Rh(0) colloids supported on TiO2: A highly active and pertinent tandem in neat water for the hydrogenation of aromatics

Article abstract of DOI:10.1039/c0gc00931h

TiO₂-supported Rh(0) nanoparticles were easily prepared in one step without calcination by a room temperature impregnation of the inorganic support with a prestabilized colloidal Rh(0) suspension. They are highly active and reusable catalysts for the hydrogenation of aromatics and chloroanisole derivatives in neat water with TOFs up to 33000 h^-1. The comparison with the analogous silica system Rh@SiO₂ was discussed showing higher catalytic selectivities and activities with Rh(0) colloids supported on TiO₂.

Full text of DOI:10.1039/c0gc00931h

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PAPER
Rh(0) colloids supported on TiO2: a highly active and pertinent tandem in
neat water for the hydrogenation of aromatics
Claudie Hubert,a,b Elodie Guyonnet Bile´,a,b Audrey Denicourt-Nowicki*a,b and Alain Roucoux*a,b
Received 20th December 2010, Accepted 15th March 2011
DOI: 10.1039/c0gc00931h
TiO2-supported Rh(0) nanoparticles were easily prepared in one step without calcination by a
room temperature impregnation of the inorganic support with a prestabilized colloidal Rh(0)
suspension. They are highly active and reusable catalysts for the hydrogenation of aromatics and
chloroanisole derivatives in neat water with TOFs up to 33000 h-1. The comparison with the
analogous silica system Rh@SiO2 was discussed showing higher catalytic selectivities and
activities with Rh(0) colloids supported on TiO2.
been reported in the literature, being divided into physical (e.g.
Introduction
sonication, microwaves, UV) or chemical (e.g. electrochemical,
Nanometer-sized transition metal particles have been intensively
impregnation, fluidized bed) routes.16,18 Classical methods for
pursued as potentially pertinent catalysts as their particu-
the preparation of supported nanocatalysts usually involve
lar properties lie between mononuclear metal complexes and
repeated oxidation and reduction steps under severe conditions.
heterogeneous bulk catalysts.1,2 Their large surface area-to-
However, in the drive towards environment-friendly processes,
volume ratio, which increases when decreasing particle size,
the synthesis of supported metal particles should be promoted
could greatly increase their specific catalytic activity.3–5 In
in a more sustainable way with room-temperature aqueous
that context, the control of the structural parameters such as
solution protocols, improving manufacturing safety as well
size, composition and morphology is of crucial importance
as decreasing production costs.19 Moreover, nanomaterials-
to tune their catalytic activities and selectivities.6,7 The most
catalyzed transformations in an aqueous reaction media remain
usual approach for the particle synthesis relies on the chemical
one of the most ideal solutions for the development of green
reduction of metallic precursors (organometallic complexes or
and sustainable protocols.20 In that context, we developed a
metal salts) in the presence of protective agents such as polymers,
clean methodology in mild conditions for the preparation of
ligands, surfactants, ionic liquids according to the nature of the
SiO2-supported metal nanoparticles and their investigation in
reaction media.8–11 Among the various catalytic applications,
arene hydrogenation in neat water. Our approach is based on the
metallic nanospecies have particularly proved to be pertinent cat-
wet21 or dry22,23 (fluidized bed) impregnation onto silica of pre-
alysts for arene hydrogenation in mild conditions.12–14 However,
stabilized colloidal suspensions, which were well-characterized
particle aggregation and catalyst deactivation could occur dur-
in terms of size, distribution and organization.24 More recently,
ing catalytic reactions with particle suspensions and the catalyst
this relevant approach was extended to the use of TiO2 anatase
separation from the reaction products, which represents a crucial
as a new support and preliminary investigation with TiO2-
parameter in the search for clean processes, could sometimes be
supported Rh(0) particles proved to be highly active catalysts
problematic. Among the approaches towards stabilizing metal
in model arenes hydrogenation.25 The choice of this support
nanoparticles for heterogeneous catalysis,15 their deposition on
was governed by its various advantages such as good stability,
solid supports (such as metal oxides, carbon or zeolites) provides
low cost, non-toxicity and photocatalytic properties.26 In this
a great scope for the development of highly active, robust and
paper, we describe the convenient preparation of Rh@TiO2
potentially recyclable catalysts.16 Supported nanocatalysts are
nanocatalyst and the further investigations in the hydrogenation
widely recognized as a mainstay of industrial catalysts, being
of functionalized or disubstituted arene compounds in neat
used in petrochemical industries and in the manufacturing of
water. Moreover, a comparison in terms of activities and
fine and speciality chemicals.17 Many preparation methods have
selectivities of Rh@TiO2 and Rh@SiO2 systems was performed
on various aromatic compounds.
