Hydrogenation of citral on activated carbon and high-surface-area graphite-supported ruthenium catalysts modified with iron

Article abstract of DOI:10.1006/jcat.2001.3394

The hydrogenation of citral has been performed over Ru-Fe catalysts supported on activated carbon and on high-surface-area graphite. It was found that selectivity to unsaturated alcohols is independent of the carbonaceous support used for ruthenium catalysts. The addition of iron enhances selectivity to unsaturated alcohols (geraniol and nerol) in a manner similar for both ruthenium catalysts, becoming maximum for the highest iron loading. Calorimetric experiments give some evidence about alloy formation in ruthenium catalysts promoted with iron. It is inferred that the surface polarity of the alloyed particles promotes the selective hydrogenation of citral toward unsaturated alcohols.

Full text of DOI:10.1006/jcat.2001.3394

Journal of Catalysis 204, 450–459 (2001)
doi:10.1006/jcat.2001.3394, available online at http://www.idealibrary.com on
Hydrogenation of Citral on Activated Carbon and High-Surface-Area
Graphite-Supported Ruthenium Catalysts Modified with Iron
�
�
�,1
B. Bachiller-Baeza, A. Guerrero-Ruiz,† P. Wang, and I. Rodr ´ı guez-Ramos
�
Instituto de Cat a´ lisis y Petroleoqu ´ı mica, C.S.I.C., Campus de Cantoblanco, 28049 Madrid, Spain; and †Departamento Qu ´ı mica Inorg a´ nica y T e´ cnica,
U.N.E.D., C/Senda del Rey s/n, 28040 Madrid, Spain
Received June 13, 2001; revised August 16, 2001; accepted August 16, 2001; published online October 25, 2001
lysts supported on activated carbon and carbon blacks (6,
The hydrogenation of citral has been performed over Ru–Fe 7). It was observed that selectivity to unsaturated alcohol
catalysts supported on activated carbon and on high-surface-area depended on the number of oxygenated surface groups;
graphite. It was found that selectivity to unsaturated alcohols is
independent of the carbonaceous support used for ruthenium cata-
lysts. The addition of iron enhances selectivity to unsaturated alco-
hols (geraniol and nerol) in a manner similar for both ruthenium
catalysts, becoming maximum for the highest iron loading. Calori-
groups did not influence selectivity. But rather, the presence
metric experiments give some evidence about alloy formation in
ruthenium catalysts promoted with iron. It is inferred that the sur-
the greater the number of oxygenated groups, the higher
the selectivity. In contrast, the same reaction was studied
over graphite-supported platinum and ruthenium catalysts,
and it was concluded that the number of oxygenated surface
of residual amounts of chloride ions anchored to the surface
oxygen groups enhances unsaturated alcohol selectivity (8).
face polarity of the alloyed particles promotes the selective hydro-
In another investigation graphite-supported catalysts were
found to be much more selective than charcoal-supported
ones in the liquid-phase hydrogenation of cinnamaldehyde
to cinnamyl alcohol (9). It was proposed that the selectivity
improvement observed was related to the electron donating
properties of the graphite, which was interacting with the
metal particles located along the edges of the basal planes.
Higher selectivity for unsaturated alcohols might also be
arranged by using a second metal component. It is well-
known that various metal salts, such as iron, tin, or germa-
nium chlorides, added to the reaction medium have a pos-
itive effect on selectivity to the unsaturated alcohol in the
hydrogenation of cinnamaldehyde over platinum catalysts
genation of citral toward unsaturated alcohols.
�c 2001 Elsevier Science
Key Words: microcalorimetry; citral; hydrogenation; ruthenium
catalysts; iron; carbon supports.
1
. INTRODUCTION
The hydrogenation of �,�-unsaturated aldehydes to un-
saturated alcohols has been extensively studied in recent
years because of the relevance of these compounds to the
pharmaceutical and fine chemicals industry (1, 2). More-
over, much work has been done to develop heterogeneous
catalysts applicable to these reactions. On one hand, these
systems avoid the production of residues and waste ob-
tained in conventional stoichiometric processes. On the
other hand, they alleviate the difficulties associated with the
separation and recycling of homogeneous catalysts. More-
over, different approaches to designing a proper catalyst
have been used to improve the generally low selectivity to
the desired alcohol. It is known that the final selectivity
of an active metal can be modified by using an adequate
support, which interacts with the metal, or by adding a sec-
ond component as a promoter. Electronic and geometric
effects have been suggested as the explanation for the im-
provement in selectivity to unsaturated alcohols (3–5).
