Polyhydroxylated ammonium chloride salt: A new efficient surfactant for nanoparticles stabilisation in aqueous media. Characterization and application in catalysis

Article abstract of DOI:10.1039/b911094a

A trihydroxyammonium chloride has proved to be an efficient protective agent for Rh(0) nanoparticles and the hydrogenation of arene compounds has been investigated. Significant formation of cyclohexanone in the reduction of anisole has been demonstrated. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b911094a

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Polyhydroxylated ammonium chloride salt: a new efficient surfactant for
nanoparticles stabilisation in aqueous media. Characterization and
application in catalysis
Claudie Hubert,a,b Audrey Denicourt-Nowicki,a,b Jean-Paul Gue´gana,b and Alain Roucoux*a,b
Received 5th June 2009, Accepted 9th July 2009
First published as an Advance Article on the web 24th July 2009
DOI: 10.1039/b911094a
A trihydroxyammonium chloride has proved to be an efficient
protective agent for Rh(0) nanoparticles and the hydrogena-
tion of arene compounds has been investigated. Significant
formation of cyclohexanone in the reduction of anisole has
been demonstrated.
(Scheme 1).13 The reaction proceeds with a non-optimized 44%
yield and the obtained ionic surfactant is highly soluble in water.
In the drive towards sustainable and environmentally friendly
chemistry, aqueous biphasic catalysis has received an increasing
interest.1 Such catalytic systems, based on two immiscible phases
(water/hydrocarbon), provide an easy method for the separation
and recycling of the catalyst by simple decantation and could
be compared to heterogeneization of catalysts on supports in
terms of recycling.2 A prerequisite for aqueous biphasic catalysis
is the solubility of the catalyst within water. The most classical
approach is the design of water-soluble homogeneous catalysts
coordinated to phosphines containing hydrophilic moieties.3 Our
original method relies on the use of metallic nanoparticles finely
dispersed in water, which offers an easy method to remove the
catalyst from the reaction product.4 During the last decade,
soluble nanoparticles have proved to be highly active nanocatalysts
in hydrogenation,5 hydrosilylation6 or carbon–carbon coupling7
reactions. Considerable efforts have been devoted to their synthesis
in order to prevent their aggregation and to control the particle
size.8 For that purpose, various stabilizers have been developed
according to the nature of the reaction media, which generally
depends on the metallic precursors. Our general approach relies
on the stabilization of metallic nanoparticles by highly water-
soluble protective agents, such as ionic surfactants. Previously,
we have described the easy synthesis of N,N-dimethyl-N-cetyl-
N-(2-hydroxethyl) ammonium (HEAX) salts, which have proved
to be efficient stabilizing agents of Rh, Ir, Ru colloids for arene
hydrogenations under mild conditions,9 of Pt nanoparticles for
asymmetric hydrogenation of ethylpyruvate10 and also of Pd
nano species for dehalogenation reactions11 and carbon–carbon
couplings.12 Herein, we report the synthesis of a new ammonium
surfactant, namely N-hexadecyl-N-tris-(2-hydroxyethyl) ammo-
nium chloride (THEA16Cl), as a protective agent for Rh(0)
nanoparticles and their use in arene hydrogenation.†
Scheme 1 Synthesis of THEA16Cl.
Catalytic aqueous suspensions of Rh(0) nanoparticles have been
easily prepared by chemical reduction of rhodium trichloride with
sodium borohydride in dilute aqueous solution of THEA16Cl.
First, the molecular ratio R = THEA16Cl/Rh has been optimized
to avoid aggregation and to provide an efficient stabilization. As
previously described for HEAX-Rh nanoparticles, a molar ratio
of 2 was used to maintain stable THEA16Cl-Rh nanoparticles.
Transmission electron microscopic (TEM) observations show
that the particles are well-dispersed on the grid and near monodis-
persed with an average diameter of 3 nm, as shown on the size
histogram resulting from the measurement of about 250 particles
found in an arbitrarily chosen area (Fig. 1). 87 percent of nano-
objects are included between 2.5 and 3.5 nm. These Rh(0) aqueous
suspensions are highly stable and can be stored without special
precautions.
Surface tension measurements have been performed on
THEA16Cl and also on the THEA16Cl-stabilized Rh(0) nanopar-
ticles in the same conditions as in catalytic reactions (Fig. 2).
Firstly, the studies on THEA16Cl demonstrated that this ammo-
nium surfactant self-aggregates into micelles above the critical
micellar concentration (CMC) of 7 ¥ 10-4 mol L-1. The interfacial
tension of aqueous solutions decreases from 55 to 35 mN m-1.
