Multifunctional mesoporous silica-supported palladium nanoparticles for selective phenol hydrogenation in the aqueous phase

Article abstract of DOI:10.1039/c4cy01036a

A simple, efficient and recoverable palladium-based catalyst was successfully prepared by immobilizing palladium nanoparticles (Pd NPs) over hydrophobic core-hydrophilic shell structured mesoporous silica microspheres. The obtained catalyst was characteri

Full text of DOI:10.1039/c4cy01036a

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Multifunctional mesoporous silica-supported
palladium nanoparticles for selective phenol
hydrogenation in the aqueous phase
Cite this: DOI: 10.1039/c4cy01036a
a
ab
Fengwei Zhang* and Hengquan Yang*
A simple, efficient and recoverable palladium-based catalyst was successfully prepared by immobilizing
palladium nanoparticles (Pd NPs) over hydrophobic core–hydrophilic shell structured mesoporous silica
microspheres. The obtained catalyst was characterized by transmission electron microscopy (TEM), N
2
adsorption–desorption, Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA)
and X-ray photoelectron spectroscopy (XPS). The as-prepared catalyst could rapidly convert phenol to
cyclohexanone with 98.5% conversion and 97.1% selectivity under mild reaction conditions in the aqueous
Received 10th August 2014,
Accepted 18th September 2014
2 3 2
phase (1 atm H , 80 °C, 3 h). The excellent catalytic performance of the Pd/MS-C @MS-NH catalyst was
probably attributed to the enhanced synergistic effect between highly dispersed Pd NPs and significantly
decreased phenol mass transfer resistance. In addition, it could be easily recovered by centrifugation and
reused 5 times without any significant loss in activity and selectivity.
DOI: 10.1039/c4cy01036a
www.rsc.org/catalysis
(
−SO
3
H), which showed excellent catalytic performance in
Introduction
22
deacetalization–Henry cascade reaction.
Core–shell structured multifunctional mesoporous materials
have recently attracted significant attention owing to their
unique characteristics, such as high specific surface area,
controllable pore structure, narrow pore size distribution, and
As is well-known, the hydrogenation of phenol is one of
the effective methods for the preparation of cyclohexanone,
which serves as an important intermediate in the synthesis
2
3
of caprolactam and adipic acid. The phenol hydrogenation
route is generally undertaken through one- or two-step
1
–5
versatile surface functionalization. Thus, they are generally
regarded as an ideal candidate for catalysis, adsorption,
separation, energy storage and conversion, drug delivery
2
4
processes. The two-step process involves hydrogenation of
phenol to cyclohexanol followed by dehydrogenation to cyclo-
6
–12
25
and optics.
Among them, as far as catalyst supports
hexanone. The one-step selective phenol hydrogenation to
are concerned, great progress has been made in their
functionalization, and the developed heterogeneous catalytic
systems displayed extremely high catalytic activity, selec-
tivity and stability in a variety of important organic
cyclohexanone, in contrast, is more advantageous in the per-
spective of energy efficiency and operation cost. Recently,
considerable research has been devoted to the development
of highly active heterogeneous catalysts for one-step selective
1
3–20
26
transformations.
For example, Suzuki et al. first reported
phenol hydrogenation. In particular, Han and co-workers
the SO H core–hydrophobic shell structured mesoporous
3
have reported the synthesis of an unprecedented commer-
cially available heterogeneous palladium and Lewis acid com-
bined catalyst, which was carried out at 50 °C under 10 atm
silica microspheres through the co-condensation/expansion
method, which exhibited enhanced catalytic activity for the
condensation reaction of 2-methylfuran and acetone. They
found that the obvious enhancement in catalytic activity due
to the incorporation of hydrophobic groups was only achieved
with core–shell structure, and little enhancement was
2
7
2
of H for liquid-phase phenol hydrogenation. However, the
environmentally unfriendly co-catalysts and organic solvents
were indispensable for the reaction process as the reaction
pressure was relatively higher.
