Citral hydrogenation over Rh and Pt catalysts supported on TiO2: Influence of the preparation and activation protocols of the catalysts

Article abstract of DOI:10.1016/j.molcata.2011.01.020

During citral hydrogenation, the products distribution obtained on Rh/TiO₂ and Pt/TiO₂ catalysts depends on their preparation and activation protocols: (i) the unsaturated alcohols (the intended products) are formed in higher quantity on samples reduced at 500 °C and more notably with Pt/TiO₂ catalyst; (ii) samples prepared by impregnation of the metallic precursor salt in HCl medium and activated at 300 °C are the only ones to lead to the formation of isopulegol as by-product. On the catalysts activated at 500 °C, these results can be explained by the presence of the SMSI effect beneficial to hydrogenate selectively the CO bond of citral towards unsaturated alcohols.

Full text of DOI:10.1016/j.molcata.2011.01.020

Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Citral hydrogenation over Rh and Pt catalysts supported on TiO2: Influence of the
preparation and activation protocols of the catalysts
Tchirioua Ekoua,b, Lynda Ekoub, Aurélie Vicentea,c, Gwendoline Lafayea,
Stéphane Proniera, Catherine Especela,∗, Patrice Marécota
a Laboratoire de Catalyse en Chimie Organique, UMR 6503, Université de Poitiers, 40 avenue du Recteur Pineau, 86000 Poitiers Cedex, France
b Laboratoire de Thermodynamique et de Physico-Chimie du Milieu, Université d’Abobo-Adjame, 02 Bp 801 Abidjan 02, Cote d’Ivoire
c Laboratoire de Catalyse en Spectrochimie de Caen, EnsiCaen, Université de Caen, 6 boulevard Maréchal Juin, 14050 Caen Cedex, France
a r t i c l e i n f o
a b s t r a c t
Article history:
During citral hydrogenation, the products distribution obtained on Rh/TiO2 and Pt/TiO2 catalysts depends
on their preparation and activation protocols: (i) the unsaturated alcohols (the intended products) are
formed in higher quantity on samples reduced at 500 ◦C and more notably with Pt/TiO2 catalyst; (ii)
samples prepared by impregnation of the metallic precursor salt in HCl medium and activated at 300 ◦C
are the only ones to lead to the formation of isopulegol as by-product. On the catalysts activated at 500 ◦C,
these results can be explained by the presence of the SMSI effect beneficial to hydrogenate selectively
the C O bond of citral towards unsaturated alcohols.
Received 19 July 2010
Received in revised form
22 November 2010
Accepted 23 January 2011
Available online 28 January 2011
Keywords:
Citral hydrogenation
© 2011 Elsevier B.V. All rights reserved.
Rh
Pt
Titania
Unsaturated alcohols
Isopulegol
1. Introduction
tion at high temperature (i.e. “strong metal-support interaction” or
SMSI effect) [19]. In the present paper, we examine the influence
Selective hydrogenation of organic substrates containing sev-
eral unsaturated functional group is an important step in the
preparation of various fine chemical products [1–3]. Particularly,
the preparation of unsaturated alcohols by selective hydrogena-
tion of the corresponding ␣, ␤-unsaturated aldehydes has a great
industrial importance and constitutes a challenging task, since the
hydrogenation of the C C bond is thermodynamically favoured
over the hydrogenation of the carbonyl group. In the present
work, the liquid-phase hydrogenation of citral (3,7-dimethyl-2,6-
octadienal) was studied, this molecule and its unsaturated alcohols
being of considerable interest in the perfumery industry [4,5]. Cit-
ral is an ␣,␤-unsaturated aldehyde including conjugated C O and
of the preparation and activation protocols (nature of the impreg-
nation medium of the precursor salt and reduction temperature) of
Rh/TiO2 and Pt/TiO2 catalysts on their performances for selective
hydrogenation of citral towards unsaturated alcohols.
