Tuning Biphasic Catalysis Reaction with a Pickering Emulsion Strategy Exemplified by Selective Hydrogenation of Benzene

Article abstract of DOI:10.1002/cctc.201801155

Although organic/aqueous biphasic catalysis reactions are widely used, it is difficult to effectively control their reaction process. Here we develop a solid particles-stabilized emulsion (Pickering emulsion) strategy to address this limitation. A challenging biphasic reaction, selective hydrogenation of benzene to cyclohexene, was investigated as a case. The developed Pickering emulsion system exhibited higher catalysis efficiency and selectivity than the conventional biphasic reaction system even under mild stirring. The cyclohexene selectivity and yield in the Pickering emulsion system reached as high as 51.2 % and 43.3 %, respectively. More importantly, the findings presented here demonstrate the feasibility that the reaction rate and selectivity of challenging biphasic reactions can be regulated by the rational adjustment of Pickering emulsion parameters including emulsion droplet size, droplet distance as well as emulsion type, which is otherwise unattainable for conventional biphasic reaction systems.

Full text of DOI:10.1002/cctc.201801155

Accepted Article
Title: Tuning Biphasic Catalysis Reaction with a Pickering Emulsion
Strategy Exemplified by Selective Hydrogenation of Benzene
Authors: Yabin Zhang, Ming Zhang, and Hengquan Yang
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Tuning Biphasic Catalysis Reaction with a Pickering Emulsion
Strategy Exemplified by Selective Hydrogenation of Benzene
Yabin Zhang,[a] Ming Zhang,[a] and Hengquan Yang*[a]
Abstract: Although organic/aqueous biphasic catalysis reactions are
widely used, it is difficult to effectively control their reaction process.
Here we develop a solid particles-stabilized emulsion (Pickering
emulsion) strategy to address this limitation. A challenging biphasic
reaction, selective hydrogenation of benzene to cyclohexene, was
investigated as a case. The developed Pickering emulsion system
exhibited higher catalysis efficiency and selectivity than the
conventional biphasic reaction system even under mild stirring. The
cyclohexene selectivity and yield in the Pickering emulsion system
reached as high as 51.2% and 43.3%, respectively. More importantly,
the findings presented here demonstrate the feasibility that the
reaction rate and selectivity of challenging biphasic reactions can be
regulated by the rational adjustment of Pickering emulsion parameters
including emulsion droplet size, droplet distance as well as emulsion
type, which is otherwise unattainable for conventional biphasic
reaction systems.
Pickering emulsions that are stabilized by solid particles are
emerging as an innovative platform for designing efficient
biphasic catalysis systems.[9] In comparison to conventional
surfactant-stabilized emulsions, Pickering emulsions are more
stable against coalescence even at high temperatures due to the
high adsorption energy of particles at oil/water interfaces.[10]
Moreover, Pickering emulsions allow their droplet sizes to tune.[11]
For example, our group had prepared Pickering emulsions with
droplet sizes ranging from 40 to 1000 μm, realizing careful
investigation of the relationship between catalytic efficiency and
droplet size.[12] Besides their tunable droplet sizes, it is
conceivable that in the Pickering emulsion systems the average
distance between two adjacent droplets can be tuned since the
droplets have high stability. This is also crucial for those multistep
biphasic catalysis reactions aforementioned. These merits and
envisioning prompt us to use Pickering emulsion systems to
address the limitations associated with challenging biphasic
catalysis reactions.
