Hydrogenation of acetophenone in the presence of Ru catalysts supported on amine groups functionalized polymer

Article abstract of DOI:10.1007/s10562-010-0448-3

Liquid phase hydrogenation of acetophenone (ACT) is studied over ruthenium catalysts (1-4 wt% Ru) supported on gel-type methacrylate-styrene resin (FCN) functionalized with C=O, -NH, and -NH₂ groups. Microscopic studies (SEM, STEM) show that th

Full text of DOI:10.1007/s10562-010-0448-3

Catal Lett (2011) 141:83–94
DOI 10.1007/s10562-010-0448-3
Hydrogenation of Acetophenone in the Presence of Ru Catalysts
Supported on Amine Groups Functionalized Polymer
•
Dorota Duraczynska Alicja Drelinkiewicz
•
´
Elzbieta Bielanska Ewa M. Serwicka
•
•
_
´
Lidia Litynska-Dobrzynska
´
´
Received: 19 July 2010 / Accepted: 3 September 2010 / Published online: 24 September 2010
Ó The Author(s) 2010. This article is published with open access at Springerlink.com
Abstract Liquid phase hydrogenation of acetophenone
(ACT) is studied over ruthenium catalysts (1-4 wt% Ru)
supported on gel-type methacrylate-styrene resin (FCN)
functionalized with C=O, –NH, and –NH2 groups. Micro-
scopic studies (SEM, STEM) show that the nature of Ru
particles depends on the level of Ru doping. At low Ru
content (1–2 wt%) ruthenium nano-clusters are formed,
while at 4 wt% Ru, metal crystallites of few nanometers in
size are observed. Catalytic reactions are performed at mild
conditions (atmospheric pressure of hydrogen, 40 °C) in
biphasic isooctane-water (IO/H2O) solvent system, and, for
comparison, in a single component solvent (isooctane or
ethanol). The use of biphasic IO/H2O solvent system
results in a dramatic improvement of selectivity. The
hydrogenation of C=O in acetophenone dominates over the
hydrogenation of aromatic ring, yielding 1-phenylethanol
with ca. 80% selectivity. Ru/FCN catalysts exhibit higher
selectivity than the reference Ru/Al2O3. This is tentatively
assigned to the steric effects induced by the polymer net-
work on migrating reactant molecules. Advantageous
influence of biphasic IO/H2O solvent system has been
attributed to the solvation of phenyl ring of acetophenone
by non-polar isooctane, which facilitates the interaction
with the catalyst surface via carbonyl group and leads to
the preferential reduction to 1-phnenylethanol.
Keywords Functional polymer ꢀ Ruthenium ꢀ
Hydrogenation ꢀ Acetophenone
1 Introduction
Catalytic hydrogenation of aromatic ketones to phenyl
alcohols constitutes an important aspect of organic syn-
thesis, affording key intermediates in the production of
fine chemicals and drugs. The hydrogenation of aceto-
phenone (ACT) is a typical example of this type of
reactions. Selective hydrogenation of ketones can be
achieved with homogeneous catalysts and most of hydride
reduction of ACT exclusively gives 1-phenylethanol, as
described in recently published reviews [1–3]. Unfortu-
nately, the problems in products separation, recovery and
reuse of catalysts create demand for heterogeneous cata-
lysts. A number of studies have already been directed to
hydrogenation of ACT using heterogeneous Pt, Pd, Rh,
and Ru catalysts in reactions carried out under liquid- or
gas-phase conditions [4–13]. Selective hydrogenation of
ACT to 1-phenylethanol (PE) is complicated by the fact
that different side reactions may take place. In conse-
quence, achieving high selectivity to 1-phenylethanol, the
desired product, is a challenging issue. On Pt and Rh
catalysts the comparable rates of C=O and phenyl ring
hydrogenations result in a mixture of various products [6,
7, 9, 10]. An exception is Pt/TiO2 catalyst which exhibit
high and selective ability for C=O activation due to the
presence of active centers formed as the result of strong
metal support interactions [14–16]. Palladium supported
catalysts, besides being highly reactive toward C=O
hydrogenation to C–OH, they catalyze the hydrogenolysis
of C–OH to give ethylbenzene as a final product [13, 17–
20]. Ruthenium supported catalysts are also tested in
´
´
D. Duraczynska ꢀ A. Drelinkiewicz (&) ꢀ E. Bielanska ꢀ
E. M. Serwicka
Institute of Catalysis and Surface Chemistry, Polish Academy
of Sciences, ul. Niezapominajek 8, 30-239 Krakow, Poland
e-mail: ncdrelin@cyf-kr.edu.pl
´
´
L. Litynska-Dobrzynska
Institute of Metallurgy and Materials Science, Polish Academy
of Sciences, ul. Reymonta 25, 30-059 Krakow, Poland
123

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D. Duraczynska et al.
