Hydroxyl Assisted Rhodium Catalyst Supported on Goethite Nanoflower for Chemoselective Catalytic Transfer Hydrogenation of Fully Converted Nitrostyrenes

Article abstract of DOI:10.1002/adsc.201801611

Control of chemoselectivity is a special challenge for the reduction of nitroarenes bearing one or more unsaturated groups. Here, we report a flower-like Rh/α-FeOOH catalyst for the chemoselective hydrogenation of nitrostyrene to vinylaniline over full conversion, which benefits the new functionalized aminostyrene because the multisubstituted aminostyrenes are usually commercially unavailable. This catalyst does not only show desirable selectivity for the vinylanilines, but also exhibits the inertness to various other reducible groups over wide reaction duration. The catalytic selectivity for the reduction of the nitro group towards vinyl group was investigated by the control experiments and FT-IR analysis. We have found that the abundant hydroxyl groups in the α-FeOOH may contribute to the improvement of catalytic activity and selectivity. Furthermore, the catalyst exhibits excellent stability and keeps its catalytic performance even after 6 cycles. (Figure presented.).

Full text of DOI:10.1002/adsc.201801611

Title: Hydroxyl Assisted Rhodium Catalyst Supported on Goethite
Nanoflower for Chemoselective Catalytic Transfer Hydrogenation
of Fully Converted Nitrostyrenes
Authors: Zenan Hu, Yongjian Ai, Lei Liu, Junjie Zhou, Gang Zhang,
Hongqi Liu, Xiangyu Liu, Zhibo Liu, Jianshe Hu, Hong-bin
Sun, and Qionglin Liang
This manuscript has been accepted after peer review and appears as an
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the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801611
Link to VoR: http://dx.doi.org/10.1002/adsc.201801611

10.1002/adsc.201801611
Advanced Synthesis & Catalysis
FULL PAPER
DOI: 10.1002/adsc.201((will be filled in by the editorial staff))
Hydroxyl Assisted Rhodium Catalyst Supported on Goethite
Nanoflower for Chemoselective Catalytic Transfer
Hydrogenation of Fully Converted Nitrostyrenes
Zenan Hua, Yongjian Aib, Lei Liua, Junjie Zhoua, Gang Zhanga,*, Hongqi Liua, Xiangyu
Liua, Zhibo Liua, Jianshe Hua, Hong-bin Suna,* and Qionglin Liang b,*
a
Department of Chemistry, Northeastern University, Shenyang 110819, People’s Republic of China. Phone:
13464046173 and E-mail: gangzhang99@126.com; Phone: 18624030315 and E-mail: sunhb@mail.neu.edu.cn
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology (Ministry of Education), Department of
b
Chemistry, Center for Synthetic and Systems Biologsy, Tsinghua University, Beijing 100084, People’s Republic of
China. Phone: 13683328687 and E-mail: liangql@tsinghua.edu.cn.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.((Please
delete if not appropriate))
Abstract. Control of chemoselectivity is a special challenge
for the reduction of nitroarenes bearing one or more
unsaturated groups. Here, we report a flower-like Rh/-
FeOOH catalyst for the chemoselective hydrogenation of
nitrostyrene to vinylaniline over full conversion, which
benefits the new functionalized aminostyrene because the
multisubstituted aminostyrenes are usually commercially
unavailable. This catalyst does not only show desirable
selectivity for the vinylanilines, but also exhibits the
inertness to various other reducible groups over wide
reaction duration. The catalytic selectivity for the reduction
of the nitro group towards vinyl group was investigated by
the control experiments and FT-IR analysis. We have found
that the abundant hydroxyl groups in the -FeOOH may
contribute to the improvement of catalytic activity and
selectivity. Furthermore, the catalyst exhibits excellent
stability and keeps its catalytic performance even after 6
cycles.
