Formylation with supercritical carbon dioxide over Ru/Al2O 3 modified by phosphines: Heterogeneous or homogeneous catalysis?

Article abstract of DOI:10.1016/j.jcat.2004.09.026

The formylation of 3-methoxypropylamine with hydrogen and supercritical carbon dioxide over Ru-based catalysts was studied. In this solventless process, carbon dioxide acts as both reactant and solvent. Interestingly, Ru/Al₂O₃ modified by the phosphine 1,2-bis(diphenylphosphino)ethane (dppe) showed a high formylation activity at 100% selectivity, comparable to those of the homogeneous catalysts RuCl ₂(dppe)₂ and RuCl₂(PPh₃) ₃. Analysis of the reaction mixture by ICP-OES and structural studies by in situ X-ray absorption spectroscopy discovered that the presence of the phosphine modifier led to the formation of a homogeneous ruthenium catalyst.

Full text of DOI:10.1016/j.jcat.2004.09.026

Journal of Catalysis 229 (2005) 144–153
www.elsevier.com/locate/jcat
Formylation with supercritical carbon dioxide over Ru/Al2O3 modified by
phosphines: heterogeneous or homogeneous catalysis?
Markus Rohr, Jan-Dierk Grunwaldt ∗, Alfons Baiker ∗
Department of Chemistry and Applied Biosciences, Swiss Federal Institute of Technology, ETH Hönggerberg, CH-8093 Zürich, Switzerland
Received 6 August 2004; revised 27 September 2004; accepted 29 September 2004
Available online 8 December 2004
Abstract
The formylation of 3-methoxypropylamine with hydrogen and “supercritical” carbon dioxide over Ru-based catalysts was stud-
ied. In this solventless process, carbon dioxide acts as both reactant and solvent. Interestingly, Ru/Al O modified by the phosphine
2
3
1,2-bis(diphenylphosphino)ethane (dppe) showed a high formylation activity at 100% selectivity, comparable to those of the homogeneous
catalysts RuCl (dppe) and RuCl (PPh ) . Analysis of the reaction mixture by ICP-OES and structural studies by in situ X-ray absorption
2
2
2
3 3
spectroscopy discovered that the presence of the phosphine modifier led to the formation of a homogeneous ruthenium catalyst.
2004 Elsevier Inc. All rights reserved.
Keywords: Formylation of amines; 3-Methoxypropylamine; Supercritical carbon dioxide; Ru/Al O ; Phosphines; X-ray absorption spectroscopy; EXAFS;
2
3
In situ spectroscopy
1. Introduction
A further step toward a “green process” is the appli-
cation of heterogeneous catalysts because of their intrin-
sic advantages concerning catalyst separation, reuse, and
handling. An interesting approach is the immobilization
of the corresponding homogeneous complexes function-
alized by silyl ether groups in an inert silica matrix, as
shown previously for RuCl2X3 (X = Ph2P(CH2)2Si(OEt)3,
Me2P(CH2)2Si(OEt)3) [8,17], and RuCl2(dppp)2 (dppp =
Ph2P(CH2)3PPh2) [18]. However, the preparation of these
Ru-silica hybrid gels is rather demanding, and more eas-
ily accessible heterogeneous catalysts would be desirable.
A simpler approach could be the modification of a supported
Ru catalyst with a suitable phosphine, which is the focus
of this study. The modification of metal catalysts by ad-
sorbed auxiliaries (modifiers) has been successfully applied
to improve the catalytic properties of a variety of metals, as
covered in a recent review [19]. A related example is the
use of phosphine modifiers in partial oxidation reactions on
Pt/alumina catalysts [20]. This prompted us to apply this
strategy for formylation with a 5 wt% Ru/Al2O3 catalyst
modified with a phosphine (dppe or PPh3). We show that
these catalysts exhibit good activity in the formylation of
3-methoxypropylamineat 100% selectivity. Extensive in situ
A step toward “green” formylation of amines is the use
of carbon dioxide and hydrogen as formylation agents in-
stead of toxic compounds such as carbon monoxide and
phosgene [1–5]. This approach provides the opportunity to
use supercritical carbon dioxide acting as both C1-building
block and solvent (solvent-free process) [6–9]. A vari-
ety of ruthenium complexes have been reported [10,11]
to be active and selective for such formylation reactions,
including RuCl2(PMe3)4 in the formylation of hydrogen
with carbon dioxide to formic acid [7], and RuCl2(PPh3)3,
Ru3(CO)12 [12], and RuCl2(dppe)2 [13] in the formylation
of dimethylamine, propylamine [14], and morpholine [15].
Among these ruthenium-based complexes, RuCl2(dppe)2
was found to be superior with respect to activity and sta-
bility [13,14,16].
*
Corresponding authors.
E-mail addresses: grunwaldt@chem.ethz.ch (J.-D. Grunwaldt),
baiker@chem.ethz.ch (A. Baiker).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2004.09.026
2004 Elsevier Inc. All rights reserved.

M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
145
EXAFS studies using a specially designed high-pressure cell
revealed that the observed catalytic behavior has to be at-
tributed to a highly active homogeneous Ru complex formed
from the Ru/alumina catalyst during reaction.
a HP-5 capillary column (30 m × 0.32 mm × 0.25 µm) and
a flame ionization detector (FID). Product identification was
achieved with a gas chromatograph (HP-6890) coupled to a
mass spectrometer (HP-5973) after separation of the com-
ponents from the reaction mixture by vacuum distillation. In
a parametric study the temperature and the amounts of cata-
lyst, mpa, hydrogen, and carbon dioxide were varied to find
the optimal reaction conditions.
2. Experimental
2.1. Catalyst materials
2.3. Characterization of in situ formed homogeneous
complex by ICP-OES
The 5 wt% Ru/Al2O3 catalyst (9001, escat 44, powder,
prereduced) was used as received from Engelhard. Note that
the ruthenium catalyst was partially oxidized, but we denote
it as Ru/Al2O3. The 5 wt% Ru/Al2O3 catalyst was modified
with dppe in two different ways. On the one hand, dppe was
added before the reaction by reflux in THF for 2 h and dried
under high vacuum; on the other hand, it was simply added
during the reaction (Section 2.2).
