Dispersion and hydrogenation activity of surfactant-stabilized Rh(0) nanoparticles prepared on different mesoporous supports

Article abstract of DOI:10.1016/j.apcata.2010.12.035

Supported Rh(0) colloidal particles were prepared by the reduction of Rh(III) ions by sodium borohydride in the presence of N-N-dimethyl-N-cetyl-N-(2- hydroxyethyl) ammonium chloride (HEA16Cl), usually used as a stabilizing agent in solution. Tested supports were Na-Al-SBA-15, SBA-15 and CMK-3. In each case, the influence of HEA16Cl was studied by comparison with blank samples. Surfactant and rhodium uptake were evaluated by means of elemental analysis and eventually thermogravimetry. Obtained materials were also characterized by XRD, N₂ sorption and TEM. Given the results, it appears that HEA16Cl promotes rhodium uptake in all cases. Most significant effects on the size and dispersion of particles were observed for the system combining HEA16Cl and Na-Al-SBA-15. All the solids prepared in this study were tested in the room temperature hydrogenation of styrene as well as that of a more demanding substrate, diphenylmethane, at 0.1 MPa of H₂. All of them were generally more active than their commercial analogue (5 wt.% Rh⁰/C). Best catalysts, i.e., those prepared from Na-Al-SBA-15 in the presence of HEA16Cl as well as CMK-3 without HEA16Cl, allowed almost 100% yield of dicyclohexylmethane within 6 h (molar substrate/Rh = 100).

Full text of DOI:10.1016/j.apcata.2010.12.035

Applied Catalysis A: General 394 (2011) 158–165
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Dispersion and hydrogenation activity of surfactant-stabilized Rh(0)
nanoparticles prepared on different mesoporous supports
Maya Boutrosa,b, Guillaume Shirley , Thomas Onfroya,c, Franck Launaya,c,∗
a
a
UPMC Univ Paris 6, Laboratoire des Systèmes Interfaciaux à l’Echelle Nanométrique, CNRS UMR 7142, 4 place Jussieu, 75252 Paris Cedex 05, France
Université Libanaise, Faculté des Sciences II, Laboratoire de Chimie Physique des Matériaux (LCPM), Campus Fanar, BP 90656, Jdeideh, Lebanon
UPMC Univ Paris 6, Laboratoire de Réactivité de Surface, CNRS UMR 7197, 4 place Jussieu, 75252 Paris Cedex 05, France
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 October 2010
Received in revised form
Supported Rh(0) colloidal particles were prepared by the reduction of Rh(III) ions by sodium borohydride
in the presence of N-N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride (HEA16Cl), usually
used as a stabilizing agent in solution. Tested supports were Na–Al–SBA-15, SBA-15 and CMK-3. In each
case, the influence of HEA16Cl was studied by comparison with blank samples. Surfactant and rhodium
uptake were evaluated by means of elemental analysis and eventually thermogravimetry. Obtained mate-
rials were also characterized by XRD, N2 sorption and TEM. Given the results, it appears that HEA16Cl
promotes rhodium uptake in all cases. Most significant effects on the size and dispersion of particles were
observed for the system combining HEA16Cl and Na–Al–SBA-15. All the solids prepared in this study were
tested in the room temperature hydrogenation of styrene as well as that of a more demanding substrate,
diphenylmethane, at 0.1 MPa of H2. All of them were generally more active than their commercial ana-
2
1 December 2010
Accepted 27 December 2010
Available online 4 January 2011
Keywords:
Rh(0) nanoparticles
SBA-15
CMK-3
Hydrogenation
Aromatic substrates
0
logue (5 wt.% Rh /C). Best catalysts, i.e., those prepared from Na–Al–SBA-15 in the presence of HEA16Cl
as well as CMK-3 without HEA16Cl, allowed almost 100% yield of dicyclohexylmethane within 6 h (molar
substrate/Rh = 100).
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
deposition onto preformed materials [16–23,30,31] or their intro-
duction in the synthesis gel of the support [24–32]. However,
the relatively low amount of metal introduced and the difficulty
in inserting the particles homogeneously within the porosity are
important drawbacks of these approaches.
