A direct imaging of amphiphilic catalysts assembled at the interface of emulsion droplets using fluorescence microscopy

Article abstract of DOI:10.1039/b713831h

An amphiphilic fluorescent catalyst Q₉[EuW₁₀O ₃₆] (Q = [(C₁₈H₃₇)₂N ⁺(CH₃)₂]), assembled in the interface of emulsion systems, was directly imaged by fluores

Full text of DOI:10.1039/b713831h

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COMMUNICATION
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A direct imaging of amphiphilic catalysts assembled at the interface of
emulsion droplets using fluorescence microscopy
Jinbo Gao, Yongna Zhang, Guoqing Jia, Zongxuan Jiang, Shouguo Wang, Hongying Lu, Bo Song and
Can Li*
Received (in Cambridge, UK) 10th September 2007, Accepted 17th October 2007
First published as an Advance Article on the web 24th October 2007
DOI: 10.1039/b713831h
multiphase systems, particularly for organic synthesis in water
using homogeneous complex catalysts or enzymes.
An amphiphilic fluorescent catalyst Q9[EuW10O36] (Q =
[(C18H37)2N+(CH3)2]), assembled in the interface of emulsion
systems, was directly imaged by fluorescence microscopy; the
catalyst shows high selectivity and activity in the oxidation of
alcohols using H2O2 as oxidant and the catalyst can be easily
separated and recycled by demulsifying.
However, it is still not well characterized where the surfactant
catalyst is distributed in an emulsion system. It is a quite important
subject to observe directly the distribution state and structure of
the surfactant-type catalyst in reaction systems. A number of
different experimental methods have been employed to character-
ize emulsion systems, such as small-angle neutron scattering
(SANS),2,5 small-angle X-ray scattering (SAXS),2,6 light scatter-
ing,2,7 fluorescence techniques,2,8 nuclear magnetic resonance
(NMR),2 and TEM etc.2 Although these methods have already
provided useful information on many emulsion systems, a direct
image of the catalyst distribution in an emulsion system has not
been reported.
One of the most important and interesting properties of surfactants
in solution is that they form submicroscopic and supramolecular
structures above a critical concentration.1,2 When introducing
surfactants into two or more immiscible liquids, the mixed systems
can form emulsions under suitable conditions.1,2 The emulsions
have been widely used as media in food, cosmetics, pharmaceutical
industries, nanoparticle synthesis, oil recovery, polymerization and
biology, etc.1–3
Herein we present a direct image of an amphiphilic catalyst in
emulsion droplets using a surfactant-type fluorescent catalyst
Q9[EuW10O36] that can be synthesized by using a combination of
hydrophilic polyoxotungstoeuropate anions and lipophilic surfac-
tant (Q = quaternary ammonium cation with long alkyl chains).{
This amphiphilic catalyst has a characteristic fluorescence emission
at 615 nm, which offers a possibility for the direct imaging of the
surfactant-type catalyst assembled at the interface of the emulsion
droplets using fluorescence microscopy.
Recently, catalysis in emulsion systems has attracted much
attention because the emulsion catalysis could play an important
role in the reactions in liquid multiphase systems, e.g., organic
reactions in water solvent. Our previous work reported the
oxidation of sulfur-containing molecules to sulfones in diesel and
the selective oxidation of alcohols to ketones, using H2O2 as
oxidant in the emulsions (Scheme 1). Surfactant-type catalysts
based on quaternary ammonium polyoxometalates (POMs),
acting as surfactants, can be assembled in emulsion droplets,
showing remarkably high selectivity and activity in the oxidation
of organic molecules.4 Emulsion catalysis could be a promising
strategy to improve the mass-diffusion limitation in liquid
Polyoxometalates (POMs) containing lanthanide elements such
as Sm3+, Eu3+, Tb3+and Dy3+ are of special interest as they can
produce strong fluorescence,9 and are usually used as fluorescent
materials. We have synthesized the surfactant-type europium-
substituted heteropolyoxotungstate (Eu-POM), Q9[EuW10O36]
(Q = dimethyl dioctadecylammonium) and the structure of the
amphiphilic catalyst is shown in Scheme 2. The luminescent
properties of Na9[EuW10O36] have been investigated system-
atically.9a,b The photoexcitation of the O A M LMCT (O A W
Scheme 1
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian,
116023, China. E-mail: canli@dicp.ac.cn; Fax: 86-411-84694447;
Tel: 86-411-84379070
Scheme 2
332 | Chem. Commun., 2008, 332–334
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ligand-to-metal charge transfer ) band of [EuW10O36]92 displays
the characteristic peaks of Eu3+ at 5D0 A 7Fj (j = 0, 1, 2, 3, 4) in
the red light region, 615 nm.9 Here, the surfactant-type fluorescent
[(C18H37)2N+(CH3)2]9[EuW10O36] catalyst can be expected to serve
as an emulsifying agent to stabilize the emulsion droplets instead of
only a simple surfactant. Fluorescence microscopy observation
was first performed with an aqueous phase with surfactant-type
fluorescent catalyst. The results are shown in Fig. 1(A). It can be
seen that the bright red spots appear in the whole region,
indicating that the whole system shows the fluorescence of the
amphiphilic catalyst because some catalyst is dissolved in water.