To overcome these problems, palladium nanoparticles
supported on a variety of organic and inorganic materials,
2
1
observed in a randomly distributed structure. Yang et al.
have also successfully prepared a yolk–shell multifunctional
2
8
29
nanoreactor with basic core (−NH
2
)
and acidic shell
such as polymers, mpg-C
3
N
4
,
N-doped ordered meso-
3
0
31
porous carbon (NOMC), carbon nanotubes (CNTs), and
3
2
carbon nanofibers (CNFs), have been developed and used
to catalyze the hydrogenation of phenol in the aqueous
phase. Nevertheless, the separation and recycling of the
polymer-supported catalyst were relatively complicated and
a
Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, PR China.
E-mail: fwzhang@sxu.edu.cn, hqyang@sxu.edu.cn; Tel: +86 351 7016082
b
School of Chemistry and Chemical Engineering, Shanxi University,
Taiyuan 030006, PR China
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the Pd NPs were inclined to aggregate due to the high surface
energy. In addition, the preparation procedures of these types
of solid supports usually involve multiple steps of prepara-
tion, high pyrolysis temperature and environmentally
unfriendly reactants, which consume a considerable amount
of time and energy. It is also very important to have the
ability to scale up the synthesis for the preparation of large
quantities of materials. Therefore, the development of a more
facile and greener heterogeneous catalyst with excellent
phenol conversion and selectivity is still a great challenge. To
our knowledge, there is no report of direct selective hydro-
genation of phenol to cyclohexanone over easily recyclable
multifunctional core–shell structured mesoporous silica in
the aqueous phase. Herein, we report a novel hydrophobic
core/hydrophilic shell structured mesoporous silica immo-
bilized Pd NP catalyst, which acts as a highly active, water-
tolerant, and easily recyclable catalyst for selective phenol
hydrogenation. More importantly, it can be easily recycled
and used repetitively more than 5 times without loss of
catalytic efficiency.
Loading of Pd NPs on hydrophobic core–hydrophilic shell
mesoporous silica
To a stirred suspension of MS-C @MS-NH (1.0 g) in 30 mL
3
2
of deionized water was added 65 mg of Pd(OAc)
ring for 4 h (the amount of Pd(OAc) adsorbed onto the solid
materials was calculated from the UV-Vis spectra) 20 mL of
NaBH (120 mg) was slowly added and the suspension was
2
. After stir-
2
4
stirred for another 2 h. The suspension was centrifuged and
washed several times with deionized water and ethanol,
resulting in Pd/MS-C
was 3.12 wt%).
3 2
@MS-NH (the loading amount of Pd
3 2
Pd/MS-C @MS-NH catalysts for selective phenol
hydrogenation
In a typical experiment, 80 mg of the catalyst (0.025 mmol),
0.5 mmol of phenol and 3 mL of deionized water were added
into the reaction vessel. The vessel was sealed and purged
three times with hydrogen to remove the air at room temper-
ature. Then, hydrogenation was performed at 80 °C under
1
atm H for 3 h. After completion of the reaction, the prod-
2
ucts were extracted with ethyl acetate, followed by analysis
using an Agilent 7890A gas chromatograph, and the products
were identified by comparison with authentic samples.
Experimental
Materials
PalladiumIJII) acetate (Pd(OAc)
silicate (TMOS, 98%) and sodium hydroxide (NaOH) were
purchased from Sinopharm Chemical Reagent Co., Ltd. Sodium
2
, ≥97%), tetramethyl ortho-
Recycling of the Pd/MS-C
3
@MS-NH
2
catalyst
@MS-NH
The recyclability of the Pd/MS-C
3
2
catalyst was
borohydride (NaBH
4
, 98%), (3-aminopropyl)trimethoxysilane
tested for phenol-selective hydrogenation in the aqueous
phase under the same reaction conditions as described
above. After each run of reaction, the catalyst was separated
from the reaction mixture by centrifugation, washed thor-
oughly with ethanol, dried at 60 °C for 2 h, and then reused
directly for the next run.
(APTMS) and n-propyltrimethoxysilane (C TMS) were obtained
3
from Aladdin Chemical Reagent Co., Ltd. Cetyltrimethyl-
ammonium chloride (CTAC, 99%) was purchased from
Nanjing Robiot Co., Ltd. All other chemicals were of analyti-
cal grade and were used without any further purification.