After reaction, selected samples were filtrated and cen-
trifuged to quantify the ruthenium and phosphorus content
of the liquid phase with inductively coupled plasma atomic
emission spectroscopy (performed at ALAB AG in Urdorf,
Switzerland).
The homogeneous RuCl2(dppe)2 catalyst was synthe-
sized according to the method of Mason et al. [21]. A sus-
pension of 1.00 g RuCl2(PPh3)3 in 20 ml of acetone was
mixed with 0.85 g dppe under an Ar atmosphere and fast
stirring. A yellow precipitate was observed after 2 min
and separated with a suction filter after 10 min of stir-
ring at room temperature. The yellow powder produced was
washed with acetone and methanol and dried in vacuum,
and its structure was confirmed with 1H- and 31P-NMR
and elemental analysis [calculated (%) for C52H48Cl2P4Ru
(968.8 g/mol): C 64.47, H 4.99, P 12.79, Cl 7.32, Ru
10.43; found: C 64.30, H 5.19, P 12.94]. RuCl2(PPh3)3
(tris(triphenylphosphine)ruthenium(II)dichloride;Fluka, pu-
2.4. Ex situ and in situ X-ray absorption spectroscopy
studies
The extended X-ray absorption fine structure (EXAFS)
and X-ray absorption near-edge structure (XANES) ex-
periments were mainly performed at the beamline X1,
HASYLAB, at DESY in Hamburg, Germany. The stor-
age ring typically operates at 4.45 GeV with a ring cur-
rent between 80 and 120 mA. A Si(311) double-crystal
monochromator was used, and higher harmonics were ef-
fectively removed by detuning of the crystals to 70% of
the maximum intensity. Three ionization chambers filled
with Ar were used to record the intensity of the incident
and the transmitted X-rays. The samples were located be-
tween the first and second ionization chambers, and a refer-
ence sample (RuCl3 pellet) was placed between the second
and third ionization chambers. Some additional experiments
were performed at the Swiss–Norwegian beamline (SNBL)
at the European Synchrotron Radiation Facility (ESRF) in
Grenoble, France. At SNBL, a Si(111) crystal was used as
a channel-cut monochromator and a double-bounce gold-
coated mirror system for rejection of higher harmonics. The
three ionization chambers were filled with Ar, N2, or Kr
in different combinations (I0 Ar, It 30% Kr and 70% N2,
Iref 30% Kr and 70% N2). At both beamlines (X1 and
SNBL), EXAFS spectra were taken under stationary con-
ditions in the step-scanning mode around the Ru K-edge
(22.117 keV) between 21,900 and 22,800 eV, with a RuCl3
pellet as a reference. Fast QEXAFS scans were recorded in
the continuous scanning mode, usually between 22,065 and
22,665 eV (0.15 s/eV). The raw data were energy calibrated
(Ru K-edge energy of the RuCl3 pellet: 22,120 eV [23],
first inflection point), background corrected, and normalized
with WINXAS 3.0 software [24]. Fourier transformation for
EXAFS data was applied to the k1-weighted functions in the
interval k = 3–12.5 Å−1 for the EXAFS data and were fitted
in R-space. Typically deviations for the coordination num-
1
rum) was used as received after structural analysis by H-
and 31P-NMR and elemental analysis [calculated (%) for
C54H45Cl2P3Ru (958.8 g/mol): C 67.64, H 4.73, P 9.69, Cl
7.39, Ru 10.54; found: C 67.36, H 4.84, P 9.72, Cl 7.56].
2.2. Catalytic formylation of 3-methoxypropylamine
The catalytic studies were performed in a 500-ml high-
pressure stainless-steel autoclave (Medimex No. 128) with
temperature control, a rupture disk, and a dosing system
for gases [22]. The chemicals 3-methoxypropylamine (mpa)
(Fluka, > 99%) and 1,2-bis(diphenylphosphino)ethane
(dppe) (Fluka, 98%) were used as received; liquid carbon
dioxide (4.5) and hydrogen gas (5.0) were supplied by Pan-
gas.
In a typical procedure, mpa, dppe, and Ru/Al2O3 were
poured into the reactor before it was closed and flushed with
hydrogen. The reactor was filled with hydrogen(p ∼ 60 bar).
After the reactor was heated to 100 ◦C, the hydrogen pres-
sure was adjusted to 80 bar, and 100 g of carbon dioxide was
added. This resulted in a total pressure of about 200 bar. The
stirring rate was fixed at 300 min−1. After a certain reac-
tion time, the reactor was cooled down and depressurized
through the outlet valve. The reaction mixture was ana-
lyzed with a gas chromatograph (HP-6890) equipped with

146
M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
ber were within 0.5, and those for the distance were within
0.02 Å.
ment (addition of hydrogen and carbon dioxide and heating
to 100 ◦C) was performed, but as without dppe.
(Safety note: The experiments described in this paper in-
volve the use of high pressure and require equipment with
an appropriate pressure rating.)
To identify the structure of the ruthenium complexes
formed, the product solution was investigated by EXAFS on
liquid samples after reaction as well. For this purpose a spe-
cial stainless-steel EXAFS cell was used for transmission
experiments. Because of the low concentration of Ru in the
product mixture (50–100 ppm), a long path length of 4 cm
was chosen, with a cross section of 6 mm × 11 mm. The
volume of the cell was 2 ml, and exchangeable Kapton win-
dows were used on both sides of the cell. The cell can be
filled from the top, and a 1 mm × 10 mm beam was focused
to the middle of the spectroscopic cell with an x, z, θ-table
(Newport). After calibration with a sample of known absorp-
tion step µ·d, the concentration of the solution could also be
determined independently of the ICP-OES measurements.