Controlled nanometer-sized metal particles have received con-
siderable attention in both industry and academia due to their
superior physical and chemical properties compared with bulk
materials [1–5]. One of the strategies to take advantage of these
properties for catalysis applications is based on the deposition of
nanostructured metal colloids onto supports with high specific sur-
face area and narrow pore size distribution. Structured mesoporous
materials are good candidates to obtain a high dispersion degree of
the metal and improved accessibility of active sites in comparison
to supports like zeolites [6–15]. At this time, mesoporous materi-
als including SBA-15 and carbon CMK-3 have been already used
as host structures for preformed metal nanoparticles [16–32], and
some research efforts have been focused on their applications as
heterogeneous catalysts. Insertion of stabilized colloidal nanopar-
ticles in the mesopore network is usually performed by their
Another pathway proposed by our group consists to prepare
the colloids directly onto the support. Indeed, we showed pre-
viously [33] that well-dispersed Rh(0) particles can be obtained
by the reduction of Rh(III)-exchanged mesoporous alumino-
silicates (Na–Al–SBA-15) by sodium borohydride in the presence
of a water soluble stabilizing agent, N,N-dimethyl-N-cetyl-N-(2-
hydroxyethyl) ammonium chloride (HEA16Cl). The efficiency of
this strategy was attributed first to the combined use of an acidic
support, Al–SBA-15, and of a quaternary ammonium salt, HEA16Cl.
The present paper aims at comparing various hexagonally ordered
siliceous or non siliceous materials in order to emphasize the
effect of the support nature and the role of HEA16Cl on the
location, the size of the nanoparticles and their catalytic activity
in arene hydrogenation under mild conditions. For this purpose,
Rh(0)-containing mesoporous SBA-15, Al–SBA-15 and CMK-3 were
synthesizedinthe presence orinthe absenceofHEA16Cl molecules.
The resulting solids were characterized by thermogravimetric ana-
lysis (TGA), X-ray diffraction, nitrogen sorption and transmission
∗ Corresponding author at: UPMC Univ Paris 6, Laboratoire de Réactivité de Sur-
face, CNRS UMR 7197, 4, place Jussieu, 75252 Paris Cedex 05, France.
Tel.: +33 1 44 27 58 75; fax: +33 1 44 27 60 33.
E-mail address: franck.launay@upmc.fr (F. Launay).
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.12.035

M. Boutros et al. / Applied Catalysis A: General 394 (2011) 158–165
159
Table 1
electron microscopy (TEM). The different solids were also tested as
catalysts in the room temperature hydrogenation of styrene and
diphenylmethane under 0.1 MPa dihydrogen pressure.
Acronyms of the different Rh(0) based-solids.
Samples
Support
SBA-15
pH adjustment
Yes
HEA16Cl
0
Rh –SBA–NH3
Yes
No
0
2
. Experimental methods
Rh –SBA–NH (B)
3
0
Rh –CMK–NH3
Yes
No
CMK-3
Yes
No
2.1. Materials synthesis
0
Rh –CMK–NH3(B)
0
Rh –Na–AlSBA
Yes
No
The different reagents, i.e., rhodium chloride hydrate
0
Na–Al–SBA-15
Rh –Na–AlSBA(B)
(
(
RhCl ·xH O, 40.8% Rh, x = 2.25, Strem), sodium borohydride
3
2
NaBH , 99%, Aldrich), aluminium isopropoxide (Al[(CH ) CHO] ,
4
3
2
3
◦
9
8%, Aldrich), Pluronic P123 ((EO)20(PO)70(EO)20
,
Mav = 5800,
washed and dried at 60 C for 24 h. Blank preparations were car-
ried out on the three supports. The blank solids were synthesised
using the procedure above but in the absence of HEA16Cl. Table 1
summarizes the acronyms of the different solids and gives some
preparation details.
Aldrich), tetraethyl orthosilicate (TEOS, ≥99%, Fluka), tetramethyl
orthosilicate (TMOS, ≥99%, Fluka), 37 wt.% fuming hydrochloric
acid (SDS), 98 wt.% sulfuric acid (Carlo Erba), sucrose (>99.5%,
Sigma), NaCl (99.9%, Carlo Erba), 30 wt.% aqueous ammonia
(
Carlo/Erba), were used as received. N,N-dimethyl-N-cetyl-N-(2-
hydroxyethyl) ammonium chloride (HEA16Cl) was prepared as
previously described in the literature [34–36].
2.2. Materials characterization
The silicon, rhodium, carbon and nitrogen compositions of
the various materials were determined by ICPAES in the CNRS
analysis center at Vernaison (France). X-ray powder diffraction
(XRD) data were recorded with a Bruker D8 Advance diffractome-
2
.1.1. Supports
SBA-15 silica was synthesized according to the method reported
by Zhao et al. [37]. In details, 4.0 g of P123 were dissolved at
4
TEOS were added. The resulting gel was stirred at 40 C for 24 h
and hydrothermally treated in a fluorinated ethylene propylene
◦
−1
◦
0 C in 140 mL of a HCl solution (1.7 mol L ). Then, 8.4 g of
ter using the Cu K␣ radiation in the 2ꢀ range between 0.5 and
◦
◦
5 . N2 adsorption–desorption isotherms were measured at liquid
nitrogen temperature using a Micromeritics ASAP 2010 instru-
ment. Before analysis, samples were degassed overnight at room
◦
(
FEP) flask at 100 C during 24 h. Finally, the solid was filtered,
◦
◦
washed with water, dried at 80 C overnight and calcined in air
temperature and 8 h at 200 C. Specific surface areas were eva-
−
1
◦
−1
◦
flow (flow = 30 mL min , heating rate = 24 C h ) at 550 C for 6 h.