aEcole Nationale Supe´rieure de Chimie de Rennes, CNRS, UMR 6226,
Results and discussion
Avenue du General Leclerc, CS 50837, 35708, Rennes Cedex 7, France.
E-mail: Alain.Roucoux@ensc-rennes.fr; Fax: +33 (0)2 23 23 81 99;
In the drive towards a sustainable and green chemistry, we
have developed an easy and reproducible method, based on
Tel: +33 (0)2 23 23 80 37
bUniversite´ Europe´enne de Bretagne
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the simple impregnation of the anatase TiO2 support by a
pre-stabilized aqueous rhodium(0) suspension (Fig. 1).25 More
precisely, colloidal solutions of rhodium(0) were easily prepared,
as previously described, by chemical reduction of rhodium
chloride salts in the presence of N,N-dimethyl-N-cetyl-N-(2-
hydroxyethyl) ammonium chloride (HEA16Cl) surfactant as
protective agent, which prevents from particle aggregation
through electrosteric stabilization.24,27 Then, the addition of
the adequate amount of titania under vigorous stirring leads
to the adsorption of the Rh(0) nanospecies on the support,
which is evidenced by the color change from black to colorless.
After filtration and several water washings, the powder was
dried overnight at 60 ◦C, leading to titania-supported Rh(0)
nanoparticles as a fine grey powder. This one-step preparation
was performed at ambient temperature, under air without any
gas treatment or calcination step. The metal loading of the
nanomaterials was determined by ICP analysis as 0.09 wt%,
corresponding to the expected one.
Fig. 2 HRTEM micrographs a) Rh@TiO2 (Scale bar is 2 nm). b)
Magnification of an area showing the lattice planes of anatase. c) EDX
spectrum of Rh(0) particles proved by deconvolution analysis.
Fig. 1 One pot TiO2-supported Rh(0) nanoparticles preparation.
The titania-supported rhodium(0) nanoparticles system
was characterized by High Resolution Transmission Elec-
tron Microscopy (HRTEM) and Energy Dispersive X-ray
spectrometry (EDX) to identify the location and size of the
metallic nanospecies. The rhodium(0) particles, observed on the
micrograph 2a (zone contrasted), present an average size of 3–
4 nm, very close to anatase one. The analyses of the fast Fourier
transformation (FFT) of the HRTEM pictures mainly revealed
the interplanar distances of 0.352 and 0.233 nm corresponding
to the (101) and (112) lattice planes of anatase as shown on the
magnification area (Fig. 2b). However, the intereticular distances
of 0.2196 and 0.1902 nm, which are characteristic of the (111)
and (200) planes lattice of Rh(0) were not clearly observed. The
presence of Rh(0) particles was confirmed by EDX (Fig. 2c –
red line) by focusing the electron beam at about 10 nm size and
the deconvolution of the Rh signal (blue line) was calculated to
prove its presence. The Cl signal was clearly observed at 2.62 keV
near to the Rh signals at 2.696 and 2.834 keV. Finally, as shown
by these microtomy experiments of a cross section of titania
spheres, the surfactant-stabilized Rh(0) colloidal suspension is
well-diffused into the matrix.
