Poly(ionic liquid) core turns hollow silica spheres into amphiphilic nanoreactor in water

Article abstract of DOI:10.1021/cm5035535

The stressing environmental concerns push us to move traditional organic reactions toward an eco-friendly way. Replacing organic solvents by water seems to be a promising solution yet is very challenging. The main obstacle is the poor solubility of many h

Full text of DOI:10.1021/cm5035535

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Article
Poly(ionic liquid) Core Turns Hollow Silica
Spheres into Amphiphilic Nanoreactor in Water
Yan Yang, Martina Ambrogi, Holm Kirmse, Yongjun Men, Markus Antonietti, and Jiayin Yuan
Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm5035535 • Publication Date (Web): 10 Dec 2014
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Poly(ionic liquid) Core Turns Hollow Silica Spheres into Am-
phiphilic Nanoreactor in Water
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†
†
‡
†
†
,†
Yan Yang, Martina Ambrogi, Holm Kirmse, Yongjun Men, Markus Antonietti and Jiayin Yuan*
†
Department of Colloid Chemistry, Max Planck Institute of Colloids and Interfaces, Research Campus Golm, D-14476, Pots
dam, Germany
‡
Institute of Physics, Humboldt University of Berlin, D-12489, Berlin, Germany
ABSTRACT: The stressing environmental concerns push us to
move traditional organic reactions toward an eco-friendly way.
Replacing organic solvents by water seems to be a promising
solution yet is very challenging. The main obstacle is the poor
solubility of many hydrophobic substrates in water, leading to a
restricted accessibility, thus inevitably ending up with low reac-
tion rates. To overcome this problem, we propose a hybrid micelle-like hydrophobic@hydrophilic nanoreactor formed by a
-
1
poly(ionic liquid) (PIL) core and a mesoporous silica shell. This nanoreactor exhibited high activity (TOF up to 414 h , 12.9 times
of that for a corresponding silica catalyst) and selectivity (100%) in the oxidation of benzyl alcohol to benzaldehyde using water as
solvent.
18-20
1. INTRODUCTION
very challenging and rarely reported.
The demands for the
synthesis of such a nanoreactor are multiple: first of all, it
must contain components or “pockets” with hydrophilic and
hydrophobic nature for aqueous dispersability and to house the
diverse reactants; Secondly, the nanoreactor should possess
component that can accommodate MNPs in a stable and high-
ly dispersed fashion; And thirdly, the nanoreactor must be
easily separable and recyclable. This reusability adds addition-
al utility value to the catalyst.
Nanoreactors with tailor-made structure and judicious choice
of compositions have shown superior performance in acceler-
ating the reaction kinetics, enhancing the selectivity, and im-
proving the recycling of the catalysts in many traditional or-
1-6
ganic solvents. Environmental concerns require chemists to
develop effective nanoreactors that can operate in more eco-
friendly reaction conditions compared to organic solvents,
such as solvent-free reactions, using water as solvent or at low
7-8
energy consumption. Significant efforts have been made to
Inspired by the micelle nanostructure, we present herein a
hybrid nanoreactor composed of a hydrophobic core and a
hydrophilic shell fabricated by poly(ionic liquid) (PIL) and
mesoporous silica, respectively. The PIL nanoparticles will
undergo a unique hydrophilic-to-hydrophobic transition upon
processing, which facilitates the fabrication of the nanoreactor
greatly. It shows significantly enhanced activity and high
selectivity in the oxidation of primary aromatic alcohols to the
corresponding aldehydes in water, using molecular oxygen as
fabricate nanoreactors that fulfill these requirements. Never-
theless, the relatively low activity of many nanoreactors, in
most cases supported noble metal nanoparticles (MNPs), re-
mains a key point to improve. The main issue lies in the poor
accessibility of the hydrophobic substrates to the water-
dispersible, catalytically active MNPs. Variable attempts have
been conducted to tackle these challenges, for example, adding
surfactants or amphiphilic agents into the aqueous medium,
building emulsion systems, modifying the hydropho-
o
the oxidizing agent at mild temperature (65 C) and pressure
9
-12
bic/hydrophilic property of the catalyst supports.
