Porous silica-encapsulated and magnetically recoverable Rh NPs: A highly efficient, stable and green catalyst for catalytic transfer hydrogenation with "slow-release" of stoichiometric hydrazine in water

Article abstract of DOI:10.1039/c7gc00986k

A core-shell structured nanocatalyst (Fe₃O₄@SiO₂-NH₂-RhNPs@mSiO₂) that is encapsulated with porous silica has been designed and prepared for catalyzing the transfer hydrogenation of nitro compounds into corresponding amines. Rh nanoparticles serve as the activity center, and the porous silica shell plays an important role in the "slow-release" of the hydrogen source hydrazine. This reaction can be carried out smoothly in the green solvent water, and the atom economy can be improved by decreasing the amount of hydrazine hydrate used to a stoichiometric 1.5 equivalent of the substrate. Significantly, high catalytic efficiency is obtained and the turnover frequency (TOF) can be up to 4373 h^-1 in the reduction of p-nitrophenol (4-NP). A kinetics study shows that the order of reaction is ～0.5 towards 4-NP, and the apparent active energy E_a is 58.18 kJ mol^-1, which also gives evidence of the high catalytic efficiency. Additionally, the excellent stability of the catalyst has been verified after 15 cycles without any loss of catalytic activity, and it is easily recovered by a magnet after reaction due to the Fe₃O₄ nucleus.

Full text of DOI:10.1039/c7gc00986k

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DOI: 10.1039/C7GC00986K
Green Chemistry
ARTICLE
Porous Silica-Encapsulated and Magnetically Recoverable Rh NPs:
a Highly Efficient, Stable and Green Catalyst for the Catalytic
Transfer Hydrogenation with “Slow-Releasing” of Stoichiometric
Hydrazine in Water
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a
a
a*
a
a
a
a,b
a
Junjie Zhou ‡, Yunong Li ‡, Hong-bin Sun , Zhike Tang , Li Qi , Lei Liu , Yongjian Ai , Shuang Li ,
www.rsc.org/
b
b*
Zixing Shao , Qionglin Liang
3 4 2 2 2
A core-shell structured nanocatalyst (Fe O @SiO -NH -RhNPs@mSiO ) that is encapsulated with porous silica has been
designed and prepared for catalyzing the transfer hydrogenation of nitro compounds into corresponding amines. Rh
nanoparticles serve as the activity center, and the porous silica shell plays an important role in the “slow-releasing” of the
hydrogen source hydrazine. This reaction can be carried out smoothly in the green solvent of water, and the atom
economy can be improved by decreasing the usage amount of hydrazine hydrate to stoichiometric 1.5 equivalent of the
-
1
substrate. Significantly, the highly catalytic efficiency is obtained and the turnover frequency (TOF) can be up to 4373 h in
the reduction of p-nitrophenol (4-NP). The kinetics study shows that the reaction order is ~0.5 towards 4-NP, and the
apparent active energy Ea is 58.18 kJ/mol, which also gives evidences of the high catalytic efficiency. Additionally,
excellent stability of the catalyst has been verified after 15 cycles without any loss of catalytic activity, and it is easy to be
recovered
by
a
magnet
after
reaction
due
to
the
Fe
3
O
4
nucleus.
usually necessary, which affects the catalytic activity.
