Hollow shell-corona microspheres with a mesoporous shell as potential microreactors for Au-catalyzed aerobic oxidation of alcohols

Article abstract of DOI:10.1039/b9nj00802k

Au-functionalized hollow shell-corona microspheres with a mesoporous shell are proposed as a microreactor for aerobic alcohol oxidation. These microreactors are constructed by template polymerization followed by hydrolysis of the shell-forming polyvinyltriethoxylsilane segment, and contain a hydrophilic corona to keep the microreactors suspended in the aqueous phase and a mesoporous chelate shell to immobilize Au nanoparticles and to increase the permeability of the microreactors. These microreactors have the ability to encapsulate and concentrate reactants, and have been demonstrated to mediate Au-catalyzed aerobic alcohol oxidation, which takes place efficiently in a quasi-homogeneous aqueous solution and under organic-aqueous biphasic conditions, since the reactants are highly concentrated within the microcavity. Other benefits of the microreactors include easy catalyst reuse, low catalyst leaching and long-term stability.

Full text of DOI:10.1039/b9nj00802k

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www.rsc.org/njc | New Journal of Chemistry
Hollow shell–corona microspheres with a mesoporous shell as potential
microreactors for Au-catalyzed aerobic oxidation of alcoholsw
Li Yang, Minchao Zhang, Yang Lan and Wangqing Zhang*
Received (in Victoria, Australia) 30th December 2009, Accepted 2nd March 2010
First published as an Advance Article on the web 28th April 2010
DOI: 10.1039/b9nj00802k
Au-functionalized hollow shell–corona microspheres with a mesoporous shell are proposed
as a microreactor for aerobic alcohol oxidation. These microreactors are constructed by
template polymerization followed by hydrolysis of the shell-forming polyvinyltriethoxylsilane
segment, and contain a hydrophilic corona to keep the microreactors suspended in the aqueous
phase and a mesoporous chelate shell to immobilize Au nanoparticles and to increase the
permeability of the microreactors. These microreactors have the ability to encapsulate and
concentrate reactants, and have been demonstrated to mediate Au-catalyzed aerobic alcohol
oxidation, which takes place efficiently in a quasi-homogeneous aqueous solution and under
organic–aqueous biphasic conditions, since the reactants are highly concentrated within the
microcavity. Other benefits of the microreactors include easy catalyst reuse, low catalyst leaching
and long-term stability.
channels therefore allow direct access to enzymes encapsulated
1. Introduction
within the interior of these vesicles.
Recently, tailor-made micro- or nanoreactors have been of
In a recent communication, we proposed a microreactor
growing interest to chemists, as they provide a confined
of Pd nanoparticles immobilized hollow shell–corona
environment within which chemical conversion can take
microspheres of poly[styrene-co-2-(acetoacetoxy)ethyl meth-
place efficiently (due to the concentrated reactants) and/or
selectively (due to its specific reaction sites).1,2 Up to now,
acrylate-co-acrylamide] (PS-co-PAEMA-co-PAM) for olefin
hydrogenation under organic–aqueous biphasic conditions.14
lots of reactors, varying in size from a few nanometers to
Herein, in order to increase the permeability of microreactor,
tens of micrometers, including inorganic ones such as
hollow shell–corona microspheres with a mesoporous shell
yolk–shell catalytic nanoreactors,3–5 and organic or polymeric
of poly[styrene-co-2-(acetoacetoxy)ethyl methacrylate-co-
ones such as vesicles6–8 and hollow spheres,9–11 have
ethylene-co-methylacrylic acid] (PS-co-PAEMA-co-PE-co-
been fabricated to provide a confined microenvironment
PMAA) are synthesized and the microreactor with Au
within which conversion takes place efficiently and selectively.
nanoparticles immobilized within it is tested for aerobic
Generally, micro- or nanoreactors should possess three
alcohol oxidation. This proposed microreactor, as shown
characteristics:12 (1) fast diffusion of both reactants and
in Fig. 1, is composed of three parts: (1) the hydrophilic
products through the shell or wall; (2) a suitable structure to
corona of poly(methacrylic acid) (PMAA); (2) the crosslinked,
restrict the catalytic species; (3) an acceptable long-term
chelating, mesoporous shell of poly[styrene-co-2-(acetoacetoxy)-
stability.
ethyl methacrylate-co-ethylene] (PS-co-PAEMA-co-PE); and
Poor permeability is the key limitation for micro- or nano-
(3) Au nanoparticles embedded in the mesoporous shell. It is
reactors (such as polymersomes) because the thick wall or shell
found that this microreactor is efficient for aerobic alcohol
hinders the movement of reactants and products. Two strate-
oxidation in quasi-homogeneous aqueous solution or under
gies have been usually used to overcome this limitation. The
organic–aqueous biphase condition.
first is to incorporate proton pumps in the polymersome
membrane.13 The second is to incorporate channels across
the thick shell through which regulation of the flux of mole-
cules in and out of the microenvironment is made. For
example, Meier’s group has developed a permeable ABA
triblock copolymer polymersome nanoreactor by incorporat-
ing the OmpF channel protein in the membrane.12 These
Key Laboratory of Functional Polymer Materials of Ministry of
Education, Institute of Polymer Chemistry, Nankai University,
Tianjin 300071, China. E-mail: wqzhang@nankai.edu.cn;
Fax: +86-22-23503510; Tel: +86-22-23509794
w Electronic supplementary information (ESI) available: Additional
experimental information. See DOI: 10.1039/b9nj00802k
Fig. 1 Structure of the microreactor.
