Preparing magnetic multicomponent catalysts via a bio-inspired assembly for heterogeneous reactions

Article abstract of DOI:10.1039/c6ra12088a

This article reports a facile synthetic approach for preparing magnetic porous catalysts, on which various inorganic compounds are alternately loaded using a pyrogallic acid (PG)-assisted layer-by-layer (LbL) coating. The PG forms an adhesion and reductive layer on the surface of hierarchically porous silica, and Pd, Al₂O₃, ZrO₂, or FeO_x can then be introduced on the surface of the material. Compared with the conventional impregnation method, the PG layer obviously improves the dispersion of catalysts and greatly reduces the blocking of the pores, which is caused by the formation of metal oxide catalysts. At the same time, the reduction properties of the layer help to reduce the diameter of the Pd nanoparticles to 5 ± 1.3 nm. The Fe³⁺ ions are partially reduced into Fe²⁺ by the PG layer to form Fe₃O₄, which endows the porous catalysts with a magnetic separation property. The synthesized multicomponent catalysts show excellent catalytic activity for various reactions, including the Suzuki Miyaura coupling, reduction, Knoevenagel condensation and Friedel-Crafts alkylation reactions. The TOF values show an improvement of 2 to 4 times over those of the catalysts prepared by the traditional impregnation method. The multicomponent magnetic catalysts also exhibit superior catalytic performance for many one-pot multistep cascade reactions.

Full text of DOI:10.1039/c6ra12088a

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Preparing Magnetic Multicomponent Catalysts via the Bio-
inspired Assembly For Heterogeneous Reactions
Received 00th January 20xx,
Accepted 00th January 20xx
Wentong Song, Shengyang Tao*, Yongxian Yu, Xuanlu Du and Shuo Wang
DOI: 10.1039/x0xx00000x
This article reports a facile synthetic approach for preparing magnetic porous catalysts, on which various inorganic
compounds are alternately loaded using a pyrogallic acid (PG)-assisted layer-by-layer (LbL) coating. The PG forms an
www.rsc.org/
2 3 2
adhesion and reductive layer on the surface of hierarchically porous silica, and Pd, Al O , ZrO , or FeOx can then be
introduced on the surface of the material. Compared with the conventional impregnation method, the PG layer obviously
improves the dispersion of catalysts and greatly reduces the blocking of the pores, which is caused by the formation of
metal oxide catalysts. At the same time, the reduction properties of the layer help to reduce the diameter of the Pd
3+
2+
3 4
nanoparticles to 5 ± 1.3 nm. The Fe ions are partially reduced into Fe by the PG layer to form Fe O , which endows the
porous catalysts with a magnetic separation property. The synthesized multicomponent catalysts show excellent catalytic
activity for various reactions, including Suzuki Miyaura coupling, reduction, Knoevenagel condensation and Friedel−Crafts
alkylation reactions. The TOF values show an improvement of 2 to 4 times over those of the catalysts prepared by the
traditional impregnation method. The multicomponent magnetic catalysts also exhibit superior catalytic performance for
many one-pot multistep cascade reactions
.
Therefore, the particle size and distribution of the active
catalytic compounds on the surface of porous silica become
the key factors in determining the reactivity and selectivity of
catalysts. Furthermore, many multistep cascade reactions
need different catalysts. The integration of various inorganics
on one porous support with high dispersion is therefore
Introduction
Uniformly dispersing nano-sized inorganic catalysts on the
surface of porous carriers is important for improving the
1
efficiency of the catalyst in heterogeneous catalysis, yet still
,
2 3
represents a significant challenge.
Reducing the size of
1
5, 16
meaningful work, but difficult to achieve.
In nature, biological systems often exhibit complicated
catalytic particles increases the utilization rate of the atoms,
which reduces the amount of expensive catalysts required,
1
7, 18
4
-8
compositions and multiple functions.
In these systems, it is
including Au, Pt and Pd.
found that the activity of catalysts can be controlled by varying
In recent years, researchers have
common for inorganics to self-assemble with the assistance of
organic molecules. For example, shells, which have high
hardness and ideal stability, are formed merely by the LbL
9
-11
the surface properties of the support materials.
Porous
silica is one of the most commonly used catalyst support
materials due to its large surface area, tunable pore structure
1
9, 20
assembly of CaCO
3
nanoparticles and proteins.
Mimicking
12 13
,
this biomineralization process can help researchers to develop
2
and chemical stability.
Immobilization of the inorganic
1-26
excellent organic-inorganic hybrid materials.
incorporation of a magnetic component (e.g. Fe
Further,
catalysts on the surface of the porous silica support is usually
achieved using the impregnation method with calcination
treatment. However, this normal impregnation method makes
it difficult to control the formation process of the inorganics in
the pores. Large particles are easily generated due to interface
migration and aggregation, but this lowers the dispersibility of
the catalysts and can even block the inner pores. Obviously,
3
O
4
) to provide
an additional function to the original material has been widely
used and there were many reports on the applications of
.
27-31
magnetite nanocomposite material for catalysts
Lately,
Messersmith et al. successfully introduced inexpensive plant
polyphenols onto solid surfaces, and the polyphenol film
3
2
1
4
showed adsorption and reduction properties with metal ions.
the catalysts
’active sites cannot then be used effectively.
Therefore, inspired by the unique adhesion and reduction
properties of plant polyphenols in nature, we developed a
general, green chemistry approach for the immobilization of
inorganic catalysts. Therefore, in our work, catalytic metal
oxides are loaded on HPS by polyphenols. This approach is
more facile, low-cost and environmental friendly. In addition,
it can be extended to the immobilization of more catalysts on
various materials for various reactions. We used PG as an
Department of Chemistry, Dalian University of Technology, Dalian 116024,
Liaoning, P.R. China.