Nanocrystalization of heterogeneous catalyst have been hailed
as a bridge between heterogeneous and homogeneous
Introduction
The reduction of nitroarenes to producing aromatic amines is
of significance because arylamines are important starting
materials and intermediates for the manufacture of a good
many of chemicals, such as dyestuffs, pharmaceutical
1
1
catalysts, but the filtration of nano-catalysts is troublesome
due to the small size. For the same reason, the leaching of
active component will decrease the catalytic activity
1
2
1
dramaticlly. In this context, the magnetic separation could be
good choice to resolve this problem. Binding the
products, agricultural chemicals and polymers. Catalytic
a
transfer hydrogenation (CTH) has attracted much more
attention on account of the advantages of less waste and high
nanoparticles to a magnetic core makes it easier for the
recovery of the nanocatalyst, which makes the catalytic
reaction greener because this technology avoids the necessity
of additional purification operations that remove catalysts
selectivity,
comparing with traditional metal-mediated
2
3
4
reduction (such as iron, zinc, tin ). Meanwhile, the reaction
conditions of CTH is much milder than that of the catalytic
hydrogenation, as handling hydrogen gas usually requires high
pressure reactor and the toxic organic solvent is also
necessary. The CTH can be promoted by homogeneous
catalysts including the transition metal complexes of noble
1
1
from waste stream. As reported, the magnetic separation
technology have been applied in various reactions, including
1
3
14-16
heck reaction,
Suzuki cross-coupling reaction,
1
asymmetric hydrogenation, asymmetric Michael addition,
1
methylene blue degradation, biodiesel transesterification,
7
18
9
5
6
7
8
9
metal Pd, Rh, Ru, Ir and non-noble metal Ni . However, the
reusability of homogeneous catalysts limits their application,
2
0
etc.
1
and mature commercial heterogeneous catalysts (eg. Pd/C )
0
Another challenging issue for the nanocatalyst is the
agglomeration that induced by their inherent quality of high
are less efficient because a large amount of noble metal is
2
1
surface energy. To minimize the surface energy, kinds of
additives have been applied for the CTH catalytic system,
2
including surfactants and polymers
2
23, 24
. Immobilizing the
nanoparticles on the surface of supporters to form “isolated
islands” is another strategy. Covering the nanoparticles with
an inert shell can prevent the agglomeration and leaching of
2
5
26, 27
28
12
active component. Carbon, silica,
alumina and ceria
have been reported as the shell material. Among these
This journal is © The Royal Society of Chemistry 20xx
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DOI: 10.1039/C7GC00986K
ARTICLE
encapsulators, silica is an ideal shell material due to their rich (Fe
Journal Name
3
O
4
@nSiO
2
-NH
2
, 1c): the core-shell structure silica coated
21
magnetite were synthesized by a precious reported method.
2
9
porous structure, good chemo and thermal stability.
3 4
Following these guidelines, we designed a novel CTH Typically, 0.2 g Fe O nanoparticles (1a) were dispersed in 50
nanocatalyst that had a magnetic core and a porous silica shell. mL HCl solution (0.125 mol/L), after ultrasonic treatment for
Although kinds of metal nanoparticles have been developed as 10 min, those magnetite were separated with a magnet and
3
activity center, including non-noble metal (Ni-B, Co, Bi-Fe washed several times with deionized water and ethanol to
0
31
3
2
33
22
34
35
36
3 4
oxide ) and precious metal (Au, Ag, Rh, Ru, Pd ), Rh remove HCl. Next, pre-treated Fe O were redispered in a
3
7
nanoparticles exhibited the highest activity. Therefore, Rh mixture of ethanol (160 mL) and water (40mL), with vigorously
NPs (2-3 nm) was chosen to fabricate a core-shell structured stirring, 4 mL ammonium hydroxide and 0.5 mL tetraethyl
nanocatalyst (Fe
NPs has the highest activity for the activation of N
utilize hydrazine hydrate as the hydrogen source, which has coated magnetite (Fe
been deemed to be an ideal hydrogen-storage material for its Fe O @nSiO with a monolayer amine groups, 0.5 mL APTES
3 4 2 2 2
O @SiO -NH -RhNPs@mSiO ). Because Rh orthosilicate were injected dropwise. The resulting solution
3
8
2
H
4
,
we was kept for stirring at ambient condition for 6 h to get silica
@nSiO , 1b). To modify the surface of
3
O
4
2
3
4
2
as high as 8.0 wt % hydrogen-storage capacity and easiness for was added into a mixture of Fe O @nSiO and isopropanol
3
4
2
3
handing. What`s more, using hydrous hydrazine as a (200 mL) under magnetic stirring. The resulting solution was
9
o
hydrogen source has shown great advantages of simple post heated at 80 C for 3 h, cooled and the product was collected.
processing, no requirement of high pressure reactor and Washed with ethanol for several times.
cleanest by-product nitrogen.
