A strategy for separating and recycling solid catalysts based on the pH-triggered pickering-emulsion inversion

Article abstract of DOI:10.1002/anie.201300534

Turn you inside out: A novel method for performing in situ separation and recycling of submicrometer-sized solid catalysts is developed based on the pH-triggered inversion of Pickering emulsions (see scheme; o=oil, w=water). Solid catalysts can be recycled 36 times without significant loss of activity. The method differs from conventional methods in terms of speed, energy consumption, catalyst separation, and recycling effectiveness. Copyright

Full text of DOI:10.1002/anie.201300534

Angewandte
Chemie
Communications
Emulsion Catalysis
Turn you inside out: A novel method for
performing in situ separation and recy-
cling of submicrometer-sized solid cata-
lysts is developed based on the pH-
triggered inversion of Pickering emul-
sions (see scheme o=oil, w=water).
Solid catalysts can be recycled 36 times
without significant loss of activity. The
method differs from conventional meth-
ods in terms of speed, energy consump-
tion, catalyst separation, and recycling
effectiveness.
H. Q. Yang,* T. Zhou,
W. J. Zhang
&&&& — &&&&
A Strategy for Separating and Recycling
Solid Catalysts Based on the pH-
Triggered Pickering-Emulsion Inversion
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1
These are not the final page numbers!

.
Angewandte
Communications
DOI: 10.1002/anie.201300534
Emulsion Catalysis
A Strategy for Separating and Recycling Solid Catalysts Based on the
pH-Triggered Pickering-Emulsion Inversion**
Hengquan Yang,* Ting Zhou, and Wenjuan Zhang
The efficient separation and recycling of catalysts is one of
accompanied with detachment of solid particles from the oil/
water interface.
[
1]
major objectives of sustainable and green chemistry. For
liquid-phase reactions, solid catalysts are usually separated
from products through filtration or centrifugation. These
separation methods are widely used but bothersome, espe-
cially when the catalyst particle sizes are in a submicrometer-
to-micrometer range, because of catalyst loss, blocking of
filters, high time- and energy-consumption, and risk of air
Herein, for the first time, we attempt to employ the
Pickering-emulsion inversion to address the “phase transfer”
of solid catalysts and explore a general concept to successively
separate and recycle submicrometer-sized solid catalysts. As
shown in Figure 1a, using an interfacially active and pH-
[2]
oxidation during the course of catalyst transport. These
limitations stimulate development of new methods that
facilitate catalyst separation and recycling, for example, the
currently extensively investigated magnetic-field-assisted sep-
[
3]
aration. We envision if a solid catalyst could be reversibly
transferred between two immiscible phases on command, for
example, an organic phase and an aqueous phase, the
successive catalyst separation and recycling would be facilely
conducted in one vessel without need of catalyst transport.
Although this idea has been successfully used to separate and
recycle homogeneous and polymer-based catalysts in biphasic
[
4,1i]
systems,
separating and recycling heterogeneous (solid particle) cata-
lysts.
there are hardly any reports on its implication for
[
2b]
The major obstacle is that for micrometer-sized
particles, the energy required for crossing the oil/water
7
interface is calculated to be as high as 10 K T (K is
B
B
Boltzmann constant) according to Youngꢀs equation, making
it thermodynamically impossible.
[5]
Figure 1. a) The strategy for catalyst separation and recycling based on
the pH-triggered emulsion inversion. b) The structural description for
an octyl–triamine bifunctionalized hairy silica microsphere. The gray
dots on teh microsphere represent catalytically active centers. c) TEM
image of SM-CN(4), scale bar: 100 nm. d) Five types of functional
silica microspheres with different loadings of octyl and triamine
groups.
Submicrometer and micrometer-sized particles with a suit-
ably wettable surface prefer to attach at the oil/water
interface, leading to Pickering emulsion [oil-in-water (o/w)
[6]
or water-in-oil (w/o)]. Pickering emulsions are emerging as
an attractive platform for designing efficient catalysis systems
[7]
because they provide a large oil/water reaction interface.
