Long chain ionic liquid-assisted synthesis of PS/Pd beads and hierarchical porous Pd-SiO2

Article abstract of DOI:10.1039/c8ra03884h

Long-chain ionic liquid, 1-hexadecyl-3-methylimidazolium chloride (C₁₆mimCl), was firstly used as a linking agent to construct polystyrene (PS)/C₁₆mimCl/palladium (Pd) beads. Subsequently, the PS/C₁₆mimCl/Pd beads were used as a macroporous templating agent and C₁₆mimCl was used as a mesoporous templating agent to prepare Pd-loaded hierarchical porous silica. A systematic study was carried out addressing the influence of the amount of C₁₆mimCl and the mass ratios of m(Pd)/m(PS) on the PS/C₁₆mimCl/Pd beads and the Pd-loaded hierarchical porous structures. The samples were characterized by electrophoresis experiments, SEM, TEM, small-angle XRD, and N₂ adsorption-desorption experiments. It was found that the coverage of citrate-coated Pd nanoparticles (Pd NPs) onto the PS beads can be simply tailored by changing the amount of C₁₆mimCl and the mass ratios of m(Pd)/m(PS). The resultant Pd-loaded hierarchical porous silica possessed a 3D ordered macroporous skeleton with a specific surface area of up to 967 m² g^-1, ordered mesoporous silica walls (SBA-3 type) and well-dispersed Pd NPs anchored on the inner walls of the spherical macroporous hollow. Importantly, the obtained Pd-loaded hierarchical porous silica exhibited an enhanced catalytic activity for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) by H₂O₂.

Full text of DOI:10.1039/c8ra03884h

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Long chain ionic liquid-assisted synthesis of PS/Pd
beads and hierarchical porous Pd–SiO2†
Cite this: RSC Adv., 2018, 8, 25141
Tianlong Wang, Ting Fu, Yuting Meng, Jing Shen *a and Tongwen Wangab
ab
ab
ab
Long-chain ionic liquid, 1-hexadecyl-3-methylimidazolium chloride (C16mimCl), was firstly used as a linking
agent to construct polystyrene (PS)/C16mimCl/palladium (Pd) beads. Subsequently, the PS/C16mimCl/Pd
beads were used as a macroporous templating agent and C16mimCl was used as a mesoporous
templating agent to prepare Pd-loaded hierarchical porous silica. A systematic study was carried out
addressing the influence of the amount of C16mimCl and the mass ratios of m(Pd)/m(PS) on the PS/
C
16mimCl/Pd beads and the Pd-loaded hierarchical porous structures. The samples were characterized
by electrophoresis experiments, SEM, TEM, small-angle XRD, and adsorption–desorption
N
2
experiments. It was found that the coverage of citrate-coated Pd nanoparticles (Pd NPs) onto the PS
beads can be simply tailored by changing the amount of C16mimCl and the mass ratios of m(Pd)/m(PS).
The resultant Pd-loaded hierarchical porous silica possessed a 3D ordered macroporous skeleton with
Received 7th May 2018
Accepted 5th July 2018
2
ꢀ1
a specific surface area of up to 967 m
g , ordered mesoporous silica walls (SBA-3 type) and well-
dispersed Pd NPs anchored on the inner walls of the spherical macroporous hollow. Importantly, the
DOI: 10.1039/c8ra03884h
obtained Pd-loaded hierarchical porous silica exhibited an enhanced catalytic activity for the oxidation of
0 0
2 2
3,3 ,5,5 -tetramethylbenzidine (TMB) by H O .
rsc.li/rsc-advances
introduction of ordered self-organized structures for synthesis
of functional nanoparticles and other inorganic nano-
Introduction
10,11
Long-chain ionic liquids (LILs), especially imidazolium-based structures.
Particularly, the supramolecular templating
LILs, have been most extensively studied for both funda- function of the LILs has been used to create various ordered
1
mental research and practical applications. The LILs consist of porous materials. For example, Adams et al. reported the
a charged hydrophilic head group and a hydrophobic ‘tail’ synthesis of mesoporous silica with 2D hexagonal structure by
12
domain, which can display not only the amphiphilic properties using the LILs as templating agent. Zhou et al. employed long-
of conventional surfactants, but also the specicity of the typical chain 1-hexadecyl-3-methylimidazolium chloride (abbreviated
short-chain room temperature ionic liquids such as low melting as C16mimCl), as template to prepare a supermicroporous
13,14
point, strong polarity and high solubility in aqueous solution lamellar silica by so-called “nanocasting” technique.
