Selective Liquid-Phase Hydrogenation of Citral over Supported Palladium

Article abstract of DOI:10.1006/jcat.1997.1817

Citral in the liquid phase was reduced in a low-pressure hydrogenator by using catalysts consisting of palladium supported on (a) a mixed 80 : 20 SiO₂/AlPO₄ system and (b) sepiolite from Vallecas (Madrid, Spain). A kinetic study provided the reaction orders in the substrate concentration and hydrogen pressure. Experimental variables such as temperature, hydrogen pressure and the type of solvent were adjusted in order to optimize the reduction process. The presence of additives of the Lewis acid type such as FeCl₂ was found to considerably alter the hydrogenation mechanism; under these conditions, the selectivity proved strongly dependent on the Fe²⁺/Pd atomic ratio. The reaction products were characterized by using gas chromatography in combination with mass spectrometry.

Full text of DOI:10.1006/jcat.1997.1817

JOURNAL OF CATALYSIS 172, 46–54 (1997)
ARTICLE NO. CA971817
Selective Liquid-Phase Hydrogenation of Citral over Supported Palladium
M. A. Aramend´ıa,1 V. Borau, C. Jime´nez, J. M. Marinas, A. Porras, and F. J. Urbano
Department of Organic Chemistry, Faculty of Sciences, University of Co´rdoba, Avenida San Alberto Magno s/n, E-14004 Co´rdoba, Spain
Received November 15, 1996; revised April 21, 1997; accepted May 22, 1997
SnCl2, FeCl3, MnCl2, or CoCl2 as additive (14, 15). The
favourable effect of the metal ions in these halides is re-
lated to their Lewis acid character, which activates the car-
bonyl group by inducing positive charge on the carbon atom
(16). Poltarzewski et al. (7), who used SnCl2 in conjunction
with Pt catalysts, put forward a second effect related to the
Citral in the liquid phase was reduced in a low-pressure hydro-
genator by using catalysts consisting of palladium supported on (a)
a mixed 80 : 20 SiO2/AlPO4 system and (b) sepiolite from Vallecas
(Madrid, Spain). A kinetic study provided the reaction orders in
the substrate concentration and hydrogen pressure. Experimental
variables such as temperature, hydrogen pressure and the type of
solvent were adjusted in order to optimize the reduction process.
The presence of additives of the Lewis acid type such as FeCl2 was
found to considerably alter the hydrogenation mechanism; under
these conditions, the selectivity proved strongly dependent on the
Fe2+/Pd atomic ratio. The reaction products were characterized by
using gas chromatography in combination with mass spectrometry.
–
formation of solid aggregates of Pt Sn particles that might
decrease the electron density at Pt sites, as in the case of
–
Ni Cu reported by Noller et al. (17).
The textural properties of the support, particle size, ex-
tent of metal dispersion, and nature of the precursor used to
synthesize the catalyst are also influential, to a smaller ex-
tent, on the selectivity of the reduction of ꢀ,ꢁ-unsaturated
carbonyl compounds (5, 18).
c
ꢀ 1997 Academic Press
On the other hand, small changes during the synthetic
process affect the catalyst performance. Thus, the addition
of some oxides such as ZnO (19) favours the reduction of
INTRODUCTION
==
the C C double bond since it leads to the formation of a
The selective reduction of ꢀ,ꢁ-unsaturated carbonyl
compounds is one of the most important research areas in
fine chemistry; an overview of the topic is provided in sev-
eral compilations (1). Crotonaldehyde (2, 3), methacrolein
(4), and, particularly, cinnamaldehyde (5–7) are the most
widely documented compounds in this respect, particularly
in relation to the use of single- and two-metal catalysts sup-
ported on various materials such as carbon, zeolites, se-
piolites, etc. The chief problem lies in obtaining catalysts
bond between the carbonyl compound and the oxide that
leaves the C C group in the optimal spatial orientation for
access by hydrogen. Conversely, the prior reduction of the
catalyst at a high temperature increases the affinity for the
O group (20).