E-mail: taosy@dlut.edu.cn; Fax: +86-411-84986035; Tel: +86-411-84986035
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
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Ltd. All the reagents mentioned above were used without
further purification.
Synthesis of HPS support. The HPS was prepared and modified
3
3
by previously reported method. The experimental details are
shown below: Firstly, PEG (8.85 g) and acetic acid (10 mM, 75
mL) were mixed together and stirred to get a homogeneous
solution. Subsequently, TMOS (30 mL) was added and stirred
for about 10 min for hydrolysis at
semitransparent sol was transferred into polythene (PE) tubes
sealed for gelation and aged (36 h) at 40 after which the
resultant gels were treated by 1 M ammonium hydroxide
solution at 110 for 9 h. Thirdly, the porous monolithic
0 ℃ . Then the
℃
℃
materials was cooled down to room temperature and
impregnated into aqueous solution of nitric acid (0.1 M) to pH
Scheme 1. Pyrogallic acid (PG) as an adhesion layer to combine the LbL coating
and impregnation method. Then, magnetic component and many other
inorganic compounds were successfully intergraded in hierarchically porous silica
=
7.0 and then evaporation-dried at 60
deionized water. The required HPS support was obtained by
calcination at 650 for 5 h in air.
Preparation of PG modified silica (HPS-PG). The as-made HPS
1.0 g) were crushed and sieved to collect 600 850
particles and then immersed into bicine buffer solution (pH =
℃ after washing by
(
HPS).
℃
adhesion layer to combine the LbL coating and impregnation
methods in this study. Then, a magnetic component and many
other inorganic compounds were successfully intergraded in
the hierarchically porous silica (HPS) (Scheme 1). HPS, which
has both macro and meso pores, has the advantages of a large
surface area and high molecular diffusion, which are beneficial
for the loading of catalysts and transfer of reactants. The PG
layer could strongly adhere to almost all types of surfaces.
Although one side of the PG layer bound to the silica surfaces
through covalent and noncovalent interactions, it further
immobilized various kinds of metal ions through coordination
or reduction. The selective chemical adsorption led to uniform
inorganic compounds. After the calcination, the dispersion of
catalytic materials was considerably improved. Surprisingly,
through the reducibility of PG, the magnetic element (FeOx)
was also fabricated in the porous silica using the LbL coating.
The multicomponent catalysts showed excellent catalytic
activity for several reactions, even including cascade reactions.
High conversion ratios, selectivity and TOF values were
achieved, indicating that they are highly efficient catalysts.
(
−
μm
-
1
7
.8, 0.1 M, 10 mL) containing PG (0.1 mg mL ) at room
temperature for 24 h, the obtained solid particles (named as
HPS-PG) were washed by water and dried over night.
In-situ generation of Pd nanoparticles modified silica support.
HPS-PG (1.0 g) particles was added into aqueous solutions of
PdCl (15 mM, 10 mL) at room temperature for 6 h, the solid
2
was washed with water, and dried in vacuum then the HPS-PG-
Pd sample was obtained and after calculation we name it as
HPS-C-Pd. Similarly uncoated silica-supported Pd nanoparticles
were fabricated using the same procedures and after
calcination we name it as HPS-Pd.
In-situ generation of metal oxides modified silica support.
HPS-PG (1.0 g) immersed into aqueous solutions of Fe(NO3)3
9
H2O (0.2 M, 10 mL)
，iso-Propyl alcohol solution of aluminium
isopropoxide (10 mM, 10 mL) and n-propanol solution of
zirconium n-propoxide (20 mM, 10 mL), respectively at room
temperature for 24 h, the solid was washed with ethanol, and
dried in vacuum at 40
℃. The resultant modified silica
Experimental Section
supports were named as HPS-PG-Fe, HPS-PG-Al and HPS-PG-Zr.
The calcining process was carried out to obtain the HPS-C-
FeOx, HPS-C-Al O and HPS-C-ZrO . Similarly, uncoated silica-
Reagents and materials. Tetramethoxysilane (TMOS) was
2
3
2
bought from the Chemical Factory of Wuhan University
supported metal oxides were fabricated using the same
procedures and after calcination we name them as HPS-FeOx,
HPS-Al O and HPS-ZrO , respectively.
(Wuhan, China). PEG (polyethylene glycol, Mw=10,000), NaOH,
Na SO were bought from the Sinopharm Chemical Reagent
2
4
2
3
2
Co., Ltd. N, N-Bis (2-hydroxyethyl) glycine, pyrogallic acid,
aluminium isopropoxide, zirconium n-propoxide, iodobenzene,
phenylboronic acid, indoles, chalcones, benzaldehyde
dimethylacetal, 4-nitrophenol (4-NPh), 4-aminophenol (4-APh)
and sodium borohydride were bought from Aladdin Chemical
Co., Ltd. Acetic acid, nitric acid, anhydrous ethanol,
hydrochloric acid were purchased from Fuyu Fine Chemical of
Tianjin Co., Ltd. Malononitrile was purchased from Shanghai
Kefeng Chemical Reagents Co., Ltd. Benzaldehyde, n-propyl
alcohol, n-butyl alcohol, methylbenzene and diethyl ether
were purchased from Tianjin Kemiou Chemical Reagent Co.,
Ltd. Ethyl cyanoacetate was purchased from J & K Scientific
Preparation of bi-functional materials by LbL coating. The
preparation of HPS-C-FeOx-C-Pd, HPS-C-FeOx-C-Al and HPS-
C-FeOx-C-ZrO were the same as HPS-C-Pd, HPS-C-Al and
HPS-C-ZrO , respectively except the HPS-PG-FeOx was used as
support. The uncoated HPS-C-FeOx was also used to support
Pd, Al and ZrO with the same procedure and we called
them HPS-C-FeOx-Pd, HPS-C-FeOx-Al and HPS-C-FeOx-ZrO
Preparation of multi-functionally and magnetically
recoverable catalysts by LbL coating. HPS-C-FeOx-C-Al was
2 3
O
2
2 3
O
2
2
O
3
2
2
O
3
2
.