2. Experimental
2.1. Catalyst preparation
The TiO2 support (Degussa P25, surface area = 50 m2 g−1) was
ground and then sieved to retain particles with sizes between
0.10 and 0.04 mm. After calcination of the support in flow-
ing air for 4 h at 500 ◦C, 1.0 wt%RhNH /TiO2, 1.0 wt%RhHCl/TiO2
and 1.0 wt%PtHCl/TiO2 monometallic ca3talysts were prepared by
impregnation using a limpid aqueous solution of RhCl3 and H2PtCl6,
respectively, in the presence of either HCl (pH 1) or NH3 (pH 11)
medium. Catalysts were dried at 120 ◦C overnight, followed by a cal-
cination in flowing air for 4 h at 300 ◦C for Rh samples and 400 ◦C for
Pt ones, then reduced in flowing pure hydrogen at 300 ◦C or 500 ◦C.
After preparation, the chlorine content on each catalyst was deter-
mined by means of potentiometry at the “Service central d’analyse”
of the CNRS.
C
C bonds as well as an isolated C C bond [6]. Heterogeneous
catalysts for the hydrogenation of ␣,␤-unsaturated aldehydes are
mostly based on supported noble metals (Pt, Ru, Rh, Pd) [6–18].
In the case of these metals are deposited on reducible support as
TiO2, the hydrogenation of the carbonyl bond can be promoted due
to the presence of partially reduced species generated upon reduc-
∗
Corresponding author.
E-mail address: catherine.especel@univ-poitiers.fr (C. Especel).
1381-1169/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2011.01.020

T. Ekou et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
83
or 500 ◦C) has no effect on the particle size distribution and then
on the mean particle size (Table 1).
Table 1
Characteristics of the 1.0 wt%Rh/TiO2 and 1.0 wt%Pt/TiO2 monometallic catalysts.
Catalyst/TiO2 Impregnation
medium
Treduction (◦C)
Cl (wt%)
Average particle
size (nm)
RhNH /TiO2 and PtHCl/TiO2 monometallic catalysts were char-
3
acterized by TPR after being pretreated in situ under oxygen
for 30 min at 300 ◦C or 400 ◦C, respectively, and cooled down to
room temperature. The TPR profiles (Fig. 2) show that the reduc-
tion of oxidized rhodium and platinum starts from the ambient
RhNH (300)
NH3
NH3
HCl
HCl
HCl
HCl
300
500
300
500
300
500
0.5
≤0.2
0.8
2.3
2.3
2.3
2.2
1.9
2.0
3
RhNH (500)
RhHCl3(300)
RhHCl (500)
PtHCl (300)
PtHCl (500)
≤0.2
temperature. For the 1.0 wt%RhNH /TiO2 catalyst, the main H2
3
0.5
consumption is located around 100 ◦C, followed by a broader and
smaller peak from 200 ◦C to 500 ◦C. For the 1.0 wt%PtHCl/TiO2 cata-
lyst, the main peak presents a maximum below 100 ◦C and a second
larger peak appears from 250 ◦C to 500 ◦C. Previous results obtained
on Rh/Al2O3 and Pt/Al2O3 catalysts showed that the rhodium and
platinum oxides are generally reduced before 250–300 ◦C [20]. Thus
the H2 consumption obtained at higher temperature for our sam-
ples can be attributed to the partial reduction of the TiO2 support,
i.e. the SMSI effect which induces TiO2−x species (x < 2). Indeed, the
total hydrogen consumptions deduced from the TPR profiles are
189 ␮mol gcatalyst−1 for 1.0 wt%Rh/TiO2 and 171 ␮mol gcatalyst−1 for
1.0 wt%Pt/TiO2, whereas the theoretical values for the reduction of
Rh2O3 and PtO2 are 146 and 102 ␮mol gcatalyst−1, respectively. The
vertical dotted lines in Fig. 2 indicate when the H2 consumptions
reach these theoretical values. If we admit that all the Rh and Pt
atoms are reduced before the support, the indexed temperature
would then correspond to the beginning of the partial reduction of
TiO2.
