range of
500–600 nm. This band is due to the surface plasmon reso-
(
nance of the metallic Au NPs and is affected by the mor-
phology of gold particles and the dielectric properties of the
1
3
deposition approach was employed (method B). Table 1
reports the gold loading in the catalysts determined by
ICP-AES, as well as the other main characteristics of these cata-
lysts. Note that the gold loading of the DP-derived samples is
consistently lower than that obtained by method B, indicating
that the latter is more efficient in terms of depositing Au par-
15
chemical environment. At this point it is interesting to
mention that there is a distinct shift in the peak maximum to
16
longer wavelengths as the Au particle size increases.
Catalytic performance
ticles onto the support. Fig. 1 shows the X-ray diffraction Following most of the previous work on dinitroaromatic
8
(
XRD) patterns of the as-prepared Au-x/TiO catalysts. It may reduction reactions, we also selected the m-dinitrobenzene
2
be noted here that compared with the XRD patterns of the (m-DNB) used as a model substrate. The problem of this par-
parent support material, the addition of Au produced no struc- ticular transformation is the large variety of reduction pro-
tural changes in the TiO2 phases. Meanwhile, no diffraction ducts that can possibly be formed (Scheme 1). Besides
lines of metal gold were observed for these Au-based catalysts.
m-nitroaniline (m-NA), m-phenylenediamine (m-PDA), azo and
Transmission electron microscopy (TEM) measurements azoxy intermediates are also typical products formed in the
1
7
were then carried out to identify the possible structures of the monoreduction. The target is to develop a selective process
five different samples. As illustrated in Fig. 2, the Au NPs are for the transformation of m-DNB into m-NA avoiding deep
all narrowly distributed and were varied in the range from 1.8 hydrogenation to m-PDA. In the preliminary study focused on
the nature of the underlying support on the performance of a
variety of supported Au NPs, the conversion of m-DNB was
Table 1 Main characteristics of the as-synthesized Au/TiO
2
catalysts
kept below 50% and observation of various reduction products
Au (wt%) UV-vis Amax (nm) BET (m g−1) DP (nm) derived from the reaction with various supported catalysts
a
2
b
Catalyst
2
under 3 MPa H at 60 °C is summarized in Table 2. Among the
Au-1.8/TiO
Au-3.9/TiO
Au-5.8/TiO
Au-7.5/TiO
Au-9.3/TiO
2
2
2
2
2
0.56
0.94
0.95
0.92
0.94
559
564
570
579
582
48
41
39
40
37
1.8
3.9
5.8
7.5
9.3
series of catalysts tested, the best performing catalyst was the
benchmark titania supported Au catalyst Au-M/TiO2 (average
gold particle size ∼2.8 nm, supplied by Mintek), which
favoured the formation of m-NA with the selectivity of 64% at
48% m-DNB conversion (Table 2, entry 1), and only traces of
a
b
Based on ICP-AES analysis. Based on TEM analysis.
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Fig. 3 XP spectra of the Au (4f) signal for Au/TiO
2
2
: (I) Au-1.8/TiO , (II)
Au-3.9/TiO
2
, (III) Au-5.8/TiO
2
, (IV) Au-7.5/TiO
2
, and (V) Au-9.3/TiO
2
.
Fig. 4 UV-vis spectra of (I) Au-1.8/TiO
2
2
, (II) Au-3.9/TiO , (III) Au-5.8/
TiO , (IV) Au-7.5/TiO , and (V) Au-9.3/TiO .
2
2
2
Fig. 2 Representative (A) TEM images and (B) Au particle size distri-
butions of (I) Au-1.8/TiO
2
, (II) Au-3.9/TiO
2
respectively.
2 2
, (III) Au-5.8/TiO , (IV) Au-7.5/
TiO , and (V) Au-9.3/TiO
2
azo and azoxy intermediates were generated. While gold with
similar average particle size (ca. 3 nm, see Table S1†) sup-
ported on other supports, such as Au/CeO , Au/ZnO, Au/Al O ,
2
2 3
2
and Au/ZrO , also exhibited an appreciable activity for
reduction of m-DNB, the formation of highly toxic azo and
azoxy-derivatives represent the main drawbacks. Moreover, it
should be noted that previously reported catalytic metals, such
as Pt, Pd, and Ru, showed good activity but much inferior
selectivity in terms of m-NA formation relative to Au under the Scheme 1 Simplified pathway for hydrogenation of m-DNB.
