Rh Catalyzed Selective Hydrogenation of Nitroarenes under Mild Conditions: Understanding the Functional Groups Attached to the Nanoparticles

Article abstract of DOI:10.1002/cctc.201901491

Modifying the surface of metal catalyst is a crucial subject for improving the heterogeneous metal catalysts. It is still a great challenge to understand the structure-activity relationship (SAR) between the surface supporting groups and the metal particles. Herein, Rh NPs supported on the magnetic silica sphere with various functional groups (?NH₂, ?SH, ?SO₃H, ?N=CH₂, ?Cl, ?NHCOCH₃ and ?NHCH₂Ph) have been prepared for catalytic hydrogenation of nitroarenes under atmospheric pressure at room temperature. We discover that the chemical state of Rh NPs depends on the supporting groups significantly. The electron-donating functionalities can effectively elevate the Rh⁰/Rh³⁺ ratio, which increase the catalytic performance both in conversion and selectivity. The amino group decorated catalyst has such a good selectivity that no dechlorination reaction is observable in the hydrogenation of 2-chloro-3-nitropyridine. What's more, by introducing magnetic Fe₃O₄ nuclei and mesoporous silica encapsulating layer, the catalyst can be recovered conveniently by an external magnet, and can be reused 10 cycles without any loss of activity.

Full text of DOI:10.1002/cctc.201901491

DOI: 10.1002/cctc.201901491
Full Papers
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Rh Catalyzed Selective Hydrogenation of Nitroarenes under
Mild Conditions: Understanding the Functional Groups
Attached to the Nanoparticles
+
[a]
+ [b]
[a]
[a]
[a]
[c]
Haimeng Tian , Junjie Zhou , Yunong Li,* Yiming Wang, Lei Liu, Yongjian Ai,
Ze-Nan Hu, Jifan Li, Rongxiu Guo, Zhibo Liu, Hong-bin Sun,* and Qionglin Liang
[a]
[a]
[a]
[a]
[a]
[c]
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Modifying the surface of metal catalyst is a crucial subject for
improving the heterogeneous metal catalysts. It is still a great
challenge to understand the structure-activity relationship (SAR)
between the surface supporting groups and the metal particles.
Herein, Rh NPs supported on the magnetic silica sphere with
electron-donating functionalities can effectively elevate the Rh /
+
3
Rh ratio, which increase the catalytic performance both in
conversion and selectivity. The amino group decorated catalyst
has such a good selectivity that no dechlorination reaction is
observable in the hydrogenation of 2-chloro-3-nitropyridine.
various functional groups (À NH , À SH, À SO H, À N=CH , À Cl, What’s more, by introducing magnetic Fe O nuclei and
2
3
2
3
4
À NHCOCH and À NHCH Ph) have been prepared for catalytic
mesoporous silica encapsulating layer, the catalyst can be
recovered conveniently by an external magnet, and can be
reused 10 cycles without any loss of activity.
3
2
hydrogenation of nitroarenes under atmospheric pressure at
room temperature. We discover that the chemical state of Rh
NPs depends on the supporting groups significantly. The
[
3]
Introduction
competitive adsorption between H and substrates. Accord-
2
ingly, specializing the SAR is instructive in the fabrication and
application of catalysts. Various strategies have been adopted
to improve performance such as alloying synergy, strong
metal support interaction (SMSI) and phase interfacial effect.
[4]
Heterogeneous catalysts hold a great situation in the hydro-
genation reactions due to their unparalleled advantages of
reusability and environment friendliness. For the reduction of
[
5]
[1]
[6]
[7]
nitro-compounds (-NO ) especially the catalytic hydrogenation,
the metal based heterogeneous catalysts have contributed
prominent progresses, which have been applied in the presence
By adjusting the arrangement of atoms on the active surface
through alloying, both adsorption and activation energy
changed in favorable directions and even for hydrogen
2
[8]
of coexisting reducible groups, such as À C=O, À C=C, À Cl and passivation metal such as gold. Li and co-workers fabricated
[2]
À COÀ NHÀ . Nevertheless, it still remains a great challenge for
directional adjustment of the structure and function of
heterogeneous catalysts. In the early research, the effect of
metal centers on the activity of catalysts was firstly explored.