Some work in this line has focused on the study of carbon-
aceous materials as supports. The gas-phase hydrogenation
of crotonaldehyde has been carried out over platinum cata-
(
10, 11). Richard et al. found volcano-type activity, a selec-
tivity curve, and an optimum molar ratio for Fe/Pt of 0.2 for
the hydrogenation of cinnamaldehyde with Pt/C catalysts
and FeCl2 as additive (10). They observed the formation of
bimetallic particles with electron-deficient iron atoms act-
ing as Lewis adsorption sites for the activation of the C O
bond. Bimetallic catalysts have also been investigated in the
hydrogenation of �,�-unsaturated aldehydes (12–18), with
tin and iron being the most effective among the promot-
ers. Moreover, mechanical mixtures of Pt–Fe2O3 possess
bifunctional properties, iron oxide coordinates the alde-
hyde molecule, and Pt activates the hydrogen (19). How-
ever, exhaustive studies on bimetallic catalysts involving Fe
as promoter are scarce. Pt–Fe/C catalysts follow a behavior
similar to that obtained by adding iron salt to the reaction
medium in the hydrogenation of cinnamaldehyde (20).
In this paper we report on a study of the hydrogenation
of citral using bimetallic Ru–Fe catalysts supported on two
==
1
To whom correspondence should be addressed. Fax: 34-91-5854760.
E-mail: irodriguez@icp.csic.es.
450
0
�
021-9517/01 $35.00
c 2001 Elsevier Science
All rights reserved

HYDROGENATION OF CITRAL
1
451
carbon materials. We have examined the influence of the 8 K �min� (22). The effluents of the reactor were analyzed
promoter and the effect of a noninteracting or inert support in a gas chromatograph provided with a thermal conductiv-
(active carbon) and of an electron donor support (graphite) ity detector, an automatic sample injection, and a Porapack
on the catalysts reactivity. In addition, the possible combi- Q column.
nation of the two effects is analyzed. Different techniques
Temperature-programmed desorption (TPD) experi-
are applied for the characterization of the catalysts; in par- ments were performed under vacuum. The apparatus con-
ticular, the microcalorimetry of CO adsorption is intended sists of a quartz bulb directly attached to a quadrupole mass
to be correlated with the electronic surface properties of spectrometer (Balzers QMG 421-C) (23). The temperature
the supported metallic clusters.
of the sample was increased from 473 to 1023 K at a rate of
�
1
1
0 K � min
.
The samples after reaction were further analyzed by
2
. EXPERIMENTAL
X-ray photoelectron spectroscopy (XPS). The spectra were
obtained on a Physical Electronics 5700 spectrometer
equipped with a hemispherical multichannel Electronics
2
.1. Catalyst Preparation
8
0-365B analyzer. The excitation source was the MgK� line
The supports used in the preparation of the catalysts were
2
(h� = 1253.6 eV, 300 W). The binding energies of Fe 2p
anactivatedcarbon(C, ICASASpain, SBET = 964m /g)and
3/2
2
and Ru 3p were referenced to the C 1s line at 284.8 eV.
a high-surface-area graphite (H, Lonza, SBET = 295 m /g).
3/2
X-ray diffraction patterns of the samples used in the re-
The activated carbon was treated with inorganic acids (HCl
and HF) and then washed with deionized water to remove
inorganic impurities. The graphite was treated to clear the
surface from oxygen groups under flow of He at 1173 K (21).
Monometallic catalysts were prepared by incipient
wetness of the supports with an aqueous solution of
RuNO(NO3)3. Bimetallic ruthenium–iron catalysts were
action were recorded using a Philips PW 1050/81 appara-
tus using a filtered CuK� radiation (� = 0.1538 nm) and a
graphite monochromator.
2
.3. Calorimetric Measurements
The heats of CO adsorption at 330 K were measured in a
prepared by a coimpregnation method with aqueous solu- differential heat-flow microcalorimeter of the Tian–Calvet
tion of RuNO(NO3)3 and Fe(NO3)3 � 9H2O. All solids were type C80 from Setaram. The adsorption vessels were linked
dried overnight at 383 K.
to a Pyrex volumetric apparatus equipped with greaseless
stopcocks that permitted the introduction of small pulses
of CO (21). The samples were activated in flowing H2 at
2
.2. Characterization of Catalysts
6
1
73 K for 2 h, and after outgassing at this temperature for
6 h were cooled down to the adsorption temperature. Then
Composition of the catalysts was determined by ICP–
AES in a Perkin Elmer 3300 PV after dissolution of the
catalysts. Metal loading of the monometallic catalysts was
also determined by burning away the carbon in air at 1073 K
and weighing the residue. The results of elemental analysis
and the Fe/Ru atomic ratios are shown in Table 1.