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microspheres were purified by centrifugation and then
dispersed in water for subsequent use.
2. Experimental
2.1 Materials
2.2.3 Preparation of the PS-co-PAEMA-co-PE-co-PMAA
hollow shell–corona microspheres with a mesoporous shell.
Hydrochloric acid (37 wt%, 3.0 mL) was added to the aqueous
dispersion of the PS-co-PAEMA-co-PVTES-co-PMAA hollow
microspheres (100 mL, containing 3.65 g of the hollow micro-
spheres), and then the dispersion was kept at room temperature
for 24 h. The hollow microspheres were purified by centrifugation
and then dispersed in aqueous hydrofluoric acid (100 mL,
B9 wt%) at room temperature for 24 h. The resultant
PS-co-PAEMA-co-PE-co-PMAA hollow shell–corona micro-
spheres with a mesoporous shell were purified by centrifugation,
washed with water, and finally dispersed in water.
Styrene (St, >98%) and methylacrylic acid (MAA, >99%)
were purchased from Tianjin Chemical Company and distilled
under vacuum before being used. Divinylbenzene (DVB,
>80%, Alfa Aesar) was washed with 5% NaOH aqueous
solution and water, followed by drying with MgSO4. The
monomers, 2-(acetoacetoxy)ethyl methacrylate (AEMA,
>95%, Aldrich) and vinyltriethoxysilane (VTES, >95%,
Tianjin Chemical Company), K2S2O8 (>99.5%, Tianjin
Chemical Company), HAuCl4ꢁ3H2O (>99.9%, Tianjin
Chemical Company), NaBH4 (>98.9%, Tianjin Chemical
Company), 1-phenylethanol (>99%, Alfa Aesar), 1-(4-methoxy-
phenyl)ethanol (>95%, Alfa Aesar), 1-(4-methylphenyl)ethanol
(>98%, Acros), 1-(2-methylphenyl)ethanol (>98%, Alfa
Aesar), 1-(4-chlorophenyl)ethanol (>97%, Alfa Aesar),
benzyl alcohol (>99%, Tianjin Chemical Company) and
benzhydrol (>99%, Alfa Aesar), were used as received.
2.3 Encapsulation of alcohol within the microspheres
An aqueous dispersion of the PS-co-PAEMA-co-PE-
co-PMAA microspheres (1.0 mL, containing 3.2 ꢂ 10ꢃ2 g of
the hollow microspheres) was added to a saturated aqueous
solution of 1-phenylethanol (6.0 ꢂ 10ꢃ2 mmol mLꢃ1, 9.0 mL)
at room temperature. After being stirred for 1 h, the mixture
was filtered with a 0.20 mm filter and the concentration of
1-phenylethanol in the filtrate was measured by HPLC. The
amount of 1-phenylethanol encapsulated within the micro-
spheres was calculated from the difference between the fresh
1-phenylethanol aqueous solution and the filtrate. The volume
percent P occupied by the encapsulated 1-phenylethanol
within the microcavity of the microspheres was calculated by
the following equation:
2.2 Synthesis of the microspheres
2.2.1 Preparation of the core template of PS-co-PMAA
microspheres. The core template of poly(styrene-co-methyl-
acrylic acid) (PS-co-PMAA) microspheres was synthesized as
described elsewhere.15 To a flask, MAA (0.430 g, 5.0 mmol)
was first dissolved in 100 mL water to form a homogeneous
solution. Subsequently, styrene (5.208 g, 50.0 mmol) was
added. The mixture was degassed with nitrogen at room
temperature, and then K2S2O8 (0.297 g, 1.1 mmol) was added.
The mixture was degassed again, and polymerization was
performed with vigorous stirring at 80 1C for 24 h under a
nitrogen atmosphere. Then the product of the PS-co-PMAA
microspheres was purified by centrifugation and washed thrice
with water. The product of PS-co-PMAA microspheres was
dispersed in 150.0 mL of water for subsequent use.
ma=ra
À
ꢀ
Á
P ¼
;
4
3
4
3
mb pðR3 ꢃ r3Þrc ꢂ p r3
where ma and mb are the weight of the encapsulated 1-phenyl-
ethanol and the PS-co-PAEMA-co-PE-co-PMAA hollow
shell–corona microspheres, ra and rc are the density of
1-phenylethanol and the wall of the hollow microspheres, R
and r are the average radius of the hollow microspheres and
the average radius of the microcavity, respectively. Herein, the
density of the wall of the hollow shell–corona microspheres,
rc, is estimated to be approximately equal to that of the bulk
2.2.2 Preparation of the PS-co-PAEMA-co-PVTES-
co-PMAA hollow microspheres. The hollow microspheres
of poly[styrene-co-2-(acetoacetoxy)ethyl methacrylate-co-
vinyltriethoxylsilane-co-methylacrylic acid] (PS-co-PAEMA-
co-PVTES-co-PMAA) were synthesized by template poly-
merization.16,17 To a flask, 0.135 g of K2S2O8 was added to
45.0 mL of the aqueous dispersion of the PS-co-PMAA micro-
spheres (containing 1.69 g of the PS-co-PMAA microspheres).