Our
etc. Increasing efforts have been made to explore the self- group has also contributed much effort to successfully synthe-
2
organized behavior of the LILs in aqueous solution. Various size the mesoporous silica with high-quality cubic gyroid and
characterization methods, such as surface tension, electrical 2D hexagonal mesoporous structures by using C16mimCl as
15
conductivity and uorescence measurement, have been used to template. These results indicated that the LILs have signi-
determine the micelle formation and the micelle aggregation cantly stronger tendency toward self-aggregation and supra-
3–6
numbers of the LILs in aqueous solution. The results indi- molecular templating in the preparation of ordered
cated that the surface activity of the LILs is slightly superior to mesoporous structure. However, their potential as modifying
that of the typical cationic surfactants. Moreover, the LILs have agent, especially the formation of LIL-functionalized polymer
been demonstrated to exhibit the behavior of both lyotropic and microspheres or their multifunctional combination for the
7,8
thermotropic liquid crystals, which offer a partially ordered synthesis of nanostructured materials is less commonly known.
9
microenvironment for organic reactions. These advantages of
The objective of the present work is twofold: rstly, long-
the LIL derivatives have been broadly applied in the chain ionic liquid, 1-hexadecyl-3-methylimidazolium chloride
C mimCl) was used as a surface functional molecule to modify
(
1
6
a
polystyrene (PS) beads, which can effectively connect small
Department of Applied Chemistry, College of Vocational Education, Yunnan Normal
citrated-coated palladium (Pd) nanoparticles (Pd NPs) to
College of Chemistry and Chemical Engineering, Yunnan Normal University, Kunming construct PS/C16mimCl/Pd beads. Secondly, based on a dual
University, Kunming 650092, China. E-mail: shenjingbox0225@hotmail.com
b
6
†
1
50092, China
templating method of the PS/C16mimCl/Pd beads and
Electronic supplementary information (ESI) available. See DOI:
0.1039/c8ra03884h
C
16mimCl, a more facile access to prepare Pd-loaded silica with
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hierarchical porous (macropore-mesopore) structures and methylimidazole (10.26 g, 0.125 mol) was mixed with an excess
catalytic activities is presented.
of 1-hexadecylchloride (33.41 g, 0.128 mol). The mixture was put
ꢁ
Recently, loading Pd NPs on suitable supports have attracted into a 250 mL ask, reuxed at 90 C for 24 h, and then cooled
great attention due to their unique catalytic properties for the down to room temperature. The product was further puried by
carbon–carbon cross-coupling reactions.
16,17
Coating Pd NPs on recrystallization in tetrahydrofuran (THF). Aer washing several
polymer beads can not only provide high dispersion and times with THF, the white crystalline powder of C16mimCl was
stabilization for small Pd NPs and large surface-to-volume ratio collected by vacuum ltration, and dried in air at room
to increase more number of catalytically active sites, but also be temperature. The structure of the obtained C16mimCl was
easily isolated and recycled from the reaction medium by identied by IR spectrum (see Fig. S1†).
18–20

ltration or centrifugation.
The traditional self-assembly
approach for the synthesis of polymer/Pd beads is based on
preformed Pd NPs sorption on polymer beads. For example,
Thompson and co-workers reported an electrostatic deposition
of colloidal noble metals (Pt, Pd, and Au) on the surface of PS
beads. Before the deposition, the non-cross-linked PS beads
were functionalized by using carboxylic acid- and amino-groups
Synthesis of PS/C16mimCl beads
Monodisperse PS beads were synthesized according to an
emulsion polymerization using sodium dodecylbenzene sulfo-
nate (SDBS) as emulsier. Typically, 21 mL of styrene was
washed in a separatory funnel four times with 20 mL of 0.1 M
NaOH, then four times with 20 mL water. 0.005 g of SDBS was
dissolved in 180 mL water in a round-bottomed ask. The
washed styrene was added, and nitrogen was bubbled to
deaerate the mixture for 20 min. 0.2328 g of potassium per-
sulfate initiator was added to the mixture, and the mixture was
21
to offer an effective connection of small noble-metal NPs.
Here, the C mimCl is designed to modify PS beads, which
1
6
forms the C mimCl-functionalized PS beads for the deposition
1
6
of the preformed citrated-coated Pd NPs.
Ordered porous materials can also be considered as an ideal
ꢁ
22,23
then heated to 70 C for 12 h with rapid stirring. The PS beads in
scaffold for loading Pd NPs.