Preparation of hydrophobic core–hydrophilic shell
Characterization
mesoporous silica
Transmission electron microscopy (TEM) was performed
using a FEI Tecnai G2 F20S-Twin with an accelerating voltage
of 200 kV. For sample preparation, the powders were dis-
persed in ethanol by sonication, and one drop of the solution
Hydrophobic core–hydrophilic shell mesoporous silica was
prepared according to the method previously described by
Suzuki et al. with some modification. Specifically, 3.52 g of
CTAC and 2.5 mL of NaOH (1.0 mol L ) were dissolved in a
mixture solution of 400 mL of deionized water and 500 mL of
2
1
−
1
was dropped onto a microgrid. N adsorption–desorption iso-
2
therms were obtained with an ASAP2020 analyzer. Before
measurement, samples were degassed under vacuum at
373 K for 6 h. The surface area of the samples was calculated
by the Brunauer–Emmet–Teller (BET) method, and the pore
volume and pore size distribution were calculated using the
Barrett–Joyner–Elmer (BJH) model. Thermogravimetric analy-
sis (TGA) was performed using a SETARAM Evolution 16/18
apparatus. The samples were heated on an alumina pan from
methanol, and then 1.20 g of TMOS and 0.25 g of C TMS
3
were added into the above solution with vigorous stirring at
room temperature. After 1 h of pre-hydrolysis, 1.20 g of TMOS
and 0.20 g of APTMS were slowly added to the suspension.
After 8 h of continuous stirring, the obtained white precipi-
tate was filtered and washed thoroughly with deionized water
and ethanol and dried at 100 °C for 6 h. To remove the sur-
factant, the as-synthesized material was dispersed in a
−
1
25 °C to 800 °C at a heating rate of 10 °C min under nitro-
gen. Fourier transform infrared (FTIR) spectra were obtained
using a Nicolet NEXUS 670 spectrophotometer (frequency
1
00 mL ethanol solution containing 1.5 mL of concentrated
HCl and the mixture was stirred at 80 °C for 12 h. The
resulting material was designated as MS-C @MS-NH . As a
control, the MS@MS-NH
under the same procedure except no C TMS was added into
−
1
range from 4000 to 500 cm ) with KBr pellets. X-ray photo-
electron spectra (XPS) was obtained with the PHI-5702 instru-
ment and the C1S line at 286.6 eV was used as the binding
energy reference.
3
2
2
and MS-NH
2
@MS-C
3
were prepared
3
the synthesis process and in turn the order of adding.
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nanoparticles with particle sizes of about 5.8 nm for
Pd/MS-C @MS-NH and 5.4 nm for Pd/MS@MS-NH2 were
Results and discussion
3
2
The synthesis route of the hydrophobic core–hydrophilic
uniformly dispersed on the surface of the multifunctional
core–shell structured mesoporous silica. However, the Pd NPs
were poorly distributed on the surface of the Pd/MS-NH @MS-C
catalyst (Fig. 1d) and they were inclined to aggregate into
larger particles (about 8.1 nm).
Scanning transmission electron microscopy (STEM) analysis
3 2
of the Pd/MS-C @MS-NH catalyst was performed to obtain
more detailed information about the structure. As shown
in Fig. 2, STEM demonstrated that the as-synthesized
Pd/MS-C @MS-NH catalyst has a typical core–shell structure,
3 2
shell structured Pd/MS-C @MS-NH catalyst is illustrated in
3
2
Scheme 1. Firstly, the hydrophobic core was prepared
through the co-condensation of TMOS and C TMS in the
2
3
3
presence of CTAC in an alkaline methanol solution. Secondly,
after a certain time of pre-hydrolysis and condensation, a
mixture of TMOS and APTMS was added to the above sus-
pension to form a layer of hydrophilic shell. The resultant
materials were extracted with hot acidic alcohol solution
to remove CTAC. Thirdly, palladium nanoparticles were
immobilized on the MS-C @MS-NH microspheres by
which was in agreement with the results of TEM mea-
surement. EDX element mapping of the sample further
showed the spatial distributions of Si, O, N and Pd in the
Pd/MS-C @MS-NH catalyst. The strong Si and O signals
3
2
impregnation and subsequent reduction.