More interestingly, if the particle surface is switchable
between hydrophilicity and hydrophobicity, Pickering emul-
sion types can be inverted between o/w and w/o. The
emulsion inversion may provide a thermodynamically feasi-
ble way of “transferring” solid catalysts because it is not
value-responsive solid catalyst, an organic/aqueous biphasic
system can be formulated in which an emulsion phase and
a neat phase coexist. The protonation and deprotonation
make the hydrophilicity/hydrophobicity of the catalyst sur-
face switchable, thus driving emulsion inversion. Before
reaction, lowering the pH value of the reaction system leads
to an o/w emulsion. Owing to the location of the catalyst
particles at the interface of the emulsion droplets, the catalyst
can efficiently convert the substrate to product. At the end of
reaction, the emulsion is inverted from o/w to w/o after raising
the pH value. The solid catalyst is “transferred” into the
bottom layer and a neat oil phase is resolved. The resolved
organic layer can simply be removed by decantation, and the
solid catalyst is “transferred” back into the upper layer after
adjusting the pH value for the next reaction cycle. This in situ
separation and recycling method may address the limitations
of the conventional filtration and centrifugation.
[8]
[
*] Prof. Dr. H. Q. Yang, T. Zhou, W. J. Zhang
School of Chemistry and Chemical Engineering
Shanxi University
Wucheng Road 92,Taiyuan 030006 (China)
E-mail: hqyang@sxu.edu.cn
[
**] This work was supported by the Natural Science Foundation of
China (20903064, 21173137), Program for the Top Young Academic
Leaders of Higher Learning Institutions of Shanxi (2011002) and
Middle-aged Innovative Talents of Higher Learning Institutions of
Shanxi (20120202)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201300534.
2
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
We first need to create a material that is interfacially
active for producing an emulsion, pH-responsive, and suitable
to be used as a catalyst support (although pH-responsive
[6a,8a,d]
emulsifiers appear in literature,
most of them are
polymers. For catalysis, inorganic materials are preferred
owing to their high stability). We used a mixture of hydro-
phobic (MeO) Si(CH ) CH and relatively hydrophilic, pH-
3
2
7
3
sensitive (MeO) SiCH CH CH (NHCH CH ) NH to modify
3
2
2
2
2
2
2
2
the silica microsphere (SM) by covalent linkage, leading to
triamine–octyl bifunctionalized hairy silica microsphere (Fig-
ure 1b). To obtain the desired surface chemistry, the molar
fraction of triamine silane in the mixture was varied from 3%
to 4% and 5% (the total amount of organosilanes was kept
constant); the resultant silica microspheres are denoted as
SM-CN(x) (x = 3–5, respectively). For comparison, we also
synthesized octyl-monofunctionalized and triamine-mono-
functionalized silica microspheres, denoted SM-C and SM-
N, respectively.
SM-CN(x) was characterized with TEM (Figure 1c) and
elemental analysis (Table S1 in Supporting Information), X-
ray photoelectron spectroscopy (XPS, Figure S1 in Support-
ing Information), and thermogravimetry (TG, Figure S2 in
Supporting Information). SM-CN(x) is spherical in morphol-
ogy and its diameter is in a range of 250–350 nm (Figure 1c).
According to the elemental analysis results, the triamine
loading on SM-N is much higher than the octyl loading on
SM-C because the alkaline triamine itself has ability to
catalyze the surface silylation (Figure 1d). For the bifunction-
alized samples, both the triamine and octyl loadings gradually
increase from SM-CN(3) to SM-CN(4) and SM-CN(5).
Notably, the molar ratio of triamine to octyl gradually
decreases, suggesting that the surface chemistry can be
influenced by changing the molar fractions of these two
organosilane.
Figure 2. The appearance of toluene/water systems in the presence of
various silica microspheres (all photographs were taken after the
samples had being stabilized for 0.5 h). A) the biphasic system
contains 4 mL of toluene, 4 mL of water, and 0.032 g of silica micro-
sphere or functionalized silica microsphere; B) adding a few drops of
À1
HCl (1 molL ) to (A) the pH value is adjusted to 3–4; C) adding a few
À1
drops of NaOH (1 molL ) to (B) and the pH value is re-adjusted to
7–8.