Temperature-programmed reduction (TPR) experi-
ments were performed using a U-shaped quartz reactor.
A high-purity mixture of 4% hydrogen in helium was fed
into the reactor and the temperature of the sample was
successive doses of CO were sent onto the sample. The
equilibrium pressure was measured by means of a Baratron
pressure transducer MKS Instrument. The calorimetric and
volumetric data were stored and analyzed using microcom-
puter processing.
2
.4. Activity Measurements
The liquid-phase hydrogenation of citral was carried out
increased from room temperature to 873 K at a rate of at atmospheric pressure under H2 flow in a five-necked
TABLE 1
Catalyst Characterization
CO adsorption
�
1
Catalyst
% Ru
% Fe
(Fe/Ru)Chem
(Fe/Ru)XPS
(�mol � g
)
CO/Ru + Fe
Ru/C
1.7
1.5
1.4
—
1.9
1.6
1.6
—
0.4
1.5
2.1
—
—
0.4
2.0
—
—
0.5
2.1
—
5.1
7.8
—
—
1.5
4.3
130
87
81
23
75
62
43
0.75
0.43
0.20
0.11
0.40
0.25
0.09
RuFe/C
RuFe2/C
Fe/C
Ru/H
RuFe/H
RuFe2/H
0.5
1.8

4
52
BACHILLER-BAEZA ET AL.
reactor fitted with a stirred head (850 rpm), a thermome- hand, Ru/C catalyst presents a well-defined second H2 con-
ter, a reflux condenser, gas entry through a glass filter, and sumption peak at higher temperatures, centered at 660 K.
a port for reactant introduction and sample extraction. The This peak is accompanied by CH4 evolution and thus could
catalysts were transferred to the reactor, after reduction be due to a partial carbon gasification process (22). For the
in the gas phase at 673 K for 2h, using the conventional Ru/H catalyst the peak is broader, but again the simultane-
Schlenk technique to preserve the catalyst from air exposi- ous evolution of methane in the same range of temperature
tion. The reactor flask containing the solvent (isopropanol) is observed. The TPR profile for the Fe/C catalyst shows a
was heated at 333 K in flowing H2 and citral was injected peak at 690 K, assigned to the reduction of surface Fe3+
citral/Ru molar ratio of 30) through the charging port. The This temperature is within the temperature range reported
.
(
distribution of products was followed by repetitive sam- for the reduction of iron supported on different carbon
pling. The samples were analyzed by gas chromatography materials (24–26).
using a MFE-20 capillary column and a flame ionization
detector.
The TPR profiles of the bimetallic catalysts are close to
that of the Ru unpromoted catalysts. Thus, only one peak,
Preliminary runs carried out at different stirring condi- at 510 K for catalysts supported on activated carbon and at
tions, loading, and catalyst grain size demonstrated the ab- 490 K for the catalysts supported on graphite, is observed.
sence of external and internal mass transfer limitations.
The reducibility of surface metal oxides has been described
as being affected by the presence of a noble metal and in
general the reduction temperature of the oxide becomes
lower (25, 27). Also, it has been reported that the close
interaction between the two metals could inhibit the re-
duction of the noble metal (27). The TPR curves show that
3
. RESULTS AND DISCUSSION
3
.1. Catalysts Characterization
The TPR profiles obtained for the monometallic and reductionofFeisshiftedtolowertemperatures, andthatthe
bimetallic Ru–Fe catalysts are shown in Fig. 1A for the cata- two metals are reduced together. This latter fact is deduced
lysts supported on activated carbon, and in Fig. 1B for from the following. On the one hand, only a single defined
the catalysts supported on high-surface-area graphite. Both peak is observed in the TPR profile at low and medium
monometallic Ru catalysts displayed a sharp peak, at 510 K temperature ranges. This peak coincides with that of ruthe-
for Ru/C and at 490 K for Ru/H, which corresponds to the nium reduction in monometallic catalysts, the temperature
reduction of the salt precursor to metallic ruthenium (21). of which is not raised by the presence of the promoter.
