Selective hydrogenations on heterogenized ruthenium complexes

Article abstract of DOI:10.1016/j.jcat.2004.06.017

The heterogenized versions of [RuCl₂(PPh₃) ₃] and [{RuCl₂(TPPMS)₂}₂] were prepared and applied in the selective hydrogenation of α, β-unsaturated aldehydes. Depending on the conditions the above heterogenized catalysts could hydrogenate selectively either the C=O or the C=C bonds, similar to the results obtained in aqueous biphasic systems. Meanwhile the heterogenized catalysts show all the advantages that we can expect from a heterogeneous catalyst: good performance, easy separation, and the possibility of recycling. The effect of H₂ pressure on the selectivity was also studied.

Full text of DOI:10.1016/j.jcat.2004.06.017

Journal of Catalysis 227 (2004) 428–435
www.elsevier.com/locate/jcat
Selective hydrogenations on heterogenized ruthenium complexes
Ágnes Zsigmond a,∗, István Balatoni a, Krisztián Bogár a, Ferenc Notheisz a, Ferenc Joó b
a
Department of Organic Chemistry and Organic Catalysis Research Group of the Hungarian Academy of Sciences, University of Szeged,
6720 Szeged, Dóm tér 8, Hungary
Institute of Physical Chemistry and Research Group on Homogeneous Catalysis of the Hungarian Academy of Sciences,
b
University of Debrecen PO Box 7, 4010 Debrecen 10, Hungary
Received 16 April 2004; revised 11 June 2004; accepted 16 June 2004
Available online 9 September 2004
Abstract
The heterogenized versions of [RuCl (PPh ) ] and [{RuCl (TPPMS) } ] were prepared and applied in the selective hydrogenation of
2
3 3
2
2 2
α,β-unsaturated aldehydes. Depending on the conditions the above heterogenized catalysts could hydrogenate selectively either the C=O or
the C=C bonds, similar to the results obtained in aqueous biphasic systems. Meanwhile the heterogenized catalysts show all the advantages
that we can expect from a heterogeneous catalyst: good performance, easy separation, and the possibility of recycling. The effect of H
pressure on the selectivity was also studied.
2
2004 Elsevier Inc. All rights reserved.
Keywords: Chemoselectivity; Heterogenized catalyst; Hydrogenation; Ru complex; α,β-Unsaturated aldehydes
1. Introduction
been introduced by Augustine and co-workers [6]. The pro-
cedure involves the attachment of a preformed metal com-
plex to a solid support using a heteropoly acid (HPA: phos-
photungstic, phosphomolybdenic, or silicotungstic acids) as
the anchoring agent. The heteropoly acids are attached to
the support by the interaction of the protons of the acid with
the basic sites of the support (alumina, carbon). Either ion
pairing or direct bonds between surface oxygen atoms of the
HPA and the metal center of the complex have been sug-
gested to account for the immobilization of the metal com-
plex onto the heteropoly acid. However, the true nature of
this linkage is still unclear. Nevertheless, the resulting cat-
alyst is at least as active as the homogeneous one, and in
addition it has the advantages of a heterogeneous system
[7,8].
The selective hydrogenation of α, β-unsaturated aldehy-
des has attracted much interest in homogeneous catalysis
because of the synthetic value of the corresponding allylic
alcohols [9]. It is also an industrially important process dealt
with in several patents and papers such as the one from
Degussa [10]. In that study the authors used the Ru com-
plexes [RuCl2(TPPTS)3] and [H2Ru(TPPTS)3] (TPPTS,
During the last decade there has been a dramatic devel-
opment within the field of selective catalysis for the produc-
tion of fine chemicals. Among these processes, selective hy-
drogenations with soluble catalysts have become more and
more important in the pharmaceutical industry, and conse-
quently in catalytic research. Both the activity and the se-
lectivity of these soluble complexes are excellent, but the
difficulties of recovering and recycling limit the usefulness
of these catalysts. Due to these limitations, an increased de-
mand has developed toward the application of heterogeneous
catalysts. The heterogeneous systems could be modified het-
erogeneous catalysts [1–3] or heterogenized homogeneous
complexes [4]. The importance of the heterogenized homo-
geneous catalysts is shown by the large number of recent
publications [5].
A new and highly efficient method for the heterogeniza-
tion of homogeneous hydrogenation catalysts has recently
*
Corresponding author. Fax: +36 62 544 200.