Keywords: Hydrogen transfer; Hydroxyl Assisted;
Nitrostyrenes; Nanoparticles; Heterogeneous catalysis
direction of green chemistry. Catalytic transfer
hydrogenation (CTH) is a promising approach for the
reduction of nitroarenes into corresponding
anilines.[5] Meanwhile, CTH using a proper hydrogen
donor instead of H2 can not only enhance the
Introduction
Functionalized aromatic amines are key chemical
feedstocks and intermediates in the synthesis of
pharmaceuticals, dyes, agrochemicals, fine chemicals, selectivity of the catalytic reaction, but also makes
and polymers.[1] They are usually produced by
chemoselective reduction of the corresponding
functionalized nitroarenes.[2] Compared with the use
of the stoichiometric reducing agents such as sodium
hydrosulfide, Fe and Zn, catalytic hydrogenation over
heterogeneous catalysts has attracted wide attention
because of its environmental friendliness.[3] However,
selectivity is the main problem in heterogeneous
catalysis because the co-reduction of other
unsaturated groups in the same molecule can also
occur simultaneously.[4] To achieve good selectivity,
the hydrogenation is usually controlled under a
relatively low conversion. Therefore, high selectivity
over full conversion has become a challenging
process handling easier because hydrogenation with
H2 requires harsh equipment for security concern.[6]
Hydrazine hydrate is a perfect hydrogen source for its
greeness because the by-products are only nitrogen
and water.[7]
The researches about aromatic amines bearing
reducible groups have been concerning, such as keto,
aldehyde, nitrile, amide and halide.[3, 8] However, the
selective reduction of the nitro group of nitrostyrenes
is a particular challenge because the hydrogenation of
vinyl group is thermodynamically prior. Many
technologies have been reported so far to improve
selectivity, including the addition of appropriate
additives,[9] modification of active metals,[10]
1
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
adjustment of the metal-support interaction,[11] and
use of relatively inert metal catalysts.[12] However,
these approaches have more or less defects, such as
inconvenient preparation and too long reaction time,
or sacrificing activity to improve selectivity, which
are not satisfactory in practical production. Therefore,
it is essential to develop a highly selective and active,
green, and easy-handled catalyst. It has been reported
that Rh(0) nanoparticles (NPs) has the highest
activity for the activation of hydrazine hydrate, while
it also has high activities in CTH reactions of organic
compounds.[10b, 13] However, metal nanoparticles tend
to agglomerate due to their inherent nature of high
surface energy, which leads to the undesirable
catalytic activity.[14]. In this work, we used a flower-
like goethite (-FeOOH) as the support to
immobilize the Rh nanoparticle, because a flower-
like hierarchical structure may address good
homogenity, permeability and robustness. The
goethite (-FeOOH) has been widely used in various
field because of its availability, eco-friendliness,
inexpensiveness and non-toxicity, including water
treatment, organic pollutant degradation, human
sensing applications, rechargeable lithium ion battery,
photocatalysis, and as electrode materials.[15] Due to
its controlable morphology and hydroxyl-rich surface,
the flower-like -FeOOH is expected as an ideal
support to assist Rh NPs in the selective reduction of
nitrostyrene.
Herein, we prepared a flower-like Rh/-FeOOH
catalyst for the selectively catalytic transfer
hydrogenation of the nitro group into corresponding
functionalized anilines. The obtained catalyst exhibits
an excellent 98% selectivity of the 3-vinylaniline
upon 100% conversion of the 3-nitrostyrene.
Similarly, other vinylanilines are also produced with
satisfactory selectivity including some new
vinylanilines that we have synthesized for the first
time. Moreover, this flower-like Rh/-FeOOH
catalyst exhibits the inertness to various reducible
groups. Based on the control experiments and FT-IR
analysis, we have demonstrated that the nitro group
has the high intrinsic activity but the vinyl group is
hardly activated in our catalytic system, which
guarantees the perfect selectivity. It is worth noting
that the hydroxyl group of the -FeOOH nano flower
may help to improve catalytic activity.
Figure 1. (a) and (b) SEM images of the -FeOOH and
Rh/-FeOOH, respectively; (c) TEM, (d) HAADF-STEM
image, (e -h) elemental mapping of the Rh/-FeOOH.
FeOOH does not change during the preparation
process of the catalyst. Moreover, there is no
diffraction peak of the lattice rhodium in the XRD
pattern of the Rh/-FeOOH, and this may be because
of the low content and small particle size (< 5 nm).