84
hydrogenation of ACT because of their high ability to
hydrogenate carbonyl group in the vicinity of conjugated
and isolated double bonds [11, 12, 21, 22]. Indeed, in the
case of citral and cinamaldehyde a highly selective
hydrogenation to the corresponding unsaturated alcohols
is observed [11]. However, when Ru catalysts are used in
hydrogenation of aromatic aldehydes or ketones, such as
acetophenone, they exhibit comparable reactivity toward
hydrogenation of carbonyl group and aromatic ring of
ACT [12, 21, 22]. In hydrogenation of ACT on Ru-cat-
alysts two alternative reaction pathways are observed
(Scheme 1) [11]. One involves hydrogenation of C=O
group to give C–OH in 1-phenylethanol (PE), the other
leads to hydrogenation of aromatic ring of ACT to form
cyclohexyl methyl ketone (CMK). Both products, PE and
CMK, undergo further but slower hydrogenation to 1-
cyclohexylethanol (CE) as a final product. Other products,
such as ethylbenzene, benzene, cyclohexene and cyclo-
hexane are also formed, but in much smaller quantities
[12]. In order to enhance selectivity of C=O hydrogena-
tion an appropriate support or modification of Ru-cata-
lysts with metal additives are needed. Results obtained by
Cerveny et al. [12] and by Wismeijer et al. [22] show that
among Ru-catalysts supported on Al2O3, SiO2, C and
TiO2, the best efficiency in selective formation of PE is
observed for Ru/TiO2 catalyst. The effect is assigned to
the possible activation of the carbonyl double bond by
Ti3? cations, formed on the TiO2 support [22]. Also, the
promotion of silica supported Ru catalysts with Cr ions
enhances the selective reduction of C=O, but the overall
activity decreases [23]. The selectivity increase is attrib-
uted to the strong interaction of the oxygen atom of C=O
group with the Cr ions. In the present work hydrogenation
of ACT is studied on Ru-supported catalysts using
biphasic solvent system composed of less polar isooctane
and polar water (IO/H2O). According to the literature, in
biphasic IO/H2O solvent system a variety of catalytic
reductions can effectively proceed under mild conditions
(ca. 50 °C, atmospheric pressure of hydrogen) and their
selectivity can be modified [24–27]. Thus, Marques et al.
[24] demonstrate that hydrogenation of propiophenone on
Pd/C catalyst in conventional ethanol solvent produces
propylbenzene as the final product, while in the biphasic
IO/H2O system propiophenone is almost selectively
reduced to 1-phenyl-1-propanol [24]. In contrast, on Pt/C
catalyst and in IO/H2O system, the hydrogenation of
acetophenone and propiophenone proceeds towards the
full reduction products, ethyl benzene and ethyl cyclo-
hexane, respectively [25, 26]. Biphasic IO/H2O system is
also successfully used for enantioselective hydrogenation
of ACT in the presence of cinchona-modified Pt/C cata-
lyst [27]. Moreover, our preliminary catalytic data dem-
onstrate the advantage of biphasic IO/H2O solvent system
in hydrogenation of ACT in the presence of Ru-catalysts
supported on methacrylate-styrene gel-type resin (FCN)
(Scheme 2). The FCN polymer is functionalized with
C=O, –NH, and –NH2 groups and is used for the prepa-
ration of palladium catalysts with finely dispersed Pd-
nanoparticles [28, 29]. Our previous results show that
functional groups of FCN resin, and in particular the N-
containing ones participate in the bonding of Pd, Ru-
species via filling metal coordination sphere [28–30].
Functional groups may also affect the electron properties
of active centers and in consequence catalytic properties
of studied Ru/FCN samples.
Homogeneous Ru(III) complexes with nitrogen and
oxygen-containing ligands like diamine, diamino alcohols,
amine carboxylic etc. are capable of catalyzing various
reactions, oxidation, hydrogenation and hydrogen transfer
reactions. Traditionally, insoluble polymer resins such as
divinylbenzene, cross-linked polystyrene have been stud-
ied as supports for heterogenization of such catalytically
active complexes. Other, but more convenient way to
prepare polymer-anchored catalytically active metal spe-
cies is the use of polymers that are functionalized with
appropriate coordinating ligands, such as b-diketones
(O,O), dipyridyl amine (N,N), diphosphine (P,P) etc. For
example, Ru immobilized on poly(4-vinylpyridine) is
reported to be an efficient catalyst for selective hydro-
genation of aromatic ring of quinoline [31]. Ru supported
on potassium methacryloyl ethylene sulphonate resin is
found to be efficient in partial hydrogenation of benzene
[32], and Ru immobilized by chloromethylated styrene-
divinyl benzene copolymer functionalized by glycine is
effective for hydrogenation of nitrobenzene [33]. This
OH
CH3
O
PE
HO
CH3
CH3
O
CE
ACT
OH
CH3
P
O
NH
NH2
CMK
O
Scheme 1 Reaction pattern of acetophenone hydrogenation in the
presence of Ru catalysts
Scheme 2 The simplified structure of FCN resin
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Hydrogenation of Acetophenone in the Presence of Ru Catalysts
85
2.4 Hydrogenation Experiments
method, based on the usage of polymer functionalized
with coordinating ligands is also applied in the present
work.