E-mail address: azsig@chem.u-szeged.hu (Á. Zsigmond).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.06.017
2004 Elsevier Inc. All rights reserved.

Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
429
tris(3-sulfonatophenyl)phosphane trisodium salt) together
with their supported aqueous-phase (SAPC) heterogenized
analogs. It is believed that the immobilization of the SAPC
catalysts (used mainly for the hydroformylation of long-
chain alkenes [11]) is due to the strong interactions between
the sulfonated phosphane ligands and the silanol groups of
the silica support within a surface water film. 3-Methyl-
2-butenal (prenal) was hydrogenated on these catalysts in
hexane with 95% selectivity. When the hydrogenation was
carried out in methanol, low activity and rapid deactivation
were observed. Since a high percentage of the metal was
recovered in the solution, it was suggested that the rapid
deactivation was caused by the dissolution of the metal
complex in the polar media. Even in the nonpolar media
it was difficult to recycle the catalyst, because of the poi-
sonous adsorption of organic compounds at the catalyst
surface. Supported iridium–phosphane catalysts appeared
more promising. These catalysts were chemoselective for
several unsaturated aldehydes, including 3-methyl-2-butenal
and all-trans-retinal, producing a suitable unsaturated al-
cohol and in the latter case they were stereoselective, too.
Recovery was easy and it was possible to use the supported
catalysts in polar reaction media. However, the activity of the
heterogenized catalysts was one or two orders of magnitude
less than that of the corresponding homogeneous catalyst.
Although recycling could be achieved, the productivity of
these heterogenized catalysts is still to be improved in or-
der to make them reasonable alternatives to unsupported
(soluble) homogeneous catalysts in the hydrogenation of
α, β-unsaturated aldehydes. These difficulties point out the
importance of the heterogenization of [RuH2(PPh3)4] and/or
[RuH2(TPPTS)4] on solid supports and the application of
these heterogenized systems in the above-noted reaction.
Similar water-soluble Ru complexes, such as [{RuCl2
(TPPMS)2}2] (TPPMS, (3-sulfonatophenyl)diphenylphos-
phane sodium salt). were applied in the hydrogenation
of α, β-unsaturated aldehydes, and depending on the pH,
various Ru–hydride complexes were established as active
catalytic species [12]. At pH < 6 the [HRuCl(TPPMS)3]
(1a) was the dominant Ru(II) complex and it catalyzed
a slow but selective hydrogenation of the C=C bond in
trans-cinnamaldehyde. Conversely, at pH > 8 the [H2Ru
(TPPMS)4] (2a) was found to be an active and selective cat-
alyst for the C=O reduction. Consequently, changing the pH
of the solution could shift the equilibrium between the two
Ru species and invert the selectivity of the hydrogenation of
trans-cinnamaldehyde.
genization of soluble complexes [13], either by synthesizing
the complex inside the zeolite or by anchoring it onto the
surface [14].
2. Experimental and methods
2.1. Materials: preparation of the catalysts
[RuCl2(PPh3)3] was purchased from Aldrich and used as
received. [{RuCl2(TPPMS)2}2] was prepared from TPPMS
and RuCl3 × 3H2O as described in the literature [12].
The purity of the TPPMS ligand was checked by NMR
spectroscopy, using a Bruker AVANCE DRX 500 MHz in-
strument. The NMR spectra showed that the sulfonated
triphenylphosphane has one SO3 group in position 3.
2.2. Anchoring the soluble complexes
The amount of 1.5 g of NaY zeolite (Aldrich) was sus-
pended in 30 mL of 96% ethanol, and 288.0 mg (0.1 mmol)
of phosphotungstic acid hydrate (PTA) was dissolved in
25 mL of ethanol. This solution was added dropwise into the
zeolite suspension with efficient stirring. The stirring was
continued for 2 days at room temperature, under an Ar at-
mosphere. The mixture was filtered and the solid residue
was suspended in 30 mL of ethanol. Ninety-six milligrams
(0.1 mmol) of [RuCl2(PPh3)3] was dissolved in 40 mL of
deoxygenated ethanol and this solution was dropped slowly,
with stirring to the suspension. The stirring was continued
for another 2 days. The mixture was filtered and washed
with ethanol, until a colorless solution was obtained. The
light brown solid material (1.6 g) was dried at 30 ◦C for 2 h
in vacuum and for 1 day under argon. A 1.45 g catalyst was
obtained, with a Ru content of 36.3 µmol/g.
The heterogenized [{RuCl2(TPPMS)2}2] catalyst was
prepared analogously using 1.5 g NaY zeolite and (0.1 mmol)
PTA; 90.1 mg (0.1 mmol Ru) of [{RuCl2(TPPMS)2}2] com-
plex was applied; and 1.5 g of the heterogenized catalyst was
obtained with a Ru content of 2.0 µmol/g.
