Palladium supported catalysts for the selective hydrogenation of sunflower oil

Article abstract of DOI:10.1016/j.molcata.2004.11.017

Selective hydrogenation of ethyl esters of traditional sunflower oil (SOEE) was carried out at low temperature (40°C) in ethanol as solvent in the presence of supported palladium catalysts. In the range of studied dispersions (12-55%), the SOEE hydrogenation reaction is insensitive to the size of the palladium particles deposited on silica, but the largest metallic particles enhance the C18:1 cis-trans isomerization. The use of various oxide supports (α-Al₂O₃, γ_c-Al₂O ₃, TiO₂, MgO, ZnO, CeO₂, CeZrO₂) to deposit palladium does not allow to improve the selectivity of the reaction toward the cis C18:1 compared to Pd/SiO₂ catalysts. On the other hand, the modification of palladium by lead, introduced by surface redox reaction (catalytic reduction), promotes the selectivity in cis C18:1. In addition, this technique of preparation involves a moderate decrease of the palladium activity compared to a traditional method of successive impregnations. The introduction of amines in the reaction medium modifies the hydrogenating properties of the Pd/SiO₂ catalyst. According to the quantity and the nature of the added amine (aliphatic with linear or ramified chain and cyclic compounds), the catalytic activity can either be unchanged or inhibited. These evolutions result from a promoter electronic effect generated by the presence of the amine and from a poison geometric effect related to the adsorption of this nitrogen-containing compound on the palladium surface. Whatever the nature of the amine, it induces an increase of the selectivity toward the cis C18:1.

Full text of DOI:10.1016/j.molcata.2004.11.017

Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
Palladium supported catalysts for the selective
hydrogenation of sunflower oil
a
a,∗
a
a
´
Bendaoud Nohair , Catherine Especel , Gwendoline Lafaye , Patrice Marecot ,
b
a,b
ˆ
ˆ
Le Chien Hoang , Jacques Barbier
a
LACCO, UMR 6503, Universite´ de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
Valagro, Valorisation des Agroressources, 40 Avenue du Recteur Pineau, 86022 Poitiers Cedex, France
b
Received 29 October 2004; received in revised form 15 November 2004; accepted 20 November 2004
Available online 25 December 2004
Abstract
Selective hydrogenation of ethyl esters of traditional sunflower oil (SOEE) was carried out at low temperature (40 ◦C) in ethanol as solvent
in the presence of supported palladium catalysts. In the range of studied dispersions (12–55%), the SOEE hydrogenation reaction is insensitive
to the size of the palladium particles deposited on silica, but the largest metallic particles enhance the C18:1 cis–trans isomerization. The use of
various oxide supports (␣-Al2O3, ␥c-Al2O3, TiO2, MgO, ZnO, CeO2, CeZrO2) to deposit palladium does not allow to improve the selectivity
of the reaction toward the cis C18:1 compared to Pd/SiO2 catalysts. On the other hand, the modification of palladium by lead, introduced by
surface redox reaction (catalytic reduction), promotes the selectivity in cis C18:1. In addition, this technique of preparation involves a moderate
decrease of the palladium activity compared to a traditional method of successive impregnations. The introduction of amines in the reaction
medium modifies the hydrogenating properties of the Pd/SiO2 catalyst. According to the quantity and the nature of the added amine (aliphatic
with linear or ramified chain and cyclic compounds), the catalytic activity can either be unchanged or inhibited. These evolutions result from a
promoter electronic effect generated by the presence of the amine and from a poison geometric effect related to the adsorption of this nitrogen-
containing compound on the palladium surface. Whatever the nature of the amine, it induces an increase of the selectivity toward the cis C18:1.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Vegetable oils; Selective hydrogenation; Palladium; Lead; Amines
1. Introduction
acid, C18:3), linoleic acid (cis-9, cis-12-octadecadienoic
acid, C18:2) and oleic acid (cis-9-octadecenoic, C18:1).
Their oxygen absorption rate is 800:100:1, respectively.