General procedure for porous silica encapsulated
Nevertheless, the hydrazine mediated CTH (eq. 1) needs magnetically recoverable Rh NPs (Fe O @nSiO -NH -
3
4
2
2
excess N H generally because the unavailable decomposition RhNPs@mSiO , 1f): 5.0 mg RhCl3—3H O was dissolved in 20 mL
2
2
4
2
3
3, 40
of hydrazine is inevitable as shown in path 1 and path 2:
.
ethanol, meanwhile, the prepared Fe O @nSiO -NH were
3 4 2 2
R
NO
2
+
1.5
N
2
H
4
H
2
O
R
NH
2
+
3.5 H O
2
(eq. 1)
dispered in 80 mL EtOH. Under vigorously stirring of
Fe O @nSiO -NH solution, the rhodium precursor solution
3
4
2
2
Path 1:
Path 2:
N
2
H .
4
H
2
O
N
2
+
H
2
+
+
H
2
O
(eq. 2)
(eq. 3)
was added dropwise, stirred overnight. To get Rh NPs
absorbed on the surface of silica, 20 mL methanol solution of
sodium borohydride (0.1 g) was mixed with above solution
immediately, and kept stirring for another 15 min to obtain 1d.
The preparation of outer layer of silica was similar with the
synthesis of inlayer silica, but the differences were the volume
of TEOS (1 mL) and the time of stirring (10 h).
H .
N
2
4
H
2
O
NH3
+
N2
2
H O
On the other hand, the treatment of waste fluid containing
hydrazine is a challenge both in technology and cost-economy
4
1
for the cellular toxicity of hydrazine.
Therefore,
stoichiometric ratio use of hydrous hydrazine in the reduction
of nitro compounds is according with the concept of green
chemistry. To our delight, due to the “slow-releasing” of
hydrazine hydrate resulting in the special porous structure, our
catalyst could be used in the reduction of nitro with
stoichiometric hydrazine hydrate. At the same time, the
reaction can be carried out in water on account of the
sufficient activity of Rh NPs. Compared with traditional organic
solvents, water is the “greenest” solvent for the huge
Porous silica layer encapsulating Rh NPs was synthesized
43
according to a published procedure. Briefly, 1.0 g PVP was
dissolved in 10 mL H O, and dropped into 50 mL Fe O @nSiO -
2
2
3
4
o
NH -RhNPs@SiO solution. The mixture was heated to 100 C
2
2
and kept for 3 h with magnetic stirring. After cooling, 10 mL
aqueous sodium hydroxide (0.1 g/mL) was added and the
mixture was stirred at r.t. for 1 h. Finally, washing with water
and ethanol completed the synthesis of the catalyst
Catalytic transfer hydrogenation reaction and product
analysis: 1 mmol of nitro compound was added into a sealed
tube with 20 mg catalyst. 2 mL water was volumed as solvent.
3
6, 42
advantage in security and economic efficiency.
o
The reaction occured at 110 C, and the yield was monitored
Experimental
by HPLC. After reaction, the catalyst was separated by an
external magnet and the product was obtained by cooling and
precipitating from water. After separation, the catalyst was
Material
Spherical nano Fe O (100~300 nm) were purchased from
3
4
Aladdin Reagent Co., Ltd., China, RhCl ·3H O, NaBH , Polyvinyl
3
2
4
Pyrrolidone (PVP) and all of the nitroarenes were provided by
Sinopharm Chemical Reagent Co.,Ltd., China. Tetraethyl
orthosilicate (TEOS), 3-aminopropyl-triethoxysilane (APTES),
hydrous hydrate (80%) were supplied by the Tianjin Yongda
Chemical Reagent Co., Ltd., China. All chemicals used were
analytically pure and used without any further purification.
General
procedure
preparation
of
Fe O @nSiO -NH -
3 4 2 2
RhNPs@mSiO (Fig. 1):
2
General procedure for preparetion of amine-functionalized
silica coated nano Fe O supported nanocomposite
3
4
2
| J. Name., 2012, 00, 1-3
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ARTICLE
reused without extra treatment at the same conditions.