2 3
O
used as support, coated with PG, followed by immersed into n-
propanol solution of zirconium n-propoxide (20 mM, 10 mL)
2
and aqueous solutions of PdCl (15 mM, 10 mL), respectively.
With the procedure mentioned above and named them as
2
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2 3 2 2 3
HPS-C-FeOx-C-Al O -C-ZrO and HPS-C-FeOx-C-Al O -C-Pd. All immersed into an oil bath equipped with a magnetic stirrer
the above mentioned calcinations were carried out in a tubular under N
2
.
furnace under nitrogen condition. The temperature program Catalytic for Friedel–Crafts reactions. Friedel-Crafts alkylation
was from room temperature to 550
℃
with a ramp of 3
℃
of indoles with chalcones was carried out. In a typical
synthesis, indole (2 mmol), chalcone (3 mmol), catalyst (0.1 g),
-1
min and maintenance for 6 h.
Characterization. The microscopic features of the samples and methylbenzene as solvent (20 mL) were mixed in a flask
were taken with a QUANTA 450 scanning electron microscope connected to a cooling condenser. Then the flask was
(SEM) at 30 kV. Transmission electron microscope (TEM) immersed into an oil bath equipped with a magnetic stirrer at
images were obtained with a Tecnai F30 electron microscope 110 ℃ for 6 h.
equipped operates at 300 kV accelerating voltage and Catalytic the cascade reactions of Knoevenagel condensation
equipped with Schottky Field emission gun (FEG). X-Ray coupled with subsequent hydrogenation process. In a typical
diffraction (XRD) patterns were obtained on a Rigaku D/MAX- procedure, the catalysts
400 X-ray powder diffraction (Japan) using Cu K radiation, DMF, followed by adding 4-nitrobenzaldehyde (0.2 mmol) and
operating at 40 kV and 10 mA. The surface elements were malononitrile (0.21 mmol). Next, the obtained solution was
analyzed by X-ray photoelectron spectroscopy (XPS, Shimadzu stirred in a stainless-steel autoclave at 80 for 20 min to
（0.1 g） were dispersed into 15 mL
2
α
℃
KROTAS AMICUS spectrometer).The nitrogen adsorption and finish the Knoevenagel condensation. Subsequently, the
desorption isotherms were measured at 77 K using an ASAP autoclave was purged with H for 4 times, and the H pressure
2
2
2
010 analysis instrument. The specific surface areas were of the autoclave was set at 0.20 MPa for the subsequent
calculated by the Brunauer-Emmett-Teller (BET) method and hydrogenation process. The reaction solvent was magnetically
the pore size were calculated using the Barrett-Joyner-Halenda stirred (245 rpm) at 50 for 3 h.
BJH) model. FT-IR spectra (4000-400 cm ) were collected on a Catalytic of deacetalization-Knoevenagel cascade reaction.
Nicolet Avatar 360 FT-IR spectrometer. Thermal gravimetric mixture of benzaldehyde dimethylacetal (4 mmol),
℃
-1
(
A
-
analysis (TGA) was carried out using TGAQ600 ramp 10
1
to 500 ℃. We used UV1000 spectrophotometer to measure mL) and the catalysts (0.2 g) was stirred at 80 ℃ for 3 h. The
℃ min malononitrile (6 mmol), anhydrous toluene (8.0 mL), H O (0.2
2
the conversion of p-Nitrophenol in the solutions. Gas reaction conversion was calculated based on benzaldehyde
chromatography (GC-7900) equipped with a capillary column dimethyl acetal since there was excess malononitrile. For the
(
PEG-20M, 30.0 m
was used to measure the conversion of the Reactant. The by GC and GC-MS.
magnetic measurements were performed on super
×320 mm×0.25 mm) and an FID detector specified time, samples were taken periodically and analyzed
a
conducting quantum interference device vibrating sample
1
magnetometer (Quantum Design SQUID) at 300 K. H-NMR
Results and Discussion
spectra were measured on
a
Varian INOVA 400M
Study of the modified HPS structures. As shown in Fig. 1A and
spectrometer with chemical shifts reported in ppm.
Catalytic Suzuki-Miyaura cross-coupling reactions. In a typical
run, iodobenzene (2.0 mmol), phenylboronic acid (2.4 mmol)
were added to 40 mL of deionized water, then NaOH (8.0
mmol) was added and stirred at 80
℃ until the chemicals are
completely dissolved. Then catalyst (0.2 g) was added to the
stirred solution in one portion, and the reaction mixture was
stirred for another 3 h. After the mixture had cooled to room
temperature, the reaction mixture was diluted with diethyl
ether (2
× 20 mL), dried with excessive Na SO , and
2 4
concentrated under reduced pressure to yield the final
product.
Catalytic activities for nitro reduction reactions. In a typical
reduction protocol, 50 mg of catalyst was added to 10 mL
water containing 0.1 mmol of nitro-compound and 1.3 mmol
4
of NaBH . The mixture was vigorously stirred at room
temperature for 10 min. The reaction was monitored by UV-vis
spectroscopy. The bright yellow solution gradually faded as the
reaction proceeded.