Therefore, partial hydrogenation of rapeseed oil to the C18:1
acid should result in a strong stabilization over against
oxygen. Moreover, the oleic acid C18:1 in the cis isomer
form, contrary to the trans C18:1 and the totally saturated
(stearic acid, C18:0) compounds, remains liquid at ambient
temperature. However, a traditional sunflower oil can contain
only 25–30% of C18:1 cis isomer, the main constituent being
the cis-9, cis-12 C18:2. The selective hydrogenation of such
vegetable oil must lead to a minimum of 80% toward cis
oleic acid to correspond to the industrial needs. Currently,
the catalyst used in the industrial hydrogenation process is a
nickel catalyst deposited on a silica support [2–5]. Studies on
the hydrogenation of different vegetable oils have been also
The vegetable edible oils hydrogenation is a very impor-
tant operation in the chemical industry. The main qualities
of natural substances are their environment compatibility
(low ecotoxicity, great biodegradability). Moreover, these
resources are renewable. The industrial applications are as
varied as pharmacy, cosmetics, plastics, detergents, lubri-
cants, etc. The aim of this study is to produce lubricants. Oils
used as lubricants have to remain liquid at ambient tempera-
ture and to be stable under oxygen atmosphere to avoid their
polymerization [1]. Main unsaturated fatty acids in vegetable
oils are linolenic acid (cis-9, cis-12, cis-15-octadecatrienoic
∗
Corresponding author. Tel.: +335 49 45 39 94; fax: +335 49 45 34 99.
E-mail address: catherine.especel@univ-poitiers.fr (C. Especel).
1381-1169/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2004.11.017

118
B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
undertaken on catalysts containing copper [6] or copper
chromite [7]. Generally, these different catalysts require a
reaction temperature above 150 ◦C and a high hydrogen pres-
sure. Recently, we have studied the selective hydrogenation
of ethyl esters of a traditional sunflower oil (SOEE) in the
presence of monometallic catalysts containing noble metals
(Pd, Pt, Ru) supported over silica, which allow to work under
gentle conditions [8]. The activity decreases in the order
Pd > Pt > Ru at 40 ◦C under 10 bar of hydrogen. However, the
three metals are not selective in cis oleic isomer since trans
oleic and stearic derivatives were produced in large amounts.
In order to promote the selectivity toward the cis oleic isomer,
noble metal can be modified by metallic additives or by addi-
tion of amines in the reaction medium. The effect of amines
during the selective hydrogenation of unsaturated hydro-
carbon compounds was the subject of many studies [9–16].
For example, it was shown that the addition of pyridine or
quinoline allows to selectively hydrogenate the acetylenic
compounds in ethylenic ones on palladium catalysts [17–23].
In the same way, the presence of amines allows to selectively
obtain monoolefins starting from diolefins on catalysts con-
taining palladium, nickel or cobalt [24,25]. These results are
generally explained by a competition of adsorption between
theamineandthehydrocarboncompoundsandbyamodifica-
tion of the metal electronic properties resulting from an elec-
tron transfer from the free nitrogen doublet toward the metal
phase.
Preliminary experiments concerning the addition of cop-
per or lead to palladium, and the addition of amines in
the reaction medium were performed [8]. The present work
completes the previous paper on the selective hydrogena-
tion of ethyl esters of traditional sunflower oil on sup-
ported palladium catalysts. We study the influence of the
following parameters: the palladium particle size, the na-
ture of the support, the addition of lead on palladium
catalyst and the addition of various amines in the reac-
tion medium during the SOEE hydrogenation on Pd/SiO2
catalyst.
hydrogen activated on palladium particles and the lead pre-
cursor (Pb(CH3COO)2) dissolved in methanol (“catalytic re-
duction” method [26]). This preparation method of bimetallic
catalysts is well known to induce a strong interaction be-
tween the two metals whatever the prepared bimetallic sys-
tem [27–29]. A known amount of the prereduced palladium
catalyst was introduced into a reactor under nitrogen and was
activated at 300 ◦C for 1 h under hydrogen. Then the solution
of the lead precursor in methanol, previously degassed un-
der nitrogen flow, was introduced onto the catalyst at room
temperature. After a 1 h reaction time under hydrogen bub-
bling, the solution was filtered out and the catalyst was dried
overnight at 100 ◦C. Finally, the bimetallic catalysts were re-
duced under hydrogen flow at 300 ◦C for 2 h (2 ◦C/min heat-
ing rate). Using the same approach, a blank Pd catalyst over
SiO2, named “Pd1 treated in CH3OH/H2” further in the text,
was also prepared in a methanol solution without addition of
the lead precursor.
2.2. Electron microscopy
Transmission electron microscopy (TEM) studies were
performed on a Philips CM 120 instrument operating at
120 kV. The Pd/SiO2 samples were ultrasonically dispersed
in ethanol and the suspension was brought onto a copper
grid with a carbon support film. The preparation of the grids
was different for the catalysts supported onto TiO2, MgO,
CeO2 and CeZrO2 because of some contrast problems. These
samples were embedded in a polymeric resin (spurr) and
cut into a section as small as 40 nm with an ultramicrotome
equipped with a diamond knife. Cuts were then deposited
on an Al grid previously covered with a thin layer of car-
bon.