The reduction of ꢀ,ꢁ-unsaturated carbonyl compounds
is also influenced by other experimental variables. Thus,
Grass et al. (21) found temperature and the type of solvent
used (ether, alcohol) to have some effect on the activity and
selectivity of the process.
Prominent among ꢀ,ꢁ-unsaturated carbonyl compounds
is citral (3,7-dimethyl-2,6-octadienal), a substrate of special
==
==
C
==
==
affording the selective reduction of C O or C C double
bonds.
The hydrogenation of ꢀ,ꢁ-unsaturated carbonyl com-
pounds with single-metal catalysts leads to the reduction
==
interest inasmuch as it possesses an isolated C C bond in
==
of the conjugate C C bond with a high selectivity (close
==
==
addition to the conjugate C O and C C groups. Its poten-
tial reduction products are schematized in Fig. 1. Citronelal
and citronelol are especially interesting to the perfume in-
dustry because of their highly pleasant odours (22). The
presence of 3,7-dimethyloctanal and 3,7-dimethyloctanol
detracts from this valuable quality; as a result, for a catalytic
process to have the desired result, it should reduce the con-
to 100% ) (8–11). On the other hand, the formation of un-
saturated alcohols calls for two-metal catalysts (the best re-
–
sults are usually obtained with Ru Sn catalysts supported
on carbon (12)). However, Richard et al. (13), using Pt
catalysts and a reaction medium containing FeCl2, accom-
==
plished the highly selective reduction of the C O group
of cinnamaldehyde. Other authors obtained similar results
for various ꢀ,ꢁ-unsaturated carbonyl compounds by using
==
==
jugate C C bond first and then the C O bond, without
altering the isolated C C bond. However, the highly se-
==
==
lective reduction of the C O group, which is unaffordable
1 To whom all correspondence should be addressed. E-mail: qo1urnaf@
uco.es.
with a single-metal catalyst, is more interesting in chemical
terms.
46
0021-9517/97 $25.00
c
Copyright ꢀ 1997 by Academic Press
All rights of reproduction in any form reserved.

SELECTIVE LIQUID-PHASE HYDROGENATION
47
FIG. 1. Scheme of the products of citral reduction.
In this work, we studied the hydrogenation of citral with
The procedure used in the experiments where FeCl2 was
Pd catalysts supported on (a) a mixed 80 : 20 SiO2/AlPO4 added to the reaction medium was as follows: an amount of
system named PM2and (b) sepiolite from Vallecas(Madrid, 50 mg of catalyst (pre-activated under a hydrogen stream)
Spain) designated PS400. We examined the influence of re- in 13.3 mL of dioxane was supplied with 5 mL of aque-
action variables (temperature, hydrogen pressure, type of ous FeCl2 at an appropriate concentration to ensure the
solvent) on the reduction sequence. The addition of a small required Fe2+/Pd atom ratio. After the temperature had
amount of FeCl2 to the reaction medium was found to al- stabilized at 303 K, an H2 pressure of 60 psi was set and
ter the sequence and increase the selectivity with which the the mixture was shaken for 2 h. Then, a volume of 1.7 mL
==
C
O group was reduced.
of citral (for a 0.5 M concentration) was added, the re-
actor was filled up with H2 and the shaking mechanism
restarted.
EXPERIMENTAL
Each reaction waspreceded bytestsintended to ascertain
the absence of intra- and interparticle diffusion under the
working conditions used. Thus, the absence of intraparticle
diffusion was confirmed by checking that:
Procedure
Reactions were conducted in a Parr Instruments Co.
model 3991 low-pressure reactor at a constant rate of
300 shakes/min. The reaction bottle, 500 mL, was wrapped
in a metal jacket internally thermostated by circulating wa-
ter. The apparatus was equipped with a pressure gauge from
which the pressure inside the bottle could be read at any
time. All reactions were carried out by using an overall so-
lution volume of 20 mL, a 0.5 M citral concentration, an
initial hydrogen pressure of 20–60 psi and a temperature
in the range 283–323 K. Prior to each reaction, the catalyst
(50 mg) was activated by passage of a hydrogen stream at
a flow-rate of 120 mL minꢁ1. The temperature programme
used comprised the following steps: (a) heating from room
temperature to 393 K at 4 K minꢁ1; (b) holding 393 K for
30 min; and (c) cooling to room temperature. The hydrogen
flow was maintained throughout the programme.