Among
(2 atm).
these solutions, micelles and emulsion systems show excellent
dispersity in water and high affinity towards hydrophobic
substrates; However, these soft matter systems suffer from
being liable to external environments and surfaces, and diffi-
The general synthetic scheme and the corresponding elec-
tron microscopy images are shown in Figure 1. The synthesis
started with PIL nanospheres (~56 nm in diameter, see SI for
synthetic approach) with a distinctive multilamellar centric
vesicular nanostructure (Figure 1B), consisting of multiple
13-14
culties associated with product separation and reuse.
Taking the advantages of heterogeneous catalysts, such as
hydrophilic and hydrophobic nanodomains, as reported previ-
1
5-17
21-24
simple separation, high mechanical stability,
recyclability,
ously.
The chemical structure is depicted in Figure 1A and
and being available to flow reactors in industry, constructing
hybrid nanoreactors with a micelle-like amphiphilic feature
and a stable framework seems to be a promising approach, yet
Scheme S1. In our initial attempts, several polymer systems
were considered as the core component, such as polymer la-
texes prepared via emulsion polymerization and microgels
A
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2
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32
from poly(N-isopropylacrylamide). Due to their very favora-
ble properties, finally PIL nanospheres were chosen. PILs are
structural rearrangement and turn hydrophobic, and no long-
33
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er dispersible in water. In addition, the ionic nature enables
the PIL to complex metal ions and locate the derived catalyti-
cally active MNPs around the reactor core. All these structural
features are beneficial to the functional nanostructure design.
2
6-31
the polymerized products of ionic liquids,
prepared here
via dispersion polymerization of vinylimidazolium ionic liquid
monomers with long alkyl chains. They are well dispersible in
water, and can stabilize MNPs in their polar domains; Howev-
er, upon environmental change, they will sporadically undergo
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Figure 1. Schematic illustration of the synthetic route to the PIL@SiO
2
–Pd nanoreactor and the corresponding TEM images at each step. (A) Chemical structure
-
of PIL (x denotes Br or dicyanamide anion), (B) PIL nanospheres, (C) after coating a layer of mesoporous silica via using micelles formed by CTAB as template,
2
(D) the formation of PIL@SiO nanostructure and (E) the incorporation of Pd NPs in the PIL core (with CTAB residues inside the mesopores of silica shell). Inset in
(B) is a cryo-TEM image. Scale bar in inset in (B) and (E) is 20 nm.
2. EXPERIMENTAL SECTION
3 2
tion containing Pd(NO ) (35 mg). After ultrasonic treatment for 20 min,
the solution was stirred for 12 h at room temperature. Ethanol was then
removed by rotation evaporation at room temperature. The obtained solids
PIL nanospheres. PIL nanospheres (56 nm and 70 nm respectively) were
made by a previously reported single step precipitation polymerization in
34
were reduced using NaBH
PIL@SiO -Pd catalyst.
Synthesis of PIL-Pd catalyst. The PIL-Pd catalyst was synthesized by
adding a Pd(NO solution (8 mL, 1 mg/mL, H O) dropwise into a disper-
4
solution, similar to the synthesis of
aqueous solutions.
2
Synthesis of PIL@SiO
dispersion solution (50 g/L, water) was added into a mixture of 40 mL of
O, 30 mL EtOH, containing CTAB (0.15 g) and NH •H O (0.05 mL, 28
2
nanospheres. In a typical procedure, 1.5 mL PIL
3
)
2
2
H
2
3
2
sion of PIL nanospheres (10 mL, 50 g/L) under stirring. After stirring for 1
h, the transparent colloid solution turned into turbid, due to the deposition
of the PIL nanospheres. Gray solid polymeric product was obtained after
freeze-drying of the mixture. It cannot be re-dispersed in water anymore.
Wash the product with water, until colorless solution was obtained. The
solid was reduced by NaBH solution (10 mL, 2 mol/L), then removed
4
from the liquid phase, washed with water for several times, and dried
under vacuum.
wt%). After ultrasonication for 15 min, the mixture was put into an oil
o
bath of 38 C. Then TEOS (0.2 mL) was added under vigorous stirring,
o
and the mixture was kept at 38 C for 24 h. Then the mixture was centri-
fuged and washed with water and ethanol for several times and dried at
room temperature.