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a
Table 2 Liquid phase reduction of m-DNB to m-NA with H
2
in the presence of various catalysts
b
Sel. [%]
b
c
c
Entry
Catalyst
T [min]
Conv. [%]
m-NA
m-PDA
m-NNB
Azo
Azoxy
1
2
3
4
5
6
7
8
9
Au/TiO
Au/ZnO
Au/Al
Au/Fe
Au/CeO
Au/ZrO
2
25
30
20
30
30
30
30
30
30
15
20
25
48
32
50
6
28
48
11
n.r.
n.r.
45
40
47
64
22
40
66
1
51
65
—
—
35
34
45
18
0
5
0
0
1
1
—
—
52
49
32
16
9
16
19
1
20
13
—
—
2
<1
8
9
6
6
4
9
—
—
4
<1
61
30
9
92
25
12
—
—
7
2
O
3
2
O
2
3
2
Au/SiO
Au/C
2
TiO
Pt/TiO
Pd/TiO
Ru/TiO
2
1
1
1
0
1
2
2
2
2
5
5
7
10
11
2
a
b
2
Reaction conditions: 0.5 mmol m-DNB, metal: 0.5 mol%, 5 mL ethanol, 3 MPa H , 60 °C. n.r. = no reaction. Conversion and selectivity
c
determined by GC. Azo stands for azo intermediate; azoxy stands for the azoxy intermediate.
present reaction conditions (Table 2, entries 10–12). In
addition, no conversion was found in the presence of the Au-
free TiO catalyst under identical conditions (Table 2, entry 9),
2
illustrating that the presence of gold was indispensable for
high catalytic activity of the title reaction.
The distinct advantages of Au/TiO prompted us to investi-
2
gate this system in more detail. It is noteworthy that the mod-
erate selectivity for m-NA (up to 66%) is maintained at up to
9
8% conversion of m-DNB within 1 hour with Au-M/TiO
2
(Table S2†). To improve the yield of the target product, screen-
ing of the reaction conditions was then carried out. Firstly,
studies on the nature of the solvent revealed that ethanol was
the solvent of choice (Table S2†). When the solvent was
changed to CH Cl , toluene, or dioxane, the conversion of
2
2
m-DNB decreased to 29%, 40%, and 31%, respectively. Sub-
sequently, the effect of hydrogen pressure on the hydrogenation
of m-DNB was tested at 60 °C (Table S3†). With increasing
hydrogen pressure from 1 to 4 MPa at 60 °C, only a marginal
advance of selectivity for m-NA from 59 to 67% was attained.
Fig. 5 Effect of the particle size in hydrogenation of m-DNB with
various Au/TiO catalysts. Reaction conditions: 0.5 mmol m-DNB,
.5 mol% Au, 5 mL ethanol, 3 MPa H , 60 °C.
2
0
2
Further investigation on the effect of the reaction temperature been detected as a result of experimental limitations, a closer
at 3 MPa revealed that the rates of the hydrogenation could be comparison of the corresponding reaction kinetics indicates a
greatly accelerated at higher temperatures (from 40 to 80 °C), as rapid attenuation of the m-DNB conversion rates with increas-
in the case of hydrogen pressure, temperature did not affect ing Au particle size. The most interesting aspect, however, is
selectivity much but affected the reaction rate (Table S3†). The that significant m-NA selectivity enhancement could be
reaction conditions with 0.5 mol% of Au/TiO at 60 °C under achieved with the catalysts containing a relatively larger mean
2
3
MPa H were used for further optimization studies.
Au particle size (>5 nm). Among them, the best performing
2
Bearing in mind that the particle size has proven especially catalyst, Au-7.5/TiO , produces the maximum yield (97%) of m-
2
1
8
influential for supported gold catalysts, we initiated a syste- NA within 4.5 hours. This result is remarkable, and becomes
matic study of a set of TiO supported Au samples with varied more relevant as the catalyst is applicable for a gram-scale
2
mean Au particle sizes ranging from ca. 2 to 10 nm. Consistent reaction of m-DNB (20 mmol scale up) for 76 h, where 98%
with most of the previous work on Au-catalyzed hydrogenation yield of m-NA was obtained (Scheme S1†). Under these con-
1
9
−1
reactions, a prominent size-dependent behavior has been ditions, an exceptionally high initial TOF value of 385 h was
−
1
identified for m-DNB conversion over these catalysts (Fig. 5). calculated. This value compares favorably with 238 h
The reaction profiles (Fig. 6) for three representative reported recently on an Au/TiO2 catalyst prepared by simple
Au-1.8/TiO , Au-3.9/TiO and Au-7.5/TiO samples showed that impregnation for the gas phase hydrogenation of m-DNB (reac-
2
2
2
8
f
the hydrogenation of m-DNB occurred through a step-wise tion at 200 °C).
process via intermediacy of m-nitrosonitrobenzene (m-NNB).