Catalytic activities for Knoevenagel condensations. Reactions
were performed with benzaldehyde and ethyl cyanoacetate at
80 ℃ or malonitrile at 25 ℃ in DMF. In a typical run, the
catalyst (0.2 g), benzaldehyde (0.1 mol), ethyl cyanoacetate or
malonitrile (0.11 mol), and DMF (15 mL) were mixed in a flask
connected to a cooling condenser. Then the flask was
Fig. 1. SEM and TEM images of the HPS (A, B). SEM and TEM images of the HPS-
PG (C, D). The scale bars in (A, C) represent 10 μm and the scale bars in (B, D)
represent 100 nm.
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Fig. 2. (A) FT-IR spectra of HPS and modified process. (B) XPS spectrum of HPS
before and after PG modification. (C) TGA plot of HPS before and after PG
modification in flowing air from 25 to 500 ℃ to determine weight loss of 3.1 %
(
wt) upon oxidation of the PG layer.
Fig. 3. TEM images of (A) HPS-C-Pd and (B) HPS-Pd and the scale bars represent
2
0 nm. HRTEM image of the (C) HPS-C-Pd nanocomposite and the scale bar
represents 5 nm. (D) Pd particle size distributions were estimated by counting 55
B, the as-made HPS matrix exhibited an interconnected particles in the the HPS-C-Pd TEM images and 48 particles in the HPS-Pd TEM
images.
1
three-dimensional (3D) macroporous structure and abundant
mesopores. The macropores and mesopores were about 2.3
μm and 14 nm in diameter, respectively. After modification
with PG, the HPS-PG maintained its regular morphology
without any blocking or cracking, and the texture of the silica
skeleton turned slightly rougher, as shown in the SEM image
(Fig. 1C). The mesopores were also well preserved according to
the TEM image (Fig. 1D), indicating that the PG layer did not
seriously affect the porous structure of the HPS.
The FT-IR, XPS and TGA results in Fig. 2 provide further
evidence for the growth of PG on HPS. Fig. 2A shows that the
-1
bands around 1625-1400 cm was assigned to the stretching
vibrations of C=C in the benzene ring from the HPS-PG
incomparison with the bare HPS, due to the formation of PG
3
4
on the surface of the silica. In the XPS curves (Fig. 2B), two
high intensity peaks representing Si could be observed from
the naked HPS at a binding energy of 150.5 eV (Si2s) and 100.2
eV (Si2p), respectively. Another two weak peaks at 532.7 eV
3
5
and 284.8 eV were attributed to O1s and C1s, respectively.
After modification with PG, the Si2p peak from the silica
substrate became weak. In contrast, the intensity of the C1s
peaks was enhanced relative to Si2p. This indicated the
formation of the PG coating on the HPS surface. The TGA plot
of the HPS and HPS-PG (Fig. 2C) in flowing air from 25 to 500
℃
and at around 100 ℃ were assigned to the absorbed water.
The TGA curve of the HPS-PG was used to determine the
weight loss upon oxidation of the PG, which was observed
from 285
is close to that of a monolayer of PG molecules.
2
Inorganic compounds, including Pd, Al , ZrO and FeOx,
℃ with a mass loss of about 3.1% (wt). This amount
2
O
3
were loaded onto the porous HPS and HPS-PG by impregnation
and calcination, respectively. The TEM and SEM results show
that the presence of PG obviously changed the dispersion state
of the inorganics. Fig. 3A and 3B show the Pd loads of the HPS
and HPS-PG, respectively. The PG layer allowed the Pd to form
Fig. 4. SEM images of PG modified materials (A) HPS-C-FeOx (B) HPS-C-Al
C) HPS-C-ZrO . The images in (B, D, E) are the corresponding uncoated silica
and HPS-ZrO , respectively. The scale bars in all of the
m.
2 3
O and
(
2
HPS-FeOx, HPS-Al
2
O
3
μ
2
images represent 10
4
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small nanoparticles, which were highly dispersed in the porous
silica. The high-resolution TEM image (Fig. 3C) shows small Pd
particles with a well-defined crystalline structure with regular
lattice spacing of d = 0.225 nm, consistent with the (111)
lattice planes of the face centered cubic. As a comparison, Fig.
3B shows that quite large and non-uniform Pd particles formed
on the bare HPS skeleton. The HPS-C-Pd (red line in Fig. 3D)
had a narrower Pd particle size distribution with better
dispersion and smaller Pd particles (5
± 1.3 nm) with the help
of the PG layer, whereas the HPS-Pd (blue line) had a broad Pd
particle size distribution from 4 to 30 nm. These results were
due to the reduction properties of the PG. Before calcination,
2
+
the Pd had already been partrly reduced to highly dispersed
Pd nanoparticles on the surface of the porous material. In the
conventional impregnation method, the nanoparticles formed
during the calcination, making it difficult to control their size.
Further investigation of the specific effect of PG on metal
oxides produced results similar to those for the above
mentioned Pd particles. As shown in Fig. 4A, compared with
the uncoated sample HPS-FeOx (Fig. 4B) , the skeleton of the
HPS-C-FeOx become thick, but the material still had a better
macroporous structure and less blocking, indicating that the
PG played an important role in improving the dispersion of
metal oxides after calcination. This result may be caused by
the combination of the PG layer with the metal ions. It has
been reported that polyphenol can coordinate with metal ions.