EDX microanalysis of palladium and lead was carried
out in the nanoprobe mode. The diameter of the probe is
5.0–7.0 nm.
2.3. Metal accessibility
Metal accessibilities of catalysts were determined by
chemisorption of hydrogen at 70 ◦C. The catalysts were first
reduced under hydrogen at 400 ◦C (2 h), then evacuated at the
same temperature under argon for 3 h. Hydrogen chemisorbs
on palladium according to the stoichiometry H/Pds = 1. The
particle size (d) was determined from the metal accessibility
value (%D) according to Hugues et al. [30], which assim-
ilated particles to cubes with one face in contact with the
support. The equation is the following:
2. Experimental
2.1. Catalyst preparation
The various supports were ground and then sieved to re-
tain particles with sizes between 0.10 and 0.04 mm. The
monometallic Pd based catalysts were prepared either via
cationic exchange of the support (SiO2) by Pd(NH3)4(NO3)2
in basic medium (pH = 11) or via simple impregnation
(Al2O3, TiO2, MgO, ZnO, CeO2, CeZrO2) using the same Pd
precursor. The monometallic catalysts were dried overnight
at 120 ◦C, then calcined in flowing air at 300 ◦C for 4 h, and
finally reduced under hydrogen flow at 400 ◦C for 4 h. The
Pd/TiO2 catalyst was reduced at 300 ◦C instead of 400 ◦C
in order to avoid the SMSI effect. Bimetallic Pd–Pb cata-
lysts were prepared by the surface redox reaction between
5 × 106M
˚
d(A) =
(1)
DSρ
with M is the atomic weight (g/mol), S the surface by mol of
Pd metal (47,776 m2/mol) and ρ is the metal volumic weight
(g/cm3).

B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
119
2.4. Hydrogenation of sunflower oil
Classicalrefinedsunfloweroil(fromVALAGRO, Poitiers)
was transformed in presence of ethanol and sodium
ethanolate, at 60 ◦C, into glycerol and ethyl esters. After the
separation of the reaction products by decantation, the sun-
flower oil ethyl esters (SOEEs) were washed and dried at
110 ◦C for 3 h under vacuum. They were then purified on
a molecular distillation apparatus at 120 ◦C (P < 10−3 mbar)
until a high purity near 98%. The hydrogenation of SOEE was
performed in liquid phase in a stirred autoclave, at constant
pressure of 10 bar and at low temperature (40 ◦C) accord-
ing to an experimental procedure previously implemented
at the laboratory [8]. A known amount of prereduced cata-
lyst was immersed into 50 mL of solvent (ethanol) without
exposure to air and introduced into the autoclave. Then a
mixture of 75 mL of SOEE in 25 mL of solvent was poured
into the autoclave and flushed with nitrogen. When the de-
sired temperature was raised, 10 bar hydrogen pressure was
introduced and the stirring was switched on (zero time for
the reaction). In some experiments, amines were introduced
directly in the reaction medium. Liquid samples were ana-
lyzed by VPC on a Varian 3400CX chromatograph equipped
with a flame ionization detector and a capillary column
DB-23 (J&W, 30 m, 0.25 mm i.d.) using nitrogen as carrier
gas.
Fig. 2. SOEE hydrogenation vs. time at 40 ◦C over 1 wt.% Pd/SiO2 catalyst
(mcatalyst = 122 mg): (᭹) % C18:2; (ꢀ) % C18:1 (tot); (ꢀ) % cis C18:1; (ꢁ)
% trans C18:1; (ꢁ) % C18:0 (mol%).
It must be recall that the aim of the study is to selectively
hydrogenate C18:2 toward cis C18:1, avoiding the cis–trans
isomerization and the complete hydrogenation to C18:0. It
should be noticed that the C18:1 compounds include the var-
ious isomers of position of the C C double bond (C C in
positions 9, 10, 11 or 12, Fig. 1). An example of the temporal
percentage profiles of the various compounds is represented
in Fig. 2. The reagent to be hydrogenated consists of a mix-
ture of C18:0, C18:1 and C18:2 in the following initial pro-
portions: C18:0, 3.72%; cis-9 C18:1, 27.49%; cis-9, cis-12
C18:2, 60.11%.
3.1. Particle size effect
3. Results and discussion
In order to achieve a series of catalysts displaying vari-
ous Pd particle sizes, two different strategies were followed:
first, the Pd loading was increased from 0.3 to 1.5 wt.%.