Al–SBA-15, with a nominal Si/Al molar ratio of 10, was synthe-
sized using the method described by Li et al. [38]. This material was
prepared as follows: 4.0 g of P123 were dissolved in 150 mL of aque-
luated using the Brunauer–Emmett–Teller (BET) method in the
P/P0 range of 0.05–0.25. Pore sizes were determined using the
Barrett–Joyner–Halenda (BJH) method applied to the desorption
0
isotherms. Thermal analysis (TG-DSC) of Rh –SBA–NH3 was per-
◦
ous HCl (pH = 1.5) at 40 C (solution A). Simultaneously, 6.4 mL of
formed on a SDT 2960 system (TA Instruments, Inc.). Measurements
−
1
TMOS and 0.88 g of aluminium isopropoxide were added to 10 mL
of aqueous HCl (pH = 1.5, solution B). Solution B was stirred at room
temperature for 3 h and added dropwise to solution A. The resulting
mixture was aged for 20 h at 40 C and hydrothermally treated in a
FEP flask at 100 C for 24 h. The resulting solid was filtered, washed,
were carried out in a constant (100 mL min ) N2 flow with a
◦
−1
heating rate of 10 C min . Transmission electron microscopy
(TEM) images were recorded with a JEOL TEM 100 CXII electron
microscope operating at an acceleration voltage of 100 kV.
◦
◦
◦
dried and calcined at 550 C for 5 h. The sodium form, Na–Al–SBA-
2.3. Catalysis test
1
1
5, was obtained by treating 1.0 g of calcined Al–SBA-15 solid by
00 mL of a 0.7 M NaCl solution at 80 C for 48 h. Na–Al–SBA-15
◦
Rh(0) containing solids were used as catalysts in the hydrogena-
tion of styrene and diphenylmethane (99%, Aldrich). A weighted
amount (50 mg) of the material was dispersed in hexane (10 mL) for
15 min prior to the introduction of the arene derivative (100 equiv.
per eq metal). The tests were carried out at room temperature under
0.1 MPa H2. Products were analysed with a Delsi Nermag gas chro-
solid was recovered by filtration, washed with distilled water and
dried 24 h at 60 C. The experimental Na/Al ratio was 0.5 [33].
The mesoporous carbon material, CMK-3, was prepared by using
SBA-15 as template and sucrose as carbon source [39]. A solution,
obtained by dissolving 1.25 g of sucrose and 0.14 g of 98 wt.% H SO4
◦
2
®
in 5.0 g of water, was contacted with 1.0 g of SBA-15. The resulting
matograph equipped with a Macherey–Nagel Optima -5 capillary
◦
◦
mixture was then treated at 100 C for 6 h and at 160 C for another
h. The recovered solid was impregnated again with a solution con-
column and a FID detector. Quantifications were performed using
0
6
n-decane as internal standard. Recycling test for Rh –CMK–NH3
◦
taining 0.8 g of sucrose, 0.09 g of 98 wt.% H SO4 and 5.0 g of water,
(B) was performed after filtration and drying (60 C) of the sample
2
and was thermally treated as described above. Finally, the obtained
recovered from the first run. There was no activation step. Condi-
tions for the second test were the same as those used for the fresh
catalyst (see above).
◦
composite material was pyrolyzed in nitrogen flow at 900 C for 6 h
−1
◦
−1
(
flow = 20 mL min , heating rate = 300 C h ). CMK-3 was reco-
vered by dissolving silica in 1 M aqueous/ethanol solution of NaOH
(
50 mL per 1.0 g of starting material). After 24 h, the solid was fil-
3. Results and discussion
◦
tered, washed with absolute ethanol and dried at 80 C during 12 h.