The lower reduction temperature for Ru/H indicates that On the other hand, the amount of hydrogen consumption
metal reduction is facilitated in this catalyst. On the other corresponding to this peak increases with the iron loading,
FIG. 1. TPR profiles. (A) Catalysts supported on carbon: (a) Fe/C, (b) Ru/C, (c) RuFe/C, and (d) RuFe2/C. (B) Catalysts supported on graphite:
(
a) Ru/H, (b) RuFe/H, and (c) RuFe2/H.

HYDROGENATION OF CITRAL
453
suggesting that the peak has to be associated with the that the initial heats of CO adsorption were very close
reduction of not only Ru but also Fe. Quantitative evalua- for the Ru/H (138 kJ/mol) and Ru/C (137 kJ/mol) cata-
tion of the relative amounts of reduced Ru and particularly lysts. In a first approach these values seem to contradict
Fe is not possible because the reduction reaction involves those of previous studies carried out with Ru catalysts
the reduction of oxides and mainly of undecomposed pre- supported on activated carbon and graphite (28, 29). A
cursors, and consequently of the NO and NO3 groups of the higher initial heat of CO adsorption was observed on ruthe-
molecules. The TPR profiles of bimetallic catalysts show nium/graphite, indicating enhanced electron density in the
also the peak corresponding to the gasification of the sup- ruthenium particles caused by electron transfer from the
port, but it is less marked than that observed for the Ru graphite. However, it was also shown that the presence of
monometallic catalysts. These results reveal the reduction oxygen groups at the surface of the graphite restrains the
in catalytic activity for methane production by the gasifica- metal-support interaction (21). In this work a high-surface-
tion of the carbon support when iron is added as promoter area graphite free of oxygen surface groups was utilized as
to Ru and can be indicative of specific metal-promoter in- support. However, the aqueous solution of the ruthenium
teractions. Therefore, the TPR patterns of the bimetallic precursor (RuNO(NO3)3) used in the preparation of the
catalysts indicate close proximity of iron and ruthenium.
catalysts has a very high oxidizing character. However, it
is known that the active sites present at the edges of the
graphitic layers of the carbonaceous materials easily re-
act with an oxidant (HNO3) and form oxygen functional
groups (30). Therefore, it can be inferred that oxygen sur-
face groups were inevitably introduced into the carbona-
ceous material during the preparation procedure, imped-
ing the electron donor ligand effect of the graphite support
over the ruthenium particles supported on it (31). In fact,
TPD experiments over the two monometallic ruthenium
catalysts show the presence of surface oxygen groups which
decompose upon heating in vacuum. The most acidic (car-
boxyls and lactones) evolve as CO2 between 473 and 800 K
and the less acidic (phenols and carbonyls) evolve as CO
above 723 K. Moreover, the activated carbon contains a
higher amount of surface oxygen functional groups. This
latter fact is expected, given that the proportion of active
sites increases as the surface area of the carbon increases.
Moreover, the calorimetric profiles for the Ru/C and
Ru/H catalysts show that the energetic distribution of the
CO adsorption sites is very heterogeneous (Fig. 2). The dif-
ferential heat of CO adsorption on Ru seems to steadily
decrease from the start, although in the range of coverage
between 0.3 and 0.6 the rate of fall is slower. On the other
hand, the Fe/C catalyst shows a plateau at the initial heat of
adsorption up to a coverage of approximately 0.2, at which
point the heat values slowly fall until physical adsorption is
reached. It is worth noting that the initial differential heat of
CO adsorption on Fe is lower than on Ru. The formation of
subcarbonyls species by the admission of CO onto reduced
3
.2. Calorimetric Measurements
The electronic properties of the surface sites of mono-
and bimetallic catalysts were studied using CO adsorption
microcalorimetry. The differential heats of CO adsorption
on Ru, Fe, and Ru–Fe catalysts with different Fe/Ru atomic
ratios, and supported on activated carbon and on graphite,
are given in Figs. 2A and 2B, respectively. It can be seen
0
samples containing Fe crystallites is expected (32, 33). The
formation of this type of species explains the lower ini-
tial heat of adsorption observed for the iron catalyst. Al-
though few measures of CO chemisorption on Fe have been
performed, the initial heat value compares well with the
data for CO adsorption on films and with that determined
on a Fe supported on MgO catalyst (34, 35). The latter
authors found a particle-size dependence in the distribu-
tion of CO adsorption heat. In particular, the amount of
weakly bound CO (80–90 KJ/mol) increases with increasing
dispersion.
FIG. 2. Differential heats of CO adsorption at 330 K. (A) Catalysts
supported on carbon: (ᮀ) Ru/C, (᭺) RuFe/C, (᭝) RuFe2/C, and (᭞) Fe/C.