Fig. 1 presents the reaction pathway of SOEE hydrogena-
tion, with the various intermediates which can be observed.
Fig. 1. Reaction pathway of SOEE hydrogenation.

120
B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
Table 1
Main characteristics and TOF (C18:2 hydrogenation) of silica supported monometallic and bimetallic catalysts containing palladium
Pdwt.% catalyst/SiO2
Tcalcination, Treduction
(◦C)
Metala (wt.%)
db (nm)
Dc (%)
MCd (×103 mol/L)
TOF C18:2 hydrogenation
(×1019 mol/h)
Pd0.3
Pd1
Pd1.5
300, 400
300, 400
300, 400
–, 300
–, 500
–, 300
0.39
0.91
1.45
0.91
1.7**
1.8**
2.0**
3.5**
7.8**
3.8*
55
53
47
28
12
–
0.030
0.070
0.037
0.070
0.070
0.070
0.070
2.08
1.92
2.25
1.99
2.12
–
Pd1 treated in CH3OH/H2
Pd1 treated in H2O/H2
Pd1 + 0.05 wt.% Pb
Pd1 + 0.10 wt.% Pb
0.91
0.91 (Pd), 0.04 (Pb)
0.91 (Pd), 0.07 (Pb)
–, 300
4.0*
–
–
a
Determined by ICP.
b
Mean particle size deduced from electronic microscopy (*) or hydrogen chemisorption (**).
Pd accessibility.
MC: metallic concentration in the reaction medium during the SOEE hydrogenation.
c
d
The principal characteristics of the resulting catalysts are
C18:2 hydrogenation were expressed as turnover frequencies
(Table 1). Whatever the metallic particle size, the accessible
atoms of the various Pd/SiO2 catalysts seem to work at a
comparable initial rate (about (2.10 0.20) × 10−19 mol/h).
This observation suggests that the particle size in the range
1.7–7.8 nm does not have an effect on the intrinsic rate of
the SOEE hydrogenation. Likewise, Fig. 3b shows no no-
table evolution concerning the C18:0 product. As for the
trans C18:1 (Fig. 3c), it tends to increase for the Pd1/SiO2
catalyst treated in H2O/H2 medium, which presents the
largest mean particle size of the Pd catalysts series (Table 1,
d = 7.8 nm).
summarized in Table 1. Pd1.5/SiO2 presents slightly larger
particle size (2.0 nm) than Pd0.3 and Pd1/SiO2, which are
similar in size (mean diameter about 1.7–1.8 nm). The sec-
ond strategy was a treatment of the Pd1/SiO2 catalyst un-
der hydrogen atmosphere in a methanol or water solution.
This treatment was suggested from previous results from
our group, which show a sintering of the metallic particles
under similar conditions on various monometallic catalysts
containing Pd, Pt or Rh [28,29,31]. In our case, this treat-
ment induces an increase of the particle size from 1.8 nm
(Pd1) to 3.5 nm (Pd1 in methanol) and 7.8 nm (Pd1 in water)
(Table 1).
Consequently, the rate of hydrogenation of the SOEE
is insensitive to the palladium particle size in the range
1.7–7.8 nm. However, larger particles should be avoided be-
cause they promote the cis–trans isomerization.
Fig. 3a gives the evolution of the C18:2 conversion as a
function of time obtained on the five different monometal-
lic catalysts. From these various curves, initial rates of
Fig. 3. SOEE hydrogenation on the various monometallic palladium/SiO2 catalysts: (a) C18:2 conversion (%) vs. time, (b) C18:0 (%) and (c) trans C18:1
(%) vs. C18:2 conversion (mol%); (᭹) Pd0.3 catalyst; (ꢁ) Pd1 catalyst; (ꢁ) Pd1.5 catalyst; (ꢂ) Pd1 treated in CH3OH/H2 catalyst; (ꢀ) Pd1 treated in H2O/H2
catalyst (for each sample, mcatalyst = 122 mg except for Pd1.5 catalyst, mcatalyst = 41 mg).