The differences in the emulsion-inversion ability are
related to the pH-responsive surface chemistry. As shown in
Figure 3d (zeta potentials), under the neutral conditions
(pH 6.8), the zeta potential SM-CN(4) was measured to be
approximately 0. Under the acidic conditions (pH 3.1), it was
+ 30.1 mV, indicating the protonation of the surface triamine.
As the pH value increased, the zeta potential gradually
decreased owing to the deprotonation of protonated tria-
mines. When the pH value reached 7.9, the zeta potential
became slightly negative (À3.1 mV), suggesting a nearly full
deprotonation of the protonated triamines. The triamine
protonation at the low pH values makes the SM-CN(4)
surface hydrophilic owing to it bearing charges, whereas the
deprotonation of protonated triamines at the high pH values
renders the SM-CN(4) surface hydrophobic. This pH-trig-
gered hydrophilicity/hydrophobicity switching was confirmed
by water contact angle measurements (Figure 3d). The water
contact angle of fresh SM-CN(4) was 1108 (Figure 3d
point a), whereas the water contact angle of SM-CN(4)
treated with an acidic aqueous solution (pH 3.9) decreased
down to 618 (point b). When the protonated SM-CN(4) was
further treated with a basic solution (pH 7.9) its water contact
angle restore the value (1098, point c). As expected, such
changes drive the SM-CN(4)- and SM-CN(3)-stabilized
emulsion inversion between o/w and w/o (the emulsion
types were verified by the conductivity measurement and
We examined the interfacial activity and emulsion-inver-
sion ability of the synthesized samples. After addition of an
equal volume of toluene into water containing 0.8 wt% of the
sample (with respect to water), and subsequent shaking
(
800 rpm for 3 min) or vigorous stirring (3000 rpm for 5 min),
different phenomena were observed for these samples
Figure 2). Bare silica microsphere (SM) precipitated at the
(
bottom, while SM-N was mainly dispersed in water layer. SM-
C, SM-CN(3), SM-CN(4), and SM-CN(5) were well distrib-
uted in the bottom layer with good stability against sedimen-
tation (at least two weeks). Optical microscopy confirmed
that the bottom layer was a Pickering emulsion phase because
spherical droplets were clearly observed (microscopic images
for SM-CN(4) in Figure 3a, microscopic images for other
samples in Figure S3 in Supporting Information). Interest-
ingly, after adding a few drops of aqueous HCl solution and
stirring, only SM-CN(3) and SM-CN(4) transferred to the
upper layer where Pickering emulsion was also observed
(
Figure 2, Figure 3b SM-CN(4) has a better emulsifying
efficiency than SM-CN(3), judging from the emulsion phase
height). More interestingly, SM-CN(3) and SM-CN(4) again
rapidly transferred back to the bottom layer after a few drops
of NaOH solution were added (Figure 2), where emulsion
droplets were still observed (Figure 3c).
[
8]
emulsion droplet test). The hydrophobicity/hydrophilicity
of SM and SM-C is almost unchangeable because of the
absence of a pH-sensitive moiety. SM-N is relatively hydro-
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
3
These are not the final page numbers!

.
Angewandte
Communications
oil volume fraction in the emulsion phase at pH 3–4, and the
resolved oil volumes at pH 7–8. These results are given in
Table S2 in Supporting Information. In the o/w emulsions (at
pH 3–4), the oil volume fractions in the emulsion phase are in
the range of 83.0–91.7% for these six solvents. The high
internal oil phase is helpful for the sufficient contact between
catalyst and reactants. At pH 7–8, 68.8–83.8% of the initial oil
can be resolved after emulsion inversion. The high fraction of
the resolved oil aids the separation of more products from the
catalyst-containing emulsion phase. Moreover, after remov-
ing the resolved oil layer, the remnant emulsion still exhibits
good stability without any coalescence (Figure S11 in Sup-
porting Information, 24 h), which allows for a sufficient time
flexibility to transfer the upper products.
These investigations establish that the SM-CN(4)-stabi-
lized emulsion system tolerates a variety of oils, has a high
internal oil phase, flexible formulation, rapid inversion, and
good ability to resolve oil. Such properties are favorable to
design efficient and recyclable catalysis systems. As a proof of
the concept, we prepared a solid catalyst Pd/SM-CN(4) by
depositing Pd nanoparticles onto SM-CN(4) and tested them.
