Ruthenium Nanoparticles in High-Throughput Studies of Chemoselective Carbonyl Hydrogenation Reactions

Article abstract of DOI:10.1002/cctc.201501069

Small (≤1.4 nm) and very active Ru nanoparticles, stabilized in a polysiloxane matrix, were prepared and studied in hydrogenation reactions by the integration of catalysis and analysis. We used our strategy to combine catalytic activity and separation selectivity in a capillary microreactor, installed in a GC-MS instrument, to develop a fast and reliable screening tool for catalysis over Ru nanoparticles. A high conversion using a low catalyst loading of 0.3 mol % and temperature and long-term stability of the catalytically active column were observed for the hydrogenation of various carbonyl compounds, which included aldehydes, ketones, and pyruvates. Additionally, we observed a high chemoselectivity for aromatic carbonyl systems. Comprehensive measurements were performed in this high-throughput experimental setup to gain important insights into the kinetics of hydrogenation reactions at the interface between heterogeneous and homogeneous catalysis.

Full text of DOI:10.1002/cctc.201501069

DOI: 10.1002/cctc.201501069
Full Papers
Ruthenium Nanoparticles in High-Throughput Studies of
Chemoselective Carbonyl Hydrogenation Reactions
Julia Gmeiner,[a] Silke Behrens,[b] Bernd Spliethoff,[c] and Oliver Trapp*[a]
Small (ꢀ1.4 nm) and very active Ru nanoparticles, stabilized in
a polysiloxane matrix, were prepared and studied in hydroge-
nation reactions by the integration of catalysis and analysis.
We used our strategy to combine catalytic activity and separa-
tion selectivity in a capillary microreactor, installed in a GC–MS
instrument, to develop a fast and reliable screening tool for
catalysis over Ru nanoparticles. A high conversion using a low
catalyst loading of 0.3 mol% and temperature and long-term
stability of the catalytically active column were observed for
the hydrogenation of various carbonyl compounds, which in-
cluded aldehydes, ketones, and pyruvates. Additionally, we ob-
served a high chemoselectivity for aromatic carbonyl systems.
Comprehensive measurements were performed in this high-
throughput experimental setup to gain important insights into
the kinetics of hydrogenation reactions at the interface be-
tween heterogeneous and homogeneous catalysis.
Introduction
Although advanced strategies to optimize the activity of cata-
lysts have been known for many decades, attention has shifted
to control the selectivity for the desired product in catalysis re-
search in recent years. This is related to an ecological focus,
pollution control, and to develop green chemistry.[1,2] In this
regard, the preparation of new catalysts, which combine ho-
mogeneous selectivity and heterogeneous recyclability, is of
great interest in catalysis.[3,4] Furthermore, research aims to-
wards new methods for the characterization of these catalysts
to improve our understanding of catalysis in quasi-homogene-
ous systems.[5] The immobilization of catalysts in a two-phase
system is one of the common technical solutions to achieve
high chemoselectivity and to make the catalytic system recy-
clable.[1] With regard to industrial applications, metal nanopar-
ticles offer many new opportunities because of their high sur-
face area and, therefore, their high activity under mild reaction
conditions. Based on the increased consumption of energy
and decreasing fossil reserves, processes that involve the hy-
drogenation of CO and CO2 receive much attention, for exam-
ple, the optimization of Fischer–Tropsch synthesis to develop
a selective processing of ordered hydrocarbons.[6] Established
processes for gasoline synthesis mainly use Co and Fe cata-
lysts. Ni and Ru catalysts are also known, especially for the pro-
duction of high-molecular-weight hydrocarbons. Furthermore,
Ru is the most active known Fischer–Tropsch catalyst, which is
of considerable interest because of its efficiency at moderate
temperatures of 1508C.[7] Here we present highly active Ru
nanoparticles for the selective hydrogenation of carbonyl
groups. There have been various studies of the chemoselective
hydrogenation of aromatic carbonyl systems.[8,9] Recently, Jiang
and Zheng described the chemoselective hydrogenation reac-
tion of aromatic aldehydes, ketones, and quinoline derivatives