For spectroscopic studies under reaction conditions, in
addition, an in situ batch reactor was designed that allows
investigation of the volume at two locations. In this way
the solid catalyst sample at the bottom and the liquid phase
10 mm above the bottom could be probed. The in situ cell
can be used up to a pressure of 250 bar and a temperature
of 200 ◦C and equipped with a burst plate (250 bar), a mag-
netic stirrer, a thermocouple, and an in/outlet [25]. For the
experiments, 100 mg of 5%-Ru/Al2O3 was loaded into the
batch reactor cell, 3.0 ml of mpa was added, and the reac-
tor was closed. After cell alignment, normal EXAFS spectra
of the solid catalyst and the solution were taken at room
temperature, while the solution was stirred with a magnetic
stirrer. Then hydrogen (purity 5.0) was added to a pressure
of 70 bar, and 4 g of liquid CO2 was introduced with the help
of a CO2 compressor (NWA PM-101) and a Rheonik mass
flow controller (RHM015). While changes in the solid mate-
rial were monitored by QEXAFS, the mixture was heated to
120 ◦C. Then the reaction mixture was cooled to room tem-
perature. After the addition of 50 mg of dppe, the same treat-
3. Results
3.1. Modification of Ru/Al2O3 with dppe and its
performance in the formylation of mpa
Table 1 gives an overview of the catalytic results obtained
during formylation under different reaction conditions. The
dppe-modified Ru/Al2O3 catalyst system showed good cat-
alytic performance, whereas runs without either catalyst or
dppe (runs 1–3) were unsuccessful. A highly active catalyst
was thus formed by modification of the Ru/Al2O3 surface
with dppe in a molar dppe:Ru ratio of 1:1. The addition
of dppe before the catalytic reaction under reflux in THF
(premodification) or its addition directly during the reaction
itself led to similar catalytic performance. Therefore most
of the experiments were carried out without premodifica-
tion. Table 1 provides some information about the influence
of parameters like temperature, the amount of carbon diox-
ide, the initial partial pressure of hydrogen, and the amounts
of Ru/Al2O3 and dppe on the conversion. The increase in
temperature led to a higher reaction rate (runs 4–6), as ex-
pected. The conversion was also influenced by the amount
of carbon dioxide added (runs 5, 7, 8), which resulted in
the best conversion of 48% with 100 g of carbon diox-
ide (run 8). Previous studies of the formylation of various
amines [14,15] under similar conditions showed that the re-
action mixture is made up of two distinguishable phases:
A dense liquid-like phase containing mainly the amine, the
product formamide and water, and on top a “supercritical”
Table 1
a
Catalytic formylation of 3-methoxypropylamine (mpa) with carbon dioxide and hydrogen using dppe-modified Ru/Al O as catalyst
2
3
◦
Run
Amount of dppe (µmol)
Amount of CO (g)
2
H
initial pressure (bar)
Temperature ( C)
Time (h)
Conversion (%)
2
b
1
0
0
100
100
100
25
25
25
50
100
25
25
25
100
100
100
100
100
80
80
80
80
80
80
80
80
40
140
180
80
80
80
80
80
100
100
100
80
3
3
3
3
3
3
3
3
3
0
0
0
2
3
b
50
50
50
50
50
50
50
50
50
14
25
37
50
100
4
5
6
7
8
18
24
35
23
48
15
38
39
16
47
67
82
63
100
120
100
100
100
100
100
100
100
100
100
100
9
10
11
12
13
14
15
16
3
3
20
20
20
20
20
a
Conditions, if not otherwise stated: 100 mmol mpa, 50 µmol Ru.
0 µmol Ru.
b

M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
147
Fig. 2. XAFS spectra of (1) Ru/Al O , compared to (2) dppe-modified
2
3
Fig. 1. Comparison of the activity in 3-methoxypropylamine formylation
of the homogeneous complexes RuCl (dppe) , RuCl (PPh ) with dppe-
Ru/Al O before reaction (pretreated with dppe and amine under reflux),
2
3
2
2
2
3 3
and (3) dppe-modified Ru/Al O after reaction.
2
3
modified Ru/Al O . Reaction conditions: 100 mmol mpa, 0.050 mmol Ru,
2
3
◦
100 g CO , 80 bar H initial partial pressure, 3 h and 100 C.
2
2
Table 2
Test series of the formylation of 3-methoxypropylamine with dppe-
modified Ru/Al O to check the possibility of the formation of a homo-
geneous catalyst
more gas-like phase containing mainly hydrogen and the
carbon dioxide (note that the term “supercritical” is exactly
defined only for pure substances [26]). Increasing the hydro-
gen initial pressure resulted in higher conversions (Table 1,
runs 5, 9–11). At a hydrogen pressure of 40 bar, 15% con-
version was reached within 3 h, whereas at 180 bar, 39%
conversion was achieved. The amount of dppe added influ-
enced the reaction as well (Table 1, runs 12–16). Conversion
increased with the addition of dppe up to 82%; the P:Ru ra-
tio (mol/L Ru per mol/L P) in this optimal case was 2:1.
When dppe was added in a P:Ru ratio of 4:1, the conversion
decreased to 63%.
2
3
a
Run
Catalytic system
Dppe-modified Ru/Al O
Time (h)
3
20
20
Conversion (%)
17
18
19
48
78
82
2
3
b
Filtered reaction mixture of run 17
Dppe-modified Ru/Al O
2
3
a
Conditions: 100 mmol mpa, 50 µmol Ru on Al O , 50 µmol dppe, 100 g
2
3
◦
CO , 80 bar H at 100 C. After 3 h (run 17), the solid catalyst was removed
2
2
and the reaction was continued in the filtered reaction mixture. The conver-
sion after 20 h (run 18) was equal to the one observed with the catalyst
present during 20 h (run 19).
b
Catalyst was filtered off after 3 h.
For comparison, related homogeneous ruthenium cata-
lysts were tested in the formylation reaction; results are
shown in Fig. 1. The homogeneous complexes RuCl2(dppe)2
and RuCl2(PPh3)3 showed a turnover frequency of 425 and
235 h−1, respectively, at 100% selectivity. Dppe-modified
Ru/Al2O3 showed a conversion of 48% under the same reac-
tion conditions. This conversion is higher than that observed
with the homogeneous catalyst RuCl2(PPh3)3 but lower than
that observed with RuCl2(dppe)2. The higher activity of
RuCl2(dppe)2 compared with RuCl2(PPh3)3 is in line with
earlier studies [14], where RuCl2(dppe)2 has been found to
be most active.
