Semipermeable organic?inorganic hybrid microreactors for highly efficient and size-selective asymmetric catalysis

Article abstract of DOI:10.1021/acscatal.7b01659

A highly efficient and size-selective organic?inorganic hybrid microreactor was fabricated successfully by assembling the semipermeable microporous silica hollow nanospheres at the interface of organic polymers containing an asymmetric transfer hydrogenation catalyst. The small micropores in the silica shells (1.0?1.5 nm) of the microreactors provide short mass-transfer channels for small molecules but prevent the diffusion of large reactants through the shell. At the same time, the combination of the outer microporous silica and inner polymers endows the microreactor with hydrophobic@ hydrophilic property, which helps its dispersion in water as well as the adsorption of reactant molecules from water, yielding an ideal microenvironment for water medium catalysis reactions. Such microreactors could efficiently catalyze the aqueous asymmetric transfer hydrogenation (ATH) of small-sized ketones with catalytic activity comparable to the homogeneous Rh-VBSDPEN catalyst (TOF 625 vs 724 h^?1). Meanwhile, the microreactors also showed size selectivity toward large-sized reactants. Our studies provide an approach for the construction of artificial bioinspired microreactors with size-selective properties and favorable catalysis microenvironments.

Full text of DOI:10.1021/acscatal.7b01659

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Article
Semi-Permeable Organic-Inorganic Hybrid Microreactors
for Highly Efficient and Size-Selective Asymmetric Catalysis
Xiaoming Zhang, Lingyan Jing, Lijuan Wei, Fengwei Zhang, and Hengquan Yang
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b01659 • Publication Date (Web): 24 Aug 2017
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Semi-Permeable Organic-Inorganic Hybrid Microreactors for
Highly Efficient and Size-Selective Asymmetric Catalysis
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Xiaoming Zhang,†# Lingyan Jing,†# Lijuan Wei,† Fengwei Zhang,‡ Hengquan Yang*†
†School of Chemistry and Chemical Engineering, Shanxi University, Taiyuan 030006, China
‡Institute of Crystalline Materials, Shanxi University, Wucheng Road 92, Taiyuan 030006, China
Supporting Information
ABSTRACT: A highly efficient and size-selective organic-inorganic hybrid microreactor was fabricated successfully by
assembling the semi-permeable microporous silica hollow nanospheres at the interface of organic polymers containing an
asymmetric transfer hydrogenation catalyst. The small micropores in the silica shells (1.0-1.5 nm) of the microreactors
provide short mass-transfer channels for small molecules, but prevent the diffusion of large reactants through the shell.
At the same time, the combination of the outer microporous silica and inner polymers endow the microreactor with
hydrophobic@hydrophilic property, which helps its dispersion in water as well as the adsorption of reactant molecules
from water, yielding an ideal microenvironment for water medium catalysis reactions. Such microreactors could
efficiently catalyze the aqueous asymmetric transfer hydrogenation (ATH) of small-sized ketones with catalytic activity
comparable to the homogeneous Rh-VBSDPEN catalyst (TOF 625 vs 724 h−1). Meanwhile, the microreactors also showed
size selectivity toward large-sized reactants. Our studies provide an approach for the construction of artificial bio-inspired
microreactors with size-selective properties and favorable catalysis microenvironments.
KEYWORDS: bioreactor mimicking; hybrid microreactors; semi-permeable; size-selective; heterogeneous asymmetric
catalysis
active catalysts, diffusion barriers caused by the structure
of solid support or low affinity of the solid surface to
organic reactants. Accordingly, it is highly desirable to
develop an efficient encapsulation method that not only
provides an encapsulated catalyst which is compatible
with homogeneous catalysts but also makes use of the
positive effects of solid supports such as pore
confinement effect or enrichment effect etc.
INTRODUCTION
The cellular environment can be regarded as
a
sophisticated reaction locus, where metabolic pathways
take place with unsurpassed efficiency and specificity. As
a long-standing goal of bio-synthetic chemistry, the
construction of assemblies that mimic the superior
qualities of natural cells is highly desirable.1-4 These
qualities often include the ability to operate in an
aqueous solution at a near physiological pH, to provide a
well-defined compartment for spatially isolated catalytic
cycles and to select one substrate in light of its size or
shape from a pool of structurally related molecules.
