Ferric hydroxide supported gold subnano clusters or quantum dots: Enhanced catalytic performance in chemoselective hydrogenation

Article abstract of DOI:10.1039/b716870e

An attempt to prepare ferric hydroxide supported Au subnano clusters via modified co-precipitation without any calcination was made. High resolution transmission electron microscopy (HRTEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the structure and chemical states of these catalysts. No Au species could be observed in the HRTEM image nor from the XRD pattern, suggesting that the sizes of the Au species in and on the ferric hydroxide support were less than or around 1 nm. Chemoselective hydrogenation of aromatic nitro compounds and α,β-unsaturated aldehydes was selected as a probe reaction to examine the catalytic properties of this catalyst. Under the same reaction conditions, such as 100 °C and 1 MPa H₂ in the hydrogenation of aromatic nitro compounds, a 96-99% conversion (except for 4-nitrobenzonitrile) with 99% selectivity was obtained over the ferric hydroxide supported Au catalyst, and the TOF values were 2-6 times higher than that of the corresponding ferric oxide supported catalyst with 3-5 nm size Au particles. For further evaluation of this Au catalyst in the hydrogenation of citral and cinnamaldehyde, selectivity towards unsaturated alcohols was 2-20 times higher than that of the corresponding ferric oxide Au catalyst. The Royal Society of Chemistry.

Full text of DOI:10.1039/b716870e

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Ferric hydroxide supported gold subnano clusters or quantum dots: enhanced
catalytic performance in chemoselective hydrogenation
Lequan Liu,a,b Botao Qiao,a,b Yubo Ma,a,b Juan Zhanga,b and Youquan Deng*a
Received 31st October 2007, Accepted 4th March 2008
First published as an Advance Article on the web 3rd April 2008
DOI: 10.1039/b716870e
An attempt to prepare ferric hydroxide supported Au subnano clusters via modified co-precipitation
without any calcination was made. High resolution transmission electron microscopy (HRTEM), X-ray
diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the
structure and chemical states of these catalysts. No Au species could be observed in the HRTEM image
nor from the XRD pattern, suggesting that the sizes of the Au species in and on the ferric hydroxide
support were less than or around 1 nm. Chemoselective hydrogenation of aromatic nitro compounds
and a,b-unsaturated aldehydes was selected as a probe reaction to examine the catalytic properties of
this catalyst. Under the same reaction conditions, such as 100 ◦C and 1 MPa H2 in the hydrogenation of
aromatic nitro compounds, a 96–99% conversion (except for 4-nitrobenzonitrile) with 99% selectivity
was obtained over the ferric hydroxide supported Au catalyst, and the TOF values were 2–6 times higher
than that of the corresponding ferric oxide supported catalyst with 3–5 nm size Au particles. For further
evaluation of this Au catalyst in the hydrogenation of citral and cinnamaldehyde, selectivity towards
unsaturated alcohols was 2–20 times higher than that of the corresponding ferric oxide Au catalyst.
hundreds or thousands of atoms, and are currently attracting
Introduction
considerable attention both in synthesis and application.19–26 It
In the past decade gold catalysts have aroused increasing interest
in the chemistry community,1–4 and numerous studies have been
has been found by Goodman et al. that Au clusters with one
dimension smaller than three atoms which was related to quantum
size effects were most active for low-temperature CO oxidation.1 In
another report of Au particles supported on a zinc oxide surface,
no significant adsorption of CO was observed until the Au particles
decreased to below 5 nm, and became stronger with the decrease in
Au particle size.14 In our previous study, we also found that ferric
hydroxide supported Au catalysts prepared by a co-precipitation
method without calcining could possess high performance for
carried out to expand new reactions catalyzed by supported nano
Au, such as the water–gas shift reaction,5 oxidation of alcohols,6
epoxidation of propene,7 direct synthesis of hydrogen peroxide,8
activation of cyclohexane,9 etc. Our research group also reported
that polymer supported gold could be an effective catalyst for
carbonylation and CO2 activation.10,11
Among factors proposed to affect the catalytic performance
of supported Au catalysts, reducing the dimension of the Au
particles was believed to be critical to convert “inert gold “ into
a highly active catalyst.12–14 This could be understood from both
physical and chemical respects. It is well known that quantum
size effects of noble metal particles can be remarkable when the
dimensions become comparable with their corresponding Fermi
electron wavelength15,16 (e.g. ∼ 0.5 nm for Au and Ag), which
could result in changes in physicochemical properties such as
optical and luminescent characteristics17 and catalytic properties18
in comparison with those of the bulk metal or even those of
metal particles > 1 nm in size. Meanwhile, remarkable surface
effects together with size effects, which become more evident as
decreasing particle size are also aspects proposed to affect the
catalytic performance. On the other hand, subnano clusters with
several to tens of atoms (the corresponding sizes are usually less
than or around 1 nm),19 possess unique physicochemical properties
rather different from a single atom as well as nano clusters with
27
selective oxidation of CO in the presence of H2 and the catalytic
activity for total CO oxidation is much higher than that of the
corresponding catalyst calcined at elevated temperatures. Similar
results were also observed by other groups.28–30 Taking into
account the unique properties of ultra-fine metal particles together
with the average sizes of Au species in these uncalcined and
calcined products being less than 1 nm and 4–20 nm respectively,
it can be proposed that the smaller size of Au species (so
called Au subnano clusters or quantum dots may be formed)
were critical for a superior catalytic performance, although
supports may also have a strong impact on the catalytic per-
formance.