A clear decrease in the whiteline at 22.12 eV is apparent
after step (3), which reveals that ruthenium was reduced
during the reaction. No change in the EXAFS spectra was
observed during modification of the Ru/Al2O3 catalysts by
treatment with amine and dppe beforehand. In the EXAFS
region (Fourier-transformed data, not shown) a clear change
in the Ru–O and the Ru–Ru backscattering was found. How-
ever, ruthenium was not completely reduced.
To elucidate the role of possible corrosion of the Ru con-
stituent and the formation of active ruthenium complexes in
solution under reaction conditions in the presence of dppe,
we performed suitable tests that allowed us to distinguish be-
tween heterogeneous and homogeneous catalysis [27]. The
reaction was stopped after a certain time, the solid catalyst
was filtered off, and we checked it to determine whether
the reaction could be continued in the filtered reaction mix-
ture (Table 2). Note that almost the same conversion was
observed irrespective of whether the Ru/Al2O3 catalyst was
filtered off after 3 h or was present during the whole reaction
time. This implies that during the first 3 h of the reaction,
a highly active Ru-based homogeneous catalyst must have
been formed.
3.2. Identification of the catalytically active species
Fig. 2 shows the ex situ X-ray absorption spectra of the
solid catalyst under different conditions:
(1) Ru/Al2O3 untreated,
(2) Ru/Al2O3 treated with dppe and amine before the reac-
tion, and
(3) dppe-modified Ru/Al2O3 after the reaction.

148
M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
Table 3
Solubility of the in situ formed catalyst from Ru/Al O and dppe in the liquid phase, compared to RuCl (PPh ) and RuCl (dppe)
2
2
3
2
3 3
2
a
Run
Catalyst
Reaction data
Time (h)
Dissolved in reaction mixture
b
−1
)
Conversion (%)
TOF (h
Ru (ppm)
P (ppm)
P/Ru (mol/mol)
20
21
22
dppe-modified Ru/Al O
3
20
20
3
3
3
48
82
78
0
3147
559
585
0
50.5
66.4
60.6
0.60
1.89
385
251
256
294
21.8
268
375
173
319
16.2
12.6
15.8
118
463
3
2
3
3
3
dppe-modified Ru/Al O
2
dppe-modified Ru/Al O
2
c
23
24
Blank test
dppe
d
0
0
25
26
27
RuCl (PPh )
36
64
91
315
425
91
2
3 3
RuCl (dppe)
3
20
133
252
4.2
4.1
2
2
2
RuCl (dppe)
2
a
◦
Conditions: 50 µmol Ru, 100 mmol mpa, 100 g CO , 80 bar H partial pressure, 100 C.
TOF based on Ru dissolved in reaction mixture.
0 mol Ru, 0 mol dppe.
2
2
b
c
d
0 mol Ru, 50 µmol dppe.
In addition, the ruthenium and phosphorus contents in the
designed batch reactor cell was used that allowed in situ
EXAFS measurements both at the bottom (solid phase) and
in the middle (liquid) of the batch reactor [25]. In the first
step the catalyst in the presence of mpa, hydrogen, and car-
bon dioxide was investigated; the results are shown in Fig. 3.
Fig. 3a shows the treatment of the catalyst in the reaction
mixture without dppe; Fig. 3b depicts the spectra taken dur-
ing the reaction in the presence of dppe.
With increasing temperature and in the absence of dppe
(Fig. 3a), Ru was reduced, indicated by the decrease in the
whiteline at 22.12 keV. The structural changes in ruthenium
were limited to the solid phase, and no Ru species were ob-
served in the liquid phase.
After a reaction time of 4.5 h (without dppe), the concen-
tration of ruthenium in the liquid phase was still negligible.
Only after an increase in the stirring rate (Fig. 4, spectrum 2)
was some Ru visible in the X-ray absorption spectrum. Some
part of the catalyst was probably suspended in the upper part
of the cell, and thus a small edge jump was observed. Note
that the XANES scans around the Ru K-edge were taken
only directly around the edge (spectra 2 and 3), and the ab-
sorption step was related to the maximum intensity observed
at the end of the experiment (Fig. 4, spectrum 4, absorption
edge of 1.0).
Hence, during this first step no catalyst was dissolved
in the liquid-like amine-rich phase. However, as soon as
dppe was added, an abrupt increase in the Ru concentration
and a further increase during heating were observed (Fig. 4,
traces 3 and 4). During this step hardly any changes in the
solid Ru catalyst (Fig. 3b) could be observed. Thus the in
situ formation of the active species in the liquid-like amine-
rich phase could be observed as soon as dppe was added
(Fig. 4) and can be related to the tendency of this ligand to
corrode the ruthenium and form Ru complexes. This behav-
ior was also supported by the observation that no catalytic
activity was observed in the absence of dppe, and no sig-
nificant amount of Ru was found in the liquid solution with
ICP-OES (Table 3). Note, furthermore, that the in situ spec-
trum after the reaction (Fig. 4, spectrum 4) and the ex situ
spectrum after the reaction (Fig. 4, trace 5) are similar. Dur-
liquid phase of the samples were measured by ICP-OES;
Table 3 gives an overview. The ruthenium concentration in
the liquid reaction mixture was about 50 ppm (50 mg/kg,
mass based) after 3 h (run 20) under standard reaction con-
ditions, as described above. The results show that after 20 h,
66 ppm (0.689 mmol/L) ruthenium was dissolved in the liq-
uid phase (run 21), compared with 60 ppm (0.631 mmol/L)
if the catalyst was filtered off after 3 h (run 22). The ruthe-
nium concentrations were corroborated by X-ray absorption
spectroscopy (edge jump at the Ru K-edge). Application of
UV–vis spectroscopy was biased by the very low Ru con-
centration, which was at the detection limit of the technique.