When the amphiphilic fluorescent catalyst was introduced into a
water–oil (dodecane) biphasic system, as shown in Fig. 1(B), bright
circular-shaped fluorescent domains were clearly observed, indicat-
ing that the amphiphilic fluorescent catalyst is distributed at the
water/oil interface of the dispersed emulsion droplets. This is a
direct image of the O/W (dodecane-in-H2O) emulsion systems
formed using the fluorescent catalyst as the surfactant. The
lipophilic quaternary ammonium cation of the surfactant catalyst
may orient to the oil side, and the hydrophilic heteropolyanions to
the aqueous side (Fig. 1(B)). The emulsion catalysis is different
from most homogeneous catalysis. In the emulsion catalytic
system, the amphiphilic catalyst is assembled at the interface of
emulsion droplets, not in the continuous phase or inside the
emulsion droplet. In other reaction media, the catalyst is usually
dissolved in one phase and the reactions are normally carried out
in a homogeneous or pseudohomogeneous phase.2
Fig. 2 Selective oxidation of 2-octanol to 2-octanone in different
reaction systems. (A) [(C18H37)2N+(CH3)2]9[EuW10O36] as catalyst in the
emulsion system, (B) Na9[EuW10O36] as catalyst in the biphasic reaction
system. Reaction conditions: 0.05 mol of 2-octanol, 0.08 mol of H2O2
(30 wt%) and 0.015 mmol of the catalyst, 80 uC.
2-octanol in a reaction time of 8 h (Fig. 2(B)), By contrast, for
2-octanol and hydrogen peroxide mixed with amphiphilic catalyst
([(C18H37)2N+(CH3)2]9[EuW10O36]) with stirring, in which the
metastable O/W emulsion (oil-in-H2O) was formed, the amphi-
philic catalytic system gave high activity and selectivity for the
oxidation of the alcohol, giving 100% of conversion of 2-octanol
and 100% selectivity under the same reaction conditions
(Fig. 2(A)). The results suggested that the emulsion reaction
system provides a high interfacial area and the reaction rates can
be greatly increased. The emulsion catalysts served as both a
catalyst to activate the substrate molecules and an emulsifying
agent to stabilize the emulsion droplets. Moreover, the amphiphilic
catalyst Q9[EuW10O36] can be easily separated and recycled from
the reaction system by demulsifying by centrifugation. By contrast,
the homogenous catalyst Na9[EuW10O36] cannot be recovered and
separated.
To test the catalytic property of the catalyst assembled in the
emulsion system, the selective oxidation of 2-octanol with
hydrogen peroxide as oxidant, as a model reaction, was
investigated both with a biphasic catalytic system and emulsion
catalytic system. The results are shown in Fig. 2. The homogenous
catalyst Na9[EuW10O36] (without (C18H37) 2N+(CH3)2) was
monitored in aqueous–oil biphasic reaction systems, showing very
slow reaction rate and giving only about 4% of conversion of
The emulsion system can also be applied to the oxidation of
other alcohols.{ The results are summarized in Table 1. The
catalyst gave a conversion of 100% for 2-pentanol, with pentanone
selectivity of 100%, and also 100% conversion for cyclopentanol.