(a) The reaction rate did not depend on the shaking rate,
which was the case above 200 shakes/min.
(b) The reaction rate was proportional to the catalyst
weight. For this purpose, we plotted the reciprocal of the
hydrogenation rate against the catalyst weight and obtained
a straight line with a correlation coefficient better than 0.99.
The reciprocal of the intercept was used to obtain the rate
of hydrogen transfer from the gaseous phase to the liquid
phase; because such a rate was higher than the highest hy-
drogenation rate obtained, we can state that the hydroen-
transfer process was not the rate-determining step. These
tests were repeated with every solvent studied and no signs
of diffusion control were observed.

´
48
ARAMENDIA ET AL.
We used a small enough particle size to ensure that no (29, 30). The mean particle size (d), metal surface area (S),
interparticle diffusion occurred.
and metal dispersion (D) of the catalysts were determined
In addition, the absence of both internal and external by transmission electron microscopy (TEM).
heat and mass transfer constraints is apparent from the
turnover frequencies obtained for two PM2 catalysts that
differed in their metal loading by a factor of 3 but had sim-
RESULTS AND DISCUSSION
ilar dispersion. The absence of any significant change in
these turnover frequencies (0.76 vs 0.81 sꢁ1) confirm that
the reaction rates are correctly measured (23, 24).
Characterization of the Catalysts
The results are given in Table 1. Note that support PM2
exhibited greater SBET and acidity values than did sup-
port PS400. Regarding metal catalysts, the most salient find-
ing was that Pd3/PS400 had a larger particle diameter than
Pd3/PM2. The two histograms in Fig. 2 show the metal parti-
cle distribution in terms of inner diameter for both catalysts.
Analysis of Reaction Products
Reaction products were analysed on a Hewlett–Packard
5890 gas chromatograph furnished with a Supelcowax-10
semi-capillary column of 30 m and 0.53 mm ID. Products
were identified on a VG Autospec high-resolution mass
spectrometer.
Hydrogenation of Citral
One of the essential steps in elucidating a reaction mech-
anism is determining the partial reaction order with respect
to each reactant. The order in the hydrogen pressure was
obtained from the initial reduction rate, rg, at a variable
initial hydrogen pressure (20–80 psi) and a temperature of
303 K, using a 0.5 M concentration of citral in tetrahydrofu-
ran and an amount of 50 mg of catalyst Pd3/PM2. The slope
of a logarithmic rg vs pressure plot provided the reaction
order for hydrogen, which was found to be 1.25 (r = 0.994).
The reaction order in citral was obtained by using vari-
able substrate concentrations in the range 0.3–0.9 M, an
amount of 50 mg of catalyst Pd3/PM2 and an initial hy-
drogen pressure of 60 psi. The order was found to be 0.36
(r = 0.998). This and the previous result are consistent with
strong adsorption of citral and weaker adsorption of hydro-
gen (31).
Synthesis and Characterization of Catalysts
The catalysts used contained 3% Pd supported on (a)
a mixed 80 : 20 SiO2/AlPO4 system or (b) Spanish sepio-
lite supplied by TOLSA S.A. (Vallecas, Madrid, Spain); the
supports were designated PM2 and PS400, respectively. The
procedures used to prepare the two solids were described in
detail elsewhere (25, 26). The metal was deposited by using
the method of “impregnation to incipient wetness,” using
Na2PdCl4 as the precursor salt. The pretreated support was
placed in flask and supplied with appropriate amounts of
precursor and aqueous sodium hydroxide. The flask was
then placed on a rotavapor and stirred for 24 h, after which
the solvent wasvacuum-evaporated. Subsequently, the mix-
ture was dried in a stove at 383–393 K for 24 h and ground to
fine particles. Finally, the powder was calcined in a furnace
at a linearly increasing temperature up to 483 K, which was
held for 30 min.