≤0.2
2.2. Transmission electron microscopy (TEM)
TEM analysis was performed on a Philips CM 120 instrument
operating at 120 kV. Samples were embedded in a polymeric resin
and cut into a section as small as 40 nm with an ultramicro-
tone fitted with a diamond knife. Cuts were then deposited on
an Al grid previously covered with a thin layer of carbon. Aver-
age particle sizes were determined by measuring at least hundred
particles for each sample analyzed, from at least five different
micrographs.
2.3. Temperature-programmed reduction (TPR)
TPR experiments were aimed to investigate the H2 consump-
tion during the reduction of the Rh/TiO2 and Pt/TiO2 monometallic
catalysts. They were done with a 1.0 vol%H2/Ar gas mixture. The
temperature range was 25–500 ◦C with a ramp of 5 ◦C min−1 and
then maintained at 500 ◦C for 1 h. The hydrogen uptake was moni-
tored by a thermal conductivity detector.
3.2. Citral hydrogenation over Rh/TiO2 and Pt/TiO2 catalysts
Fig. 3 presents the main reaction pathways that occur during
citral hydrogenation. The reduction of citral can lead to a variety
of products. A first step is the reduction of either the C O or the
conjugated C C bond to produce geraniol and nerol (unsaturated
alcohols) or citronellal, respectively. Consecutive hydrogenation
leads to citronellol and finally to 3,7-dimethyl octanol. Beside these
reactions, processes of cyclization or of reaction with the solvent
(alcohol) can lead to other by-products like isopulegol or acetals,
respectively.
2.4. Citral hydrogenation
The citral hydrogenation was performed in liquid phase in a
300 mL stirred autoclave (Autoclave Engineers, fitted with a sys-
tem for liquid sampling), at 70 ◦C and at constant pressure of 7 MPa.
Prereduced catalysts (400 mg) were immersed into 90 mL of sol-
vent (isopropanol, 99%) without exposure to air before introduction
into the autoclave. After a first flush with nitrogen and a sec-
ond with hydrogen, the temperature was raised to 70 ◦C under
3 MPa of hydrogen. Then, a mixture of substrate (3 mL of citral, i.e.
17.6 mmol) and of isopropanol (10 mL) was loaded into the auto-
clave through a cylinder under a 7 MPa hydrogen pressure. Zero
time was taken at this moment and stirring was switched on. Liquid
samples were analyzed by gas chromatography on a Thermofinni-
gan chromatograph equipped with a flame ionization detector and
a capillary column DB-WAX (J&W, 30 m, 0.53 mm i.d.) using nitro-
gen as carrier gas. Preliminary runs carried out at different stirring
conditions, loadings and catalyst grain sizes have demonstrated the
absence of external and internal diffusional limitations.
3.2.1. Influence of the impregnation method of monometallic
Rh/TiO2 catalyst
The hydrogenating properties of RhHCl/TiO2 and RhNH /TiO2 cat-
3
alysts prepared by impregnation of the rhodium salt on the support,
either in HCl (pH 1) or NH3 (pH 11) medium, and both activated
at 300 ◦C were compared. Fig. 4 presents the temporal concen-
tration profile of the reactant and the main products during citral
hydrogenation on these catalysts.
Citronellal is the main product formed over the RhNH /TiO2
catalyst under our experimental conditions. The formation 3of cit-
ronellol and 3,7-dimethyloctanol (listed as “others products”) is
also observed at longer reaction times. The formation of the
intended products (i.e. the unsaturated alcohols) remains rather
low indicating that the C C/C O adsorption competition of the cit-
ral molecules is mainly in favor of the C C bond on this catalyst.
On RhHCl catalyst, citronellal is rapidly formed and then for the
biggest part converted into isopulegol. It becomes the main prod-
uct on this sample, reaching 50 mol%, whereas it was not detected
on RhNH3 catalyst. Once citral is totally converted, isopulegol is then
hydrogenated into menthol (listed as “others products”).