2.3. Catalyst characterization
FTIR spectra of the support, the Ru complexes, and the
heterogenized samples were recorded on a Bio-Rad FTS–
65 A spectrophotometer, in the range of 400–4000 cm−1, in
KBr pellets. XRD spectra were obtained on a Philips PW–
1830 diffractometer. The metal content of the anchored cat-
alysts was determined using a JOBIN YVON 24-type ICP-
AES instrument; 250 mg catalyst samples were dissolved in
4 mL conc HNO3.
Considering all of these former results and the environ-
mental and economic aspects, heterogenization of the above-
noted catalytic systems has become a challenge. In this paper
we report our efforts to heterogenize onto a zeolite support
the [RuCl2(PPh3)3] and [{RuCl2(TPPMS)2}2] complexes
and to apply them in the hydrogenation of α, β-unsaturated
aldehydes. NaY zeolite was chosen as a support, since our
former experience in this field has shown that the three-
dimensional zeolite is generally applicable for the hetero-
2.4. Hydrogenation experiments
Trans-cinnamaldehyde and crotonaldehyde were hydro-
genated in a batch reactor of 30 mL capacity, at 65 ◦C

430
Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
and 0.4 MPa hydrogen pressure. For the hydrogenation
of the C=C bond, acidic conditions were applied: 10 mg
(10.42 µmol) [RuCl2(PPh3)3] or 300 mg (10.89 µmol Ru)
[RuCl2(PPh3)3]/NaY was added to 3 ml of 96% ethanol, fol-
lowed by 3 µL (20 µmol) Et3N and 7.8 mg (30 µmol) PPh3.
The catalyst precursors were prehydrogenated overnight.
Then 50 µl of substrate (0.396 mmol trans-cinnamaldehyde
or 0.603 mmol crotonaldehyde) was injected and the reac-
tor was pressurized with H2 and the reaction was initiated
by starting the stirring. For the C=O reduction, we have
adjusted a basic condition, with the same amount of Ru pre-
cursor and 13.5 mg (0.05 mmol) PPh3 together with 6 µL
(0.04 mmol) Et3N. Samples were taken every hour from the
reaction mixture, and the products were analyzed by cap-
illary gas chromatography (Hewlett Packard 5890 Series II)
using a DB-5 column at 145 ◦C. Reactions with the TPPMS-
containing catalysts were done the same way using 12.3 mg
(0.03 mmol) or 20.5 mg (0.05 mmol) TPPMS for the two
different conditions.
Fig. 1. The FTIR spectra of [RuCl (PPh ) ] and [RuCl (PPh ) ]/PTA/
2
3 3
2
3 3
NaY: 1, [RuCl (PPh ) ]; 2, [RuCl (PPh ) ]/PTA/NaY; 3, PTA/NaY.
2
3 3
2
3 3
Similar spectra were obtained for [{RuCl2(TPPMS)2}2]
and its heterogenized analog. The comparison of the spectra
leads to the same conclusion as that for [RuCl2(PPh3)3].
The XRD spectra of the zeolite and the heterogenized
samples were taken as well. The similarity of these spectra
shows no change of the zeolite structure during the catalyst
preparation.
2.5. Catalysts recycling
In order to determine the metal complex concentration
of the heterogenized catalysts, samples were dissolved in
conc HNO3. The metal content of these solutions were de-
termined by ICP-AES. The solid catalysts were found to
contain 36.3 µmol/g and 2.0 µmol/g Ru in the case of
the anchored [RuCl2(PPh3)3] and [{RuCl2(TPPMS)2}2], re-
spectively. This is a fairly large difference in view of the
same heterogenization procedure. It seems that the neutral
[RuCl2(PPh3)3] can be more easily incorporated to the PTA
layers containing large polytungstate anions than the bulky
[{RuCl2(TPPMS)2}2] which carries negatively charged sul-
fonated ligands.
The heterogenized catalysts were used in several subse-
quent runs. After the reactions the catalyst was recovered by
filtration under Ar, washed with ethanol, dried in Ar, and
then reused.
3. Results and discussion
With the aim of developing active, chemoselective het-
erogeneous catalysts, we have prepared the anchored
[{RuCl2(TPPMS)2}2] and [RuCl2(PPh3)3] catalysts, using
the method developed by Augustine and co-workers [7]. In
all cases phosphotungstic acid was used as anchoring ma-
terial. The heterogenized complexes were characterized by
spectroscopic methods and applied as catalysts in the hydro-
genation of α, β-unsaturated aldehydes. To our knowledge
this is the first example of heterogenization of these com-
plexes by this relatively new method and applying them for
catalysis of hydrogenation reactions.