Particularly, bimodal meso-
emulsion were collected by centrifugation.
porous–macroporous inorganic materials possess increased
mass transport through its macropores as well as retaining
a specic surface area on the level of ne mesoporous struc-
The PS bead solid was redispersed in water under ultra-
ꢀ1
sound. 50 mL of C mimCl aqueous solution (2.985 g L ) was
1
6
24
added dropwise and slowly to 10 mL of the PS bead dispersion
tures. In previously reported preparation of Pd NPs-loaded
ordered porous silica, the commonly used method is simple
impregnation with metal salt solutions into porous channels
and further reduction to form Pd NPs in the porous silica
channels. However, this method oen causes the partial plug-
ging and destruction of the porous system because of crystal-
lization of the metal salt during the calcination process to
remove the pore-forming templates. Some improved
approaches have been reported to enhance the stabilization of
the Pd NPs-loaded hierarchical porous structure. For instance,
Zhou et al. reported the cyclodextrin (CD)-based homogeneous
incorporation of Pd NPs into silica with bimodal pore struc-
ꢀ
1
(
4.686 g L ) with stirring for 4 h at room temperature to obtain
ꢀ
1
the dispersion (0.781 g L ) of the C16mimCl-coated PS beads
PS/C16mimCl). In order to explore the effect of the amount of
adding C16mimCl on the surface charge of the PS/C16mimCl
beads, the different volumes of the C16mimCl solution (V(C16
(
-
mimCl) ¼ 1, 25, 75 and 100 mL) were also added dropwise to the
PS bead dispersion.
Synthesis of PS/C mimCl/Pd beads
1
6
Pd nanoparticle (Pd NP) dispersion was prepared by using
sodium citrate as a reduction agent as well as a stabilizing agent
25
ture. In the present synthesis of the Pd-loaded hierarchical
porous silica, the prepared PS/C16mimCl/Pd beads and long-
chain C16mimCl were employed as dual templates to create
27
according to a reported method. Briey, 7.5 mL of PdCl2
ꢀ1
solution (0.99 g L ), 15 mL of sodium citrate solution (60 g
ꢀ
1
L
), and 52.5 mL of water were mixed with stirring. The
3D macroporous silica skeleton with ordered mesoporous walls.
mixture was heated to boil, and kept boiling and stirring for 6 h.
The color of the citrate-coated Pd NP (Pd/Cit.NP) dispersion
The inverse replicas of the PS/C mimCl/Pd beads resulted in
1
6
spherical voids, in which Pd NPs were anchored on the inner
walls of the spherical voids, and the inverse replicas of the
ꢀ1
(
0.0599 g L ) turned from yellow to brown.
The PS/C16mimCl/Pd beads were synthesized using the PS/
C
16mimCl leaded to ordered mesoporous silica channel walls.
C mimCl beads as nuclei for subsequent adsorption deposi-
1
6
The method that employs the LILs not only as a connector but
also as a templating agent in one system should provide
important guidelines for the multifunctional applications of the
LILs in the synthesis of nanostructured materials.
tion of Pd/Cit.NPs according to the compositions listed in
Table 1. The different volumes of the Pd/Cit.NP dispersion
Table 1 Synthesis recipe of PS/C16mimCl/Pd(x)
Experimental section
Sample
PS/C16mimCl (mL)
Pd/Cit. (mL)
Synthesis of C mimCl
1
6
PS/C16mimCl/Pd(6%)
PS/C16mimCl/Pd(12%)
PS/C16mimCl/Pd(18%)
PS/C16mimCl/Pd(24%)
20
20
20
20
15.62
31.24
46.86
62.48
Long-chain ionic liquid, 1-hexadecyl-3-methylimidazolium
chloride (C mimCl) was prepared according to a route re-
1
6
15,26
ported in the literature.
All chemicals were purchased from
Acr ¨o s and used as received. As a typical synthesis, 1-
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V(Pd/Cit.NP) ¼ 15.62, 31.24, 46.86 and 62.48 mL) (0.0599 g L
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ꢀ
1
(
)
Pd–SiO
2
(18%) catalysts were also studied by tracking the
were added dropwise to 20 mL of the PS/C mimCl dispersion changes in UV-vis absorption spectrograph of the reaction
1
6
ꢀ1
(
0.781 g L ), respectively. The mixture was sufficiently stirred system under different cycles.
for 4 h to obtain the PS/C mimCl/Pd beads. The mass ratios of
1
6
m(Pd)/m(PS) were calculated based on the mass of Pd in the Pd/
Cit.NP dispersion and the mass of PS in the PS/C16mimCl
dispersion to be 6%, 12%, 18% and 24%, respectively. The four
PS/C16mimCl/Pd bead samples were coded as PS/C16mimCl/
Pd(x), x ¼ 6%, 12%, 18% and 24%, respectively.