The TiO2-supported nanomaterials were evaluated in the
catalytic hydrogenation of various aromatic compounds in
water under hydrogen pressure (1 or 30 bar H2) and at
room temperature. The conversion was determined by gas
chromatography analysis. The turnover frequencies (TOFs) were
calculated considering an optimized reaction time for complete
conversion of the substrate and based on the introduced amount
of metal and not on the exposed surface metal and thus may be
underestimated. As previously reported by Janiak et al.,28 the
catalytic activity results not only from the exposed surface metal
atoms since the surface can restructure, atoms can shift positions
during heterogeneous processes and partial aggregation could
occur during catalysis, modifying the fraction of surface atoms.
These changes in the surface render the determination of the
number of surface atoms difficult. The catalytic performances
of the Rh@TiO2 nanocatalyst in the hydrogenation of mono- or
di-alkylsubstitued and/or functionalized arene derivatives are
reported in Table 1.
First, the Rh@TiO2 catalyst is very active at atmospheric
pressure. In all cases, a total conversion of the substrate in
the totally hydrogenated compound was observed with quite
interesting TOFs ranging from 72 h-1 to 476 h-1. In the series
of benzene, toluene, o-xylene (entries 1, 3, 5), the increased
reaction time and decreased catalytic activity are typical of
the increasing steric hindrance of the alkyl groups, which
hamper the approach of the substrate towards the metallic
nanospecies. This could also be explained by the difference
of solubility of these arenes in water (benzene > toluene >
p-xylene) as reported by Matsumoto et al.29 In addition, the
hydrogenation of functionalized arene derivatives such as anisole
and ethylbenzoate (entries 11 and 13) was performed with
complete conversion. Moreover, in the case of anisole, the
formation of phenol or cyclohexanone as by-products was not
observed, contrary to the Rh(0) colloidal suspension which
gives a significant ratio of cyclohexanone (20–30%).30 The
hydrogenation of di-substituted benzene derivatives was also
performed with total conversion (entries 5, 7, 9). In all cases,
the selectivity of the reaction remained unchanged. Indeed, as
usually observed with pure heterogeneous catalysts22,31 and with
our classical aqueous Rh(0) suspension,13,27 the hydrogenation
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a
Table 1 Hydrogenation of non halogenated aromatic compounds under 1 or 30 bar H2
Entry
Substrate
Productsb (%)
P (bar H2)
t (h)
TOFc (h-1)
1
2
3
4
5
6
7
8
Benzene
Benzene
Toluene
Toluene
o-Xylene
o-Xylene
m-Xylene
m-Xylene
p-Xylene
p-Xylene
Anisole
Anisole
Ethylbenzoate
Ethylbenzoate
Diphenylmethane
Diphenylmethane
Naphtalene
Naphtalene
Pyridinee
Cyclohexane (100)
Cyclohexane (100)
Methylcyclohexane (100)
Methylcyclohexane (100)
1,2-Dimethylcyclohexane cis (90)/trans (10)
1,2-Dimethylcyclohexane cis (95)/trans (5)
1,3-Dimethylcyclohexane cis (80)/trans (20)
1,3-Dimethylcyclohexane cis (85)/trans (15)
1,4-Dimethylcyclohexane cis (64)/trans (36)
1,4-Dimethylcyclohexane cis (67)/trans (33)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Ethylcyclohexanoate (100)
Ethylcyclohexanoate (100)
Dicyclohexylmethane
Dicyclohexylmethane
Decalin cis (91)/trans (9)
Decalin cis (92)/trans (8)
1
30
1
30
1
30
1
30
1
30
1
30
1
30
1
0.7
0.01
1.5
0.03
4
0.25
4.6
0.42
4.6
0.42
1.8
0.07
2
0.09
20
3
24
3
20
3
476
33300
222
11100
83
1333
72
793
72
9
10
11
12
13
14
15
16
17
18
19
20
793
185
4761
167
3704
33d
30
1
30
1
222
23d
185d
17d
Piperidine
Piperidine
Pyridine
30
111d
a Reaction conditions: Rh@TiO2 (1 g, 0.09% wt, 8.8 ¥ 10-6 mol), substrate (9.6 ¥ 10-4 mol), H2O (10 mL), RT, stirred at 1700 min-1. b Determined
by Gas Chromatography analysis. c Turnover frequency defined as the number of moles of consumed H2 per mole of introduced rhodium per hour
and calculated considering 100% conversion of the substrate after an optimized time. d Underestimated value according to a total conversion but a
non-optimized time. e 50% conversion after 6 h.