Based on this strategy, in this work we report that ferric
hydroxide supported Au subnano cluster catalysts could be more
effective for the chemoselective hydrogenation of aromatic nitro
compounds containing carbonyl, halogen, olefin and ester as
well as nitrile groups, and a relatively high selectivity towards
unsaturated alcohols was also obtained in a further evaluation
of the hydrogenation of a,b-unsaturated aldehydes. Furthermore,
this preparation method is time and energy saving, and supported
Au catalysts prepared in this way may also be expanded into other
reactions with unexpected catalytic performance.
aCentre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou, 730000, China. E-mail:
ydeng@lzb.ac.cn; Fax: +86-931-4968116; Tel: +86-931-4968116
bGraduate University of Chinese Academy of Sciences, Beijing, 100039,
China
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Selective hydrogenation of substrates with a range of functional
groups represents an important process in the manufacture of
fine chemicals, such as dyes, cosmetics, pharmaceuticals, etc.31–33
Specific examples include the chemoselective hydrogeneation of
nitro componds and hydrogenation of a,b-unsaturated aldehydes
to the corresponding allylic alcohols. Aromatic amines, especially
in the presence of other functional groups are important industrial
intermediates for a variety of specific and fine chemicals.34
Catalytic hydrogenation of nitro compounds is a classic route
to afford the corresponding aromatic amines. The reduction
of simple nitro compounds is readily carried out with various
commercial catalysts, but the selective reduction of the nitro
group with H2 in the presence of other reducible groups is
more challenging. Very recently, Corma and Serna35 reported that
gold supported on TiO2 or Fe2O3 catalyzed the chemoselective
hydrogenation of nitroarenes, suggesting that properly prepared
oxide supported Au could be a versatile catalyst for various
reactions with excellent catalytic performance. In this work, we
report that Au subnano clusters supported on ferric hydroxide
exhibited enhanced catalytic performance in the hydrogenation of
nitro compounds (Scheme 1).
at 100 ◦C so as to ensure that no changes occurred in the support
structure. The other catalysts, Au/Fe2O3-4 and Au/Fe2O3-WGC,
were outgassed to 0.1 Pa at 200 ◦C.
Au loadings in the catalyst samples were measured using
atomic absorption spectroscopy (AA240, Varian). High resolution
transmission electron microscopic (HRTEM) investigations were
carried out using a JEOL JEM-2010 electron microscope. The
Au catalyst powder was suspended in toluene using ultrasonic
dispersion for 5–10 min and then the resulting solution was
dropped onto a holey-carbon film supported by a 300 mesh copper
grid.
X-Ray diffraction (XRD) measurements for the structure
determination were carried out on a Siemens D/max-RB powder
X-ray diffractometer. Diffraction patterns were recorded with Cu-
Ka radiation (40 mA, 40 kV) over a 2h-range of 10◦ to 75◦ and
a position-sentient detector using a step size of 0.01◦ and a step
time of 0.15 s.
The X-ray photoelectron (XPS) spectroscopy analyses were per-
formed with a VG ESCALAB 210 instrument. Mg-Ka radiation
at a pass energy of 150 eV at an energy scale calibrated versus
the C 1s peak at 284.6 eV arising from adventitious carbon were
used. The surface composition of the samples was determined
from the peak areas of the corresponding lines using a Shirley-
type background and empirical cross section factors for XPS. The
sample powders were mounted on double-sides adhesive tape. The
pressure in the analysis chamber was in the range of 10−9 torr
during data collection and the sample was maintained at such a
low pressure for 24 h without heating before being analyzed.