Fig. 5. XPS spectrum of HPS after surface modification. (A) typical XPS wide scan
spectra of HPS-PG-Pd. (B) high-resolution XPS spectra of Pd 3d. (C) typical XPS
wide scan spectra of HPS-PG-Fe. (D) high-resolution XPS spectra of Fe 2p and a
3/2
1/2
shakeup satellite XPS peak observed between Fe 2p and Fe 2p . (E) typical
XPS wide scan spectra of HPS-PG-Al. Inset shows a spectra of Al 2p. (F) typical
XPS wide scan spectra of HPS-PG-Zr. Inset shows a spectra of Zr 3d.
3
+
To study the interactions between Fe ions and PG polymer,
FT-IR spectra analysis was conducted. As shown in Fig. 2A,
3
after the HPS-PG adsorbed Fe ions, a new sharp absorption
+
around 719 eV, which is typical of the maghemite phase,
3
-1
9, 40
band at 1384 cm appeared, which was assigned to the
stretching vibrations of C-O-Metal, confirming that the metal
suggested the phase of the Fe
3
O
4
.
In addition, XPS studies
of HPS-PG-Al and HPS-PG-Zr confirmed the presence of
3
6
4
1, 42
ions had coordinated with the hydroxyl groups.
The
aluminum and zirconia elements (inserts in Fig. 5E and 5F).
As Fig. 6 shows, the N adsorption-desorption isotherms of
the unmodified and modified HPS monoliths all belong to type-
redundant ions caused by physical absorption were washed off
and homodisperse metal oxides were obtained after
2
4
3
calcination at 550
the bare HPS and HPS-PG, the results were similar (Fig. 4C and
D, and Fig. 4E and 4F, respectively), which were because of
℃. When Al O and ZrO were loaded onto
2 3 2
IV isotherm according to IUPAC classification. It indicates that
mesopores of HPS are maintained after modification. After
loading the catalysts, the surface areas of all of the examples
changed, as shown in Table 1. The HPS and HPS-PG had BET
4
the PG helped highly dispersed metal oxides particles in the
porous silica. After the modification of Al O and ZrO , the
2
-1
2
3
2
surface areas of 223.66 and 224.27 m g and pore volumes of
blocking of pores in the bare HPS was so serious that virtually
no macropores could be observed in the final products. This
would be harmful for the transfer of reactants in the pores.
XPS was carried out to further analyze the surface
composition of the synthesized component in the hierarchical
porous silica (Fig. 5). A wide range XPS of the four examples
(Fig. 5A, 5C, 5E and 5F) clearly shows four dominant peaks
corresponding to C1s, O1s, Si2s and Si2p. These indicated the
formation of the PG coating on the HPS surface. The XPS
spectra for Pd 3d peak (Fig. 5B) can be de-convoluted into two
pairs of doublets. A comparison of the relative areas of
0
2+
integrated intensity of Pd and Pd shows that the Pd mainly
37, 38
0
exists as Pd in the HPS-PG-Pd (
～
68 %).
This result
0
confirms the generation of metallic Pd particles and the
reductive properties of PG. The high-resolution XPS spectra for
Fe 2p is shown in Fig. 5D. Double peaks with binding energies
3
/2
1/2
of Fe 2p and Fe 2p at 711.1 and 724.9 eV are visible in the Fig. 6.
2
N adsorption-desorption isotherms of HPS, HPS-PG, and the subsequent
modified materials.
binding energy range of 705−735 eV. The satellite peak at
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Table 1. Structural properties of the HPS and modified HPS and the metal element
content of various modified materials measured by ICP.
S
BET
-1
D
p
V
p
3
Metal element
Sample
2
-1
(
m g )
(nm)
(cm g ) content (wt %)
HPS
223.66
224.27
204.51
213.6
14.16
14.42
13.87
13.67
13.92
12.42
1.069
1.074
0.919
0.942
1.052
0.928
--
HPS-PG
--
HPS-C-FeOx
HPS-C-Pd
5.74
0.129
0.142
0.271
HPS-C-Al
2
O
3
223.26
217.43
Fig. 8. (A) Raman spectra of the HPS-Fe O (red) and HPS-C-FeOx (blue)
2
nanocomposites. (B) magnification image of the green rectangular area in (A).
3
HPS-C-ZrO
2
The red line is hematite based on the characteristic peaks at 220, 289, 403, 681
−
1
−
1
and 1316 cm ; the blue line is magnetite, as revealed by the peaks at 630 cm .
Raman spectra were collected at room temperature.
3
.069 and 1.074 cm g , respectively, indicating that the thin layer behaved as a weak paramagnet displaying no saturation
-1
1
PG layer did not block the pores in the HPS. The SBET of or coercivity (Fig. S2). In the presence of the PG layer, the
different inorganics modified by HPS were about 204 to 220 saturation magnetization value of HPS-C-FeOx was 0.46 emu
2
-1
-1
m
g
and the pore size distribution before and after the g . After immobilization and calcination of the PG twice, the
modification of the HPS slight shrank from 14.16 to 11.07 nm magnetization of the samples increased from 0.46 to 0.95 emu
-1
and have uniform mesopores with narrow pore size g .
distribution (as shown in Fig. S1), which were large enough for
Raman spectroscopy was used to confirm the type of
most organic molecules to diffuse through. The element magnetic FeOx in the porous silica. Fig. 8 presents the in situ
content was measured using ICP-AES. The Fe content in the Raman spectra of HPS-Fe and HPS-C-FeOx. HPS-Fe
HPS-C-FeOx was 7.5 % (wt). The Pd, Al and Zr contents in the exhibits five bands, 220, 289, 403, 681 and 1316 cm , which
2
O
3
2 3
O
−
1
4
5
HPS-C-Pd, HPS-C-Al
2 3 2
O and HPS-C-ZrO
2 3
were 0.129% (wt), are typical γ-Fe O features. The additional typical peak at
-1
0
.142% (wt) and 0.271% (wt), respectively. The higher Fe 630 cm , for HPS-C-FeOx, probably because of the surface
content in the HPS-C-FeOx may have been induced by the Fe was partially reduced to Fe due to the reducing property
3
+
2+
3
+
44
reported stronger interaction between PG and Fe ions.