The deposited metal salt was reduced in a hydrogen flow
system consisting of a furnace into which the material to
be reduced was inserted within a glass U-tube. The gas
flow-rate was set at 120 mL minꢁ1. The reduction pro-
gramme started at 373 K, which was held for 1 h and then
raised linearly at 2 K minꢁ1 up to 493 K, which was held for
10 min. Then, the U-tube was removed from the furnace
and allowed to cool to room temperature under a constant
flow of hydrogen. Thermal programmed reduction profiles
confirmed that the temperature programme used ensured
complete reduction of palladium on the surface of the
support (27).
In order to complete the kinetic study, activation para-
meters such as the energy (Ea), enthalpy (1H #) and entropy
of activation (1S#) were calculated from the Arrhenius and
Eyring equations.
Table 2 lists the results obtained for catalyst Pd3/PM2
in four different solvents, using a citral concentration of
TABLE 1
Textural, Acid–Base and Metal Properties of the Supports
and Catalysts
Properties
Textural
Sesp Vp rp
Acid–basic
Metalic
Acidity
Basicity
d
˚
A
S
D
%
Supports and metal catalysts were characterized from ni-
trogen adsorption–desorption isotherms recorded on a Mi-
cromeritics ASAP 2000 porosimeter. Their specific surface
area, was obtained by using the BET method (28). The
acid–base properties of the supports were determined by
using a thermal programmed desorption–mass spectrome-
try method with two different types of probe (pyridine for
acid sites and CO2 for basic sites) as described elsewhere
Catalyst m2 ꢂ gꢁ1
A
ꢂmolPY ꢂ gꢁ1 ꢂmolCO2 ꢂ gꢁ1
m2 ꢂ gꢁPd1
˚
˚
A
PM2
Pd3/PM2
402 0.36 141
241 0.30 50
115
—
—
—
—
62
—
81
—
18
PS400
Pd3/PS400
121 0.36 141
71 0.32 180
84
—
46
—
—
144
—
35
—
8
Note. Specific surface area, SBET; pore volume, Vp; mean pore radius,
rp; mean particle diameter, d, metal surface area, S; metal dispersion, D.

SELECTIVE LIQUID-PHASE HYDROGENATION
49
mass spectrometry) thereby decreasing the reduction rate.
The effect arises from sterically hindered access of the sub-
strate to the catalytic site. In any case, the exact nature of the
solvent effect on the reduction of citral is rather complex
and requires further investigation (32, 33).
The time course of the hydrogenation process was mon-
itored by recording hydrogen uptake and product distribu-
tion profiles (Figs. 3a and 3b, respectively) at an initial citral
concentration of 0.5 M in THF and an initial H2 pressure of
60 psi, using 50 mg of catalyst Pd3/PM2 and a temperature
of 303 K. The hydrogen uptake profile (Fig. 3a) consists of
three distinct portions of different slope that correspond
to as many reduction rates. The first portion (the steepest),
==
corresponds to the reduction of the C C bond conjugated
==
with the C O bond, which takes place at the highest rate.
The second, of gentler slope, represents the period dur-
==
ing which the isolated C C bond is reduced. Finally, the
third portion corresponds to the reduction of the carbonyl
group to an alcoholfunction. Therefore, the reduction ofthe
==
conjugate C C bond takes place first, at a high rate that
FIG. 2. Metal particle size distribution histograms for the two cata-
lysts studied: (A) Pd3/PM2; (B) Pd3/PS400
.