The catalysts containing metallic active sites such as gold,
nickel, platinum and palladium nanoparticles have unsatisfied
performance in terms of activity or selectivity due to the
highly dispersed CeO -supported Ni catalyst with SMSI, which
2
[
9]
favors the H dissociation and adsorption of nitroarenes. While
2
3
+
the Pt-Fe -OH interface could suppress the further hydro-
[10]
genation of imine to amine. There are also methodologies
that provide various protocols for the selective reduction of
nitro compounds through special pathway, such as space
[11]
confined sterically absorption, surface diversities of coordina-
[12]
[13]
tion sites,
special location of active metal sites
and
[14]
+
modifying surface hydrophobic properties. For humble metal-
based catalysts, it has been proposed that the partial surface
oxidization benefits the vertical adsorption of nitroarenes or
reduction intermediates, which favor the selectivity at the cost
of activity. With the development of modern characterization
techniques, the single atom catalyst has been regarded as a
better model to shed light the precise construction of the
SAR. An example is that the single atomic Pt on the four-fold
hollow sites on phosphomolybdic acid, each single Pt atom is
stabilized by four oxygen atoms with positive charge. It exhibits
excellent performance in the hydrogenation of nitrobenzene
and cyclohexanone.
Modifying the electron density of the nanoparticles via the
[
a] H. Tian, Dr. Y. Li, Y. Wang, Dr. L. Liu, Dr. Z.-N. Hu, J. Li, R. Guo, Z. Liu,
Dr. H.-b. Sun
Department of Chemistry
Northeastern University,
Shenyang 110819 (P. R. China)
[15]
E-mail: sunhb@mail.neu.edu.cn
liyunong@mail.neu.edu.cn
+
[b] Dr. J. Zhou
Department of Electrical Engineering
Tsinghua University
Beijing 100084 (P. R. China)
[16]
[c] Dr. Y. Ai, Dr. Q. Liang
Key Laboratory of Bioorganic Phosphorus Chemistry & Chemical Biology
(
Ministry of Education)
[17]
Department of Chemistry, Center for Synthetic and Systems Biology
Tsinghua University
Beijing 100084 (P. R. China)
[18]
+
corresponding supports is an alternative strategy. In a recent
report, Xiao and co-workers introduced Au, Pt or Ni nano
[
] These authors contributed equally to this work.
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201901491
particles on the support of TiO with the single atom dispersed
2
ChemCatChem 2019, 11, 1–11
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Full Papers
Sn, and the SnÀ OÀ Ti provide oxygen vacancies to enhance the
supported surface groups is thoroughly investigated. We
discovered that the different groups act not only as the anchor
site, but also the surface groups have a great influence on
the electron properties of the rhodium nanoparticles. It can be
systematically summarized that catalysts with electron-rich
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[19]
selectivity. Su et al. reported that the platinum nanoparticles
loaded on diverse functionalized carbon nanotubes exhibited
[28]
[20]
different activity. By doping with oxygen and nitrogen into
carbon nanotubes, the catalysts were increased by two and four
times, respectively. Triphenylphosphine is verified positive for
Pd-based hydrogenation catalyst, both on enhancing surface
groups, especially for À NH , are superior to those electron-
2
withdrawing opposites.
[
21]
electron density and splitting PdÀ H bonds.
It was also
reported that the electron density of ultrathin platinum nano-
wires can be promoted by ethylenediamine coating. This favors Results and Discussion
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the adsorption of electron-deficient reactants over electron-rich
[22]
products, then the selectivity could be effectively improved.
To further illustrate the morphological properties, the shape
and size distribution of these catalysts were surveyed by high
resolution transmission electron microscopy (HR-TEM). As
It is noticeable that dechlorination usually occurs in the
catalytic hydrogenation of chloro-containing nitro arenes, which
[23]
brings the problem of chemical selectivity. Rhodium nano-
particles have shown excellent performance in catalytic transfer
shown
in
Figure 1
and
Figure S1–S2,
both
[
24]
hydrogenation of nitroarenes using hydrazine hydrate,
[25]
[26]
sodium borohydride and alcohols as hydrogen source. The
leaching of Rh particles that leads the reduced lifetime can be
[27]
resolved by covering another layer of mesoporous silica.