For all preparations, the nominal NaBH /Rh and HEA16Cl/Rh
4
2
.1.2. Rh(0) containing solids
The support (SBA-15, Na–Al–SBA-15 or CMK-3) (1.0 g) was dis-
molar ratio used were rigorously the same as those of the opti-
mized synthesis of 2–3 nm Rh(0) colloidal particles in aqueous
solution [36] (i.e., 2.5 and 2, respectively). The theoretical value of
rhodium loading onto the different supports was 1.0 wt.%. Accord-
ing to elemental analysis results reported in Table 2, Rh(III) uptake
by the different supports is facilitated in the presence of the cationic
stabilizing agent. Indeed, Rh loadings of samples prepared with
HEA16Cl are between 1.5 and 3.5 times higher than those of the
corresponding blank materials. Table 2 also indicates that the inser-
persed in 25 mL of distilled water in the presence of HEA16Cl
−4
(
(
0.063 g, 1.8.10 mol) for 24 h. Then, rhodium(III) chloride hydrate
0.024 g) was contacted with the solid for 2 h. A pH adjustment to
9
.5 (addition of a few drops of 30 wt.% aqueous NH ) was necessary
3
in the case of SBA-15 and CMK-3. Reduction occurred instanta-
neously after NaBH4 (10 mg) introduction as shown by a colour
change from yellow to black. After 2 h, the solid was filtered,

1
60
M. Boutros et al. / Applied Catalysis A: General 394 (2011) 158–165
Table 2
Physicochemical properties of Rh -containing samples.
0
Sample
Rh (wt.%)
N (wt.%)
SBET
Vp
(cm
Dp
(nm)
2
−1
3
−1
g )
(
m
g
)
SBA-15
–
0.55
0.15
–
0.2
–
912
495
734
1.03
0.99
1.00
6.7
7.6
6.9
0
Rh –SBA–NH3
0
Rh –SBA–NH3(B)
CMK-3
–
0.96
0.58
–
0.2
852
653
878
0.73
0.57
0.86
3.4
3.3
3.2
0
Rh –CMK–NH3
0
Rh –CMK–NH3(B)
Na–Al–SBA-15
–
0.90
0.40
–
0.2
–
785
870
795
1.03
1.24
0.99
7.9
7.0
8.2
0
Rh –Na–AlSBA
0
Rh –Na–AlSBA(B)
tion of rhodium is more difficult on pure silica than on the other
supports.
Differences in Rh loading, observed on blank samples (prepared
in the absence of the stabilizing agent) may be understood by
referring to a model based on electrostatic interactions between
the negatively charged supports and Rh complexes in solution.
Indeed, the pH of the solution was about 6 for Na–Al–SBA-15, 9 for
SBA-15 and CMK-3 and reported isoelectric point values of pure
silica, silica–alumina (5 wt.% Al O ) and carbon are 2 [40], 2–5
0
Fig. 1. Thermogravimetric analysis of Rh –SBA–NH3.
2
3
[
41] and 3–4 [42], respectively. In these conditions, it is concei-
vable that the cationic aqua (Na–Al–SBA-15) or ammine (SBA-15,
CMK-3) rhodium complexes are adsorbed on the support through
electrostatic interactions. Differences in rhodium uptake may be
due to variations of the surface concentration of negative charges
According to the above-mentioned data, all supports adsorbed
approximately the same amount of HEA16Cl. However, rhodium
uptake of pure silica support is inferior to those obtained with
Na–Al–SBA-15 and CMK-3. So, it can be concluded that, even if
HEA16Cl molecules play a determining role in the uptake of the
rhodium species, the nature of the support is also very important.
Such conclusion agrees with previous observations [33]. Indeed, we
already showed that modification of Al–SBA-15 surface through
H /Na exchange induces an increase of the final Rh loading
from 0.44 to 0.90 wt.%. Arrangement of the adsorbed surfactant
molecules and, hence their ability to capture Rh(III) complexes,
probably differ with the intrinsic nature of the support as well as
with its textural properties.
and thus of the nature of the support. In the specific case of
0
Rh –CMK–NH (B) sample, Rh content is in agreement with those
3
of various activated carbons prepared by Daza et al. [43].
The systematic increase in Rh loading as the result of the
addition of the cationic stabilizing agent may correspond to the for-
mation of admicelles [44,45]. Hence quaternary ammonium groups
of HEA16Cl molecules would potentially interact with negatively
charged surfaces leading to adsorption and the formation of a
bilayer through tail to tail interactions. Resulting admicelles would
+
+
ꢀꢀ
3+
lead to Rh uptake through NR4+. . .Cl . . . Rh
3
+
−
“
association [46].
Aqueous Rh(III) species would be adsolubilized during the 2 h stir-
ring period prior to the reduction step. Taking into account the fact
that most of the polar heads of the surfactant are oriented to the
solution, one admicelle could stabilize more Rh(III) species than
one negative surface charge.