Fig.
porous structure can be distinguished easily. The interface
between Fe core and silica layer is clear, and the thickness
of inlayer silica shell is approximately 20 nm. While, rhodium
3 4 2 2 2
1 The preparation of magnetic porous Fe O @nSiO -NH -RhNPs@mSiO
3 4
O
Fig. 4 XRD patterns of Fe
3 4 2 2 2
O @SiO -NH -RhNPs@mSiO (1f) compared with standard
spectra of Fe and SiO .
3
O
4
2
Fig. 2 FT-IR spectra of nano Fe
3
O
4
(1a), Fe
3
O
4
@SiO
2
(1b), Fe
(1f).
3 4 2 2
O @SiO -NH (1c),
Fe @SiO -NH -RhNPs (1d), Fe @SiO
O
3 4
2
2
O
3 4
2
-NH -RhNPs@mSiO
2
2
3 4 2
nanocatalyst (black: Fe O ; blue: SiO ; yellow: Rh NPs).
Result and discussion
Characterization of catalysts
Fourier transform infrared spectroscopy (FT-IR) has been
carried out to verify the structure of Fe @nSiO -NH
RhNPs@mSiO (Fig. 2). The adsorption peaks at 1084 cm is
assigned to asymmetrical stretching vibration of Si-O-Si, which
is absent in Fe NPs. While two peaks appear at 801 and 476
3
O
4
2
2
-
-1
2
3
O
4
-1
cm due to the stretching vibration and bending vibration of
Si-O, respectively. The evidence shows the coating of silica.
Meanwhile, absorption bands are also observed at 2976 and
-
1
2
932 cm , which attributed to asymmetrical stretching
Fig. 3 HR-TEM images: a) as prepared 1d; b) after 10 times used 1d; c) as prepared 1f;
d) after 15 times used 1f; e) 1d at 5 nm scale; f) nitrogen adsorption-desorption
isotherm of 1f.
-1
2
vibration of –CH groups, and 1568 and 1465 cm bands
37
confirmed the existence of –NH
To illustrate the size, shape and morphological characteristcs
of magenetic separation nano catalyst, Fe @nSiO -NH
RhNPs (1d), Fe @nSiO -NH -RhNPs@mSiO (1f) were
scrutinized by high resolution transmission electron
microscopy (HR-TEM). As we can see (Fig. 3), Fe @nSiO
NH -RhNPs and Fe @nSiO -NH -RhNPs@mSiO were
regularly in the spherical shape with an average diameter of
69 nm and 367 nm (by LPSA in SI). And there is an obvious
2
groups.
nanoparticles (2-3 nm mainly) are anchored on the surface of
inlayer silica equally. Unfortunately, without the protection of
porous silica layer, Rh NPs in 1d will be washed away gradually
in reaction then 1d loses its original activity (Fig 3b). At the
same time, the structure of 1f is stable enough with locating
Rh NPs between the two layers of silica (Fig 3d). The distance
between Rh atom layers was measured to be 2.16 A and 1.92
3
O
4
2
2
-
3
O
4
2
2
2
3
O
4
2
-
2
3
O
4
2
2
2
4
4
A of the type {111} and {200}, respectively.
2
difference between porous silica layer and inlayer silica,
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ARTICLE
The Brunauer-Emmet-Teller surface area of as-prepared 1f
Table 1 Optimization of reaction condition
Journal Name
HO
NO
2
+
N H —H O
2
4
2
HO
NH2
o
d
Entry
Catalyst
Hydrogen
source
eq.
Solvent
T/ C
Time
Conv./Sele.
1
2
3
4
5
6
7
8
9
1a
1b
1c
1d
1e
1f
1f
1f
1f
1f
1f
1f
N
N
N
N
N
N
N
N
N
N
N
N
2
2
2
2
2
2
2
2
2
2
2
2
H
H
H
H
H
H
H
H
H
H
H
H
4
4
4
4
4
4
4
4
4
4
4
4
·H
·H
·H
·H
·H
·H
·H
·H
·H
·H
·H
·H
2
2
2
2
2
2
2
2
2
2
2
2
O
O
O
O
O
O
O
O
O
O
O
O
1.5
1.5
1.5
2.0
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
H
H
H
H
H
H
2
2
2
2
2
2
O
O
O
O
O
O
110
110
110
110
110
110
110
110
90
60 min
60 min
60 min
4 min
-/-
-/-
-/-
100/>99
-/-
10 min
7 min
100/>99
60/>99
100/>99
100>99
100/>99
43/>99
20/>99
MeOH
10 min
7 min
isopropanol
H
H
H
H
H
2
2
2
2
2
O
O
O
O
O
10 min
15 min
15 min
10 min
10
11
12
13
70
50
rt.