As shown in the FT-IR spectra of the -FeOOH and
Rh/-FeOOH (Fig. S1b), the bands between 900
cm−1 and 500 cm−1 can be attributed to the Fe-OH
bending vibrations and Fe-O stretch,[17] which once
again proves the formation of the -FeOOH phase
and the richness of the hydroxyl group.
The morphology and structure of the samples were
investigated using SEM and TEM (Fig. 1). The SEM
image in Fig. 1a shows that the self-assembled -
FeOOH nanorods presents
a
flower-like 3D
hierarchical architecture with a diameter of about 1.4
um. In Fig. 1b and 1c, the SEM and TEM image
demonstrate that the flower-like structure maintains,
merely the nanorods slightly incline and the
hierarchical structure do not collapse. The image in
Fig. 1d depicts the clear lattice fringes with
interplanar distance of 0.23 and 0.27 nm,
corresponding to the (200) and (130) crystal planes of
the -FeOOH, respectively. It is worth noting that the
lattice fringes of no Rh nanoparticles could be found,
we speculate that the active Rh particles have been
entrapped in the inner structure of the FeOOH.
However, the Rh nanoparticles can be seen clearly
with the average size of 2.5 nm (Fig. S2). The high-
Results and Discussion
The XRD was used to analyze the phase
composition and structure of the samples. In Fig. S1a,
the peaks centered at 17.80o, 21.22o, 26.32o, 33.24o,
34.70o, 36.65o, 41.19o, 53.24o, 59.02o, and 61.39o can
be indexed to pure orthorhombic phase of -FeOOH
(JCPDS No. 029-713),[16] which demonstrates the
crystal structure of the -FeOOH. The XRD pattern
of Rh/-FeOOH is same to that of the pure -FeOOH,
and this indicates that the crystal structure of the -
2
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
angle annular dark-field imaging (HAADF) scanning
transmission electron
have almost no pore structure. The Rh/-FeOOH has
a higher BET surface area of 85.7 m2/g than the -
FeOOH (39.6 m2/g), which contributes from the
reduction of NaBH4.
The hydrogenation of 3-nitrostyrene was chosen as
the probe reaction to evaluate the catalytic
performance under various reaction conditions. Fig. 3
Figure 2. XPS survey spectrum (a) of the Rh/-FeOOH.
The corresponding narrow region of XPS spectra: (b) Fe
2p, (c) O 1s, and (d) Rh 3d.
Figure 3. Reaction network of the catalytic reduction of 3-
nitrostyrene.
microscopy (STEM) image and elemental mapping
further demonstrate the distribution of the elements O,
Fe, and Rh in the Rh/-FeOOH, which confirms the
embedded Rh. These results also prove the successful
preparation of the Rh/-FeOOH catalyst with
combining the XRD and FT-IR results (Fig. 1e-h).
In order to further investigate the surface chemical
composition and elemental states of the Rh/-FeOOH
catalyst, the XPS measurement was conducted. As
presented in Fig. 2a, the XPS spectrum of the catalyst
demonstrates the presence of Fe, O and Rh elements.
The high-resolution Fe 2p spectrum in Fig. 2b shows
two peaks centred at 711.0 and 724.1 eV,
corresponding to Fe 2p3/2 and Fe 2p1/2, respectively,
which is well consistent with the binding energy
value of Fe3+ in -FeOOH.[15] Moreover, the satellite
peaks centred at 718.7 eV and 733.6 eV are attributed
to the Fe3+.[18] In Fig. 2c, the two peaks of the O 1s
spectrum at 529.6 and 530.9 eV can be assigned to
corresponding =O, -OH, respectively.[19] In the high-
resolution Rh 3d spectrum, the two peaks in Fig. 2d
with binding energies of 306.8 and 311.6 eV can be
attributed to Rh0.[20] It is worth noting that the peak
positions of the Rh0 have shifted compared to that of
the reported literature,[20a] which indicates a strong
interaction between Rh and -FeOOH. The peaks
located at 308.2 eV and 313.2 eV can be ascribed to
the oxidized Rh (Rh2O3),[21] and this is inevitable
when the surface of the Rh NPs exposes to air. Even
it may be because Rh0 is oxidized by Fe3+ of the -
FeOOH.