The mixture was firstly degassed with nitrogen at room
temperature for 30 min with vigorous stirring. Subsequently,
the mixture was heated to 80 1C, and then the mixture of
styrene (1.043 g, 10.0 mmol), AEMA (1.071 g, 5.0 mmol),
VTES (0.952 g, 5.0 mmol), MAA (0.430 g, 5.0 mmol) and the
cross-linker DVB (0.163 g, 1.0 mmol) was added dropwise.
The polymerization was performed with vigorous stirring for
24 h at 80 1C to form coated microspheres. The resultant
coated microspheres were purified by centrifugation and
washed first with water followed with N,N-dimethylformamide
(DMF). Then, the coated microspheres were dispersed in DMF
at 40 1C for 72 h to remove the core template. The elimination
of the core template was judged by dropping the DMF solution
into water until no turbidity or deposition could be seen. Lastly,
the resultant PS-co-PAEMA-co-PVTES-co-PMAA hollow
polymer materials, 1 g cmꢃ3
.
2.4 Preparation of microreactor by immobilization of Au
nanoparticles on the microspheres
To a flask, an aqueous solution of HAuCl4 (2.0 mmol Lꢃ1
,
25.0 mL) and an aqueous dispersion of the PS-co-PAEMA-co-
PE-co-PMAA microspheres (15.0 mL, containing 0.25 g of the
microspheres) were added. The mixture was stirred for 2 h,
and then the pH adjusted to B7 with 0.05 mol Lꢃ1 aqueous
NaOH. Subsequently, cold aqueous NaBH4 (20.0 mmol Lꢃ1
,
10.0 mL) was slowly added with vigorous stirring. The final
dispersion was adjusted to 100.0 mL with water, wherein the
Au concentration is 0.50 mmol Lꢃ1
.
2.5 Catalyst testing
2.5.1 Typical procedures for aerobic alcohol oxidation.
Alcohol (1.0 mmol), suitable base (3.0 mmol), the
aqueous dispersion of the microreactor (10.0 mL, containing
a
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5.0 ꢂ 10ꢃ3 mmol Au catalyst), and a given volume of toluene
(only under the biphasic conditions) were added to a 50 mL
tube-like glass reactor equipped with a reflux condenser. The
resulting mixture was stirred at 80 1C and the oxidation was
carried out with bubbling O2 (ca. 0.05 L minꢃ1) at atmospheric
pressure. The progress of the oxidation was monitored by high
performance liquid chromatography (HPLC).
performed on a Micromeritics Gemini V system at 77 K.
The UV-vis absorption spectra were recorded on a TU-8110
UV-vis spectrophotometer at 25 1C. AAS analysis was performed
on a Solaar AAS 2 atomic absorption spectrometer.
3. Results and discussion
3.1 Synthesis and characterization of hollow shell–corona
microspheres with a mesoporous shell
2.5.2 Catalyst recycling of the microreactor. Oxidation of
1-phenylethanol under toluene–aqueous biphasic condition
was chosen to evaluate the recycling of the microreactor. After
the oxidation was completed (3 h), the organic phase was
decanted and then analyzed with HPLC, then the same
amounts of 1-phenylethanol (1.0 mmol) and toluene (2.0 mL)
were added to the aqueous phase containing the microreactor,
and the next run of oxidation was performed at 80 1C with
bubbling of O2 at atmospheric pressure for 3 h, under the same
oxidation conditions as for the first run.
The microspheres synthesized are composed of three parts: (1)
a hydrophilic corona of PMAA, which keeps the microspheres
suspended in aqueous solution; (2) a chelating mesoporous
shell of PS-co-PAEMA-co-PE, in which the chelate segment of
PAEMA is used to immobilize the Au nanoparticles (the PS
segment is introduced to increase the thermal stability of the
hollow microspheres, since the glass transition temperature
(Tg) of the PAEMA segment is very low (B3 1C),18 and the PE
segment results from the hydrolysis of the PVTES segment to
produce the mesopore in the shell); and (3) a hydrophobic
microcavity to encapsulate and concentrate the alcohol during
catalysis.
To detect the Au catalyst leaching into the organic phase,
the organic phase containing the oxidation product (aceto-
phenone) was analyzed by atomic absorption spectrum (AAS).
To diagnose aggregation of the immobilized Au nanoparticles,
the aqueous dispersion of the recycled microreactor was first
characterized by UV-vis absorption spectroscopy and then by
transmission electron microscopy (TEM). To check the stability
of the PS-co-PAEMA-co-PE-co-PMAA hollow shell–corona
microspheres in the catalytic oxidation, the aqueous dispersion
of the recycled microreactor was first frozen in liquid nitrogen,
freeze-dried to remove water, and then characterized by solid-
state 13C NMR.