3.1.1. Structural and textural characterizations of the materials
All samples are characterized by the three diffraction peaks
((1 0 0), (1 1 0) and (2 0 0)) attributed to a 2D-hexagonal structure
and there was almost no change of the cell parameters upon Rh
0
Thermogravimetric and elemental analyses of Rh –Na–Al–SBA
incorporation. The significant decrease of the intensities of the
0
0
and Rh –SBA–NH samples are consistent with the presence of
diffraction peaks observed in the particular case of Rh –SBA–NH
3
3
+
0
HEA16 cations. Indeed, values of the molar C/N ratio in both solids
and Rh –SBA–NH (B) materials may be related to a partial disso-
3
are equal to 24. The slight difference between the theoretical and
the experimental value could be due to some contamination by
P123 (expected ratio of C/N is 20). Thermogravimetric studies of
lution of the silica support following the addition of ammonia (pH
adjustment). Nitrogen adsorption–desorption isotherms of SBA-15
and Na–AlSBA-15-based materials (Fig. 2a and b) exhibit type IV
isotherms with a H1 hysteresis loop characteristic of structured
materials of the SBA-15 type.
0
0
Rh –Na–Al–SBA [33] and Rh –SBA–NH are also consistent with
3
0
the presence of both molecules. Analysis of Rh –SBA–NH is similar
3
0
to that of Rh –Na–Al–SBA sample (Fig. 1).
Modification of the isotherms of the SBA-15-based solids upon
rhodium incorporation (Fig. 2a) are related to a decrease of the
specific surface area and an increase of the average pore diameter
(Table 2) during the catalyst preparation. As proposed by Galarneau
et al. [49], such modifications can be attributed to a partial solu-
bilisation of the walls thus inducing the loss of the microporous
component of the total surface area and an increase of the pore
The derivative of the thermogravimetric curve shows four peaks,
◦
at 65, 250, 350 and 540 C, respectively. The first one corresponds
to the desorption of physisorbed water. Others are associated to
the decomposition of HEA16Cl and a residue of Pluronic P123 not
◦
eliminated by calcination at 550 C [47,48]. In both cases, the ove-
rall percentage loss (physisorbed water not included) determined
by TG analysis is about 8–9%. Elemental analysis would suggest
the presence of 5 wt.% of HEA16Cl (based on nitrogen amount)
and about 2% of Pluronic P123. Nitrogen loading of all Rh-based
0
diameters. Textural properties of Rh –Na–AlSBA and its corres-
ponding blank material are not remarkably different from those
of their parent, Na–AlSBA-15. A slight increase of specific surface
−1
0
solids is 0.2 wt.% (0.14 mmol g ) whereas theoretical one is 0.24%
area and pore volume was observed in the case of Rh –Na–AlSBA.
−1
(
0.18 mmol g ). Actually, it can be concluded from elemental anal-
This sample is characterized by a deformation of its hysteresis loop
0
ysis of the different solids recovered after the reduction step, that
c.a. 80% of the HEA16Cl contacted with the supports is adsorbed.
in comparaison with those of Na–Al–SBA-15 and Rh –Na–AlSBA(B)
(Fig. 2b).

M. Boutros et al. / Applied Catalysis A: General 394 (2011) 158–165
161
b
1000
a
900
9
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
00
0
8
7
6
5
4
3
2
1
00
00
00
00
00
00
00
00
0
0
0.2
0.4
0.6
0.8
1
0
0.2
0.4
0.6
0.8
1
P/P0
P/P0
6
00
00
00
00
00
00
0
c
5
4
3
2
1
0
0.2
0.4
0.6
0.8
1
P/P0
◦
0
Fig. 2. N2 adsorption–desorption isotherms of Rh(0) containing mesoporous materials and their supports at −196 C. (a) (–) SBA-15, (ꢀ) Rh –SBA–NH3 (B) and (×)
0
0
0
0
0
Rh –SBA–NH3 (b) (–) Na–Al–SBA-15, (ꢀ) Rh –Na–AlSBA (B) and (×) Rh –Na–AlSBA. (c) (–) CMK-3, (ꢀ) Rh –CMK–NH3 (B) and (×) Rh –CMK–NH3.
Isotherms of CMK-3 support and CMK-3-based samples (Fig. 2c)
is of type IV with a H2 hysteresis loop characteristic of this type of
3.1.2. TEM study
mesoporous solids [39]. According to Fig. 2c and Table 2, CMK-3 and
Transmission electron microscopy analyses allowed to ver-
ify that pore channels of SBA-15 and CMK-3-based samples
are still structured after NH3 treatment. The absence of sta-
bilizing agent molecules leads to a poor dispersion of Rh(0)
in SBA-15 and Na–Al–SBA-15-based materials. As shown in
Fig. 3b and d, significant amounts of Rh(0) nanoparticles are
formed on the external surface of the SBA-15 and Al–SBA-
15 solids. Rhodium is present as large particles (average
0
Rh –CMK–NH (B) samples have similar textural properties. How-
3
0
ever, Rh –CMK–NH , prepared with HEA16Cl, is characterized by
3
lower surface area and pore volume. The important modification
0
of the textural properties during the synthesis of Rh –CMK–NH3
cannot be explained only by the incorporation of the metallic
nanoparticles. It could also be related to a filling up or a blocking of
some pores by HEA16Cl molecules.