Characterization
A eld emission scanning electron microscopy (FEI Nova
NanoSEM 450) was used to observe the morphology of the
samples. Transmission electron microscopy (TEM) images were
obtained by using a JEM-2100 electron microscope at an accel-
eration voltage of 200 kV. The specimens for TEM were prepared
by dropping a small drop of the solutions onto a carbon-coated
copper grid. Electrophoresis experiments were performed on
a Nanjing Sangli DYL-3 electrophoresis apparatus with 15 V of
external voltage. X-ray diffraction (XRD) patterns were measured
on a TTR III powder X-ray diffractometer using Cu Ka radiation
Synthesis of Pd-loaded hierarchical porous silica
The Pd-loaded hierarchical porous silica was prepared in acid
medium by a sol–gel procedure using tetraethylorthosilicate
(TEOS) as silicon source, the prepared PS/C16mimCl/Pd beads
and C mimCl as macroporous and mesoporous structural
1
6
templating agents, respectively. In a typical synthesis proce-
dure, C mimCl and HCl were added to the PS/C mimCl/Pd
ꢁ
ꢀ1
(wavelength 0.154 nm) at a rate of 0.05 2q s and operated at
1
6
16
40 kV and 30 mA. Nitrogen sorption experiments were con-
ducted using a Micromeritics Tristar 3000 automated gas
adsorption analyzer. A UV-vis spectrophotometer (SHIMADZU
UV-1780) was used to collect absorption spectra of the solution
in catalytic reactions. A TENSOR27 Fourier transform infrared
dispersion under mild magnetic stirring. Aer homogeniza-
tion of the mixture, TEOS was added dropwise at room
temperature. The molar compositions of the starting mixtures
were n(TEOS)/n(C16mimC)/n(HCl)/n(H
The resulting mixtures were stirred at room temperature for
h and then aged for 12 h. Aer the sol–gel treatment, the
2
O) ¼ 0.23/0.2/56/504.
(FT-IR) spectrometer was employed for recording IR spectra.
2
mixtures were ltered, washed with deionized water, dried
under atmosphere at room temperature, and nally calcined by
Results and discussion
ꢁ
a temperature programmed route at 300 C for 2 h, and then,
Characterization of C mimCl layer assembled onto PS beads
1
6
ꢁ
ꢁ
ꢀ1
5
50 C for 2 h with a temperature ramp of 2 C min under
PS/C16mimCl beads were prepared by adding the C16mimCl
static air condition to remove the templates. The resultant
sample based on the PS/C16mimCl/Pd(12%) beads and
ꢀ1
aqueous solution (2.985 g L ) to PS bead dispersion (4.686 g
ꢀ
1
L
). It was found that a drop by drop and slow adding way for
C
16mimCl dual templates is coded as Pd–SiO
2
(12%), and the
the C mimCl aqueous solution is especially important for
1
6
sample based on the PS/C mimCl/Pd(18%) beads and
1
6
preventing the micelle formation of C16mimCl in the disper-
sion. The cmc of C16mimCl at 298 K was reported to be
C mimCl dual templates is coded as Pd–SiO (18%).
1
6
2
28
1
.21 mM, and the concentration of one drop of the C16mimCl
aqueous solution in the suspension is about 0.043 mM, which is
In order to test the catalytic activities of the Pd-loaded hier- well below the cmc of C16mimCl. Hence, this adding way is
archical porous silica (Pd–SiO
(12% or 18%)), the catalytic favorable to coat the individual C16mimCl molecule onto the PS
oxidation of 3,3 ,5,5 -tetramethylbenzidine (TMB) by H O was efficiently. Moreover, we found that C mimCl is freely soluble
Catalysis studies of Pd-loaded hierarchical porous silica
2
0
0
2
2
16
employed as a model reaction. Briey, in each of glass tubes, in water at room temperature, which increases the possibility of
ꢀ
1
2
H
00 mL of TMB aqueous solution (10 mmol L ), 200 mL of the direct interaction between individual C16mimCl molecule
ꢀ1
2
O
2
solution (250 mmol L ) and 1560 mL of PBS buffer and PS bead. In order to understand the effect of the amount of
16mimCl, different volumes of the C16mimCl aqueous solution
solution (pH ¼ 5) were added with stirring. Then, the Pd-
C
loaded hierarchical porous silica catalysts (1 mg) were added (V(C16mimCl) ¼ 1, 25, 50, 75, and 100 mL) were added dropwise
to the above 1960 mL solutions. With constant magnetic stir- and slowly to the PS dispersion (10 mL). Fig. 1 shows that the
ring, the mixture color changed from colorless to blue. Aer zeta potential (z) as a function of the V(C16mimCl). Two features