Selective hydrogenation of benzene to cyclohexene has been
proven as an important route for producing cyclohexene, an
important intermediate chemical for producing cyclohexanol,
caprolactam and adipic acid.[13] Compared to other routes, this
method has many advantages such as inexpensive feedstock,
atomic economy and energy saving.[14] However, the selective
Introduction
Organic/aqueous biphasic catalytic system is an important
platform for many chemical transformations such as synthesis of
fine chemicals, biocatalysis and biomass refining.[1] Despite
extensive applications, biphasic systems often suffer from low
reaction efficiency due to their high mass transfer resistance.[2] To
improve their mass transfer rate, vigorous mechanical agitation
has to be employed.[3] By doing so, one of the liquids is
homogenized into liquid droplets, which are dispersed in the other
liquid under incessant high-speed agitation.[4] However, the liquid
droplets generated by mechanical agitation are often large in size,
and at the same time the droplets are very unstable and prone to
coalescence, driven by the high interfacial tension.[5] For example,
despite of the agitation speed, the average droplet sizes no longer
decreased after they were downsized to ca. 270 μm.[6] Moreover,
the droplet size, the average distance between droplets and the
homogeneity of the biphasic reaction systems are still difficult to
control precisely through mechanical agitation.[7] Consequently,
the reaction outcomes such as conversion, selectivity and yield
are unlikely to control at a desired level. This is especially true for
those multistep biphasic reactions, of which the conversion,
selectivity and yield are sensitive to the mass transport.[8] In this
context, it is urgent to explore new efficient methods that enable
the biphasic catalysis reaction to take place at a controllable level.
Scheme 1. (a) Schematic description of an O/W Pickering emulsion strategy for
selective hydrogenation of benzene; (b) microscopic presentation for mass
transfer during the selective hydrogenation of benzene in the O/W Pickering
emulsion reaction system. d is the droplet diameter; a is the average distance
between the droplet interface and the catalyst particle; D is the distance
between droplets.
[a]
Y. Zhang, M. Zhang, Prof. H. Yang
School of Chemistry and Chemical Engineering
Shanxi University, Wucheng Road 92
Taiyuan (PR China)
E-mail: hqyang@sxu.edu.cn
Supporting information for this article is given via a link at the end of
the document.
1
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hydrogenation of benzene to cyclohexene is thermodynamically
unfavorable since its Gibbs free energy change is −23 kJ mol−1,
which is much higher than that for the complete hydrogenation of
available TiO2 nanoparticles (NPs) are a widely used catalyst
support due to the high stability and low-cost.[18] However, the
commercial TiO2 nanoparticles are too hydrophilic to stabilize
benzene to cyclohexane (−98 kJ mol−1).[15] Over past twenty years, Pickering emulsions. To render them hydrophobicity, we used two
it has been demonstrated that this challenging limitation can be
overcome by exploiting the kinetics of organic/aqueous biphasic
reaction.[16] The selectivity toward cyclohexene in the biphasic
system can be improved significantly. The reason is that water
can promote rapid departure of cyclohexene from water to organic
phase due to its low solubility in water, thereby avoiding further
hydrogenation to cyclohexane.[17] We envision that if the key
parameters of the biphasic selective hydrogenation of benzene
such as the droplet size and the droplet distances can be tuned,
the cyclohexene selectivity and yield would be improved further.
Herein, we use Pickering emulsion strategy to perform the
biphasic reaction at a controllable level, which is for the first time
exemplified by selective hydrogenation of benzene to
cyclohexene, as shown in Scheme 1a. The impacts of the
emulsion type, droplet size and the average distance between two
adjacent droplets on the cyclohexene selectivity and yield were
investigated in detail. The cyclohexene yield in the O/W Pickering
emulsion system was shown to have a significant increase in
comparison to its conventional biphasic counterpart system. The
results presented here well demonstrate that the selectivity and
yield of multistep biphasic catalysis reactions can be tuned
through regulating Pickering emulsion droplet size, distance
between droplets as well as Pickering emulsion types.
doses of (MeO)3SiCH3 to modify the surface of TiO2: 0.4 mmol/g
and 4 mmol/g (with respect to TiO2). The latter modified TiO2 was
expected to be more hydrophobic than the former because of high
loading of methyl group. The morphology of TiO2 NPs and the
modified TiO2 NPs was observed with scanning electron
microscopy (SEM) and transmission electron microscopy (TEM).