Chemists have been trying to mimic them by selective
encapsulation of catalysts in artificial microreactors.5-8
On the other hand, the size or shape-selective ability,
which is an important and basic concept in the bio-
synthetic system, is seldom achieved in the reported
artificial microreactors so far.17-20 Such a concept is widely
adopted in nature as most enzymes can highly selectively
convert the right molecules that match their active sites
in size or shape. Though shape- or size-selective catalysis
phenomenon was already found in microporous zeolites
or MOFs materials, and even used in chemical industry,
the design of artificial microreactors possessing size or
shape-selective properties while maintaining favorable
reaction microenvironments still remains a challenge due
to the inherent limitations of traditional mimicking
methods.21-24
Enzymes,
homogeneous
metal
catalysts
and
organocatalysts have been loaded into lipid vesicles,
dendrimers, supramolecular architectures, sol-gel
materials or silica scaffolds for various purposes.9-16
Although these techniques were successfully applied, the
resultant solid catalysts often show decreased reactivity
and selectivity compared to their homogeneous
counterparts due to the altered microenvironment of the
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Colloidosomes, which are usually formed by the self-
2
assembly of nanoparticles at the immiscible liquid–liquid
or gas-liquid interface during emulsification, have
exhibited great promise in the development of artificial
bio-microreactors.25-32 Such synthetic capsules inherit the
characteristics of emulsions, including bi-phase nature,
compartmentalization effects, large surface area and
remarkable interfacial stability. Active ingredients such as
drugs, proteins, gas bubbles, catalyst or even living cells
could be encapsulated into the colloidosome capsules
easily through in situ processes.33-42 And, the isolated
inner space provides pretty good chances for the design of
reaction microenvironment. It has also been found that
the large interface area of colloidosome microcapsules
facilitates the phase transfer of molecules across the
interface, and can improve interfacial reaction efficiency
by several orders of magnitude compared to bulk
systems.43-45 Moreover, the spontaneous partitioning of
colloidal particles at liquid-liquid or gas-liquid interfaces
produces shell-like micro-structures. And by applying
certain nanoparticles, microcapsules with semi-
permeable-membrane-bound could be fabricated.20, 46, 47
Owing to these unique properties, colloidosome
microcapsules have great potential as new robust
microreactors with desired catalytic microenvironment
and size-selective properties.
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Figure 1. Schematic illustration of the preparation of semi-
permeable organic-inorganic hybrid microreactors with a
Pickering emulsion polymerization method.
RESULTS AND DISCUSSION
Formation and Characterization of Semi-permeable
Organic-Inorganic Hybrid Microreactor
The
semi-permeable
organic-inorganic
hybrid
microreactors were prepared through
a
Pickering
emulsion polymerization method, as illustrated in Figure
1. The whole process comprises two steps. First, an oil-in-
water (o/w) Pickering emulsion is formulated by
vigorously stirring a mixture of organic phase and
aqueous suspension of solid emulsifier. During this
process, hollow silica nanosphere that was synthesized
through a single micelle template method was chosen as
an emulsifier based on the following considerations.52-53
Such nanospheres possess a small particle size in the
range of 20-30 nm and a hollow nanostructure with a
ultra-thin shell (TEM, Figure 2a), which might be
beneficial for the diffusion of molecules. Abundant
micropores with dimension of 1.0 to 1.5 nm or even
smaller are present on the shell (N2 sorption and pore size
distribution, Figure 3c, d), thereby providing an excellent
test system to examine the effect of micropores on the
retention or diffusion of molecules. Furthermore, the
hollow nanosphere has hydrophilic surface properties
(water contact angle 29o, Figure S1a), which is favorable
for formation of an oil-in-water Pickering emulsion.54 As
shown in Figure S1b, after screening compositions, the
Pickering emulsion was formulated with octanol as oil
phase and aqueous suspension of silica hollow
nanospheres as continous phase. The oil phase contained
a given amount of styrene (Sty) and divinylbenzene (DVB)
monomers and initiator necessary to form cross-linked
polymers. The hydrophobic catalyst, Rh-VBSDPEN, was in
situ encapsulated into the polymersomes by dissolving it
into the octanol oil phase. The mass ratio of silica
nanospheres to organics (octanol + Sty + DVB + Rh-
VBSDPEN) in the system is 1:5.92, and this mixture was
sheared at 28500 rpm for 2 min to form a hollow silica
nanosphere-stabilized emulsion. In this proportion, the
amount of interfacial nanoparticles is much higher than
In this work, we describe an organic–inorganic hybrid
colloidosome microcapsule with semi-permeability and
ideal interior microenvironments as efficient microreactor
for asymmetric catalysis. The capsules are composed of a
shell made up with closely-packed size-sieving inorganic
silica
colloidal
particles
and
inner
organic
macromolecular polymer layer with asymmetric catalyst
in the frameworks. Controlled permeability of the shells
of microcapsules enables size-selective catalysis and the
interior organic layer endows the catalytic active sites
with desired microenvironments. Moreover, the unique
hybrid structure ensures long-term retention of the
catalysts during catalytic cycles as well as the rapid
diffusion of substrates and products into and out of
microreactors. The hydrophobic polymers in the
hydrophilic silica layer might induce
a core-shell
structure with hydrophobic polymer as a core and
hydrophilic silica as a shell. Such a unique structure has
been demonstrated to benefit aqueous reactions since the
hydrophilic surface enables the particles to be well
dispersed in water, while the hydrophobic core helps to
absorb organic molecules from the surrounding aqueous
environment.48-51 As a proof of this concept, we use the
aqueous asymmetric transfer hydrogenation (ATH)
reaction catalyzed by Noyori-Ikariya catalyst (Rh, Ru or Ir
complexes of TsDPEN) as a model to investigate the
availability of colloidosome microcapsule platform. Our
results demonstrated convincingly that the organic-
inorganic microcapsule could be utilized as efficient fast-
acting and size-selective microreactors.