Table 3
Reactant distribution vs time in the catalytic hydrogenation of styrene with Rh(0) containing heterogeneous catalysts.
Samples
Rh (wt %)
3 h
6 h
Styrene conv. (%) [T.O.N.]a
Sel. (%)
EB
Styrene conv. (%) [T.O.N.]
Sel. (%)
EB
EC
EC
0
Rh –Na–AlSBA
0.90
0.40
100[400]
100[256]
0
48
100
52
100[400]
100[400]
0
0
100
100
0
Rh –Na–AlSBA (B)
0
Rh –SBA–NH3
0.55
0.15
100[202]
100[229]
66
57
34
43
100[400]
100[400]
0
0
100
100
0
Rh –SBA–NH3 (B)
0
Rh –CMK–NH3
0.96
0.58
n.d.
100[229]
100[310]
100[277]
57
30
41
43
70
59
100[355]
100[400]
100[394]
15
0
2
85
100
98
0
Rh –CMK–NH3 (B)
0
b
Rh –CMK–NH3 (B)
0
c
Rh /SiO2
1
1
5
99[174]
97[118]
0[0]
75
93
–
25
7
–
99[243]
99[144]
98[146]
51.5
85
84
48.5
15
16
0
c
Rh /C
Commercial Rh /C
0
d
◦
Conditions. Catalyst (50 mg), hexane (10 mL), T = 25 C, P(H2) = 0.1 MPa, substrate/Rh = 100.
a
T.O.N. expressed as number of mol of H2 consumed per number of mol of Rh.
Recycled catalyst.
b
c
Samples synthesized by incipient wetness impregnation of SiO2 (Aerosil 200) or activated carbon (Bioinvest) by aqueous RhCl3 followed by a reduction step treatment
◦
under H2 at 220 C.
d
Supplied by Aldrich.

1
62
M. Boutros et al. / Applied Catalysis A: General 394 (2011) 158–165
0
0
0
0
0
0
Fig. 3. TEM images of (a) Rh –SBA–NH3, (b) Rh –SBA–NH3 (B), (c) Rh –Na–AlSBA, (d) Rh –Na–AlSBA (B), (e) Rh –CMK–NH3 and (f) Rh –CMK–NH3 (B) samples.
0
diameter = 7 nm for Rh –SBA–NH (B), Fig. 3b) or aggregates
obtained on CMK-3. Histograms of the silica-based materials show
3
0
(
Fig. 3b and d).
a narrow size distribution centred at 3–4 nm for Rh –Na–AlSBA
0
The use of HEA16Cl induces the presence of larger amounts
of small nanoparticles located inside the mesopore channels
average diameter = 5 and 3 nm for Rh –SBA–NH3 and
Rh –Na–Al–SBA, respectively, Fig. 3a and c). Small aggregates
on the external surface (not shown) are still observed but to a
lesser extent than in the absence of HEA16Cl. In general, Rh(0)
particles formed on CMK-3 solids (Fig. 3e and f) are smaller
than those observed on the two other supports (Fig. 3a–d). Size
distribution histograms of the particles in samples prepared with
HEA16Cl are presented in Fig. 4. Clearly, the best metal dispersion is
and a broader one centred at 6–7 nm for Rh –SBA–NH3. In the case
0
0
of Rh –CMK–NH3 and Rh –SBA–NH3, the mean diameters of the
particles are very close to the pore aperture of the support, i.e., 3
and 6 nm, respectively.
0
(
0
In conclusion, TEM studies have shown that the use of HEA16Cl
facilitates the preparation of well-dispersed Rh(0) nanoparticles.
Indeniably, the size of the nanoparticles prepared with similar
NaBH4/Rh ratio is influenced by the pore aperture of the meso-
porous supports (6.7, 3.4 and 7.9 nm for SBA-15, CMK-3 and
Na–Al–SBA-15, respectively). However, the average size of the

M. Boutros et al. / Applied Catalysis A: General 394 (2011) 158–165
163
a
b
c
35,0
H2
3H2
0
0
3
2
2
1
1
0,0
5,0
0,0
5,0
0,0
Rh /support
Rh /support
Styrene
Ethylbenzene(EB)
Ethylcyclohexane (EC)
Scheme 1.
0
chemicals. We previously showed that Rh –Na–AlSBA efficiently
catalyze the hydrogenation of various aromatic substrates (styrene,
anisole, toluene, m-xylene and tetralin) at room temperature at
atmospheric pressure of dihydrogen [33]. The aim of the present
study is to look at the influence of the support (Na–Al–SBA-15,
SBA-15 and CMK-3) as well as of the use of HEA16Cl. Comparative
tests were performed with styrene (Table 3) and diphenylmethane
5
0
,0
,0
1
1
1
2
2
2
3
3
3
4
4
4
5
5
5
6
7
8
8
8
9
10
10
10
11
11
11
Size (nm)
(Table 4).