shows three typical products for the catalytic
reduction of 3-nitrostyrene (3-NS, 1a). The
hydrogenation reaction proceeds through two routes:
one is reduction of the nitro group to obtain the target
product 3-vinylaniline (3-VA, 2a), and the other is
the reduction of vinyl group to give an unexpected
by-product 3-ethylnitrobenzene (3-ENB, 3a). Both of
the routes give the final by-product 3-ethylaniline (3-
EA, 4a). It can be observed that the -FeOOH
showed poor activity while the Rh/-FeOOH catalyst
exhibited 100% conversion and 98% selectivity to 3-
vinylaniline 2a, and the TOF value could reach up to
3000 h-1 (Table 1, entries 1 vs. 2). This indicates that
Rh NPs provide catalytic activity. Less hydrazine
hydrate could lead the reaction slow down, but more
hydrazine hydrate would not further accelerate the
reaction either (entries 3 and 4). Upon the tested
solvents, isopropanol was the best one (entries 5-7).
This could be attributed to the adsorption of nitro to
the surface hydroxyl. With the decreasing of solvent
polarity, the nitro group trends to combine with the
-FeOOH, thus the reaction proceeds more smoothly.
Water is better than methanol and ethanol, this could
be explained as most nitrostyrenes have weak affinity
with water, and water is helpful to stable the surface
hydroxyl of -FeOOH. To further investigate the
effect of hydroxyl groups on the catalytic activity, we
used the Rh/Fe3O4 catalyst to catalyze the reduction
of 3-nitrostyrene 1a under the same conditions, and
the results were summarized in Table S1. We found
that Rh/-FeOOH had higher catalytic activity than
the Rh/Fe3O4 catalyst, and this may be because the
surface of -FeOOH has more hydroxyl groups than
that of Fe3O4 according to the results of the FT-IR in
The N2 adsorption-desorption isotherm curves
were presented in Fig. S3. It has been observed that
the -FeOOH and Rh/-FeOOH catalyst both belong
to type I isotherms, and this indicates the samples
3
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
Fig. S4. Moreover, the Rh/-FeOOH had higher
catalytic activity than the Rh/g-C3N4 catalyst (Table
S1, entry 3 and 4). The interaction between Rh and -
FeOOH, as confirmed by XPS above, may also
contribute to the improvement of catalytic activity.
The effect of temperature on catalytic performance
was very remarkable, and the reaction becomes
slower as the temperature decreased (entries 8-11).
Therefore, the optimum reaction conditions were
fixed.
It
is
noteworthy
that
a
large
Table 1. Optimization of the reaction conditions.a
a)
Reaction condition: 3-nitrostyrene (1a, 0.5 mmol), catalyst (0.2 mol% Rh), solvent (2 mL), appropriate hydrazine
b)
hydrate was added. 1 mmol of N2H4·H2O was added to the reaction mixture again and continued to react for 1 h after
the conversion reached 100% at 10 min. c) Selectivity = yield/conversion, conversion and selectivity were analyzed by GC
(internal standard: o-xylene). TOF [h-1] = converted number of molecules of substrate / (theoretical number of Rh atom
d)
in catalysts × reaction time (h)).
number of intermediates existed when the conversion
was less than 100%,[22] and they are responsible for
low selectivity. The 3-ethylnitrobenzene 3a was not
detectable in our catalytic reaction, and 97%
selectivity to 3-vinylaniline 2a was still achieved with
prolonged reaction time or adding one more portion
of hydrazine hydrate again after complete conversion
of 3-nitrostyrene 1a (entries 12 and 13). These results
clearly indicated that the Rh/-FeOOH catalyst
favored the reduction of the nitro group instead of the
vinyl group.
In order to make this conclusion solid, the control
experiments were carried out with styrene and
nitrobenzene (Table 2). Although the reaction time
was extended, styrene was only converted by 7.6%,
while nitrobenzene reached complete conversion in
15 min (entries 1 and 2). Moreover, the same result
could be obtained using an artificial mixture of
nitrobenzene and styrene as the substrates (entry 3).
These results confirmed that catalytic transfer
hydrogenation of nitro had higher intrinsic activity
than olefin, which was almost inert.