(The TEM images of Pd/SM-CN(4) are in Figure S12a–c in
the Supporting Information. The Pd loading is 0.46 wt% and
its particle sizes are in the range of 2–4 nm. XPS investiga-
II
0
tions confirm that Pd was fully reduced to metallic Pd in
Figure S12d. The N sorption isotherm is reflected in Fig-
ure S12e.)
The hydrogenation of olefin (styrene) in the organic/
aqueous biphasic system was used as a model reaction to
evaluate the catalysis efficiency and the ability of separating
and recycling catalyst (reaction conditions: 0.1 mol% Pd with
2
Figure 3. Microscopic image, zeta potentials, and water contact angles
of SM-CN(4). a) Microscopic image of the SM-CN(4)-stabilized emul-
sion, neutral pH, scale bar: 200 mm. b) Microscopic image of the SM-
CN(4)-stabilized emulsion, pH 3–4, scale bar: 200 mm. c) Microscopic
images for the SM-CN(4)-stabilized emulsion, pH 7–8, scale bar:
2
00 mm. d) Zeta potentials of SM-CN(4) at different pH values and
water contact angles of SM-CN(4) treated with different pH value
water, see text for details.
respect to substrate, ambient pressure of H , room temper-
2
ature, and 1200 rpm). Before reaction, the water pH value
was adjusted to 3–4. The Pd/SM-CN(4)-stabilized emulsion
(
the droplets are 100–400 mm in size and the viscosity is
À1
philic and its surface is more hydrophilic after protonation.
The low molar ratio of triamine/octyl makes SM-CN(5)
relatively hydrophobic, and the protonation does not makes
its surface sufficiently hydrophilic. The hydrophobicity/
hydrophilicity of SM-N and SM-CN(5) is changeable but
not in an appropriate range (The contact angles switchable in
11.3 MPas at a shear rate of 300 s ) was formed in the upper
layer (o/w) after shaking or vigorous stirring. At the end of
reaction, the solid catalyst Pd/SM-CN(4) was still in the upper
layer (Figure S13 in Supporting Information). To separate the
product from the catalyst, a few drops of NaOH solution was
added and the pH value re-adjusted to 7–8. Pd/SM-CN(4) was
observed to transfer into the lower layer within several
minutes. The o/w emulsion changed to the w/o emulsion and
the extra oil was quickly resolved. The resolved upper organic
layer could be transferred through a simple decantation. The
conversion of styrene was more than 99% within 1.5 h (the
first bar in Figure 4. This reaction efficiency is slightly lower
than that for using pure ethyl acetate as solvent (99%
conversion, 1 h). We further evaluated the catalytic efficiency
of this emulsion system through a comparison with three
other biphasic systems that are schematically described in
Figure 4 (their actual reaction states are reflected in Fig-
ure S13 in Supporting Information). Interestingly, the con-
version obtained with the Pd/SM-CN(4)-stabilized o/w emul-
sion system was much higher than that of the system without
emulsification (the second histogram, 52% conversion. At
the end of the reaction, some of the solid catalyst was found at
the two phase boundary where a small amount of droplets was
formed due to stirring). Moreover, it is much higher than that
[
8f,9]
the vicinity of 908 is likely to invert emulsion).
In addition to the toluene/water system, SM-CN(4) is
a good emulsifier for using ethyl acetate, benzene, ether,
dichloromethane, and trichloromethane as the oil phase
(
Figure S4–9 in Supporting Information). In these systems,
all Pickering emulsions can be inverted easily between o/w
and w/o through regulation of the pH value (changing
temperature was found to be ineffective for inversion of our
emulsion system). The process was accomplished within less
than six minutes under stirring. Most importantly, the
inversion was highly reversible in a wide range of oils. After
1
0 cycles the Pickering emulsion was still formed and the neat
organic phase could be well resolved (Figure S4–9). Addi-
tionally, the emulsion inversion was observed in the presence
of different fractions of SM-CN(4) (Figure S10 in Supporting
Information).
For the above formulated 1:1 (v/v) biphasic systems where
an emulsion phase and a neat phase coexist, we estimated the
4
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!