1H NMR and 13C NMR spectra were recored on an INOVA-
400 MHz NMR spectrometer, and the chemical shifts were
reported in parts per millon (ppm, d). Qualitative analyses were
also conducted with a HP 6890/5973 GC-MS with chemstation
containing a NIST mass spectral database.
Scheme 1 Selective hydrogenation of aromatic nitro compounds.
Experimental
Catalyst preparation
A series of ferric hydroxide or oxide supported catalysts with differ-
ent Au loadings (1.0–1.1 wt%) were prepared by co-precipitation.
Dilute aqueous solutions of HAuCl4·4H2O (0.1 g ml−1) and
Fe(NO3)3·9H2O (1 mol l−1) were mixed uniformly, then added
drop-wise into aqueous Na2CO3 solution (1 mol l−1) with vigorous
stirring at 20 ◦C until the pH of the solution was adjusted to 8.0–
8.5. After 2 h stirring and 2.5 h aging, the resulting precipitate
was filtered off and washed with distilled water, and dried at room
temperature (ca. 15–20 ◦C) for 24 h. Then, the resulting ferric
hydroxide supported Au was treated in two different ways: (i)
dried at 110 ◦C for 0.5 h (denoted as Au/Fe(OH)x) or (ii) calcined
at 400 ◦C for 2 h (denoted as Au/Fe2O3-4). For the propose of
comparison, 2.6 wt% Au/Co(OH)x as well as blank Fe(OH)x
Catalytic performance testing
Catalytic experiments of chemoselective hydrogenation of nitro
compounds were carried out in a 90 ml autoclave with a glass tube
inside equipped with magnetic stirring. In each reaction, 0.2 g sub-
strate and 4 ml tetrahydrofuran (ethanol for 2-chloronitrobenzene,
distilled before use) as solvent were placed into the autoclave
together with 0.1 g catalyst. After being sealed, the reactors were
flushed three times with 4 MPa hydrogen and then pressurized at
1 or 2.5 MPa. Finally, each autoclave was heated to the required
temperature (90–150 ◦C) and a slight increase in pressure was
observed. No further hydrogen was added to the autoclaves during
the experiment.
◦
were prepared in this manner, dried at 110 C for 0.5 h without
Selective hydrogenations of a,b-unsaturated aldehydes were
carried out in a 90 ml autoclave equipped with magnetic stirring.
A mixture of 4 ml alcohol as solvent, 0.2 g substrate, 0.085 g
1-butanol or 1-octanol serves as internal standard and 0.1 g as-
prepared catalyst were successfully charged into the autoclave.
After being flushed two times with 4 MPa hydrogen, the re◦action
mixtures were pressured at 2 MPa and heated to 65–100 C for
different substrates.
being calcined at higher temperatures. 4.4 wt% Au/Fe2O3 bought
from the World Gold Council (denoted as Au/Fe2O3-WGC) and
Raney-Ni prepared with reference to an earlier report36 were also
employed. All reagents used in this procedure are analytical grade
and were used without further purification.
Sample characterization
BET surface areas (SBET) were obtained by physisorption of N2
at 77 K using a Micromeritics ASAP 2010. It must be mentioned
that the Au/Fe(OH)x catalyst samples were outgassed to 0.1 Pa
Quantitative analyses were conducted with a HP 6820 GC
equipped with a FID detector. The GC yields were obtained using
an internal standard method.
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Table 1 Basic physicochemical properties of the supported Au catalysts
BET surface area/m2 g−1
Mean pore diameter/A
˚
Catalysts
Au loading/wt%
Support
Au/Fe(OH)x
Au/Fe2O3-4
Au/Fe0O3-WGC
1.0
1.1
4.4
Fe(OH)x
a-Fe2O3
a-Fe2O3
243.3
20.1
39.0
32.7
106.6
109.1
Results and discussion
Typical data such as Au loadings, BET surface areas, etc. are
summarized in Table 1.