B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
121
Table 2
Main characteristics and TOF (C18:2 hydrogenation) of Pd catalysts supported on various oxides
Pd0.3 catalyst/support
Support surface
area (m2/g)
Tcalcination, Treduction
Metala
(wt.%)
db (nm)
Dc (%)
Accessible atom
TOF C18:2 hydrogenation
(◦C)
(×10
g
)
(×1019 mol/h)
−19
cata
SiO2
200
9
215
48
297
40
127
87
300, 400
300, 400
300, 400
300, 300
300, 400
300, 400
300, 400
300, 400
0.39
0.32
0.30
0.38
0.30
0.28
0.30
0.31
1.7**
1.6**
1.3**
2.0*
55
59
70
47
38
26
26
29
1.21
1.07
1.18
1.01
0.65
0.41
0.44
0.51
2.08
3.52
3.70
3.29
2.62
3.80
3.69
3.35
␣-Al2O3, SCS 9
␥c-Al2O3 GFS
TiO2
MgO
ZnO
2.4*
3.5**
3.6*
CeO2
CeZrO2
3.2*
a
Determined by ICP.
b
Mean particle size deduced from electronic microscopy (*) or hydrogen chemisorption (**).
Pd accessibility.
c
3.2. Support effect
sicity of the various oxides established by Martin and Duprez
[32]: MgO ꢂ CeO2 > ZrO2 > ␥c-Al2O3 > SiO2, we observe
that the catalysts producing higher amounts of C18:0 and
trans C18:1 compounds (series 4d, 4f) are more basic than
the alumina, silica and titanium oxide supports (series 4c,
4e). Nevertheless, the results cannot be explained only by
this characteristic of the oxides because the more basic sup-
port (MgO) does not lead to higher percentages of C18:0
and trans C18:1 compared to a less basic support such as the
cerium oxide for example.
The aim of the study was to examine the influence of the
nature of the support on the catalytic properties of supported
palladium for the SOEE hydrogenation. The amount of Pd
was fixed at 0.3 wt.%. The main characteristics of the differ-
ent catalysts are summarized in Table 2.
Fig. 4a and b represent the C18:2 conversion versus time
during the SOEE hydrogenation on the various supported
Pd0.3 catalysts. According to Fig. 4a and b, respectively, two
tendencies become apparent concerning the activity in SOEE
hydrogenation:
In conclusion, this study shows that the use of various ox-
ides to prepare supported palladium catalysts did not improve
the selectivity of the reaction of C18:2 hydrogenation toward
cis C18:1.
• The Pd0.3 catalysts supported on the two aluminas, as well
as titanium oxide, present a conversion slightly higher than
the Pd0.3/SiO2 catalyst.
• The Pd0.3 catalysts supported on magnesium oxide, zinc
oxide, cerium oxide as well as cerium–zirconium, tend to
be less active than the Pd0.3/SiO2 catalyst.
3.3. Modification of the palladium by lead addition
Two bimetallic Pd–Pb catalysts were prepared from
the monometallic Pd1/SiO2 catalyst by catalytic reduction
(Table 1). The blank catalyst “Pd1 treated in CH3OH/H2”
prepared under the same operating conditions, but in ab-
sence of the lead precursor, allows to estimate the influ-
ence of the preparation method of bimetallic catalysts by
catalytic reduction (CR), on the hydrogenating properties of
the Pd1/SiO2 catalyst. EDX microanalysis of Pd–Pb/SiO2
catalysts reduced at 300 ◦C indicates that both metals are de-
posited into closed contact. The mean particle size of the
Pd–Pb catalysts is approximately 50% larger than obtained
fortheparentcatalyst(Table1), indicatingthatsomesintering
is taking place during the preparation of bimetallic catalysts.
Indeed, this phenomenon of sintering has been previously
reported in the course of bimetallic catalyst preparation by
catalytic reduction [28,33].
sFor comparison with the CR bimetallic catalysts, a
1 wt.% Pd–0.05 wt.% Pb/SiO2 catalyst was also prepared
by a traditional technique of successive impregnations (SI)
using the same Pb(CH3COO)2 precursor. The blank and
the bimetallic samples were reduced at 300 ◦C for 2 h. The
comparison of the blank “Pd1 treated in CH3OH/H2” and
the CR Pd–Pb bimetallic catalysts indicates that the addition
of lead involves a small decrease of the C18:2 conversion
The reaction of SOEE hydrogenation being insensitive
to the particle size in the 12–55% dispersion range, which
includes the whole of studied catalysts, it is then appropriate
to express the activities as turnover frequencies (Table 2). The
results indicate that the support effect is really not significant
despite the extremely variable specific surface areas, from
9 m2/g (␣-Al2O3) to 297 m2/g (MgO), and the difference in
acido-basic properties of the supports [32].
