Nanoparticle-based catalysis using supramolecular hydrogels

Article abstract of DOI:10.1002/adsc.201100888

A thermoregulated catalytic process has been elaborated using a ruthenium-nanoparticle catalyst embedded into a supramolecular cyclodextrin-based hydrogel matrix. Upon heating, the gel phase turned into a sol phase in which alkenes could be efficiently hydrogenated. Upon cooling, the reaction products and the metallic catalyst could be easily separated. The reusability of the system was clearly demonstrated. Copyright

Full text of DOI:10.1002/adsc.201100888

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DOI: 10.1002/adsc.201100888
Nanoparticle-Based Catalysis using Supramolecular Hydrogels
Bastien Lꢀger,a Stꢀphane Menuel,a Anne Ponchel,a Frꢀdꢀric Hapiot,a
and Eric Monfliera,*
a
Universitꢀ Lille Nord de France, CNRS UMR 8181, Unitꢀ de Catalyse et de Chimie du Solide, UCCS UArtois, Facultꢀ
des Sciences Jean Perrin, rue Jean Souvraz, SP18, F-62307 Lens, France
Fax : (+33)-3-2179-1755; phone: (+33)-3-2179-1772; e-mail: eric.monflier@univ-artois.fr (homepage:
www.uccs.univ-artois.fr)
Received: November 15, 2011; Revised: January 19, 2012; Published online: April 26, 2012
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201100888.
Abstract: A thermoregulated catalytic process has
been elaborated using a ruthenium-nanoparticle
catalyst embedded into a supramolecular cyclodex-
trin-based hydrogel matrix. Upon heating, the gel
phase turned into a sol phase in which alkenes
could be efficiently hydrogenated. Upon cooling,
the reaction products and the metallic catalyst
could be easily separated. The reusability of the
system was clearly demonstrated.
creasing interest, particularly in the development of
stimuli-responsive materials.[6] One of the main ad-
vantages of supramolecular over polymer hydrogels
lies in the non-covalent intermolecular interactions
between the hydrogel components that allows for
thermal reversibility. Generally speaking, the hydro-
gel becomes an isotropic solution when heated and
returns to a gel state when cooled at room tempera-
ture.[7] However, hydrogels have also been described
that turn into a clear solution upon cooling to room
temperature and are re-formed when heated to the
gelling temperature.[8] Thus, by controlling the tem-
perature, the self-assembled hydrogel texture can be
switched from gel to sol at will. Astonishingly, the
thermoresponsive character of hydrogels has never
been taken into account in catalysis.
Keywords: cooperative effects; cyclodextrins; host-
guest systems; inclusion compounds; supramolec-
ular chemistry
In this study, we took advantage of the hydrogel
Hydrogel applications have become very popular in template properties to access size-controlled metallic
recent years, especially in biology and medicine.[1] nanoparticles (NPs). Moreover, the hydrogel thermo-
However they currently have an impact on other
ACHTUNGTRENrNGNU eversibility allows for the stabilization of NPs at
areas such as materials[2] or catalysis.[3] For instance, room temperature and their activation at high tem-
polymer hydrogels proved especially effective as tem- perature. More precisely, once metallic NPs have
plates for in situ metal nanoparticles (NPs) syntheses. been embedded into a supramolecular hydrogel
Once the NPs are embedded in the polymer matrix, matrix, the system was heated above the sol-gel tran-
the nanocomposite hydrogels could even be used as sition temperature. A metal-catalyzed reaction could
a reactor in the metal-catalyzed reduction of nitro- then take place in the sol phase (Figure 1). Upon
phenols[4] or hydrogen production.[5] Parallel to classic cooling, the catalyst-containing hydrogel and the
hydrogelators, supramolecular gels have gained in- products could be separately recovered in two distinct
Figure 1. Thermoregulated catalytic system using a metallic NPs-containing supramolecular hydrogel matrix.
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transfer between the aqueous NPs-containing phase
and the substrate-containing organic phase. Indeed,
free a-CD and 1 could act as supramolecular carrier
and as surfactant, respectively.
Incorporation of RuCl3 within the hydrogel net-
work followed by a subsequent reduction of the Ru
Scheme 1. Thermoresponsive 1·ACTHNUTGRNEUNG(a-CD)2 hydrogel obtained salt in the presence of NaBH4 led to gel-embedded
by self-assembling of 1 and two a-CDs.