3.2. Catalytic properties of the catalysts
[{RuCl2(TPPMS)2}2] is water soluble and in aqueous
(biphasic) systems under H2 a pH-dependent equilibrium
exists between its monohydrido [HRuCl(TPPMS)3] (1a)
and dihydrido [H2Ru(TPPMS)4] (2a) derivatives. These two
Ru–hydrides have different selectivities in the hydrogenation
of α, β-unsaturated aldehydes. Namely, the monohydrido
derivative hydrogenates the C=C double bond, while the di-
hydrido species is a selective catalyst for the hydrogenation
of the C=O double bond [12]. Consequently, simply adjust-
ing the pH of the aqueous phase can change the selectivity
of the hydrogenation of trans-cinnamaldehyde.
3.1. Physical properties of the catalysts
The heterogenized catalysts were characterized with
the usual spectroscopic methods. The FTIR spectra of
the support (PTA/NaY), the [{RuCl2(TPPMS)2}2] and the
[RuCl2(PPh3)3] complexes, and the heterogenized samples
were all taken.
3.3. Hydrogenation of trans-cinnamaldehyde
The comparison of these spectra (Fig. 1) shows con-
vincingly that [RuCl2(PPh3)3] is anchored on the NaY sup-
port. The spectrum of the heterogenized catalyst displays the
bands at 1493, 1440, 1076 cm−1, which are characteristic for
the [RuCl2(PPh3)3] complex.
The purpose of this study was the synthesis of het-
erogenized ruthenium–phosphane catalysts having all the
advantages of heterogeneous systems, i.e., easy handling
and the possibility of recycling. The properties of the het-
erogenized catalysts, [{RuCl2(TPPMS)2}2]/PTA/NaY and

Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
431
Scheme 1. The hydrogenation reaction of trans-cinnamaldehyde. Abbreviations: Uald, unsaturated aldehyde (trans-3-phenylprop-2-enal); Sald, saturated
aldehyde (3-phenylpropanal); Ualc, unsaturated alcohol (3-phenylprop-2-en-1-ol); Salc, saturated alcohol (3-phenylpropan-1-ol).
Table 1
Table 2
The product distribution of the hydrogenation of trans-cinnamaldehyde on
homogeneous and heterogenized Ru–phosphane catalysts under basic con-
ditions
The product distribution for three subsequent runs of the hydrogenation of
trans-cinnamaldehyde on (2b)/NaY catalyst
Runs
Time
(h)
Conversion
(%)
Product distribution (%)
Catalyst
Time
(h)
Conversion
(%)
TOF
Product distribution (%)
Sald
Salc
Ualc
Sald
Salc
Ualc
1
2
3
1
1
1
32.7
27.8
25.7
4.7
3.1
2.1
10.4
11.7
12.0
84.9
85.2
85.9
(2a)
1
3
1
3
1
3
1
3
22.1
68.4
5.8
15.0
30.7
93.9
32.7
92.3
8.5
38.3
11.7
11.9
11.5
9.1
12.1
12.0
7.0
5.5
4.7
3.0
0
0
0
0
10.5
11.6
10.4
11.9
88.5
90.9
87.9
88.0
82.5
82.9
84.9
85.1
(2a)/NaY
(2b)
is in a good agreement with our earlier findings [13,14],
i.e., that the heterogenized catalysts showed about the same
or higher reaction rate than the homogeneous counterparts,
leading to higher specific activities than those of the ho-
mogeneous analogs. Indeed, in the present experiments, the
sulfonated derivative had substantially higher specific activ-
ity than its homogeneous counterpart while with the PPh3-
containing derivatives no difference could be seen (Table 1).
Although in the present case this behavior still awaits expla-
nation; however, these observations are not without prece-
dent [15].
Considering the selectivity in the above conditions all
the catalysts have a fairly good selectivity for the C=O
hydrogenation. In other words, the synthetically important
product, the unsaturated alcohol, is formed in high yields.
However, in addition to the C=O hydrogenation, all cata-
lysts showed some activity in C=C hydrogenation, too.
As it was expected on the basis of the biphasic exper-
iments, the consecutive reaction did not take place on the
sulfonated catalysts, while on the other catalyst the complete
hydrogenation has occurred, especially with higher conver-
sions.