of xylenes yields the kinetically favoured cis-product with a
ratio ranging from 65 to 90% according to the isomer. The
cis-product is also the major product with p-xylene but the
ratio of the trans-isomer increases in the same range as with
colloidal and supported systems (35 and 36% respectively).
The formation of the trans isomer, commonly observed as the
minor product, could be explained by a successive dissociation
and re-association phenomenon of a partially hydrogenated
intermediate on the catalyst surface. Whatever isomer of xylene
is considered, it can be noticed that the values of cis/trans
ratio are higher than unity and increase with hydrogen pressure,
suggesting that trans products are formed through a “roll-over”
mechanism.32 The selective hydrogenation was also carried out
with bicyclic arenes. The diphenylmethane was totally converted
into dicyclohexylmethane after 20 h (entry 15); the conversion
was complete after 3 h into a 1 : 1 mixture of mono-and di-
hydrogenated products. The hydrogenation of naphthalene was
observed with a total conversion after 24 h (non optimized time)
in the decalin isomers with a cis/trans ratio of 91/9 (entry 17),
which is quite consistent with the one observed in the literature
with Rh nanoparticles entrapped in aluminium oxyhydroxide
nanofibers.33 After 6 h, all the naphthalene was converted in a
mixture of tetralin (68%) and decalins (32%). It is interesting
to notice that the substrate did not strongly interact with the
particle surface, limiting the tetralin hydrogenation, as observed
with the Pt/TiO2 system described by Kurokawa et al.34 The
pyridine was also investigated as an heteroaromatic compound
and shows a total conversion into piperidine in 20 h (entry 19).
The increase in hydrogen pressure up to 30 bar H2 (entries 2, 4,
6, 8, 10, 12, 14) dramatically increases the catalytic activity, with
TOFs up to 33000 h-1 observed in the case of benzene. Finally,
the Rh@TiO2 catalyst is also more active on bicyclic substrates
such as diphenylmethane and naphthalene (entries 16 and 18)
and heteroaromatics such as pyridine (entry 20).
The selective hydrogenation is also possible for anthracene
(Scheme 1). Some results have been described in the litera-
ture with Rh(0) nanocatalysts.35,36 Due to the non-solubility
of the substrate in water, the reaction was performed in a
cyclohexane/water mixture. After 20 h, a yield of 87% was
observed and the three formed products were analyzed by
GC-MS: the one produced by the hydrogenation of the central
ring 2, the two side rings 3 and only one side ring 1.
Scheme 1 Anthracene hydrogenation with Rh@TiO2 catalyst.
The hydrogenation of chloroanisole isomers were also inves-
tigated in neat water under hydrogen pressure (1 or 30 bar H2)
and room temperature. The reaction leads to the formation of
two products, which were identified by Gas Chromatography
analysis: the expected methoxycyclohexane 1 and also small
traces of cyclohexanone 2. The results are gathered in Table 2.
First, in all cases, the substrate is totally converted in
methoxycyclohexane (95–98%) with TOF around 70–80 h-1 at
atmospheric pressure (entries 1, 3, 5). Very small traces (<5%)
of cyclohexanone were detected, compared with the results
obtained with classical colloidal Rh(0) suspensions which gave
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a
Table 2 Hydrogenation of chloroanisoles with Rh@TiO2
Entry Substrate
1/2b
P (bar H2) t (h) TOFc (h-1)
1
2
3
4
5
6
o-Chloroanisole
95/5
1
4.6
0.03 11000
83
0.04 8333
4.6 72
0.03 11000
72
o-Chloroanisole
m-Chloroanisole
m-Chloroanisole
p-Chloroanisole
p-Chloroanisole
100/0 30
96/4
100/0 30
98/2
100/0 30
1
4
1
a Reaction conditions : Rh@TiO2 (1 g, 0.09% wt, 8.8 ¥ 10-6 mol),
substrate (9.6 ¥ 10-4 mol), H2O (10 mL), RT, stirred at 1700 min-1.
b Determined by Gas Chromatography analysis. c Turnover frequency
defined as number of moles of consumed H2 per mole of introduced
rhodium per hour and calculated considering 100% conversion of the
substrate after an optimized time.