PIL@SiO nanospheres with different shell thickness (10, 13 and 20 nm)
2
and sizes (78, 102 and 112 nm) were achieved through a similar way by
using a larger amount of TEOS (0.4, 0.5 and 1.0 mL) and PIL nanospheres
with different sizes as precursors (56, 70 and 70 nm respectively).
Synthesis of PIL@SiO -Pd catalyst. Incorporation of Pd NPs into the
2
PIL part as the nanoreactor core: The Pd NPs were incorporated through
an anion exchange method (and a partial complexing effect of the Pd ions
Catalytic Tests. In a typical oxidation, 0.5 mmol of substrate and 0.0048
mmol of Pd catalysts (calculated by weight) and 1.3 mL of H O were
2
added into a 20 mL Schlenk tube, which was fitted with a magnetic stirrer
o
and an oxygen balloon. The reaction was performed at 65 C in an oil bath
with magnetic stirring (stirring rate: 900 r.p.m.) for a given time. When
the reaction was finished, the liquid phase of the reaction mixture was
with the charged PIL chains). A Pd(NO
was added dropwise into a aqueous dispersion of PIL@SiO
40 mL, 15 mg/mL, H O) under vigorous stirring. After 10 min, the liquid
3
)
2
solution (5 mL, 1 mg/mL, H
2
O)
2
nanospheres
collected by filtration. Then the liquid mixture was extracted by CH
2
Cl
2
(
2
for 3 times, and the mixture of CH Cl was dried with anhydrous Na SO
2
2
2
4
phase was removed by centrifugation. The gray solid was further washed
with water for several times to remove free salts. The solid was re-
dispersed in water (20 mL) with the assistance of ultrasonic, in which
for 2 h, and used for GC–MS analysis. The oven temperature of GC was
o
programed increasing to 300 C.
For the recycling test, all the solids were collected by centrifugation after
reaction. The residual catalyst was washed with water for several times
and used directly for the next catalytic reaction. The reaction time for the
third and fourth recycling is 10 h.
4
NaBH solution (2 mL, 5 mol/L) was added quickly. The solution was
kept in ultrasonic bath for another 5 min, before the liquid phase was
removed by centrifugation. The gray solid was further washed with water
and ethanol for several times, and dried under high vacuum till constant
weight.
3. RESULTS AND DISCUSSION
Synthesis of MCM41-Pd catalyst. The synthesis of MCM41 silica
The PIL nanospheres were then coated by a thin layer of
mesoporous silica through the assistance of surfactant
cetyltrime-thylammonium bromide (CTAB, which can form
1
nanospheres were according to a literature method. The MCM41-Pd
catalyst was made through an immersion and reduction method as follows:
1 g MCM41 silica nanospheres were immersed into 30 mL ethanol solu-
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nomenon of the self-adaptive ability of PIL nanospheres under
micelles in water and act as templates for mesopores, as
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shown in Figure 1) in basic solution. The inorganic shell acts
as an “exoskeleton” and provides high mechanical/chemical
stability and hydrophilic nature to the nanoreactor, while the
residual CTAB aggregates inside the mesopores play double
roles, i.e. preventing the diffusion of PIL chains out of the
silica shell and meanwhile assisting the transmission of hydro-
phobic molecules from water to the hydrophobic PIL core,
similar to the solubilization effect of micelles in water. The
details will be discussed later in the surface nature investiga-
tion part. The transmission electron microscopy (TEM) image
confinement in silica.
The dimension and shell thickness of the PIL@SiO are eas-
2
ily tunable by changing the size of the used PIL nanosphere
templates and the amount of silica precursor (TEOS) added for
the silica shell formation (Figure 2). PIL@SiO nanospheres
2
with larger sizes (78, 102 and 112 nm) and thicker shells (10,
13 and 20 nm) were also made and visualized in Figure 2 (for
experimental details, see SI). From the TEM images it is clear
that all these nanospheres contain a PIL core and a mesopo-
rous silica shell with radial cylindrical mesopores (~2 nm in
diameter), which provide the shortest way for mass transport
across the silica shell.