As shown in Figure 1, TiO2 NPs show approximately a cubic
morphology with a mean diameter of 21 nm although they are
somewhat aggregated. After modification, TiO2 NPs still retain
their original morphology without severe aggregation. The
modified TiO2 was characterized with FT-IR spectroscopy. From
the FT-IR spectrum in Figure 2a, peaks at 2850−2950 cm−1 and
1485 cm−1, corresponding to C-H stretching vibrations and C−H
scissoring vibrations, are clearly observed. This is an indication
that methyl groups were successfully grafted on the surface of
TiO2 NPs. The intensity of these two characteristic peaks of TiO2-
C(4) is higher than that of TiO2-C(0.4), which accords with the
experimental dose. TG measurement was performed to further
examine the loading of methyl group and the thermal stability of
the emulsifiers. As shown in Figure 2b, TG curves exhibit that
three stages of weight loss. The first stage was observed in the
range of 50−150 oC, which was associated to the loss of
physically adsorbed water. The second stage of weight loss
occurred in the range of 150−400 oC, which was mainly related to
the decomposition of methyl groups. The third stage from 400 to
o
700 C was mainly caused by the condensation dehydration of
Results and Discussion
hydroxyl groups. According to the TG results, the methyl loadings
are estimated to be about 0.22 mmol/g (1.13% weight loss) for
TiO2-C(0.4) and 0.33 mmol/g (1.99% weight loss) for TiO2-C(4),
respectively. These loadings are broadly consistent with the
results determined by elemental analysis (Figure 2c). To examine
the surface wettability of the modified TiO2 NPs, we measured
their water contact angles. As shown in Figure 2d, the water
contact angle for the unmodified TiO2 is only 30.3o. The contact
angle for TiO2-C(0.4) increases to 89.2o, confirming its partially
hydrophobic. In contrast, the contact angle for TiO2-C(4) reaches
Preparation and characterization of interface-active particles.
In order to transform a conventional benzene/water biphasic
system to a Pickering emulsion system, we first need to prepare
particles with proper wettability, which are used as emulsifiers to
stabilize an O/W or W/O Pickering emulsion. Commercially
Figure 1. (a) SEM image of unmodified TiO2; (b) SEM image of methyl-modified
TiO2 with a methyl loading of 0.4 mmol g-1; (c) TEM image of unmodified TiO2;
(d) TEM image of methyl-modified TiO2 with a methyl loading of 0.4 mmol g-1.
Figure 2. (a) FT-IR spectra of TiO2-C(0.4) and TiO2-C(4); (b) TGA curves of
TiO2-C(0.4) and TiO2-C(4); (c) results of elemental analysis of TiO2-C(0.4) and
TiO2-C(4); (d) water contact angles of the unmodified TiO2, TiO2-C(0.4) and
TiO2-C(4).
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as high as 107.8o, being hydrophobic. Such a change in the
surface wettability is also in agreement with the changes in the
loading of hydrophobic methyl group.
Preparing Pickering emulsions for selective hydrogenation
of benzene
A Ru/TiO2 catalyst for the selective hydrogenation of benzene was
prepared by loading Ru nanoparticles on the above unmodified
TiO2 via an impregnation-reduction process.[19] The TEM images
in Figure S1 clearly show that Ru nanoparticles are uniformly
dispersed on the surface of TiO2. Their average size is estimated
to be about 3.3 nm. The water contact angle of the Ru/TiO2
catalyst was measured to be 33.5o, suggesting its surface is
highly hydrophilic (Figure S2). After vigorously stirring a mixture
of benzene, water containing ZnSO4·7H2O (3 g, as inorganic
additive)[19] and the Ru/TiO2 catalyst (0.15 g) in the presence or
absence of the emulsifier TiO2-C(0.4) or TiO2-C(4), different
phenomena were observed. For the case of the absence of the
Figure 4. (a) Kinetic profiles of the selective hydrogenation of benzene over
Ru/TiO2 in a conventional biphasic system; (b) kinetic profiles of the selective
hydrogenation of benzene over Ru/TiO2 in the W/O Pickering emulsion system;
(c) kinetic profiles of the selective hydrogenation of benzene over Ru/TiO2 in the
O/W Pickering emulsion system; (d) reaction results obtained at the maximum
yield of cyclohexene, calculated based on the reaction within the first 60 min for
the conventional biphasic system and the O/W Pickering emulsion system, the
first 50 min for the W/O Pickering emulsion system. Reaction conditions: 0.15 g
catalyst, 15 mL benzene, 30 mL H2O, 3 g ZnSO4·7H2O, 5.0 MPa H2, 140 oC,
1000 rpm for the conventional biphasic reaction system and 700 rpm for the
Pickering emulsion reactions, 1wt% emulsifier (with respect to the dispersed
phase) for Pickering emulsion systems.