It is observed that Au/Fe(OH)x possesses a relatively higher
BET surface area of 243.3 m2 g−1, while after being calcined at
400 ◦C the BET surface area decreased greatly to 20.1 m2 g−1 which
is comparable to that of Au/Fe2O3-WGC, 39.0 m2 g−1. Au loadings
were increased slightly after being calcined at elevated temperature
and this should be related to losing the adsorbed water and the
transformation of ferric hydroxide into ferric oxide, although the
same quantity of gold precursors were initially used. Meanwhile,
calcination significantly enlarged the mean pore diameters of
˚
˚
Au/Fe(OH)x, from 32.7 A to 106.6 A after being calcined at
400 ◦C.
Fig. 2 X-Ray diffraction patterns for supported Au catalysts: (a) 1.0 wt%
Au/Fe (OH)x, (b) 1.1 wt% Au/Fe2O3-4. (ꢀ) hematite; (ꢁ) metallic gold.
In order to get explicit details of Au particle size, 1.0 wt%
Au/Fe(OH)x and 1.1 wt% Au/Fe2O3-4 were examined with
HRTEM, Fig. 1. Au species were invisible over 1.0 wt%
Au/Fe(OH)x, suggesting that the Au species were highly dispersed
into or on Fe(OH)x, or more precisely, Au species in or on the
Fe(OH)x were subnano clusters containing a few Au atoms, and
no structural changes between 1.0 wt% Au/Fe(OH)x and pure
Fe(OH)x (not shown here) could be discerned. However, after
being calcined at 400 ◦C, Au species of 3–5 nm in size were clearly
visible and aggregation of the supports was directly observed.
at 38.2 and 44.4 (2h/◦) was noted. It can be concluded that,
after calcination at 400 ◦C, formation of a-Fe2O3 crystallites was
evident and Au crystallites could also be detected, which suggested
that calcination at elevated temperatures would cause Au species
aggregation.
To gain insight into the chemical states of the Au species, the as-
prepared catalysts were also characterized by X-ray photoelectron
(XPS) spectroscopy (Fig. 3). It could be seen that the chemical
state of the surface Au species on Au/Fe(OH)x was, as expected,
a mixture of Au+ (B.E. 4f7/2 = 86.0 eV) and metal Au (B.E. 4f7/2
=
83.8 eV), and the corresponding ato◦m ratio Au+/Au was 0.33.
However, after being calcined at 400 C, the Au species changed
Fig. 1 HRTEM images of supported gold catalysts: (a) 1.0 wt% Au/Fe
(OH)x, (b) 1.1 wt% Au/Fe2O3-4.
Structural characteristics of the catalysts have also been inves-
tigated by X-ray diffraction (XRD). Fig. 2a showed no evident
characteristic diffraction peaks of Au, indicating that the Au
species in Au/Fe(OH)x were ultra-highly dispersed, which was
consistent with the HRTEM image. Besides, it could also be seen
that ferric hydroxide in this low-temperature prepared catalyst
was amorphous. These results are in agreement with our previous
findings.37 After being calcined at 400 ◦C, XRD patterns showed
significante changes in Au/Fe2O3-4, Fig. 2b. It showed a well-
resolved reflection pattern which was attributed to hematite with
the main diffraction peaks at 24.2, 33.2, 35.6, 49.5 and 54.1 (2h/◦).
Moreover, the characteristic diffraction pattern of metallic gold
Fig. 3 XPS spectra of supported Au catalysts: (a) 1.0 wt% Au/Fe (OH)x,
(b) 1.1 wt% Au/Fe2O3-4.
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to mainly to metal Au (B.E. 4f7/2 = 83.7 eV), which was almost
identical to the Au/Fe2O3-WGC (B.E. 4f7/2 = 83.6 eV).
Chemoselective hydrogenation of nitro compounds
Catalytic performances of the as-prepared ferric hydroxide
supported Au catalysts as well as other catalysts treated in
traditional manners were tested. Firstly, hydrogenation of 4-
nitrobenzaldehyde was tested, Table 2. Pure Fe(OH)x gave only
lower activity, but after incorporation of Au into the Fe(OH)x, the
hydrogenation activity increased greatly and 96% conversion with
selectivity of 99% could be achieved over 1.0 wt% Au/Fe(OH)x.