(2b)/NaY
◦
Experimental conditions: 0.4 MPa H , 65 C, 0.05 mmol PPh or TPPMS,
2
3
0.04 mmol Et N, 0.396 mmol substrate, 10.28 µmol (2a) or 0.6 µmol
3
(2a/Na), 10.42 µmol (2b), 10.89 µmol (2b/NaY).
[RuCl2(PPh3)3]/PTA/NaY were investigated in the hydro-
genation of trans-cinnamaldehyde. During this study we
have examined the existence of the Ru–hydridecomplexes—
(1a), (2a), as well as [HRuCl(PPh3)3] (1b) and [H2Ru
(PPh3)3] (2b)—on the support and their catalytic effect on
the hydrogenation of trans-cinnamaldehyde.
3.3.1. Selective hydrogenation of C=O bonds
For the hydrogenation reaction we have applied two
slightly different conditions, based on the results obtained
under homogeneous conditions [12]. In our ethanolic solu-
tions the basicity was adjusted by Et3N. For the C=O reduc-
tion, basic conditions were applied (see Experimental). The
catalyst mixture was prehydrogenated for sufficient time to
ensure the formation of the catalytically active Ru–hydrides.
The products of the reaction are shown on Scheme 1 (no
other products were found during the hydrogenation reac-
tion), and the results are collected in Table 1.
3.3.2. Catalyst recycling
The main advantage of using heterogenized catalysts
is the possibility of recycling. We have used our [RuCl2
(PPh3)3]/NaY catalyst in several subsequent runs and the re-
sults were collected in Table 2.
As Table 2 shows, in the three subsequent runs, the
performance of the catalyst has not changed significantly.
A slight decrease in conversion was observed, due to the mi-
nor loss of catalyst from run to run. However, the selectivity
is about the same in each subsequent run; as a matter of fact,
a slight increase in selectivity was observed.
It is seen that the heterogenized catalysts—both the sul-
fonated and the nonsulfonated derivatives—were active in
the hydrogenation of trans-cinnamaldehyde in alcoholic so-
lution. Comparing the performance of the heterogenized and
the homogeneous catalysts, we must use the TOF values,
since the concentrations of Ru complexes are different for
different catalysts. Considering these values the heteroge-
nized catalysts have about the same or higher activity than
the homogeneous analogs. The higher activity of (2a)/NaY

432
Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
Table 3
The product distribution in the hydrogenation of trans-cinnamaldehyde on
homogeneous and heterogenized Ru–phosphane catalysts under acidic con-
ditions
Catalyst
Time
(h)
Conversion
(%)
TOF
Product distribution (%)
Sald
Salc
Ualc
(1a)
1
3
1
3
1
3
1
3
22.1
54.5
10.1
30.6
9.5
25.5
10.8
32.7
8.5
66.7
3.6
63.1
62.2
58.8
58.5
43.8
41.6
55.8
53.9
8.9
9.5
19.7
19.8
5.8
6.7
9.8
10.4
28.0
28.3
21.5
21.7
50.4
51.8
34.4
35.7
(1a)/NaY
(1b)
(1b)/NaY
3.9
(1a), [HRuCl(TPPMS) ]; (1b), [HRuCl(PPh ) ]. Experimental conditions:
3
3 3
◦
0.4 MPa H , 65 C, 0.03 mmol PPh or TPPMS, 0.02 mmol Et N,
2
3
3
0.396 mmol substrate, 10.28 µmol (1a) or 0.6 µmol (1a/NaY), 10.42 µmol
(1b), 10.89 µmol (1b/NaY).
Fig. 2. The effect of H pressure on the selectivity of the hydrogenation of
2
trans-cinnamaldehyde catalyzed by (1b) and (1b)/NaY. Selectivity: unsatu-
rated alcohol (Ualc)/saturated alcohol (Sald).
This set of experiments allows us to conclude that
our heterogenized catalysts have the expected advantages,
namely good performance—approximately the same ac-
tivity and similar selectivity as that of the homogeneous
counterparts—together with easy handling and efficient re-
cycling.
[RuH(H2)(TPPMS)4]+, can also take part in the hydrogena-
tion reaction [20].
Under the conditions of Fig. 2 (ethanolic solution, acidic
conditions) at 4 bar H2 pressure, we have obtained a prod-
uct ratio of cinnamyl alcohol:3-phenylpropanal = 36:54 in
the hydrogenation of trans-cinnamaldehyde with the hetero-
genized catalyst (1b)/NaY. Based on the results obtained in
aqueous biphasic systems, we have also studied the effect
of H2 pressure on the catalysts’ selectivity, and—indeed—
these measurements showed a remarkable pressure effect on
the selectivity. For comparison, experiments were made un-
der the same conditions with the soluble (1b) catalyst, too
(Fig. 2).