1f
1f
1f
1f
NaBH
4
4.0
rt.
rt.
3 h
24 h
1 h
100/>99
90/>99
2/>99
-/-
b
1
4
H
2
-
-
-
MeOH
c
1
1
5
isopropanol
ethylene
glycol
-
-
110
110
c
6
1 h
2
was investigated to be 15.50 m /g using nitrogen adsorption-
(entry 4). While covering the Rh nanoparticles with another
a
reaction condition: 4-NP (1 mmol), solvent (2 mL), catalyst silica layer gave no yield as a result of no access to the active
b
c
(
20 mg); the pressure of H was 1 atm; isopropanol and site (entry 5). Porous core-shell structure catalyst gave a full
2
d
a
Conditions: nitroarenes (1.0 mmol) hydrazine hydrate (1.5
ethylene glycol acted as hydrogen source and solvent;
conversion and selectivity were determined by HPLC.
o
mmol), catalyst 1f (20 mg), water (2 mL), 110 C, 7-30 min;
b
desorption measurement. It is clear that the adsorption- determined by GC-MS.
desorption isotherm (fig 3f) belongs to a type IV classification conversion of p-nitrophenol to p-aminophenol in 7 mins (entry
45
coupling with a hysteresis loop, indicating mesoporous 6). When using water or isopropanol as solvent, there is no
structure with an average pore diameter of 9.50 nm calculated difference in yields. However, the conversion declined in
MeOH system (entries 6, 7 and 8). Temperature is a key factor.
by employing the Barrett- Joyner-Halenda model.
o
o
An X-ray diffraction (XRD) pattern in the wide-angle region is With the decrease of temperature from 110 C to 50 C, yield
displayed as shown in Fig. 4. There are no intense or broad descended, and at the same time, reaction time extended
diffraction peaks of rhodium as the result of low loading and
very small nanoparticles. Amorphous Silica is also affirmed due
to short and wide characteristic diffraction peaks.
The magnetic properties of the Fe O , Fe O @SiO -NH -
2
3
4
3
4
2
RhNPs and Fe O @SiO -NH -RhNPs@mSiO have been
3
4
2
2
2
determined by vibrating sample magnetometer (VSM) system
at room temperature, with the field sweeping from 10,000 to
+
10,000 Oe. Their magnetization saturation values are 71.87,
0.0 and 39.6 emu/g, respectively. No obvious remanence and
5
coercivity are shown. The VSM date clearly indicated both of
them were superparamagnetic. The catalysts can be separated
easily using an external magnet as shown in Fig. 5.
The performance of Fe O @nSiO -NH -RhNPs in nitrobenzene
3
4
2
2
reduction
In our work, p-nitrophenol was chosen as model substrate
under various conditions (table 1). Without the loading of Rh
nanoparticles, none p-aminophenol was detected (entry 1-3). Fig. 5 Room temperature hysteresis loops of nano Fe O (1a), Fe O @SiO -NH -RhNPs
3
4
3
4
2
2
After anchoring Rh nanoparticles to the surface of (1d), Fe
Fe O @nSiO -NH , 100 % conversion can be achieved in 4 mins
3 4 2 2 2
O @SiO -NH -RhNPs@mSiO (1f).
3
4
2
2
4
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C7GC00986K
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ARTICLE
(
entries 6, 9, 10, 11 and 12). Not only was hydrazine hydrate The stability of both Fe @SiO
3
O
4
2 2
-NH -RhNPs (1d) and
selected as hydrogen source, but also kinds of reductive agents Fe @SiO -NH -RhNPs@mSiO (1f) were tested (Fig. 6).