Fig. 2 shows the synthesis of the microspheres. Firstly, the
core template of PS-co-PMAA microspheres is synthesized by
one-stage soap-free polymerization as described elsewhere.15
Subsequently, polymerization of styrene, AEMA, VTES and
MAA in the presence of the DVB cross-linker on the sacrificial
template of the PS-co-PMAA microspheres forms coated
microspheres of PS-co-PMAA/PS-co-PAEMA-co-PVTES-co-
PMAA. Then the coated microspheres are dispersed in DMF
to remove the core template, producing hollow shell–corona
microspheres of PS-co-PAEMA-co-PVTES-co-PMAA. These
are then dispersed in aqueous HCl, in which the shell-forming
segment of PVTES is hydrolyzed to form siloxanes, and then
the resultant siloxane component is removed with hydrofluoric
acid to form mesopores in the shell, producing PS-co-PAE-
MA-co-PE-co-PMAA hollow shell–corona microspheres with
a mesoporous shell. It should be pointed out that PVTES is
easily hydrolyzed both in acidic and basic aqueous solu-
tion.19–21 Herein, mesopores are introduced into the shell of
the microspheres through hydrolysis of the PVTES segment in
aqueous acid, which is useful to avoid ionization of the PMAA
corona in basic aqueous solution. It is believed that the
mesopores randomly locate in the shell-layer as shown in
Fig. 1, since the mesopores result from the hydrolysis of the
random copolymer of PS-co-PAEMA-co-PVTES.
2.5.3 Lifetime of the microreactor. To explore the lifetime
of the microreactor, oxidation of 1-phenylethanol employing
very dilute suspension of the microreactor at a molar ratio of
substrate/Au of 10 000 : 1 was tested. To a 50 mL tube-like
glass reactor equipped with a reflux condenser, 10.0 mL
of aqueous dispersion of the microreactor containing
1.0 ꢂ 10ꢃ3 mmol of Au catalyst, 30 mmol of KOH, 10.0 mmol
of 1-phenylethanol and 2.0 mL of toluene were added. The
oxidation at 80 1C was started by bubbling through O2
(ca. 0.05 L minꢃ1) at atmospheric pressure, and the oxidation
was monitored by HPLC.
2.6 General characterizations
TEM was conducted by using a Philips T20ST electron
microscope at an acceleration voltage of 200 kV, whereby a
small drop of the colloidal dispersion was deposited onto a
piece of copper grid and dried at room temperature under
atmospheric pressure. Fourier-transform infrared spectra
(FTIR) were determined on a Bio-Rad FTS 135 spectrometer
on a potassium bromide pellet and the diffuse reflectance
spectra were scanned over the range 400–4000 cmꢃ1. The
solid-state 13C NMR measurements were performed on a
Varian Infinityplus wide-bore (89 mm) NMR spectrometer
at spin rate of 13 kHz, which was equipped with a double-
resonance HX CP/MAS probe. HPLC analysis was performed
on a LabAlliance PC2001 system equipped with a C18 column
and a UV-vis detector using a mixture of CH3CN and water
(6 : 4 by volume) as eluent. The nitrogen adsorption was
It is found that the coating monomers of styrene, AEMA,
VTES, MAA and the cross-linker DVB are almost
quantitatively converted to the coated microspheres. After
the subsequent removal of the core template and the hydrolysis
of the PVTES segment, the weight of the resultant
PS-co-PAEMA-co-PE-co-PMAA hollow shell–corona micro-
spheres with a mesoporous shell is about 59 wt% of the coated
microspheres.
Fig. 3 shows the TEM images taken during the synthesis of
the microspheres. Fig. 3A shows the core template of the
microspheres (average diameter B230 nm), Fig. 3B shows the
coated microspheres (average diameter B330 nm), and
Fig. 3C shows the hollow microspheres (average diameter
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Fig. 2 Synthesis of the PS-co-PAEMA-co-PE-co-PMAA microspheres.
B320 nm, average wall thickness B52 nm, extent of micro-
cavity B210 nm). Clearly, the size of the PS-co-PAEMA-
co-PVTES-co-PMAA hollow microspheres is a little smaller
than that of the coated microspheres, indicating a slight
shrinkage of the coating materials during dissolution of the
core template. In addition, we also observed something within
the hollow microspheres; we think that some of the coating
monomers diffuse into the swollen core template of the
PS-co-PMAA microspheres, and that these polymerize in the
presence of the cross-linker DVB to form insoluble polymeric
materials. Fig. 3D shows the microspheres with a mesoporous
shell, and indicates that their size, B310 nm, is almost as same
as those of the hollow microspheres (Fig. 3C), while the wall
thickness, B50 nm, is slightly smaller.
which the distribution of the pore size is calculated using the
Barrett–Joyner–Halenda (BJH) method. As shown in Fig. 4B,
the shape of the N2 isotherms corresponds to a mesoporous
material with pore size distribution centered at 2 nm. Here it
should be pointed out that the average size of the mesopores
in aqueous solution should be larger than 2 nm, since the
PS-co-PAEMA-co-PE-co-PMAA hollow shell–corona micro-
spheres become swollen in aqueous solution due to the
hydrophilic PMAA corona.