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usual cases for achieving a full coverage of the droplets.
distributed over the surface of these microspheres,
2
The optical microscope image of emulsion droplets is
shown in Figure S1c. Evidently, the resulting emulsion
droplets are spherical in morphology, and polydisperse in
size. The formed Pickering emulsions were sufficiently
stable against coalescence without visible separation of
the dispersed phase after standing for 3 d, which is
important for the following polymerization step. In the
second step, the monomers dissolved in the oil phase
polymerized and formed polymer@silica composites upon
thermal annealing the Pickering system at 70 oC for 20 h.
After polymerization, the composite was washed with a
series of ethanol/water solvents and dried under vacuum,
the assembled “colloidosomes” were structurally
reinforced into robust free-standing capsules. As shown
in Figure S1d, the microcapsule structure could still be
completely covering its surface, indicating that the as-
made silica hollow nanosphere is sufficiently wettable by
the dispersed oil phase without further modification that
is often required to generate stable emulsions from
inorganic colloids. This dense over-layer renders the
capsules with additional structural stability, and strongly
suggests these could be utilized in size-selective
applications where molecular access is controlled by the
highly regular micropores of the silica hollow
nanospheres as previously reported for catalysis by
encapsulating noble metal NPs.55 Figure 2e shows that the
obtained hybrid spheres are hollow structured, small
nanospheres are found in the inner surface of the wall,
which might be a result of phase separation of the
polymer during polymerization as octanol chosen is a
poor solvent for the cross-linked polymer network,
leading to phase separation and precipitation at the
interface and thus locking the silica nanospheres into the
capsule surface. The nature of the oil phase is very
important for the formation of hollow structure. For
example, replacing octanol with a good swelling reagent
towards organic polymers, like toluene, yields core-shell
structures under the same reaction conditions (Figure S2).
This is due to the fact that toluene can swell the polymer
network preventing effective phase separation for the
formation of hollow capsule. Polymer nanoparticles with
particle size of 500-600 nm could also be obtained
without the formation of Pickering emulsion aisle (Figure
2f). However, such polymer catalyst doesn’t possess the
unique hollow structure and densely packed silica shell.
The organic-inorganic microreactors were further
characterized by FT-IR, thermogravimetric analysis (TGA)
and nitrogen adsorption. FT-IR was applied to verify the
composition of the hollow microcapsules. As shown in
Figure 3a, the characteristic bands from silica can be
observed at around 3447, 1631, and 1104 cm−1 respectively
for ν (O–H), δ(O–H), and ω (Si–O) in the spectra of
hybrid capsules. The bands around 2858–3061 cm-1, 1418–
1510 cm-1 and 678–750 cm-1 that are assigned to the C–H
stretching vibrations, the C=C vibrations and breathing
vibrations of the phenyl ring are also observed, suggesting
the successful incorporation of polymers in the
microcapsules. The organic group content and thermal
stability of capsules was investigated using the
thermogravimetric analysis (TGA) technique (Figure 3b).
The capsules largely decompose in a single step in the
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maintained intactly even in the presence of
a
demulsification reagent ethanol. The morphology and
size of the capsules are similar to those of the initial
Pickering emulsion droplets, confirming that the
microreactors are originated from precursor emulsion
droplets.