Angewandte
Chemie
Figure 5. Photographs of the pH-triggered catalyst separation and
recycling based on the emulsion inversion. Before: at the beginning of
reaction (the pH value is adjusted to 3–4). After: at the end of reaction
(the pH value is adjusted to 7–8).
Figure 4. The results of the styrene hydrogenations in different bipha-
sic systems. Pd/SM-CN(4) (o/w): using Pd/SM-CN(4) as catalyst, o/w
emulsion was formed at pH 3–4 after emulsification. Pd/SM-CN(4)
ure S15). The selected photographs for the reaction cycles are
displayed in Figure 5. Throughout the 36 reaction cycles, the
product–catalyst separation and catalyst recycling were con-
ducted facilely by only adjusting the pH values, thus circum-
venting the aforementioned limitations of conventional
separation methods. The yields are presented in Figure 6.
(
phase boundary): using Pd/SM-CN(4) as catalyst (pH 3–4, without
emulsification), some of the solid catalysts was found at the biphasic
boundary. Pd/SM-CN(4) (w/o): using Pd/SM-CN(4) as catalyst, w/o
emulsion was formed after emulsification, without addition of acid
and base (pH 7). Pd/SM (no emulsion): using Pd/SM as catalyst
(
unmodified silica sphere as support), an emulsion was not found in
this reaction system. The actual catalyst distributions in the biphasic
systems are shown in Figure S13 in Supporting Information.
of the Pd/SM-CN(4)-stabilized w/o emulsion system (the third
bar in Figure 4, 62% conversion. This is also confirmed by
other types of reactions (Figure S14 in Supporting Informa-
tion), five-times higher than those of the Pd/SM-catalyzed
biphasic system (the last bar in Figure 4, at pH 3–4, no
emulsion, 19% conversion). The comparison of these results
clearly indicates that the Pd/SM-CN(4)-stabilized o/w emul-
sion system has a much higher catalytic efficiency than the
other biphasic systems. The reaction efficiency difference of
the emulsion systems mainly originates from mass transfer
effects (Table S3 in Supporting Information).
The conversions of various olefins reach 90–99% within
5
h (Table S4 in Supporting Information), indicating the good
Figure 6. The recycling results of the Pd/SM-CN(4)-catalyzed hydro-
genation of styrene in the ethyl acetate/water system (0.5–2 h. The
reaction conditions are included in the Supporting Information).
efficiency of the emulsion system. Moderate yields were
achieved in the first reaction cycles because a part of product
was sacrificed in the lower emulsion layer. To recycle the
catalyst, the fresh substrate was added and the pH value was
re-adjusted to 3–4. The solid catalyst quickly came back into
the upper layer. All the second reaction cycles still proceeded
efficiently without decrease in activity (Table S4 in Support-
ing Information). Note that the “yields” for the second
reaction cycles increased by up to 80–102% because the
continuous phase of emulsion was already supplied in the first
reaction cycles. Moreover, the emulsion catalysis could be
extended to other systems such as toluene/, ether/, and
benzene/water systems (Table S5 in Supporting Information).
Quite impressively, the emulsion catalysis system is highly
recyclable, which is highlighted by 36 reaction cycles (Fig-
Although the ethylbenzene yield was moderate (77%) in the
first reaction cycle, 86–108% “yields” could be obtained from
the second to the thirty-sixth reaction cycles (the variation of
yields is mainly due to the effects caused by the produced
NaCl and the different phase separation time for each
reaction cycle; “yields” over 100% are because the partial
product comes from the previous reaction cycle). The high
yields and excellent recyclability are indications of the high
effectiveness of our strategy. To our knowledge, it is one of the
best results among various methods for catalyst recycling.
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
To further justify the high effectiveness of this method, we
determined the residual Pd in the resolved organic layer with
ICP-AES. In the first reaction cycle, the residual Pd concen-
tration was determined as 0.1 ppm. In the sixteenth and
thirty-sixth cycles, the residual Pd concentrations were below
[
1] a) P. Tundo, A. Perosa, Chem. Soc. Rev. 2007, 36, 532; b) V. K.