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of the obtained PIL@SiO nanosphere is shown in Figure 1C.
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Solid nanospheres with an average size of 66 nm (as deter-
mined from TEM characterization) were obtained. This de-
fines an average thickness of the silica shell to be 4-5 nm.
Interestingly, small irregular inner voids (indicated by white
arrows) are already visible at this stage in these PIL nano-
spheres, indicating the inner PIL core starts the structural
rearrangement, induced by the silica shell. Upon prolonging
the reaction time to 24 h, nanospheres with a shrunken hydro-
phobic hollow PIL core and a hydrophilic mesoporous silica
jacket were obtained (Figure 1D).
It was demonstrated previously that PILs are excellent sup-
37-40
ports and stabilizers for noble MNPs.
The complexing
ability (also partially the anion exchange) of the PIL was used
in our study to bind Pd ions onto the surface of PIL nano-
spheres in the hollow silica, which were subsequently reduced
41-42
by NaBH to generate Pd NPs.
As shown in Figure 1E, the
4
Pd NPs are mainly located around the PIL core of the nano-
reacotor, with uniform size (~2 nm in diameter) and are high-
ly dispersed (denoted as PIL@SiO -Pd).
2
The catalytic performance of the fabricated PIL@SiO -Pd
2
nanoreactor was investigated by a model reaction, the selec-
tive oxidation of primary aromatic alcohols to the correspond-
ing aldehydes in water. To illustrate the advantages of our
nanoreactor design, two additional catalysts were synthesized
and adopted as control experiments: a reference mesoporous
silica material (MCM41 particles with pore size 2.4 nm) and
PIL nanospheres (without silica shell) loaded with Pd NPs
(denoted as MCM41-Pd and PIL-Pd, respectively). Their TEM
images are displayed in Figure S1. The MCM41-Pd catalyst
showed a spherical mesoporous nanostructure (~100 nm in
diameter) with Pd NPs (3-10 nm) deposited on the sphere
surface (due to its small pore size). The PIL-Pd is composed of
aggregated PIL spheres with Pd NPs of 2-4 nm.
The nitrogen sorption isotherms of MCM41-Pd and
PIL@SiO -Pd display typical type-IV isotherms with a capil-
2
2
Figure 2. TEM images of PIL@SiO nanospheres with different sizes and
lary condensation in the relative pressure range P/P of 0.4-0.6
0
shell thickness. The size and shell thickness are: 78 and 10 nm (A, D), 102
and 13 nm (B, E) and 112 and 20 nm (C, F), respectively. The scale bar in D-
F is 10 nm.
and 0.4-0.9, respectively (Figure S2). The Brunauer–Emmett–
2
-
Teller (BET) specific surface area for MCM41-Pd is 542 m g
1
.
In comparison, the PIL-Pd as a soft matter structure is poor-
2 -1
ly porous (18 m g ). The PIL@SiO -Pd bears a medium BET
2
2
-1
surface area of 110 m g (Table S1), underlining the combi-
nation of the two sides as a building principle. According to
Density Functional Theory (DFT) method, MCM41-Pd and
A probable explanation for this structural rearrangement
phenomenon is expected to stem from the uniquely structured
PIL nanospheres, bearing a multilamellar centric vesicular
morphology, as reported in Figure 1B and our previous
PIL@SiO -Pd showed peaks at 2-3 nm in their pore size dis-
2
21
tribution curves, which correspond to the mesopores in the
silica nanospheres (for MCM41-Pd) or the shells (for
work. Such nanostructure is constructed by multiple alternat-
ing hydrophilic (the charged polymer backbone part) and
hydrophobic (the long alkyl side chain part) nanodomains,
similar to a multilayer liposome. This PIL polymer is intrinsi-
cally hydrophobic due to the long alkyl chain and insoluble in
water. However, their nanospheres are well-dispersed in water
due to the preferential organization of the charged ionic back-
bone on outer ring, which provides electrostatic repulsion.