It can be seen from the data of Fig. 2 that the increase in
hydrogen pressure caused a substantial increase in the selec-
tivity of both the soluble and the heterogenized catalysts to-
ward the formation of the unsaturated alcohol product. This
also implies, though does not prove unequivocally, that the
same ruthenium–hydride complexes, [HRuCl(PPh3)3] and
[H2Ru(PPh3)4], act as catalytic species on the surface of
the support, which are already known from solution phase
hydrogenations of trans-cinnamaldehyde. As a result, the
selectivity in this system can be affected not only by ma-
nipulating the basicity of the solution phase, but by varying
the hydrogen pressure, as well. Consequently, the syntheti-
cally important unsaturated alcohol, 3-phenylprop-2-en-1-ol
can be produced either by using basic conditions or under
acidic conditions using a higher H2 pressure.
3.3.3. Selective hydrogenation of C=C bonds
Under acidic conditions (see Experimental), the selective
hydrogenation of C=C was expected. Table 3 shows the re-
sults obtained under these conditions.
Table 3 shows clearly that all of the catalysts were ac-
tive in the hydrogenation reaction under this condition, too.
Similar to the case of C=O hydrogenation, the specific ac-
tivities on the heterogenized catalysts were about the same
or higher than in the homogeneously catalyzed reactions.
Both the sulfonated catalysts and the heterogenized non-
sulfonated catalyst showed some selectivity in favor of the
hydrogenation of C=C bond, with 3-phenylpropanal (the
saturated aldehyde) as the major product. However, with
(1b) the C=O hydrogenation remained predominant, even
under acidic conditions.
3.3.4. The effect of hydrogen pressure on the selectivity
Under aqueous/organic biphasic conditions [12] the hy-
drogenation of trans-cinnamaldehyde catalyzed by water-
soluble Ru(II)–phosphane complexes at pH 3.04 produced
a 51:49 mixture of cinnamyl alcohol and 3-phenylpropanal
at 1 bar H2 pressure. However, with increasing H2 pressure
this ratio increased, too, and at 10 bar H2 pressure it reached
93:7. This dramatic pressure effect on selectivity could be
explained by the different pressure dependence of the rate of
C=C hydrogenation, catalyzed by [RuHCl(TPPMS)3] and
that of C=O hydrogenation, catalyzed by [RuH2(TPPMS)4].
The increase in H2 pressure changes the molecular dis-
tribution of Ru among its various hydride species. How-
ever, it is also possible that some other Ru complexes, e.g.,
3.4. Hydrogenation of crotonaldehyde
The hydrogenation of an unsaturated aliphatic aldehyde,
crotonaldehyde, was also studied to examine the effect of
the structure of the starting material. It was interesting to

Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
433
Scheme 2. The hydrogenation of crotonaldehyde. Abbreviations: Uald, unsaturated aldehyde (trans-but-2-en-1-al); Sald, saturated aldehyde (butanal); Ualc,
unsaturated alcohol (but-2-en-1-ol); Salc, saturated alcohol (butan-1-ol).
Table 4
Table 5
The product distribution in the hydrogenation of crotonaldehyde under ba-
sic conditions with soluble and heterogenized Ru(II)–phosphane complexes
Product distribution in the hydrogenation of crotonaldehyde under acidic
conditions with soluble and heterogenized Ru(II)–phosphane catalysts
Catalyst
Time
(h)
Conversion
(%)
TOF
Product distribution (%)
Catalyst
Time
(h)
Conversion
(%)
TOF
Product distribution (%)
Sald
Salc
Ualc
Sald
Salc
Ualc
(2a)
1
3
1
3
1
3
1
3
64.0
100
37.5
120.6
8.0
0.6
1.6
6.9
3.6
7.2
10.6
11.7
1.8
2.3
7.4
8.1
95.8
91.2
82.5
80.5
87.6
86.7
90.6
89.7
(1a)
1
3
1
3
1
3
1
3
14.9
50.8
14.8
49.8
11.0
42.4
12.0
43.4
8.7
90.5
90.0
86.5
84.2
81.0
78.2
63.0
60.0
8.5
9.0
9.5
11.2
9.0
10.3
11.4
12.5
1.0
1.0
4.0
(2a)/NaY
(2b)
12.0
35.9
13.8
42.0
28.5
91.1
(1a)/NaY
(1b)
148.7
6.4
7.8
4.6
10.6
11.0
2.0
10.0
11.5
25.6
27.5
(2b)/NaY
15.8
(1b)/NaY
6.6
2.2
◦
◦
Experimental conditions: 0.4 MPa H , 65 C, 0.05 mmol PPh or TPPMS,
Experimental conditions: 0.4 MPa H , 65 C, 0.03 mmol PPh or TPPMS,
2
3
2
3
0.04 mmol Et N, 0.603 mmol substrate, 10.28 µmol (2a) or 0.6 µmol
0.02 mmol Et N, 0.603 mmol substrate, 10.28 µmol (1a) or 0.6 µmol
3
3
(2a/NaY), 10.42 µmol (2b), 10.89 µmol (2b/NaY).