(
B) Catalysts supported on graphite: (ᮀ) Ru/H, (᭺) RuFe/H, and (᭝)
RuFe2/H.

4
54
BACHILLER-BAEZA ET AL.
The initial heats of adsorption for the Ru–Fe catalysts not appear in the XRD patterns even in the samples with
supported on activated carbon are lower than that for Ru/C, higher iron content. This latter fact implies that the size
22 kJ/mol. The two promoted bimetallic catalysts with of the metallic crystallites remains below the size limit of
1
different Fe/Ru ratios display similar heat-of-adsorption XRD detectability (<4 nm) and therefore they could not
shapes falling between the two monometallic extremes be detected. The TPR profiles of the bimetallic catalysts
throughout the coverage range (Fig. 2A). Moreover, the have shown the simultaneous reduction of the two metals,
increase in the Fe/Ru ratio results in a calorimetric curve supporting the formation of a Ru–Fe alloy.
approaching that of the iron monometallic catalyst. On the
3
.3. Activity Measurements
other hand, the addition of Fe to the Ru/C catalyst reduced
also the extent of CO adsorption on the surface. The sat-
uration coverages of carbon monoxide on the investigated
catalysts are tabulated in Table 1. It can be seen that the sat-
uration extent of carbon monoxide is almost constant with
increasing content of iron for catalysts supported on acti-
vated carbon. As for the bimetallic catalysts supported on
high-surface-area graphite, the initial heats of CO adsorp-
tion are lower than that for the monometallic ruthenium,
and they decrease with increasing iron content, from 143 to
The desired products of citral hydrogenation are the un-
saturated alcohols geraniol and nerol, which are obtained
throughhydrogenationoftheC == Obondofthetwoisomers
E and Z of citral. However, different products, from the iso-
lated or the conjugated C == C double-bond reduction and
successive hydrogenation of the primary products, can be
produced as displayed in the reaction scheme (Scheme 1).
The reaction of citronellal with the alcoholic solvent and its
cyclization to isopulegol have been also proposed to occur.
Citral hydrogenation at 333 K was carried out over the
mono- and bimetallic catalysts after reduction at 673 K. The
results of the test are summarized in Table 2. The slope of
the linear region of the plots of conversion vs. time was used
to determine the initial rates for each catalyst. It can be seen
that the initial activities for the two monometallic Ru cata-
lysts are similar. However, Fe/C presents negligible activity
under our experimental condition. The selectivity for the
unsaturated alcohols (geraniol and nerol) is the same for
the two ruthenium catalysts (38 and 39%) and follows the
same trend in the whole range of conversion (Fig. 3). More-
over, mainly one of the two unsaturated alcohols, geraniol,
is produced by hydrogenation of the C == O bond. Similar se-
lectivities to unsaturated alcohols have been found (38) for
activatedcarbon-supportedruthenium catalysts with differ-
ent metal particle sizes. However, these results, in a first ap-
proach, contrast with those previously reported by Gallezot
and co-workers (9, 39). These authors found selectivities to
the unsaturated alcohol in the hydrogenation of cinamalde-
hydeinliquidphaseforgroupVIIImetalssupportedongra-
phite that were higher than for those supported on activated
1
33 and 112 kJ/mol for 0.5 and 2.1 Fe/Ru ratios, respectively
(Fig. 2B). Concerning the saturation coverage, while for ac-
tivated carbon-supported catalysts the amount of adsorbed
CO is constant with increasing Fe loading, for graphite-
supported catalysts it decreases with increasing Fe/Ru ra-
tio (Table 1). It can be concluded that the addition of Fe
to Ru/C and Ru/H catalysts results in a weakening of the
CO bound states and a decrease in the amount of CO ad-
sorbed. Surface analysis of catalysts by XPS shows that the
surface of the metal crystallites is enriched in Fe compared
to the bulk composition, with the iron enrichment increas-
ing with increasing Fe/Ru ratio (Table 1). This fact agrees
well with data reported for carbon-supported and unsup-
ported Ru–Fe alloys (36, 37). This surface-iron enrichment
is to be expected in Ru–Fe alloys from the fact that iron
has a lower surface energy than does ruthenium (36). Pre-
vious M o¨ ssbauer spectroscopy results obtained on carbon-
supported Ru–Fe catalysts showed the formation mainly of
the Ru–Fe alloy and also of a small amount of segregated
metallic iron (25).