The different reaction pathways observed on these two cata-
lysts presenting the same particle size are bound to be due to
their preparation procedure, i.e. their impregnation medium. In
fact, isopulegol is a by-product of citral hydrogenation resulting
from the isomerisation of citronellal in protonic medium [21,22]
and generally favoured in hydrophobic solvents [23,24]. Then,
RhHCl/TiO2 catalyst prepared in HCl medium possesses acid sites
3. Results and discussion
3.1. Characterizations of the Rh/TiO2 and Pt/TiO2 catalysts
The prepared catalysts and their characteristics are listed in
Table 1 with their code name, depending on their impregnation
medium (HCl or NH3) and their reduction temperature (300 ◦C
or 500 ◦C). For each catalyst, TEM image analysis reveals small
and well-dispersed particles on the TiO2 surface and a narrow
particle size distribution. Examples of characteristic TEM images
are given in Fig. 1 for PtHCl/TiO2 catalyst. TEM results in Table 1
indicate that the average particle size of the Rh/TiO2 samples is
independent of the impregnation medium since all the Rh-based
catalysts present particle size around 2.2–2.3 nm. Moreover, what-
ever the monometallic catalyst, the reduction temperature (300 ◦C

84
T. Ekou et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
Fig. 1. TEM images and metal particle size distributions of 1.0 wt%PtHCl/TiO2 catalyst reduced at (a) 300 ◦C and (b) 500 ◦C.
20 a.u.
(a)
20 a.u.
(b)
102 µmol/gcatalyst
146 µmol/gcatalyst
0
0
100 200 300 400 500
100
300
500°C (1 h)
200
500
400
500°C (1 h)
Temperature (°C)
Temperature (°C)
Fig. 2. TPR profiles of (a) 1.0 wt%RhNH /TiO2 and (b) 1.0 wt%PtHCl/TiO2 catalysts.
3

T. Ekou et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
85
Fig. 3. Reaction scheme for citral hydrogenation.
necessary for the isopulegol formation. Conversely, the impregna-
tion of RhCl3 salt in NH3 medium followed by a reduction at 300 ◦C
allows one to synthesize Rh/TiO2 catalyst without such acid sites.
Previous studies have reported that the acidity was induced by the
presence of remaining chlorine on the catalyst surface [23]. In fact,
by inductive effect, the presence of chlorine ions allows decreasing
the electronic density of the metal particles and its close vicinity,
and consequently increasing the acidic character of the catalyst.
This is consistent with the fact that RhHCl/TiO2 catalyst leads to isop-
ulegol since it presents higher chlorine content than RhNH /TiO2
3
after reduction at 300 ◦C (Table 1).
3.2.2. Influence of the reduction temperature of monometallic
Rh/TiO2 catalysts
The results stated above showed that both RhHCl/TiO2 and
RhNH /TiO2 monometallic catalysts after reduction at 300 ◦C pos-
3
100
100
(b)
(a)
80
80
60
40
20
0
60
40
20
0
0
30
60
90
120
0
30
60
90
120
Time (minute)
Time (minute)
Fig. 4. Citral hydrogenation on 1.0 wt%Rh/TiO2 catalysts reduced at 300 ◦C prepared in (a) NH3 and (b) HCl medium: (᭹) citral; (ꢀ) citronellal; (ꢁ) citronellol; (ꢂ) unsaturated
alcohols; (*) isopulegol; and ( ) others products.

86
T. Ekou et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
100
80
60
40
20
0
100
(b)
(a)
80
60
40
20
0
0
30
60
90
120
0
30
60
90
120
Time (minute)
Time (minute)
Fig. 5. Citral conversion as function of time on 1.0 wt%Rh/TiO2 catalysts prepared in (a) NH3 and (b) HCl medium: reduced at (᭹) 300 ◦C and (ꢀ) 500 ◦C.
100
80
sess sites able to hydrogenate C O and C C double bonds of citral
and of its secondary products. Besides, a competitive adsorption
must exist between the ␣,␤-unsaturated aldehyde and its sec-
ondary products. The effect of the reduction temperature of both
catalysts on their catalytic properties was investigated. They were
reduced at 500 ◦C with the aim of favoring the C O hydrogenation
of citral.