Figure 2. (a) TEM image of hollow silica nanospheres; (b)
SEM image of organic-inorganic hybrid microcapsules; (c)
magnification of
a hybrid microcapsule showing the
spherical structures; (d) the densely packed out-layer; (e) a
broken microcapsule revealing the hollow interior and inner
surface structure and (f) polymer nanoparticles without the
formation of Pickering emulsion aisle.
Scanning electron microscope (SEM) images of the
hybrid microcapsules reveal the formation of individual
intact spherical structures with a size distribution of 20 ±
12 μm, together with few broken spheres (Figure 2b).
Most of the microspheres are stable under the working
conditions of the SEM analysis, suggesting moderate
mechanical stability. The size of the hybrid spheres is
polydispersed, which may be due to the emulsification
process. The image of hybrid hollow spheres is shown in
Figure 2c, which clearly shows the spherical structures
and the presence of silica building blocks on the surface
of the capsules. Under a higher magnification (Figure 2d),
silica hollow nanospheres are observed to be randomly
o
temperature range of 340–500 C, and a small step is also
o
observed between 500–650 C. The polymer contents is
determined to be 45 wt% on the basis of the weight loss
after full decomposition. Nitrogen adsorption isotherms
at 77 K demonstrate that the capsules are porous with an
apparent BET surface area of 112 m2/g and a pore volume
of 0.989 cm3/g. Its isotherm is distinct from that of pure
silica hollow nanospheres, reflecting the overall
contribution of the polymer component. DFT pore size
distribution analysis reveals that abundant micropores
existed in the microcapsules, with size similar with the
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important role of the semi-permeable shell structure,
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δ
(C=C)
(Ar-H)
a
b
σ
Hybrid microreactor
since FITC, as an organic molecule, tends to be adsorbed
by the inner polymer component due to interactions
between the abundant organic functionalities of the
hybrid framework particles and the aromatic dye. On the
other hand, oil soluble Nile Red with molecular size of 1.4
nm*0.6 nm was loaded into the capsules by in situ
dissolving a small amount of such fluorescent dye in the
octanol phase of interfacial active silica-stabilized
45%
Polymer
Hybrid microreactor
88%
δ
(O-H)
SiO2 nanospheres
ν
(O-H)
Polymer
σ(C-H)
ϖ
(Si-O)
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4000 3500 3000 2500 2000 1500 1000 500
-1
0
200
400
600
800
1000
Pickering
emulsions
during
synthesis.
After
o
Wavenumbers (cm )
Temperature/ C
c
d
polymerization and washing with EtOH to remove non-
encapsulated dyes, the capsules were re-dispersed in
EtOH to assess the dye release. As shown in Figure 4b,
the Nile Red is observed in the capsules and mainly
concentrated around and/or within the interior of the
capsule shell as evidenced by the observed red halo,
which is consistent with a strong interaction between the
shell and the dye, since polymers are distributed in the
inner surface of the capsules. Notably, no dye molecules
were observed in the continuous ethanol solution,
demonstrating the successful encapsulation of the
fluorescent dye molecules and the high accumulation
ability of the interior of the capsule shells to the organic
dyes. Such a result also indicates that the small
micropores of silica shell component can regulate dye
access through the porous polymer shell into the external
medium.
1600
1400
1200
1000
800
600
400
200
0
12
10
8
SiO nanospheres
2
6
SiO nanospheres
4
2
2
Hybrid microreactor
Hybrid microreactor
0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
0
10 20 30 40 50 60 70 80
Pore size/nm
Relative pressure (P/Po)
Figure 3. (a) FT-IR spectra of SiO2 hollow nanosphere,
polymer and hybrid microreactor; (b) TG of hybrid
microreactor and polymer; (c) nitrogen adsorption–
desorption isotherm and (d) pore size distributions
calculated from the adsorption branches using a nonlocal
density functional theory (NLDFT) method of the hollow
silica nanospheres and hybrid microreactors.
interfacial silica nanospheres, demonstrating that the
pores of silica nanospheres remain accessible in the
composite capsules. The mesopores corresponding to the
hollow cavity of the silica nanospheres was disappeared,
which might be attributed to the filling by polymers.
