Hydrogenation of hydrophobic substrates catalyzed by gold nanoparticles embedded in Tetronic/cyclodextrin-based hydrogels

Article abstract of DOI:10.1039/c8nj06081a

Hydrogenation of alkenes, alkynes and aldehydes was investigated under biphasic conditions using Au nanoparticles (AuNP) embedded into combinations of α-cyclodextrin (α-CD) and a poloxamine (Tetronic90R4). Thermo-responsive AuNP-containing α-CD/Tetronic90R4 hydrogels are formed under well-defined conditions of concentration. The AuNP displayed an average size of ca. 7 nm and a narrow distribution, as determined by TEM. The AuNP/α-CD/Tetronic90R4 system proved to be stable over time. Upon heating above the gel-to-sol transition temperature, the studied catalytic system allowed hydrogenation of a wide range of substrates such as alkenes, alkynes and aldehydes under biphasic conditions. Upon repeated heating/cooling cycles, the Au NP/α-CD/Tetronic90R4 catalytic system could be recycled several times without a significant decline in catalytic activity.

Full text of DOI:10.1039/c8nj06081a

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MENUEL, B. LEGER, S. Noel, E. Monflier and F. Hapiot, New J. Chem., 2019, DOI: 10.1039/C8NJ06081A.
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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
rsc.li/njc

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Hydrogenation of hydrophobic substrates catalyzed by gold
nanoparticles
hydrogels
embedded
in
Tetronics/cyclodextrin-based
Received 00th January 20xx,
Accepted 00th January 20xx
M. Chevrya, S. Menuela, B. Légera,*, S. Noëla, E. Monfliera and F. Hapiota,*
DOI: 10.1039/x0xx00000x
Hydrogenation of alkenes, alkynes and aldehydes was investigated under biphasic conditions using Au nanoparticles (AuNP)
embedded into combinations of -cyclodextrin (-CD) and poloxamines (Tetronics®90R4). Thermo-responsive AuNP-
containing -CD/Tetronics®90R4 hydrogels are formed under well-defined conditions of concentration. AuNP displayed an
average size of ca. 7 nm and a narrow distribution, as determined by TEM. The AuNP/-CD/Tetronics®90R4 system proved
to be stable over time. Upon heating above the gel-to-sol transition temperature, the studied catalytic system allowed
hydrogenation of a wide range of substrates such as alkenes, alkynes and aldehydes under biphasic conditions. Upon
repeated heating/cooling cycles, the Au NP/-CD/Tetronics®90R4 catalytic system could be recycled several times without
significant decline in catalytic activity.
www.rsc.org/
a)
OH
Introduction
n = 6: native -CD
n = 7: native -CD
n = 8: native -CD
H
O
Since the emergence of synthetic catalysts during the 19th century,
the search for effective catalytic systems have been receiving great
attention. Although significant progresses have been made in terms
of catalytic activity and selectivities, efforts should still be made to
both recover and recycle the catalyst once the reaction is complete.
During the past twenty years, we have been developing catalytic
systems capable of ensuring high catalytic performance and
reusability of organometallic or metallic catalysts.1-3 Most of our
strategies rely on cyclodextrins (CDs) which are cyclic
oligosaccharides consisting of glucopyranose units linked by -D-1,4-
glycosidic bonds (Fig. 1).4 Among the latest developments, one of our
idea is to benefit from the ability of -CD (6 glucopyranose units) to
form thermoresponsive hydrogels by inclusion of organic molecules
or polymer chain within their hydrophobic cavity.5 At room
temperature, such hydrogels are in the gel phase wherein they can
efficiently stabilize organometallic and nanoparticle catalysts. When
they are heated beyond their gel-to-sol transition temperature, the
catalyst-containing gel phase turn into a sol phase. Upon stirring of
the resulting catalyst-containing sol phase and a liquid organic
substrate, the latter is converted into the corresponding product.