emulsifier TiO2-C(0.4) or TiO2-C(4), this system is still
a
conventional biphasic suspension since no droplets were
observed with optical microscopy (Figure S3). The Ru/TiO2
catalyst particles were distinctly dispersed in the bottom layer
(water phase), due to their high hydrophilicity. In the case of the
presence of the emulsifier TiO2-C(0.4), droplets with mean size of
60 μm appeared (Figure 3a). This emulsion is confirmed to be an
oil-in-water Pickering emulsion because the fluorescence
microscopy reveals the distribution of the oil-soluble Nile red in oil
(benzene) droplet (Figure 3c). Different from the case of TiO2-
C(0.4), the TiO2-C(4)-stabilized Pickering emulsion belongs to a
water-in-oil type with droplets size of 56 μm (Figure 3b), which is
also supported by fluorescence microscopy image, in which
water-soluble FITC-dextran is distributed within the droplets
(Figure 3d). The dependence of emulsion type on the emulsifier
agrees with the general principle that partially hydrophobic
particles tend to stabilize oil-in-water Pickering emulsions, while
hydrophobic particles is favorable to stabilize water-in-oil
Pickering emulsions.
Impact of reaction system
Next, we investigated the selective hydrogenation of benzene in
the above three systems. The reactions were conducted at 140
oC under 5 MPa H2. Their reaction kinetics were monitored with
GC, as shown in Figure 4a-c. In these three systems, with time
the benzene concentration gradually decreased, the cyclohexane
concentration increased, and the cyclohexene concentration first
increased and then shrunk. This tendency indicates that
cyclohexene is the intermediate product of this two-step reaction,
which can be further hydrogenated to cyclohexane. However,
these three systems differ in reaction rate. For the conventional
biphasic system, high-speed agitation (at 1000 rpm) had to be
implemented to obtain a satisfactory reaction efficiency.
Cyclohexene in 37% yield at the benzene conversion of 83.6%
was given within 60 min (Figure 4d). For the Pickering emulsions,
we found it unnecessary to employ such a high-speed agitation
since they already have sufficiently high reaction interface areas.
Accordingly, a rate of 700 rpm was implemented. In the W/O
Pickering emulsion system, the maximum yield of cyclohexene
was 38% at the benzene conversion of 83% within 50 min (Figure
4d). Although the W/O Pickering emulsion system is almost equal
to the conventional biphasic system in terms of the selectivity and
conversion, the W/O Pickering emulsion system even at lower
stirring rate proceeded faster when considering the reaction time.
Figure 3. (a) Optical microscopy image of the O/W Pickering emulsions
stabilized by TiO2-C(0.4), inset showing its appearance; (b) optical microscopy
image of the W/O Pickering emulsion stabilized by TiO2-C(4), inset showing its
appearance; (c) fluorescence confocal microscopy image of the O/W Pickering
emulsion with the oil phase dyed by oil-soluble Nile Red; (d) fluorescence
confocal microscopy image of the W/O Pickering emulsion with the water phase
dyed by water-soluble FITC-dextran. The Pickering emulsion comprises of 0.15
g catalyst, 15 mL benzene, 30 mL H2O, 3 g ZnSO4·7H2O and 1 wt% emulsifier
(with respect to dispersed phase).