Shell Permeability and Interior Microenvironment
The permeability of the microreactor shell is critical for
their potential in size-selective applications, and here we
used fluorescent dyes to visualize the molecular uptake
and release. As an effective probe of shell permeability,
the probe molecule should ideally be inaccessible to the
silica micropores. Therefore, we firstly selected a large
water soluble dye, fluorescein isothiocyanate (see in
Figure S3, FITC, 1.0*0.9*0.7 nm3, considering the solvation
effect, the actual size of a moving molecule might be
larger than the theoretical molecular size obtained from
the calculation), to assess the reactor permeability. By
immersing the synthesized microcapsules in the dye
solution, the diffusion ability of FITC from the bulk
solution to the interior was investigated. After washing
with water several times, fluorescence confocal
microscopy image (CLSM) was recorded. As shown in
Figure 4a, CLSM image show that the interior and shell of
the capsules remain almost totally dark, suggesting that
the large dye molecule can’t diffuse through the dense-
packed microporous silica hollow nanosphere layer into
the interior of the capsule. This also confirms the
Figure 4. Shell permeability properties of hybrid
microreactors. (a) CLSM image of microreactors after
soaking in FITC solution; (b) CLSM image of microreactors
loaded with Nile Red; (c) time vs. adsorption curves of hybrid
microreactors and polymer towards aqueous solution of p-
aminophenol.
For further investigating the permeability of the porous
shell, a water soluble small molecule p-aminophenol was
also selected to assess the adsorption capacity of these
microcapsules. According to the adsorption test in Figure
4c, the organic-inorganic microporous capsules could
efficiently adsorb p-aminophenol from the aqueous
surroundings, and exhibited a faster adsorption rate than
the pure polymers, which could be attributed to the
unique polymer@silica hollow structures. The hydrophilic
silica nanoparticles at the interface confer the
hydrophilicity to the microcapsule surface (water contact
angle 30o) for its good dispersion in water, while the
interior hydrophobic polymer (water contact angle 100o)
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endows the capsules with good ability to absorb organic
To further evaluate the high activity of hybrid
2
molecules. The good dispersibility in water also shortens
the diffusion distance and benefits the adsorption.
Furthermore, the hydrophobic@hydrophilic structures
provide the unique microenvironment of the interior
catalyst. The final saturated adsorption capacity is a little
lower than the polymers, due to the lower organic group
contents. This adsorption test indicates the good
permeability of the shell to small molecules and
enrichment effects towards organic substrates in water.
microcapsule catalyst, the dependences of acetophenone
conversion on reaction time over the microcapsule
catalyst and its homogeneous counterpart are compared,
as shown in Figure 5a. Notably, the solid microcapsule
catalyst exhibited a comparable reaction rate with the
homogeneous Rh-VBSDPEN catalyst, while the polymer
catalyst showed relatively lower efficiency. After
normalization, as shown in Figure 5d, the hybrid
microreactor catalyst affords a turnover frequency (TOF)
of 625 h−1, which approaches that for the homogeneous
catalyst (724 h-1) and 1.4 times higher than that for the
polymer catalyst (449 h-1). These comparisons
demonstrate that the encapsulated polymer-based-
catalyst is as active as the homogeneous counterpart. The
high activity might be attributed to its unique
microstructures. On one hand, the catalyst features a
hydrophobic polymer core and a hydrophilic silica shell.
The hydrophilic shell would ensure good dispersion of the
solid catalyst in water and benefits the contact with water
dissolved hydrogen source HCOONa. The hydrophobic
inner would provide desired microenvironment for the
catalysis and drive the adsorption of organic reactants
from water. The kinetic barriers caused by the access of
organic substrates to catalytically active sites would be
thus alleviated. Such a phenomenon has also been
observed in other catalytic systems, such as carbon
nanotube, amphiphilic polymer, and superhydrophobic
catalyst.51,64,65 On the other hand, the nanometer-sized
shells with short, unhindered microchannels might also
favor the mass-transfer, and improve the catalytic
performance. To verify this advantage, adsorption test of
an amphipathic molecule p-aminophenol was performed
(Figure 4c). The solid microreactor could quickly enrich
the adsorbate from its aqueous solution, and has a high
adsorption capacity.