reacting at different time intervals, the mixtures were scanned should be noted. Firstly, the uncoated PS bead dispersion
by UV-vis absorption spectrograph. The control experiment produced a zeta potential of about ꢀ7 mV. The presence of
was also carried out by mixing the solution of TMB and H
2
O
2
a small quantity of C16mimCl (1 mL) caused a reversal in zeta
in the absence of the Pd-loaded hierarchical porous silica potential (+14.5 mV). The multiple tests of adding small quan-
catalyst. For comparison, the catalytic activities of PS/ tity of C16mimCl, even 0.2 mL of C16mimCl, leaded to the zeta
C
C
16mimCl/Pd(12%),
PS/C16mimCl/Pd(18%),
and
PS/ potential reversal from negative charge to positive charge,
16mimCl/Pd(24%) beads were studied respectively under implying a hypothetical structure of the exposure of imidazole
identical reaction conditions. According to the amount of PS/ head groups of C16mimCl toward the surrounding solution, and
C mimCl/Pd bead dispersion used for the synthesis of 1 mg the hydrophobic long alkyl chain extending to the surface of PS
1
6
Pd–SiO (12% or 18%), the volume of the bead dispersion bead. This result showed that the hydrophobic interaction
2
tested for the catalytic activity was controlled to be 2 mL. between the PS beads and the alkyl chain of C16mimCl is
Moreover, the reusability of the prepared Pd–SiO
2
(12%) and stronger than the electrostatic interaction between the PS beads
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Characterization of Pd NPs assembled onto PS/C16mimCl
beads
In order to reveal the effect of the amount of Pd NPs, PS/
C
16mimCl/Pd(x) beads with x ¼ 6%, 12%, 18% and 24% (x is
different mass ratios of m(Pd)/m(PS)) were prepared by mixing
corresponding volumes (15.62, 31.24, 46.86 and 62.48 mL) of
ꢀ
1
citrate-coated Pd NPs suspensions (0.0599 g L , z-potential of
ꢀ
1
ꢀ
44 mV) with 20 mL of PS/C mimCl dispersion (0.781 g L ,
16
z-potential of +60 mV), respectively. Fig. 2 shows SEM images
of PS/C mimCl/Pd(18%) beads. Compared with the uncoated
1
6
PS beads with a narrow size distribution and a smooth particle
surface (see Fig. S2†), PS/C16mimCl/Pd (18%) exhibited
a coarse surface in each bead, indicating that a layer of Pd
coating with a few clusters was deposited on the surface of PS/
Fig. 1 Zeta potential as a function of the volumes of adding C16mimCl
aqueous solution for the C16mimCl-coated PS beads.
C
16mimCl beads. These SEM images are similar to the images
3
1
of PS spheres covered with dense and uniform Au NPs.
Fig. 3 further shows TEM images of PS/C mimCl/Pd(x) bead
1
6
and the imidazole head groups of C16mimCl. Secondly, the
increase in the feeding V(C16mimCl) increases the zeta potential
of the resultant PS/C mimCl beads. When the V(C mimCl) is
with different coverage. Compared to PS/C mimCl/Pd(6%)
1
6
with an extremely sparse coverage, PS/C16mimCl/Pd(12%)
Fig. 3A and B) and PS/C16mimCl/Pd(18%) (Fig. 3C and D)
1
6
16
(
50 mL, the highest value of zeta potential (+60 mV) can be ob-
exhibited dense and uniform Pd NPs (about 10 ꢂ 3 nm)
dispersed onto the C16mimCl-functionalized PS beads with
a good coverage. It can be found that with the increasing of the
mass ratios of m(Pd)/m(PS) from 18% to 24%, the Pd NPs on the
PS beads became more denser (Fig. 3E and F), and some
aggregate phenomenon of the Pd NPs in the local position onto
PS beads can be observed. This may be because of adding too
much Pd NPs, which exceeds the matching quantity of Pd NPs
and PS/C16mimCl beads. The best matching ratios of the mass
ratios of m(Pd)/m(PS) are 12% and 18%.
tained, which is consistent with the value of zeta potential
measured from the PS latex particles with poly(allylamine
hydrochloride) (PAH) or poly(diallyldimethylammonium chlo-
29
ride) (PDADMAC) as outer shell prepared by LbL method. The
high zeta potential onto the PS/C16mimCl beads suggests that
a close-packed array of the C16mimCl molecules can form onto
the PS beads. Sastry et al. considered that the increase in the
alkyl chain length of ionic liquids leads to the parallel packing
of chains in the interior of aggregates and the cationic rings
with delocalized p-electron clouds favor high aggregation
In brief, there are two steps for the synthesis of the PS/
C mimCl/Pd beads. As shown in the Scheme 1, the rst step is
30
number.