The reaction was carried out with similar substrate/Rh molar
ratio (1 0 0) in hexane. Such conditions correspond to those used
in the case of non supported Rh(0) colloids stabilized by HEA16Cl
70,0
6
5
4
3
2
1
0,0
0,0
0,0
0,0
0,0
0,0
[
36]. Whatever the catalyst considered, the external double bond
of styrene was more easily reduced than the aromatic ring of the
molecules (Scheme 1). Ethylcyclohexane (EC) and the intermediate,
ethylbenzene (EB), are the only products obtained.
All the Rh(0) containing mesoporous materials turned out
to be much more active than either commercial Rh /C or
conventionally synthesized reference catalysts (Rh /SiO , Rh /C).
Excepted for Rh –CMK–NH3 sample, complete conversion of
styrene into ethylcyclohexane was reached in less than 6 h. Most
active catalysts are Rh –Na–AlSBA and Rh –CMK–NH (B). The effi-
ciency of SBA-15-based solids does not depend on the presence or
absence of HEA16Cl. The quite low activity of Rh –SBA–NH and
Rh –SBA–NH3 (B) can be understood as the result of the presence
of a lot of aggregates on the external surface as well as particles
with larger average sizes in these samples.
Differences between HEA16Cl-containing samples and blank
ones are more significant but contradictory in the cases of
Na–Al–SBA-15 and CMK-3 supports. With Rh –CMK–NH , ethyl-
cyclohexane yield was 85% after 6 h. The lower activity of
0
0
0
2
0
0
0
3
0
,0
6
7
9
0
3
Size (nm)
0
45,0
4
3
3
2
2
1
1
0,0
5,0
0,0
5,0
0,0
5,0
0,0
0
3
0
0
Rh –CMK–NH compared to that of Rh –CMK–NH (B) can be inter-
3
3
preted by the steric hindrance induced by HEA16Cl molecules
surrounding Rh(0) nanoparticles in a limited volume space (Dp
of CMK-3 is 3.4 nm). Indeed, it has to be noted that the total
pore volume of CMK-3 has been strongly decreased (by ≈20%)
when HEA16Cl was used. Variations were weaker for Na–Al–SBA-
5
0
,0
,0
0
1
5 supports. As the result, lower performance of Rh –CMK–NH
3
0
compared toRh –Na–AlSBAcould beexplained by the low diffusion
rate of styrene molecules to the surface of Rh(0) nanoparticles.
6
7
9
Size (nm)
0
Previously, it was shown that the activity of Rh –Na–AlSBA
0
is not affected significantly over three successive tests [33]. In
Fig. 4. Nanoparticles size distribution histograms in (a) Rh –SBA–NH3, (b)
Rh –Na–AlSBA and (c) Rh –CMK–NH3 samples.
0
0
the present study, despite a little bit slower reaction kinetic,
0
Rh –CMK–NH (B) recovered from the first test was shown to be
3
still efficient in a second run (Table 3) carried out under similar
conditions (ethylcyclohexane yield = 98% instead of 100% after 6 h).
The rate decrease observed is probably due to partial leaching of
rhodium.
colloids formed in Na–Al–SBA-15 is smaller than the average meso-
pore diameter which implies that a greater control is exercised by
HEA16Cl on this support.
0
0
Given that Rh –Na–AlSBA and Rh –CMK–NH samples are
3
3
.1.3. Catalytic performance of the materials
characterized by similar dispersions and particles sizes, it can be
concluded that differences in the catalytic activity observed are
related to the average pore diameters of the supports. Changes in
the nature of the interaction of HEA16Cl with the surfaces of the
The conversion of aromatic cycles into cyclohexyl groups is
a goal of important interest both at the industrial or laboratory
scale [50–52]. Formation of cyclohexane from benzene for adipic
acid synthesis or removal of aromatic derivatives from fuels are
mainly performed in the presence of Ni and Pt-based catalysts
under harsch conditions [53]. Ru or Rh based materials represent
convenient alternatives [52,54–58]. They can work under milder
conditions and are particularly adapted to the synthesis of fine
support and of the metal particles could also be mentioned.
0
Catalytic
performances
of
Rh –Na–AlSBA
and
0
Rh –CMK–NH (B) samples are a little bit lower than those of
3
the Rh(0) colloids stabilized by HEA16Cl [33,36]. Comparisons
with other heterogeneous catalytic systems based either on
Rh(0) nanoparticles or Rh complexes are not easy. Reported

1
64
M. Boutros et al. / Applied Catalysis A: General 394 (2011) 158–165
Table 4
Reactant distribution vs time in the catalytic hydrogenation of diphenylmethane with Rh(0) containing heterogeneous catalysts.