3 4
of PG to form Fe O . This suggests that the systematic increase
These results indicate that the PG modified surface could in the saturated magnetization was due to the increased Fe
3 4
O
4
6
effectively realize the immobilization of various kinds of content in the samples.
inorganic catalysts with low loading amounts (less than 0.5% enhanced the magnetization to a higher degree, reaching 1.01
The immobilized Pd greatly
-1
except for Fe).
Based on the outstanding performance of the PG layer for the noble metal induced the reduction of the non-noble metal
immobilizing inorganics, we attempted to integrate different
emu g . This result is in agreement with earlier reports that
components in a porous silica support by PG layer assisted LbL
deposition. It is interesting that the LbL assembly was found to
be beneficial for increasing the magnetics property of FeOx.
The magnetic behavior of the samples was measured at 300 K
in the applied magnetic field ranging from -20000 to 20000 Oe,
2 3
as shown in Fig. 7. The asprepared HPS-Fe O without the PG
Fig. 7. Magnetization curves at 300 K in the applied magnetic field ranging from
20000 to 20000 Oe. The insert shows the magnetic separation-redispersion
process of the HPS-C-FeOx-C-Pd catalyzed 4-nitrophenol (4-NPh) to 4-
aminophenol (4-APh).
Fig. 9. TEM images of (A) the HPS-C-FeOx-C-Pd and (B) the HPS-C-FeOx-Pd and
the scale bars represent 20 nm. (C) SEM image of the HPS-C-FeOx-C-Pd
nanocomposite and the scale bar represents 10 μm. (D) Pd particle size
distributions were estimated by counting 45 particles in the the HPS-C-FeOx-C-Pd
TEM images and 36 particles in the HPS-C-FeOx-Pd TEM images.
-
6
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4
7, 48
2+
Herein, the noble metal was Pd , Table 2. Catalytic activity test of various Pd catalysts in the Suzuki-Miyaura coupling
ions by reducing agent.
the non-noble metal ion was Fe and the reductant should reaction of iodobenzene and phenylboronic acid.
have been PG. As shown in the inset of Fig. 7, the HPS-C-FeOx-
3
+
C-Pd aggregated within a few seconds after an external
magnetic field was applied, redispersed quickly again via
shaking when the magnetic field was removed, demonstrating
the magnetic separation property of the material, which is
Pd counts
Time Conv.
TOF
Entry
Cat.
-1
particularly desirable for its practical applications in catalysis.
The Raman spectrum of the HPS-C-FeOx (Fig. 8A) is used to
further investigat the transformation of the PG layer to carbon
on the silica support. The broad Raman bands (Fig. 8A)
(wt %)
(h)
(%)
(h )
1
2
3
4
HPS-C-Pd
HPS-Pd
0.1288
0.1162
0.1377
0.1305
3
4
99
33
99
68
272.7
75.5
−
1
HPS-C-FeOx-C-Pd
HPS-C-FeOx-Pd
2.5
4
306.1
138.6
appearing at 1340 cm (denoted as D-band) generally refer to
2
the sp planar and conjugated structures. The narrow G-band
49
-1
appearing at 1585 cm indicates a disordered graphitic
structure. The D/G ratio ( 0.84) is slightly smaller than the
Reaction condition: iodobenzene (2 mmol), phenylboronic acid (2.4 mmol),
～
NaOH (8 mmol) and Cat. (0.2 g). TOF was calculated by moles of product per
reported values of TiO @Carbon prepared by the in situ molar Pd per hour. Conversion was determined by GC and the product were
2
1
polymer encapsulation
-
graphitization method, which is determined by the GC-MS and the H NMR spectrum (400 MHz, CD
3
CN) which are
5
0
included in Fig. S5, Fig. S6 and Fig. S7 in the Supporting Information.
indicative of the transformation of the PG layer to carbon.
The effect of FeOx on the loading of other components was
examined. Fig. 9A and 9B show the corresponding TEM images
of the HPS-C-FeOx-C-Pd and HPS-C-FeOx-Pd and the results are
the same as in Fig. 3A and 3B. The HPS-C-FeOx-C-Pd, which
was produced by the PG assisted LbL deposition, showed high
Pd dispersion compared with the non-PG-coated HPS-C-FeOx.
Other bi- and multi-functional catalysts based on the magnetic
2 3
Table 3. Catalytic activity test of various Al O catalysts in the Knoevenagel
condensations of benzaldehyde and ethyl cyanoacetate or malonitrile.
a
b
R′
R′
Al Counts
carrier were prepared by the LbL coating. The N sorption
2
Entry
Cat.
Conv. TOF
Conv. TOF
(
wt %)
isotherms of the catalysts are shown in Fig. S3. The pore
diameter and pore volume showed few changes, and the
values were similar to the bare HPS (Table S1). The LbL coating
bi- and multi-functional materials still had better macroporous
structures and less blocking (as shown in Fig. S4), which may
have been caused by the low loading amounts of the
elements. Therefore, the LbL coating process does not induce
the pore blocking of the support materials, and can ensure the
transfer of substances during the catalytic reaction.