An important feature of the hybrid microreactor
catalyst is the ease of separation by simple centrifugation
while retaining its catalytic activity and enantioselectivity
after multiple cycles of use. As shown in Figure 6, the
hybrid solid microreactors could be recycled over five
consecutive reactions, while the ee values and
conversions were well maintained despite longer reaction
times. Since the textural parameters and polymer content
remain almost the same as the fresh solid catalyst (see N2
sorption and TG measurements of recycled catalyst, in
Figure S4), the decreased activity was unlikely related to
the textural parameters or loss of polymer unit. To further
reveal the deactivation reasons, ICP analysis was also
performed, which shows 6.8% of Rh loading in total was
leached. However, such a low metal leaching amount was
not enough to account for the decreased activity. The
decreased activity might be relevant to the deterioration
of active chiral catalysts caused by their decomposition
under reaction conditions.59,60,66
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The Size-Selective Asymmetric Transfer Hydrogenation
Reactions
As a type of highly active and enantioselective catalysts,
the chiral N-sulfonylated diamine-based organometallic
catalysts have been studied extensively in the aqueous
asymmetric transfer hydrogenation (ATH) of aromatic
ketones.56-60 And due to the advantages of the high
catalytic performance of the reaction system, operational
simplicity, safety, and the easy availability of reductants,
this reaction is promising for practical applications. For a
heterogeneous catalytic process, focuses are mainly
concentrated on the development of highly active solid
catalyst or cascade reaction systems, seldom reports can
be found about the size-selective catalysis.61-63 Here, with
the valuable semi-permeable microcapsule solid catalyst
in hand, we examined systemically their size-selectively
catalytic activities and enantioselectivities.
Since the molecular size of hydrogen source HCOONa
is so small that the size effects can be negligible, and the
size-selectivity could be explored with different aromatic
ketones. Here, we chose three different sized compounds,
including acetophenone, 1-acetylnaphthalene and 3, 5-
dibenzyloxyacetophenone, as substrates to assess the
catalytic performance of semi-permeable organic-
inorganic hybrid microreactors and their size-selectivity.
For comparison, the pure polymer catalyst without shell
structure and homogeneous Rh-VBSDPEN were also
evaluated. The reactions were carried out at 40 oC in
water, and monitored by GC or HPLC with a chiral
column. As shown in Table 1 (entry 1), for the small
substrate acetophenone, the solid microcapsule catalyst
could smoothly catalyze the reaction with a 98%
conversion and 95% ee within 1.5 h, which is almost the
same as its corresponding homogeneous Rh-VBSDPEN
catalyst (entry 2). This indicates that the activity of
complex catalyst is maintained after being incorporated
into the hybrid microcapsules and also suggests that
acetophenone as small reactant can readily diffuse
through the framework micropores of the outer shell.
However, under the identical reaction conditions, the
polymer catalyst gives only 86% conversion and 94% ee
using cetyltrimethylammonium bromide (CTAB) a phase
transfer reagent during the reaction (entry 3). The lower
activity might be related to its nonporous structure and
hydrophobic surface properties, which are not beneficial
for the exposure of active site and for mass transport in
aqueous solution.
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Figure 5. Kinetic plots of asymmetric transfer hydrogenation of acetophenone (a), 1-acetylnaphthalene (b) and 3, 5-
dibenzyloxyacetophenone (c) catalyzed by different catalysts (homogeneous catalyst, black; microcapsule solid catalyst, red;
polymer catalyst, blue); turn over frequencies (TOFs) for different catalysts with different substrates, acetophenone (d), 1-
acetylnaphthalene (e) and 3, 5-dibenzyloxyacetophenone (f); time vs adsorption curves of hybrid microcapsules and polymer
towards aqueous solution of different substrates, acetophenone (g), 1-acetylnaphthalene (h) and 3, 5-dibenzyloxyacetophenone
(i), the total dosages of acetophenone, 1-acetylnaphthalene and 3, 5-dibenzyloxyacetophenone were 60 mg, 42 mg and 42 mg,
respectively and the absorbents were 50 mg (see SI, experimental section).
ATH of 1-acetylnaphthalene over the homogeneous Rh-
100
80
60
40
20
0
VBSDPEN was similar to that observed for acetophenone,
reaching a full conversion after 1.5 h (Figure 5b). In this
case, however, much lower activity was found over the
hybrid microcapsule catalyst and only 51% of conversion
could be obtained, which is similar to the polymer
catalyst (TOF 146 vs 176 h-1, Figure 5e). Interestingly, when
a larger compound 3, 5-dibenzyloxyacetophenone was
utilized as the substrate, much lower conversion was
achieved with the hybrid microcapsule catalyst (about
17%, Figure 5c). This activity is even lower than that of the
pure polymer catalyst, with which 43% of final conversion
could be obtained within the same reaction time. After
normalization, as shown in Figure 5f, the hybrid
microcapsule catalyst only affords a TOF of only 11 h−1. In
comparison, 561 h−1 and 65 h−1 of TOF values could be
obtained with the homogeneous Rh-VBSDPEN and pure
polymer catalysts. So, the activity of these catalysts is in
1
2
3
4
5
Recycling times
Figure 6. Recyclability of the hybrid microreactor catalyst in
the asymmetric transfer hydrogenation of acetophenone (the
reaction time for cycles 1, 2, 3, 4 and 5 is 1.5 h, 3 h, 8 h, 12 h,
and 16 h, respectively).