RuNPs, namely RuNPs@1·ACHTNUTRGNENG(U a-CD)2. The morphology
and size of the resulting colloidal nanocomposite hy-
drogel particles were determined by transmission
phases. Additionally, once the reaction was complete, electron microscopy (TEM). The TEM micrographs
NPs did not aggregate but remained homogeneously revealed spherical RuNPs in a Gaussian distribution
dispersed in the 3-D hydrogel matrix allowing for with an average diameter of approximately 1.6 nm
their reusability. The robustness and stability of the and a homogeneous dispersion without any aggre-
catalyst were thus greatly improved.
gates (Figure 2).
Herein, the concept has been validated in an
The particles size is smaller than that observed
RuNPs-catalyzed hydrogenation of alkenes. The using surfactants[11] or ionic liquids[12] as RuNPs stabi-
supramolecular gel was prepared from a mixture of lizing agents, thus highlighting the effective control
the
(CH2)12OC6H3-3,5-(OMe)2]+ (BrÀ) (1) and a-cyclo- over the RuNPs growth. The RuNPs being in a con-
dextrins (a-CD).[9]
strained environment, their size and shape are likely
Once mixed in water, 1 and a-CD resulted in modulated by the volume of the hydrogel nano-re-
self-assembled [3]pseudorotaxane 1·(a-CD)2 gions.
N-alkylpyridinium
amphiphile
[py-N- exerted by the hydrogel internal network structure
a
ACHTUNGTRENNUNG
(Scheme 1), characterized by a sol-gel transition tem-
Viscosity measurements of the nanocomposite ma-
perature of 428C.[9] In fact, the amphiphilic character terial yielded a sol-gel transition temperature of 398C
of 1 was masked by inclusion of the alkyl chain into (Figure 3) below which the gel predominated and
the CD cavities resulting in the formation of a hydro- above which a liquid phase was obtained (Figure 4).
gel. The choice of 1 as a hydrogel component has Moreover, RuNPs@1·ACTHNURGTNEUNG(a-CD)2 is considered thixotrop-
been guided by two properties. First, pyridinium and ic as the viscosity curve profiles show (i) almost com-
dimethoxyphenyl groups have been reported to stabi- plete reversibility when varying the shear rate and (ii)
lize RuNPs in aqueous media.[10] Second, we antici- a time-dependent decrease in the viscosity induced by
pated that, at high temperature, the association con- flow.[13]
stant between the a-CDs and 1 would be sufficiently
The catalytic performances of the RuNPs@1·ACHTUNGTRENNUNG(a-
lowered to dissociate, at least partially, the two a-CDs CD)2 thermoresponsive hydrogel have been evaluated
and 1. The resulting mixture of “free” CDs and 1 was in hydrogenation of hydrophobic and hydrophilic al-
expected to have a significant impact on the mass kenyl substrates (Table 1). When the reaction pro-
Figure 2. a) Transmission electron micrograph and b) size distribution of RuNPs@1·ACTHNURTGNENUG(a-CD)2.
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Nanoparticle-Based Catalysis using Supramolecular Hydrogels
Table 1. RuNPs-catalyzed hydrogenation of terminal alk-
AHCTUNGTRENNUNG
enes.[a]
Entry
Substrate
H2
[bar]
Temp.
[8C]
TOF
ACHTUNGTRENNUNG
[hÀ1][b]
N
1
2
3
4
5
6
2-methyl-3-buten-2-ol
1-decene
1-undecene
1-hexadecene
1-decene
1-decene
1-decene
1-decene
1-undecene
1-undecene
10
10
10
10
10
40
40
40
10
40
10
40
10
40
20
20
20
20
50
50
50
50
50
50
50
50
50
50
28
4
5
6
133
300
300
300
80
240
50
200
100
350
7[c]
8[d]
9
10
11
12
13
14
1-hexadecene
1-hexadecene
2-methyl-3-buten-2-ol
2-methyl-3-buten-2-ol
Figure 3. Viscosity curves as a function of temperature of 1·-
&
^
ACHTUNGTRENNUNG(a-CD)2 ( ) and RuNPs@1·ACHTUNTGERN(NUGN a-CD)2 ( ).
[a]
Conditions: 1.9ꢂ10À5 mol catalyst; 5 mL water; stirring
at 750 rpm; substrate/Ru=100.
Determined by GC analysis after total conversion of the
substrate. TOF=turnover frequency defined as number
of mol of converted substrate per mol of total ruthenium
per hour.
[b]
[c]
[d]
Second run using the recovered hydrogel of entry 6.
Third run using the recovered hydrogel of entry 7.