(1a/NaY), 10.42 µmol (1b), 10.89 µmol (1b/NaY).
see whether the same phenomena can be observed as in the
hydrogenation of trans-cinnamaldehyde.
As shown by the data in Table 5, the C=C selectivity
is much better than in the case of trans-cinnamaldehyde
under the same conditions. In the hydrogenation of croton-
aldehyde the saturated aldehyde is the major product on all
catalysts, under the acidic conditions. The selective C=C
bond hydrogenation is definitely pronounced on the sul-
fonated catalysts, where all the other products amount to
less than 20%. This is in contrast to the selectivity of the
same catalyst under basic conditions where the C=O hy-
drogenation is dominant. The results obtained with the two
different starting materials show the generality of the selec-
tivity change, which becomes even more pronounced with
crotonaldehyde. It is noted here, that in the hydrogenation
of trans-cinnamaldehyde, the C=O hydrogenation remains
predominant even under acidic condition.
For a better comparison we have used the same exper-
imental protocol for this substrate as in the case of trans-
cinnamaldehyde. Table 4 shows the product distribution of
the hydrogenation of crotonaldehyde under basic conditions.
As seen in Table 4 both the soluble, (2a) and (2b), and the
heterogenized (2a)/NaY and (2b)/NaY catalysts were active
in the hydrogenation of crotonaldehyde. Generally the ac-
tivities were somewhat higher than they were in the case of
trans-cinnamaldehyde. In this system both of the heteroge-
nized catalysts—(2a)/NaY, (2b)/NaY—had higher TOF than
the homogeneous catalysts. The product distribution is about
the same as that of the homogeneously catalyzed reactions.
On the (2b)/NaY catalyst not only the activity was higher
but also the selectivity.
The heterogenized catalyst (2b)/NaY was used in three
subsequent runs and we had similar observations as in the
case of trans-cinnamaldehyde. In these recycling experi-
ments the heterogenized catalyst did not loose its activity to
a significant extent and the product distributions were very
similar from run to run. In other words the selectivity did not
change considerably; as a matter of fact, it increased slightly
in favor of the formation of the unsaturated alcohol. Consid-
ering the conversion, we have observed a similar trend as in
the case of trans-cinnamaldehyde, which can be caused by
some loss of catalyst.
Once again, it can be concluded, that these heterogenized
Ru–phosphane catalysts deliver all the expected advantages:
good performance, easy separation, and the possibility of ef-
ficient recycling.
3.5. General remarks on the hydrogenation of
α, β-unsaturated aldehydes with Ru–triphenylphosphane
catalysts anchored on NaY zeolite
Selective hydrogenation of α, β-unsaturated aldehydes is
an industrially important process. In the case of aldehydes
with short carbon chains (e.g., 3-methyl-2-butenal) the cat-
alyst recycling problem, which is a main drawback of ho-
mogeneous catalysis, has been overcome by the use of a
We have also tested the hydrogenation of crotonaldehyde
under acidic conditions. The results are shown in Table 5.

434
Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
biphasic process in which the reactants are in the organic
phase and the water-soluble catalyst, [RuCl2(TPPTS)3], is
found in the aqueous phase. However, in the case of aldehy-
des with long carbon chains the use of liquid–liquid biphasic
systems leads to poor results, probably caused by the low
solubility of the reactant in the aqueous phase.
The observed similarities in the selectivity of the ho-
mogeneous and the heterogenized samples suggest that the
mechanism of hydrogenation is also similar [20].
4. Conclusions
In our study we have prepared the heterogenized analogs
of [RuCl2(PPh3)3] and [{RuCl2(TPPMS)2}2]. In the hydro-
genation of trans-cinnamaldehyde and crotonaldehyde these
catalysts showed good activity and selectivity (both compa-
rable to the homogeneous catalysts), easy separation, and the
possibility for recycling with no appreciable loss of perfor-
mance. In some instances, the heterogenized catalysts had
higher activity and selectivity than the homogeneous coun-
terparts (e.g., (2b)/NaY for both aldehydes and (1b)/NaY for
trans-cinnamaldehyde). This latter finding is in accord with
our earlier observations [13,14].