Once the reaction is complete, the system is cooled down below the
gel-to-sol transition temperature. The hydrogel returns to the gel
=
O
n
OH
HO
H
H
b)
c)
H3C
CH3
H
H
(O-CH2CH2)m-(O-CHCH2)n
(CH2CHO)n-(CH2CH2O)m-H
N
CH2CH2 N
(O-CH2CH2)m-(O-CHCH2)n
H3C
(CH2CHO)n-(CH2CH2O)m-H
CH3
H3C
CH3
H
H
(O-CHCH2)m-(O-CH2CH2)n
(CH2CH2O)n-(CH2CHO)m-H
N
CH2CH2 N
(O-CHCH2)m-(O-CH2CH2)n
H3C
(CH2CH2O)n-(CH2CHO)m-H
CH3
Fig. 1 Structure of a) cyclodextrins (CDs) and b) conventional sequential poloxamines and
c) reverse-sequential poloxamines.
The concept has been developed for several applications. For
example, Rh-catalysts embedded within -CD/PEO (polyethylene
oxide) hydrogels proved to be effective in the hydroformylation of
very hydrophobic alkenes (>C12).6,7 Ruthenium nanoparticles (RuNP)
dispersed in a supramolecular CD-based hydrogel matrix showed
great catalytic activity in hydrogenation of alkenes.8 Interesting
phase and the catalyst and the product are easily recovered results were also obtained with -CD and poloxamines (also known
separately in two different phases.
as Tetronics® macromolecules). The latter consist of block
copolymers based on hydrophilic PEO and hydrophobic
polypropylene oxide (PPO) attached on an ethylene diamine spacer.9
Their X-shaped structure differs from the linear chains of poloxamers
(Pluronics® macromolecules). Conventional sequential Tetronics®
consist of PEO blocks at the outer sphere and PPO blocks attached to
the diaminoethylene core, while reverse-sequential poloxamines
a. Univ. Artois, CNRS, Centrale Lille, ENSCL, Univ. Lille, UMR 8181, Unité de Catalyse
et de Chimie du Solide (UCCS), F-62300 Lens, France.
† Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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show PPO blocks at the periphery and PEO attached to the central programmable controller. The sample volume was slightly adapted
View Article Online
DOI: 10.1039/C8NJ06081A
to the volume of the sample chamber. The program parameters and
results analysis were controlled with the Rheocalc@ PC Software.
The UV-vis spectrophotometry analysis was carried out with a Perkin
Elmer Lambda 19 UV-Vis spectrophotometer. 0.25 mL of the reaction
mixture (with or without -CD) are diluted in 2 mL of deionized water
in order to get a transparent pink-purple solution. Transmission
Electron Microscopy (TEM) was performed on a Tecnai microscope
(200 kV). A drop of the colloidal suspension was deposited onto a
carbon coated copper grid. Metal particle size distributions have
been determined from the measurement of around 400 particles
found in arbitrarily chosen area of the images using the program
SCION Image.
diamino moiety (Fig. 1). -CDs slide along the PPO/PEO copolymer
blocks and selectively accommodate the PEO blocks. The threaded
-CDs form stacked nanocylinders that further aggregate into
nanosized columnar -CD domains through non-covalent
interactions (Fig. 2).10,11
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Colloidal suspension synthesis
A Tetronics®90R4 solution (1250 mg in 2.5 mL of deionized water,
0.17 mmol) was stirred at 40 °C during 30 minutes at 1000 rpm. Then,
2.5 mL of an AuCl3 solution ([Au] ranging from 0.1 to 2 mM) were
added and the resulting solution was stirred at 40 °C during 1h at
1000 rpm. Finally, 725 mg (0.75 mmol) of -CD were added and the
mixture was stirred at 40 °C during 30 min at 1000 rpm. The flask was
put in the fridge overnight to facilitate the sol-to-gel transition.
Fig. 2 Polypseudorotaxane aggregates of -CD / reverse-sequential poloxamines
(Tetronic®90R4).
Using Rh-catalyst embedded within -CD/Tetronics® hydrogels,
hydrophobic alkenes were efficiently converted into their
corresponding aldehydes.12 The catalytic performance remains
unchanged upon recycling as the -CD/Tetronics® hydrogel matrix
prevents the oxidation of the phosphane (ligand stabilizing the Rh
catalytic species).
Catalytic experiment
All hydrogenation experiments have been performed using a Parker-
Autoclave Engineers stainless steel autoclave containing the
AuNP@-CD/Tetronics®90R4 colloidal suspension in a glass vessel.