Conversions decreased sharply over 1.1 wt% Au/Fe2O3-4 and
moderately over 4.4 wt% Au/Fe2O3-WGC, though with com-
parable selectivity for 4-aminobenzaldehyde. When the amount
of 4.4 wt% Au/Fe2O3-WGC charged was increased to 0.25 g,
>99% conversion with selectivity of 98% was obtained. This
result was consistent with an earlier report.12 Trace amounts of (4-
aminophenyl)methanol, 4-aminotoluene and p-nitrotoluene were
detected as by-products over 1.0 wt% Au/Fe(OH)x, while the main
by-products resulting from Au/Fe2O3-4 and Au/Fe2O3-WGC were
4-aminotoluene (12% and 0.6% respectively) and p-nitrotoluene
(4.7% and 0.3% respectively), indicating those catalysts calcined
at elevated temperatures have similar selectivity. In the case of
blank Fe(OH)x, 51.8% 4-aminotoluene and 38.2% p-nitrotoluene
were also detected. From the composition of byproducts, it can
be speculated that hydrogenation of 4-nitrobenzaldehyde over
nano Au began at the nitro group, then a small amount of
carbonyl hydrogenation occurred, and p-nitrotoluene may have
resulted from active sites on the support. It is worth noting
that the TOF value over 1.0 wt% Au/Fe(OH)x reached 251,
which, though relatively low, is still six times higher than that
of 4.4 wt% Au/Fe2O3-WGC. As for 2.6 wt% Au/Co(OH)x,
inferior catalytic performance both in activity and selectivity was
obtained in comparison with 1.0 wt% Au/Fe(OH)x, indicating
that the support also plays an important role in catalytic activity.
In the case of Raney-Ni, 19% (4-aminophenyl)methanol and
16.3% complete hydrogenation by-product, 4-aminotoluene, were
detected, indicating a poor selectivity for the desired amine.
Hydrogenation of aromatic nitro compounds with carbonyl,
chlorine, alkene and nitrile groups over these catalysts was
also investigated. Propably due to electron-accepting effects,
p-nitroacetophenone,
2-chloronitrobenzene
and
ethyl-4-
nitrocinnamate could be selectively hydrogenated under more
mild reaction conditions as compared with the hydrogenation
of 4-nitrobenzaldehyde. As can be seen, >99% conversions
with high selectivities were obtained in hydrogenation of the
substrates over 1.0 wt% Au/Fe(OH)x. The main by-products
resulting from 1.0 wt% Au/Fe(OH)x in the hydrogenation
of p-nitroacetophenone were 4-ethylbenzenamine and 4-
vinylbenzenamine, while 0.1% and 14% 4-ethylbenzenamine as
the only byproduct were detected over Au/Fe2O3-4 and Au/Fe2O3-
WGC respectively. As for ethyl-4-nitrocinnamate, the reduction
of the nitro group without further hydrogenation of the alkene
and ester carbonyl can also be successfully reached. Catalytic hy-
drogenation of 2-chloronitrobenzene to afford the corresponding
chloroaniline is an important procedure affording intermediates
for fine chemical use, such as pharmaceuticals, dyes, herbicides and
pesticides.38 Investigations have been carried out over Pt,39,40 Ag41
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and Au42,43 catalysts in order to avoid the accompanying pollution
resulting from the use of acid agents in stoichiometric reducing re-
actions, or from the use of toxic and unsafe hydrogenation agents,
such as hydrazine. Nevertheless, undesired hydrodechlorination
reactions are usually observed over those metal catalysts.38,39,44 In
our case, >99% conversion with no dechlorination reaction was
successfully achieved under a low hydrogen pressure and reaction
temperature, while distinct decreases in conversion over 1.1 wt%
Au/Fe2O3-4 and 4.4 wt% Au/Fe2O3-WGC were obtained. The
only by-product in this reaction was N-ethyl-chloroaniline and
its proportion increased with the calcination temperature the
catalysts been treated at, from 0.5% for Au/Fe(OH)x to 20%
for Au/Fe2O3-4. It is worth noting that it has been reported
that the by-product N-ethyl-chloroaniline could not be further
hydrogenated to the desired product by increasing the reaction
time.42 In the case of 4-nitrobenzonitrile, when the reaction was
carried out at 100 ◦C for 2 h, only 15% of conversion was obtained.
It increased to 48% upon raising the reaction temperature to
150 ◦C, indicating the harsh reaction conditions required
for substrates with nitrile groups, which were inconsistent with
another report.35 Characterizations of the main products are given
below.