Hydrogenation experiments are carried out in an agitated
batch glass reactor at constant atmospheric pressure of
hydrogen and temperature of 40 °C [30]. A biphasic sol-
vent system, composed of isooctane and water (29 dis-
tilled) in 1:1 volume ratio is used. In a typical
hydrogenation test the concentration of ACT is 0.137 mol/
dm3 (referred to isooctane phase) and the concentration of
catalyst is 10 g/dm3, referred to biphasic IO/H2O solvent
system. Before the hydrogenation experiment the catalyst,
wetted with water, is activated inside the reactor in a flow
of hydrogen for 45 min at 20 °C and 45 min at the tem-
perature of reaction (40 °C). For comparison, in hydroge-
nation tests isooctane or ethanol are used as a single
solvent. In the course of catalytic runs the samples of
reaction mixture (organic phase) are periodically with-
drawn and analyzed by GC method [30]. The contents of
ACT, PE, CMK, and CE are determined by comparison
with calibration curves. From the data of GC analysis
acetophenone conversion (C, %) and selectivity [S(i)] to
products, 1-phenylethanol [S(PE)], cyclohexyl methyl
ketone [S(CMK)] and 1-cyclohexylethanol [S(CE)] are
calculated from the equations:
Here, the role of biphasic IO/H2O solvents system in
hydrogenation of ACT is studied using Ru/FCN catalysts
of various Ru content. For comparison, the reaction is also
studied in the presence of c-Al2O3 supported Ru.
2 Experimental
2.1 Materials
Acetophenone is purchased from Sigma-Aldrich, 1-phen-
ylethanol, 1-cyclohexylethanol, cyclohexyl methyl ketone
are from Fluka, and isooctane is from POCH. All chemi-
cals are used as purchased.
2.2 Preparation of Catalysts
The FCN resin (3% crosslinking degree) (77 wt% styrene,
2.78 wt% N content) [28] swells very well in THF and this
solvent is used as a medium during the preparation of
Ru-composites (1, 2 and 4 wt% Ru) according to the pre-
viously described procedure [30]. Thus, FCN resin pre-
swollen in THF is treated with appropriate amount of
aqueous solution of RuCl3 under slow mixing up to the
complete incorporation of ruthenium species. After wash-
ing and drying in air, the as-received composites are
reduced by NaBH4 solution in THF:CH3OH (9:1 volume
ratio) using 10-fold excess. The resulting black Ru-poly-
mer beads are washed with THF/acetone several times and
dried in air. 2%Ru/Al2O3 catalyst (c-Al2O3, BET surface
area 155 m2/g, particles ca. 100 lm) is prepared by
impregnation method using aqueous solution of RuCl3,
followed by reduction with NaBH4 [30].
C ½%ꢁ ¼ n0ðACTÞ ꢂ ntðACTÞ=n0ðACTÞ
SðiÞ½%ꢁ ¼ ntðNiÞ=n0ðACTÞ ꢂ ntðACTÞ
where, n0(ACT) and nt(ACT) are the moles of acetophe-
none initially present in the reactor and the moles of ACT
remaining at time t, respectively; nt(Ni) are number of
moles of individual reaction products, PE, CMK, CE, at
reaction time t.
2.5 Determination of the Partitioning of Reaction
Mixture
In two-phase isooctane-water system the distribution of all
the reagents (ACT, PE, CMK, and CE) in both solvent
phases is determined by GC method. The known amounts
of particular reagents (137 9 10-5 mol of ACT,
199 9 10-5 mol of PE, 34 9 10-5 mol of CMK, and
43 9 10-5 mol of CE) are added into 20 cm3 of IO/H2O
(1:1 by volume). The whole lot is vigorously mixed with a
mechanical shaker for 45 min. The samples of liquids from
both phases are withdrawn immediately after the comple-
tion of shaking, and after 5, 15, 30, and 60 min. The
concentrations of ACT, PE, CMK, and CE in both isooc-
tane and aqueous phases are determined using GC method.
The equilibrium concentrations of ACT, PE, CMK, and CE
are established just after stopping the mixing. The isooc-
tane phase (top layer) contains 92.7% of ACT, 55.6% of
PE, 100% of CMK, and 93.0% of CE. From these data the
2.3 Characterization Methods
The reduced Ru/FCN catalysts are characterized by XRD,
SEM and STEM techniques. X-ray diffraction patterns
(XRD) are obtained with a Siemens D5005 diffractometer
using Cu Ka radiation. Morphology of catalysts is studied
by means of Field Emission Scanning Electron Microscope
JEOL JSM—7500 F equipped with the X-ray energy dis-
persive (EDS) system. The secondary electron detector
provides SEI images and back scattered electron detector
provides BSE (COMPO) micrographs. K575X Turbo
Sputter Coater is used for coating the specimens with
chromium (deposited film thickness—20 nm). STEM
studies are performed on FEI Tecnai G2 transmission
electron microscope at 200 kV equipped with EDAX EDX
and HAADF/STEM detectors.
