A surfactant-assisted preparation of well dispersed rhodium nanoparticles within the mesopores of AlSBA-15: Characterization and use in catalysis

Article abstract of DOI:10.1039/b802548g

Well dispersed and efficient Rh(0) hydrogenation catalysts were obtained by the reduction of Rh(iii)-exchanged mesoporous aluminosilicates by sodium borohydride in the presence of N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride. The Royal Society of Chemistry.

Full text of DOI:10.1039/b802548g

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A surfactant-assisted preparation of well dispersed rhodium nanoparticles
within the mesopores of AlSBA-15: characterization and use in
catalysiswz
c
c
d
Maya Boutros,ab Audrey Denicourt-Nowicki, Alain Roucoux, Leon Gengembre,
´
Patricia Beaunier, Antoine Gedeon and Franck Launay*ab
e
ab
´
´
Received (in Cambridge, UK) 14th February 2008, Accepted 10th March 2008
First published as an Advance Article on the web 15th April 2008
DOI: 10.1039/b802548g
Well dispersed and efficient Rh(0) hydrogenation catalysts were
obtained by the reduction of Rh(III)-exchanged mesoporous
aluminosilicates by sodium borohydride in the presence of
N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride.
particles are obtained by the reduction of Rh(III)-exchanged
mesoporous aluminosilicates (AlSBA-15) by sodium borohy-
dride in the presence of a water soluble stabilizing agent, N,N-
dimethyl-N-cetyl-N-(2-hydroxyethyl) ammonium chloride
(HEA16Cl).6 Ligands are usually used to confine nanoparticles
in mesoporous solids12 but, to our knowledge, examples of our
strategy in the literature only concern montmorillonite or iron
oxides.13 The hydrogenation activity of the new heterogeneous
catalysts towards aromatic rings is compared with that of the
previously reported HEA16Cl-stabilized aqueous dispersions of
the same particles (Rh0/HEA16Cl).6 This approach could be an
alternative way to investigate heteroaromatic cycle reduction
and has potential applications for water remediation like the
denaturation of aromatic pollutants such as atrazine.14
Controlled nanometre-sized metal species have attracted great
attention in the field of catalysis because of their unique
chemical and physical properties.1 Particles hosted in porous
supports are mainly prepared by the decomposition of organo-
metallic compounds2 or the reduction of metallic salts.3 In the
latter case, impregnation methods are limited by the tendency
of the metal to agglomerate on the external surface. In parallel,
the control of the size of metallic particles by the use of
protective agents in solution is now well established.4 Among
noble metal nanoparticles, dispersions of rhodium(0) colloids
are known to be efficient hydrogenation catalysts.5,6 Up to now,
most of the methods used for the heterogenization of Rh(0)
colloids consist in the impregnation of solids like polymers,7
silica8 and alumina.9 Attempts to take advantage of the high
specific surface area of mesostructured supports have also been
described. The nanoparticles are either deposited onto pre-
formed materials10 or included in their synthesis gel.11 How-
ever, these approaches can be handicapped by the relatively low
amounts of metal introduced and the difficulty of inserting the
particles homogeneously within the pore network.
The nominal NaBH4/Rh and HEA16Cl/Rh molar ratios
(2.5 and 2, respectively) tested are rigorously the same as in the
optimized preparation of aqueous 2–3 nm Rh(0) colloids. In
this first attempt, we have considered that the dispersion of the
Rh(0) particles nucleated on the support should be improved
by the anchorage of the rhodium(III) ion precursors prior to
their reduction. Cation exchange on a H-AlSBA-15 type
material or its sodium form, Na-AlSBA-15, has been tested
in the present work. Two routes based on simultaneous (route
I) or distinct (route II) additions of rhodium(III) chloride and
NaBH4 were proposed (Scheme 1). Except for a blank
test (Rh0/Na-AlSBA(IIB)), the support (H-AlSBA-15 or
Na-AlSBA-15) was first impregnated with HEA16Cl for
24 h. The molar Si/Al ratio of the aluminosilicates tested
was about 17 and the maximum sodium uptake from the
NaCl solution should correspond to 0.07 mol of Na per 100 g
(1.6 wt%). In our conditions, the sodium loading was half
what was expected (0.80% or 0.035 mol Na per 100 g) but was
still enough in order to incorporate 1 wt% of Rh(III) (0.01 mol
Rh per 100 g) on the basis of Rh3+/3 Na+ exchanges.
An alternative solution would imply a stabilizing agent-
assisted reduction of rhodium salts directly on the surface of
the support. In this study, we show that well dispersed Rh(0)
a
`
UPMC Univ Paris 06, UMR 7142, Systemes Interfaciaux a
l’Echelle Nanome´trique, F-75005 Paris, France
`
b
`
`
CNRS, UMR 7142, Systemes Interfaciaux a l’Echelle
´
Nanometrique, F-75005 Paris, France.