The outstanding catalytic efficiency for m-DNB monohydro-
While no significant formation of other intermediates has genation under the above mentioned test conditions over the
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Scheme 2 Simplified reaction scheme for hydrogenation of m-DNB.
1 2
the present study, specific activities r and r were respectively
measured using m-DNB and m-NA as reactants over these
samples. Shown in Scheme 2 is the simplified reaction pathway
involving two sequential steps for m-DNB hydrogenation. As
shown in Fig. 7, a good correlation is observed between the
specific activity ratio r
stressed that, r /r varies between different particle sizes. The
highest value of r /r , corresponding to the highest m-NA selec-
1 2
/r and the Au particle size. It should be
1
2
1
2
tivity, is achieved with the largest Au particle size catalyst, con-
sistent with the superior selectivity as found for larger Au
particles with respect to smaller ones. These results are consist-
ent with a dependence of selectivity on the gold particle size as
shown in Fig. 5. Taken together, it appears that the Au-7.5/TiO
2
catalyst can substantially facilitate the cumulative formation of
m-NA relative to that of m-PDA, presumably via a preferential m-
DNB adsorption/activation relative to other reaction intermedi-
ates, which appears to be the key factor for achieving the
superior selectivity for m-NA in this reaction.
As stability is critical for the efficient use of any catalyst, the
reusability of the Au-7.5/TiO sample was investigated in the
2
hydrogenation of m-DNB. After the first hydrogenation was
complete, the crude reaction mixture was allowed to settle and
the supernatant clear product mixture was removed from the
reactor. A fresh charge of reactants was added to the catalyst
residue retained in the reactor and the subsequent run was
continued. This procedure was followed for four subsequent
runs and the results are shown in Fig. 8. Our Au-7.5/TiO
2
cata-
lyst showed almost the same selectivity with slight decrease in
Fig. 6 Time course of the hydrogenation of m-DNB over (a) Au-1.8/
TiO
2
, (b) Au-3.9/TiO
2
, (c) Au-7.5/TiO
2
. Reaction conditions: 0.5 mmol
, 60 °C.
m-DNB, 0.5 mol% Au, 5 mL ethanol, 3 MPa H
2
2
Au-7.5/TiO catalyst clearly reflects a delicate balance between
the various kinetic rates associated with the title reaction. Very
recently, on investigating the effect of Au particle size on gas
2
phase m-DNB hydrogenation over a series of Au/TiO catalysts
prepared by a simple impregnation method, Keane et al. dis-
closed that the dependence of hydrogenation performance on
the Au particle size can be accounted for by the resulting elec-
tronic modification of the Au particles, which is suggestive of
Fig. 7
of product (m-NA) accumulation; r
m-PDA) generation. Reaction conditions: 0.5 mmol m-DNB, 0.5 mmol
better understand the essential role of the Au particle size in m-NA, 0.5 mol% Au, 5 mL ethanol, 3 MPa H
, 60 °C.
r
1
/r
2
as a function of the mean particle size. r
1
: the reaction rate
2
: the reaction rate of by-product
8
f
having a strong impact on m-DNB adsorption/activation. To
(
2
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Table 3 Selective monoreduction of dinitrobenzene derivatives
a
2
affording the corresponding nitroanilines over Au-7.5/TiO
b
b
Conv.
[%]
Sel.
[%]
Entry
1
Substance
Product
t [h]
4.5
99
97
2
2.5
7.5
2.5
4.5
>99
96
c
3
99
98
97
83
93
4
5
Fig. 8 Recycling of the catalyst for the synthesis of m-NA from m-DNB.
e
Reaction conditions: 0.5 mmol m-DNB, Au-7.5/TiO
2
(0.5 mol% Au with
, 60 °C, 4.5 h. (■) conversion
2
83
respect to m-DNB), 5 mL ethanol, 3 MPa H
of m-DNB, (□) selectivity to m-NA.
d
9
1
8
5
e
e
6
3.5
98
72
conversion for m-DNB mono-hydrogenation even after the
fourth recycle. Inductively coupled plasma (ICP-AES) analysis
showed the absence of Au species in the filtrate after the reac-
tion (detection limit: <7 ppb), confirming that no leaching
occurred during the reaction. XPS results (Fig. S2†) show no
obvious changes in the metallic state of Au after the above suc-
d
e
9
6
24
7
8
2.5
9.5
97
93
cessive runs. Moreover, TEM images of Au-7.5/TiO after the
2
reuse (Fig. S1†) showed that no aggregation of Au NPs
occurred, and the average diameters, as well as size distri-
butions of the Au NPs, were similar to those of the fresh one,
e
>99
89
d
e
supporting the durability of Au-7.5/TiO
experiments.