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D. Duraczynska et al.
86
distribution coefficients: 1.08 for ACT and CE, and 1.80
for PE are calculated and used for the determination of the
total concentrations of reagents during the catalytic test on
the basis of GC analysis of organic phase only.
3 Results and Discussion
3.1 Characterization of Ru/FCN Catalysts
Preparation and characterization of Ru/FCN composites
used in the present study in the capacity of catalysts is
described in detail in our previous work [30]. Briefly, the
results obtained by number of techniques (FT-IR, XPS,
DSC, SEM, EDS) show that functional groups of FCN
polymer, in particular the amine groups –NH, and –NH2,
participate in the coordination of Ru(III) ions. The coor-
dination sphere of ruthenium trapped within the FCN
matrix contains both Cl and N-ligands in various propor-
tions depending on Ru loading in the polymer. Introductory
catalytic experiments show no hydrogenation of ACT when
as-prepared Ru/FCN composites are tested. In the presence
of NaBH4 reduced 2%Ru/FCN catalyst hydrogenation of
ACT is effective, however only when reaction is performed
in biphasic IO/H2O solvent system. On the other hand, in
ethanol, THF, or isooctane as the only solvent the reaction
is very slow, practically no measurable under conditions of
catalytic tests [30]. Therefore present studies are under-
taken to shed more light on the advantageous role of
biphasic IO/H2O solvent system in the hydrogenation of
ACT. Here, reaction is studied in the presence of reduced
Ru/FCN catalysts and for the comparison 2%Ru/Al2O3
catalyst is tested.
Ru-particles in reduced Ru/FCN catalysts are charac-
terized by XRD and electron microscopy (SEM and TEM)
techniques. The SEM studies are carried out in SEI and
BSE (COMPO) modes. SEI (secondary electron imaging)
is appropriate for studying the morphology of catalyst. The
BSE (COMPO) registration mode provides information
about the distribution of Ru in the catalyst because the
brightness observed in BSE images is strongly related to
atomic number. BSE (COMPO) images of reduced
Ru/FCN composites with 1, 2 and 4 wt% Ru are displayed
in Fig. 1. A number of irregular ‘‘bright’’ spots of various
shapes and sizes randomly oriented and distributed
throughout the polymer beads can be observed in all cat-
alysts. However, no reflections associated with the parti-
cles of crystalline Ru appear in their XRD diffraction
pattern (Fig. 2a). Thus, irregularly shaped ‘‘bright’’ spots
observed in BSE images may be considered to be associ-
ated with the presence of aggregated nano-particles of
ruthenium. In order to clarify this supposition, the ‘‘areas
of bright spots’’ are examined at higher magnification and
Fig. 1 Electron microscope images (BSE registration mode) of
(a) 1%Ru/FCN, (b) 2%Ru/FCN, and (c) 4%Ru/FCN catalysts
the representative SEI and BSE images are shown in
Fig. 3. The SEI image (Fig. 3a) shows that polymer is
composed of almost spherical grains of similar shape and
size within the range 100–150 nm. The BSE (COMPO)
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Hydrogenation of Acetophenone in the Presence of Ru Catalysts
87
(a)
1%Ru/FCN
2%Ru/FCN
4%Ru/FCN
5
15
25
35
45
2 Theta
(b)
Fig. 3 The BSE (a) and SEI (b) micrographs of 1%Ru/FCN
2%Ru/Al2O3
much brighter. Upon increasing Ru loading up to 2 wt%
Ru, apart from ‘‘bright areas of polymer’’ the separated
bright spots can be observed showing that ruthenium
aggregates start to form. EDS analysis of Ru carried out in
marked boxes (1, 2, 3) indicates the presence of ruthenium
even in the areas apparently almost free of bright spots.
However, electron diffraction shows no evidence of any
crystalline Ru phase in both 1%Ru/FCN and 2%Ru/FCN
catalysts, which points to the presence of Ru-containing
nano-clusters.
5
15
25
35
45
2 Theta
Fig. 2 XRD diffraction pattern of Ru/FCN (a) and 2%Ru/Al2O3
(b) catalysts
image (Fig. 3b) reveals that, among spherical polymer
grains, agglomerates of much brighter grains exist, which
indicates that the concerned areas are substantially enri-
ched in ruthenium.