4
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
The reason of the high selectivity over our catalyst
FeOOH catalyst could adsorb the nitro group rather
than the vinyl group.
was supplied by the substrate adsorption and FT-IR
analysis (Fig. 4). In the selective adsorption
experiment, the catalyst was mixed with substrates
solution for 10 h at room temperature and then
collected for the measurement. The asymmetric and
symmetric IR vibration frequencies of −NO2 at 1526
and 1348 cm−1 could be found when nitrobenzene
was chosen as the only absorbance. For the
adsorption of styrene, no C=C vibration peak of the
vinyl group was presented (liquid styrene shows it at
1630 cm-1 in Fig S5.). The same result can be found
on the use of the
To check the functional group versatility of the
Rh/-FeOOH catalyst system, various nitroarenes
with unsaturated functional groups were studied
under optimized reaction conditions, and the results
were summarized in Table 3 and 4. In Table 3,
Ortho-, meta-, and para-vinyl nitrobenzenes were
smoothly converted to corresponding aminostyrenes
with 100% conversion and 98% selectivity, and the
TOF value was up to 3000 h-1 (entries 1-3). In more
cases, multi-substituted aminostyrenes are not usually
commercially available, therefore the full conversion
Table 2. Control experiments with nitrobenzene and
styrene as the substrates over the Rh/-FeOOH catalyst.a
Table 3. Chemoselective reduction of various
nitrostyrenes.a
a)
Reaction conditions: hydrazine hydrate (1.0 mmol.), the
Rh/-FeOOH catalyst (0.2 mol% Rh), Isopropanol (2 mL),
o
b)
c)
110 C, 15 min. Reaction time 1h. Conversion was
analyzed by GC (internal standard: o-xylene).
Figure 4. The FT-IR spectra of absorbed species on the
Rh/-FeOOH catalyst. Substrate: (a).nitrobenzene;
(b).styrene; (c).3-nitrostyrene; (d).nitrobenzene + styrene.
a)
Reaction condition: nitrostyrenes (0.5 mmol), the Rh/-
mixture as a substrate. When 3-nitrostyrene 1a was
used as the probe molecule, only two peaks of −NO2
were observed while no vibrations of the C=C band
appeared, this may be because the adsorption of nitro
group obliges the unabsorbed C=C in an IR inactive
conformation (similarly, tilted CO adsorbed on metal
is IR inactive).[23] These results meant that the Rh/-
FeOOH catalyst (0.2 mol% Rh), hydrazine hydrate (1.0
mmol), Isopropanol (2 mL), 110 C.
Selectivity = yield/conversion, conversion and selectivity
were analyzed by GC (internal standard: o-xylene). All
reactions could reach 100% conversion.
converted number of molecules of substrate / (theoretical
number of Rh atom in catalysts × reaction time (h)).
o
b)
Isolated yields.
c)
TOF [h-1] =
5
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
of the corresponding substrates is required because of
halogen inertness also appeared in Table 4, and the
selectivity of hydrogenated the monosubstituted or
disubstituted halogen substrates at different positions
of the aromatic ring to corresponding anilines could
achieve a desirable > 99% with the conversion of
100% and a desired TOF value (table 4, entries 1-6).
Moreover, the catalyst still showed excellent
selectivity and conversion for other nitroarenes
substituted with some unsaturated groups, including
acid, ester and amide (table 4, entries 7-9). What’s
more, the Rh/-FeOOH catalyst could smoothly
their
scarcity.
We
synthesized
several
multisubstituted nitrostyrenes to test the broad
functionality tolerance. To our delight, the Rh/-
FeOOH catalyst could easily achieve an excellent
conversion and selectivity with a high TOF value for
the functionalized vinylanilines that are synthesized
for the first time (entries 4-9). It is worth noting that
the selectivity for the reduction of the nitrostyrenes
substituted with halogen groups at complete
conversion could still reach 97%, which indicated
that the dehalogenation reaction did not occur in our
catalytic experiments. The same result of
Table 4. Chemoselective reduction of the nitroarenes with
diverse functional groups.a
Figure 5. The reusability of the Rh/-FeOOH catalyst.
a)
Reaction condition: nitroarenes (0.5 mmol), the Rh/-
FeOOH catalyst (0.2 mol% Rh), hydrazine hydrate (1.0
mmol), Isopropanol (2 mL), 110 C.
o
b)
Isolated yields.