2
3
O
4
2
2
were tested. The conversions with sodium borohydride or Because of the outstanding magnetic property, no filtration or
hydrogen gas as the hydrogen source could also reach 100 % centrifuge was required after reaction. The catalysts can be
and 90 %, only if the reaction time would be prolonged to 1 fixed on an external magnet within one minute, meanwhile,
hour and 24 hours respectively.
poured the mother liquid out directly and it can be reused
Following the evaluation of reaction scope, several without any post-processing. Both of those catalysts show
nitroarenes was investigated as shown in table 1 under excellent catalytic efficiency, however, with the increasing of
optimized condition (table 2). All of them can be converted to cycle index, progressively declining catalytic effect could be
3 4 2 2
their corresponding amino products smoothly. Meanwhile, observed for catalyst Fe O @SiO -NH -RhNPs. Generally, the
reducible groups are tolerant in our catalytic protocol such as deactivation mainly caused by the leaching or the
acid, ester and amino. In the presence of hydrazine hydrate, agglomeration of active site-RhNPs. The Rh amounts loaded in
dehalogenation may occur to halogenated anline depending Fe O @SiO -NH -RhNPs before and after being used can be
3
4
2
2
4
6-48
on the the position of halogen at benzene ring,
however, determined by ICP with 1.127 wt% and 0.6287 wt%
no dechloration was observed in the reduction of chloro respectively. This indicates that the leaching of Rh is the main
nitrobenzene in our catalytic system, no matter ortho, mete, reason. It can be also verified by HR-TEM of recycled 1d (Fig.
para position or monosubstituted, disubstituted chloro 3d), in which no large particle of Rh can be observed.
substrates (3, 4, 5, 6). The catalytic reduction by hydrazine can Nevertheless, Fe O @nSiO -NH -RhNPs@mSiO could keep its
3
4
2
2
2
also be applied for heterocycle containing nitroarenes with original performance even after 15 cycles which could be put
good yields. Some products can be separated by crystallization down to the fixation and stability of porous shell of silica, and
after cooling down because of low solubility in water. In a the content of Rh decreased only from 0.5339 wt % to 0.5146
wt %. From the change of Rh-containing, we make sure that
the main reason of the deactivation of catalyst is the losing of
Rh element.
The comparison with other reported works in the reduction
of nitroarenes (mainly 4-NP) has been made to show the
advantages of our protocol (table 3). It is quite clear that we
have incomparable superiorities in terms of the hydrazine
dosage, reaction time and solvent selection. Herein, the merits
of our reaction system can be concluded as the followings: (1)
The catalyst system is clean because non-toxic solvent is used;
(
2) No excess reductant is required and no residual reductant
is discarded; (3) The silica shell protected catalyst is still
efficient enough that the TOF in the reduction of 4-NP is as
-1
high as 4373 h ; (4) The encapsulated catalyst is stable and
Eq
Entry
Substrate
Catalyst
Solvent
Time
Ref
4
5
9
0
1
4-NP
Ni-Co BMNPs
4.0
H2O
1.0 h
2
3
4
5
6
7
8
9
4-NP
4-CNB
4-NP
4-NP
4-CNB
4-NP
4-NP
4-CNB
4-NP
4-NP
O-MoS
2
2.0
3.4
3.0
4.0
3.0
4.3
7.0
6.0
27
EtOH
H2Or
EtOH
EtOH
EtOH
MeOH
EtOH
EtOH
MeOH
H2O
1.0 h
45 min
1.0 h
4.0 h
40 min
10 min
3 h
2
5
2
1
Ag/Fe
2
3
O /rGO
MoS
2
3
5
3
2
Au/TiO
2
2 3
Fe O /ACF
Fig. 6 a) The conversions and selectivities, b) TOFs in the recycling of catalytic system
Fe @nSiO -NH -RhNPs (1d) and Fe @nSiO -NH -RhNPs@mSiO (1f).
53
2
Co-Co B
O
3 4
2
2
O
3 4
2
2
2
5
3
3
4
3
0
2
Co–Mo C/AC
scaled synthesis of
6
that is an intermediate for many fine
Au/TiO2
16 h
chemicals, the COD (chemical oxygen demand) of the mother
liquid water was about 600, while in industry the COD value
can be 3000 due to the excess usage of hydrazine hydrate. A
simple adsorption that remove soluble organic compound with
resin will decrease COD to 50 that fully fit the wastewater
emission demand.