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Figure 5. (a) Benzene conversion, cyclohexene selectivity and cyclohexene yield versus average droplet sizes in O/W Pickering emulsions. Pickering emulsions
were formulated with different stirring rates (before reaction): 1000 rpm (d=270 μm), 3000 rpm (d=176 μm), 10000 rpm (d=100 μm) and 20000 rpm (d=60 μm).
Raction conditions: 0.15 g catalyst, 15 mL benzene, 30 mL H2O, 3 g ZnSO4·7H2O, 5.0 MPa H2, 140 oC and 700 rpm. (b) Benzene conversion, cyclohexene selectivity
and cyclohexene yield versus average distances between two adjacent droplets in the O/W Pickering emulsion (homogenization at 20,000 rpm, 2 min). Reaction
conditions: 0.15 g catalyst, 15 mL benzene, 5.0 MPa H2, 140 oC, 700 rpm, a certain amount of ZnSO4·7H2O (1.5 g, D=7 μm; 3 g, D=18 μm; 4.5 g, D=44 μm).
This may be explained by the fact that the use of benzene as
continuous phase is favorable to increase the solubility of H2 in
the reaction system.[20] The O/W Pickering emulsion system
exhibited reaction rate slower than the W/O system because the
continuous phase (water) has low solubility toward H2, but slightly
faster than the conventional biphasic system. Notably, the O/W
Pickering emulsion system gave a cyclohexene yield of 43.3% at
a benzene conversion of 84.6% within 60 min (Figure 4d). These
results clearly demonstrate that the formulated O/W Pickering
emulsion system outperforms two others in terms of the
cyclohexene selectivity and yield even under low-speed agitation.
It is worth noting that at the end of reaction, the droplets were still
clearly observed (Figure S4). This is just a feature of Pickering
emulsions, namely higher stability even at high temperatures due
to the extremely high adsorption of particles at the oil/water
interface, which is inaccessible for molecular surfactant-stabilized
emulsions. Notably, the Pickering emulsion in which the Ru/TiO2
particles were positioned just at the interfaces led to a low
cyclohexene yield because the particle partially protrude in the oil
phase, having no stagnant water layer (Figure S5).[17b]
downsized remarkably. Notably, although the droplet sizes in the
above Pickering emulsion systems were found to undergo an
increase after the reaction (414, 278, 189 and 135 μm,
respectively), the order for the droplet size was not altered (Figure
S7). The growth of droplets can be explained by the possibility
that high temperatures accelerate Ostwald ripening and the
polarity of oil phase is changed as products generated.[22] These
Pickering emulsion systems with different droplet sizes led to
different reaction outcomes, as presented in Figure 5a. When the
average droplet size was changed from 270 to 176 and 100 μm,
the corresponding benzene conversion increased from 69.1% to
76.4 and 80.6%, the cyclohexene selectivity increased from 42.9%
to 43.5% and 48.7% and its yield increased from 29.6% to 33.2%
and 39.2%. Impressively, when the droplets were further
downsized to 60 μm, the benzene conversion further increased
up to 84.6%, and the cyclohexene selectivity and yield increased
up to 51.2% and 43.3%, respectively. These results highlight the
impact of the droplet size on the conversion, selectivity and yield
of the selective hydrogenation of benzene (Figure 5a).
For the O/W Pickering emulsion system, the catalysis reaction
takes place in the continuous phase (outside the droplets) since
the Ru/TiO2 catalyst particles are distributed in water as discussed
above. The oil−water interfacial area significantly increases with
decreasing droplets size.[12] In parallel, as the droplet size reduces,
the numbers of the droplets increase. Consequently, the average
distance (D, Scheme 1b) between two adjacent droplets also
declines from 81 to 53, 30 and 18 μm (calculation is provided in
Supporting Information). The two factors all facilitate the benzene
diffusion to the catalyst surface through the aqueous phase,
resulting in the reaction acceleration and the increase in
conversion. At the same time, the presence of excessive benzene
molecules on the catalyst surface benefits desorption of
cyclohexene molecules from the catalyst surface due to the
competitive adsorption.[17c] As a result, the further hydrogenation
Impact of emulsion droplet size
To clarify the role of Pickering emulsions, we firstly examined the
O/W Pickering emulsion systems with different emulsion droplet
sizes under the same conditions. The adjustment of emulsion
droplet size was achieved by varying the stirring rate during
emulsification since it is well established that the emulsion
droplets size decreases as the shear rate increases in a certain
range.[21] When the stirring rate was varied from 1000 to 3000,
10000 and 20000 rpm, the average size of the resultant droplets
decreased from 270 to 176, 100 and 60 μm (Figure S6). Further
increasing the stirring rate, the emulsion droplets size no longer
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of cyclohexene into cyclohexane is hindered, and high selectivity
and yield of cyclohexene is thus obtained.