STEM image of catalyst with high Ru loading, 4%Ru/
FCN, shows a qualitative difference (Fig. 5). Clearly
observable bright spots of metallic ruthenium are visible
(Fig. 5a) and the electron diffraction pattern originating
from crystalline ruthenium (Fig. 5b) is obtained. Although
the size of such Ru-crystalline particles is within wide
range, their sizes do not exceed few nanometers. Thus,
upon increasing ruthenium loading in the FCN polymer
the nature of Ru-particles is changed from very highly
dispersed Ru-nanoclusters to nanocrystallites of few
The nature of Ru particles in catalysts of various
Ru-content is characterized by the STEM micrographs in
which ruthenium particles appear as bright spots and/or
areas against darker background originating from the
polymer matrix (Figs. 4 and 5). At the lowest Ru loading in
polymer, 1%Ru/FCN (Fig. 4), no separated particles of Ru
can be distinguished but the whole polymer matrix appears
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D. Duraczynska et al.
88
Fig. 4 STEM micrograph of 1%Ru/FCN and 2%Ru/FCN catalysts and electron diffractions
nanometers in size. This may be explained taking into
account that amine –NH, and –NH2 groups of FCN resin
(Scheme 2) participate in the immobilization of Ru(III)
ions. In the coordination sphere of Ru(III)-species immo-
bilized by FCN resin both Cl and N-ligands are observed,
however in various proportions depending on the Ru con-
tent introduced into polymer [30]. At low loading of Ru
(1 and 2 wt%) the contribution of N-groups of polymer in
the coordination sphere of Ru-species is high and it
decreases as the loading of Ru grows to 4 wt%. In catalysts
with low Ru content (1 and 2 wt%) highly dispersed
Ru-nanoclusters are observed showing that high contribu-
tion of N-groups of polymer in the Ru-species favors the
formation of highly dispersed Ru catalysts. On the other
hand, functional groups may affect the electron properties
of Ru-centers and in consequence catalytic properties of
Ru/FCN samples. The influence of functional groups,
however, would be to some extent determined by the
size of Ru-particles. It may be expected that the higher
Ru dispersion, the more important effect of functional
groups of polymer matrix.
isolated ‘‘white spots’’ appear whereas most of them form
aggregates of size within the range 10–50 nm. In the XRD
diffraction pattern of 2%Ru/Al2O3 the broad and low
intense reflexes of crystalline Ru located at 38.1° and 41.1°
appear (Fig. 2b) indicating the presence Ru particles of size
of few nanometers. Thus, the ‘‘white spots’’ observed in
the SEM micrograph are the aggregated nanoparticles of
metallic Ru.
3.2 Catalytic Experiments
3.2.1 Hydrogenation of ACT in the Presence
of 2%Ru/Al2O3 Reference Catalyst
The first part of present work concentrates on ACT
hydrogenation in the presence of 2%Ru/Al2O3 catalyst.
Reaction is studied in ethanol and isooctane as the only
solvents and then in biphasic IO/H2O solvent system. In all
experiments the same catalyst concentration (10 g/dm3) is
used. At this amount of catalyst no hydrogenation of ACT
is observed in isooctane solvent whereas reaction is
effective in ethanol and IO/H2O solvent system. The con-
versions of ACT obtained in ethanol and in biphasic
IO/H2O solvents are compared in Fig. 7a, and the
The electron microscopic images of 2%Ru/Al2O3 cata-
lyst are displayed in Fig. 6. The ‘‘white spots’’ of Ru par-
ticles of various shapes and size are observed. Only a few
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Hydrogenation of Acetophenone in the Presence of Ru Catalysts
89
100
80
60
40
20
0
(a)
IO/H2O
ethanol
0
50
100 150
200 250 300
350 400
t [min]
(b)100
80
60
40
20
0
S (PE)
ethanol
IO/H2O
S (CMK)
S (PE)
Fig. 5 STEM micrograph of 4%Ru/FCN (a) and electron diffraction
(b)
0
20
40
60
80
100
C [%]
Fig. 7 Hydrogenation of ACT in the presence of 2%Ru/Al2O3 in
ethanol and biphasic IO/H2O solvent system. Conversion of ACT vs.
reaction time (a) and selectivity to particular reaction products against
ACT conversion (b), (catalyst concentration 10 g/dm3, ACT:
0.137 mol/dm3)
ACT conversion is attained after ca. 320 min of reaction.
The rate of ACT hydrogenation distinctly grows in
biphasic IO/H2O solvent system and after comparable
reaction time (ca 300 min) almost 70% of ACT conversion
is attained. Furthermore, induction periods appear, espe-
cially clearly observable in IO/H2O solvents system. It
should be stressed that in the case of ruthenium catalysts
induction periods are quite common, especially in reactions
carried out at low hydrogen pressure in nonaqueous media.
At high hydrogen pressures, induction periods rarely occur,
regardless of solvent used [34].
Fig. 6 Electron microscope images (BSE registration mode) of
2%Ru/Al2O3 catalyst
selectivities to PE, CMK and CE are reported in Fig. 7b. In
ethanol solvent, the hydrogenation of ACT is effective, but
the reaction proceeds at a very low rate and about 20%
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D. Duraczynska et al.