Dioumaev, R. M. Bullock, Nature 2000, 424, 530; c) M. Wende,
J. A. Gladysz, J. Am. Chem. Soc. 2003, 125, 5861; d) D. J. Cole-
Hamilton, Science 2003, 299, 1702; e) M. Mokhadinyana, S. L.
Desset, D. B. G. Williams, D. J. Cole-Hamilton, Angew. Chem.
2012, 124, 1680; Angew. Chem. Int. Ed. 2012, 51, 1648; f) P. Pollet,
R. J. Hart, C. A. Eckert, C. L. Liotta, Acc. Chem. Res. 2010, 43,
0
.1 ppm. After 36 reaction cycles, spherical emulsion droplets
were still observed (Figure S16 in Supporting Information).
The recyclability is attributed to the strong adsorption of the
solid catalyst at the oil/water interface. This adsorption means
that the energy obstacle for detachment from the interface is
so high that the catalyst cannot leach into the resolved oil
1237; g) V. Chechik, R. M. Crooks, J. Am. Chem. Soc. 2000, 122,
1243; h) Z. Wang, G. Chen, K. L. Ding, Acc. Chem. Res. 2009, 42,
322; i) S. Ikegami, H. Hamamoto, Chem. Rev. 2009, 109, 583;
j) D. E. Bergbreiter, J. Tian, C. Hongfa, Chem. Rev. 2009, 109, 530.
2] a) Y. F. Yang, N. Priyadarshani, T. Khamatnurova, J. Suriboot,
D. E. Bergbreiter, J. Am. Chem. Soc. 2012, 134, 14714; b) M.
Zeltner, A. Schꢁatz, M. L. Hefti, W. J. Stark, J. Mater. Chem. 2011,
[
[
[
[5]
phase. Moreover, SM-CN(4) was generalized to load other
metals such as Rh. The resultant Rh/SM-CN(4) catalyst also
showed high activity, and could be recycled by the pH-
triggered strategy (Figure S17 in Supporting Information).
In summary, by tailoring the surface chemistry by using
combined organosilanes, we have prepared interfacially
active silica microspheres for the formation of Pickering
emulsions. With this multifunctional silica as a catalyst
support, the formulated catalyst-stabilized emulsion system
shows much higher efficiency than its analogous biphasic
systems. More importantly, based on Pickering-emulsion
inversion, we have successfully developed a conceptually
novel method for in situ separate and recycling of submi-
crometer-sized solid catalysts by tuning the pH value. This
method is time/energy saving for the catalyst separation and
recycling. Its high effectiveness is highlighted by 36 reaction
cycles and negligible catalyst loss. The overall efficiency of
a chemical process may be significantly improved and the
work-up method may be simplified because the successive
catalyst separation and recycling can be processed in one
reaction vessel. The pH-triggering for separation and recy-
cling of solid catalysts can probably be extended to thermo-
2
1, 2991.
3] a) S. Shylesh, V. Schunemann, W. R. Thiel, Angew. Chem. 2010,
22, 3504; Angew. Chem. Int. Ed. 2010, 49, 3428; b) H. Q. Yang,
1
Y. W. Wang, Y. Qin, Y. Z. Chong, Q. Z. Yang, G. Li, L. Zhang, W.
Li, Green Chem. 2011, 13, 1352.
4] a) S. L. Desset, D. J. Cole-Hamilton, Angew. Chem. 2009, 121,
1500; Angew. Chem. Int. Ed. 2009, 48, 1472; b) A. Buhling, P. C. J.
Kamer, P. W. N. M. Van Leeuwen, Organometallics 1997, 16,
3
027; c) C. I. Fowler, C. M. Muchemu, R. E. Miller, L. Phan, C.
OꢀNeill, P. G. Jessop, M. F. Cunningham, Macromolecules 2011,
4, 2501; d) P. G. Jessop, S. M. Mercer, D. J. Heldebrant, Energy
4
Environ. Sci. 2012, 5, 7240; e) L. Phan, J. R. Andreatta, L. K.