Thus, it is of great significance for improving catalytic perform-
ance by investigating the influence of electronic properties on
the catalyst. Based on these considerations, we fabricated a
series of Rh-based catalysts, which are the functionalized
magnetic silica spheres supported Rh nanoparticles. The typical
structure and synthesis strategy to fabricate the amino group
decorated catalyst Fe O @SiO À NH -RhNPs@mSiO is shown in
3
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2
2
Scheme 1. The Fe O core endow the catalyst with favorable
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Figure 1. HR-TEM images: (a) Fe
3
O
4
@SiO
; EDS mapping of (e)
À RhNPs, (f) Fe, (g) Si, (h) N and (i) Rh.
2
À NH À RhNPs; (b–d)
2
Fe
Fe
3
O
O
4
@SiO
@SiO
2
À NH
2
À RhNPs@mSiO
2
3
4
2
À NH
2
Scheme 1. Preparation of Fe
3
O
4
@SiO
2
À NH
2
-RhNPs@mSiO
2
.
Fe O @SiO À NH À RhNPs and Fe O @SiO À NH À RhNPs@mSiO
3
4
2
2
3
4
2
2
2
have a spherical shape with an average diameter of 260 nm and
00 nm, respectively. It is clear that the Fe O nanospheres in
4
3
4
Fe O @SiO À NH À RhNPs are covered by a layer of silica shell,
3
4
2
2
recoverability. The inner silica shell is produced through
hydrolysis of TEOS to provide sufficient space for linking the
functional groups with silica coupling reagent, e.g. amino
which provides the location for binding amino groups and Rh
nanoparticles, as shown in Figure 1a. What’s more, it can
[
29]
effectively prevent the agglomeration of the particles.
propyl tri-ethoxyl silicane (APTES) for À NH . After the active
When another layer of silica shell is encapsulated on the
nanospheres, the interface of the layered structure can be easily
distinguished (Figure 1c), where the Rh nanoparticles with an
average diameter of about 3 nm are evenly dispersed.
2
metals are embedded in the silica shell, another mesoporous
silica shell is encapsulated. The pores are made through
extracting the Cetyltrimethyl Ammonium Bromide (CTAB) that
are embedded in the silica shell. As a benchmark, the À NH
The
Rh
loading
on
Fe O @SiO À NH À RhNPs
and
2
3
4
2
2
group decorated catalyst showed the best performance for the
selective catalytic hydrogenation of nitroarenes. Based on the
in-depth study of XPS, the SAR between the catalysts and
Fe O @SiO À NH À RhNPs@mSiO are 1.187 wt% and 0.5863 wt%
3
4
2
2
2
detected by ICP respectively. The existence of metallic Rh can
be further confirmed due to the lattice fringe on the catalysts
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Figure 2. (a) XRD patterns of Fe
Fe @SiO À RhNPs@mSiO
À NH
3
O
2
.
4
@SiO
2
À NH
2
À RhNPs@mSiO
2
compared with standard spectrum card of Fe
3
O
4
. (b) Nitrogen adsorption-desorption isotherm of
3
O
4
2
2
0
3+
[a]
Table 1. Rh /Rh ratio and Rh binding energies of different catalysts.
[a] The data was obtained from XPS results.
measured to be 0.221 nm, assigned to the (111) of metallic Rh,
respectively, as shown in Figure 1d.
The EDS layered images and the distribution of Fe, Si, N and
Rh are shown in Figure 1e–1i. The Fe O4 solid cores are
pore diameter of the catalysts is measured to be 2.6 nm by BJH
(Barrett-Joyner-Halenda) model. Moreover, the BET (Brunauer-
Emmett-Teller) surface area of Fe O @SiO À NH À RhNPs@mSiO
3
4
2
2
2
2
is calculated to be 302.992 m /g.
3
encapsulated by SiO , which shows a bigger hollow spherical
The elemental status and electronic properties of catalysts
were investigated by X-ray photoelectron spectroscopy (XPS).