Upon external environmental variation, such as silica coating,
the contact of the hydrophilic outer surface with water is re-
placed by silica-PIL interface. To fit the new environment, the
PIL chains started rearrangement to rebalance its surface ten-
sion, which initiated a structural evolution. This is the phe-
PIL@SiO -Pd), respectively. These results are also in good
2
agreement with the TEM characterization. The chemical com-
position of these three catalysts was verified by Fourier trans-
form infrared (FTIR) spectroscopy, as shown in Figure S3.
In the next step, the surface nature and hydropho-
bic/hydrophilic behavior of these three catalysts were studied.
The surface character was first investigated according to their
dispersity in different solvents. Toluene and water were cho-
sen as hydrophobic and hydrophilic models, respectively. The
three catalysts (same weight) were individually placed into the
same amount of water. After slightly shaking, it is seen from
the photographs (Figure 3A) that a uniform dispersion of the
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PIL@SiO -Pd nanospheres in water was found, meanwhile a
large part of the MCM41-Pd catalyst deposited at the bottom
amphiphilic behavior, well dispersed in water while keeping
high affinity toward benzyl alcohol molecules.
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of the bottle. This completely opposite appearance stems from
the structural difference of these two catalysts: one is hollow
and smaller while the other is denser and larger. In the case of
PIL-Pd, it fails to be dispersed (floating on the surface of
water) due to the aforementioned intrinsic hydrophobic nature
of the PIL polymer.
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Figure 4. STEM-HAADF images of a PIL@SiO nanosphere (A) after the
platinum(II) acetylacetonate (dissolved in benzyl alcohol) adsorption experi-
ment and the corresponding STEM-EDXS maps of the Pt (B), Si (C), O (D)
and N (E) signals. F is the combined image of (B) to (E) and additionally F1 is
the signal of fluorine, to show the background, since the material does not
contain fluorine.
2
Figure 3. Photographs illustrating the solution behavior of PIL@SiO -Pd,
MCM41-Pd and PIL-Pd catalysts. The three catalysts in water (A), toluene +
water (B) and water + benzyl alcohol (C). The photographs were taken 30 s
and 2 min after shaking the mixture strongly in B and C, respectively.
This unconventional “reach-through” van der Waals interac-
tion between the hydrophobic substrate and PIL core is very
probably facilitated by the “bridge” function of the CTAB
residue in the silica mesopores. To prove this effect, dye ad-
sorption experiments (Nile-red dissolved in benzyl alcohol,
The central construction philosophy of our nanoreactor lies
in providing a flexible, adjustable hydrophobicity to bind the
substrates around the Pd NPs inside the hydrophilic silica
shell. Therefore, the same amount of toluene was added into
these three aqueous bottles, and the mixture was shaken vigor-
ously. After 30 seconds, it is interesting to observe that the
0.5 mg/mL) of PIL@SiO , MCM41-Ext (without CTAB) and
2
MCM41-As (with CTAB) were designed for a visual detection
(Figure S5-6, Experiment details in SI). The rapid color
change of PIL@SiO and MCM41-As compared to MCM41-
2
Ext after a short dye adsorption time indicates the difference in
adsorption rate indeed stems from CTAB. It is well-known
that micelles can transfer and enrich hydrophobic molecules
into their hydrophobic core in water. According to the adsorp-
tion experiments mentioned above, the CTAB residue con-
fined in the thin porous silica shell showed similar solubiliza-
tion function of free micelles in water. Since the nanoreactor
resembles and behaviors as micelle (the amphiphilicity and
high affinity towards hydrophobic molecules in water) with
the assistance of residue CTAB inside silica shell, a quick
transmission of the hydrophobic substrates through the silica
mesopores to the hydrophobic PIL core is expected.
PIL@SiO -Pd catalyst was suspended as a foamy emulsion at
2
the interface of toluene and water, meanwhile the MCM41-Pd
catalyst remained in the aqueous phase (Figure 3B). This
foam-like suspension formed by PIL@SiO -Pd at the liquid
2
interface is very stable, without any significant change even
after 5-day storage (Figure S4). This tells that the PIL@SiO2-
Pd acts as a Pickering stabilizer, i.e. it is turning amphiphilic
in this situation. Coincidently, the PIL-Pd catalyst stays at the
interface too, however due to the non-dispersity of PIL in
neither water nor toluene.