The respective TEM images of MS-C
3
@MS-NH
2
,
Pd/MS-C @MS-NH , Pd/MS@MS-NH and Pd/MS-NH @MS-C
3
2
2
2
3
3
2
are shown in Fig. 1. From the TEM image of MS-C @MS-NH
across the sphere confirmed the presence of mesoporous
silica, while the N and Pd signals detected in the surface
region clearly suggested that they were evenly distributed on
the surface of core–shell structured mesoporous silica. More-
over, our previous XPS elemental analysis results also have
3
2
(
Fig. 1a), highly uniform microspheres with an average
diameter of 340 nm, a shell thickness of 30 nm and perpen-
dicularly oriented channels about 2.7 nm in diameter can be
seen. For Pd/MS-C
Fig. 1b and c), the products were composed of uniform meso-
porous microspheres with a particle size of 350 nm, which
were similar to those of MS-C @MS-NH microspheres. Addi-
tionally, it can be clearly seen that most of the palladium
3 2 2
@MS-NH and Pd/MS@MS-NH catalysts
(
shown that the –NH groups were mainly concentrated in the
2
shell for the Pd/MF@MN catalyst (Pd NPs supported on
fluoro-functionalized core and amino-functionalized shell
3
2
1
6
structured mesoporous silica).
In the FT-IR spectra of MS-C @MS-NH
microspheres (Fig. 3A), the peaks around 3422 cm , 1404
3
2 3 2
and Pd/MS-C @MS-NH
−
1
−
1
and 1072 cm were assigned to the surface-absorbed water
molecules, C–H bending and Si–O–Si stretching modes,
−
1
respectively. The bands at 2926 and 2847 cm can also be
seen in these two materials, which should be attributed to
the saturated C–H stretching vibrations of propyl and amino-
propyl groups, respectively. Meanwhile, the presence of N–H
symmetric bending vibration at 1633 cm further confirmed
the existence of amine groups. The results demonstrate that
propyl and amino groups were successfully incorporated into
the mesoporous silica microspheres and in the IR spectrum
3 2
Scheme 1 The synthesis procedure of the Pd/MS-C @MS-NH catalyst.
−
1
of Pd/MS-C
immobilizing Pd NPs on the MS-C
3
@MS-NH
2
, almost no change occurs after
@MS-NH microsphere
3
2
Fig. 1 TEM images of (a) MS-C
3
@MS-NH
2
, (b) Pd/MS-C
3
@MS-NH
2
,
Fig. 2 (a) Images of STEM and EDX of the Pd/MS-C
3 2
@MS-NH catalyst
(
c) Pd/MS@MS-NH and (d) Pd/MS-NH
2
2
@MS-C .
3
and (b) the corresponding elemental mapping of Si, O, N and Pd.
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3 2 3 2
Fig. 3 (A) FTIR spectra of (a) MS-C @MS-NH and (b) Pd/MS-C @MS-NH
and (B) the TGA curve of Pd/MS-C
3
@MS-NH .
2
3 2
Fig. 5 XPS spectrum of the Pd/MS-C @MS-NH catalyst.
surface. The thermal stability of the Pd/MS-C @MS-NH cata-
3
2
lyst was investigated by TGA under nitrogen. As shown in
Fig. 3B, the TGA curve of Pd/MS-C @MS-NH exhibits 1.5 wt%
3
2
weight loss at around 150 °C due to the loss of adsorbed water
molecules, the second weight loss takes place above 150 °C,
and about 14.1% weight loss was observed between 150 °C
and 650 °C, indicating that propyl and amino groups were
successfully incorporated into the mesoporous silica. The
sharp peaks centered at 341.6 and 336.4 eV, which were
ascribed to Pd 3d3/2 and Pd 3d5/2, respectively. The results
demonstrated that all of the Pd(II) species were completely
converted into Pd(0) nanoparticles through chemical reduc-
4
tion with NaBH .
TGA curve of Pd/MS-C
thermal stability up to 150 °C.
