Pickering emulsions based on supramolecular hydrogels: Application to higher olefins' hydroformylation

Article abstract of DOI:10.1021/cs4002282

Supramolecular hydrogels elaborated from a mixture of native α-cyclodextrin and poly(ethylene glycol)s in water proved to be effective media for higher olefins Rh-catalyzed hydroformylation due to the formation of Pickering emulsions.

Full text of DOI:10.1021/cs4002282

Letter
pubs.acs.org/acscatalysis
Pickering Emulsions Based on Supramolecular Hydrogels:
Application to Higher Olefins’ Hydroformylation
Jonathan Potier,† Stephane Menuel,† Marie-Helene Chambrier,† Laurence Burylo,† Jean-Franco
̧
is Blach,†
́
́
̀
Patrice Woisel,‡ Eric Monflier,† and Frederic Hapiot*,†
́
́
†Lille Nord de France, CNRS UMR 8181, Unite
Perrin, rue Jean Souvraz, SP18, 62307 Lens Cedex, France
‡Universite
́ ́
Lille Nord de France, ENSCL, CNRS UMR 8207, Unite des Materiaux et Transformations (UMET), USTL, 59655
́
de Catalyse et de Chimie du Solide (UCCS) UArtois, Faculte
́
des Sciences Jean
́
Villeneuve d’Ascq Cedex, France
S
* Supporting Information
ABSTRACT: Supramolecular hydrogels elaborated from a mixture of native
α-cyclodextrin and poly(ethylene glycol)s in water proved to be effective media
for higher olefins Rh-catalyzed hydroformylation due to the formation of
Pickering emulsions.
KEYWORDS: hydrogel, cyclodextrins, Pickering emulsion, biphasic catalysis, hydroformylation
Scheme 1. Formation of a Supramolecular Hydrogel from α-
CD and High-Molecular-Weight PEG
fter having been successfully used in biology and
a
1−6
Amedicine,
supramolecular hydrogels currently find
applications in other areas, such as materials7−10 or catalysis.11
In particular, supramolecular hydrogels have emerged as a
promising class of hydrophilic media for catalysis because of the
ease with which they could be implemented.12−14 They were
readily accessible by physical bindings between two comple-
mentary compounds through noncovalent interactions. In
catalysis, the resulting three-dimensional cross-linked macro-
molecular networks have especially been used in the gel phase
as solid-like structures to confine reactants in an organized
environment and make them react selectively.15−20 Astonish-
ingly, catalytic applications in the sol phase have not been
exploited so far. Recently, we have been very interested in
several publications by Li et al.12,21,22 They showed that
supramolecular hydrogels built up from poly(ethylene glycol)
(PEGs) and α-cyclodextrin (α-CD) contained nanosized
columnar α-CD domains (nanocrystallites) acting as physical
cross-links alongside PEG chains not included within the α-CD
cavity (Scheme 1). Concurrently, Saito et al. showed that CD-
based precipitates could help to form Pickering emulsions, that
is, particle-stabilized emulsion.23 The latter have recently gained
renewed interest because they were considered an efficient
alternative to solve mass transfer limitations in biphasic
system.24 Contrary to amphiphilic surfactants that form stable
emulsions and require special equipment for separation,25,26
Pickering emulsions act as emulsifiers and can be advanta-
a
PEG thread into the α-CD cavity resulting in necklace-like
polypseudorotaxanes.
geously recovered by filtration once the reaction is complete.
Taking into account the previous observations, we wondered
how relevant the presence of crystallites in the sol phase of a α-
CD/PEG hydrogel could be to form Pickering emulsions
through contacts with an organic phase. Herein, we highlighted
Received: March 26, 2013
Revised: June 12, 2013
© XXXX American Chemical Society
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Letter
for the first time the ability of α-CD/PEG crystallites to form
Pickering emulsions in the presence of an oil phase. The
obtained Pickering emulsions found application in organo-
metallic catalysis. They proved especially effective in the highly
challenging aqueous Rh-catalyzed hydroformylation of higher
olefins.