Fig. 5 shows the solid-state 13C CPMAS NMR spectra
taken during the synthesis of the microspheres. Lines A, B,
C and D are the spectra of the core template of the PS-co-PMAA
microspheres, the coated microspheres of PS-co-PMAA/
PS-co-PAEMA-co-PVTES-co-PMAA, the PS-co-PAEMA-
co-PVTES-co-PMAA hollow microspheres and the PS-co-
PAEMA-co-PE-co-PMAA hollow shell–corona microspheres
with a mesoporous shell, respectively. Comparing spectrum C
with spectrum B, the relative intensities in chemical shift of
127 ppm and 40 ppm are weakened, suggesting removal of the
core of PS-co-PMAA microspheres and formation of hollow
microspheres. In addition, as indicated by the insets above
spectrum C, the characteristic chemical shifts due to the
corresponding carbons are present, which further conforms
their constitution, and this is also confirmed by FTIR (Fig. S1w).
To confirm formation of mesoporous structure in the shell,
the dried PS-co-PAEMA-co-PE-co-PMAA hollow shell–
corona microspheres were further characterized by nitrogen
adsorption. Fig. 4A shows their N2 isotherms at 77 K, from
3.2 Encapsulation of alcohol within the microspheres
Due to the hydrophilic PMAA corona, the microspheres can
be easily dispersed (and remain suspended) in aqueous solution.
It is found that, similar to other polymeric capsules,22–24 these
microspheres can encapsulate and concentrate guest organic
molecules. Here, we encapsulate a hydrophobic alcohol
(1-phenylethanol) by way of example. After addition of the
microspheres into a saturated aqueous solution of 1-phenyl-
ethanol at room temperature, we found that the concentration
of 1-phenylethanol within the hollow microspheres was almost
389 times higher than those in water, the encapsulated 1-phenyl-
ethanol taking up 100% of microcavity space. We point out
that the percentage of the encapsulated guest molecules is
estimated by the extent of the microcavity measured with
TEM, and so this should be a little higher than the actual
value, since the microspheres will become swollen in aqueous
solution due to the hydrophilic PMAA corona. As shown in
Fig. 2, the microspheres contain a hydrophilic corona and a
hydrophobic shell, and therefore we think that the micro-
environment in the microcavity of the hollow microspheres is
Fig. 3 TEM images of (A) the core template of the PS-co-PMAA
microspheres, (B) the microspheres coated with PS-co-PMAA/
PS-co-PAEMA-co-PVTES-co-PMAA, (C) the hollow microspheres
coated with
a mesoporous shell of PS-co-PAEMA-co-PVTES-
co-PMAA, and (D) the hollow shell–corona microspheres with a
mesoporous shell of PS-co-PAEMA-co-PE-co-PMAA.
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Fig. 4 (A) N2 isotherms of the hollow shell–corona microspheres with a mesoporous shell at 77 K, and (B) the size distribution of the mesopores.
size distribution of the immobilized Au nanoparticles. It can
clearly be seen that the Au nanoparticles are uniformly
dispersed on the hollow microspheres, and that the average
size of the Au nanoparticles is 5.1 nm. Thus, we deduce that
the Au nanoparticles are selectively embedded mainly on the
shell-layer, since the chelate segment of PAEMA is primarily
located in the shell of the microspheres. Furthermore, the Au
nanoparticles are believed to be outside the mesopores (as
shown in Fig. 1), since they are larger than the mesopores.
It is well documented that UV-vis spectroscopy can be used
to diagnose the aggregation state of Au nanoparticles.27,28 For
example, highly dispersed 5–20 nm Au nanoparticles exhibit
an absorbance peak at B520 nm.27 As the gold particle size
decreases, a hypsochromic shift of the characteristic absorbance
occurs, and no sharp absorbance peak is observed within the
UV-vis range when the size of gold nanoparticles falls below
3 nm.28 Fig. 7 shows the UV-vis absorption spectra of an
Fig. 5 13C CPMAS NMR spectra of (A) the core template of the
PS-co-PMAA microspheres, (B) the microspheres coated with
PS-co-PMAA/PS-co-PAEMA-co-PVTES-co-PMAA, (C) the hollow
microspheres coated with a mesoporous shell of PS-co-PAEMA-co-
PVTES-co-PMAA, and (D) the hollow shell–corona microspheres
with a mesoporous shell of PS-co-PAEMA-co-PE-co-PMAA.
aqueous dispersion of the microreactors, in which a distinct
absorption peak around 520 nm is clearly observed, indicating
formation of B5 nm Au nanoparticles.