Note that the Ru concentration in the liquid phase was very
similar, irrespective of whether the reaction was stopped af-
ter 3 h, and whether the Ru/Al2O3 catalyst was removed or
was present in the reactor for 20 h. The background con-
centrations of ruthenium and phosphorus were significantly
lower during blank runs (Table 3, runs 23, 24), and no con-
version was observed. If we relate TOF to the amount of
ruthenium dissolved in the liquid phase, a very high rate
of 3148 h−1 is estimated based on a ruthenium concentra-
tion of 51 ppm in the reaction mixture. For comparison, the
Ru concentration of the homogeneouscatalyst RuCl2(dppe)2
was also determined. After a reaction time of 3 h, 133 ppm
Ru (corresponding to 1.386 mmol/L) (run 26) was found,
which increased to 252 ppm (2.577 mmol/L in run 27) after
20 h, reflecting the time dependence of the solubility of the
catalyst in the amine-rich liquid-like phase. And in case of
the homogeneous catalyst RuCl2(PPh3)3, a concentration of
385 ppm Ru and 375 ppm P (according to a P/Ru ratio of 3)
was detected (run 25). Note that the concentration of Ru was
very small but sufficient to catalyze the formylation of mpa.
3.3. Spectroscopic study of the in situ formation of the
homogeneous Ru catalyst
To understand the formation of the homogeneous cata-
lyst, the changes in the solid Ru/Al2O3 catalyst and in the
liquid phase were investigated. For this purpose, a specially

M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
149
(a)
Fig. 4. Solubility and formation of the catalytic active species monitored
by XANES in liquid phase during reaction. (1) at 0 min, without dppe;
◦
(2) at 4.5 h, without dppe, increased stirring rate, 120 C; (3) at 30 min,
◦
◦
with dppe, 120 C; (4) after reaction, with dppe, 25 C; (5) ex situ spectrum
◦
after reaction with dppe, 25 C; absorption of spectra 1–4 are calibrated to
the normalized spectrum 4; simultaneously recorded to Fig. 3.
(b)
Fig. 3. (a) Reduction of Ru/Al O , monitored in situ by X-ray absorption
2
3
◦
spectroscopy, solid phase (1) before reaction at 40 C as reference and at
120 C after (2) 0 min, (3) 30 min, (4) 60 min, (5) 150 min reaction time.
◦
(b) In situ monitoring during reaction over Ru/Al O , solid phase (1) before
2
3
◦
◦
reaction at 40 C as reference and with dppe at 100 C after (2) 150 min,
(3) 160 min, (4) 250 min. Conditions: 10 ml batch reactor cell, 3 ml mpa,
Fig. 5. X-ray absorption near-edge structure at the Ru K-edge of
70 bar H , total pressure 120 bar, rest CO .
2
2
RuCl (dppe) , liquid product mixture (dotted black line), dppe-modified
2
2
Ru/Al O , liquid product mixture (solid black line), RuCl (dppe) , solid
2
3
2
2
complex (dotted gray line) and RuCl (PPh ) , liquid product mixture (solid
2
3 3
ing cooling and depressurizing no remarkable change was
detected.
gray line). Reference spectrum: solid RuCl (dppe) .
2
2
son, with the homogeneous catalyst RuCl2(dppe)2 in liquid
phase and as a solid (see corresponding catalytic results in
Table 1). For structural identification RuCl2(PPh3)3 in the
liquid reaction mixture was also investigated. The analyti-
cal results indicate that the structure of the catalyst formed
in situ from Ru/Al2O3 and dppe is similar to the structure of
RuCl2(dppe)2 under reaction conditions, whereas the struc-
ture of the solid RuCl2(dppe)2 seems to be different, proba-
bly as a consequence of the replacement of the chloride by
hydrogen and by the amine under reaction conditions. Note
that a change in the XANES region is also due to the change
in multiple scattering paths that also influence the near-edge
structure [28,29]. From the variation of the XANES region
of RuCl2(PPh3)3 it can also be inferred that the structure
3.4. Structural identification of the homogeneous Ru
complex
To gain information about the structure of the active
species formed in situ, ex situ XANES and EXAFS spec-
tra were recorded. Because of the low concentration of 50–
100 ppm in the reaction mixture, a liquid cell with a 4-cm
path length was constructed. The cell was open on top to
eliminate possible gas bubbles in the highly viscous product
mixture, and Kapton foils were used as X-ray transparent
windows on both sides of the liquid cell to keep the liquid in
the beam.
Fig. 5 shows the XANES region of the dppe-modified
Ru/Al2O3 catalyst in the liquid product mixture in compari-

150
M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
(a)
Fig. 6. Comparison of the Fourier transformed EXAFS spectra
1
(k weighted) at the Ru K-edge of RuCl (dppe) in reaction mixture from
2
2
reaction with mpa (dotted black line), dppe-modified Ru/Al O in reaction
2
3
mixture (solid black line), RuCl (dppe) , solid phase (dotted gray line) and
2
2
RuCl (PPh ) , in reaction mixture (solid gray line).
2
3 3
of the catalyst formed in situ from Ru/Al2O3 and dppe is
more similar to the structure of RuCl2(dppe)2 than to that of
RuCl2(PPh3)3. Only a slight shift in the Ru K-edge is found,
indicating that the oxidation states of the two complexes are
similar [23].
Fig. 6 shows the corresponding Fourier transformed
k1-weighted extended X-ray absorption fine structure
(EXAFS) spectra. Similar to the near-edge spectra, signif-
icant differences of the as-prepared solid catalysts RuCl2-
(PPh3)3 and RuCl2(dppe)2 and the corresponding Ru com-
plexes after reaction (liquid product mixture) are evident.
The backscattering is lower for both Ru–Cl and Ru–P, and
the peak is shifted to lower R values (spectra not cor-
rected for the phase shift). The spectra of the catalysts
exhibit backscattering peaks at 1.8 and 2.5 Å for all cata-
lysts. Whereas RuCl2(dppe)2 in reaction mixture and dppe-
modified Ru/Al2O3 show similar backscattering from the
nearest-neighbor atoms, the solid RuCl2(dppe)2 exhibits a
large peak at 1.8 Å and only a very small one at 2.5 Å. The
corresponding spectrum of RuCl2(PPh3)3 is quite different
(Fig. 6).