Fig. 3 Recycling experiments.
These results prove again the efficient adsorption of the metallic
nanospecies on the titania surface since a loss of catalytic activity
due to metal leaching is usually observed in the case of weak
interactions between the particles and the support. Moreover,
the absence of lixiviation was checked by elemental analysis of
the supported catalyst after hydrogenation, which presents a
same metal ratio (0.09%) after the recycling experiments.
Finally, the catalytic activities of Rh@TiO2 system were
compared to those of Rh@SiO2 catalyst prepared by the
same methodology in the hydrogenation of the same mono-
or di-substitued and/or functionalized arene derivatives at
room temperature and under atmospheric pressure in water
(Table 3). In all cases, the hydrogenation of aromatic com-
pounds proceeds with higher TOFs using the Rh@TiO2 catalyst
than the Rh@SiO2 one. In the case of xylenes (entries 5–
10), whatever the isomer is, the cis/trans ratio is higher with
a higher ratio of cyclohexanone.30 The increase in hydrogen
pressure to 30 bar H2 leads to a dramatic increase in catalytic
activities, with TOFs up to 11000 h-1, and in selectivity with
100% methoxycyclohexane (entries 2, 4, 6).
Recycling experiments were carried out with the Rh@TiO2
catalyst in the hydrogenation of typical substrates such as
toluene, anisole and o-xylene at room temperature and atmo-
spheric pressure in water (Fig. 3). After each run, the catalyst
was easily recovered by filtration and dried at 60 ◦C before
another run with new addition of the substrate. Three runs were
performed with no significant loss of activity. Moreover, we have
checked in the case of toluene the absence of catalytic activity
with the aqueous phase after filtration of the solid catalyst which
could be due to putative metallic species obtained by desorption.
a
Table 3 Hydrogenation of arene derivatives: Rh@TiO2 vs. Rh@SiO2
Entry
Catalyst
Substrate
Productsb (%)
t (h)
TOFc (h-1)
1
2
3
4
5
6
7
8
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Rh@TiO2
Rh@SiO2
Benzene
Benzene
Toluene
Toluene
Cyclohexane (100)
Cyclohexane (100)
Methylcyclohexane (100)
Methylcyclohexane (100)
1,2-Dimethylcyclohexane cis (90)/trans (10)
1,2-Dimethylcyclohexane cis (95)/trans (5)
1,3-Dimethylcyclohexane cis (80)/trans (20)
1,3-Dimethylcyclohexane cis (90)/trans (10)
1,4-Dimethylcyclohexane cis (64)/trans (36)
1,4-Dimethylcyclohexane cis (75)/trans (25)
Methoxycyclohexane (100)
Methoxycyclohexane (100)
Ethylcyclohexanoate (100)
Ethylcyclohexanoate (100)
Methoxycyclohexane (95)/Cyclohexanone (5)
Methoxycyclohexane (64)/Cyclohexanone (36)
Methoxycyclohexane (96)/Cyclohexanone (4)
Methoxycyclohexane (60)/Cyclohexanone (40)
Methoxycyclohexane (98)/Cyclohexanone (2)
Methoxycyclohexane (65)/Cyclohexanone (35)
0.7
3.8
1.5
2.5
4
4
4.6
4
4.6
4
1.8
3
476
113
222
149
83
93
72
93
72
93
185
124
167
162
72
68
83
75
72
68
o-Xylene
o-Xylene
m-Xylene
m-Xylene
9
p-Xylene
10
11
12
13
14
15
16
17
18
19
20
p-Xylene
Anisole
Anisole
Ethylbenzoate
Ethylbenzoate
o-Chloroanisole
o-Chloroanisole
m-Chloroanisole
m-Chloroanisole
p-Chloroanisole
p-Chloroanisole
2
2.3
0.7
5.5
1.5
5
4
5.5
a Reaction conditions : Rh@TiO2 or Rh@SiO2 (1 g, 0.09% wt, 8.8 ¥ 10-6 mol), substrate (9.6 ¥ 10-4 mol), H2O (10 mL), 1 bar H2, RT, stirred
at 1700 min-1. b Determined by Gas Chromatography analysis. c Turnover frequency defined as the number of moles of consumed H2 per mole of
introduced rhodium per hour and calculated considering 100% conversion of the substrate after an optimized time.