Similarly to the PS-co-PAEMA-co-PE-co-PMAA hollow
shell–corona microspheres themselves, the microreactors can
also be easily dispersed in aqueous solution due to the
hydrophilic PMAA corona, suggesting that they are a quasi-
homogeneous catalyst. In addition, we also found that the
microreactor is selectively dispersed in the aqueous phase (as
shown by the insets in Fig. 7) when a hydrophobic solvent
such as toluene is added into the aqueous dispersion. This
feature makes the microreactor especially suitable for organic–
aqueous biphasic catalysis, as discussed later.
generally hydrophobic. Thus, the encapsulation of 1-phenyl-
ethanol is mainly ascribed to the hydrophobic microenvironment
provided by the microspheres.
3.3 Preparation of microreactor by immobilization of Au
nanoparticles on the microspheres
Since the PAEMA segment provides the chelate ligands of the
b-diketone for coordination with metal precursors,18,25 polymeric
materials containing the PAEMA segment can be used to
immobilize metal catalysts. We have successfully immobilized
Pd nanoparticle catalysts on PAEMA-based polymers and
studied their applications for the C–C coupling reaction and
hydrogenation.14,26 In the current work, we found that Au
nanoparticles can be easily immobilized on the microspheres.
When HAuCl4 was added to the aqueous dispersion of the
microspheres, Au ions were coordinated with the chelate
ligands in the PAEMA segment, and subsequently reduced
by aqueous NaBH4, which resulted in the formation of Au
nanoparticles immobilized on the hollow microspheres. Fig. 6
shows the TEM image of the resultant microreactor and the
3.4 Aerobic alcohol oxidation
The oxidation of alcohols to aldehydes or ketones is a pivotal
functional group transformation in organic chemistry.29–31
Recently, aerobic alcohol oxidation has attracted much
interest, and various Au catalysts immobilized on inorganic
materials32–36 and polymers37–41 have been investigated as
potential catalysts. It was concluded that the oxidation follows
the Langmuir–Hinselwood reaction mechanism, that is, the
higher concentration of the reactants in a confined region, the
faster the oxidation.42 As discussed above, the present hollow
shell–corona microspheres with a mesoporous shell can
encapsulate and concentrate alcohols in water. Therefore,
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Fig. 6 (A) TEM image of the microreactors, and (B) the size distribution of the immobilized Au nanoparticles.
the microreactor is diluted by toluene, but meanwhile it has an
advantage that the diffusion of the oxidation product out of
the microreactor can be accelerated by extraction due to the
toluene phase, as shown in Fig. 8. It is found that the
oxidation under biphasic conditions is most efficient at a
toluene–water volume ratio of 1 : 5 (Fig. S2).
Fig. 9 shows the dependence of yield of acetophenone with
respect to time during the alcohol oxidation performed in
aqueous phase and under toluene–aqueous biphasic conditions,
employing the optimized base KOH (Fig. S3). Clearly, an
almost quantitative yield of acetophenone is achieved in 2 h at
80 1C employing the optimized base, whether the oxidation is
performed in the aqueous phase or under toluene–aqueous
biphasic conditions. However, the initial oxidation under
toluene–aqueous biphasic conditions is faster than in the
aqueous phase, which affords a higher turnover frequency
(TOF) value (667 hꢃ1 vs. 317 hꢃ1, as calculated by the number
of Au atoms after the first 5 min).
Fig. 7 The UV-vis spectra of the aqueous dispersion of the fresh and
recycled microreactor. Insets: optical images of the aqueous dispersion
of the microreactor.
aerobic alcohol oxidation within the present microreactor is
expected to be efficient due to concentrated reactants.
By way of example, we studied the aerobic oxidation of
1-phenylethanol within the present microreactor in aqueous
phase and under toluene–aqueous biphasic conditions. As
shown in Fig. 8A, in aqueous solution, alcohol is trapped in
the hollow shell–corona microspheres through the mesoporous
shell due to the hydrophobic microenvironment, and oxidation
of alcohol with concentrated reactants occurs by the action of
the Au nanoparticles. During the oxidation, the product
(acetophenone) accumulates within the microreactor and
gradually diffuses into the aqueous phase through the meso-
porous shell. Clearly, there exists an alcohol–ketone dispersion
balance between the water and within the microreactor. Under
the toluene–aqueous biphasic conditions, the alcohol toluene
solution forms one phase, while water (containing the micro-
reactor) forms the other. The alcohol and toluene molecules
first diffuse into the water and are then encapsulated in the
microreactor, within which oxidation takes place. The product
then diffuses into the water and is extracted into the organic
phase.
To evaluate the effect of the mesopores or permeability of
the microreactor on alcohol oxidation, a reference micro-
reactor of Au nanoparticles immobilized hollow shell–corona
microspheres of PS-co-PAEMA-co-PAM is synthesized (Fig. S4).
These are the same as the microreactors discussed earlier, but
they have no mesopores in the shell layer.14 The size of the Au
nanoparticles immobilized on the reference microreactor,
5.7 nm, is very close to those on the microporous microreactors.