0.5 M, an initial hydrogen pressure of 60 psi and temper-
atures between 293 and 323 K. As a rule, the higher the
dielectric constant of the solvent was, the higher was the
activation energy obtained. These results suggest a strong
influence of the solvent on the reduction temperature. Ac-
cording to Grass et al. (21), the effect arises from (a) the
acid–base properties of the solvent; (b) its electrical proper-
ties (a high dielectric constant favours alternative reactions
over hydrogenation owing to polarity effects); and (c) the
substrate–solvent reaction.
Our attempts at correlating rg values with the acid–base
properties of the solvent led to no conclusive results, so
the latter can have no appreciable effect on the reaction.
On the other hand, the dielectric constant was found to
significantly affect the results, which differed markedly at
a given temperature depending on the solvent. Thus, the
solvents with low ꢃ values (dioxane and cyclohexane) led
to high reduction rates that decreased with increase in the
solvent polarity (e.g., in methanol).
FIG. 3. Hydrogen uptake (a) and product distribution profiles (b)
for the reduction of citral with molecular hydrogen. Reaction conditions:
amount of catalyst (Pd3/PM2), 50 mg; solvent, tetrahydrofuran; temper-
ature, 303 K; citral concentration, 0.5 M (᭺, E-citral; ᭢, Z-citral; ꢀ, cit-
The effect of the substrate–solvent reaction is interesting
when the solvent is an alcohol, which forms an acetal with
the carbonyl group of citral (compunds characterized by ronelal; ᭝, dihydrocitronelal; ᭜, citronelol).

´
50
ARAMENDIA ET AL.
TABLE 2
Kinetic and Activation Parameters for the Hydrogenation of Citral with Catalyst Pd3/PM2
in Different Solvents
Kinetic parameters
Activation parameters
Solvent
rg
TON
sꢁ1
Ea
1H #
kJ ꢂ molꢁ1
1S#
J ꢂ kꢁ1 ꢂ molꢁ1
ꢃ
T/ꢃC
mmol ꢂ sꢁ1 ꢂ gꢁPd1
kJ ꢂ molꢁ1
MeOH
32.6
293
303
313
323
1.0
1.3
2.5
3.5
0.59
0.76
1.47
2.05
40.0 ꢄ 7.2
20.0 ꢄ 2.5
8.7 ꢄ 0.4
36.5 ꢄ 7.2
ꢁ9.6 ꢄ 2.5
CHA
2.0
293
303
313
323
7.7
11.7
13.8
17.3
4.52
6.87
8.10
17.9 ꢄ 2.5
6.1 ꢄ 0.4
19.8 ꢄ 5.2
ꢁ166.2 ꢄ 8.4
ꢁ202.0 ꢄ 1.5
ꢁ165.8 ꢄ 16.9
10.16
DIO
2.2
293
303
313
323
13.4
14.6
16.3
17.8
7.87
8.57
9.57
10.45
THF
7.6
293
303
313
323
3.9
5.1
5.7
9.9
2.29
2.99
3.35
5.81
22.4 ꢄ 5.2
Note. MeOH, metanol; CHA, cyclohexane; DIO, dioxane; THF, tetrahidrofurane; ꢃ, dielectric con-
stant; rg, initial rate; TON, turnover frequency; Ea, 1H #, and 1S#, energy, enthalpy, and entropy, of
activation, respectively. Reaction conditions: amount of catalyst (Pd3/PM2), 50 mg; hydrogen pressure,
60 psi; citral concentration, 0.5 M.
==
decreases as the reaction develops and the isolated C
bond and carbonyl group are reduced.
C
E/Z selectivity, SE/Z as
SE/Z
The product distribution profile (Fig. 3b) shows changes
in the reactant and product concentrations as a function of
the reaction time. At the beginning, citral is reduced highly
selectivelyto citronelalvia the double bond conjugated with
the carbonyl group. The amount of product formed peaks
(change of slope, in Fig. 3a) and then decreases, concomi-
tantly with an increase in the amount of dihydrocitronelal,
which suggests that the former is reduced to the latter. The
proportion of citronelol (obtained by reduction of the car-
bonyl group of dihydrocitronelal) is very low but increases
slightly as the hydrogenation proceeds.
mmol of E consumed
mmol of E consumed + mmol of Z consumed
=
ꢅ 100.