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Rh@SiO2. The hydrogenation of chloroanisole isomers (entries
15–20) leads to the formation of two products: the expected
methoxycyclohexane and small (<5%) or significant (35–40%)
quantities of cyclohexanone according to the catalyst,
respectively Rh@TiO2 or Rh@SiO2. The methoxycyclohexane
was obtained from the tandem dehalogenation-hydrogenation
reaction. The cyclohexanone has already been observed in acidic
media via the formation of an hemiacetal during the hydrogena-
tion of anisole with aqueous colloidal Rh(0) suspensions.30 In
contrast, no cyclohexanone was observed in the hydrogenation
of anisole with the Rh@SiO2 or Rh@TiO2 catalyst (entries
11–12). In this case, the formation of cyclohexanone during
the hydrogenation of chloroanisole isomers could be explained
by the release of HCl in the reaction media during the
dehalogenation, which constitutes the first step of the tandem
dehalogenation-hydrogenation reaction (Scheme 2).37 The dif-
ference of cyclohexanone ratio between both catalysts could be
explained by a pH value of the aqueous phase near 6.5 with
Rh@TiO2 and near 5 with Rh@SiO2. The initial pH difference
value between both catalysts and the release of HCl in the
reaction media are responsible for a more acidic solution (pH =
3.9) with the Rh@SiO2 system, which favours the formation of
cyclohexanone. As the kinetics of the hydrogenation reaction is
faster with the Rh@TiO2 catalyst, compared to Rh@SiO2, the
formation of cyclohexanone is not favoured.
materials. These new properties are under current investigation
in the laboratory.
Experimental
a) General
RhCl3·3H2O was obtained from Strem Chemicals. All the
organic compounds were purchased from Acros Organics or
Sigma-Aldrich-Fluka and used without further purification.
The titania was purchased from Millenium Chemicals. The
-1
photocatalyst PC500 has a BET-surface area of 287 m2 g
with 100% anatase and a primary particle size of 5–10 nm.39,40
The silica Si80 was obtained from Merck and presents the
following characteristics: mean diameter: 80 mm, pore diameter
5.4 nm, specific area 490 m2 g-1, pore volume 0.77 cm3 g-1 and
internal porosity 58%. Water was distilled twice before use by
conventionalmethod. Thehydroxyethylammonium chloridesalt
HEA16Cl, which was used as a protective agent for the Rh(0)
nanoparticles in aqueous solution, was synthesized as previously
described in the literature and fully characterized.27,41 All gas
chromatography analyses of catalytic reactions were performed
using a Carlo Erba GC 6000 with a FID detector equipped with
a Factor Four column (30 m, 0.25 mm i.d.).
b) Preparation of Rh@TiO2
A suspension of titania (19 g) in 40 mL of deionized water
is stirred vigorously during two hours. Furthermore, 50 mL
of surfactant-stabilized Rh(0) suspension13 (1.9 ¥ 10-4 mol of
Rh) are added under vigorous stirring. The system is kept
under stirring during two hours. After filtration and three water
washings, the grey powder is dried overnight at 60 ◦C. Anal.
found: Rh, 0.09.
c) Preparation of Rh@SiO2
Scheme 2 Formation of cyclohexanone during the chloroanisole
hydrogenation.