Scheme 2 Transformation of the a,b-unsaturated aldehydes.
has been found that supported Au catalysts exhibited potential
application in controlling the intermolecular selectivity in the
hydrogenation of a,b-unsaturated aldehydes,53–54 and a number
of studies have been carried out.55–62 In this work, we further
evaluated the catalytic properties of ferric hydroxide supported Au
catalyst in the hydrogenation of acrolein, citral and cinnamalde-
hyde.
The results obtained for the hydrogenation of these a,b-
unsaturated aldehydes on gold supported catalysts are reported
in Table 3. As can be seen, in the case of acrolein, no allyl alcohol
was obtained on Au/Fe(OH)x, Au/Fe2O3-4 and Au/Fe2O3-WGC
catalysts under the present reaction conditions, and propanal
became the main product in all these cases. This may be caused by
the high hydrogen coverage which was reported to be harmful
to the chemoselectivity towards unsaturated alcohols.63 As for
the hydrogenation of citral, the main products obtained on
Au/Fe(OH)x were unsaturated alcohols (geraniol and nerol, E
and Z forms of 3,7-dimethyl-2,6-octadienol, respectively), derived
from the hydrogenation of theE and Z isomers of citral. Citronellal
1
4-Aminobenzaldehyde: H NMR (400 MHz, CDCl3): d 9.70 (s,
1H), 7.65 (d, J = 6.60 Hz, 2H), 6.64 (d, J = 6.59 Hz, 2H), 4.60 (bs,
2H); 13C NMR (100 MHz, CDCl3): d 190.3, 152.6, 132.2, 127.0,
113.9; MS m/z: 122 (M+ + 1, 5%), 106 (M+ − 15, 100%).
1
1-(4-Aminophenyl)ethanone: H NMR (400 MHz, CDCl3): d
7.79 (d, J = 6.69 Hz, 2H), 6.63 (d, J = 6.74 Hz, 2H), 4.16 (bs, 2H),
2.48 (s, 3H); 13C NMR (100 MHz, CDCl3): d 196.5, 151.1, 130.8,
127.7, 113.6, 26.1; MS m/z: 135 (M+, 48%), 120 (M+ − 15, 100%).
2-Chlorobenzenamine: 1H NMR (400 MHz, CDCl3): d 6.60–7.30
(m, 4H, C6H4), 3.7 (bs, 2H); 13C NMR (100 MHz, CDCl3): 142.9,
129.4, 127.6, 119.3, 119.0, 115.8; MS m/z: 127 (M+, 100%), 92
(M+ − 35, 20%).
Ethyl 3-(4-aminophenyl)acrylate: 1H NMR (400 MHz, CDCl3):
d 7.60 (d, J = 16.1 Hz, 1H), 7.32 (d, J = 8.2 Hz, 2H), 6.61 (d,
J = 8.1 Hz, 2H), 6.19 (d, J = 16.0 Hz, 1H), 4.17 (q, J = 7.1 Hz,
2H), 3.90 (br, 2H), 1.32 (t, J = 6.8 Hz, 3H); 13C NMR (100 MHz,
CDCl3): d 167.7, 148.6, 144.8, 140.5, 129.8, 128.6, 124.1, 122.5,
113.7, 61.0, 60.1; MS m/z: 191 (M+, 71%), 146 (M+ − 45, 100%).
=
(3,7-dimethyl-6-octenal) derived from the hydrogenation of C C
=
adjacent to C O in citral and citronellol (3,7-dimethyl-6-octenol)
were the other products. On Au/Fe2O3-4 catalyst, citronellal
became the main product accompanied by an obvious decrease
in the proportion of unsaturated alcohols, from 63% to 25%. As
for Au/Fe2O3-WGC, a similar hydrogenation product distribution
was obtained. The catalytic activity of Au/Fe2O3-4 is slightly
increased as compared with Au/Fe(OH)x, which may be caused
by the increase in Au particle size as reported by Claus et al.64
However, it is worth noting that the unsaturated alcohols, geraniol
and nerol, are the main products of Au/Fe(OH)x, indicating the
high selectivity of this catalyst towards the hydrogenation of the
Hydrogenation of a,b-unsaturated aldehydes
Selective hydrogenation of a,b-unsaturated aldehydes to the
corresponding unsaturated alcohols attracts much interest in
heterogeneous catalyt research owing to its importance in the fine
chemical, pharmaceutical and cosmetic industries.31,32,45 However,
=
from the aspect of thermodynamics, hydrogenation of the C C
=
=
bond is favoured over the C O group (stronger negative free
C O bond.