Impact of the droplet distance
We further investigated the influence of the average distance
between two adjacent droplets (D) on the reaction outcome. The
adjustment of droplet distance (D) was achieved by varying water
volume while fixing the amount of benzene. As shown in Figure
5b, with varying the water volume from 15 to 30 and 45 mL, the
corresponding mean distance (D) increases gradually from 7 to
18 and 44 μm (calculation is provided in Supporting Information).
It should be noted that the increase in the volume of water also
causes increase of droplet size from 50 to 60 and 100 μm (Figure
S8). This is understandable because the emulsion droplet size is
related to the volume of biphasic system when homogenized with
the same stirring rate.[23] As the water amount increases, the
benzene conversion decreases from 86.2% to 84.6% and then to
75.7%, and the cyclohexene selectivity increases from 42.3% to
51.2% and 53.4%, as shown in Figure 5b.
The changes in the benzene conversion and cyclohexene
selectivity with the droplet distance (D) can be explained as
follows. The average distance (a, Scheme 1b) between the oil-
water interface and the catalyst particle increases as the droplet
distance (D) increases. This means the transport distance of
benzene from the oil droplet to the catalyst surface is increased.
At the same time, with the increase in the droplet distance (as a
result of increasing water volume), the H2 transport from the
gaseous phase to the catalyst surface is slowed down.[24]
Therefore, the benzene conversion diminishes upon increasing
the droplet distance (D). For cyclohexene, with increasing the
droplet distance (water volume), the possibility of cyclohexene re-
adsorbing back to the catalyst surface is reduced, avoiding its
further hydrogenation. In parallel, the lowered H2 transport rate
also is beneficial for preventing further hydrogenation of
cyclohexene.[17b] These factors work together, resulting in the high
cyclohexene selectivity. Taking into account simultaneously the
benzene conversion and cyclohexene selectivity with the droplet
distance (D), it is easy to understand the change of the
cyclohexene yield. These findings suggest that there is an optimal
droplet distance for obtaining a maximum cyclohexene.
Figure 6. (a) Recycling results of the selective hydrogenation of benzene in
O/W Pickering emulsion system; (b) optical microscopy images of O/W
Pickering emulsion in the first reaction cycle, the fifth reaction cycle and the sixth
reaction cycle. Reaction condintions are the same as those in Figure 4 except
that 0.1 g fresh emulsifier was added in the sixth cycle.
conversion of benzene decreased from 51.5% to 50.6%, and the
cyclohexene selectivity increased from 50.5% to 54.1%).[25] This
can be explained by the fact that the coalescence of Pickering
emulsion droplets is aggravated with elevating reaction
temperature, evidenced by the observation that the droplets size
increases from 108 to 116, 135, 176 and 206 μm after reaction
(Figure S9), while in the conventional biphasic system the
droplets size largely relies on the agitation rate.
Reusability
The reusability of the catalyst and solid emulsifier in the Pickering
emulsion system is evaluated. At the end of the first reaction, the
Pickering emulsion reaction system was demulsified through
centrifugation, leading to phase separation. The upper layer of
organics was isolated through decantation. The catalyst together
with the emulsifier at the bottom layer were collected, and washed
thoroughly deionized water. After being dried, the mixture of
catalyst and emulsifier was directly used to the next reaction cycle.