90
A substantial difference can be seen regarding the
100
80
60
40
20
0
(a)
selectivity of ACT hydrogenation (Fig. 7b). In reaction
media, ethanol and IO/H2O solvent system, ACT is
hydrogenated via both reaction pathways. The reduction of
C=O to form phenylethanol (PE) as well as the hydroge-
nation of aromatic ring yielding cyclohexyl methyl ketone
(CMK) take place (Scheme 1). Moreover, 1-cyclohexyl
ethanol is also formed. In both solvent systems, the
reduction of C=O predominates over the hydrogenation of
aromatic ring, and this predomination is much higher in
biphasic IO/H2O solvent system. In ethanol, selectivity to
PE attain ca. 60% and that to CMK is ca. 40% (Fig. 7b).
Owing to low rate of ACT conversion in ethanol, the data
are obtained at ca. 40% conversion of ACT only. This
selectivity pattern in ethanol solvent is similar to that
reported by Cerveny et al. [12] who demonstrated com-
parable selectivities to PE and CMK on 5%Ru/Al2O3
catalyst.
4%Ru/FCN
2%Ru/FCN
0
50
100 150 200 250
300 350 400
t [min]
100
80
60
40
20
0
(b)
On the other hand, in IO/H2O solvent system the rate of
ACT conversion is distinctly higher and the raction is
studied up to almost complete conversion of ACT. In such
biphasic IO/H2O solvent system selectivity to PE is much
higher compared to that of CMK. Selectivity to PE as high
as ca. 80% is reached at the beginning of the reaction.
However, as the reaction progresses the selectivity to PE
slowly but continuously decreases whereas the selectivity
to 1-cyclohexylethanol (CE) grows. This may suggest that
as PE is formed, it is consumed via hydrogenation of
aromatic ring to form 1-cyclohexylethanol (CE). As
Fig. 7b shows, selectivity to CMK is only 20% and prac-
tically does not change up to the almost complete con-
version of ACT. This indicates that only 20% from the
reacted ACT is transformed via aromatic ring hydrogena-
tion whereas 80% is reacted via C=O reduction. Thus, in
biphasic IO/H2O system the rate of ACT hydrogenation
and the selectivity to PE are higher compared to the
reaction in ethanol. This reveals promoting effect of
biphasic IO/H2O solvent system in activity of 2%Ru/Al2O3
and in selective reduction of C=O in ACT to give
1-phenylethanol (PE).
4%Ru/FCN
S(PE)
2%Ru/FCN
S(CMK)
S(CE)
0
20
40
60
80
100
C [%]
Fig. 8 Hydrogenation of ACT in the presence of 2%Ru/FCN and
4%Ru/FCN catalysts in biphasic IO/H2O solvent system. Conversion
of ACT vs. reaction time (a) and selectivity to particular reaction
products against ACT conversion (b) (catalyst concentration 10 g/dm3,
ACT: 0.137 mol/dm3)
in particular, a pronounced induction period observed in
the presence of 4%Ru/FCN makes difficult calculation and
comparison of reaction rates. Selectivity patterns (Fig. 8b)
shows that on both Ru/FCN catalysts ACT is preferentially
transformed via hydrogenation of C=O to give unsaturated
alcohol, PE. The selectivity to CMK is only ca. 11–12% on
2%Ru/FCN and slightly lower 6–7% on 4%Ru/FCN cata-
lyst. Similarly to 2%Ru/Al2O3, the selectivity to CMK is
practically stable up to almost complete conversion of
ACT. As the reaction progresses the selectivity to PE
slowly decreases whereas that of CE grows. Thus, polymer
supported Ru/FCN catalysts show clear preference for
catalyzing C=O reduction rather than hydrogenation of
3.2.2 Hydrogenation of ACT in the Presence of Ru/FCN
Catalysts
The conversion of ACT in the presence of Ru/FCN cata-
lysts is displayed in Fig. 8a. Reaction is studied at catalyst
concentration of 10 g/dm3. Under such conditions the
activity of 1%Ru/FCN is very low and only 2% of ACT
conversion is achieved after 300 min of reaction time (data
not shown). The hydrogenation of ACT over 2%Ru/FCN
and 4%Ru/FCN catalysts is much more effective, the
activity of 4%Ru/FCN being distinctly higher than that of
2%Ru/FCN. A different ‘‘shape’’ of conversion plots and,
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Hydrogenation of Acetophenone in the Presence of Ru Catalysts
91
phenyl ring, and this preference is somewhat stronger for
the 4%Ru/FCN catalyst. This may be associated with the
differences in the nature of Ru particles in both catalysts. In
2%Ru/FCN catalyst ruthenium exists mainly in the form of
well dispersed nano-clusters, while in 4%Ru/FCN catalyst,
Ru particles, although still not larger than few nanometers,
exhibit crystalline character. Moreover, in the presence of
both 2%Ru/FCN and 4%Ru/FCN catalysts, predomination
of C=O reduction over aromatic ring hydrogenation is
higher compared to that of 2%Ru/Al2O3.