Scheme 1. Proposed mechanism for the chemoselective
reduction of nitrostyrenes by the Rh/-FeOOH catalyst.
Selectivity = yield/conversion, conversion and selectivity
were analyzed by GC (internal standard: o-xylene). All
c)
reactions could reach 100% conversion.
TOF [h-1] =
converted number of molecules of substrate / (theoretical
number of Rh atom in catalysts × reaction time (h)).
convert 2,4-dinitrophenol 1s to 2-amino-4-
nitrophenol 2s with 98% selectivity (table 4, entry 10).
6
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
was carried out on Vertex 70 Spectrometer. X-ray
photoelectron spectroscopy (XPS) was conducted on an
ESCA Lab MKII X-ray photoelectron spectrometer using
an Mg KR radiation excitation source. Transmission
electron microscope (TEM), high-angle annular dark-field
scanning TEM (HAADF-STEM) were performed on a
JEOL JEM-2100F field emission electron microscope.
Nitrogen adsorption-desorption isotherms (American
Micromeritics ASAP 2020 sorptometer) were measured at
77 K to determine the specific surface area of the samples
using the Brunauer-Emmett-Teller (BET) analysis method
while the pore-size distribution curves were obtained by
the Barret-Joyner-Halenda (BJH) equation based on the
desorption data. The Rh content of the sample was
determined by inductively coupled plasma atomic emission
spectroscopy (ICP-AES).
The stability and recyclability of the Rh/-FeOOH
catalyst were further studied in the same reaction, and
it can be reused at least 6 times without any loss in
catalytic activity and chemoselectivity (Fig. 5). The
XRD of the recovered catalyst was the same as that
of the fresh catalyst (Fig. S1a) and the leaching of
rhodium was not particularly obvious, as the rhodium
contents decreased slightly from 0.43 wt% to 0.31
wt% before and after 6 cycles through ICP-AES
analysis. Therefore, these results confirm that the
Rh/-FeOOH catalyst has excellent stability and
recyclability.
A plausible reaction mechanism is proposed for the
chemoselective reduction of nitrostyrenes by the
Rh/-FeOOH catalyst (scheme 1). Initially, the
hydrazine hydrate and -NO2 group of the
nitrostyrenes instead of the C=C bond were absorbed
on the surface of the catalyst, and then the active H*
species produced by decomposition of N2H4·H2O
reduced the nitro group to corresponding anilines.
The typical general pathway for the reduction of
nitroaromatics was identified as: Ar-NO2Ar-
NOAr-NHOHAr-NH2.[7c]
Preparation of the -FeOOH nanoflower
The flower-like -FeOOH was synthesized according to
the reported literature.[16] In a typical experiment (scheme
S1), FeSO4·7H2O (5.56 g) was dissolved in a mixture of
deionized water (160 mL) and ethanol (40 mL) under
sonication. Then urea (1.2 g) was added and the resulted
solution was stirred at ambient temperature for 10 minutes.
o
Next, the mixture was refluxed for 6 h at 90 C. After
being cooled down, the precipitate was collected by
filtration and washed several times with deionized water.
Finally, the produced flower-like -FeOOH was dried
overnight at 60 oC.
Conclusion
In conclusion, a series of nitrostyrenes are
Preparation of the Rh/-FeOOH, Rh/Fe3O4
chemoselective
transfer
hydrogenated
to
and Rh/g-C3N4
corresponding functionalized vinyl amines. For the
reduction of 3-nitrostyrene 1a, the catalyst exhibits a
superior selectivity of 98% towards 3-aminostyrene
2a at a conversion of 100%. According to the control
experiments and FT-IR analysis, we confirm that this
catalyst is only active with regard to the -NO2 group
and is inert with respect to the C=C bond. We also
confirm that the rich hydroxyl groups in the -
FeOOH hierarchical nanoflower may help to improve
catalytic activity. Moreover, the catalyst shows
outstanding reusability with at least 6 cycles.