Cyclohexanol and 1-phenylpropanol can be completely oxidized to
the corresponding cyclohexanone and propiophenone with a
selectivity of 100% under the same reaction condition. The
amphiphilic catalyst shows remarkable activity and selectivity in
the oxidation of alcohols to ketones in the emulsion catalytic
system, and the TON (turnover number) is about 5000, while the
Table 1 Selective oxidation of alcohols in O/W emulsion systems
using the fluorescent molecule [(C18H37)2N(CH3)2]9[EuW10O36] as
catalyst
Substrates
Time (h)
Conversion (%)
Selectivity (%)
2-Pentanol
2-Octanol
Cyclopentanol
Cyclohexanol
1-Phenylpropanol
a
5
8
5
3
5
100
100
100
100
100
100
100
100
100
100
Fig. 1 Fluorescence microscope images of the emulsion systems. (A)
15 ml of H2O and 0.001 g of the amphiphilic [(C18H37)2N+(CH3)2]9-
[EuW10O36] catalyst. (B) 2 ml of dodecane (oil), 15 ml of H2O and 0.001 g
Reaction conditions: 0.05 mol of alcohol, 0.08 mol of H2O2
(30 wt%) and 0.01 mmol of [(C18H37)2N(CH3)2]9[EuW10O36] catalyst,
80 uC. The catalyst can be recycled.
of the amphiphilic [(C18H37) N+(CH3)2]9[EuW10O36] catalyst.
2
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and dried at 40 uC in vacuum for 24 h to produce the amphiphilic
Q9[EuW10O36]?32H2O catalyst. IR (KBr, cm21): n = 2953, 2918, 2850,
1484, 1468, 1380, 945 (W–Od), 870 (W–Ob–W), 842 (W–Oc–W), 814 (W–
Oc–W), 721 (W–Oc–W). Anal. calc.: for [(C18H37)2N+(CH3)2]9-
[EuW10O36]?32H2O (C342H792N9O68W10Eu, 8110.33): C 50.65, H 9.84, N
1.55; found: C 51.21, H 9.71, N 1.62%.
{ Oxidation of alcohols in an O/W emulsion system. The selective oxidation
of alcohols in the O/W emulsion system was typically carried out as
follows. A 100-ml Erlenmeyer flask was charged with 0.05 mol of alcohol,
0.08 mol of H2O2 (30 wt%) and 0.01 mmol of the Q9[EuW10O36]?32H2O
catalyst. This mixture was heated to 80 uC under vigorous stirring, and the
turbid W/O emulsion was formed. After the reaction was complete, the
water and oil layer was separated by centrifugation, and then analyzed by
GC.
TON is only about 200 in the biphasic reaction systems. The
results indicate that the state of the amphiphilic catalyst, assembled
at the interface of the emulsion droplets, not only maintains the
stability of the emulsion droplets but also provides a higher
interfacial surface area where the reaction takes place, resulting in
a higher reaction encounter probability and a greater interphase
mass transport relative to liquid–liquid biphase reaction systems.
In summary, an amphiphilic fluorescent catalyst Q9[EuW10O36]
(Q = [(C18H37)2N+(CH3)2]) was synthesized and a metastable
emulsion was formed when the catalyst was added to biphasic
systems and the emulsion droplets formed in the O/W emulsion
systems were directly imaged by fluorescence microscopy. This
provides direct evidence that the amphiphilic catalyst is assembled
at the interface of the emulsion systems. The emulsion reaction
medium functions as highly dispersed nanoreactors and behaves
like a homogeneous catalyst. The catalyst shows high selectivity
and activity in the oxidation of alcohols using H2O2 as oxidant.
Moreover, the emulsion catalysts can be easily separated and
recycled by demulsifying.
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We acknowledge the financial support from the National
Science Foundation of China (NSFC Grant No. 20503031).
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Notes and references
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{ Preparation of amphiphilic catalysts based on quaternary ammonium
europium-substituted heteropolyoxotungstate Q9[EuW10O36]?32H2O. The
polyoxotungstoeuropate Na9[EuW10O36]?32H2O was prepared as described
by Peacock and Weakley and Sugeta and Yamase.9a,b The preparation of
amphiphilic Q9[EuW10O36]?32H2O catalysts is as follows: 0.3 mmol of
Na9[EuW10O36]?32H2O was dissolved in 15 ml of water, then a quaternary
ammonium salt (2.7 mmol, Q = [(C18H37) 2N+(CH3)2] ) in 30 ml of alcohol
(95%) was added dropwise into the above aqueous solution with stirring. A
white precipitate was immediately formed. After continuously stirring for
6 h, the white precipitate was collected by filtration, washed with cold water
334 | Chem. Commun., 2008, 332–334
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