reaction enthalpy of 35 kJ mol−1). Besides, for kinetic reasons,
In the hydrogenation of cinnamaldehyde (E-3-phenyl-
propenal), Au/Fe(OH)x exhibited a relatively high selectivity of
82% towards the cinnamic alcohol (E-3-phenylprop-2-enol). As
for Au/Fe2O3-4 and Au/Fe2O3-WGC catalysts with 3–5 nm size
Au particles, hydrogenation of cinnamaldehyde led mainly to the
formation of saturated aldehyde, and the selectivity to cinnamic
alcohol was relatively low, 4% and 13% respectively. It seems that
selectivities toward the formation of unsaturated alcohol can be,
at least to a certain degree, correlated with the morphological
properties of gold, particularly the Au particle size.
=
=
the C C bond is more reactive than the bond of the C O group.
Consequently, these conjugated functional groups are particularly
difficult to hydrogenate to the corresponding unsaturated alcohols,
and the saturated aldehyde or saturated alcohol are often the
exclusive reaction products. A simple illustration is shown in
Scheme 2.
=
Selective hydrogenation of C O in a,b-unsaturated aldehydes
has only been achieved in a number of cases, for example with
heterogeneous Pt, Ag, Co and Ru catalysts.46–52 In recent years, it
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Table 3 Comparison of the catalytic performances for chemoselective hydrogenation of a,b-unsaturated aldehydes
Selectivity (%)
Au
Reaction
time/h
Saturated
Saturated Unsaturated
Substrates
Acrolein
Catalysts
loadings/wt%
T/◦C
Conversion (%)
aldehyde a alcohol b
alcohol c
Others d
Au/Fe(OH)x
Au/Fe2O3-4
Au/Fe2O3-WGC
Au/Fe(OH)x
Au/Fe2O3-4
Au/Fe2O3-WGC
Au/Fe(OH)x
Au/Fe2O3-4
1.0
1.1
4.4
1.0
1.1
4.4
1.0
1.1
4.4
65
65
65
100
100
100
100
100
100
3
3
3
11
11
11
12
12
12
89
67
73
93
98
88
86
98
81
76
75
70
24
41
24
11
71
72
1
—
—
12
30
13
6
—
—
—
63
25
61
82
4
23
25
23
1
4
2
1
6
10
Citral
Cinnamaldehyde
19
5
Au/Fe2O3-WGC
13
a Saturated aldehyde: propanal for acrolein, citronellal for citral and hydrocinnamaldehyde for cinnamaldehyde. b Saturated alcohol: 1-propanol for
acrolein, citronellol for citral and 3-phenylpropanol for cinnamaldehyde. c Unsaturated alcohol: allyl alcohol for acrolein, geraniol and nerol for citral
and cinnamic alcohol for cinnamaldehyde. d Others: acrylic acid and formyldihydropyran for acrolein, 1,1-diethoxy-3,7-dimethylocta-2,6-diene and
ethyl-3,7-dimethyloct-6-enoate for citral and 1-(3,3-diethoxypropyl)benzene for cinnamaldehyde.
Commun., 2004, 904; G. Lue, R. Zhao, G. Qian, Y. Qi, X. Wang and J.
Conclusions
Suo, Catal. Lett., 2004, 97, 115.
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Soc., 2005, 127, 4182.
Au catalysts could exhibit enhanced activity and selectivity in
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It was found for the first time that ferric hydroxide supported
the chemoselective hydrogenation of aromatic nitro compounds.
While after calcining at elevated temperatures and with aggrega-
tion of Au species (although the size only increased to 3–5 nm)
Au/Fe2O3-4 exhibited an inferior catalytic performance. Though
the detailed mechanism is not clear at this stage, it is proposed
that Au subnano clusters play an important role in the enhanced
catalytic performance, though the supports may also make a
contribution. This catalyst preparation strategy may be extended
to other reactions with satisfactory results.
12 M. Haruta, S. Tsubota, T. Kobayashi, H. Kageyama, M. J. Genet and
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Acknowledgements
This work was financially supported by the National Sciences
Foundation of China (No. 20773146). The authors would like
to thank Mr Chen for the HRTEM investigation, Ms L. He for
the XRD investigation, Ms L. Gao for the XPS invesigation, Ms
Q. Wu for the Au loading analysis, Ms J. Li for the measurement
of BET adsorption and Ms X. Wei for the NMR spectroscopic
investigation.
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