The survey XPS result of Fe O @SiO À NH À RhNPs presented in
2
structure in Figure 1g, and can be distinguished easily in
Figure 1f. The presence of highly dispersed Rh confirms the fact
that the Rh nanoparticles have been loaded successfully
between the two silica layers of the catalysts as shown in
Figure 1i.
3
4
2
2
Figure 3a verifies the existence of O, Rh, Si and C. The peaks of
N and Fe were difficult to be detected because of the small
amount for N and compact silica shell for Fe. Moreover, to
understand the relationship between the electronic properties
of Rh NPs and functional groups, we synthesized a series of
diverse functionalized catalyst with different silica coupling
In
order
to
discuss
the
characteristic
of
Fe O @SiO À NH À RhNPs@mSiO , a powder X-ray diffraction
3
4
2
2
2
pattern analysis (PXRD) was shown in Figure 2a. The character-
istic diffraction peaks of the catalyst can be assigned to the
planes of Fe O accurately. A short and wide peak is observed in
reagent. Besides À NH , we investigated the functionalities
2
including À SH, À SO H, À N=CH , À Cl, À NHCOCH and À NHCH Ph.
3
4
3
2
3
2
the range of 2θ=19–23°, which is considered as the character-
The Rh 3d regions of catalysts varied with different functional
groups are also presented in Figure 3b–3 h. The peaks at 307.2
[30]
istic peak of amorphous silica.
Nevertheless, there is no
0
3+
obvious diffraction peaks of rhodium that can be observed due
to the low loading and small size of the metal nanoparticles.
The nitrogen adsorption/desorption isotherms and the pore
size distribution of the catalysts are presented in Figure 2b. It is
clearly that the type IV hysteresis loop can be observed in
Figure 2b, from which we can conclude that a large number of
mesoporous have been created by CTAB in solid-liquid
extraction processing with boiling ethanol, and the average
and 308.7 eV in Figure 3b can be ascribed to Rh and Rh ,
0
3+
respectively, and the Rh /Rh ratio of each catalyst is listed in
0
3+
Table 1. Higher Rh /Rh
ratio could be observed for the
Fe O @SiO À NH À RhNPs with electron rich amino groups in
3
4
2
2
0
3+
Table 1, entry 1. Obviously, the Rh /Rh
ratio of
Fe O @SiO À NHPhÀ RhNPs are apparently lower than other
3
4
2
catalysts owing to the majority of oxidation state rhodium, as
showed in Table 1, entry 5. However, neither
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Figure 3. XPS spectra: (a) survey result of Fe
Fe @SiO À RhNPs, Fe @SiO À SO
À N=CH
3
O
4
@SiO
2
À NH
2
À RhNPs; (b–h) Rh 3d regions of Fe
3
O
4
@SiO
2
À NHCOCH
2
À NH
2
-RhNPs, Fe
3
À RhNPs and Fe
3
O
4
@SiO
2
À SH-RhNPs,
3
O
4
2
2
3
O
4
2
3
HÀ RhNPs, Fe
3
O
4
@SiO PhÀ RhNPs, Fe @SiO
2
À NHCH
2
3
O
4
3
O
4
@SiO
2
À Cl-RhNPs respectively.
Fe O @SiO À NHCOCH À RhNPs nor Fe O @SiO À ClÀ RhNPs shows
Catalytic Activity Tests
3
4
2
3
3
4
2
obvious peaks that could be detected in Rh 3d regions, this
may be due to their poor coordination ability, which makes it
We examined the catalytic hydrogenation of nitro compounds
using p-nitrophenol as the model substrate under ambient
conditions in water and isopropanol, which are commonly used
in this reaction. As shown in Table 2 entries 1–4, compared with
isopropanol, the conversion is higher in water system using
Fe O -Rh NPs and Fe O @SiO -Rh NPs as catalyst. Once the silica
[31]
difficult to load Rh NPs. The above results suggest that Rh
NPs on the catalysts with electron-donating groups have a
0
3+
higher Rh /Rh ratio because of its electron rich properties,
which have a positive effect on loading metal NPs on the
catalysts.
3
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[
a]
Table 2. Catalytic hydrogenation of p-nitrophenol with diverse catalysts under mild conditions.