range of catalyst concentration an induction period is
observed making the calculation of hydrogenation rate
complex (Fig. 9a). Nevertheless, the rate of ACT conver-
sion increases as the concentration of 4%Ru/FCN catalyst
grows up to 10 g/dm3. At higher concentration, 12.5 g/dm3,
the rate decreases, which is most likely associated with
limited supply of hydrogen to the catalyst surface. There-
fore in all experiments the concentration of catalyst 10 g/
dm3 is used. The concentration of 4%Ru/FCN does not
affect the selectivity pattern of ACT hydrogenation
(Fig. 9b) because PE is formed as the major product. The
selectivity to CMK, product of aromatic ring hydrogenation
is very low, ca. 6–7% and practically does not change
during the hydrogenation experiment.
The effect of 4%Ru/FCN catalyst concentration
(7.5–12.5 g/dm3) is shown in Fig. 9. Within the whole
(a) 100
The hydrogenation is also studied using 2-times lower
initial ACT concentration (Fig. 10). The induction periods
are observed in both solutions but induction period is
longer at lower initial ACT concentration. However, both
conversion curves vs. reaction time are parallel (Fig. 10),
which shows that the rates of ACT conversion do not
depend on initial ACT concentration. Moreover, initial
ACT concentration does not influence substantially the
selectivity of C=O reduction, which is still high regardless
of initial ACT concentration.
80
10 g/dm3
12.5 g/dm3
60
40
7.5g/dm3
3.3 Recovery and Reuse
20
A frequently encountered problem of polymer supported
catalysts is the decrease of their activity due to the
leaching of metal particles under catalytic test. In order
to determine whether Ru/FCN are stable under reaction
conditions the 2%Ru/FCN catalyst is recovered and
reused twice. After each hydrogenation test, the catalyst
is separated by filtration, washed with acetone, dried in
air, and then subjected to the next catalytic cycle. Fresh
ACT solution is added and the same hydrogenation
conditions are applied. The decrease of ACT content
(mol %) against reaction time in the first and the third
catalytic runs are practically the same (Fig. 11a). The
selectivity to PE as well as to CMK (Fig. 11b) after 3
reaction cycles are the same as those of fresh catalyst
showing that high selectivity towards carbonyl group in
ACT remains unchanged.
0
0
100
200
300
400
t [min]
(b) 100
80
60
40
20
0
S ( PE)
7.5g/dm3
10 g/dm3
12.5 g/dm3
This set of experiments proves stable activity of 2%Ru/
FCN in recycling hydrogenation experiments which most
likely results from the multidentate character of FCN resins
(Scheme 2). As mentioned before, in FCN matrix chelating
functional groups (–NH, –NH2, C=O) appear, able to fill
coordination sites of ruthenium making the metal centre
strongly bound to the support. This is in agreement with the
literature data [35] showing that leaching of the metal from
the polymer can be reduced significantly by using polymer
functionalized by chelating ligands.
S (CMK)
0
20
40
60
80
100
C [%]
Fig. 9 The effect of 4%Ru/FCN catalyst concentration. Conversion of
ACT vs. reaction time (a) and selectivity to particular reaction products
against ACT conversion (b) (ACT concentration 0.137 mol/dm3)
123

´
D. Duraczynska et al.
92
100
80
60
40
20
0
(a)
100
80
60
40
20
0
0.137 g/dm3
"1"
"3"
0.068 g/dm3
0
50
100
150
200
250
300
0
100
200
300
400
500
t [min]
t [min]
100
80
60
40
20
0
(b) 100
80
60
40
20
0
S (PE)
0.068 g/dm3
0.137 g/dm3
"1"
"3"
S (CMK)
0
20
40
60
80
100
0
20
40
60
80
100
C [%]
C [%]
Fig. 10 The effect of initial ACT concentration. Conversion of ACT
vs. reaction time and selectivity to particular reaction products against
ACT conversion in the presence of 4%Ru/FCN catalyst (catalyst
concentration 10 g/dm3)
Fig. 11 Successive use of 2%Ru/FCN catalyst in hydrogenation of
ACT. The change of ACT content vs. reaction time (a) and selectivity
to PE and CMK against conversion of ACT in first (‘‘1’’) and third
(‘‘3’’) catalytic experiment (b) (catalyst concentration 10 g/dm3,
ACT: 0.137 mol/dm3)
3.4 The Role of the Bi-phasic Solvent System
than in alcohol solvents [37]. Moreover, apart from polarity
of solvents, the nature of support in Pd catalysts is found to
affect the rate and selectivity of ACT hydrogenation [38].