We have prepared the heterogenized [{RuCl2(TPPMS)2}2]
and [RuCl2(PPh3)3] catalysts, using a heteropoly acid (phos-
photungstic acid) as an anchoring medium. To our knowl-
edge this is the first example of preparing these catalyst via
this heterogenization method. These catalysts were applied
successfully in the hydrogenation of trans-cinnamaldehyde
and crotonaldehyde. In general, there was no substantial dif-
ference in the performance of the homogeneous and the
heterogenized catalysts in the hydrogenation of these two
substrates. In particular, the heterogenized catalysts showed
about the same activity as their homogeneous counterparts
and at the same time their selectivity toward the formation
of the unsaturated alcohol product was found approximately
twice as high as the selectivity of the corresponding homo-
geneous catalysts. In addition, it was possible to use these
heterogenized catalysts in three subsequent runs, without
any significant loss of activity, and even with a slight in-
crease in selectivity.
The hydrogen pressure had a dramatic effect on the se-
lectivity of the hydrogenation of trans-cinnamaldehyde with
both the soluble and the heterogenized catalysts. Under
acidic conditions, with increasing H2 pressure the C=O
selectivity was increased significantly. Presently, this pres-
sure effect is not completely clear and further studies are in
progress in our laboratories in order to establish its mecha-
nistic background.
At this stage of the study no data exist on solvent ef-
fects in this reaction. Nevertheless, we expect that our het-
erogenized complexes work in aqueous solutions, too. This
could extend the use of these Ru–phosphane catalysts, con-
sidering that only [{RuCl2(TPPMS)2}2] is soluble in water.
The basic finding, though, is that the method of heteropoly
acid-mediated anchoring of soluble complexes onto hetero-
geneous supports, introduced by Augustine et al. [6,7], is
indeed a viable method for the preparation of heterogenized
homogeneous catalysts with no noticeable alteration of the
chemistry and the catalytic performance of the homogeneous
catalysts. Therefore—despite the lack of complete under-
standing of the molecular mechanism of the catalyst–support
interaction—the use of heteropoly acids can be expected to
yield further active and stable heterogenized homogeneous
(solid) catalysts.
In the literature there are various experimental results and
theories concerning the relation of the activity of a catalyst
in its dissolved and heterogenized state [16]. Immobiliza-
tion generates catalysts that are generally more complex than
their homogeneous analogs. It is therefore not surprising
that, in practice, the effect of immobilization is still very
unpredictable. Unfortunately, most heterogenized homoge-
neous catalysts are less active, than in homogeneous solu-
tions and/or loose part of their activity upon recycling [17].
However, in some cases the activities and the chemo- and
enantioselectivities of the heterogenized systems were found
superior to those of the corresponding homogeneous cata-
lysts [18]. Thomas et al. [19] suggest that in some cases the
improved performance of a heterogeneous catalyst can be
explained by the “confinement concept,” according to which
the substrate’s favorable interaction with both the pore wall
and the catalyst molecule is the key point. Conversely, lower
activities can be due to the reduced or even blocked mass
transport in the small pores, imposing limits to the effective
range of the substrates that can be utilized [16]. “Site iso-
lation,” i.e., attaching a catalyst to a support in a way that
the catalytic sites can no longer interact with each other, is
another concept that might explain the better performance
of heterogenized catalysts [18b]. In this case, effective func-
tional site isolation, and thus the formation and protection
of highly active catalytic species, was strongly dependent on
the tight attachment of the ligand to the support, the low sur-
face concentration of the ligands, and the chemical nature of
the catalyst precursor.
The fact that the heterogenized samples were active un-
der both basic and acidic conditions shows that both of the
two different Ru–hydride complexes were able to form dur-
ing the prehydrogenation. The red, monohydrido derivative
catalyzes a slow C=C hydrogenation, while the yellow di-
hydrido complex selectively hydrogenates the C=O bond;
this latter reaction is much faster with the heterogenized cat-
alysts than with the homogeneous analogs.
Acknowledgments
The authors are grateful for the financial support of the
National Science Fund of Hungary (OTKA T0 34126 and

Á. Zsigmond et al. / Journal of Catalysis 227 (2004) 428–435
435
TS 044690). Special thanks are due to Prof. Robert L. Au-
gustine, Center for Applied Catalysis, South Orange, NJ,
for the valuable discussions considering his heterogenization
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