Fig. 4c–f represent the evolution of the C18:0 and trans
C18:1 molar percentages, respectively, as a function of the
C18:2 conversion during the SOEE hydrogenation on the var-
ious catalysts separated into two series according to their be-
havior compared to the Pd0.3/SiO2 system. The first series
of catalysts (Fig. 4c and e) includes the Al2O3 supports and
TiO2. These samples present similar selectivities in C18:0
and trans C18:1. The second series of catalysts MgO, ZnO,
CeO2 and CeZrO2 (Fig. 4d and f), shows more important
amounts of C18:0 and C18:1 trans isomers, compared with
the percentages obtained on the Pd0.3/SiO2 sample at iso-
conversion of C18:2. These differences cannot be related to
the structural properties of the supports nor to the size of the
metallic particles. If we examine the classification of the ba-

122
B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
Fig. 4. SOEE hydrogenation on the monometallic Pd0.3 catalysts (mcatalyst = 122 mg) supported on various oxides: (a and b) C18:2 conversion (%) vs. time,
(c and d) C18:0 (%) and (e and f) trans C18:1 (%) vs. C18:2 conversion (mol%); (ꢁ) SiO2; (ꢃ) ␣-Al2O3; (᭹) ␥c-Al2O3; (×) TiO2; (+) MgO; (♦) ZnO; (ꢁ)
CeO2; (ꢀ) CeZrO2.
(Fig. 5a). The result can be explained by the deposit of
inactive lead atoms on the surface of the palladium particles.
This decrease is all the more significant as the content
of the deposited lead is high (Fig. 5a). The deposit of a
comparable lead content (0.05 wt.%) by the SI method led
to a more severe poisoning of the catalyst activity. This
phenomenon also means a strong interaction between the
two metals introduced by successive impregnations. The
different behaviors between the two modes of preparation
can be explained by a selective deposit of lead on particular
sites of palladium when this one is introduced by catalytic
reduction. Similar results have been observed during the
selective hydrogenation of isoprene on bimetallic catalysts
PdFe/Al2O3 prepared by successive impregnations and by
a surface controlled reaction [34]. Moreover, the addition of
lead by both method of preparation does not influence the
proportion in C18:0 (Fig. 5b) but decreases the formation of
trans C18:1 isomers (Fig. 5c). Then, the addition of lead is
beneficial to limit the C18:1 cis–trans isomerization during
the SOEE hydrogenation on palladium. Furthermore, the
hydrogenation rate of palladium is slightly inhibited when
this inactive additive is introduced by catalytic reduction.
3.4. Addition of amines in the reaction medium
The effect of amine addition was studied on the Pd1/SiO2
catalyst. The different amines are listed in Table 3 with the
introduced quantities and the pKa value of their conjugated
acid. The amine/Pdtotal molar ratio is also specified in Table 3
for each experiment. The various amines, introduced directly
in the reaction medium, are likely to be found in the hy-
drogenated oil after reaction. These compounds can be then
eliminated by acidic hydrolysis. However, it should be spec-
ified again that the considered application is located in the
nonfood field.
3.4.1. Addition of primary aliphatic amines
The hydrogenation tests were performed in the presence
of methylamine and butylamine (Table 3, entries 1 and 2) in
order to examine the influence of the length of the alkyl chain
of the amine on the activity and selectivity of the Pd1/SiO2
catalyst. In addition, different amounts of these amines were
introduced to observe the effect of their concentration on the
reaction. Fig. 6a represents the C18:2 conversion in presence
of methylamine as a function of time. From this figure, it can

B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
123
Fig. 5. SOEE hydrogenation on bimetallic Pd–Pb/SiO2 catalysts (mcatalyst = 122 mg): (a) C18:2 conversion (%) vs. time, (b) C18:0 (%) and (c) trans C18:1 (%)
vs. C18:2 conversion (mol%); (ꢃ) Pd1/SiO2; (ꢂ) Pd1 treated in CH3OH/H2; (ꢁ) Pd1–Pb0.04/SiO2 (CR); (ꢀ) Pd1–Pb0.07/SiO2 (CR); (+) Pd1–Pb0.05/SiO2 (SI).
be seen that the addition of methylamine strongly inhibits the
activity of palladium, this effect being all the more significant
as the quantity of methylamine is high. On the other hand,
the introduction of this amine does not have any effect on the
percentage of C18:0 (Fig. 6c). Moreover, Fig. 6e indicates
that the presence of methylamine allows to decrease the for-
mation of trans C18:1. However, the increase of the quantity
of this amine does not induce a more significant drop in the
formation of trans C18:1.