However, as shown in Fig. 1, further increasing the V(C16
mimCl) to 75 mL, the value of the zeta potential for the PS/
16mimCl beads is obviously decreased (18.5 mV). When the
V(C mimCl) is 100 mL, corresponding zeta potential value
1
6
-
to get C mimCl-functionalized PS beads by a hydrophobic
1
6
interaction between the PS beads and the alkyl chain of
16mimCl to expose imidazole head-groups of C16mimCl
molecules to the surrounding solution. In the second step, the
PS/C16mimCl beads with positive charge can serve as active sites
to link the negatively charged citrate-coated Pd NPs by means of
an electrostatic interaction. Thus, the C16mimCl are applied as
C
C
1
6
becomes 13.6 mV. This is because when the concentration of
C mimCl is too high, the excess of C mimCl in the dispersion
1
6
16
increase the viscosity of the colloidal solution, and reduce the
uidity of PS/C16mimCl beads in the electrophoresis experi-

‘
bridges’ to connect the PS beads and citrate-coated Pd NPs. It is
ments, resulting in the decrease of the zeta potential values.
Therefore, we conrm that when the amount of C16mimCl is the
appropriate value (V(C16mimCl) ¼ 50 mL), the optimal value of
zeta potential (+60 mV) can be obtained.
possible to manipulate the uniformity and coverage of the Pd
NPs on the PS beads by controlling the amount of C mimCl
and the matching mass ratios of m(Pd)/m(PS).
1
6
Fig. 2 SEM images of PS/C16mimCl/Pd(18%) beads with increasing magnification from (A) to (C).
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Fig. 3 TEM images of (A and B) PS/C16mimCl/Pd(12%), (C and D) PS/C16mimCl/Pd(18%), and (E and F) PS/C16mimCl/Pd(24%).
is due to the lling of mesoporous walls by capillary conden-
sation. In the previous publication of ordered mesoporous silica
15
structures templated by C mimCl, the hysteresis phenom-
1
6
enon at relatively high p/p values was observed and attributable
0
to the lling of a secondary pore structure, which resulted from
grain boundaries or small cavities between adjacent ordered
regions. In our sample, the interconnected spherical voids and
the Pd NPs onto the walls can be regarded as the grains, and
their boundaries with the ordered mesoporous silica channels
might create small cavities, therefore, resulting in the more
visible and broad hysteresis loop. The BET surface area, total
pore volume at relative pressure of 0.98 and mean pore size of
Scheme 1 Schematic diagram of the synthesis procedure of PS/
16mimCl/Pd beads using C16mimCl as a connector.
C
Characterization of Pd-loaded hierarchical porous silica
As shown in the Scheme 2, in the prepared route of Pd-loaded
2
ꢀ1
3
ꢀ1
the calcined Pd–SiO
.0 nm, respectively.
TEM image of the calcined Pd–SiO
2
(12%) are 967 m g , 0.68 cm g , and
hierarchical porous silica (Pd–SiO ), PS/C mimCl/Pd beads
2
16
3
were used as a macroporous structure templating agent. The
inverse replicas of PS/C16mimCl/Pd beads resulted in spherical
voids, in which Pd NPs were anchored on the inner walls of the
spherical voids. The C16mimCl was used as a mesoporous
structure templating agent. The inverse replicas of the
2
(12%) (Fig. 5A) reveals
alternating superposition array of the spherical voids (560 ꢂ 10
nm), in which the Pd NPs can be clearly observed with dark
particles. A wall of ordered mesoporous channel array (100
direction) (Fig. 5B) can be also observed in the enlarged view of
Fig. 5A. However, it should be pointed out that there is only
ordered mesoporous silica channel array in the local border of
the sample as depicted in Fig. 5C. It can be found that Pd–
C
16mimCl micelles leaded to ordered mesoporous silica
channel walls.
Fig. 4 shows small-angle XRD pattern and N adsorption–
2
desorption isotherm of the calcined Pd–SiO (12%) prepared
2
2
SiO (18%) has similar TEM images (Fig. S3†), but Pd particle
using PS/C16mimCl/Pd(12%) beads and C16mimCl as dual
templates. The sample exhibited one well-resolved diffraction
peak (100) in the 2q range between 2 and 6 (Fig. 4A), which is
number in the spherical voids increased signicantly. These
TEM images exhibit direct and conclusive evidence of the
coexistence of Pd NPs and well-dened hierarchical porous
structures. Furthermore, the small-angle XRD curve of Pd–
32–34
associated with p6mm hexagonal symmetry (SBA-3 type),
indicating a mesoporous structure with short range order. This
may be due to a thin wall in the macroporous silica skeleton,
thus, only one diffraction peak can be observed. The sample
displayed a type IV adsorption isotherm with an obvious
SiO (18%) appeared some characteristics of cubic phase
2
(Fig. S4†). In classical micelle chemistry, the transformation
from hexagonal to cubic mesoporous phase can be explained in
terms of the local effective surfactant packing parameter: g ¼ V/
hysteresis loop at a relatively high p/p
IUPAC, indicating the presence of open pores (Fig. 4B). A steep
0
values according to
36
0
a l, where V is the total volume of the surfactant chains plus
35
0
any cosolvent organic molecules between the chains, a is the
0
increasing occurs at a relative pressure 0.30 < p/p < 0.43, which
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2
Scheme 2 Schematic diagram of the prepared route of Pd-loaded hierarchical porous silica (Pd–SiO ) using PS/C16mimCl/Pd beads and
C
16mimCl as dual templates.