The Fourier-transformed spectra were fitted with Ru–Cl
and Ru–P shells calculated with FEFF 6.0 [30]; the results
are listed in Table 4. Some of the experimental and fitted
data are compared in Fig. 7. The structural data obtained for
the solid RuCl2(PPh3)3 (entry 1 of Table 4) are in good ac-
cordance with the literature [31]. For the solid homogeneous
catalyst, three different Ru–P bond lengths, 2.23, 2.37, and
2.41 Å, were reported. Only two Ru–P distances (2.23 and
2.38 Å, Table 4) could be distinguished with EXAFS. Only
one Ru–Cl bond length was detected at 2.44 Å by EXAFS,
which shows the limitation of our method. While the typical
uncertainty is estimated to be 0.02 Å for the distance and
about 0.5 for the coordination number, the superposition of
two shells of the same backscatterer with similar distances
(b)
1
Fig. 7. Fourier-transformed and k -weighted Ru K-edge EXAFS spectra
1
(k · χ(k)) of (a) dppe-modified Ru/Al O (20 h) and (b) RuCl (dppe)
2
3
2
2
after reaction: radial distribution function together with the imaginary
part—measured data (solid line), fitted data (dotted line).
in particular could not be resolved sufficiently. The results
from the EXAFS spectra of solid RuCl2(dppe)2 show that
the different structure is appropriately reflected. Both Ru–P
and Ru–Cl distances are found. The bond length of Ru–Cl
(2.42 Å) in the solid RuCl2(dppe)2 calculated from the EX-
AFS fit is in the same range (2.436 Å) as that published in
the literature [32]. Furthermore, we found only one averaged
Ru–P bond, instead of the two different Ru–P bonds (2.389
and 2.369 Å) reported.
Significant structural changes were detected for the com-
plexes formed during the formylation reaction of mpa. Af-
ter the use of RuCl2(PPh3)3 in the reaction, a Ru–P bond
length of 2.22 Å and an additional contribution that could be
fitted with a Ru–Ru distance at 2.93 Å were found, corre-
sponding to the two peaks observed in Fig. 6. This shows
that there are only slight changes in the Ru–P neighbors
in the liquid (after reaction) or solid phase, but the Ru–Cl
coordination changes. All bond lengths of the complexes
RuCl2(dppm)2 [33], RuCl2(dppe)2 [32], RuCl2(dppp)2 [34],

M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
151
Table 4
Structural data extracted from EXAFS spectra of different ruthenium phosphine complexes as solid and in liquid phase. Ru-concentration in the liquid phase
2
∼ 50–100 ppm, N: coordination number, R: distance of the corresponding neighbor, ꢀσ : Debye–Waller factor
d
Catalyst
Ru–P
Ru–Cl
Residual
2
2
2
2
N
R (Å)
ꢀσ (Å )
N
R (Å)
2.44
ꢀσ (Å )
0.0027
a
RuCl (PPh )
1.0
2.0
4
4.1
3.6
3.3
2.23
2.38
2.29
2.22
2.32
2.35
0.0025
0.0025
0.0013
0.0044
0.0063
0.0060
2.3
1.4
2
3 3
a
RuCl (dppe)
2
2
–
–
2.42
2.93
–
0.0006
0.0060
2.0
7.2
9.4
3.2
2
2
b
RuCl (PPh )
2
3 3
b
RuCl (dppe)
–
–
2
2
b,c
Dppe-modified Ru/Al O
2
–
3
a
Solid catalyst.
Liquid solution measured after reaction with mpa.
b
c
2
Additional shell: Ru–N or Ru–O: N = 0.8, R = 2.07 Å, ꢀσ = 0.006.
d
Quality of the fit according to Ref. [24].
RuBrCl(dppe)2 [35], and RuClH2(dppp)2PF6 [36] reported
in the literature are between 2.340 and 2.441 Å; depending
on the ligand used, the Ru–Cl bond length changes from
2.407 to 2.436 Å. In the liquid phase the bond length of
Ru–P in RuCl2(dppe)2, 2.32 Å, is in the expected range (the
decrease may result from additional oxygen/nitrogen neigh-
bors in the complex), but no Ru–Cl bond is indicated in the
spectra. Thus a change in the structure of RuCl2(dppe)2 in
the liquid phase is observed during the reaction. There are
two feasible explanations for this observation: The Cl atoms
could have been replaced by hydrogen to form a hydrido
complex [37–39], or a partial amino (or hydroxo-) complex
was formed with hydrogen and mpa present in the liquid
phase. Moreover, this may be due to similar Ru–P and Ru–Cl
distances in the complexes formed. However, the Ru–P dis-
tance is usually shorter than the Ru–Cl distance (see refer-
ences above and [40,41]). Note also that the catalyst was
investigated after air exposure. Moreover, the EXAFS analy-
sis is limited because several species may be present, and
EXAFS averages over these species.
The reaction mixture containing dppe-modifiedRu/Al2O3
exhibited a Ru–P bond length of 2.35 Å. This shows that—
as already concluded from the near-edge region and the
Fourier-transformed EXAFS spectra—a homogeneous Ru-
dppe-based catalyst forms. No Ru–Cl is observed, as ex-
pected. The structure of this catalyst formed in situ seems
to resemble that of the observed RuCl2(dppe)2 in the liquid
phase, but amino and oxygen neighbors were also found in
the first shell of the dppe-modified Ru/Al2O3 system. The
EXAFS fit of the spectrum improved significantly when
nitrogen/oxygen neighbors (the two atoms have similar
backscattering amplitudes, inasmuch as they are neighbors
in the periodic table) were also used. The formation of such
a complex is reasonable because the catalyst is dissolved in
the amine-rich phase during the reaction.
carbon dioxide and hydrogen. The catalyst shows good ac-
tivity and 100% selectivity comparable to the performance
of the homogeneous ruthenium-based phosphine complexes
RuCl2((PCH3)3)4 and RuCl2(dppe)2. The study indicates
that the formylation of amines with hydrogen and carbon
dioxide can be extended to amines containing an ether group.