Figure 4. Photographs of RuNPs@1·
perature (left) and 508C (right).
ACHTUNGTERN(NGNU a-CD)2 at room tem-
ceeded in the gel phase at room temperature, very phases (Scheme 2). The reusability of the catalytic
low catalytic activities were measured. In these condi- system has been successfully examined without any
tions, the better result was logically obtained in hy- loss of its catalytic activity. For example, with 1-
drogenation of a water-soluble substrate, namely 2- decene as substrate, the TOF remained constant (en-
methyl-3-buten-2-ol (Table 1, entry 1, TOF=28 hÀ1). tries 6–8) after three consecutive substrate reloadings
Conversely, when the reaction proceeded at 508C in and sol-gel transitions. TEM of aliquots of the recov-
the sol phase under stirring, high turnover frequencies ered hydrogels after each reaction confirmed the
(TOF) were obtained whatever the nature of the sub- RuNPs stability when embedded into the hydrogel
strate. Note that no isomerization of the C=C double network in these catalytic conditions. Indeed, no var-
bonds could be observed during the reaction.
iation in the RuNPs size could be observed (see the
The catalysis results were rationalized on the basis Supporting Information).
of two distinct and yet complementary aspects: (i) the
To sum up, these results demonstrate that a supra-
enhanced probability of contact between components molecular cyclodextrin-based hydrogel matrix allows
in the sol phase and (ii) the presence of high “free the stabilization at room temperature and the activa-
CD and 1” proportions due to the partial dissociation tion at high temperature of RuNPs catalysts. Futher-
of a self-assembled [3]pseudorotaxane 1·ACTHNUTRGNEUNG(a-CD)2 at more, the RuNPs catalyst stabilized inside the hydro-
high temperature.[14] At 508C, an alkyl chain length- gel matrix can be easily recovered and recycled by
dependence was observed whatever the H2 pressure a simple phase separation. Extension of the concept
(Table 1, compare entries 5, 9 and 11 or entries 6, 10 to other catalytic reactions is currently on-going.
and 12). Actually, the shorter the substrate alkyl
chain, the higher is the TOF. This logically results
from the more marked hydrophobicity of long alkyl
chains towards the hydrophilic hydrogel and high-
Experimental Section
lights the interfacial nature of the catalytic process.
Once the reaction was complete, the mixture was
Colloidal Suspension Elaboration
cooled at room temperature to recover RuNPs@1·
ACHTUNGERTN(NUNG a-
Ruthenium chloride hydrate RuCl3·xH2O (5 mg, 0.02 mmol)
CD)2 and the hydrogenated product into two distinct was dissolved in 5 mL deionized water. a-CD (487 mg,
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Scheme 2. Thermoregulated RuNPs-catalyzed hydrogenation of alkenes using the 1·ACHTNUTRGNEUNG(a-CD)2 hydrogel matrix. During the re-
action, the “free CD and 1” molecules contribute to the mass transfer between the aqueous phase and the organic phase.
0.5 mmol, 25 equiv.) was added to the solution and
1 (120 mg, 0.25 mmol, 12.5 equiv.) was then poured onto the
obtained mixture. The solution was heated at 508C and
NaBH4 (6 mg, 0.15 mmol, 7.5 equiv.) was quickly added. The
reduction was characterized by a color change from brown
to black. After addition of NaBH4, the colloidal suspension
was immediately cooled to 108C. The obtained colloidal sus-
pension in the gel state was kept at low temperature without
any nanoparticles aggregation even after several monthsꢃ
storage.
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General Procedure for Hydrogenation
A stainless steel autoclave was heated at 508C and charged
with 5 mL standard Ru(0) colloidal suspension. After a col-
loidal suspension in the “sol” state was obtained, the sub-
strate (3.8 mmol, 100 equiv.) was poured into the autoclave
and hydrogen was admitted to the system up to the desired
pressure [10<P(H2)<40 bar]. The mixture was magnetical-
ly stirred (750 rpm) at 508C. The reaction was monitored by
analyzing aliquots of the reaction mixture with a Shimadzu
GC-17A gas chromatograph, equipped with a methylsilicone
capillary column (30 mꢂ0.32 mm) and a flame ionization
detector.
Recycling Procedure
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After complete conversion of the substrate, the products
were extracted at 508C by successive liquid-liquid extrac-
tions and decantations with heptane until all organic com-
pounds had been removed from the hydrogel phase (moni-
toring by GC analysis). Residual heptane was removed
under vacuum. The hydrogel-embedded Ru colloidal sus-
pension could then be reused.
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