The mixture was heated up to 50 °C with a thermostated oil bath in
order to work beyond the gel-to-sol transition temperature. The
substrate was added, the reactor was purged three times with
nitrogen and hydrogen was fed to the system at constant pressure
up to 40 bar. The reaction composition was determined after taking
0.25 mL of the reaction mixture which was diluted in 1 mL of water.
The organic products were extracted with 0.5 mL of chloroform and
In the light of the previous results, we have sought to extend the use
of -CD/Tetronics® hydrogels to nanoparticle catalysts, especially
gold nanoparticles (AuNP). Although combination of AuNP,
hydrogels and CDs has already been reported in the literature,13-18
few has been devoted so far to the application of the resulting
system in catalysis.19 Reported herein is the AuNP-catalyzed
hydrogenation of alkenes. Emphasis is especially on the reusability of
the catalytic system.
analyzed by gas chromatography with
a Varian 3900 gas
Materials and methods
Reagents
chromatograph, equipped with a CP-Sil-5B (30 m × 0.25 mm x 0.25
µm) and a flame ionization detector, using decane or dodecane as
external standard.
Gold (III) chloride (99.9% - Au) was purchased from Strem Chemicals.
Styrene, 1-heptyne, 1-octyne, 1-nonyne, 1-octene, 1-decene, 1-
dodecene, trans-2-octene,  and  -pinenes were purchased from
Acros. Cinnamaldehyde and benzaldehyde were purchased from
Fischer. Tetronics®90R4, allylbenzene, phenylacetylene, methyl
oleate, methyl linoleate and citronellal were purchased from Sigma
Aldrich. -CD was purchased from Wacker. All the chemicals and
solvents were used as received without further purification.
For the recycling experiments, after the first run, all the reaction
compounds (styrene and ethylbenzene) were extracted several
times with chloroform (5mL) until complete elimination of the
organic products. Then, the organic phase was separated from the
colloidal suspension after cooling down below the gel-to-sol
transition temperature and the traces of chloroform were removed
by purging the hydrogel phase with nitrogen during 10 minutes.
Finally, after heating up to 50 °C, AuNP@-CD/Tetronics®90R4
colloidal suspension was reloaded with styrene and dihydrogen and
reused in the autoclave under the same experimental conditions (40
bar H2, 50 °C).
Characterization techniques
A Brookfield Model LVDVII+Pro viscometer with the SC4-25 spindle
at 20 rpm was used in this study. Viscosity was measured for
temperatures ranging from 4 to 85 °C into a SC4-13RPY sample
chamber with embedded RTD temperature probe. The temperature
was controlled with a TC502 circulating bath refrigerated using a
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Conversely, small size and narrow GaussianViewdiAsrttircilbe uOtniloinne
DOI: 10.1039/C8NJ06081A
characterized AuNP with a mean diameter of 7.7 nm after 60 min.
Results and discussion
Longer reaction time had deleterious effect on both the AuNP size
and their distribution. Indeed, increase in the AuNP size and loss of
the Gaussian shape of the size distribution were noticed after 90 min,
indicative of the relative poor stability of the system over time in the
absence of -CD.