The reaction rates for mpa in terms of the TOF are
comparable to or better than those achieved with other
primary amines, such as propylamine in the presence of
RuCl2((PCH3)3)4 with 52 h−1 [11]. Turnover frequencies of
secondary amines like diethylamine are between 440 h−1 [3]
and 360,000 h−1 [13], depending on the catalyst used and the
experimental conditions.
Carbon dioxide acts as both reactant and solvent. CO2 is
not present as a single “supercritical” phase, but exists in
two phases, an amine-rich CO2 phase with H2 dissolved and
a CO2/H2 phase [14]. The volume of the amine-rich CO2
phase with dissolved H2 increases with increasing carbon
dioxide in the system. Higher partial pressure of hydrogen
affects the dissolved amount of H2 in the amine-rich CO2
phase and results, combined with the higher density, in a bet-
ter mixing of reactants in the amine-rich CO2 phase and thus
finally in better conversion and TOF.
Interestingly, it was found that dppe-modified Ru/Al2O3
showed good catalytic performance in the formation of
3-methoxypropylformamide, similar to those of the homo-
geneous catalysts RuCl2(dppe)2 and RuCl2(PPh3)3. This be-
havior could be traced to the formation of active free Ru
complexes as a result of the interaction of the supported
Ru particles with the strongly complexing phosphine (dppe).
This interaction leads to some corrosion of the Ru particles.
The formation of a soluble complex during reaction could
be proved in different ways. On the one hand, X-ray ab-
sorption spectroscopy and ICP-OES showed the presence
of dissolved Ru in the product mixture. On the other hand,
the reaction could be continued if the solid material was fil-
tered off after a certain reaction time (Table 2). Only a small
fraction of the ruthenium is dissolved, resulting in a concen-
tration of 50–100 ppm in the product mixture. This shows
that the homogeneous Ru-based catalyst formed is highly
4. Discussion
Ru/Al2O3 modified by dppe was found to be highly active
for the formylation of 3-methoxypropylamine (mpa), with

152
M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
active (TOF based on the amount of dissolved Ru is about
3100 h−1, Table 3, run 20).
Structural analysis of the Ru complex formed by EXAFS
spectroscopy indicates that a Ru(dppe)2X2 catalyst is formed
that may contain either additional hydrido- or amino/hydro-
xo-ligands. The formation of a hydrido complex was previ-
ously indicated in other studies [37–39,51], and the mecha-
nism for the Ru-catalyzed formylation of amines proposed
by Jessop et al. [11] can be further confirmed. Based on
this, we can assume that a hydrido complex is probably
formed from Ru/Al2O3 and dppe in the liquid phase un-
der reaction conditions. CO2 is activated by insertion into
the metal–hydrogen bonding of the hydrido complex. In the
presence of mpa, the complex reacts further to form the
product 3-methoxypropylformamide, and water is formed
as a by-product.
The present study shows that modification of supported
transition–metal catalysts with a phosphine may corrode the
metal and result in free metal phosphine complexes, which
may disguise the intrinsic catalytic properties of the mod-
ified supported metal catalyst. Similar behavior has been
observed when another phosphine, triphenylphosphine, was
applied. Thus it seems to be of paramount importance to test
for this possibility when supported metal catalysts are ap-
plied together with phosphines.
The formation of the ruthenium complex, which is act-
ing as homogeneous catalyst, could be monitored in situ by
X-ray absorption spectroscopy. In principle, there are only a
few techniques, such as EXAFS spectroscopy, UV–vis spec-
troscopy, NMR spectroscopy, and infrared spectroscopy, that
make such studies possible [42–46]. Because of the low
concentration of the homogeneous catalyst, structural stud-
ies using UV–vis were not successful. The concentration
of the target species was also too low for NMR and IR
spectroscopy. Because of the good penetration of X-rays
at 20 keV, EXAFS studies in the transmission mode were
possible. X-ray absorption spectroscopy has the advantage
that the concentration and the structure of the Ru species
can be determined element-specifically and under reaction
conditions. Such studies are rare under such rigorous con-
ditions (150 bar, 120 ◦C) as applied here and have only
recently been reported for solid [47] and homogeneous cat-
alysts [48,49]. Here we found that the X-ray absorption
technique, combined with an appropriate high pressure cell,
allowed monitoring of the solid catalyst at the bottom and
identification of the in situ formed complex in the liquid
phase in the upper part of the reactor at the same time.
During reaction, the ruthenium constituent in the solid
catalyst is reduced. However, ruthenium can only be found
in the liquid phase if dppe is added to the system. This indi-
cates corrosion of the ruthenium particles in the presence of
hydrogen and dppe. It is probably the structure of the dppe
ligand and thus the possibility of forming a highly stable
chelate complex that ameliorate the corrosion of the Ru and
thus the in situ formation of the Ru complex I.
Such an in situ formation of a homogeneous catalyst from
a supported metal catalyst has rarely been reported. Köh-
ler et al. recently reported on the in situ formation of a
Pd catalyst in Heck reactions [50]. Moreover, in this case
ligands/reactants in the solution led to the formation of an
active homogeneous catalyst. This example underlines the
necessity for appropriate in situ spectroscopic tools to judge
between homogeneous complexes and surface modification
on metal particles as active species for catalysis.
The amount of dissolved ruthenium in the liquid phase
was quantified by ICP-OES and corroborated by XAS. The
concentration of ruthenium was very low, in the range of
50–100 ppm, and no redeposition like that in Ref. [50]
was observed. The TOF (3147 h−1) related to the dissolved
ruthenium species turns out to be significantly higher than
the TOF (324 h−1) based on the total amount of ruthe-
nium present in the reactor. We can conclude that the ruthe-
nium species formed in situ has a higher intrinsic activity
than the homogeneous catalyst RuCl2(dppe)2. The reason
for the better activity may be found in the different struc-
tures of dissolved RuCl2(dppe)2 and the species formed in
situ from Ru/Al2O3 and dppe. Furthermore, the solubility of
RuCl2(dppe)2 in the reaction mixture may affect its catalytic
performance.