A portion of RhCl3·3H2O (8.0 mg) was dissolved in 40 mL
of ethanol. Then this solution was added dropwise to a
blend of ethanol (100 mL) and the prepared -FeOOH
(200 mg) at room temperature. The mixture was kept
stirring for another 3 h. Next, methanol (20 mL) solution
of NaBH4 (0.1 g) was injected into the above solution
immediately, and the reaction mixture was stirred for
another 20 min. Finally, the Rh/-FeOOH was collected
by filtration, washed several times with ethanol, and dried
o
at 60 C for 6 h. The Fe3O4 was prepared by the co-
precipitation method and g-C3N4 was obtained by the
calcination (more details see supporting information).[20b]
The preparation of Rh/Fe3O4 and Rh/g-C3N4 were similar
to that of the Rh/-FeOOH, and their Rh contents are
consistent.
Experimental Section
The synthesis of nitrostyrenes
Materials
In
a
typical
experiment,[24]
2
mmol
of
methyltriphenylphosphonium bromide was dissolved in
dichloromethane (10 mL), then DBU (2.2 mmol) was
added. After refluxing for 30 min, the nitrobenzaldehyde
(1.0 mmol) in dichloromethane (3 mL) was added to this
solution. After refluxing for additional 3 h, the reaction
mixture was washed with HCl solution (0.1 M, 30 mL),
dried over anhydrous sodium sulfate, and evaporated to
afford the crude product. The product was
chromatographed over silica gel by elution with
dichloromethane and petroleum ether to give pure the
nitrostyrene, which was determined by GC-MS, 1H and 13C
NMR.
RhCl3·3H2O, sodium borohydride (NaBH4), ferrous sulfate
heptahydrate (FeSO4·7H2O), and the most of nitroarenes
were purchased from Sinopharm Chemical Reagent Co.,
Ltd., China. All of nitrostyrenes were synthesized by
ourselves.
Nitrobenzaldehydes,
methyltriphenylphosphonium
Diazabicyclo[5.4.0]undec-7-ene
bromide,
(DBU),
1,8-
and
urea
hydrazine hydrate (85%) were provided by Aladdin
Reagent Co., Ltd., China. All of the chemicals were of
analytical grade and were used without any further
purification.Experimental Details
Characterization methods
Selective catalytic reduction of the nitroarenes
X-ray powder diffraction patterns (XRD) of the samples
were taken on an X’Pert Pro MPDDY2094 diffractometer
equipped with graphite monochromatized Cu KR radiation
(l¼ 1.54178 A). Fourier transform infrared (FTIR) spectra
A mixture of nitro compound (0.5 mmol), 0.2 mol%
catalyst (based on Rh) and N2H4·H2O (1 mmol) was placed
in a sealed tube with isopropanol (2 mL) as the solvent.
7
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10.1002/adsc.201801611
Advanced Synthesis & Catalysis
The reaction was performed at 110 oC with vigorous
stirring. After completion of the reaction, the catalyst was
recovered by centrifugation, and the product was analyzed
by gas chromatography (GC) with o-xylene as the internal
standard. The residue was purified by TLC (silica)
separation with ethyl acetate and petroleum ether to
produce the desired product. The recycled catalyst was
washed several times with ethanol, and directly used for
the next run.
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Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21872020, 81872835 and
21621003), the Ministry of Science and Technology
(Nos.2017YFC0906902 and 2017ZX09301032), Macau Science
and Technology Development Fund (129/2017/A3 and
089/2013/A3). The China Scholarship Council (No.
201806085012) is also acknowledged.
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9
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Advanced Synthesis & Catalysis
FULL PAPER
Hydroxyl Assisted Rhodium Catalyst Supported on
Goethite Nanoflower for Chemoselective Catalytic
Transfer Hydrogenation of Fully Converted
Nitrostyrenes
Adv. Synth. Catal. Year, Volume, Page – Page
Zenan Hua, Yongjian Aib, Lei Liua, Junjie Zhoua,
Gang Zhanga,*, Hongqi Liua, Xiangyu Liua, Zhibo
Liua, Jianshe Hua, Hong-bin Suna,* and Qionglin
Liang b,*
10
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