To prevent the formation of larger AuNP, another AuNP synthesis
was carried out by addition of -CD (0.75 mmol) in the Au/Tetronics®
solution after 60 min. Upon heating at 40 °C, -CD slided along the
copolymer chain. Upon cooling down the system to room
temperature, the sol phase turned into a AuNP-containing hydrogel
(AuNP@-CD/Tetronics®90R4). UV analysis revealed that addition of
-CD provoked a blue shift of max from 532 to 527 nm (Fig. 4),
indicative of a change in the environment around the AuNP.22,24
Synthesis of hydrogel-embedded AuNP
Given our previous results on the use of Tetronics® in
hydroformylation,12 the reverse-sequential Tetronic®90R4 (Mw
7200, 16 EO and 18 PO units per arm) was considered as poloxamine
in the present study. Contrary to procedures described in the
literature for which HAuCl4 was used as Au precursor,20 we used
AuCl3 whose fast reduction in an aqueous solution of Tetronics®90R4
(0.17 mmol in 5 mL water) led to AuNP at 40 °C. A small sacrificial
quantity of Tetronics® acted as reductive agent thanks to terminal
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hydroxyl groups of Tetronics®21 and imparted
a pink color
characteristic of the presence of AuNP to the solution (Supporting
Information, Fig. S1). The AuNP embedded in the hydrogel matrix
(AuNP@Tetronics®90R4) were characterized by UV spectrometry
([Au]=0.5 mM). UV spectra displayed in Fig. 3 confirmed the
reduction of gold salts by the terminal hydroxyl groups of Tetronics®
as a strong absorption band appeared at 532 nm due to the surface
plasmon resonance of AuNP.22 Interestingly, a red shift was observed
from 532 to 547 nm after 90 min, which could be explained by an
increase in the nanoparticle size.23
Fig. 4 Liquid UV spectra of hydrogel embedded AuNPs. [Au] = 0.5 mM,
Tetronics®90R4 (1250 mg) and –CD (750 mg) in water (5 mL) at 40°C at different
reaction times, considering the start time t0 as the instant where -CD was added
to the reaction medium.
t0 ;
t0 + 1 h ;
t0 + 3 h ;
t0 + 5 h ; t0 + 18 h ;
 after 10 consecutive sol/gel transitions.
However, no further variation of max was noticed for longer reaction
time after addition of -CD, suggesting that the -CD/Tetronics®
combination led to a better AuNP stabilization over time than
Tetronics® used alone. The ability of the -CD/Tetronics® matrix to
protect efficiently the Au NPs from aggregation results from the
additional effects of steric stabilization due to PEO/PPO polymer
chains,25-27 nitrogen atoms from ethylene diamine spacer28 and the
presence of -CD.29-34 Further evidence of the increased stability of
AuNP@-CD/Tetronics®90R4 regarding the system without -CD
was given by repeated heating/cooling cycles. We first determined
the sol/gel transition temperature, which was established at 25 °C by
viscosity measurements (Supporting Information, Fig. S3). While the
viscosity of the gel was slightly influenced by the presence of AuNP,
no variation of the viscosity was noticed in the sol phase.
Fig. 3 Liquid UV spectra of hydrogel embedded AuNPs. [Au] = 0.5 mM, and
Tetronics®90R4 (1250 mg) in water (5 mL) at 40°C at different reaction times,
considering the start time t0 as the instant where -CD was added to the reaction
medium.  t0;
t0 + 15 min;  t0 + 30 min; t0 + 60 min;
t0 + 90 min; t0
+ 18 h.
AuNP@Tetronics®90R4 were further characterized by transmission
electronic microscopy (TEM). The bimodal distribution of AuNP
observed after 30 min reaction time (two populations respectively
centered at a mean diameter of 3.9 and 8.1 nm) suggested that the
growth of AuNP was incomplete (Supporting Information, Fig. S2).
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Secondly, upon heating and cooling down the system repeatedly, no
modification of the AuNP shape or size was detected by TEM,
indicative of the robustness of the AuNP@-CD/Tetronics®90R4
system (Supporting Information, Fig. S4). Crystalline domains are
shown on TEM image at a magnification of 490K with the presence
of distinct lattice planes, which can be further analyzed using
reduced Fast Fourier Transform (FFT)-derived diffraction patterns
(Supporting Information, Fig. S5). The FFT-derived diffraction pattern
indicates that the diffraction spot could be identified as Au
nanocrystals with a face-centered-cubic (fcc) structure, based on the
reflection from the (111) planes having a typical d-spacing of 0.23
nm.
Various concentrations of AuCl3 ranging from 0.1 mM to 2 mM were
also considered to assess the variation of the AuNP shape, size and
distribution (Fig. 5). Transmission electron micrographs revealed that
the spherical shape of AuNP was not altered upon increasing [Au].