Hydrogels are prepared from aqueous saturated solutions of
native α-CD and various amounts of PEGs with molecular
weight of 20 000 g/mol (PEG20000) and 35 000 g/mol
(PEG35000). The α-CD concentration is kept constant, and
the PEG concentration is varied to yield a series of hydrogels
with different stoichiometries. Their stability is evaluated by
successive heat and cool cycles. Whatever the nature of the
PEG (PEG20000 or PEG35000), α-CD/PEG hydrogels with a
2:1 stoichiometry prove to be inappropriate because a
precipitation of the polypseudorotaxanes occurs. Conversely,
sol−gel transitions always takes place for α-CD/PEG hydrogels
with a 1:1 stoichiometry, even after 10 successive heat and cool
cycles. For both α-CD/PEG20000 (H1) and α-CD/PEG35000
(H2) hydrogels in a 1:1 stoichiometry, a sol−gel transition
temperature of ca. 38 °C is measured. The existence of
crystallites is confirmed over a wide temperature range from
room temperature to 85 °C. First, the observed turbid sol is
indicative of still existing packed α-CD/PEG polypseudorotax-
ane domains in the mixture, as already described in the
literature.27 Second, X-ray diffraction measurements clearly
show the existence of crystallites in both the gel and the sol
(ESI) because no significant change in the diffraction spectra
can be noted below and over the sol−gel transition
temperature. Third, optical microscopy reveals crystalline
objects even at high temperature (ESI). Information is also
collected on the supramolecular hydrogels’ behavior in the
presence of an oil phase. Optical microscopy realized on a 1-
decene/H1 mixture clearly shows three different phases (Figure
1a).
Figure 1. (a) Three-phase system at 80 °C of a 1-decene (0.6 mL)/H1
(3 mL) mixture. The benchmark chromophore Red 1 has been added
within the mixture to help visualization of the Pickering emulsion. (b−
f) Optical microscopy of a 1-decene (0.6 mL)/H1 (3 mL) mixture.
(b) 1-Decene (upper yellow organic phase) at 80 °C. (c) Pickering
emulsion (middle pink phase) at 80 °C. (d) H1 sol phase at 80 °C
(crystallites-containing phase). (e) Pickering emulsion sampled at 80
°C and observed at 40 °C. (f) Pickering emulsion sampled at 80 °C
and observed at 20 °C.
Almost no crystallites can be observed in the upper organic
phase (Figure 1b). Conversely, crystallites and an oil in water
(O/W) emulsion coexist in the middle phase (Pickering
emulsion, Figure 1c). The hydrogel lower phase, for its part,
reveals the exclusive presence of crystallites (Figure 1d). An
aliquot of the O/W emulsion sampled at 80 °C and observed at
20 °C is especially illustrative of the crystallites’ adsorption onto
the droplet surface (Figure 1f). Indeed, when cooling the
system to room temperature, the small droplets observed at 80
°C coalesce to give larger droplets (compare Figure 1c, e, and
f). Concurrently, the crystallites’ size also increases. Crystallites
are located at the water/oil interface.
The α-CD/PEG hydrogels have been used as reaction media
in the Rh-catalyzed hydroformylation of 1-decene (1), 1-
dodecene (2), 1-tetradecene (3), 1-hexadecene (4), and 1-
octadecene (5) (Figure 2). The reaction proceeds at 80 °C
under 50 bar CO/H2 with Rh(CO)2(acac) as a rhodium
precursor. The multiphase gas−liquid-sol system consists of
syngas, the substrate-containing organic phase, and the
supramolecular hydrogel in the sol phase. Immobilization of
Rh-species within the supramolecular hydrogel is achieved
thanks to a water-soluble ligand; namely, the trisodium salt of
the trisulfonated triphenylphosphane (TPPTS).
Figure 2. Rh-catalyzed hydroformylation of higher olefins 1−5.
Conditions: Rh/TPPTS/substrate = 1:5:140, 80 °C, 50 bar CO/H2, 3
h reaction time.
Blank runs show very little conversion of 1-alkenes in the
presence of α-CD because of the precipitation of α-CD/
substrate complexes in the sol. Similarly, the use of PEGs as
reaction media also leads to very low conversions. Conversely,
because of the formation of a Pickering emulsion, the
conversions obtained using α-CD/PEGs supramolecular hydro-
gels H1 and H2 are significantly improved when compared
with those obtained without any additive (solely water) or with
the separated components (α-CD or PEGs). For example,
conversion of 1-decene is 16-fold that observed in neat water
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and 4-fold that measured using PEGs as additives. All the
studied olefins take benefit from the α-CD/PEG crystallites-
stabilized Pickering emulsion, irrespective of their alkyl chain
length; however, a regular decrease in the conversion is
observed when increasing the length of the substrate alkyl
chain. Anyway, as clearly demonstrated by the increase in
catalytic activity, the presence of α-CD/PEGs crystallites in the
medium favors the existence of a particle-stabilized emulsion
for which the surface contact at the aqueous/organic interface is
significantly improved, resulting in accelerated reaction rates
between the terminal olefins and the Rh catalyst.