the order of Rh-VBSDPEN
microreactor catalyst. However, the trend of conversions
>
polymer
>
hybrid
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and TOF over these catalysts is very different from the
the most common and mature immobilization
2
3
small substrate (acetophenone) and moderate sized 1-
acetylnaphthalene. These results suggest a size-selective
catalytic process, which might be attributed to the
variation in molecular size of the substrates and the semi-
permeability of the microporous shell to regulate their
access. To solidify this conclusion, the molecular sizes of
these investigated molecules were calculated. Based on
the optimized geometries, the sizes of acetophenone, 1-
acetylnaphthalene and 3, 5-dibenzyloxyacetophenone
were estimated to be 0.68 nm*0.499 nm, 0.81*0.68 nm
and 1.6 nm*1.4 nm (see in Figure S3), respectively.
Apparently, 3, 5-dibenzyloxyacetophenone is much larger
than the former substrates. The size of acetophenone is
smaller than the aperture size of silica shell, and these
molecules could pass through the micropores of the
dense packed silica outer layer to access the microreactor
interior. As the molecular size increases, the diffusion
resistance exacerbates, resulting in decreased catalytic
activity with 1-acetylnaphthalene. For the larger molecule
3, 5-dibenzyloxyacetophenone, the size is nearly equal to
the pore size, so it is expected to be excluded by the
micropores of the reactor shell, resulting significantly
increased diffusion barrier stemming from the potential
blockage of pores. It is likely that the large substrate pass
through the defects within the outer layer, which is
responsible for the observed albeit very low activity. To
further reveal the size-selective-ability, the catalytic
performance of the microcapsule catalyst in asymmetric
transfer hydrogenation of a mixture of acetophenone and
3, 5-dibenzyloxyacetophenone was also examined. As
shown in Figure S5, under the same reaction conditions,
small substrate acetophenone could be smoothly
converted to desired chiral aromatic alcohol with a
conversion as high as 96%, while only 8.3% of conversion
was obtained for 3, 5-dibenzyloxyacetophenone, further
confirming the size-selectivity of the microcapsule
reactors.
strategies.67
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7
8
9
Table 1 Asymmetric transfer hydrogenation of ketones
catalyzed by microreactor catalyst a
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Entry
Reaction
time (h)
Substrates
Conv.
(%)b
Ee (%)b
1
2c
3d
4
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
1.5
5
H
H
98
>99
86
95
95
94
94
94
94
93
95
94
94
91
H
4-F
>99
>99
>99
98
5
4-Cl
4-Br
4-Me
4-OMe
3-F
6
7
8
79
9
>99
>99
90
10
11
12
13
3-Br
3-Me
4-i-Bu
4-i-Bu
39
95
95
95
a Reaction conditions: solid catalyst (2.5 µmol Rh), HCOONa
(0.17 g, 2.5 mmol), ketones (0.5 mmol) and 2.0 ml H2O,
b
reaction temperature (313 K).
Analysis by GC using a
c
Agilent-113-2532 Cyclodex-B chiral column. Homogeneous
Rh-VBSDPEN as catalyst. Pure polymer catalyst with CTAB
d
as phase transfer reagent.
Encouraged by the superior performance of hybrid
microcapsule catalyst, other ketones have been
intensively investigated (Table 1). The results show that
various ketones are efficiently converted into chiral
alcohols with high ee values (91-95%) using HCOONa as
the reductant and water as the solvent. However, the
reaction rates for different substrates vary considerably.
For example, with electron-withdrawing substituent
groups, including -F, -Cl, -Br, ketones can be converted
completely within 1.5 h, but for electron-rich aromatic
ketones with -CH3 or -OCH3 substituent groups, the
activities are a little lower, and only 98 % and 79 % of
conversions could be obtained within the same reaction
time (entries 4-8). The position of the substituted groups
also has significant influence on the reaction efficiency.
With meta-substituted -F or -Br, ketones can be
converted completely similarly (entries 9, 10). However,
with meta-substituted -CH3, only 90% of conversion
could be got (entry 11). The decreased activity might also
be related with its spatial size. To clarify this, a larger
substituted group, i-Bu, was also investigated. Under
identical reaction conditions, only 39% of conversion was
obtained, much lower than other substrates (entry 12).