hydrocarbon chains of long-chain ionic liquid, the volume
fraction of the hydrocarbon chain should increase. The increase
in V results in the increase of the value of g. Thus, it is possible
that the transformation from hexagonal p6mm to atypical cubic
Ia3d structure can be observed in the small-angle XRD curve of
2
Pd–SiO (18%).
Catalytic performance of Pd-loaded hierarchical porous silica
To evaluate the catalytic activities of Pd–SiO (12%) and Pd–
SiO (18%), we employed the oxidation of 3,3 ,5,5 -tetrame-
thylbenzidine (TMB) by H as a model system. In the previous
publication, the oxidation of TMB by H can produce diazo
dianiline, which exhibited two typical absorption peaks at
2
0
0
2
2 2
O
2 2
O
3
70 nm and 652 nm, and induced a color change from colorless
37,38
to blue in solution.
In comparison with the control experi-
ment of the oxidation of TMB by H O in the absence of catalyst
2
2
(Fig. 6A), Pd–SiO
2
2
(12%) and Pd–SiO (18%) used for the oxida-
tion reaction can facilitate the reaction effectively within certain
time (Fig. 6B and C), and the catalytic rate of Pd–SiO (18%) is
faster than that of Pd–SiO (12%), suggesting that increasing the
amount of Pd NPs in the catalyst is benecial to promote the
catalytic performance for the oxidation of TMB by H . A color
2
2
2 2
O
change from colorless to blue in the solution (Fig. 6D) can be
observed corresponding to the control experiment, the catalytic
experiments with Pd–SiO
that Pd–SiO (12%) and Pd–SiO
adsorption–desorption activity. For comparison, parallel experiments were performed
(12%) prepared using PS/C16mimCl/Pd(12%)
2
(12%) or Pd–SiO
2
(18%), indicating
2
2
(18%) have enhanced catalytic
Fig. 4 (A) Small-angle XRD pattern and (B) N
isotherms of Pd–SiO
2
2
to monitor the catalytic behavior of the synthesized PS/
16mimCl/Pd(x) beads. Fig. 7 shows the successive UV-vis
absorption spectra of oxidation reaction of TMB with H
catalyzed by adding PS/C mimCl/Pd(12%), PS/C mimCl/
beads and C16mimCl as dual templates.
C
2 2
O
1
6
16
effective head group area at the micelle surface, and l is the
kinetic surfactant tail length. When the g value is increased
above critical values, mesophase transitions occur. For
example, a structural change from hexagonal p6mm to cubic
Ia3d structure should be accompanied by an increase in g from
Pd(18%), and PS/C mimCl/Pd(24%) beads, respectively. It was
1
6
observed that the obtained PS/C16mimCl/Pd(x) beads exhibited
signicant absorption peaks at 370 nm and 652 nm, indicating
that the composite beads had an enhanced catalytic activity
compared with the control experiment without catalyst
33
1/2 to 1/2–2/3. For the synthesis of Pd–SiO
2
(18%), PS/
(
Fig. 6A). The absorption peak intensity of PS/C16mimCl/
Pd(12%) beads (Fig. 7A) is similar to that of Pd–SiO (12%)
Fig. 6B). PS/C mimCl/Pd(18%) beads showed higher absorp-
C mimCl/Pd(18%) beads have more Pd particles compared to
1
6
2
PS/C mimCl/Pd(12%) beads, which can provide a denser
1
6
(
1
6
reaction system. Meanwhile, PS/C mimCl/Pd beads possess
1
6
tion peak intensity (Fig. 7B) than PS/C mimCl/Pd(12%) beads
1
6
a certain hydrophobic property, which can be regard as
a ‘cosolvent organic molecules’. When the beads are close to the
(Fig. 7A), but had lower absorption peak intensity than Pd–
SiO (18%) (Fig. 6C). The high catalytic activity of Pd–SiO (18%)
2
2
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Fig. 5 TEM images of (A) Pd–SiO (12%), (B) an enlarged local view of (A), and (C) the local border of the sample.
compared to PS/C16mimCl/Pd(18%) beads may be attributable
to its relatively distinctive hierarchical porous structure.