The N adsorption–desorption isotherms and the corre-
sponding pore size distribution (inset) of MS-C @MS-NH
and Pd/MS-C @MS-NH are shown in Fig. 4. Both the nitro-
3
@MS-NH
2
further represents good
The catalytic activity of different heterogeneous catalysts
was evaluated for the selective phenol hydrogenation and the
results are summarized in Table 1. As can be seen, in the
2
3
2
absence of the catalyst and in the presence of MS-C
at 80 °C for 6 h in the aqueous phase, no product was
detected (Table 1, entries 1 and 2). For Pd/MS-NH @MS-C
and Pd/MS@MS-NH catalysts, the reaction proceeds slowly
3 2
@MS-NH
3
2
gen adsorption and the desorption can be classified as type
IV isotherms with a small hysteresis loop. The specific sur-
face area, total pore volume and pore size of MS-C @MS-NH
2
3
2
with 38.2% and 78.8% phenol conversion and >99% and
98.7% cyclohexanone selectivity, respectively (Table 1, entries
3 and 4). As expected, when the reaction time increased from
3 h to 6 h, the conversion of phenol increased to 83.8% and
>99%, while the selectivity for cyclohexanone decreased to
98.6% and 97.5%, respectively (Table 1, entries 5 and 6). It is
worth mentioning that the reaction proceeds smoothly
with 98.5% phenol conversion and 97.1% cyclohexanone
3
2
2
−1
3
−1
were calculated to be 731 m g , 0.375 cm g and 2.54 nm,
respectively. The high specific surface area and well-developed
mesoporous channels suggest that it was a good candidate as
support for loading active Pd NPs. The specific surface area,
total pore volume and pore size were 494 m g , 0.258 cm g
and 2.38 nm, respectively, after immobilizing Pd NPs on
MS-C @MS-NH microspheres, indicating that Pd loading
2
−1
3
−1
3
2
had little effect on the specific surface area and pore size
distribution of Pd/MS-C @MS-NH .
The catalyst was also characterized by XPS in order to
ascertain the oxidation state of Pd in the catalyst. Fig. 5
shows the Pd 3d spectrum of the catalyst. It can be seen that
3 2
selectivity in the presence of Pd/MS-C @MS-NH within 3 h
(Table 1, entry 7). Apparently, the hydrogenation rate of this
new heterogenized catalyst was higher than that of its quasi-
3
2
2
6a
homogeneous counterpart (1 atm H
2
, 90 °C, 16 h).
The
excellent catalytic performance of Pd/MS-C @MS-NH was
3
2
the Pd binding energy of Pd/MS-C
3
@MS-NH
2
presented two
probably due to the hydrophilic shell being not only highly
Table 1 Selective hydrogenation of phenol to cyclohexanone by differ-
ent catalystsa
b
b
Entry Catalyst
T (°C) t (h) Conv. (%) Sel. (%)
1
2
3
4
5
6
7
—
MS-C
Pd/MS-NH
Pd/MS@MS-NH
Pd/MS-NH @MS-C
Pd/MS@MS-NH
Pd/MS-C @MS-NH
80
80
80
80
80
80
80
6
6
3
3
6
6
3
—
—
—
—
3
@MS-NH
2
2
@MS-C
3
3
2
38.2
78.8
83.8
>99
98.5
>99
98.7
98.6
97.5
97.1
2
2
2
3
a
Reaction conditions: 0.5 mmol of phenol, 80 mg of catalyst
b
(
0.025 mmol) and 3.0 mL of H
2
O under 1 atm H
2
.
The conversion
Fig. 4 Nitrogen adsorption–desorption isotherms and pore size
distributions of (a) MS-C @MS-NH and (b) Pd/MS-C @MS-NH .
3 2 3 2
and selectivity were determined using an Agilent 7890A gas
chromatograph.
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dispersed and stabilized Pd NPs through amino groups but
also beneficial to the uniform dispersion of the heteroge-
neous catalyst in water. Moreover, the hydrophobic core
could rapidly concentrate phenol to the surroundings of Pd
NP active sites. Along with phenol being hydrogenated to
cyclohexanone, it constantly diffused into the aqueous solu-
tion from the mesoporous channel and avoided the further
hydrogenation of cyclohexanone. Therefore, the catalyst can
achieve the efficient, green and selective hydrogenation of
phenol.