This new protective agent exhibits a classical behaviour similar to
HEAX. Secondly, measurements of THEA16Cl in the presence of
nanoparticles, which give the surface tension of the free surfactant,
have proved that the surfactant does not possess a critical micellar
concentration. We could assume that THEA16Cl is all around
the particles, forming a double layer at the surface, as previously
characterized by Chen14 and El Sayed15 with ammonium salts
stabilized Cu and Ni NPs or Au nanorods.
THEA16Cl has been easily synthesized by quaternarization of
hexadecylamine with chloroethanol in an aqueous solution of
NaOH at reflux from a procedure adapted from the literature
Hydrogenation of benzene derivatives using THEA16Cl-
stabilized Rh(0) nanoparticles has been performed under biphasic
conditions at atmospheric hydrogen pressure and room tempera-
ture. The reaction was monitored by gas chromatographic analysis
and the turnover frequency (TOF) was defined as the number of
moles of consumed H2 per mole of rhodium per hour. The results
are summarized in Table 1.
aEcole Nationale Supe´rieure de Chimie de Rennes, CNRS, UMR 6226,
Avenue du Ge´ne´ral Leclerc, CS 50837, 35708, Rennes Cedex 7, France.
E-mail: alain.roucoux@ensc-rennes.fr; Fax: +33 223-23-81-99
b, Universite´ Europe´enne de Bretagne
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Table 1 Hydrogenation of benzene derivatives with THEA16Cl-stabilized Rh(0) nanoparticlesa
Entry
Substrate
Product (%)b
TOF (h-1)c
1
2
3
4
5
6
7
Benzene
Toluene
Anisole
Ethylbenzoate
o-Xylene
m-Xylene
p-Xylene
Cyclohexane (100)
Methylcyclohexane (100)
300
100
120
86
81
81
Methoxycyclohexane/cyclohexanone (78/22)
Ethylcyclohexanoate (100)
1,2-Dimethylcyclohexane (cis/trans = 92/8)
1,3-Dimethylcyclohexane (cis/trans = 80/20)
1,4-Dimethylcyclohexane (cis/trans = 66/34)
83
a Reaction conditions: Rh (3.8 ¥ 10-5 mol), THEA16Cl (7.6 ¥ 10-5 mol), substrate (3.8 ¥ 10-3 mol), H2O (10 mL), 1 bar H2, RT. b Determined by GC
analysis. c Turnover frequency defined as mol of H2 per mol of Rh per h.
Fig. 2 Tensiometry of THEA16Cl and THEA16Cl-stabilized Rh(0)
nanoparticles.
activity. Indeed, only 12% of the toluene was reduced in 3 h.
Steric and electronic effects have been observed as arenes sub-
stituted with electron-withdrawing groups (entry 4) react slower
than compounds possessing electron-donating group (Entry 3).
A complete hydrogenation is also achieved with disubstituted
benzene derivatives (entries 5–7), leading to the formation of
the thermodynamically less favourable cis compounds, as usually
observed in heterogeneous systems.16 Finally we have observed
the formation of methoxycyclohexane and of about 22% of cyclo-
hexanone in the reduction of anisole (entry 3). This formation,
not previously described in the literature to our knowledge,
has been explained by further NMR experiments to elucidate
the mechanism. THEA16Cl-stabilized Rh(0) nanoparticles were
prepared in D2O. A 25 mL round bottom flask was charged with
this colloidal solution, and then anisole was added dropwise.
Anisole was hydrogenated at atmospheric pressure and room
temperature. After hydrogenation, nanoparticles were adsorbed
on silica and the filtered aqueous phase was analyzed by 1H
NMR. The formation of MeOH was proved by a peak found at
3.2 ppm, which was confirmed by the addition of 2 mL of methanol
in the NMR tube. On the basis of these results, we have proposed
the following mechanism (Scheme 2). Cyclohexanone was formed
in acid conditions from the partially hydrogenated product, which
leads in water to the formation of hemiacetal. This last compound
decomposes in cyclohexanone and methanol.
Fig.
1
Transmission electron micrograph and size distribution of
Rh/THEA16Cl suspension.
Benzene (entry 1) and monofunctionalized arene derivatives
(entries 2–4) have been totally hydrogenated with quite interesting
TOFs, ranging from 300 h-1 to 86 h-1. A reaction was carried
out without metal and no hydrogenation product was detected.
The reduction of the metal in the absence of a surfactant shows
the formation of aggregates, proving the decrease in catalytic
Scheme 2 Proposed mechanism of cyclohexanone formation during the
hydrogenation of anisole.
In conclusion, THEA16Cl, a trihydroxyalkylammonium salt
possessing an alkyl chain of 16 carbons, provides an efficient
electrosteric stabilization for Rh(0) nanoparticles in aqueous
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media. The aqueous catalytic systems have proved to be active
in the hydrogenation of various arene derivatives with interesting
TOFs. Relevant result has been explained as the cyclohexanone
formation starting methoxybenzene.