On the basis of the heat of CO adsorption data presented
above, it appears that the surface of the metallic particles
in the Ru–Fe catalysts is possibly an alloyed phase. This is
inferred from several aspects of the data. First, the differ-
ential heat curves of the bimetallic catalysts fall monotoni-
cally between the two monometallic extremes. Segregation
of the two metals would imply initial ruthenium-like heat,
since CO is expected to preferentially adsorb on ruthenium
sites at low coverage due to its higher CO adsorption heat.
Furthermore, the differential heat would fall off as the iron
sites became more important. Second, the total CO up-
take is nearly half that expected for a surface composed
of two segregated metals. This can be explained by either
a decrease in the metal surface area or a change in the
CO adsorption stoichiometry on the Ru–Fe alloyed phase.
CleardiffractionpeaksassignedtoRu, Fe, orRuFealloydid
TABLE 2
Catalytic Properties in the Hydrogenation of Citral
Activity
(�mol � g� � s� 1
1
)
S
a
(%)
Catalyst
G+N
Ru/C
1.12
1.02
0.97
0
39
52
69
—
38
66
73
RuFe/C
RuFe2/C
Fe/C
Ru/H
RuFe/H
RuFe2/H
1.24
1.07
0.41
a
Selectivity to geraniol and nerol at 70% conversion.

HYDROGENATION OF CITRAL
455
SCHEME 1
carbon, particularly for Pt and Ru. They attributed this fact
to an electron transfer from the graphite to the metal par-
ticles grafted on edge planes that modify the mode of ad-
sorption of the organic molecule. However, they use oxygen
prefunctionalized graphite and charcoal supports to pre-
pare the catalysts by ion exchange with metal ammine chlo-
ride precursors. These facts suggest that on one hand, the
presence of oxygen functional groups at the surface of the
graphite impedes the electron donor effect of the macroli-
gand graphite, but on the other hand, it provides anchoring
sites for the chloride anions. As has been earlier shown (8)
for graphite-supported platinum and ruthenium catalysts,
the presence of a small amount of residual chloride in
the catalyst promotes the selective hydrogenation of �,�-
unsaturated aldehydes toward unsaturated alcohols. The
calorimetric experiments over Ru/C and Ru/H samples do
not show changes in the differential heat of CO adsorption.
From here, it can be inferred that the electron density of
Ru particles in the catalyst supported on graphite is similar
to that supported on activated carbon. So, modifications in
selectivity to geraniol and nerol are not expected. In spite
of the similarity in unsaturated alcohol selectivity, some-
what different behavior in the hydrogenation reaction was
found for these two catalysts. Figure 4 shows the compo-
sition of the reaction mixture as a function of conversion
FIG. 3. Evolution of selectivity toward geranio + nerol with conver-
sion in the citral hydrogenation at 333 K for (ᮀ) Ru/C, (᭺) RuFe/C, (᭝)
RuFe2/C, (᭞) Ru/H, (᭛) RuFe/H, and (ଝ) RuFe2/H.

4
56
BACHILLER-BAEZA ET AL.
FIG. 4. Hydrogenation of citral at 333 K over Ru/H (A) and Ru/C (B). (ᮀ) Citronellal, (᭺) geraniol, (᭛) nerol, (᭞) citronellol, (᭝) 3,7-
dimethyloctanol, ( �+ ) other products, and (� ) acetals of citronellal.
for Ru/C and Ru/H. In the first step of the reaction, Ru/H as discussed above, although the original carbon materials
produces citronellal and unsaturated alcohols as the main used as support have clean surfaces, during impregnation
products. A small amount of citronellol is also obtained. with the oxidizing solution of the metal precursor, a certain
At higher reaction times, the consumption of the primary number of surface oxygen groups were created. The TPD
products by further hydrogenation to citronellol and finally experiments have shown that the surface oxygen groups
to 3,7-dimethyloctanol is observed. The secondary reac- are mainly carboxyl groups, which confer acidic character
tions of citronellal, forming acetals and isopulegol, occur to the carbon surface. Moreover, given that the proportion
to a lesser extent, with the formation of the latter being of active sites increases as the surface area of the carbon
almost inhibited. On Ru/C catalysts, acetals of citronellal increases, the content of surface oxygen functional groups
were formed in the first stages of the reaction. Citronel- is higher in the activated carbon. Therefore, it can be under-
lal was obtained in a lower amount than on a Ru/H cata- stood that an activated carbon-supported ruthenium cata-
lyst. The formation of acetals leads to the hydrogenation lyst produces more reactions catalyzed by acid sites than
of the terminal C == C double bond, producing the acetals does a Ru/H catalyst. These results agree with those re-
of dihydrocitronellal (included in other products), and sub- ported by Milone et al. (41). They found a relationship be-
sequently less citronellol is obtained. These results agree tween the chemical nature of the support and the product
with those previously reported for the hydrogenation of distribution of the citronellal hydrogenation.