E-mail: franck.launay@upmc.fr; Fax: +33 1 44 27 55 36;
Tel: +33 1 44 27 59 38
c
d
e
Elemental analysis results show that the rhodium content of
the solid obtained from Na-AlSBA-15 and HEA16Cl (Rh0/
Na-AlSBA(II)) is 0.90 wt%. This value is twice those of Rh0/
Na-AlSBA(IIB) (0.40%) and Rh0/H-AlSBA(II) (0.44%).
Clearly, the combined use of the cationic stabilizing agent
and of a sodium-exchanged support facilitates the adsorption
of Rh(III) on the surface. Moreover, rhodium(III) ions and
NaBH4 have to be added separately (route II). Indeed, 90%
of the rhodium introduced is retained in the Rh0/Na-AlSBA(II)
solid as against 73% for Rh0/Na-AlSBA(I). Protons or sodium
ions of the support are probably exchanged by HEA16 ca-
tions.15 The resulting solids covered by interfacial aggregates
´
Equipe Chimie Organique et Supramoleculaire, CNRS-UMR 6226
‘‘Sciences chimiques de Rennes’’, Ecole Nationale Superieure de
´
Chimie de Rennes, Avenue du Gal Leclerc, F-35700 Rennes, France
´
Unite de Catalyse et de Chimie du Solide, CNRS-UMR 8181,
Universite des Sciences et Technologies de Lille, Bat. C3, F-59655
´
ˆ
Villeneuve d’Ascq, France
UPMC Univ Paris 06, UMR 7609, Laboratoire de Reactivite de
Surface, F-75005 Paris, France
´
´
w Electronic supplementary information (ESI) available: Experimental
section; physicochemical properties of the different Rh0/AlSBA-15
samples; TEM images of Rh0/H-AlSBA(II); XPS and thermogravi-
metric analysis of Rh0/Na-AlSBA(II); results of styrene hydrogena-
tion. See DOI: 10.1039/b802548g
z In memory of Sir D.H.R. Barton on the 10th anniversary of his
death.
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Scheme 1 Synthesis routes used for surfactant-assisted preparation
of Rh0/SBA-15 solids.
Fig.
1
TEM images of (a) Rh0/Na-AlSBA(I), (b) Rh0/Na-
AlSBA(IIB), (c) Rh0/Na-AlSBA(II) and of a microtome section of
(d) Rh0/Na-AlSBA(II).
composed of the adsorbed surfactant molecules with their
cationic head-groups oriented toward the centre of the pores
(e.g. ‘‘reversed’’ monolayered structures) could be involved in
the ‘‘adsolubilization’’ of Rh3+. Then, the uptake of the
metallic cations would proceed via electrostatic interaction with
chloride anions of HEA16Cl during the 2 h stirring period
before reduction. As expected, the hexagonal pore structure of
the parent supports is retained in the new materials. The
textural properties of the resulting solids are not markedly
different from those of their parents (H or Na-AlSBA-15) and
are not very helpful in discriminating the location of the
particles due to the relatively low Rh contents (o1 wt%).
Experimental procedures have also dramatic effects on the
dispersions of metal particles as shown by transmission electron
microscopy experiments. Direct examination of the materials
prepared from Na-AlSBA-15 (Fig. 1a–c) clearly revealed differ-
ences in the size and location of the metal related to the effect of
the addition time of the rhodium salt (Rh0/Na-AlSBA(I), Rh0/
Na-AlSBA(II)) and of the prelimirary impregnation of the solid
by HEA16Cl (Rh0/Na-AlSBA(IIB)). The simultaneous addition
of Rh(III) and NaBH4 leads to a very poor particle dispersion
(Rh0/Na-AlSBA(I), Fig. 1a). The delayed introduction of the
reductant turned out to be more appropriate. Dispersed rho-
dium(0) particles mainly located inside the pore channels (Fig. 1c)
are obtained in the case of Rh0/Na-AlSBA(II), obviously as the
result of the use of HEA16Cl and of a period of 2 h between the
introduction of Rh(III) ions and of the reductant into the solution.
Incontestably, HEA16Cl is helpful in locating the particles inside
the pores. In its absence, rhodium aggregates are formed on the
external surface of the solid (Rh0/Na-AlSBA(IIB), Fig. 1b). The
mean diameter of the colloids is smaller than the pore aperture, in
agreement with their surfactant-assisted formation. Metal parti-
cles in Rh0/H-AlSBA(II) (see ESI) and Rh0/Na-AlSBA(II) sam-
ples are similar in size but large amounts of aggregates are also
produced in the solids prepared from H-AlSBA-15.
by X-ray photoelectron spectroscopy indicates that the photo-
peaks related to the Rh 3d core level, i.e. Rh 3d3/2
(312.7 eV) and Rh 3d5/2 (307.5 eV vs. C 1s (285 eV)) are slightly
shifted to higher energies compared to those of Rh(0). XPS
measurements also point out significant amounts of organic
materials. HEA16Cl has been identified through the N 1s and C
1s signals at 403.3 and 285 eV, respectively. Values of the molar
C/N ratios determined by elemental analyses for Rh0/Na-
AlSBA(I), Rh0/Na-AlSBA(II) and Rh0/H-AlSBA(II) are be-
tween 24 and 29, in agreement with the presence of HEA16
cations (expected C/N ratio = 20) and traces of Pluronic P-123.