2
in the recycling
99
0
1
Building upon these results, we started to extend this novel
Au-based liquid hydrogenation protocol to a diverse range of
structural dinitrobenzene derivatives. As depicted in Table 3,
the reaction proceeds successfully and was remarkably selective
for the synthesis of a variety of corresponding nitroanilines,
regardless of the electron-withdrawing or electron-donating
character of the groups attached to the aromatic rings, indicat-
ing the wide scope and synthetic utility of this catalytic system.
The benefits of this catalytic process are more evident when
9
8.0
5.5
99
99
94
1
0
95
a
Reaction conditions: substrate (0.5 mmol), 0.5 mol% Au, 5 mL
b
ethanol,
2
3 MPa H , 60 °C. Conversion and selectivityc were
other easily reducible groups are present in the dinitro- determined by GC using n-dodecane as the internal standard. 5 mL
d
e
toluene. Total selectivity of nitroanilines. Ratio of two partially
hydrogenated isomers.
benzenes. For instance, the preferential formation of desired
nitroanilines with selectivity over 90% and at conversion levels
above 90% is observed when the nitrile or halide functional
groups are included within the reactants (Table 3, entries 6, 7,
groups, it is worth noting that 2,4-dinitrobenzene derivatives
might produce two isomeric partially hydrogenated products,
but still exhibited excellent selectivities for their mono-
reduction derivatives (Table 3, entries 5, 6, 8).
1
0), whereas the hydrogenation of these easily reducible groups
is avoided. In fact, the selective mono-reduction of dinitroben-
zene derivatives is practically independent of the position of the
substitutions within the reactants, as can be seen by comparing
the yields of nitroanilines when the NH , CH or a halide group
2
3
is bonded to the different position of the aromatic ring for
dinitrobenzenes (Table 3, entries 4–9). More interestingly, as the
Conclusions
complexity of the process notably increases in the case of dini- Titania supported gold catalysts were applied to the mono-
trobenzene compounds substituted with other functional hydrogenation of m-DNB to m-NA in the liquid phase. Investi-
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gation of recovery and reuse of the gold catalysts in the m-DNB the boiling sodium citrate suspensions and boiled under
reduction reaction, and careful microstructural analysis to reflux for 2 h, a colour change was observed, suspension
examine the distribution and the mean particle size of sup- initially white turned amaranth. Then, the solid was washed
ported gold NPs revealed an interesting relationship between thoroughly with deionized water and air-dried at 65 °C for
the mean gold particle size and chemoselectivity. Relatively 16 h. Subsequently, it was calcined either at 500 °C or 550 °C
large Au NPs (5 to 10 nm) seem to be more selective for mono- for 4 h in static air, the thus obtained catalysts were denoted
reduction of m-DNB than small NPs (<5 nm). The catalyst as Au-7.5/TiO
bearing the mean particle size of 7.5 nm showed the highest
performance. A wide diversity of the functionalized dinitro-
2 2
and Au-9.3/TiO , respectively.
Catalyst characterization
benzenes could be successfully reduced to the corresponding
nitroanilines using this reduction system. The catalytic system
Elemental analysis. The Au loading of the catalysts was
presented here, which enables highly specific monoreduction measured by inductively coupled plasma atomic emission
of dinitroaromatics under exceedingly mild conditions, may spectroscopy (ICP-AES) using a Thermo Electron IRIS Intrepid
contribute to the design of more efficient catalytic systems and II XSP spectrometer. The detection limit for Au is 7 ppb.
may provide a new strategy for the development of green
organic transformations through a careful choice of the tured CeO
X-ray diffraction (XRD). The crystal structures of meso-struc-
2
were characterized with powder X-ray diffraction
support and fine turning the particle size.
(XRD) on a Bruker D8 Advance X-ray diffractometer using the
Ni-filtered Cu Kα radiation source at 40 kV and 40 mA.
Transmission electron microscopy (TEM). TEM images for
the supported gold catalysts were taken with a JEOL 2011 elec-
tron microscope operating at 200 kV. Before being transferred
into the TEM chamber, the samples dispersed with ethanol
Experimental
Catalyst preparations
Method A for Au/TiO preparation. A modified deposition– and deposited onto a carbon-coated copper grid and then
2
precipitation (DP) procedure has been performed for the quickly moved into the vacuum evaporator. The size distri-
preparation of the Au-1.8/TiO
2
catalyst as follows: briefly, 1.0 g bution of the metal nano-clusters was determined by measur-
2
−1
TiO (Degussa P25, specific surface area: 45 m g , nonporous ing about 200 random particles on the images.