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[
a] Reaction conditions: 1 mmol p-nitrophenol, 3 mL solvent, 1 bar H
2
, 40 mg catalyst (29 wt%), the reaction was carried out at room temperature for 16.5 h
and conversion was detected by HPLC and GC-MS. [b] The metal/substrate molar ratio is 0.47 mmol%. [c] The metal/substrate molar ratio is 0.23 mmol%.
Figure 4. (a) The conversion of p-nitrophenol as a function of time catalyzed by Fe
2-chloro-3-aminopyridine as a function of time in the presence of catalysts with different functional groups. (The yield=conversion × selectivity) (c) Selectivity
as a function of Rh /Rh ratio and (d) Rh /Rh ratio as a function of F value over catalysts functionalized with different groups.
3
O
4
@SiO
2
À NH
2
À RhNPs and Fe
3
O
4
@SiO
2
À NH
2
À RhNPs@mSiO
2
. (b) The yield of
0
3+
0
3+
shell is covered with a layer of amino groups, the reaction in
isopropanol is better. We attribute this to the hydrophobicity of
amino propyl in consideration of the hydroxyl groups on the
Fe O or silica surface. Herein, isopropanol is selected as the
solvent of the catalytic hydrogenation. To estimate the
influence of the mesoporous silica shell on the activity of the
catalyst, the hydrogenation of p-nitrophenol catalyzed by
Fe O @SiO À NH À RhNPs and Fe O @SiO À NH À RhNPs@mSiO
3
4
2
2
3
4
2
2
2
was examined and the results are shown in Figure 4a. The
reaction could be completed within 6 h when Fe O @SiO À NH -
3
4
3
4
2
2
RhNPs was applied. After covered with an outer mesoporous
shell, the activity gently decreased due to the lower metal/
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[
a]
Table 3. The effect of surface groups over Rh NPs on catalytic hydrogenation of 2-chloro-3-nitropyridine.
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[
a] Reaction conditions: 1 mmol 2-chloro-3-nitropyridine, 3 mL solvent, 1 bar H
2
, 40 mg catalyst (25 wt%), the reaction was carried out at room temperature
for 22 hours. [b] Conversion and selectivity were detected by HPLC and GC-MS, respectively.
substrate ratio (0.23 mmol% vs. 0.47 mmol%). However, the
outer silica shell can effectively elevate the stability of the
catalyst.
The catalysts with different functional groups are evaluated
with the hydrogenation of 2-chloro-3-nitropyridine, as its
dechlorination usually occurs severely in this reaction. The
reaction time was fixed to 22 hours, and most of the catalysts
achieved full conversion (Table 3 entries 1–5) except the Cl- and 1–4). 2-chloro-3-nitropyridine could be reduced to 2-chloro-3-
Table 3, entries 6–7 can be explained as low loading of Rh,
which the XPS results has shown.
The general applicability of the catalysts was investigated
by introducing a variety of nitro-compounds in our reaction
systems (Table 4). There is no doubt that those substrates with
a simple functional group could be converted to their
corresponding amino compounds completely (Table 4, entries
[32]
À NHCOCH decorated ones, whose poor performance can be
aminopyridine smoothly, and our catalytic protocol exhibits
perfect tolerance toward chlorine as shown in Table 4, entry 7.
When other substrates with halogen at different position were
reduced to their corresponding amines, our catalytic system
exhibits outstanding conversion and selectivity. Besides chlor-
ine, other reducible groups such as acid and amino on
nitroarenes showed excellent tolerant properties in our catalytic
reactions, and all of these functionalized nitrobenzenes were
converted into aniline without any by-products. The fabulous
performance of our catalyst can be attributed to the following
reason: the electron rich amino groups can serve as an electron
donor to enrich the electron of the Rh NPs, and the nitro group,
rather than chlorine or other reducible groups, is prone to be
adsorbed on Rh NPs with high electron density due to its strong
3
ascribed to the fact that Rh atoms have not been successfully
coordinated with the electron withdrawing groups from entries
6
–7, which has been verified by XPS. However, there are
apparent disparities in the selectivity for the catalysts with
different functional groups. Then we monitored the reactions of
various catalysts, and the results are shown in Figure 4b and
Figure S5-S6. It is clear that Fe O @SiO À NH À RhNPs and
3
4
2
2
Fe O @SiO À N=CH À RhNPs exhibit excellent selectivity. Thus,
3
4
2
2
0
3+
we can conclude that the catalysts with higher Rh /Rh ratios
[21,33]
have better selectivity for reduction of nitro-groups.