The same kind of experiments conducted in the presence
of butylamine shows that even the addition of 0.40 mL of
this amine (leading to an amine/Pdtotal molar ratio compa-
rable to the addition of 0.15 mL of methylamine, Table 3)
does not induce such a strong deactivation of the Pd1/SiO2
catalyst (Fig. 6b). Moreover, in the case of the addition of
small quantity of butylamine (0.15 mL), no deactivation is
observed. Concerning the C18:0 (Fig. 6d) and trans C18:1
(Fig. 6f), these products present the same evolution in pres-
ence of methylamine or butylamine: no inhibition of the
formation of C18:0 but a significant decrease of the trans
C18:1.
In conclusion, addition of primary aliphatic amines in the
reaction medium induces a more or less important deactiva-
tion according to the length of the chain of the amine and the
introduced quantity. For a similar amine/Pdtotal molar ratio,
the methylamine deactivates more strongly the catalyst than
thebutylamine. ThepKa valuesoftheconjugatedacidofthese
two amines being comparable, this result can be explained by
Table 3
Quantities of introduced amines in the reaction medium during the SOEE hydrogenation on the Pd1/SiO2 catalyst, amine/Pdtotal molar ratios and values of the
pKa of the conjugated acid of each amine
Entry
Amine
pKa
Formula
Introduced volume (mL)
Amine/Pdtotal molar ratio
1
2
Methylamine
Butylamine
10.65
10.77
H3C NH2
H3C (H2C)3 NH2
0.15, 0.30
0.15, 0.25, 0.40
377, 755
131, 217, 351
3
4
5
Triethylamine
Piperidine
Pyridine
10.70
11.12
5.21
0.10, 0.20, 0.25, 0.60
62, 124, 155, 347
0.30, 0.60
264, 528
0.20, 0.70
215, 752

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B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
Fig. 6. SOEE hydrogenation on the Pd1/SiO2 catalyst in presence of primary amines: (a and b) C18:2 conversion (%) vs. time, (c and d) C18:0 (%) and (e and
f) trans C18:1% vs. C18:2 conversion (mol%); (ꢁ) 0.15 mL methylamine; (ꢀ) 0.30 mL methylamine; (᭹) 0.15 mL butylamine; (ꢁ) 0.25 mL butylamine; (+)
0.40 mL butylamine; (ꢃ) without amine.
a higher coverage of the metallic phase by the methylamine
because of its shorter carbonaceous chain. The more com-
pact fitting of the methylamine on the surface of palladium
particles would thus inhibit more strongly the adsorption of
the C18:2 unsaturated ethyl ester. Concerning the selectivity
of the reaction, the addition of these two amines has no effect
on the formation of the C18:0 saturated ester, but allows to
decrease the formation of the trans C18:1 compound.
3.4.2. Addition of tertiary aliphatic amines
In order to examine the assumption of the steric hindrance
of the amines on the catalyst surface, the study was continued
in the presence of a tertiary aliphatic amine, the triethylamine
(Table 3, entry 3). The conversion of C18:2 in presence of
different amounts of triethylamine is given in Fig. 7a. From
this figure, it can be seen that contrary to the addition of
methylamine or butylamine, the presence of triethylamine
Fig. 7. SOEE hydrogenation on the Pd1/SiO2 catalyst in presence of different amounts of triethylamine: (a) C18:2 conversion (%) vs. time, (b) trans C18:1
(%) vs. C18:2 conversion (mol%); (ꢁ) 0.10 mL; (ꢁ) 0.20 mL; (᭹) 0.25 mL; (ꢀ) 0.60 mL; (ꢃ) without amine.

B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
125
Fig. 8. SOEE hydrogenation on the Pd1/SiO2 catalyst in presence of cyclic amines: (a and b) C18:2 conversion (%) vs. time and (c and d) trans C18:1 (%) vs.
C18:2 conversion (mol%); (᭹) 0.30 mL piperidine; (ꢁ) 0.60 mL piperidine; (ꢁ) 0.20 mL pyridine; (ꢀ) 0.70 mL pyridine; (ꢃ) without amine.
does not inhibit the C18:2 hydrogenation. Concerning the
selectivity of the reaction, no evolution of the C18:0 was
observed while the trans C18:1 decreases after addition of
the triethylamine (Fig. 7b).
3.4.3. Addition of cyclic amines
The cyclic amines were often used as additives in the reac-
tion medium during selective hydrogenations [13,14,17–23].