Horvey, C. F. Edie, A. Luco, A. Mirchandani, D. J. Darensbourg,
P. G. Jessop, J. Org. Chem. 2008, 73, 127; f) A. Behr, G. Henze, R.
Schomacker, Adv. Synth. Catal. 2006, 348, 1485; g) D. E. Berg-
breiter, Y. C. Yang, C. E. Hobbs, J. Org. Chem. 2011, 76, 6912;
h) A. Behr, L. Johnen, A. J. Vorholt, ChemCatChem 2010, 2, 1271;
i) Y. Brunsch, A. Behr, Angew. Chem. 2013, 125, 1627; Angew.
Chem. Int. Ed. 2013, 52, 1586.
[
5] a) K. Du, E. Glogowski, T. Emrick, T. P. Russell, A. D. Dinsmore,
Langmuir 2010, 26, 12518; b) S. Melle, M. Lask, G. G. Fuller,
Langmuir 2005, 21, 2158.
[6] a) R. Aveyard, B. P. Binks, J. H. Clint, Adv. Colloid Interface Sci.
[
8]
2003, 100, 503; b) S. Tsuji, H. Kawaguchi, Langmuir 2008, 24,
3300; c) N. X. Yan, M. R. Gray, J. H. Masliyah, Colloids Surf. A
2001, 193, 97; d) L. A. Fielding, S. P. Armes, J. Mater. Chem. 2012,
22, 11235; e) E. S. Read, S. Fujii, J. I. Amalvy, D. P. Randall, S. P.
triggering, making our method more generally accessible to
sustainable and green catalysis.
Armes, Langmuir 2004, 20, 7422.
7] a) S. Crossley, J. Faria, M. Shen, D. E. Resasco, Science 2010, 327,
68; b) S. Wiese, A. C. Spiess, W. Richtering, Angew. Chem. 2013,
Experimental Section
[
Preparation of SiO -CN(x): 1.0 g of SiO microsphere (dried at 1258C
2
2
for 4 h) was dispersed into toluene (5 mL). The required amount of
MeO) SiCH CH CH (NHCH CH ) NH and (MeO) Si(CH ) CH
3
1
25, 604; Angew. Chem. Int. Ed. 2013, 52, 576; c) P. A. Zapata, J.
Faria, M. P. Ruiz, R. E. Jentoft, D. E. Resasco, J. Am. Chem. Soc.
012, 134, 8570; d) L. Leclercq, A. Mouret, A. Proust, V. Schmitt,
P. Bauduin, J. M. Aubry, V. Nardello-Rataj, Chem. Eur. J. 2012, 18,
4352.
(
3
2
2
2
2
2
2
2
3
2
7
were simultaneously added. After heating under reflux at 1108C for
h under N2 atmosphere, the material obtained was isolated by
2
4
centrifugation, washed five times with toluene and dried. The total
amount of organosilanes was kept at 1.5 mmol. The molar fraction of
triamine silane in the total silylating reagents was changed from 3%
to 4% and 5%. The modified silica microspheres are accordingly
denoted as SM-CN(x), where x = 3, 4, and 5.
1
[
8] a) B. P. Binks, J. A. Rodrigues, Angew. Chem. 2005, 117, 445;
Angew. Chem. Int. Ed. 2005, 44, 441; b) B. P. Binks, J. Philip, J. A.
Rodrigues, Langmuir 2005, 21, 3296; c) Z. G. Cui, C. F. Cui, Y.
Zhu, B. P. Binks, Langmuir 2012, 28, 314; d) G. Q. Sun, Z. F. Li, T.
Ngai, Angew. Chem. 2010, 122, 2209; Angew. Chem. Int. Ed. 2010,
49, 2163; e) G. Kaptay, Colloids Surf. A 2006, 283, 387; f) B. P.
Binks, P. D. I. Fletcher, B. L. Holt, P. Beaussoubre, K. Wong, Phys.
Chem. Chem. Phys. 2010, 12, 11954; g) H. Liu, C. Y. Wang, S. W.
Zou, Z. J. Wei, Z. Tong, Langmuir 2012, 28, 11017.
Received: January 21, 2013
Revised: April 1, 2013
Published online: && &&, &&&&
[
9] B. P. Binks, J. H. Clint, Langmuir 2002, 18, 1270.
Keywords: biphasic reactions · catalyst separation ·
heterogeneous catalysis · Pickering emulsions
.
6
www.angewandte.org
ꢀ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2013, 52, 1 – 6
These are not the final page numbers!