-1
-1
(%)
(h )
(%)
(h )
1
2
3
4
HPS-C-Al
2
O
3
0.1417
0.2901
0.1735
0.1928
99
419.2
100 9432.0
52 2396.5
99 7626.2
58 4020.6
HPS-Al
2
O
3
46
99
57
95.2
342.4
177.4
HPS-C-FeOx-C-Al
2
O
3
2 3
HPS-C-FeOx-Al O
a
Reaction condition: benzaldehyde (0.1 mol), ethyl cyanoacetate (0.11 mol), DMF
b
(
15 mL) and Cat. (0.5 g) at 80
℃
for 9 h. Reaction condition: benzaldehyde (0.1
for 1 h.
mol), malonitrile (0.11 mol), DMF (15 mL) and Cat. (0.25 g) at 25
℃
Study of catalytic performance of modified HPS. The catalytic Conversion was determined by GC and the product were determined by the GC-
1
MS and the H NMR spectrum (400 MHz, CD
3
CN) which are included in Fig. S8 -
activity of the obtained catalysts was explored through rapid
Suzuki Miyaura coupling reaction, Knoevenagel condensation,
Fig. S13 in the Supporting Information.
Friedel-Crafts alkylation, nitrophenol reduction reaction and
Table 4. Friedel−Crafts alkylation of indoles with chalcones catalyzed by various ZrO
2
cascade reactions. The catalytic activity of the Pd loaded catalysts.
catalyst was examined in one of the representative Pd
catalyzed Suzuki Miyaura coupling reactions and the results
are summarized in Table 2. The different catalyst design
strategies show different catalytic activity. The TOF for the
HPS-C-Pd and HPS-C-FeOx-C-Pd (Entry 1, 3) is higher,
Zr Counts
wt %)
Conv.
(%)
TOF
representing a 2-4 times improvement over the other catalysts
Entry 2, 4). The PG coating thus represents a novel approach
Entry
Cat.
-1
(
(h )
(
1
2
3
4
HPS-C-ZrO
2
2.272
2.315
1.908
2.012
99
32
13.2
6.9
to achieving the high dispersion of Pd nanoparticles. Given a
similar Pd content, the smaller particle size leads to a larger
exposed surface area and high reactivity.
HPS-ZrO
2
HPS-C-FeOx-C-ZrO
HPS-C-FeOx-ZrO
2
～
100
15.7
8.4
The multicomponent catalysts, which contained Al
ZrO , exhibited similar results to those of the above mentioned
Pd catalyst. In the Al
method (Table 3), the deposited PG layer played an important (20mL) and Cat. (0.1 g) at 110
2 3
O or
2
56
2
2
O
3
-catalyzed Knoevenagel condensation Reaction condition: indoles (2 mmol), chalcones (3 mmol), methylbenzene
. Values at t = 6 h. Conversion was determined
℃
1
by GC and the product were determined by the GC-MS and the H NMR spectrum
role in improving the dispersion of catalysts, therefore, led to a
higher TOF value representing a 2-4 times improvement over
(
400 MHz, CD
3
CN) which are included in Fig. S14 - Fig. S16 in the Supporting
Information.
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Table 5. One-Pot Knoevenagel condensation-hydrogenation multistep cascade reaction (a) and tandem Deacetalization Knoevenagel condensation (b) catalyzed by various multi-
functional catalysts.
(
a)
(b)
Entry
Cat.
Conv. of 1 (%)
Yield of 3 (%)
Yield of 4 (%)
Conv. of 1 (%) Yield of 2 (%) Yield of 4 (%)
1
2
3
4
5
6
7
8
HPS
17
17
99
1
Trace
Trace
21
～
3
Trace
1
Trace
21
HPS-C-Al
2
O
3
～
99
22
100
22
---
---
---
99
99
HPS-C-Pd
HPS-C-Al -C-Pd
HPS-C-FeOx-C-Al
HPS-C-ZrO
HPS-C-Al
HPS-C-FeOx-C-Al
---
---
2
O
3
～
1
99
---
---
2
O
3
-C-Pd
～
100
Trace
---
～
100
---
---
2
---
---
---
---
86
13
2
O
3
-C-ZrO
2
---
---
2
97
2
O
3
-C-ZrO
2
---
---
～
100
Trace
～
100
a
Reaction condition: 4-nitrobenzaldehyde (0.2 mmol) , malononitrile (0.21 mmol), DMF (15 mL) and Cat. (0.2 g) with H
2
pressure 0.2 MPa, stirred (245 rmp) at 80
℃
b
for 3 h. Reaction condition: benzaldehyde dimethylacetal (4 mmol) malononitrile (6 mmol), H
2
O (0.2 mL) and toluene (5 mL) at 80 ℃ for 3 h. Conversion was
1
determined by GC and the product were determined by the GC-MS and the H NMR spectrum (400 MHz, CD
3
CN) which are included in Fig. S17 - Fig. S22 in the
Supporting Information.
the traditional impregnation method catalyst. The HPS-C-FeOx- with the help of PG layers using the LbL coating method. These
C-ZrO was evaluated by catalyzing the Friedel Crafts novel magnetically recyclable heterogeneous catalysts
alkylation (Table 4), which retained higher activity and a TOF exhibited superior catalytic performance for one-pot cascade
value 2 times that of the HPS-C-FeOx-ZrO . The ICP results reactions, such as the Knoevenagel condensation-
2
-
2
showed that the content of the catalytic compounds did not hydrogenation cascade reaction and deacetalization-
vary too much in the different catalysts. The Al and Zr Knoevenagel cascade reaction. The results showed that 99%
elements were even smaller in the LbL-prepared catalysts. All yield of the final product with 100% selectivity was achieved
of these results indicate that the PG-coated materials led to after 3 h, indicating that the tandem reaction (Knoevenagel
higher efficiency due to the high dispersion and stability of the condensation-hydrogenation hybrid catalysts) was successful.