1
0
Ni-B NPs
9 h
11
Our catalyst
1.5
7 min
this work
magnetically recovered.
The “slow-releasing” of Hydrazine
Many methods have been developed to reduce the emission
of excess hydrazine in the CTH, such as supporting N on
3 4 2 2
The reusability of Fe O @nSiO -NH -RhNPs
2 4
H
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ARTICLE
Journal Name
5
5-57
some carriers
. However, none of them provides a formation of hydrogen gas (path 1, eq. 2). Another critical
2 4
stoichiometric N H solution in the CTH reaction. To achieve factor is temperature. As the apparent activation energy of the
this, not only the highest active site should be chosen to CTH reaction (58 kJ/mol for 4-NP) is higher than that of the
construct efficient catalyst, but also the appropriate chemical generation of hydrogen from the Rh-catalyzed decomposition
5
8
62
of hydrazine (46 kJ/mol) , the higher temperature is
environment is strictly limited.
In our protocol, we take the attitude that the porous silica propitious to the reduction reaction rather than the hydrogen
shell plays an important role in the “slow-releasing” of generation. And our results confirmed that a moderate
hydrazine from the liquid phase to the surface of Rh- temperature resulted in lower conversion in despite of the
5
9
nanoparticles. A plausible mechanism is proposed as shown extending reaction time. What`s more, higher temperature
in Fig. 7. Because of core-shell and porous structure as well as benefited the effect of hydrogen spillover.
the acidic nature of silicon dioxide, hydrazine hydrate is
absorbed on the channel of the silica shell, hence, the diffusion
of hydrazine from the liquid phase to the surface of Rh-particle
Conclusions
is slow.
In summary, a core-shell structured catalyst Fe
3 4 2 2
O @SiO -NH -
The initial rate method was used to determine the reaction
order. The constants of reaction rate were 0.01248, 0.02216,
RhNPs@mSiO has been developed for the catalytic hydrogen
2
transfer reaction, and it has been used in the reduction of nitro
compounds to synthesize corresponding amines in water. The
catalyst is so effective that only stoichiometric hydrogen
source, namely, hydrazine hydrate was used because the
porous silica shell contributes the “slow-releasing” of
reductant to the catalytic active site, as well as the hydrogen
spillover effect between Rh-nanoparticles and silica shell. The
main reason of the deactivation of catalyst is the leaching of
-1
o
o
o
0
.04743 mol/L
the presence of excess usage of hydrazine hydrate results in
.5 order reaction of p-nitrophenol to p-aminophenol, while in
·min at 40 C, 50 C, 60 C, respectively. In
0
th
1
a literature it had been reported as 0 order . And the
-1
apparent active energy (Ea) was counted to be 58.18 kJ mol
using Arrhenius Equation.
If the nitrocompound is reduced on bare metal nanoparticles
at
active RhNPs that are anchored on the surface of Fe
3
O
4
@SiO
2
-
Table 3 The comparison with reported works
NH . However, the activity of Fe @SiO -NH -RhNPs@mSiO
2
3
O
4
2
2
2
could be stable over 15 (certainly more, theoretically 387, see
SI) cycles owing to the protection of porous silica shell.
Magnetic separation makes it easier for recycling the
heterogeneous catalyst. Therefore, the high efficiency, the
good chemoselectivity and the wonderful recoverability are
meritorious features of the catalyst. The green solvent and the
stoichiometric dosage of hydrogen donor also provide an
environmentally friendly strategy for the reduction of nitro
compounds.
Acknowledgements
This work was financially supported by National Natural
Science Foundation of China (21235004, 21175080) and the
Ministry of Science and Technology (2013ZX09507005).
Fig. 7 The possible pocession for CTH of aromatic nitro compounds.
a very low concentration, the kinetics order for the
st 60
nitrocompound is 1 . The difference of the kinetics order
indicates that the diffusion is the main reason that provides
the “slow-releasing” effect. As the reaction is very fast
Notes and references
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