[2]
As noted earlier, citronelal is the reaction product of the
highest industrial interest of all obtained in the process.
Table 3 illustrates the influence of solvent, temperature, and
hydrogen pressure on Scitronelal and SE/Z. Selectivities were
determined at 10 and 25% of conversion. The standard re-
action conditions were 0.5 M citronelal in dioxane, 303 K,
50 mg of catalyst Pd3/PM2, and 60 psi (initial H2 pressure).
From the results it follows that low pressures and tempera-
tures, in combination with methanol as the solvent, are the
best in order to ensure high values of both types of selec-
tivity.
In examining the influence of variables such as tempera-
ture, initial hydrogen pressure, solvent, and catalysts on the
process, we defined the selectivity towards reaction prod-
ucts (i) that show in Fig. 1, as
As regards the influence of the support, the last section
in Table 3 shows initial rate (rg), turnover frequency (TON)
and selectivity data (Scitronelal and SE/Z) for reactions con-
ducted with 0.5 M citral in dioxane, an initial H2 pressure
mmol i
Si =
ꢅ 100,
[1]
mmol products of reaction
where (i) is citronelal, dihidrocitronelal, citronelol, geran- of 60 psi, a temperature of 303 K and 50 mg of catalyst
iol, nerol, and 3,7-dimetiloctanol. Pd3/PM2 or Pd3/PS400. The turnover, TON, was greater for
On the other hand, citral has two geometric isomers, Z catalyst Pd3/PS400 because it had a smaller metal surface
and E. It wasthusalso interestingto determine the influence and larger Pd particles (i.e., a lower electron density) than
of the previous parameters on the changes in both isomers catalyst Pd3/PM2. However, neither the reaction sequence
during the reduction process. To this end, we defined the nor selectivity differed between the two catalysts (citronelal

SELECTIVE LIQUID-PHASE HYDROGENATION
51
lyst pores, all dependent on the morphology of the metal
surface) (5, 6); (b) electronic, which arise from the differ-
ential electron density of palladium metal particles (high in
small particles and low in small ones) (35, 36); and (c) steric,
or related to the molecule of the structure to be reduced
(37–40). The reduction of citral with Pd catalysts is affected
by geometric and steric factors, but scarcely by electric fac-
tors, which are highly influential on the reduction of cin-
namaldehyde, however. All this favours the reduction of
TABLE 3
Influence of the Temperature, Hydrogen Pressure, Solvent, and
Metal Support on the Initial Rate (rg), Turnover Frequency (TON),
and Selectivities (Scitronelal and SE/Z) of the Process
Scitronelal/% SE/Z/%
rg
TON
sꢁ1
mmol ꢂ sꢁ1 ꢂ gꢁPd1
a
b
a
b
Temperature
ꢃC
293
303
313
323
13.4
14.6
16.3
17.8
7.87
8.57
9.57
94
88
80
76
84
81
73
69
79 74
77 69
73 60
68 60
==
==
C
C double bonds over the C O group, as is the case in
most reductions of ꢀ,ꢁ-unsaturated carbonyl compounds
10.45
with a single-metal catalyst (8–11).