Two various cyclic amines are presented in this work, the
piperidine and the pyridine. The introduced quantities are
listed in Table 3, entries 4 and 5, respectively. The C18:2 con-
versions versus time on the Pd1/SiO2 catalyst are reported in
Fig. 8a (piperidine) and b (pyridine). We can notice that the
presence of cyclic amine involves a more or less important de-
crease of the catalyst activity. In particular, the pyridine does
not deactivate the Pd1/SiO2 catalyst for a amine/Pdtotal molar
ratio = 215 (Fig. 6b, 0.20 mL introduced) whereas piperidine
involves a drop of conversion for a comparable amine/Pdtotal
molar ratio (Fig. 6a, 0.30 mL introduced, molar ratio = 264).
These differences could be explained by the weaker pKa value
of the conjugated acid of pyridine compared to piperidine and
thus by a weaker adsorption of the former on the palladium
surface.
These results confirm that the effects observed on
the hydrogenating activity of a Pd/SiO2 catalyst depend
on the nature of the aliphatic amine. For a compara-
ble amine/Pdtotal molar ratio, the inhibiting effect of the
amine on the C18:2 conversion is all the more high as
the hydrocarbon fragment of the amine is small: methy-
lamine > butylamine > triethylamine. In addition, the pKa
value of the conjugated acid of these three amines being sim-
ilar, these different behaviors can be explained by a lower
coverage of palladium surface by the amine (amine/Pdsurface
ratio) having the highest steric hindrance. The absence of in-
hibiting effect on the conversion of the C18:2 in the case of
triethylamine probably results from a donor effect of nitro-
gen toward the metal (positive electronic effect) which com-
pensates the loss of active surface related to the amine ad-
sorption (negative geometric effect). So, the metallic surface
which is accessible to the reagents decreases in the presence
of amines but it would be more active because of a weaker
adsorption of C18:2 resulting from the donor effect of the
amine. Indeed, kinetic studies have shown a zero order for
the hydrogenation of this ester meaning a strong adsorption
on the metal [35]. This modification of the electronic proper-
ties of the metal also would explain the limited formation of
trans C18:1. Beneficial effects of nitrogen containing com-
pounds addition were already observed during the selective
liquid phase hydrogenation of alkynes, explained by a ligand
effect, which would decrease the complexation strength of
the highly unsaturated hydrocarbons [9–11].
Concerning the reaction products, the presence of the
cyclic amines does not involve modifications on the forma-
tion of the C18:0 on the Pd1/SiO2 catalyst. On the other hand,
the two cyclic amines induce a diminution of the trans C18:1
(Fig. 8c and d). These evolutions are similar with those pre-
viously observed in the case of the aliphatic amines.
4. Conclusion
The objective of this work was to selectively hydrogenate
C18:2 ethyl esters of the sunflower oil (SOEE) toward
C18:1 cis ethyl oleate on supported palladium catalysts, for
a nonfood use such as lubricants. The influence of various

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B. Nohair et al. / Journal of Molecular Catalysis A: Chemical 229 (2005) 117–126
parameters related to the catalyst structure was evaluated in
order to improve the selectivity of the reaction. In the range
of studied dispersions (12–55%), the reaction of SOEE
hydrogenation is insensitive to the size of the palladium
particles deposited on silica. On the other hand, the largest
metallic particles enhance the C18:1 cis–trans isomerization.
In addition, the use of various oxides (␣-Al2O3, ␥c-Al2O3,
TiO2, MgO, ZnO, CeO2, CeZrO2) to deposit palladium did
not improve the selectivity of the reaction obtained in the
presence of the Pd/SiO2 catalyst. By opposite, some supports
led to higher selectivities in trans C18:1. The modification of
palladium by lead addition showed that this inactive deposit
improves the selectivity in cis C18:1, the conversion being
only slightly decreased when this additive is introduced
by surface redox reaction (catalytic reduction). Finally,
the introduction of amines modifies the hydrogenating
properties of supported palladium catalyst. According to the
quantity and the nature of the added amine (aliphatic with
linear or ramified chain, cyclic compounds), the activity of
the Pd/SiO2 catalyst can either be unchanged or inhibited.
These evolutions would result from a promoter electronic
effect generated by the presence of the amine and from a
poison geometric effect related to adsorption of this nitrogen-
containing compound on the palladium surface. Whatever the
nature of the amine, the selectivity in cis C18:1 is improved
compared to the nonmodified monometallic Pd catalyst.
In conclusion, this study relating to the selective hydro-
genation of ethyl ester of sunflower oil on palladium catalysts
shows that it is difficult to be freed from the secondary re-
action of cis–trans isomerization of the required compound
(C18:1). All the results obtained in this work show that the
best control of this selectivity was obtained by introduction
of amines into the reaction medium.
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