The observation is in line with literature data that establish that the
AuNP size is dependent upon the nature of the stabilizer (anionic,
cationic…) or reducing agent concentration.35,36 In fact, both -CD
and Tetronics®90R4 participate in the controlled growth of the AuNP
through a “soft template” effect.37-39 The presence of Tetronics®-
constitutive nitrogens probably reinforce the stabilization of AuNP in
the hydrogel matrix because of interactions between nitrogen atoms
and the AuNP surface.40-42 The average AuNP size was only slightly
affected by variation in Au concentration, with mean diameter
ranging from 7.4 to 7.7 nm. Conversely, the distribution was more
influenced by the initial AuCl3 concentration. From [Au] ranging from
0.5 to 1.5 mM, the distribution resembled a Gaussian curve while it
was more polydisperse at [Au] = 2.0 mM.
a) [Au] = 0.1 mM
b) [Au] = 0.5 mM
c) [Au] = 1 mM
d) [Au] = 1.5 mM
e) [Au] = 2 mM
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DOI: 10.1039/C8NJ06081A
Øm= 7.7  1.9 nm
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Øm= 7.7  1.3 nm
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0
Diameter(nm)
15
Øm= 7.7  1.8 nm
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Diameter(nm)
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Øm= 7.5  1.6 nm
AuNP-catalyzed hydrogenation of styrene
15
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5
The catalytic performance of AuNP@-CD/Tetronics®90R4 was
assessed in hydrogenation of alkenes under 40 bar of H2 at 50 °C (sol
phase) for 30 min. Styrene was first chosen as a model substrate
(Table 1). No trace of ethylcyclohexane could be detected once the
reaction was complete. Ethylbenzene was exclusively formed,
indicative of the high chemoselectivity of the reduction. Increasing
the concentrations of AuCl3 led to a regular decrease in conversion.
We first thought that increasing the amount of AuCl3 would lead to
larger AuNP that would be responsible for the drop in conversion.
However, as no significant variation of the AuNP size was noticed by
TEM within the considered concentration range (Fig. 5), the reason
of the drop in conversion probably lies in the increasing amount of
chloride that limited the diffusion of the substrate at the
aqueous/organic interface (salting-out effect).43-45 Acidification of
the medium by increasing amounts of chloride could also be a valid
reason. Moreover, previous DFT studies clearly show that a strong
interaction between chloride and Au(111) surface has been
calculated46,47 and could explain these catalytic results. Note that the
viscosity of the -CD/Tetronics®90R4 hydrogel could not be
incriminated in the sol phase as it did not vary upon addition of AuNP,
irrespective of the Au concentration (Supporting Information, Fig.
S3).
0
Diameter(nm)
15
Øm= 7.4  2.2 nm
10
5
0
Diameter(nm)
Fig. 5 TEM images and corresponding size distributions of AuNPs embedded in -
CD/Tetronics®90R4 hydrogel at (a) [Au] = 0.1 mM ; (b) [Au] = 0.5 mM ; (c) [Au] = 1 mM ;
(d) [Au] = 1.5 mM and (e) [Au] = 2 mM.
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Table 1 AuNP-catalyzed hydrogenation of styrene in α-CD/Tetronic®90R4 hydrogel.a
around 40% upon recycling. To explain the decrease in catalytic
DOI: 10.1039/C8NJ06081A
activity observed after the first run, additional experiments were
View Article Online
Entry
[AuCl3] (mmol/L)
Conv. (%)b
carried out. The organic phase recovered from the first run was
reused at 50 °C under 40 bar of H2 with fresh styrene in -
CD/Tetronics®90R4 hydrogel without AuNPs (Fig. 7). As shown by the
orange line in Fig. 7, the organic phase recovered after the first run
was catalytically active. The increase in conversion suggested that a
small proportion of AuNP was not properly immobilized within the
hydrogel matrix just after the reduction of gold salts, and migrated
into the organic phase during the first run (leaching). However, the
remaining AuNPs in the hydrogel matrix were well stabilized, as the
conversion no longer varied using the organic phase resulting from
the second run. This was confirmed by TEM images of AuNPs
embedded in hydrogels resulting from runs 1, 2 and 3 (Fig. 8). The
distribution remained roughly constant throughout the catalytic
process, thus demonstrating the robustness of the AuNP@-
CD/Tetronics®90R4 system.