The dispersity of these solid catalysts in the mixture of water
and benzyl alcohol was investigated similarly. It is interesting
to find that 2 min after shaking the mixture strongly, the
To further prove the enrichment of the hydrophobic mole-
cules inside the PIL@SiO2 nanospheres in water, another
hydrophobic molecule platinum(II) acetylacetonate (soluble in
benzyl alcohol) was used instead of Nile-red to render a de-
tectable signal-to-noise ratio in Scanning Transmission Elec-
tron Microscopy (STEM) and Energy Dispersed X-ray Spec-
troscopy (EDXS) measurements (details see SI). The results
are shown in Figure 4. Assisted by the mapping technique, the
distribution of Pt, Si, O and N elements (Figure 4) is the same,
PIL@SiO -Pd catalyst was transferred from the up water
2
phase to the bottom benzyl alcohol phase (density 1.04 g/mL),
while the MCM41-Pd catalyst was still trapped in water (Fig-
ure 3C). This is exciting and very unconventional, as the PIL-
phase is essentially trapped within the exoskeleton of the silica
shell. Speculatively, the van der Waals interactions seem to be
able to “reach-through” the shell. For the PIL-Pd catalyst, its
aggregates became soft and sticky due to its partial swelling
and attached on the bottle wall. Combining the above phe-
thus proving the location of Pt inside the PIL@SiO nano-
2
sphere. Since the sample is free of fluorine, the map of fluo-
rine element was made as a contrast to show the background.
Therefore combining the results of adsorption experiments and
STEM-EDXS, it is clear that the PIL@SiO2 nanosphere
nomenon, it is obvious that the PIL@SiO -Pd displays an
2
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Table 1. The reaction kinetic profiles (conversions as a func-
showed strong affinity towards hydrophobic molecules in
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water with the assistance of the residual CTAB inside the
mesopores of silica shell. This high affinity has accelerated the
mass transport of the hydrophobic substrates and led to an
tion of time) of PIL@SiO -Pd and MCM41-Pd towards both
substrates were recorded to provide more reaction details. It is
2
clearly seen that PIL@SiO -Pd showed much higher activity
2
enriched substrate concentration inside the PIL@SiO nano-
than MCM41-Pd catalysts towards both substrates (Figure 5).
The MCM41-Pd catalyst presents a turnover frequency (TOF)
2
spheres.
-
1
of 21 and 32 h for benzyl alcohol and 4-methoxybenzyl
alcohol (entries 1 and 4, Table 1), respectively. This catalytic
Table 1. Catalytic performance of different catalysts on the
activity is comparable to other values reported in literature
[
a]
selective oxidation of aromatic alcohols.
43-44
towards both substrates.
In contrast, the PIL@SiO -Pd
2
catalyst delivers a much higher catalytic activity here, for
example, a TOF of 2.9, 12.9 times (entries 2 and 5, Table 1) of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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0
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0
1
2
3
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7
8
9
0
O
O
OH
1 mol% Pd, H
65 oC, 5 h
O
2
H
OH
that for the MCM41-Pd catalyst. This catalytic activity is
R
45-46
R
R
unusually high with regard to the literature results. Moreo-
ver, it can be seen from the table that both PIL@SiO -Pd and
2
1
2
3
MCM41-Pd catalysed the reaction to form aldehydes at a high
Entry Catalyst
R in 1 Conv. Sel. [bo] f Pd
TOF
(h )
selectivity (100%), but for PIL@SiO -Pd at a conversion of
2
[
b]
[c]
-1 [d]
(%)
2(%)
(wt%)
100%. In comparison, the PIL-Pd catalyst showed, as ex-
pected, negligible activity towards both substrates (entries 3
and 6, Table 1), which is mainly caused by its poor dispersity
in water.
1
2
3
4
5
6
MCM41-Pd
MeO 34
100
1.39
0.41
0.85
1.39
0.41
0.85
21
60
n.d.
32
414
n.d.
PIL@SiO -Pd MeO > 99 100
2
PIL-Pd
MCM41-Pd
MeO trace n.d.
H
H
H
28
100
As a typical advantage of heterogeneous catalysts, the
PIL@SiO -Pd
> 99 100
trace n.d.