60
Fig. 5 displays the citral conversion as function of time for both
catalysts reduced at 300 ◦C and 500 ◦C. First, a deactivation of the
catalysts occurs during the first few minutes, explained either by a
polymerization of carbonaceous species, or by a decarbonylation of
citral and/or unsaturated alcohols leading in both cases to a poison-
ing of the active sites [25–30]. For both RhHCl/TiO2 and RhNH /TiO2
catalysts, whatever the reduction temperature, citral is totall3y con-
verted after 90 min reaction time. However, the samples reduced at
500 ◦C are fewer actives than their counterparts reduced at 300 ◦C,
particularly for the RhHCl/TiO2 catalyst (Fig. 5b). This phenomenon
is explained by the presence of partially reduced support species
(TiO(2−x) (x < 2)) generated after reduction at high temperature,
which can cover part of the metallic surface. For the RhNH /TiO2
monometallic catalyst, an increase of the reduction tempe3rature
from 300 ◦C to 500 ◦C involves an increase of the unsaturated alco-
hols and citronellol formation (Fig. 6a compared to Fig. 4a). This
phenomenon is linked to the use of the TiO2 support since the
selectivity to unsaturated alcohols of previously studied Rh/Al2O3
catalysts was proved to be independent of the reduction temper-
ature, remaining in all cases rather low (1–2%) [25]. In the case
40
20
0
0
30
60
90
120
Time (minute)
Fig. 7. Citral conversion as function of time on 1.0 wt%PtHCl/TiO2 catalysts reduced
at (᭹) 300 ◦C and (ꢀ) 500 ◦C.
of the RhHCl/TiO2 sample, citronellal and citronellol are formed in
larger extend after reduction at 500 ◦C to the detriment of isop-
ulegol (Fig. 6b compared to Fig. 4b). This phenomenon cannot be
explained by an evolution of the metal particle size since both sam-
ples present similar average particle diameters (Table 1). The drop
of isopulegol formation after reduction at high temperature can be
100
(a)
80
100
(b)
80
60
40
20
0
60
40
20
0
0
30
60
90
120
0
30
60
90
120
Time (minute)
Time (minute)
Fig. 6. Citral hydrogenation on 1.0 wt%Rh/TiO2 catalysts reduced at 500 ◦C prepared in (a) NH3 and (b) HCl medium: (ꢀ) citronellal; (ꢁ) citronellol; (ꢂ) unsaturated alcohols;
(*) isopulegol; and ( ) others products.

T. Ekou et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
87
100
80
60
40
20
0
100
(a)
(b)
80
60
40
20
0
0
30
60
90
120
0
30
60
90
120
Time (minute)
Time (minute)
Fig. 8. Citral hydrogenation on 1.0 wt%PtHCl/TiO2 catalysts reduced at (a) 300 ◦C and (b) 500 ◦C: (ꢀ) citronellal; (ꢁ) citronellol; (ꢂ) unsaturated alcohols; (*) isopulegol; and
) others products.
(
explained by the removal of chloride ions (Table 1) that means the
elimination of acid sites on the catalyst surface.
RhHCl/TiO2 catalyst and explained by the acidity decrease after high
reduction temperature (500 ◦C). Nevertheless, it should be noticed
that isopulegol formation is rather limited on PtHCl/TiO2 catalyst
reduced at 300 ◦C compared to the RhHCl/TiO2 sample prepared and
activated in the same conditions (Fig. 8a vs Fig. 4b).
Finally, on both Rh/TiO2 monometallic catalysts, the SMSI effect
is slightly beneficial on the unsaturated alcohols formation. Never-
theless, once the citral conversion reaches 100%, the unsaturated
alcohols keep being hydrogenated. Thus, the presence of TiO(2−x)
species does not poison the C C hydrogenation sites, but modifies
the C C/C O adsorption competition by enhancing the activation
of the oxygen electronic doublet of the carbonyl function [31–33].