citral on supported ruthenium catalysts (40). Furthermore,
isopulegol (included in other products) is formed on the ported on activated carbon and on graphite does not have
Ru/C catalyst by cyclization of citronellal. a pronounced effect on initial activity except for RuFe2/H,
The use of iron as a promoter in ruthenium catalysts sup-
The reactions of isomerization of citronellal forming which presents a marked decrease (Table 2). Concerning
isopulegol and the reaction of citronellal with the solvent selectivity, we can see in Table 2 and Fig. 3 that the addition
forming acetals have been associated with the existence of iron favors the production of geraniol and nerol. The in-
of acid sites in the catalyst (39, 41, 42). It has been sug- crease in selectivity to these compounds with increasing
gested that the catalyst is a bifunctional system, where iron loading is much clearer for the samples supported
isomerization occurs on the surface of the support and the on activated carbon than for those supported on high-
hydrogenationofC == CandC == Obondsoccursonthemetal surface-area graphite. For the former catalysts, selectivity
particles (41). Carbon materials with clean surfaces do not increases from 39% for Ru/C to 52 and 69% for the 0.4
present acid sites and therefore the occurrence of these and 2.0 Fe/Ru ratios, respectively. In graphite-supported
reactions would be inhibited on these supports. However, catalysts, selectivities obtained for the two iron loadings

HYDROGENATION OF CITRAL
457
FIG. 5. Hydrogenation of citral at 333 K over RuFe/C (A) and RuFe/H (B). (ᮀ) Citronellal, (᭺) geraniol, (᭛) nerol, (᭞) citronellol, (᭝) 3,7-
dimethyloctanol, ( �+ ) other products, and (� ) acetals of citronellal.
are 66 and 73%. The product distribution displayed for the the coimpregnation of the supports during the preparation
bimetallic catalysts also changed with respect the mono- procedure.
metallic counterpart. It is observed in Fig. 5A for RuFe/C
that geraniol is the main product of the reaction and that
the citronellal formed is subsequently hydrogenated to cit-
ronellol after 50% of citral conversion. Compared with the
4
. CONCLUSION
The increase in selectivity to unsaturated alcohols for the
monometallic Ru/C catalyst, the increase in the initial cit- Ru–Fe catalysts is in line with that previously reported for
ronellal selectivity is accompanied by a decrease in selectiv- supported group VIII metal catalysts promoted with a sec-
ities to acetal of citronellal and to other secondary products. ond element, particularly Sn and Fe. The enhancement of
That is, a mitigation in the acid site catalyzed reactions is selectivity in hydrogenating the carbonyl group has been as-
observed. Selectivity to geraniol and nerol decreases after sociated with the presence of polarity at the metal surface
total conversion of citral due to its further hydrogenation (3). And two mechanisms have been proposed for the pro-
to citronellol and to cis- and trans-3,7-dimethyl-octen-2-ol. moting effect in the bimetallic catalysts: (i) The promoter
Similar patterns were obtained for RuFe2/C, but the sec- acts as an electrophilic or Lewis site activating the C == O
ondary reactions of citronellal were totally inhibited. The bond and favoring its adsorption and hydrogenation, or
decrease in the acid site catalyzed reactions for RuFe/C and (ii) the promoter acts as electron-donor ligand, increasing
RuFe2/C catalysts correspond with a reduction in the sur- the electron density of the active metal, which reduces the
face population of oxygen groups on the samples, as seen adsorption of the C == C bond and its hydrogenation. How-
in TPD experiments. In the case of the catalysts supported ever, themodeofactionandthestateofthepromoteradded
on graphite, both catalysts show comparable profiles and were not definitively known under working conditions and
in Fig. 5B it is displayed for RuFe/H. Geraniol is the main modern techniques were used to improve the catalyst char-
product but citronellal was also detected. The most impor- acterization.
tant fact is that the small amount of citronellal acetals de-
From the techniques applied in the present work to char-
tected with Ru/H is absent with both RuFe/H and RuFe2/H. acterize the catalysts, the formation of Ru–Fe alloy can be
The two isomers of 3,7-dimethyl-octen-2-ol were observed inferred. It is clear from the TPR experiments that the two
after the disappearance of citral. From the reduction of the metals are in close interaction. The reduction of Fe is shifted
secondaryreactionscatalyzedbyacidsitesoverthebimetal- to lower temperatures and it could be considered that the
lic catalysts, a diminution in the surface oxygen group pop- iron is present in a totally reduced state. Our calorimetric
ulation of these catalysts can be inferred. This is explained experiments have shown that the promoted catalysts sup-
by the less oxidizing character of the solution employed for ported on activated carbon and on graphite exhibit heats of

4
58
BACHILLER-BAEZA ET AL.