Silica (18.5 g) was vigorously stirred in 40 mL of deionized water
during 2 h. Furthermore, 50 mL of Rh(0)@HEA16Cl (1.9 ¥
10-4 mol of Rh) were added under vigorous stirring and the
system was kept under stirring for two hours. The catalyst was
recovered by filtration,◦then washed several times with distilled
water and dried at 60 C overnight. Previous TEM analyses22
have evidenced well-dispersed Rh(0) particles with an average
size of 5 nm. Anal. found: Rh, 0.09.
In conclusion, TiO2-supported Rh(0) nanoparticles were
easily prepared by a simple, environmentally-friendly and
reproducible procedure based on wet impregnation of the
support with a pre-stabilized colloidal suspension, under mild
conditions without calcination step in water. The nanocatalysts
presenting low metal loading (0.09 wt%) proved to be very active
and recyclable in the hydrogenation of mono- or di-substituted
and/or functionalized arene derivatives at atmospheric pressure
with TOF up to 470 h-1 in neat water at atmospheric hydrogen
pressure and room temperature. A dramatic increase in catalytic
activities with TOF up to 33000 h-1 were achieved when
increasing the pressure up to 30 bar H2. The catalytic activities
of the Rh@TiO2 system proved to be higher than those of
Rh@SiO2 on a large variety of aromatic derivatives. The use of
TiO2 anatase, well-known for its high photocatalytic activity,38
offers great opportunities for the development of photocatalytic
processes and further investigations are ongoing concerning the
remediation of toxic arene compounds from aqueous effluents,
combining tandem dehalogenation-hydrogenation reaction and
photocatalytic activity using metallic nanoparticles doped-TiO2
d) General procedure for hydrogenation under atmospheric
hydrogen pressure
Reactions are carried out under standard conditions (20 ◦C,
1 atm of H2). A 25 mL round bottom flask, charged with
Rh@TiO2 catalyst (1 g, 10 mL of water) and a magnetic stirrer,
is connected with a gas burette (500 mL) with a flask to balance
the pressure. The flask is closed by a septum, and the system
is filled with hydrogen. The appropriate aromatic substrate
([Substrate]/[Rh] = 100) is injected through the septum and the
mixture is stirred at 1700 min-1. The reaction is monitored by gas
chromatography analyses. Based on the amount of introduced
rhodium, turnover frequencies (TOFs) are calculated for 100%
conversion.
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e) General procedure for hydrogenation under hydrogen pressure
12 A. Roucoux and K. Philippot, in The Handbook of Homogeneous
Hydrogenation, ed. J. G. d. Vries and C. J. Elsevier, Wiley-VCH,
2007, pp. 217–256.
The stainless steel autoclave was charged with the Rh@TiO2
catalyst (1 g, 10 mL of water) and a magnetic stirrer. The
appropriate substrate ([Substrate]/[Rh] = 100) was added into
the autoclave and dihydrogen was admitted to the system
at constant pressure (up to 30 bars). The conversion was
determined by gas chromatography analysis. Based on the
amount of introduced rhodium, the TOF was determined for
100% conversion.
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d) Transmission electron microscopy (TEM) analysis
After embedding of the sample in a resin (AGAR 100) and
treatment at 70 ◦C during 48 h for polymerization, the solid
has been cut into ultrathin lamellas (70 nm) with a diamond
knife (Leica Ultracut UCT). A TEM analysis was then realized
after deposition of the lamellas onto a carbon covered copper
grid using a microscope operating at 200 kV with a resolution
of 0.18 nm (JEOL JEM 2011 UHR) equipped with an EDX
system (PGT IMX-PC).
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Acknowledgements
The authors are grateful to the Region Bretagne for financial
support (Grant to C. Hubert) and the Universite´ Europe´enne de
Bretagne (UEB). The authors are indebted to Patricia Beaunier
from UPMC for HRTEM images and EDX analyses.
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