catalyzed by ionic-liquid-stabilized Ru nanoparticles.[8] In this
study, we used our established strategy of on-column reaction
gas chromatography (ocRGC) to investigate in detail the de-
pendence of the catalyzed reaction on the temperature, the
pressure of the reactive carrier gas (H2), the catalyst loading,
and the length of the active catalyst capillary. The utilization of
the chromatographic technique described here allows us to
perform high-throughput experiments and achieve the direct
separation and quantification of the reaction mixture, which
provides easy access to kinetic data and, therefore, improves
our understanding of catalysis in quasi-homogeneous sys-
tems.[10–14] This experimental setup was already used by Trapp
et al. for detailed studies of palladium.[15,16] There are various
procedures to prepare, characterize, and apply Ru nanoparti-
cles.[4] Somorjai et al. reported the synthesis of poly(vinylpyr-
rolidone)-stabilized Ru nanoparticles by the polyol reduction of
Ru(acac)3 (acac=acetylacetonate) and their use for CO oxida-
tion.[17] Further suitable precursors are [Ru(cod)(cot)] (cod=1,5-
cyclooctadiene, cot=1,3-cyclooctatetraene) or RuCl3, which
form small ligand- or polymer-stabilized Ru nanoparticles with
a mean diameter of 1.5–6 nm. The protecting shell influences
both the stability and the activity of the generated parti-
cles.[4,18] Several strategies have been applied for the stabiliza-
[a] J. Gmeiner, Prof. Dr. O. Trapp
Organisch-Chemisches-Institut
Ruprecht-Karls-Universität Heidelberg
Im Neuenheimer Feld 270, 69120 Heidelberg (Germany)
E-mail: trapp@oci.uni-heidelberg.de
[b] Dr. S. Behrens
Institut für Katalyseforschung und Technologie
Karlsruhe Institute of Technology
Herrmann-von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen
(Germany)
[c] B. Spliethoff
Max-Planck-Institut für Kohlenforschung
Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr (Germany)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201501069.
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tion of nanoparticles, which include the use of polymers, den-
drimers, ligands, and ionic liquids.[19]
The concept presented here includes the in situ generation
of highly active Ru nanoparticles stabilized by a polymer
matrix for application in the selective hydrogenation reactions
of carbonyl groups. [Ru(methylallyl)2cod] was chosen as a suit-
able precursor for the preparation of the Ru nanoparticles. Re-
cently, Prechtl et al. investigated this precursor in the hydroge-
nation of arenes under mild conditions (50–908C, 4 bar H2).
These Ru nanoparticles had a mean diameter of 2.1–3.5 nm
and were stabilized using imidazolium ionic liquids.[20]
Figure 1. a) TEM image of the isolated Ru nanoparticles with a metal con-
centration of 1.0”10À7 molmLÀ1. Scale bar=10 nm. b) Size distribution of
the prepared Ru nanoparticles with a mean diameter of (1.4Æ0.2) nm.
Results and Discussion
hydrogenation experiments and will be discussed in detail (see
Supporting Information).
In this study, we prepared Ru nanoparticles by a method that
has been established in our research group for the synthesis of
Pd nanoparticles, in which a stabilizing polymeric matrix is ob-
tained by using the copolymers methylvinyldimethylpolysilox-
ane (MVPS; 4.5% Si(O)(CH3)(CH=CH2) groups) and hydrido-
methyldimethylpolysiloxane (HMPS; 25.7% Si(O)(CH3)H groups)
that takes advantage of the metal as a hydrosilylation catalyst
to promote the crosslinking of these copolymers.
The reduction of the RuII precursor [Ru(methylallyl)2cod] is
performed by the coordination of the Ru ions to the vinyl
groups of MVPS (1) and the addition of HMPS (2). Consequent-
ly, the polymeric matrix is built up by the crosslinking Ru-cata-
lyzed hydrosilylation reaction (Scheme 1).[15,16]
For the on-column GC measurements, a coating solution
that contained 4 mg of the copolymers HMPS and MVPS and
33.4 mg of the RuII precursor in 1 mL diethyl ether was pre-
pared to give a 250 nm film on the inner wall of the fused-
silica capillary with an inner diameter of 250 mm (Scheme 1).
The ratio of HMPS to MVPS was 3:1 to ensure an adequate
concentration of HMPS for both the reduction of the metal
ions and the formation of the stabilizing matrix as well as the
immobilization to the capillary surface to achieve a stable film.