Solvent effect can be manifested among others, in inter-
actions with the reactants molecules via the solvation
effect. From the experiments of competitive hydrogenation
of non-polar and polar substrates, a general conclusion has
been formulated, indicating that a polar solvent enhances
adsorption of a non-polar reactant at the catalyst surface
while a non-polar solvent promotes adsorption of a polar
reactant [34, 36]. Due to such solvent–substrate solvation
In liquid phase catalytic reactions the type of solvent fre-
quently influences the rate and selectivity of reaction. The
effect has usually a complex nature and may be related to
reactant-solvent, product-solvent and/or catalyst-solvent
interactions [34, 36]. Often, the solvent effect is rational-
ized by correlating the reaction rates and product distri-
bution with solvent polarity or dielectric constant [13, 19].
For instance, the activity of Pd/AlPO4 in hydrogenation of
ACT in various alcohols decreases when increasing the
alcohol dielectric constant [18]. Similarly, on Raney Ni
catalyst the ACT hydrogenation is faster in cyclohexane
123

Hydrogenation of Acetophenone in the Presence of Ru Catalysts
93
effect, the selectivity and the type of product formed may
change significantly. In reactants possessing polar groups,
interaction with the solvent can affect different fragments
of the molecules, depending on the solvent polarity.
The hydrogenation of p-phenylphenol, which contains
less polar unsubstituted phenyl ring and more polar
OH-substituted phenyl rings, is a good illustration [34]. In
the polar water–acetic acid solvents, p-cyclohexylphenol is
the major product, because OH-substituted phenolic ring is
more strongly solvated and thus less readily adsorbed on
the catalyst surface. Conversely, more effective solvation
of less polar unsubstituted phenyl ring by less polar
cyclohexane solvent resulted in preferential adsorption of
OH-substituted phenolic ring and preferential formation of
phenylcyclohexanol.
4 Conclusions
Ruthenium catalysts supported on amine functionalized
methacrylate-styrene FCN resin prove very active and
highly selective in the hydrogenation of carbonyl group in
acetophenone. In the presence of 4%Ru/FCN catalyst
1-phenylethanol is formed with ca. 80% selectivity. The
essential feature of the catalytic set-up is the use of a
biphasic isooctane/water solvent system. An important
aspect of reaction carried out in the biphasic isooctane/
water solvent is that acetophenone becomes transformed
preferentially via hydrogenation of C=O group rather than
hydrogenation of aromatic ring. The role of biphasic iso-
octane/water solvent system has been attributed to the
solvation of acetophenone phenyl ring by non-polar iso-
octane, which enhances appropriate orientation of the
reactant and facilitates adsorption and catalytic transfor-
mation of polar carbonyl group at the surface of the cata-
lyst. Superior selectivity to 1-phenylethanol over polymer-
supported ruthenium catalyst with respect to the reference
Ru/Al2O3 catalyst is tentatively assigned to steric effects
induced by the polymer chains which enhance favorable
orientation of acetophenone molecule towards Ru active
sites.
In biphasic IO/H2O system the reduction of C=O pre-
dominates over the hydrogenation of aromatic ring in the
ACT substrate. Moreover, the reaction in biphasic IO/H2O
system is faster than in ethanol.
While trying to find a rationale for the observed
phenomenon, one should recall that in biphasic IO/H2O
solvent system the overwhelming majority of ACT is
contained in less polar isooctane. On the other hand, the
primary function of the water is believed to be making
the surrounding of Ru centers more hydrophilic, simi-
larly to what is postulated for benzene–water system
used for hydrogenation of Ru-supported catalysts [39,
40]. Under such conditions the ACT molecule assumes
orientation in which its less polar part, i.e. phenyl ring,
is directed towards non-polar isooctane, while the more
polar carbonyl group is exposed toward Ru centers.
Additionally, the enhanced hydrophilicity of catalyst
surface may facilitate interaction of Ru-centers with the
lone pair of the electrons on the carbonyl oxygen, which
polarizes the C=O bond, and hence promotes its hydro-
genation. For the same reason, in biphasic IO/H2O sys-
tem, the adsorption of ACT molecule at the catalyst
surface in a flat configuration, involving simultaneous
adsorption of aromatic ring and carbonyl group, is much
less likely. Thus, the observed effect of biphasic IO/H2O
system can be explained by its role in controlling the
orientation of ACT molecule and enhancing the hydro-
philicity of the catalyst surface. This favors catalyst–
ACT interaction via carbonyl group and preferential
reduction of ACT to PE.
Acknowledgements D. D. acknowledges the financial support from
the POL-POST DOC II (project D037/H03/2006) and grant NN204
249034 (project 2490/B/H03/2008/34). The authors thank Prof.
W. Bukowski and dr A. Bukowska from Rzeszow University of
Technology for preparation of polymeric support.
Open Access This article is distributed under the terms of the
Creative Commons Attribution Noncommercial License which per-
mits any noncommercial use, distribution, and reproduction in any
medium, provided the original author(s) and source are credited.
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