Furthermore, of the three composite beads, PS/C mimCl/ where c
ln(c /c) ¼ kt
(1)
0
1
6
0
is the initial concentration of the TMB solution, c is the
Pd(24%) had the lowest absorption peak intensity (Fig. 7C). This reactant concentration at time t and k is the rate constant.
may be due to some aggregate phenomenon of Pd NPs in the Fig. S5† shows a correlation between A/A and reaction time for
local position onto PS beads (Fig. 3E and F), leading to the the oxidation reaction of TMB by H (A and A are absorbance
at time t and absorbance at the initial stage). For the pseudo
, the concentra- rst-order kinetics, the c/c can be measured from the relative
because A/A is in proportion to c/
. Thus, according to the linear relationship of ln(A/A ) versus
pseudo-rst-order kinetics with respect to TMB can be applied time (t), the rate constant k can be calculated. The calculated
0
2
O
2
0
reduction of its catalytic activity.
In the oxidation reaction of TMB by H
2
O
2
0
tion of H
2
O
2
was greatly higher than that of TMB, so it is intensity of absorbance A/A
0
0
consider as a constant during the reaction process, and the
c
0
0
ꢀ1
ꢀ1
as shown by:
2
rate constants are 0.0562 min (Pd–SiO (18%)), 0.0436 min
Fig. 6 Successive UV-vis absorption spectra of oxidation reaction of TMB by H
and (C) Pd–SiO (18%). (D) Corresponding digital photos of the samples.
2 2 2
O (A) without catalyst, and catalyzed by adding (B) Pd–SiO (12%)
2
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Fig. 7 Successive UV-vis absorption spectra of oxidation reaction of TMB by H
O
2 2
catalyzed by adding (A) PS/C16mimCl/Pd(12%), (B) PS/
C
16mimCl/Pd(18%), and (C) PS/C16mimCl/Pd(24%) beads.
Table 2 Recycling of the catalysts
Conclusions
ꢀ
1
Sample
Cycle
Rate constant (min )
In summary, we report a exible approach for the controllable
synthesis of PS/C mimCl/Pd beads using long-chain 1-
Pd–SiO
2
(12%)
(18%)
1
2
3
4
1
2
3
4
0.0454
0.0451
0.0435
0.0387
0.0543
0.0538
0.0521
0.0468
16
hexadecyl-3-methylimidazolium chloride (C mimCl) as
1
6
a connector. The amount of C16mimCl and the mass ratios of
m(Pd)/m(PS) were systematically studied to optimize the
coverage of Pd NPs. When m(Pd)/m(PS) ¼ 12% and 18%, the
citrate-coated Pd NPs are well-dispersed onto PS beads to form
well-dened PS/C16mimCl/Pd beads. Moreover, the Pd-loaded
hierarchical porous silica with a 3D ordered macroporous
skeleton and ordered mesoporous silica walls (SBA-3 type) was
prepared using the dual templates of PS/C mimCl/Pd beads
Pd–SiO
2
1
6
ꢀ1
(Pd–SiO
2
(12%)), and 0.0274 min (without catalyst), respec-
(18%)
and C16mimCl. The inverse replicas of the PS/C16mimCl/Pd
bead template led to well-dispersed Pd NPs anchored on the
inner walls of the spherical macroporous voids. Furthermore,
the obtained Pd-loaded hierarchical porous silica with a high
tively. It can be clearly seen that the addition of Pd–SiO
and Pd–SiO (12%) signicantly fast the oxidation reaction. It is
2
2
accepted that for the effective promotion of the oxidation
reaction, TMB should be concentrated and adsorbed on the
surface of the Pd-loaded hierarchical porous silica catalyst due
to the high surface area and ordered mesoporous channels.
Meanwhile the macroporous spherical voids can increase TMB
transport, facilitating access to reactive sites of Pd NPs
anchored on the inner walls of the spherical voids. Aer the
2
ꢀ1
surface area of up to 967 m g exhibited enhanced and Pd
loading capacity-dependent catalytic efficiencies.
Conflicts of interest
There are no conicts to declare.
completion of the catalytic reaction, the Pd–SiO (12%) and Pd–
2
SiO (18%) catalysts were recovered simply by centrifugation and
2
ꢁ
then dried at room temperature, and nally calcined at 550 C Acknowledgements
for 2 h under static air conditions to remove the catalytic reac-
We gratefully acknowledge the nancial support from National
Natural Science Foundation of China (21564018 and 21363029),
and Yunnan Provincial Science and Technology Department
tion products. The recovered catalysts were used again in
successive cycles (four times) under identical reaction condi-
tions. Table 2 shows the rate constants of the catalytic oxidation
reaction aer four cycles. The rate constants were tted
according to the pseudo-rst-order kinetics (formula (1)). The
results showed that there is no appreciable change of the rate
constants for the recovered Pd–SiO (12%) and Pd–SiO (18%)
(2015FD014).
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