A careful analysis of the reaction profiles in time shows that
the conversions level off after 4 h reaction time, whatever the
substrate (ESI). These results can be explained by previous
studies on Pickering emulsions showing that particles can
strongly adsorb at the aqueous/organic interface and inhibit the
mass transfer between the two phases.28−30 In the present case,
α-CD/PEG crystallites play a role similar to particles and
saturate the interface. A very different behavior would have
been observed if α-CD/PEG mixtures had behaved as
surfactants. Indeed, no mass transfer limitation is observed
using surfactant-based emulsions.31,32 However, this saturation
phenomenon can be easily overcome by successive depressu-
rization−pressurization sequences and heat and cool cycles.
Indeed, after partial depressurization to 5 bar CO/H2, the oil
droplets coalesce upon cooling to 20 °C and creep into the
upper organic phase (decantation). When subsequently heating
the system at 80 °C under 50 bar CO/H2, the conversion
increases again for another 4 h period. The depressurization−
pressurization sequences and heat and cool cycles can be
repeated until most CC double bonds are converted. Figure
3 is illustrative of this step-by-step conversion with 4 as a
substrate.
Accordingly, the substrate conversion depends on only the
interfacial saturation with time. Subjected to partial depressu-
rization, the Pickering emulsion is broken, and dynamics of
exchange at the aqueous/organic interface is then recovered.
In terms of chemoselectivity, the aldehyde proportion is
always higher in the presence of hydrogel H1 (ESI). Effects are
especially marked for short alkyl chain olefins, such as 1-decene,
for which the chemoselectivity rises from 59% without any
additive (mainly alkene isomers are formed alongside the
expected aldehydes) to 95% with H1 as a reaction medium.
The regioselectivity, for its part, remains rather constant,
whatever the substrate (in the 2.3−2.9 range), suggesting that
the hydrogel network does not significantly modify the
equilibriums existing between the rhodium catalytic species
(ESI).33,34 The latter being unaffected, the aldehyde
proportions are similar to those obtained without any hydrogel.
This observation contrasts sharply with what is usually
observed when β-CD derivatives and TPPTS are mixed
together in water. A significant decrease in the l/b ratio is
observed in that case because of the formation of a β-CD/
TPPTS supramolecular complex.33,34 In the present case, the
utilization of α-CDs instead of β-CDs prevents the formation of
such complexes and warrants the catalyst integrity.
Using H2 as reaction media, conversions are adversely
affected when compared with H1 (Figure 2). Indeed, though
the α-CD/PEG ratio between H1 and H2 remains constant,
lengthening the PEG chains disfavors the crystallites’ formation
because they experience more difficulties in threading into the
α-CDs cavity. No significant variations can be measured using
H2 in terms of chemo- and regioselectivities when compared
with H1.
This preliminary investigation highlights the potential
applications of supramolecular hydrogels, especially in the
field of aqueous catalysis. The present study shows that O/W
emulsions can be efficiently stabilized by α-CD/PEG
crystallites. The resulting increase in contact surface between
the substrate-containing organic phase and the catalyst-
containing aqueous phase allows for a significant improvement
in the catalytic performances. Thus, the existence of Pickering
emulsions resulting from supramolecular hydrogel/oil phase
mixtures is of prime importance and could play a pivotal role in
improving the catalytic performance in many other systems.
Further investigations are currently ongoing to extend the
scope of the concept to other organometallic reactions.
For each cycle, 40−45% of remaining 4 can be sequentially
converted, thus explaining the shape of the green curve.
Moreover, the initial catalytic activity is recovered after each
depressurization−pressurization cycle, thus demonstrating the
integrity of the Rh catalyst during the course of the reaction.
ASSOCIATED CONTENT
■
S
* Supporting Information
Preparation of hydrogels, viscosity measurements, optical
micoscopy, X-ray diffraction measurements, and details of
catalytic experiments. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Figure 3. Conversion variation observed in Rh-catalyzed hydro-
formylation of 4 submitted to (i) one catalytic run (red curve) and (ii)
successive depressurization−pressurization sequences and heat and
cool cycles (green curve). Conditions: Rh/TPPTS/4 = 1:5:140, 80 °C,
50 bar CO/H2.
*Phone: +33 3 21 79 17 73. Fax: +33 3 21 79 17 55. E-mail:
frederic.hapiot@univ-artois.fr.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
J.P. is grateful to the Region Nord-Pas-Calais and the Centre
́
National de la Recherche Scientifique (CNRS) for financial
support (2010−2013).
Letter
■
ABBREVIATIONS
H1, α-CD/PEG20000 hydrogel; H2, α-CD/PEG35000;
TPPTS, sodium salt of the trisulfonated triphenylphosphane
■
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