understand the effect of different functional groups on
catalysts, field inductive parameter (F), which is used to describe
the electronic inductive effect, was introduced and presented
in Table 3. With the enhancement of F, the group’s electron
cloud density decreases then results the lower Rh /Rh ratios,
which lead to the lower selectivity. Low F value of 0.08 and 0.10
can be observed in entries 1–2, and corresponding high
selectivity can be obtained. With the increase of F value, the
To
[34]
[35]
electron-withdrawing properties. Consequently, excellent se-
lectivity could be obtained.
0
3+
The reusability of Fe O @SiO À NH À RhNPs@mSiO
3
4
2
2
2
selectivity decreases in entries 3–7. The À SH and À SO H
3
contained catalysts give acceptable selectivity (entries 3 and 4).
The magnetic properties of Fe O , Fe O @SiO À NH À RhNPs and
3
4
3
4
2
2
It is perhaps due to the dissociation properties of À SH and Fe O @SiO À NH À RhNPs@mSiO at an applied field of 10000 Oe
3
4
2
2
2
À SO H. The low selectivity of catalysts containing À NHCH Ph
are shown in Figure 5a. There is no obvious hysteresis
phenomenon can be observed, which clearly demonstrates that
all of those materials are superparamagnetic. Thus, the catalysts
can be separated easily by an external magnet from a solution.
3
2
0
3+
could be the result of a poor Rh /Rh ratio, as the coordination
as well as steric hindrance effect of benzylamine group impedes
3
+
the reduction of Rh . The poor performance of catalysts in
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[
a]
Table 4. The hydrogenation of nitroarenes catalyzed by Fe
3
O
4
@SiO
2
À NH
2
À RhNPs@mSiO
2
.
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[
a] Reaction conditions: nitroarenes (1 mmol), hydrogen (1 atm), catalyst (40 mg, 0.23 mmol% Rh), isopropanol (3 mL), room temperature. [b] Conversion and
selectivity were determined by HPLC and GC-MS using trimethylbenzene as external standard.
Figure 5. (a) Room temperature hysteresis loops of Fe
Fe @SiO À RhNPs@mSiO
À NH
3
O, Fe
3
O
4
@SiO
2
À NH
2
-RhNPs and Fe
3
O
4
@SiO
2
À NH
2
À RhNPs@mSiO
2
. (b) The reusability of
3
O
4
2
2
2
.
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To investigate the stability and reusability of the
avoid dehalogenation or hydrogenation of other reducible
groups.
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Fe O @SiO À NH À RhNPs@mSiO , the catalyst was employed in
3
4
2
2
2
hydrogenation of 2-chloro-3-nitropyridine at the optimized
reaction conditions. After each cycle test, the catalyst can be
separated by an external magnet instead of centrifugation or Conclusions
filtration because of its excellent paramagnetic properties. As
shown in Figure 5b, with the increasing number of cycles, there
is no loss of catalytic performance that can be observed. The
catalyst can still maintain its inherent activity and selectivity
even after being reused up to 10 times. The outstanding
stability of the catalyst could be attributed to the following two
points: (1) the active centers of the catalyst in hydrogenation
reaction are Rh nanoparticles, which are linked and fixed by the
amino functional groups on the surface of the inner silica shell.
In addition to the physical fixation effect of the amino group,
electrons migration plays an important role in protecting the Rh
nanoparticles from oxidation. (2) The catalyst is encapsulated
by a layer mesoporous silica shell, which can effectively prevent
the losing of Rh NPs. Besides, the sandwich structure consisting
of two silicon shells retard agglomerating of Rh NPs, which
usually lead to the deterioration of catalytic performance.