PIL@SiO -Pd catalyst can be reused by simple centrifugation
2
2
PIL-Pd
and washing. The reusability was tested by using benzyl alco-
hol as a model substrate. As shown in Figure 6, this novel
amphiphilic nanoreactor can be recycled for at least 4 sequen-
tial runs, with slight loss of activity in the fourth run, which
stems from the loss of the catalyst during the catalyst recycling
procedure via centrifugation. There is no obvious CTAB loss
after recycling experiment (Figure S7). It is worth mentioning
that the selectivity towards benzaldehyde is constantly above
[
a]
Reaction conditions: aromatic alcohol/Pd (mmol) /H
2
O
(mL):
The conversion and
Pd content was determined by
o
[b]
0
.5/0.0048/1.3, oxygen balloon (~2 atm), 65 C, 5 h.
[
c]
selectivity were determined by GC-MS.
[
d]
inductively coupled plasma atomic emission spectroscopy (ICP-AES). TOF
(The converted number of substrate molecules)/(total amount of Pd at-
=
om)/(reaction time, h), Figure 5.
9
9%, which is quite good for such a long-run recycling exper-
iment.
Figure 6. The conversion of benzyl alcohol and selectivity of benzaldehyde in
recycle tests. Time for the last two recycles is 10 h.
Figure 5. The reaction profiles of PIL@SiO
2
-Pd and MCM41-Pd towards
The reason for the dramatic enhancement in catalytic activi-
benzyl alcohol (a and d) and 4-methoxybenzyl alcohol (b and c) respectively.
ty and recyclability of the PIL@SiO -Pd catalyst in our opin-
2
ion a natural outcome of its exquisite nanostructure configura-
tion, i.e. an incompatible, but adaptive hydrophobic core and a
hydrophilic shell coupled in a hybrid nanoreactor to work
The selective oxidation of aromatic alcohols, especially
benzyl alcohol to benzaldehyde, is a key reaction for the syn-
thesis of many high value-added fine chemicals, such as
pharmaceuticals, agrochemicals, and perfumery industries.
The well-designed nanoreactor was tested in the selective
oxidation of primary aromatic alcohols to aromatic aldehydes,
whose catalytic performance was compared with that of
MCM41-Pd and PIL-Pd. The results were summarized in
synergistically. Specifically, the PIL@SiO -Pd catalyst is
2
highly dispersible in water through the assistance of the hy-
drophilic silica layer. The hydrophobic substrates can be ac-
cumulated in the PIL core through the van der Waals interac-
tions as proven by dye adsorption experiments (Figure S5-6,
Figure 4). This provides a local enrichment and close proximi-
E
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Chemistry of Materials
Page 6 of 7
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reactor in aqueous environment, overcoming the common
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tive sites. This advancement leads to outstanding catalytic
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Supporting Information.
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substrates adsorption results in different solvents. This material is
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2
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AUTHOR INFORMATION
Corresponding Author
(27) Lu, J.; Yan, F.; Texter, J., Progr. Polym. Sci., 2009, 34, 431-
448.
(28) Mecerreyes, D., Progr. Polym. Sci., 2011, 36, 1629-1648.
(29) Texter, J., Macromol. Rapid Commun., 2012, 33, 1996-2014.
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Mater. 2014, 29, 78-80.
31) Yan, F.; Texter, J. Angew. Chem. Int. Edit. 2007, 46, 2440-
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(32) Guntari, S. N.; Khin, A. C. H.; Wong, E. H. H.; Goh, T. K.;
Blencowe, A.; Caruso, F.; Qiao, G. G. Adv. Funct. Mater. 2013, 23,
159-5166.
33) Rozik, N.; Antonietti, M.; Yuan, J. Y.; Tauer, K. Macromol.
*
E-mail: jiayin.yuan@mpikg.mpg.de
(
Notes
(
The authors declare no competing financial interest.
ACKNOWLEDGMENT
The authors would like to thank the Max-Planck Society for the
financial support and Dr. Markus Drechsler for the cryo-TEM
measurement.
5
(
Rapid Commun. 2013, 34, 665-671.
(34) Koebe, M., Drechsler, M., Weber, J., Yuan, J. Y. Macromol
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