We thank the Region Bretagne for financial support (grant to
C. Hubert) and the Universite´ Europe´enne de Bretagne (UEB). We
also thank Jean-Paul Rolland (University of Rennes 1) for TEM
experiments.
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colloidal Rh(0) suspension and aromatic substrate ([substrate]/[metal] =
100), is connected to a gas burette (500 mL) and a flask to balance pressure.
Then, the system is filled with hydrogen and the mixture is magnetically
stirred at 1500 mn-1. The reaction is monitored by gas chromatography
analyses. Turnover frequency (TOF) is determined for 100% conversion.
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Notes and references
† NMR: 1H NMR and 13C NMR spectra were recorded on a BRUCKER
Avance III 400 spectrometer at 400.13 MHz for 1H and 100.61 MHz
for 13C. Chemical shifts are given in d-units (ppm) measured downfield
from tetramethylsilane (TMS) as external reference. Chromatography: all
gas chromatographic analyses were performed using a Carlo Erba GC
6000 with a FID detector equipped with an Alltech AT1 column (30 m,
0.25 mm i.d.). Helium was used as a carrier gas and the inlet pressure
depend on the nature of the substrate. The injections were done at 220 ◦C
and the temperature detector was fixed at 250 ◦C. TEM: TEM study
was conducted using a Philips CM 12 Transmission Electron microscope
at 100kV (UMR CNRS 6026–Universite´ de Rennes I). The sample was
prepared by a dropwise addition of the stabilized rhodium colloids in water
onto a copper sample mesh covered with carbon. The colloidal dispersion
was partially removed after 1 min using cellulose before transferring to
the microscope. The picture was obtained at X 100 000. Measurement
of about 250 particles was made with a program SCION Image (NIH)
R
and was analysed with Excelꢀ program providing histogram of the
nanoparticles size distribution. Tensiometry: surface tension of aqueous
colloidal solutions was measured with a drop tensiometer. A syringe with
a U-shaped needle was lowered into a sample cell containing an aqueous
solution of rhodium nanoparticles protected by the surfactant. An air
bubble was produced from the syringe. The dynamic surface tension was
measured by filming the rising bubble and analysing the contour of the
bubble with a Tracker instrument from IT Concept. The surface tension
was thus determined at room temperature for several concentrations
of surfactants. N-Hexadecyl-N-tris-(2-hydroxyethyl) ammonium chloride
THEA16Cl. In a two necked flask, hexadecylamine (16.7 g, 62 mmol) and
chloroethanol (19.5 g, 230 mmol) were heated to reflux. Then, NaOH
(8.16 g, 204 mmol) in distilled water (75 mL) was added dropwise via an
additional funnel. After 24 h at reflux, the reaction mixture was cooled
down and extracted with chloroform (3 ¥ 50 mL). The organic phase
was dried on MgSO4 and chloroform was removed. The residue was
recrystallized from EtOH–EtOAc to give the final product as a white
powder with a moderate yield (44%). M.p. 94–96 ◦C. 1H NMR (400 MHz,
DMSO-d6, 25 ◦C, TMS) d/ppm: 0.84 (t, J = 7 Hz, 3 H, H16¢), 1.17–1.32
(m, 26 H, H3¢-15¢), 1.66–1.68 (m, 2 H, H2¢), 3.38–3.40 (m, 2 H, H1¢), 3.48–3.50
(m, 6 H, H2), 3.73–3.79 (m, 6 H, H1), 5.44 (t, J = 6 Hz, 3 H, OH). 13C NMR
(100 MHz, DMSO-d6, 25 ◦C, TMS) d/ppm: 13.91 (C16¢), 21.32–31.25 (C2¢–
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C
15¢), 54.51 (C1), 60.13 (C2), 60.61 (C1¢). IR (KBr) n/cm-1: 3300 (C–OH).
HRMS: C22H48NO3 m/z calcd: 374.36342, m/z found: 374.3625 (2 ppm).
Synthesis of THEA16Cl-stabilized Rh(0) nanoparticles. The suspensions
were prepared under nitrogen at 20 ◦C. To an aqueous solution of the
surfactant THEA16Cl (312 mg, 7.6 ¥ 10-2 mol, 2 equiv., in 90 mL H2O) was
added in a flask containing 36 mg of sodium borohydride (9.5 ¥ 10-2 mol,
2.5 equiv.). Then this solution was quickly added under vigorous stirring to
an aqueous solution (10 mL) of the precursor RhCl3·6H2O (100 mg, 3.8 ¥
10-2 mol) to obtained an aqueous Rh(0) colloidal suspension (100 mL).
The reduction occurs instantaneously and is characterized by a change
colour from red to black. The solutions are stable under stirring for several
months. General procedure for hydrogenation under atmospheric hydrogen
7358 | Dalton Trans., 2009, 7356–7358
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The Royal Society of Chemistry 2009
©