Prolonging the reaction time to 5 h, a much higher
The size-selective effects of the microreactors were also
further confirmed by the adsorption tests. Different sized
substrates were examined, as shown in Figure 5g and 5h,
the microcapsule catalyst has a strong adsorption capacity
(calculated to be 368 and 293 mg/g) for small substrate
acetophenone and 1-acetylnaphthalene, while a small
quantity
of
the
large
substrate
3,
5-
dibenzyloxyacetophenone was adsorbed (Figure 5i, about
55 mg/g). This result further demonstrates the increased
blockage barrier of microreactors towards larger molecule.
Compared with the hybrid microcapsule material, the
polymer exhibited higher and faster adsorption rate due
to its good wettability to organics. However, due to the
hydrophobic properties, water soluble hydrogen source
might be not easy to access the polymer framework.
Although size-selective catalysis has been reported with
MOFs or zeolites structures, this work provides an
alternative strategy for the formation of size-selective
solid asymmetric catalysts, since polymerization is one of
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1. Wang, Z. J.; Clary, K. N.; Bergman, R. G.; Raymond, K. N.;
conversion, about 95%, could be got (entry 13). These
results further demonstrate the efficiency of our solid
catalyst and its size-selective properties.
Toste, F. D. Nat. Chem. 2013, 5, 100−103.
2. Marguet, M.; Bonduelle, C.; Lecommandoux, S. Chem. Soc.
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CONCLUSION
4. Marguet, M.; Edembe, L.; Lecommandoux S. Angew. Chem.
Int. Ed. 2012, 51, 1173−1176.
In summary, we have developed an organic-inorganic
hybrid microreactor possessing unique semi-permeable
9
5. Chi, Y. G.; Scroggins, S. T.; Frechet, J. M. J. J. Am. Chem. Soc.
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and
hydrophobic@hydrophilic
properties
with
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encapsulated asymmetric homogeneous Noyori–Ikariya
catalyst. The developed solid catalyst exhibited excellent
catalytic performance (TOF 625 h-1, 95% ee) in the
asymmetric transfer hydrogenation of small aromatic
ketones, demonstrating that the immobilization process
did not compromise their catalytic performance, which
might be attributed to the synergy of appropriate semi-
permeable pore size and finely engineered hydrophobic
microenvironment around the catalyst centers. Size-
selective permeability of the capsule shell allows the
creation of size-selective reactors. Substrates with large
sizes did not show noticeable conversion, because of the
significantly increased diffusion barrier stemming from
the potential blockage of micropores. This method might
be readily adaptable to a broad range of other catalysts
and is appealing for the development of novel functional
microscale ensembles that mimick bio-microreactors.
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ASSOCIATED CONTENT
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J.; Monteiro, M. J.; Liu, J. NPG Asia Mater. 2016, 8, e240.
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AUTHOR INFORMATION
Corresponding Author
*
hqyang@sxu.edu.cn
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final
version of the manuscript. #These authors contributed
equally.
21. Cui, T. L.; Ke, W. Y.; Zhang, W. B.; Wang, H. H.; Li, X. H.;
Chen, J. S. Angew. Chem. Int. Ed. 2016, 55, 9178−9182.
22. Laursen, A. B.; Hojholt, K. T.; Lundegaard, L. F.; Simonsen,
S. B.; Helveg, S.; Schuth, F.; Paul, M.; Grunwaldt, J. D.; Kegnoes,
S.; Christensen, C. H.; Egeblad, K. Angew. Chem. Int. Ed. 2010,
49, 3504−3507.
Supporting Information
Experimental section; Appearance of the Pickering emulsion
and its optical micrograph; SEM image; The molecular
structure and size of fluorescent probes and reactant
molecules; The N2 sorption and TG measurements of
recycled catalyst; The GC and HPLC traces for various
substrates. This material is available free of charge via the
Internet at http://pubs.acs.org.
23. Zhang, W. N.; Lu, G.; Cui, C. L.; Liu, Y. Y.; Li, S. Z.; Yan, W.
J.; Xing, C.; Chi, Y. R.; Yang, Y. H.; Huo, F. W. Adv. Mater. 2014,
26, 4056−4060.
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Li, Y. W. Angew. Chem. Int. Ed. 2016, 55, 5019−5023.
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ACKNOWLEDGMENT
This work is supported by the Natural Science Foundation
of China (21603128), the Natural Science Foundation for
Young Scientists of Shanxi Province (2016021034), the
Scientific Research Start-up Funds of Shanxi University
(rsc723).
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