Finally, as with Rh/TiO2 catalysts, the high temperature reduc-
tion generates TiO(2−x) (x < 2) moieties on PtHCl/TiO2 sample, which
activate the C O bond of citral. However, the SMSI effect is more
marked in the case of Pt after reduction at 500 ◦C since more than
50 mol% of unsaturated alcohols are obtained after 1 h reaction
time. In fact, the beneficial effect of the SMSI phenomenon on
the unsaturated alcohols selectivity is known to be more impor-
tant on Pt or Ir-based catalysts [12,34,35]. Moreover, in a previous
study concerning the preparation and characterization of bimetallic
Rh–Ge and Pt–Ge catalysts supported on TiO2 [20], we tested the
catalytic performances of the samples for the cyclohexane dehy-
drogenation, a structure insensitive reaction, under atmospheric
pressure and at 270 ◦C. The catalytic activities of the monometal-
lic Rh and Pt catalysts measured before their modification by Ge
addition indicated lower values for all catalysts reduced at 500 ◦C
compared to 300 ◦C. These results evidenced the SMSI effect devel-
oped by TiO2−x species covering one part of the metallic active
surface of the samples reduced at high temperature. Moreover, we
observed that the decrease of activity was more pronounced with
Pt than with Rh catalysts, suggesting that Pt is more sensitive than
Rh to the SMSI effect. According to Herrmann [36], the sensitivity
to the SMSI effect which varies with the nature of the metal in the
order Pt > Rh, is explained by a greater number of unfilled d-orbitals
per metal atoms in the case of Rh.
3.2.3. Influence of the reduction temperature of monometallic
PtHCl/TiO2 catalyst
Fig. 7 shows the evolution of the citral conversion as function of
time for the 1.0 wt%PtHCl/TiO2 catalyst reduced at 300 ◦C and 500 ◦C.
As observed for Rh/TiO2 catalysts, the sample reduced at 300 ◦C is
more active than the one reduced at 500 ◦C, but both are rapidly
deactivated. The deactivation process is then independent of the
metal nature (Rh or Pt). The curves reported in Fig. 8 indicate that
the increase of reduction temperature from 300 ◦C to 500 ◦C allows
one to double the amount of unsaturated alcohols, while this phe-
nomenon is not observed by using a classical support as alumina for
example. The opposite behavior is observed for the hydrogenation
of the conjugated C C double bond leading to citronellal forma-
tion. For both reduction temperatures, citronellal decreases while
citronellol increases as function of time, indicating the hydrogena-
tion of the carbonyl function of citronellal. Moreover, isopulegol
is obtained in noticeable quantity on the Pt/TiO2 after reduction
at 300 ◦C, whereas reducing the catalyst at 500 ◦C leads almost to
its disappearance. This phenomenon was previously observed on
4. Conclusion
Rh and Pt monometallic catalysts supported on titania oxide
were prepared by impregnation of precursor salt in various media
(NH3 or HCl), and reduced either at 300 ◦C or at 500 ◦C (high temper-
ature required to generate SMSI effect). Whatever the preparation
and activation protocols, TEM analysis revealed small (1.9–2.3 nm)
well-dispersed particles of Rh and Pt on the support. TPR exper-
iments highlighted the partial reduction of TiO2 species above
300 ◦C as reduction temperature. Selective hydrogenation of citral
was performed at 70 ◦C on all samples and under 7 MPa hydrogen
pressure. When Rh and Pt salts are impregnated in HCl medium,
reducing the resulting catalysts at low temperature (300 ◦C) leads
mainly to citral cyclization towards isopulegol, due to the presence
of acidic sites induced by chloride species. A reduction of these cat-
Fig. 9. Schematic representation of the adsorption mode of citral on M/TiO2 cata-
lyst reduced at high temperature (500 ◦C): ( ) support in a partially reduced state
TiO(2−x) (x < 2).

88
T. Ekou et al. / Journal of Molecular Catalysis A: Chemical 337 (2011) 82–88
alysts at high temperature (500 ◦C) allows one to decrease chlorine
content and consequently isopulegol formation. For all catalysts,
the presence of TiO(2−x) moieties (x < 2) on the Rh and Pt active sur-
face after reduction at 500 ◦C involves a slight decrease of the citral
conversion. Nevertheless, the SMSI effect promotes the formation
of unsaturated alcohols (intended products), this effect being more
pronounced with Pt/TiO2 catalyst. The TiO(2−x) species are respon-
sible for the activation of the C O group of citral according to the
schematic representation proposed in Fig. 9.
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