Fig. 10 shows the dependence of yield of acetophenone with
respect to time during the alcohol oxidation performed in
aqueous phase and under toluene–aqueous biphasic conditions
employing the present microreactor and the reference micro-
reactor, respectively. The result indicates that the micro-
reactors with a mesoporous shell are much more efficient than
the reference microreactor under aqueous condition. However,
under toluene–aqueous biphasic conditions, the microreactor
is as efficient as the reference microreactor. The reason for this
is possibly due to the faster diffusion of acetophenone (due to
the mesoporous shell) out of the microporous microreactor
compared to the reference microreactor, when under aqueous
conditions. However, under biphasic conditions, the diffused
acetophenone is quickly extracted into the organic phase, and
the diffusion is accelerated whether the microreactor has a
mesoporous shell or not, and therefore the two microreactors
have similar catalytic efficiency.
One would expect there to be a balance of reactants between
the toluene solution, aqueous solution, and the microreactors.
Clearly, compared with oxidation performed in aqueous
phase, oxidation performed under toluene–aqueous biphasic
conditions has a disadvantage that the alcohol confined within
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Fig. 8 Alcohol oxidation within the microreactor (A) in the aqueous phase and (B) under toluene–aqueous biphasic conditions.
with the Microgel-Au or Au/DMA-resin catalyst, the present
microreactor affords a competitive TOF value (Table 1, entries
1–4). We found that aerobic oxidation of 1-phenylethanol
within the microporous microreactor at 25 1C proceeds with
a slightly lower TOF value than for the Au/PS catalyst
(Table 1, entries 5–6). However, it should be pointed out that
various parameters such as the size of Au nanoparticles,38 the
nature of the support,43 and the dissolution of O2 in the
reaction mixture44 can affect the catalytic activity of Au
nanocatalyst for aerobic alcohol oxidation.
To further evaluate the microreactor, aerobic oxidations of
several typical alcohols within the microreactor under organic–
aqueous biphasic conditions were explored. For the secondary
alcohols that have either an electron-deficient or electron-rich
para-substituent, such as 1-phenylethanol, 1-(4-methoxy-
phenyl)ethanol, 1-(4-methylphenyl)ethanol, 1-(4-chlorophenyl)-
ethanol and benzhydrol, the oxidation within the microreactor
runs efficiently, almost quantitative yields are achieved in
30–120 min, and the microreactor affords a TOF ranging from
100 to 400 hꢃ1 (Table 2, entries 1–5). In contrast, the oxidation
of 1-(2-methylphenyl)ethanol within the microreactor is
less efficient, possibly due to the steric hindrance of the
meta-substituent (Table 2, entry 6), and above 99% yield is
achieved when the oxidation extends to a relatively long time
Fig. 9 The dependence of yield of acetophenone with respect to time
during the alcohol oxidation performed in aqueous phase (&) and
under toluene–aqueous biphasic conditions (K). Reaction conditions:
10.0 mL of aqueous dispersion of the microreactor containing
5.0 ꢂ 10ꢃ3 mmol of Au catalyst, 1.0 mmol of 1-phenylethanol,
3.0 mmol of KOH, 2.0 mL of toluene (only under organic–aqueous
biphasic conditions), bubbling O2 at 0.05 L minꢃ1, 80 1C, HPLC yield.
Up to now, several Au nanocatalysts immobilized on or
stabilized with polymeric materials such as microgel,39 polystyrene
(PS),40 and dimethylamino-functionalized resin (DMA-resin)41
have been proposed for aerobic alcohol oxidation. Compared
Fig. 10 The dependence of yield on time in the oxidation of 1-phenylethanol performed within the microporous microreactor (K) and the
reference microreactor (&), (A) in aqueous solution and (B) under toluene–aqueous biphasic conditions. Reaction conditions: 10.0 mL of aqueous
dispersion of microreactor containing 5.0 ꢂ 10ꢃ3 mmol of Au catalyst, 3.0 mmol of KOH, 1.0 mmol of 1-phenylethanol, 2.0 mL of toluene (only
under organic–aqueous biphasic conditions), bubbling O2 at 0.05 L minꢃ1, 80 1C, HPLC yield.
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Table 1 Aerobic alcohol oxidation employing different immobilized noble-metal nanocatalysts
Entry
Catalyst
Conditions
Yield (%)
TOF (hꢃ1 b
)
1a
2a
3d
4e
5a
6f
Microreactor (this work)
Microreactor (this work)
Microgel-Au
Au/DMA-resin
Microreactor (this work)
Au/PS
80 1C, H2O–toluene, KOH, bubbling O2, 0.50 mol% catalyst, 2 h
80 1C, H2O, KOH, bubbling O2, 0.50 mol% catalyst, 2 h
60 1C, H2O, NaOH, 1.5 atm O2, 0.2 mol% catalyst, 1 h
60 1C, EtOH–H2O, KOH, 5 atm O2, 0.5 mol% catalyst
25 1C, H2O–toluene, KOH, bubbling O2, 1.0 mol% catalyst, 3 h
Room temperature, 1 atm O2, benzotrifluoride–H2O, 1 mol% catalyst, 3 h
98
98
75
—
60
96
100 [200c]
100 [160c]
375
120
20
32
a
b
See the Experimental section for details. TOF calculated from the total number of Au atoms at the end of the reaction. TOF calculated by the
c
d
total Au atoms at 75% yield. Reference catalyst in ref. 40. Reference catalyst in ref. 41. Reference catalyst in ref. 42.