CO adsorption intermediate between the two monometal-
lic extremes. As discussed above, this fact implies a Ru–Fe
alloy formation.
2. Bauer, K., and Garbe, D., “Common Fragance and Flavor Materials.”
VCH, Weinheim, 1985.
3
4
5
6
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The formation of an alloy and its role in the hydrogena-
tion of unsaturated aldehydes has been a controversial
issue. In the hydrogenation of citral with Rh–Sn catalysts
supported on silica, and prepared using an organometallic
route or by impregnation and oxidation-reduction activa-
tion, the alloy was almost unselective toward unsaturated
alcohols and no electronic effects due to electron transfer
from Sn to Rh were present (15, 18, 43). On the other hand,
Claus (4) found the highest selectivity to crotylalcohol
in the hydrogenation of crotonaldehyde over supported
group VIII metal M–Sn/SiO2 (M = Rh, Pt, or Ru) catalysts
7
8
9
. Coloma, F., Narciso-Romero, J., Sep u´ lveda-Escribano, A., and
Rodr ´ı guez-Reinoso, F., Carbon 36, 1011 (1998).
. Bachiller-Baeza, B., Guerrero-Ruiz, A., and Rodr ´ı guez-Ramos, I.,
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41, 171 (1988).
1
1
1
1
1
1
1
1
1
1
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(1995).
prepared using controlled surface reaction. The author
�
�
�+
considers that the Rh � Sn bimetallic sites of the alloyed
phase are the selective sites for the C == O hydrogenation (4).
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(
1997).
In the case of iron, Pt–Fe alloys over carbon prepared ex situ
or by adding FeCl2 to the solution have shown excellent val-
ues of selectivity in the hydrogenation of cinnamaldehyde
4. Coq, B., Figueras, F., Moreau, C., Moreau, P., and Warawdekar, M.,
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(10, 44).
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The catalytic results presented in this paper give addi-
1
03, 4431 (1999).
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74, 246 (1998).
tional support for evidence of the important role of al-
loyed phase in determining unsaturated alcohol selectivity.
Thus, alloy particles of ruthenium with the more elec-
tropositive metal, iron, exhibit surface polarity, which is
necessary for interacting with the oxygen atom of the
carbonyl group of the �,�-unsaturated aldehyde. There-
fore, mechanism (i), described above, is considered to
account for the promoting effect of iron. So, the iron
1
8. Coup e´ , J. N., Jord a˜ o, E., Fraga, M. A., and Mendes, M. J., Appl. Catal.
A 199, 45 (2000).
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2
2
In conclusion, the Ru–Fe alloy supported on activated
carbon and on graphite is very selective for the production 24. Ill a´ n-Gomez, M. J., Raymundo-Pi n˜ ero, E., Garc ı´ a-Garc ´ı a, A., Linares-
Solano, A., and Salinas-Mart ´ı nez de Lecea, C., Appl. Catal. B 20, 267
of unsaturated alcohols (geraniol + nerol) by the selective
(
1999).
hydrogenation of citral. This selectivity is independent of
the carbonaceous support used, probably due to the pres-
ence of oxygen groups at the edges of the graphitic layers,
whichrestrainsthe electrontransferfromgraphite to metal-
lic particles. Moreover, the oxygen groups with acidic char-
acter created at the surface of the carbonaceous supports
catalyze secondary reactions, whose extension is a function
of their population.
2
5. Guerrero-Ruiz, A., Sep u´ lveda-Escribano, A., and Rodr ´ı guez-Ramos,
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2
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ACKNOWLEDGMENTS
1. Rodr ı´ guez-Reinoso, F., Guerrero-Ruiz, A., Moreno-Castilla, C.,
Rodriguez-Ramos, I., and L o´ pez-Gonzalez, J. D., Appl. Catal. 23, 299
The financial support of the CICYT of Spain under project MAT-1999-
005 is acknowledged. One of the authors, P. W., thanks the Spanish
Agency of International Cooperation for a scholarship grant.
(
1986).
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1
3
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(
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(1979).
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