Phydrogen
psi
20
40
60
80
11.3
12.9
14.6
15.7
6.63
7.57
8.57
9.22
97
94
88
82
86
83
81
75
83 78
81 76
77 69
76 61
Effect of FeCl2 in the Reaction Medium
Solvent
Catalyst
MeOH
THF
CHA
DIO
1.3
5.1
11.7
14.6
0.76
2.99
6.87
8.57
99
97
92
88
96
91
87
81
78 74
85 73
81 69
77 69
Citral was also reduced in a reaction medium contain-
ing FeCl2 in an Fe2+/Pd ratio of 0–2. Figures 4a, b show
the variation of the TON and the selectivity of the differ-
ent reaction products (citral, citronelal, dihydrocitronelal,
Pd3/PM2
Pd3/PS400
14.6
8.7
8.57
11.82
88
82
81
73
77 69
75 67
Note. Conversion: a = 10% ; b = 25% . Standard conditions: amount of
catalyst (Pd3/PM2), 50 mg; hydrogen pressure, 60 psi; temperature, 303 K;
citral concentration, 0.5 M; solvent, dioxane.
and dihydrocitronelal were the main reaction products with
both).
Richard et al. (5), using cinnamaldehyde and Pt catalysts,
found the selectivity to be markedly dependent on parti-
cle size. Thus, large particles led to the unsaturated alcohol
preferentially, whereas small particles led to the saturated
aldehyde. On the other hand, our experiments with citral
showed neither turnover nor selectivity to depend on par-
ticle size, consistent with the previous findings of Galvagno
et al. (34) for citral. The explanation for the divergence
with the results of Richard et al. lies in steric factors related
to the molecule to be reduced. Thus, the stiffness of the
cinnamaldehyde molecule (where the phenyl ring is conju-
==
==
gated with C C and C O bonds) favours its adsorption
==
==
via the C C bond on small particles and the C O bond
on large particles. On the other hand, the citral molecule
–
can rotate freely about the C4 C5 bond, thus facilitating
==
adsorption via the C2 C3 bond, whatever the particle size.
SE/Z always exceeded 50% , so isomer E was reduced to
a greater extent than was isomer Z. This is logical, taking
into account that the reduction rate, calculated from the
distribution profiles for the two isomers at similar concen-
trations, was greater for isomer E than for isomer Z (e.g.,
6.2 vs 4.1 mmol/s ꢂ gPd as calculated from the distribution
profile of Fig. 3b at a 0.15 M concentration of each isomer).
As a rule, the reduction of an ꢀ,ꢁ-unsaturated carbonyl
compounds with a single-metal catalyst is influenced by var-
ious types of factors, namely: (a) geometric (e.g., support
FIG. 4. Variation of the turnover frequency (TON) (a), and selectivity
(b) in the reduction of citral with Pd3/PM2 in the presence of variable
amounts of FeCl2. Reaction conditions: amount of catalyst (Pd3/PM2),
50 mg; solvent, dioxane; temperature, 303 K; citral concentration, 0.5 M;
conversion, 25% (᭺, (E + Z)-citral; ꢀ, citronelal; ᭝, dihydrocitronelal; ᭜,
structure, extent of metal dispersion, accessibility of cata- citronelol; ᭢, unsaturated alcohol (geraniol + nerol)).

´
ARAMENDIA ET AL.
52
the reduction rate decreased as a result. On the other hand,
an excessive proportion of Fe atoms (an Fe2+/Pd mole ratio
above 1) would turn the two-metal catalyst into a single-
metal (Fe) catalyst of selectivity similar to that obtained
with the single-metal Pd catalyst (initial Fe2+/Pd ratio = 0)
(see Fig. 4b).
Figure 6 shows the hydrogen uptake (a) and products
distribution (b) profiles for a reaction carried out at 303 K
and an initial Fe2+/Pd ratio of 1. As can be seen, the
amount of unsaturated alcohol (geraniol + nerol) increased
with increasing hydrogen uptake, whereas that of citronelal
peaked and then decreased. The amount of citronelol ob-
tained was very small, consistent with the results in the ab-
sence of FeCl2.
For easier comparison of the performance of the three
most frequently used catalysts, Fig. 7 shows the selectivity
profiles (obtained from Eq. [1]) for Pd3/PS400 (a), Pd3/PM2
(b), and Pd3/PM2 doped with FeCl2 in a unity Fe2+/Pd mole
ratio (c). Catalyst Pd3/PM2, was the most selective towards
FIG. 5. Model for the selective adsorption–reduction of citral on a Pd
catalyst doped with FeCl2.
geraniol, and nerol) as a function of the Fe2+/Pd ratio at
25% of conversion.