Comprehensive studies of polymer-stabilized Pd nanoparticles
for application in ocRGC prove that with this ratio no changes
in the size and shape and no agglomeration of the particles
were observed at elevated temperatures (2008C) and after
treatment with H2. Therefore, we can conclude that the Ru cat-
alyst embedded on the column shows similar characteristics as
that in solution (Figure 1).[16]
The fused-silica capillaries were coated using the static
method described by Grob[21] and then heated to 1908C at
a rate of 0.5 KminÀ1 under a slow stream of H2 (10 kPa) to
achieve the complete conversion of the reactants as well as
a sufficient permanent bonding of the remaining SiÀH groups
to the capillary surface silanol groups. In previous studies of
catalytically active capillaries coated with Pd nanoparticles, we
added the slightly polar dimethyldiphenylpolysiloxane copoly-
mer (GE-SE-52, 5% phenyl groups) to improve the stability of
the coating and the reproducibility of the on-column hydroge-
nation measurements.[16] Notably, we experienced a decrease
in the catalytic activity of the Ru nanoparticles on the addition
of GE-SE-52. The inhibiting effect in the on-column reaction
chromatographic measurements can be explained by the stabi-
lization of the nanoparticles and the shielding of the catalyti-
cally active sites.[22] The coated catalyst capillaries were instal-
led between a preseparation column (GE-SE-30, 50 cm, i.d.
250 mm, 500 nm film thickness) and a separation column (GE-
SE-52, 25 m, i.d. 250 mm, 250 nm film thickness) to integrate
catalysis and analysis in a single experimental step (Figure 2).
The loading of Ru nanoparticles is calculated to be 2.1”
10À13 molcmcapillaryÀ1, which corresponds to a catalyst loading of
0.3 mol% using a 50 cm long catalytically active column. Hy-
drogen was used as the reactive carrier gas. A reactivation pro-
cess was performed after each ocRGC experiment to sustain
Scheme 1. Preparation of Ru nanoparticles embedded in a polymeric matrix
for application in on-column reaction gas chromatography.[16] Full conver-
sion was achieved for 3 by using a 50 cm long catalytic capillary (catalyst
loading 0.3 mol% Ru).
The clear solution that contains the metal precursor and the
mixture of the copolymers in anhydrous diethyl ether turns
black within 2 h at room temperature, which corresponds to
a fast reduction of the metal precursor. The Ru nanoparticles
were characterized by high-resolution transmission electron
microscopy (TEM). We observed spherical and crystalline parti-
cles with a narrow size distribution of (1.4Æ0.2) nm. It can be
seen from the TEM picture shown in Figure 1 that the particles
are isolated separately in the polymeric matrix. As a result of
the protecting polymer layer, no agglomeration was observed.
The small size of the particles determined by TEM corresponds
well with the high catalytic activity observed in the following
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long-term stability of the catalytic capillaries was also exam-
ined. The microreactor can be reused over 200 times without
a significant loss in conversion, which corresponds to a turn-
over number (TON) >60000 (Figure 3b). We monitored the
dependence of the conversion on the temperature and the
inlet hydrogen pressure to obtain a 3D dataset (Figure 3c)
that shows the expected trends, namely, high conversions at
increased reaction temperatures and lower inlet pressures,
which corresponds to increased reaction times. Here, conver-
sions of up to 100% were observed for reaction temperatures
ꢁ808C. The effect of the decrease of the hydrogen pressure
has only a minor influence on the conversion, which can be ex-
plained by a smaller impact on the contact time of the sub-
strate with the catalytically active column. These observations
confirm that the selectivity and activity of the Ru nanoparticles
is mainly based on their nature, size, and stabilization in the
polymeric matrix. This makes our high-throughput micro-flow-
reactor system comparable with a classical batch reactor
system and allows the fast reaction screening of various cata-
lytic systems.[10–16]
Figure 2. Experimental setup: The reaction capillary is coupled between
a preseparation column (50 cm, GE-SE-30) and a separation column (25 m,
GE-SE-52).
the reproducibility and activity of the catalyst column. There-
fore, the capillary was heated to 1608C at 40 kPa H2 inlet pres-
sure for 10 min. We observed the total conversion of cyclohex-
anecarboxaldehyde (3) in a broad temperature range (50–
1008C) at various inlet pressures (50–90 kPa) by using an only
50 cm long catalytically active column (Scheme 1). With regard
to the extremely low reaction time of 5 s, which corresponds
to the contact time of the substrate with the catalytically
active column, we developed a nanoparticle-based catalyst
system with a very high activity for the hydrogenation of car-
bonyl compounds.