active metal oxides. In addition, no catalysts were observed in Similar yields (acid-base hybrid catalysts) of the final product
the filtrate at the detectable limits of ICP. These results show were observed for the other one-pot reaction (Table 5) with
that there was little leaching, which contributed to the the same reaction time. A blank experiment without the
stability and recyclability of the catalyst.
heterogeneous catalyst was also performed (entry 1), and the
To further improve the multi-functional features of the results showed that the cascade reaction yield was only a trace
catalyst, we successfully combined two types of catalytic active in the absence of the hybrid catalyst.
sites with magnetic response properties on a single platform
The catalysts have the important characteristics of easy
recycling and reusability. As Fig. 10A shows, the modified
samples revealed a complete conversion of 4-NPh to 4-APh
within 10 min. To determine the durability of the catalyst, it
was recovered by simple magnetic separation and
subsequently reused after regeneration. After 10 cycles, the
conversion rate was still higher than 99% (Fig. 10B). The
samples without the help of PG were reused, but showed an
obvious loss of catalytic activity, with the conversion rate
reduced to about 60% after 10 cycles. This indicates that the
PG layer helps to stabilize the catalysts even though the layer
changes to carbon with the following calcination. The carbon
coating catalysts retained higher activity and demonstrated
Fig. 10. (A) Time-dependent evolution of UV−vis absorption spectra showing 4-NPh to
4
-APh catalyzed by HPS-C-FeOx-C-Pd at room temperature. (B) Recycling performance
of the four catalysts.
8
| J. Name., 2012, 00, 1-3
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improved stability compared with the uncoated catalysts. It
may also be possible to functionalize the carbon to provide
additional improvements in catalytic performance.
7
8
9
N. Fan, Y. Yang, W. Wang, L. Zhang, W. Chen, C. Zou and S.
Huang, ACS Nano, 2012, , 4072-4082.
Y. Yang, W. Wang, X. Li, W. Chen, N. Fan, C. Zou, X. Chen, X.
Xu, L. Zhang and S. Huang, Chem. Mat., 2013, 25, 34-41.
L. Srisombat, A. C. Jamison and T. R. Lee, Colloid Surf. A-
Physicochem. Eng. Asp., 2011, 390, 1-19.
6
Conclusions
10 H. N. Pham, A. E. Anderson, R. L. Johnson, T. J. Schwartz, B.
J. O’Neill, P. Duan, K. Schmidt-Rohr, J. A. Dumesic and A. K.
Datye, ACS Catal., 2015, 5, 4546-4555.
In summary, we present a general chemical approach for
improving the traditional method of preparing catalysts loaded
on porous silica, using PG, a polyphenol from plants. The PG
layer, when adhered to the surface of silica, remarkably
reduces the particle size of the catalysts and increases their
dispersion, which helps to avoid the blocking of the pores in
the silica support. Furthermore, the reducibility and
1
1
1
2
H. Y. Liu, L. Y. Zhang, N. Wang and D. S. Su, Angew. Chem.
Int. Edit., 2014, 53, 12634-12638.
W. Jing, L. Yuyun, C. Jia, L. Yuhui, Y. Qin, P. Gaoxiang, Y.
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N. Yu and J. L. Kong, Small, 2014, 10, 109-116.
adhesiveness of the PG layer allows various magnetic and 14 Y. Lee and T. G. Park, Langmuir, 2011, 27, 2965-2971.
1
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F. Zhang, H. Y. Jiang, X. Y. Li, X. T. Wu and H. X. Li, ACS
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catalytic inorganics to be successfully immobilized on the
surface of the support by the biomineralization. The PG acts as
both an adsorbing agent and a reducing agent, resulting in a
multifunctional catalyst containing highly dispersed catalytic
active sites and magnetic response properties. It shows
excellent catalytic activity in the Suzuki Miyaura coupling
reaction, nitrophenol reduction reaction, Knoevenagel
condensation reaction and Friedel-Crafts alkylation. The TOF
values are several times higher than those achieved by the
4
6, 5602-5608.
1
1
8
9
H. Lee, S. M. Dellatore, W. M. Miller and P. B. Messersmith,
Science, 2007, 318, 426-430.
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Li, H. Mohwald and X. H. Yan, ACS Appl. Mater. Interfaces,
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S. S. Shankar, A. Rai, B. Ankamwar, A. Singh, A. Ahmad and
can be easily recycled and reused, thus showing good potential
for practical applications. The strategy presented here, which
uses a PG layer to achieve the LbL coating, can be used to
adjust the surface environment of the conventional support
material and help to overcome some of the agglomeration of
particles. Furthermore, these magnetically recoverable
2
7
2
2
2
2
2
2
2
1
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Acknowledgements
This work was supported by National Natural Science
2816.
Foundation of China (21473019, 51273030) and the 28 C. Wang, J. Chen, X. Zhou, W. Li, Y. Liu, Q. Yue, Z. Xue, Y. Li,
A. A. Elzatahry, Y. Deng and D. Zhao, Nano Research, 2014,
Fundamental Research Funds for the Central Universities
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Page 11 of 11
RSC Advances
View Article Online
DOI: 10.1039/C6RA12088A
A facile synthetic approach for preparing magnetic porous catalysts, on which various
inorganic compounds loaded by pyrogallic acid (PG) assisted layer-by-layer (LbL)
coating.