Samples
Rh (wt.%)
6 h
24 h
DPM conv. (%) [T.O.N.]a
Sel. (%)
CPM
DPM conv. (%) [T.O.N.]
Sél. (%)
CPM
DCM
DCM
0
Rh –Na–AlSBA
0.90
0.40
51[195]
13[39]
72.5
100
27.5
0
99[588]
46[180]
2
69.5
98
30.5
0
Rh –Na–AlSBA (B)
0
Rh –CMK–NH3
0.96
0.58
48[162]
67[237]
87.5
82
12.5
18
93[351]
100[600]
74
0
26
100
0
Rh –CMK–NH3 (B)
0
b
Rh /SiO2
1
1
5
27[90]
2[6]
0[0]
89
100
0
1
0
0
86[324]
15[51]
5[18]
74
87
80
26
13
20
0
c
Rh /C
Commercial Rh /C
0
c
◦
Conditions. Catalyst (50 mg), hexane (10 mL), T = 25 C, P(H2) = 0.1 MPa, substrate/Rh = 100.
a
T.O.N. expressed as number of mol of H2 consumed per number of mol of Rh.
b
◦
Samples synthesized by incipient wetness impregnation of SiO2 (aerosil 200) or activated carbon (Bioinvest) by RhCl3·xH2O followed by reduction under H2 at 220 C.
c
Aldrich.
0
0
Rh/support
Rh/support
Diphenylmethane
DPM)
Cyclohexylphenylmethane
(CPM)
Dicyclohexylmethane
(DCM)
(
Scheme 2.
experiments are often performed at higher H2 pressure (0.5 MPa)
or at higher temperature (60 C). Solvents are not always the
described protocol involving Na–Al–SBA-15 and HEACl as a stabi-
lizing agent [33]. In this study, it is clear that, whatever the support
(silica, aluminosilica or carbon), prior adsorption of the quaternary
ammonium salt then facilitates that of rhodium (III). According to
elemental and thermogravimetric analyses, nearly 80% of the ini-
tial amount of the stabilizing agent is retained by the solids. The
increase in adsorption capacity of the resulting materials is a priori
related to admicellisation phenomena. These however seem to vary
from one sample to another. In all cases, the nature of the support
is the determining factor. Indeed, adsorption capacities with and
without HEA16Cl are classified in the same order.
Generally, metal nanoparticles prepared in the presence of
HEA16Cl are smaller and more dispersed at least on the silicic and
aluminosilicic materials of the SBA-15 type. In the case of CMK-
3, particle size is controlled by the average pore diameter and
HEA16Cl present on the solid reduces the catalytic activity. Never-
theless, all Rh(0)-based materials prepared in this study are much
better catalysts in the hydrogenation of aromatic rings than com-
mercial analogues tested under the same conditions (PH2 = 0.1 MPa
and ambient temperature). Further studies of the Na–Al–SBA-
15/HEA16Cl system are in progress in order to better understand
interactions of HEA16Cl with Na–Al–SBA-15 as well as to take
advantage of the latter to develop new selectivities.
◦
same. Buil et al. [54] recently described the preparation of 2 wt.%
Rh(0)/Al O from the reduction of a Rh(I) complex in the presence
2
3
of Al O . Hydrogenation of the exocyclic double bond was very
2
3
efficient but, despite less difficult catalysis test conditions (Sub-
◦
strate/Rh = 50, T = 60 C), such material was not able to allow the
complete hydrogenation of styrene into ethylcyclohexane within
4
h (T.O.N. = 70).
Hydrogenation of diphenylmethane (DPM), into dicyclohexyl-
methane (DCM) (Scheme 2) was also investigated under similar
conditions. Na–Al–SBA-15 and CMK-3-based solids were tested in
0
0
0
comparison with Rh /SiO , Rh /C and commercial Rh /C. What-
2
ever the test considered, cyclohexylphenylmethane (CPM) was
observed as a reaction intermediate.
Diphenymethane is a more demanding substrate as corrobo-
rated by the low conversion of DPM after 24 h in the presence of
0
the reference materials and Rh –Na–AlSBA(B) (Table 4).
Total or quasi-total conversion of DPM into DCM was observed
with the best catalysts for styrene hydrogenation. Actually, perfor-
0
0
mances of Rh –Na–AlSBA and Rh –CMK–NH (B) samples compare
3
well with those of other reported catalytic materials. For exam-
ple, Maegawa et al. [52] recently described the use of Rh(0)/C
as a catalyst to get DCM from DPM. It has to be noted that best
◦
conditions (isopropanol as the solvent, T = 60 C, P = 0.5 MPa and a
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