We further investigated the activity of the Ru nanoparticle
system in various hydrogenation reactions and studied their re-
action kinetics. Based on the Langmuir–Hinshelwood mecha-
nism, we calculated the activation parameters DG°, DH°, and
DS° as well as the reaction rate constants by using a first-
order reaction rate law. The calculation was performed using
temperature-dependent measurements. The activation enthal-
py DH° of the reaction was obtained from the slope, and the
activation entropy DS° was obtained from the intercept of the
Eyring plot. Deviations were calculated by error-band analy-
sis[16] of the linear regression.[23]
Next we studied the hydrogenation reaction of ethylpyru-
vate (5) in detail (Figure 3). The dependence of the conversion
on the length of the reactor capillary is shown in Figure 3a.
The conversion decreases with the length of the reactor capil-
lary as expected because of the reduced contact time. The
Figure 3. On-column hydrogenation reaction of 5. a) Turnover dependent on the length of the catalyst capillary at 908C and 70 kPa H2. b) Recyclability of Ru
NPs for the hydrogenation of 5 at 908C and 70 kPa H2 (catalyst loading 0.3 mol%). c) Three-dimensional dataset in the temperature range of 50–1008C using
inlet pressures of 50, 60, 70, 80, and 90 kPa H2.
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additive in the catalyst system is necessary for a chemoselective
hydrogenation.[8,24] Recently, Jiang and Zheng described ionic-
liquid-stabilized Ru nanoparticles and observed no catalytic ac-
tivity without the addition of a base.[8] The Gibbs activation
energy was in the range of 83.1 and 94.9 kJmolÀ1, which is in
very good agreement with measurements of on-column hydro-
genation reactions using metal nanoparticles performed previ-
ously.[16] The lowest activation energy of almost DG° =
81 kJmolÀ1 was determined for 3, which explains the great ac-
tivity for this system (Figure 1). For benzoylmethyl formate,
a high DG° value was determined as well, however, high con-
versions of up to 88% were observed. These results can be ex-
plained by the very high retention of the substrate in the sta-
tionary phase of the catalytically active capillary, which is sup-
ported by the contact time of 30 s. In comparison, benzalde-
hyde has a reaction time of only 3 s.
Figure 4. Eyring plot to determine the activation parameters of the hydroge-
nation reaction of 5 over Ru nanoparticles (catalyst loading 0.3 mol%).
The Eyring plot for the hydrogenation reaction of 5 obtained
by temperature-dependent measurements is shown in
Figure 4. The obtained low activation enthalpy DH° =
54.4 kJmolÀ1 and the highly negative entropy value DS° =
À109 JmolÀ1 KÀ1 corroborate the high catalytic activity and
high temperature dependence of the hydrogenation reaction.
In addition, the small error bands[16] demonstrate the high re-
producibility and reliability of the hydrogenation measure-
ments in the capillary-type microreactor employed here.
Our results are in agreement with thermodynamic data for
hydrogenation reactions of carbonyl compounds catalyzed ho-
mogenously published in the literature. For example, for the
hydrogenation of cyclohexanone, we found an activation barri-
er of 83 kJmolÀ1, which is in very good agreement with the
time-dependent measurements of the homogeneous hydroge-
nation reaction using the cationic complex [RuH(CO)(NC-
Me2)(PPh3)2]+, in which a Gibbs activation energy of 85 kJmolÀ1
was found.[25] This result demonstrates the suitability of ocRGC
to study a catalytic system to determine activation parameters
with high precision, and in this case, the quasi-homogeneous
nature of the Ru nanoparticles.[26] At the interface of this ho-
mogeneous and heterogeneous system we are able to obtain
the selectivity observed for homogeneous catalysis in
The systems studied in the hydrogenation reaction of car-
bonyl compounds are summarized in Table 1. We observed
high conversions in a range of 90–100% at moderate tempera-
tures using a capillary catalyst loading of 0.3 mol%. A very low
conversion of only 12% for the aromatic ketone, acetophe-
none, was observed. It is well known that in this case a basic
the hydrogenation reaction of carbonyl compounds
by the use of a heterogeneously immobilized cata-
lyst.[1]
Table 1. Results of the on-column hydrogenation measurements using Ru nanoparti-
cles.