Fig. 6a shows the evolution of phenol conversion and
cyclohexanone selectivity with reaction temperature. As can
be seen, the conversion of phenol increased from 85.2% to
Fig. 7 Recycling tests of the Pd/MS-C
3
@MS-NH
2
catalyst in selective
phenol hydrogenation. Reaction conditions: 0.5 mmol of phenol,
.0 mL of H O, 80 mg of catalyst, 1 atm H , 80 °C, 3 h.
9
9
9.6% as the reaction temperature increased from 50 °C to
0 °C. Although the conversion of phenol increased rapidly
3
2
2
from 50 °C to 80 °C, the selectivity for cyclohexanone reduced
gradually. Notably, a further increase in reaction temperature
hardly improved the phenol conversion, but the selectivity for
cyclohexanone significantly decreased from 98.2% to 86.5%.
The influence of reaction time on phenol conversion and
cyclohexanone selectivity is shown in Fig. 6b. The reaction
was accompanied by a rapid increase in phenol conversion
and a slight decrease in cyclohexanone selectivity in the first
reach 98.5% and the selectivity for cyclohexanone could
reach 97.1%.
Additional advantages of Pd/MS-C @MS-NH prepared
3
2
here were stability and ease of recycling because it can be
facilely separated by centrifugation. Fig. 7 shows the recycla-
bility of Pd/MS-C @MS-NH for selective hydrogenation of
3
2
phenol in the aqueous phase. It is worth mentioning that the
catalyst still exhibited excellent catalytic activity even after
running for more than five cycles. However, the slight
decrease in phenol conversion with further recycle times
should be due to the gradual leaching of Pd NPs from the
catalyst surface (the Pd content in the catalyst was found
to be 2.97 wt% based on ICP-AES analysis, indicating that
ca. 4.8% of Pd was leached from the catalyst surface). The
results clearly indicate that the high catalytic performance
should be attributed to the small particle size and good
dispersion of Pd NPs, high specific surface area and hydro-
phobicity of the core.
3
h, suggesting that phenol was mainly hydrogenated to
cyclohexanone in the first step. With the further increase in
reaction time (from 3 h to 8 h), the phenol conversion no
longer increased and cyclohexanone selectivity decreased
gradually (from 97.1% to 88.2%), illustrating that the conver-
sion of cyclohexanone into cyclohexanol plays a vital role in
the second step. The influence of the S/C molar ratio on the
conversion of phenol and selectivity for cyclohexanone is
shown in Fig. 6c. When the molar ratio of S/C (substrate/
catalyst) was 80 : 1, the conversion of phenol was low due to
the inadequate number of active sites. The conversion
increased as the S/C molar ratio decreased from 80 : 1 to 20 : 1,
and a conversion of 98.5% was achieved. However, the conver-
sion hardly changed with a further decrease of the S/C molar
ratio. Considering the cost of the catalyst and reaction rate,
the suitable amount of the S/C molar ratio was 20 : 1. Based
on the above results, the optimum conditions of selective
phenol hydrogenation were obtained: reaction time 3 h,
reaction temperature 80 °C and S/C molar ratio 20 : 1. Under
the optimized conditions, the conversion of phenol could
Conclusion
In conclusion, a simple and environmentally friendly
approach was reported for the synthesis of hydrophobic
core–hydrophilic shell structured Pd/MS-C @MS-NH
3
2
nanocomposite catalysts, which exhibited superior catalytic
activity and stability toward the selective hydrogenation of
phenol in water. The reasons could be attributed to the
Fig. 6 Conversion of phenol and the selectivity for cyclohexanone as functions of (a) reaction temperature, (b) reaction time and (c) S/C molar
ratio. Reaction conditions: 0.5 mmol of phenol, 3.0 mL of H O and 1 atm H .
2 2
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synergistic effect derived from binary organic functional
groups since the amino groups promoted the uniform disper-
sion of Pd NPs inside the pore channels while the propyl
groups decreased the diffusion limitation of phenol in water.
Thus, it can be seen that this synthesis strategy provided
a useful idea for the fabrication of other nanoparticle-
supported catalysts with high specific surface area, ordered
mesoporous channel and hydrophobic/hydrophilic function-
ality, which should be highly promising in diverse catalytic
reactions.
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Acknowledgements
This work was supported by the Scientific Research Start-up
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