The effect of Lewis acids on the reduction of carbonyl
compounds with Pd catalysts was previously studied (41).
The selectivity change introduced by FeCl2 arises from the
presence of positively charged ferrous ions on the surface
of Pd particles. These species may originate from unre-
duced Fe2+ ions or electron-deficient Fe0 atoms resulting
from electronegativity differences between iron and palla-
dium (Fig. 5). Surely, only a few of initial ferrous ions ap-
pear either as Fe0 atoms over palladium or unreduced Fe2+
ions associated with palladium particles. The rest of ferrous
ions stay in solution, far from the palladium, and they do
not take part in the reaction. In futher works, we will at-
tempt to determine the percentage of initial ferrous ions
that take part in the reaction. The interaction of positively
charged iron with the carbonyl group causes an electron to
==
be transferred from the oxygen atom in the C O group of
citral, which is thus made electron-deficient. Next, an elec-
tron rearrangement takes place by which one of the elec-
trons in the pair that forms the double bond is transferred
–
to the oxygen atom while the other is used to form a C H
bond between the carbonyl carbon and a proton bonded
to the more electronegative metal (Pd). Finally, the forma-
tion of a bond between the second proton and the oxygen
causes the alcohol to be released (13, 42). In this context,
Coq et al. (32) claim that low-coordinated acid sites (Snꢄ+
,
Zr3+) involved in a two-metal catalyst (usually with Ru)
==
favour anchoring via the C O double bond and, hence, its
reduction.
The reduction rate and the proportion ofeach reaction in-
gredient varied with the Fe2+/Pd ratio. Thus, the maximum
amount of unsaturated alcohol (geraniol + nerol) was ob-
tained at an initial atomic Fe2+/Pd ratio close to unity. On
the other hand, rg increased linearly with increase in such
a ratio up to Fe2+/Pd = 1, above which it decreased sharply
with further increases. The origin of this variation pattern
for rg lies in the above-described mechanism. Thus, below
an Fe2+/Pd ratio of 1, the presence of Fe2+ facilitates the
emergence of electric factors, which are absent from the
single-metal catalyst. On the other hand, in the presence of
a large amount ofFe2+ (viz. at an Fe2+/Pd ratio above unity),
FIG. 6. Hydrogen uptake (a) and product distribution (b) profiles for
the reduction of citral with catalyst Pd3/PM2 in a reaction medium contain-
ing FeCl2 at an Fe2+/Pd ratio of 1. Reaction conditions: amount of catalyst
(Pd3/PM2), 50 mg; solvent, dioxane; temperature, 303 K; citral concen-
tration, 0.5 M (᭺, (E + Z)-citral; ꢀ, citronelal; ᭝, dihydrocitronelal; ᭜,
the dissociative cleavage of H2 on Pd atoms is hindered and citronelol; ᭢, unsaturated alcohol (geraniol + nerol)).

SELECTIVE LIQUID-PHASE HYDROGENATION
53
nied by side reactions which, together with steric hindrance,
decrease the rate at which the substrate can access active
sites.
The hydrogenation of the E (trans) isomer is favored over
that of the Z (cis) isomer as a result of the adsorption of the
latter on the catalyst surface being sterically hindered.
The presence of Lewis acid additives such as FeCl2 in-
duces an electron transfer between the two metals (Fe
and Pd) arising from their differential electronegativity;
==
the effect favours adsorption of citral via the C O group
and, hence, increases the selectivity towards geraniol and
nerol.
ACKNOWLEDGMENTS
The authors gratefully acknowledge funding of this research by the
Consejer´ıa de Educacio´n y Ciencia de la Junta de Andalucı´a and the Di-
reccio´n General de Investigacio´n Cientı´fica y Te´cnica (DGICyT) in the
framework of Project PB92-0816.
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