We further investigated the influence of para sub-
Substrate
C
[%]
k[c]
DG°[d]
DH°
DS°
stituents in the chemoselective hydrogenation of
benzaldehyde (Figure 5). Although Jiang et al. ob-
served a strong solvent dependence in their studies
of ionic-liquid-stabilized Ru nanoparticles,[8] we ob-
served a 100% chemoselective conversion with a cat-
alyst loading of 0.3 mol%. Moreover, we noticed
a small influence of the substituent.[27] As expected,
an electron-withdrawing group has a positive impact
on the reaction, whereas electron-donating groups
lead to a decreased conversion. This effect was also
seen by substitution in the a-positions. We already
mentioned that a basic additive is required for the
hydrogenation of acetophenone, however, the con-
version for trifluoroacetophenone increases signifi-
cantly, which can be attributed to the electron-with-
drawing substitution. As a result of the electron-do-
nating influence in phenyl formate, we observe again
a decreased conversion compared with the unsubsti-
tuted benzaldehyde.
[10À1 sÀ1
]
[kJmolÀ1
]
[kJmolÀ1
]
[JmolÀ1 KÀ1
]
100[a]
3.5
3.5
0.05
0.9
–
83.1
83.3
94.9
93.0
86.9
81.7
39.5Æ0.8
39.6Æ1.6
55.2Æ2.3
52.6Æ1.4
54.4Æ0.3
34.1Æ1.1
À146Æ6
À147Æ12
À132Æ12
À135Æ8
À109Æ1
À135Æ8
88[a]
12[a]
88[b]
100[a]
100[b]
–
An increase of the catalyst loading to 0.6 mol% led
to an increased activity and an almost 100% chemo-
selective conversion for all substrates studied (Fig-
ure 6b). The obtained TEM picture of a Ru loading of
[a] Conversion C at 1008C and 70 kPa using a catalyst loading of 0.3 mol%. [b] Con-
version C at 908C and 70 kPa using a catalyst loading of 0.3 mol%. [c] Reaction rate
constants reported at 908C and 70 kPa. [d] Gibbs activation energy at 258C.
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tential for the examination of various industrially relevant pro-
cesses.
Experimental Section
Coating of fused-silica columns
[Ru(methylallyl)2cod] (0.1 mg), HMPS (9 mg), and MVPS (3 mg) were
dissolved in anhydrous diethyl ether (3.00 mL). The fused-silica ca-
pillary (i.d. 250 mm) was coated by the static method described by
Grob[21] using this solution to give a 250 nm film on the inner wall
of the capillary. Immobilization was achieved under a stream of H2
(10 kPa) using a temperature program starting at 408C for 10 min
then heating to 1908C for 10 h at a rate of 0.5 KminÀ1
.
On-column reaction gas chromatographic hydrogenation
measurements
The hydrogenation measurements were performed by on-column
reaction gas chromatography. The catalyst capillary (10–50 cm) was
coupled between a preseparation column, coated with GE-SE-30
(50 cm, i.d. 250 mm, 500 nm film thickness), and a separa-
tion column, coated with GE-SE-52 (25 m, i.d. 250 mm,
250 nm film thickness). H2 was used as the reactive carri-
er gas. All measurements were repeated three times at
each temperature and pressure. After each measurement
the column was reactivated at 1608C and 40 kPa H2 for
10 min.
Figure 5. a) Influence of para substitution on the chemoselective hydrogena-
tion reaction of benzaldehyde. b) Influence of substitution in the a-position.
Acknowledgements
Figure 6. Doubled catalyst loading of 2.1”10À7 molmLÀ1. a) TEM image of the Ru nano-
particles. Scale bar=10 nm. b) Elution profiles of the studied substrates with the use of
a 50 cm catalytically active column (catalyst loading 0.6 mol% Ru) at 908C and 70 kPa H2.
This work was supported by the European Research
Council under Grant Agreements No. StG 258740
(AMPCAT).
2.1”10À7 molmLÀ1 shows a higher concentration of particles
with a comparable mean diameter of (1.4Æ0.2) nm (Support-
Keywords: heterogeneous
catalysis
·
·
high-throughput
hydrogenation ·
ing Information). The increased number of particles were no
longer embedded separately in the polymer matrix, but no ag-
gregation was observed (Figure 6a). The prepared catalyst
column shows a high activity in the selective carbonyl hydro-
genation, whereas the aromatic system is hydrogenated to
a lower extent. These results confirm that the particles main-
tain the expected quasi-homogeneous behavior and the de-
sired chemoselectivity in the studied reaction if the number of
available catalytically active Ru sites is increased.[28]
screening
·
homogeneous catalysis
nanoparticles
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