The more HEA16Cl is retained, the more rhodium is present.
From the nitrogen weight percentages, it can be estimated that
60 to 80% of the initial amount of HEA16Cl is recovered on the
solid. Thermogravimetric analyses are also consistent with the
presence of both molecules (weight loss between 473 and 823
K). In the case of Rh0/Na-AlSBA(II), HEA16Cl and Pluronic
P-123 account for a 9% weight loss. Such information is
coherent with the ‘‘adsolubilization’’ of rhodium and the
involvement of these molecules in the stabilization of the
particles prepared by the heterogeneous nucleation mode.
TEM images of thin microtome sections of Rh0/Na-AlS-
BA(II) (Fig. 1d) confirm the good dispersion of the Rh(0)
particles. The size distribution histogram indicates that the
mean diameter is 2.7 nm (Fig. 2) near to that of Rh(0) particles
prepared in water using the same ratios of stabilizing agent and
reductant to Rh. Like in solution, HEA16Cl is involved in the
control of the particle size. The analysis of Rh0/Na-AlSBA(II)
Fig. 2 Size distribution histograms of the Rh0/HEA16Cl colloids and
of Rh0/Na-AlSBA(II) (behind).
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Table 1 Hydrogenation of arenes over Rh0/Na-AlSBA(II) and Rh0/Na-AlSBA(IIB)
Entry
Substrate
Product(s) (%)
Yields after 3 h (%) (time for completion/h)
Completion time/h
Rh0/Na-AlSBA(II)
Rh0/Na-AlSBA(IIB)
Rh0/HEA16Cl colloids
1
Styrene
Ethylbenzene (EB)
Ethylcyclohexane (EC)
Methoxycyclohexane
Methylcyclohexane
1,3-Dimethylcyclohexanea
Decalinb
0 (3)
100
100 (3)
70 (6)
56 (6)
32 (24)
48 (6)
52
66 (6)
—
—
(4)
2
3
4
5
Anisole
Toluene
m-Xylene
Tetralin
(3.6)
(3.6)
(4.3)
—
0 (424)
a
b
Conditions: catalyst (50 mg); hexane (10 mL); Substrate : Rh (100 : 1); 298 K, PH2 0.1 MPa. cis : trans, 80 : 20. cis : trans, 87 : 13.
The AlSBA-15-supported Rh(0) particles were tested as
catalysts in the hydrogenation of styrene at atmospheric
pressure of dihydrogen. The reaction was carried out with
1 : 100 ratio of Rh/substrate at 298 K in hexane. Under these
conditions, the activity of the catalysts changes in the follow-
ing order: Rh0/Na-AlSBA(II) E Rh0/H-AlSBA(II) 4 Rh0/
Na-AlSBA(I) 4 Rh0/Na-AlSBA(IIB). Preliminary investiga-
tions of the recycling showed that the activity of Rh0/
Na-AlSBA(II) is not significantly affected over three successive
tests. The activity of Rh0/Na-AlSBA(II) towards styrene and
anisole (Table 1, entries 1 and 2) also turned out to be higher
than that of the aqueous dispersion of Rh0/HEA16Cl colloids
tested under two-phase conditions. In fact, the complete
conversion of anisole into methoxycyclohexane is performed
in a shorter time (3 h) than over commercially available
heterogeneous Rh(0) catalysts (Rh0/C or Rh0/Al2O3).16
Other substrates, i.e. the mono (entry 3) and dialkylarene
derivatives (entries 4,5), are less easily hydrogenated over Rh0/
Na-AlSBA(II). Cis isomers are the major products as usually
observed with heterogeneous systems. However, cis-1,3-dimethyl-
cyclohexane is formed to a lower extent than in solution (cis :
trans, 90 : 10). Differences between the activities and selectivities
of Rh0/Na (or H)-AlSBA(II) and Rh0/HEA16Cl colloids could
be reasonably attributed to an effect of the support, thus illustrat-
ing the heterogeneous nature of the catalysts. Moreover, leaching
of the HEA16Cl-stabilized particles into the hexane phase cannot
be reasonably envisaged, due to the difficulties encountered in the
extraction of the aqueous colloids by organic solvents.
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AlSBA-15 using HEA16Cl as an electrosteric stabilizing agent
and NaBH4 as a reductant allows better control of the final
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This journal is The Royal Society of Chemistry 2008
2922 | Chem. Commun., 2008, 2920–2922