2
7
1
0% anatase and 30% rutile, purity >99.5%) was added to
00 mL of an appropriate amount of aqueous solution of powders were packed into a round cell with quartz window.
UV-vis diffuse reflectance (UV-vis DRS). The sample
HAuCl at a fixed pH = 8 by adjusting with 0.2 M NaOH. The UV/VIS-DRS measurements were performed by a Cary 400
4
mixture was aged for 2 h at 80 °C under vigorous stirring, after spectrometer (Varian) equipped with a praying mantis diffuse
which the suspension was cooled to room temperature. Exten- reflectance accessory (Harrick) using BaSO
4
as a reference.
sive washing with deionized water was then followed until it X-ray photoelectron spectroscopy (XPS). XPS data were
was free of chloride ions. The samples were dried under re-corded with a Perkin Elmer PHI 5000C system equipped
vacuum at room temperature for 12 h before calcination at with a hemispherical electron energy analyzer. The spectro-
3
00 °C for 4 h.
meter was operated at 15 kV and 20 mA, and a magnesium
Method B for Au/TiO
2
preparation. In a typical colloidal anode (Mg Kα, hν = 1253.6 eV) was used. The C 1s line
deposition (CD) preparation, to the aqueous solution of (284.6 eV) was used as the reference to calibrate the binding
HAuCl , an appropriate amount of PVA was added using a energies (BE).
4
mass ratio Au–PVA of 1.5 : 1 at 25 °C under vigorous stirring.
The obtained solution was then stirred vigorously for 10 min.
A subsequent rapid injection of an aqueous solution of NaBH4
Catalytic activity test
(0.1 M, with molar ratio Au–NaBH
4
of 1 : 5) resulted in the
General procedure for the liquid phase reduction of m-DNB
colour of the reaction mixture immediately turning from pale compounds. The mixture of m-DNB (0.5 mmol), appropriate
yellow to dark brown, indicating the formation of Au NPs. The amounts of catalysts (Au: 0.5 mol%), ethanol (5 mL) and
TiO
under stirring for 6 h. After that the solid was washed reactor. It was purged with H
thoroughly with deionized water and dried under vacuum at introduced to the desired pressure. The reaction temperature
5 °C overnight. Finally, the samples were calcined at 450 °C was maintained by a thermostat under continuous stirring of
2
support was then added to the colloidal gold solution n-dodecane (10 µL) as the internal standard was put into the
2
for five times and then H was
2
2
or 500 °C for 4 h in static air. The thus obtained catalysts were 800 rpm. For kinetic studies changes in hydrogen pressure
denoted as Au-3.9/TiO and Au-5.8/TiO , respectively. On the (1 to 4 MPa) and temperature (40 to 100 °C) were made. Liquid
2
2
other hand, the Au-7.5/TiO
2
2
and Au-9.3/TiO catalysts with a samples were taken periodically from the reactor; m-DNB con-
relatively larger mean Au particle size have been prepared version and product selectivity were determined by a GC-17A
according to the well-known citrate-reduction procedure: 0.5 g gas chromatograph equipped with a HP-5 column (30 m ×
TiO was added to 106 mL of 2.2 mM sodium citrate and was 0.25 mm) and a flame ionization detector (FID) using n-decane
2
boiled under reflux with vigorous stirring, then an appropriate as an internal standard. Reactants and products were identi-
amount of aqueous solution of HAuCl was quickly added to fied by comparison with samples, and GC–MS coupling.
4
4168 | Green Chem., 2014, 16, 4162–4169
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Paper
L. H. Lu, Chem. Lett., 2008, 37, 974; (c) F. Cárdenas-Lizana,
S. Gómez-Quero, N. Perret and M. A. Keane, Catal. Sci.
Technol., 2011, 1, 652; (d) F. Cárdenas-Lizana, S. Gómez-
Quero, C. J. Baddeley and M. A. Keane, Appl. Catal., A,
2010, 387, 155; (e) F. Cárdenas-Lizana, S. Gómez-Quero and
M. A. Keane, Catal. Lett., 2009, 127, 25; (f) F. Cárdenas-
Lizana, S. Gómez-Quero, H. Idriss and M. A. Keane,
J. Catal., 2009, 268, 223.
Acknowledgements
Financial support by the National Natural Science Foundation
of China (21273044), the Research Fund for the Doctoral
Program of Higher Education (2012007000011) and Science &
Technology Commission of Shanghai Municipality (08DZ2270
5
00 and 12ZR1401500) is kindly acknowledged.
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