1
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0.1
0.5
1
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1.5
2
aConditions: styrene (50 equiv/Au), -CD (0.75 mmol, 1250 mg), Tetronics®90R4 (0.17
b
mmol, 725 mg), water (5 mL), 40 bar H2, 50 °C, 30 min, 1000 rpm. determined by GC
analysis.
It should be noticed that to have a better understanding of the
catalytic process, control experiments have been carried out. First,
AuCl3 was solubilized in water without any additive and the resulting
yellow solution was stirred under standard experimental conditions.
After 1 h, the color of the solution remained unchanged. No
reduction of the gold precursor was detected under hydrogen
pressure. Moreover, in the case of styrene hydrogenation, no
catalytic activity was obtained with AuCl3 alone. Another control
experiment consisted in solubilizing of AuCl3 int water in the
presence of Tetronics®. After addition of styrene and considering the
same experimental conditions described above, the color of the
solution changed after 1 h of reaction from yellow to deep purple
with metal sedimentation. Indeed, because of the presence of
Tetronics®, it was possible to reduce the metallic precursor during
the catalytic test. Nevertheless, the Au NPs sedimentation resulted
in reproducibility issues and the catalytic result was not relevant.
These control experiments clearly show Au NPs are the active
catalyst and the addition of -CD is mandatory to avoid any
aggregation of Au NPs during the catalytic process in sol state.
Other substrates
Given the promising results obtained in the AuNP-catalyzed
hydrogenation of styrene, the substrate scope of AuNP@-
Recyclability of the catalytic system
The reusability of the nanoparticle catalytic system was assessed
using styrene as substrate under 40 bar of H2 at 50 °C (Fig. 6).
Fig. 7 Variation of the conversion over time.  organic phase control
(carried out without catalyst),  organic phase after run 1,  organic phase
after run 2. Conditions: See Fig. 6.
CD/Tetronics®90R4 was evaluated using different alkenes under 40
bar of H2 at 50 °C for 1h. It should be noticed that the comparison
with previous reported works is not so obvious. Indeed, most of the
catalytic systems reported in the literature using gold nanoparticles
for hydrogenation reaction in gas phase or liquid phase are
heterogeneous catalysts where influence of the support on the
catalytic activity and selectivity is not negligible.48-53
Fig. 6 Recyclability of AuNP@-CD/Tetronics®90R4 in hydrogenation of styrene over 5
consecutive runs. aConditions: styrene (0.125 mmol, 13 mg), AuNP@-
CD/Tetronics®90R4 ([Au]= 0.5 mM in 0.75 mmol -CD + 0.17 mmol Tetronics®90R4),
water (5 mL), 40 bar H2, 50 °C, 40 bar of H2, 30 min.
After each run, the organic phase was removed and fresh styrene
was added on the hydrogel mixture. The catalytic system displayed
good reusability as the conversion remained roughly the same
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hydrogenated at all within 1h (entry 8). Interestingly,VbieewnAzratilcdleeOhnyldinee
DOI: 10.1039/C8NJ06081A
gave a low 5% conversion with excellent selectivity in benzyl alcohol,
Øm= 7.7  1.3 nm
Table
2 AuNP-catalyzed hydrogenation of terminal and internal alkenes in α-
CD/Tetronic®90R4 hydrogel.a
8
9
Entry
Substrate
Conv.
(%)b
Sel.b
0
Diameter(nm)
10
11
12
13
14
15
16
17
18
19
20
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60
1
2c
3
4
5
6
7
8
9
styrene
styrene
39
31d
21
22
19
20
34
0
100% ethylbenzene
100% ethylbenzene
100% octanee
b)
c)
d)
20
15
10
5
Øm= 7.6  1.5 nm
1-octene
1-decene
100% decanee
100% dodecanee
100% octane
1-dodecene
trans-2-octene
allylbenzene
cinnamaldehyde
benzaldehyde
0
Diameter(nm)
100% propylbenzene
-
25
20
15
10
5
Øm= 8.2  1.1 nm
5
100% benzyl alcohol
61% 1-heptene
39% heptane
10
11
12
13
1-heptyne
1-octyne
8
53% 1-octene
47% octane
0
18
14
15
Diameter(nm)
58% 1-nonene
42% nonane
1-nonyne
15
10
5
Øm= 7.8  1.6 nm
74% styrene
phenylacetylene
26% ethylbenzene
99% 3,7-
dimethyloctanal
0
14
citronellal
54
1% 3,7
dimethyloctanolf
Diameter(nm)
Fig. 8 TEM images and size distributions of AuNPs embedded in -CD/Tetronics®90R4
hydrogel a) before catalysis and b) after run 1, c) after run 2 (b), and d) after run 3 for
hydrogenation of styrene. Conditions: see Fig 6.