In summary, the rhodium NPs based core-shell structured
catalyst were investigated in the application of nitro com-
pounds hydrogenation. By tuning the kind of supporting
groups, the active sites’ properties have been verified, and the
SAR between catalytic site and supported surface groups has
been systematically investigated with XPS. The electron-donat-
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ing groups with a lower F value, especially for À NH , can
2
effectively increase the electron density of catalytic sites and
0
3+
the ratio of Rh /Rh , which results in the promotion of activity
and selectivity. The explanation is that the electrophilic nitro
groups, instead of chlorine or other reducible groups on
nitroarenes, are inclined to be adsorbed on the surface of
electron rich Rh NPs. Owing to high catalytic performance of
Fe O @SiO À NH À RhNPs@mSiO , the reaction can go smoothly
3
4
2
2
2
with high yield and selectivity under mild reaction conditions.
The catalyst exhibited excellent stability even after 10 cycles
due to the protection of mesoporous silica shell. The signifi-
cance of electron-donating support groups can provide a
promising methodology for enhancing catalytic performance in
other reactions.
Mechanism
Based on those experimental data presented above and
[
36]
previous literatures,
a plausible reaction mechanism is
proposed in Figure 6. Initially, molecular H diffuse through the
2
mesoporous silica, then it is absorbed and activated followed Experimental Section
by producing of active H species on Rh NPs. Meanwhile, the
nitroarenes are adsorbed on Rh NPs as well. In virtue of the
electron rich properties of Rh NPs, the electrophilic nitro-groups
are prone to be adsorbed on Rh NPs and come into contact
with those active H species to produce the intermediates of
nitrosobenzenes and hydroxylamines. Finally, those intermedi-
ates are reduced to corresponding amines and desorbed from
Rh NPs due to the reduced electrophilicity, which effectively
Materials
Fe nano-sphericals with diameter of 100–300 nm are provided
O
4
3
by Aladdin Reagent Co., Ltd, China. Hydrochloric acid, NaBH4,
RhCl ·3H O, cetyltrimethyl ammonium bromide (CTAB) and all of
3
2
the nitro compounds are purchased from Sinopharm Chemical
Reagent Co., Ltd, China. Tetraethyl orthosilicate (TEOS), ammonia,
3-aminopropyl-triethoxysilane (APTES), 3-mercaptopropyltrimethox-
ysilane, 3-chloropropyltrimethoxysilane, glacial acid and carbon
tetrachloride are purchased from the Tianjin Yongda Chemical
Reagent Co., Ltd, China. All chemical reagents are analytic grade
and used without any further purification.
Instrumentation
The materials morphology was observed by JEM-2100F high
resolution transmission electron microscopy (HR-TEM). The scan-
ning transmission electron microscopy (STEM) and energy-disper-
sive X-ray spectroscopy (EDS) are accessories of the HR-TEM
apparatus. X-ray photoelectron spectroscopy (XPS) measurements
were performed using a Thermo ESCALAB 250, and the binding
energy determination was based on the carbon contamination C 1s
at 284.6 eV. A VSM (cryogenic Mini-CFM measurement system) was
used to measure the magnetic property of the sample. The samples
X-ray diffraction (XRD) experiments were recorded on a PANalytical
Empyrean, diffractometer using Cu Kα radiation. Room-temperature
Fourier-transform infrared (FT-IR) spectra was obtained by using a
À 1
Bruker Vertex spectrometer in the region of 30000–10 cm with a
À 1
spectral resolution of 16 cm
using a KBr pellet. Nitrogen
adsorption-desorption isotherms (American Micromeritics ASAP
Figure 6. Possible mechanism for the reduction of nitroarenes with atmos-
pheric H .
2
2020 sorptometer) were measured at 77 K to determine the specific
ChemCatChem 2019, 11, 1–11
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as un raf lay cs ei s amr e ea t ho of dt .h e samples using the Brunauer-Emmett-Teller (BET) Acknowledgements
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This work was financially supported by the National Natural
Science Foundation of China (Nos. 21872020, 81872835 and
General Fabrication Procedure of Rh-based Catalysts:
2
2
1621003), the Ministry of Science and Technology (Nos.