e
f
Table 2 Aerobic alcohol oxidation within the microreactor under organic–aqueous biphasic conditionsa
Entry
Alcohol
Product
Time (min)
Yieldb (%)
TOFc (hꢃ1
)
1
120
98
100
2
3
60
30
>99
>99
200
400
4
5
6
60
>99
>99
200
400
30
780
>99
>99
15
7d
105
110
a
b
c
See the reaction conditions indicated in the Experimental section. HPLC or 1HNMR yield. TOF is measured based on total Au atoms at the
d
end of the oxidation. The oxidation is performed in aqueous solution.
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of 780 min. The aerobic oxidation of a typical primary alcohol
of benzyl alcohol within the microreactor in aqueous solution
is also studied and the microreactor affords a moderate TOF
of 110 hꢃ1 (Table 2, entry 7). It is found that benzaldehyde is
first formed, and then this is further oxidized into benzoic acid.
It should be pointed out that the oxidation of benzyl alcohol
is performed in aqueous solution instead of under organic–
aqueous biphasic conditions, since the oxidation product
(benzoic acid) is easily soluble in water. For aliphatic alcohols
such as cyclohexanol, the oxidation within the microreactor
runs inefficiently, similarly to those employing general Au
nanocatalysts.39–41
The recycling of the microreactors and leaching of Au
nanocatalyst were checked using the oxidation of 1-phenyl-
ethanol under toluene–aqueous biphasic conditions at 80 1C as
a typical example. After the oxidation was just complete (3 h),
the organic phase was removed by simple decantation and
1-phenylethanol was reloaded. We found that the micro-
reactors can be recycled at least 6 times without losing activity.
AAS analysis of the organic phase indicates that the Au
catalyst leaching into organic phase can be ignored. In addition,
no aggregation of the immobilized Au nanoparticles was
confirmed by UV-vis measurements (Fig. 7) or TEM observations
(Fig. 11), which shows that the average size of the Au
nanoparticles in the recycled microreactor, 5.3 nm, is very
similar to those in the fresh microreactor.
Fig. 12 The dependence of yield on time in the oxidation of
1-phenylethanol under toluene–aqueous biphasic conditions for the
microreactor endurance testing. Reaction conditions: 10.0 mL of
aqueous dispersion of the microreactor containing 1.0 ꢂ 10ꢃ3 mmol
of Au catalyst, 30 mmol of KOH, 10.0 mmol of 1-phenylethanol,
2.0 mL of toluene, bubbling O2 at 0.05 L minꢃ1, 80 1C, HPLC yield.
The lifetime or endurance of the present microreactor was
also tested by greatly diluting the microreactors in alcohol
(molar ratio of alcohol–Au catalyst of 10 000 : 1). As shown in
Fig. 12, the microreactor can be used for 80 h at least under
the present alcohol oxidation.
4. Conclusions
Furthermore, from the TEM image of the recycled micro-
reactor (Fig. 11), it is clearly observed that the size and
morphology of the hollow shell–corona microspheres are
almost unchanged after 6 cycles of oxidation. This is ascribed
to the fact that the PS-co-PAEMA-co-PE-co-PMAA hollow
microspheres are cross-linked by 4 mol% DVB (see Experi-
mental section), which makes the hollow microspheres stable.
13C CPMAS NMR analysis of the recycled microreactor
further suggests that the hollow microspheres in the recycled
microreactor have a similar composition to the fresh PS-co-
PAEMA-co-PE-co-PMAA hollow microspheres (Fig. S5). All
these results confirm the stability of the PS-co-PAEMA-co-
PE-co-PMAA hollow microspheres and the reusability of the
present microreactor.
Shell–corona hollow microspheres with a mesoporous shell of
PS-co-PAEMA-co-PE-co-PMAA have been synthesized. They
contain a hydrophilic corona of PMAA to keep the hollow
microspheres suspended in aqueous solution, and a mesoporous
chelating shell to immobilize the Au nanocatalyst and to
increase the permeability of the microreactor. The hollow
shell–corona microspheres can encapsulate organic molecules
such as 1-phenylethanol, and it is found that the concentration
of 1-phenylethanol within the microspheres is much higher
than in water. The catalyst (5.1 nm Au nanoparticles) is
immobilized on the microspheres, and the resulting micro-
reactor is demonstrated to be efficiently catalyze the aerobic
oxidation of alcohols under quasi-homogeneous aqueous
solution and under organic–aqueous biphasic conditions. Other
benefits of the microreactor include easy catalyst reuse, low
catalyst leaching and long-term stability. We anticipate that the
present microreactor has promising potential in catalysis.
Acknowledgements
The financial support by National Science Foundation of
China (No. 20974051), Tianjin Natural Science Foundation
(No. 09JCYBJC02800), and the Program for New Century
Excellent Talents in University (No. NCET-06-0216) is gratefully
acknowledged.
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