66% cis-pinane
15
16
-pinene
-pinene
15
9
34% trans-pinane
Only one publication is reporting the use of solvent dispersed gold
NPs into aqueous phase containing surfactants such as SDS or CATB
for the hydrogenation of anthracene.54 The so-obtained conversions
of unsaturated compounds are low and could be explained by the
fact that gold is known to have i) a high energy barrier for hydrogen
dissociation and ii) the least stable chemisorption state in
comparison with metal such as Ni or Pt.55 Styrene was rapidly
converted (39%) to yield exclusively ethylbenzene (Table 2, entry 1).
Addition of NaCl significantly affected the conversion (31%, entry 2)
as already described for styrene (vide supra). Linear alkenes were
equally converted into their corresponding alkanes, irrespective of
64% cis-pinane
36% trans-pinane
17
18
methyl oleate
0
0
-
-
methyl linoleate
aConditions: alkene/Au = 100, AuNP@-CD/Tetronics®90R4 ([Au]= 0.5 mM in 0.75 mmol
-CD + 0.17 mmol Tetronics®90R4), water (5 mL), 40 bar H2, 50 °C, 1 h, 1000 rpm.
bdetermined by H NMR. with NaCl (17 mg added during the synthesis of the colloidal
suspension). AuNPs at the interface (colorless aqueous phase). no isomerization. no
citronellol.
1
c
d
e
f
the length of the alkyl chain (entries 3-5). Trans-2-octene gave similar indicative of the ability of the catalytic system to reduce C=O as well
conversion, suggesting that AuNP also reacted with its less accessible
carbon-carbon double bond (entry 6). While allylbenzene gave 34%
conversion (entry 7), conjugated C=C of cinnamaldehyde was not
(entry 9). The versatility of the AuNP@-CD/Tetronics®90R4 was
also demonstrated in hydrogenation of alkynes. Mixtures of alkenes
and alkanes were obtained, alkenes being logically intermediates
before conversion into alkanes (entries 10-12). Phenylacetylene was
selectively hydrogenated into styrene and ethylbenzene, with a
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styrene/ethylbenzene selectivity ratio of 74/26, without alteration of
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Conclusions
With the aim of recycling nanoparticle catalysts, we developed a
catalytic system consisting of AuNP embedded in -
CD/Tetronics®90R4 hydrogels. AuNP displayed an average size of
ca.7 nm and a narrow distribution, as determined by TEM. The
AuNP/-CD/Tetronics®90R4 system proved to be stable over time.
Upon heating above the gel-to-sol transition temperature, the
studied catalytic system proved versatile, as a wide range of
substrates such as alkenes, alkynes and aldehydes could be
converted under biphasic conditions. The best results were obtained
in AuNP-catalyzed hydrogenation of citronellal (54% conversion
within 1h, 99% selectivity in 3,7-dimethyloctanal). Interestingly,
upon repeated heating/cooling cycles, the AuNP@-
CD/Tetronics®90R4 catalytic system could be recycled several times
without significant decline in catalytic activity.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
Chevreul Institute (FR 2638), Ministère de l’Enseignement Supérieur
et de la Recherche, Région Nord – Pas de Calais and FEDER are
acknowledged for supporting and funding partially this work. The
TEM facility in Lille (France) is supported by the Conseil Regional du
Nord Pas de Calais and the European Regional Development Fund
(ERDF). We are grateful to Dr. A. Addad for his contribution in setting
up the TEM measurements. We also thank Dr. Nicolas Kania and D.
Prevost for technical assistance. The authors gratefully acknowledge
the financial support from the Major Domain of Interest (DIM) "Eco-
Energy Efficiency" of Artois University.
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DOI: 10.1039/C8NJ06081A
Graphical abstract
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