In the second reaction cycle, a Pickering emulsion with relatively
uniform droplets could still be formulated, delivering the
advantages of the solid emulsifier that is recyclable. As presented
in Figure 6a, in the second reaction cycle, a cyclohexene yield of
42.3% with 52.4% selectivity was given. From the third to firth
reaction cycles, the selectivity and yield for cyclohexene gradually
declined slightly. For the fifth cycle, the cyclohexene yield and
benzene conversion were reduced to 34.0% and 64.2%,
respectively. We believe that these decreases in the cyclohexene
yield and selectivity are caused by the loss of catalyst and the
emulsifier due to the multiple transferring and separation.
Emulsion droplets in the fifth cycle were observed to become
larger due to the loss of emulsifier, in comparison to the first
reaction. Therefore, in the sixth reaction, 0.1 g of fresh emulsifier
was added as a supplement. As expected, the droplet size return
to being as small as that in the first reaction cycle (Figure 6b). The
Impact of reaction temperature
Table S1 shows the impact of reaction temperature on selective
hydrogenation of benzene in the O/W Pickering emulsion system.
When the temperature was varied from 120 to 130, 140, 150 and
160 oC, the conversion of benzene decreased from 89.0% to
85.9%, 84.6%, 74.4% and 70.2%, and the cyclohexene selectivity
increased from 44.6% to 47.0%, 51.2%, 53.4% and 54.1%. The
decrease in reaction rate is attributed to the decreased solubility
of H2 in water. The cyclohexene selectivity is improved upon
increasing the reaction temperature, which is explained by the
possibility that the high temperature promotes cyclohexene
desorption from the catalyst surface. Notably, in comparison to
the conventional biphasic system, the benzene conversion and
cyclohexene selectivity in the Pickering emulsion are more
sensitive to the reaction temperature. (The conventional biphasic
system, when the temperature was varied from 120 to 160 oC, the
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cyclohexene yield and selectivity came back to 38.4% and 54.8%,
respectively. After the six reaction cycles, the total TON for Ru
reaches as high as ca. 5000 mol mol-1, which is higher than that
in the reported conventional biphasic counterpart system (1493
mol mol-1).[19]
Young Sanjin Scholar and Doctoral education innovation project
in Shanxi Province (2017-8).
Keywords: biphasic reactions• Pickering emulsions • droplet
size • selective hydrogenation
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Conclusions
In summary, we have successfully demonstrated that the key
parameters of biphasic catalysis reactions including the droplet
size and droplet distance can be tuned with the Pickering
emulsion strategy. Such a tuning is curial for the mass transport-
sensitive, multistep catalysis reactions such as selective
hydrogenation of benzene. As the case study shows, the
Pickering emulsion system gave much higher cyclohexene
selectivity (51.2%) and yield (43.3%) than the conventional
biphasic system even under lower-speeded stirring. The catalyst
together with the solid emulsifier could be recycled effectively.
After six reaction cycles, the selectivity and yield of cyclohexene
were still as high as 54.8% and 38.4%, respectively. Impressively,
this strategy and the case study presented here will help to
understand the behavior of biphasic catalysis and push biphasic
reactions toward a more efficient and controllable level because
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Acknowledgements
This work is supported by the Natural Science Foundation of
China (U1510105, 21733009 and 201573136), the Foundation for
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Entry for the Table of Contents (Please choose one layout)
Layout 1:
FULL PAPER
An O/W Pickering emulsion system is
Y. B. Zhang, M. Zhang, and H. Q. Yang*
developed
hydrogenation
for
the
selective
to
Page No. – Page No.
of benzene
cyclohexene. Using this system, the
reaction rate and selectivity of
challenging biphasic reactions can be
regulated by the rational adjustment of
Tuning Biphasic Catalysis Reaction
with a Pickering Emulsion Strategy
Exemplified
by
Selective
Hydrogenation of Benzene
Pickering
emulsion
parameters
including emulsion droplet size,
droplet distance as well as emulsion
type.
8
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