017YFC0906902 and 2017ZX09301032). The China Scholarship
Fe O @SiO À NH À RhNPs: the preparation method of silica coated
3
4
2
2
[
24]
Fe O can be found elsewhere.
3
In brief, 50 mL diluted HCl
4
Council (No. 201806085012) is also acknowledged.
solution are added into 0.2 g Fe O powders, following by ultra-
3
4
sonic treatment to disperse agglomerated particles. After washing
with deionized water to remove residual acid, those magnetic
spheres are redistributed in a mixed solution of 40 mL water and Conflict of Interest
1
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1
60 mL ethanol, and 4 mL ammonia water is used to adjust pH
value. Secondly, 0.5 mL tetraethyl orthosilicate (TEOS) is added into
above mixture under stirring and kept for 6 h. The prepared
The authors declare no conflict of interest.
Fe O @SiO is collected using an external magnet. For amino-
3
4
2
functionalization, 0.5 mL 3-aminopropyl-triethoxysilane is added
dropwise into 200 mL isopropanol with Fe O @SiO inclusion under
Keywords: Electron density
nanoparticles · Atmospheric hydrogen · Catalytic hydrogenation
·
Functional groups
·
Rh
3
4
2
8
0°C for 3 h. After magnetic separation from the suspension, the
solid are redispersed in 80 mL ethanol. 5 mg RhCl ·3H O are
dissolved in 20 mL ethanol and added dropwise into the suspen-
3
2
3
+
sion and keep on stirring overnight. To reduce the Rh , 0.1 g
sodium borohydride is dissolved in 20 mL methanol and added into
the mixture. After stirring for 30 min, the product is separated and
washed with ethanol for several times.
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3
4
2
2
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[
37]
manufactured according to the literature.
Typically, 0.3 g
Cetyltrimethyl Ammonium Bromide (CTAB) is dissolved in the
mixed solution of 60 mL ethanol and 2.5 mL ammonia water. At the
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2
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reaction between amino and acetic oxide at 80°C in 60 mL CCl
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in 50 mL methanol for 3 h, following by the adding of 0.15 g NaBH4.
After another 6 h, H O is used to quench.
2
Fe O @SiO À SH: the procedure is much the same as
3
4
2
Fe O @SiO À NH except that the APTES is replaced by 0.5 mL 3-
3
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mercaptopropyltrimethoxysilane.
Fe O @SiO À SO H: the obtained Fe O @SiO À SH is redispersed into
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0 mL hydrogen peroxide (30 wt%), and stirs for 12 h under room
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Fe O @SiO À Cl: in this case, 0.5 mL 3-chloropropyltrimethoxysilane
3
4
2
acts as the corresponding silane coupling reagent instead of APTES.
General catalytic hydrogenation of nitro compounds: 1 mmol
nitro compound, 40 mg catalyst and 3 mL isopropanol are added in
a 50 mL round bottomed flask with a magnetic stir bar inside.
Before reaction starting, air in the flask is replaced with hydrogen
for three times in a balloon. Then the reaction is conducted at
room temperature. The conversion and selectivity are detected by
GC-MS and HPLC.
6485.
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[
Manuscript received: August 15, 2019
Accepted manuscript online: August 31, 2019
Version of record online: ■■■, ■■■■
[
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© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPERS
Functionalized surfaces: Rh nano-
particles supported on magnetic
silica spheres with various functional
groups were tested in the catalytic
hydrogenation of nitroarenes under
mild conditions. Electron-donating
groups can effectively increase the
electron density of catalytic sites and
H. Tian, Dr. J. Zhou, Dr. Y. Li*, Y. Wang,
Dr. L. Liu, Dr. Y. Ai, Dr. Z.-N. Hu, J. Li, R.
Guo, Z. Liu, Dr. H.-b. Sun*, Dr. Q. Liang
1
– 11
Rh Catalyzed Selective Hydrogena-
tion of Nitroarenes under Mild Con-
ditions: Understanding the Func-
tional Groups Attached to the
Nanoparticles
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0
3+
the ratio of Rh /Rh , which results in
the promotion of the selectivity of
the catalyst for hydrogenation at
room temperature under atmospher-
ic conditions.