5. Conclusions
Modification of Ru/Al2O3 by the addition of a phos-
phine (dppe) has been shown to result in homogeneous
Ru catalysts that are highly active in the formylation of
methoxypropylamine with carbon dioxide and hydrogen.
The catalytically active species were not the phosphine-
modified ruthenium particles, but a homogeneous chlorine-
free Ru-dppe complex that formed under reaction condi-
tions. The presence of dppe led to dissolution (corrosion)
of ruthenium and probably to the formation of a highly ac-
tive homogeneous Ru(dppe)2X2 complex, which was even
sufficiently active to achieve high conversion at very small
concentrations (50 ppm Ru in the reaction mixture).
X-ray absorption spectroscopy proved to be a valuable
technique for identifying the role of solid and liquid Ru
species in the catalytic reaction. The formation of the ho-
mogeneous catalyst could be monitored under reaction con-
ditions, and the structural identification of the homogeneous
Ru-dppe complex could be achieved, even down to a con-
centration of less than 100 ppm in the product mixture.
Acknowledgments
The authors thank the Bundesamt für Energie (BFE) for
financial support and HASYLAB (DESY, Hamburg) for of-
fering beam time for the in situ XAS studies at beamline X1.
The Swiss–Norwegian Beamline (ESRF, Grenoble) is ac-
knowledged for offering beam time for ex situ XAS studies.
The staff at beamline X1 (Mathias Herrmann, Julia Wienold)

M. Rohr et al. / Journal of Catalysis 229 (2005) 144–153
153
and SNBL (H. Emerich, W. van Beek) and M. Caravati and
M. Ramin are acknowledged for their support during the
measurements. Thanks are also due to Dr. C. Beck for dis-
cussion and ICP-OES, P. Trüssel and R. Mäder (ICAB, me-
chanical workshop), and M. Wohlwend (ICAB, electronic
workshop).
[25] J.-D. Grunwaldt, M. Ramin, M. Rohr, A. Michailovski, G.R. Patzke,
A. Baiker, in preparation.
[26] R. Wandeler, N. Künzle, M.S. Schneider, T. Mallat, A. Baiker,
J. Catal. 200 (2000) 377.
[27] R.A. Sheldon, M. Wallau, I.W.C.E. Arends, U. Schuchardt, Acc.
Chem. Res. 31 (1998) 485.
[28] O. Sipr, G. Dalba, F. Rocca, Phys. Rev. B 69 (2004) 134,201.
[29] D. Bazin, J.J. Rehr, J. Phys. Chem. B 107 (2003) 12,398.
[30] S.I. Zabinsky, J.J. Rehr, A. Ankudinov, R.C. Albers, M.J. Eller, Phys.
Rev. B 52 (1995) 2995.
References
[31] S.J. LaPlaca, J.A. Ibers, Inorg. Chem. 4 (1965) 778.
[32] T.S. Lobana, R. Singh, E.R.T. Tiekink, J. Coord. Chem. 21 (1990) 225.
[33] A.R. Chakravarty, F.A. Cotton, W. Schwotzer, Inorg. Chim. Acta 84
(1984) 179.
[1] M. Halmann, Chemical Fixation of Carbon Dioxide: Methods for Re-
cycling CO , CRC Press, Boca Raton, FL, 1993.
2
[2] W. Leitner, Angew. Chem., Int. Ed. Engl. 34 (1995) 2207.
[3] P.G. Jessop, T. Ikariya, R. Noyori, Chem. Rev. 95 (1995) 259.
[4] M. Aresta, E. Quaranta, CHEMTECH (1997) 32.
[5] A. Baiker, Appl. Organomet. Chem. 14 (2000) 751.
[6] A. Behr, Angew. Chem., Int. Ed. Engl. 27 (1988) 661.
[7] P.G. Jessop, T. Ikariya, R. Noyori, Nature 368 (1994) 231.
[8] O. Kröcher, R.A. Köppel, A. Baiker, Chimia 51 (1997) 48.
[9] P.G. Jessop, W. Leitner, Chemical Synthesis Using Supercritical Flu-
ids, Wiley/VCH, Weinheim, 1999.
[10] P.G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 116
(1994) 8851.
[11] P.G. Jessop, Y. Hsiao, T. Ikariya, R. Noyori, J. Am. Chem. Soc. 118
(1996) 344.
[34] M.R.M. Fontes, G. Oliva, L.A.C. Cordeiro, A.A. Batista, J. Coord.
Chem. 30 (1993) 125.
[35] E.P. Perez, R.M. Carvalho, R.H.A. Santos, M.T.P. Gambardella, B.S.
Lima-Neto, Polyhedron 22 (2003) 3289.
[36] B. Chin, A.J. Lough, R.H. Morris, C.T. Schweitzer, C. D’Agostino,
Inorg. Chem. 33 (1994) 6278.
[37] M.K. Whittlesey, R.N. Perutz, M.H. Moore, Organometallics 15
(1996) 5166.
[38] M.G. Basallote, J. Duran, Inorg. Chem. 38 (1999) 5067.
[39] K.A. Lenero, M. Kranenburg, Inorg. Chem. 42 (2003) 2859.
[40] E. Lindner, S. Al-Gharabli, I. Warad, H.A. Mayer, S. Steinbrecher, E.
Plies, M. Seiler, H. Bertagnolli, Z. Anorg. Allg. Chem. 629 (2003)
161.
[12] S. Schreiner, J.Y. Yu, L. Vaska, Inorg. Chim. Acta 147 (1988) 139.
[13] O. Kröcher, R.A. Köppel, A. Baiker, Chem. Commun. 5 (1997) 453.
[14] L. Schmid, A. Canonica, A. Baiker, Appl. Catal. A: Gen. 255 (2003)
23.
[41] O. Kröcher, R.A. Köppel, M. Fröba, A. Baiker, J. Catal. 178 (1998)
284.
[42] M. Hunger, J.W. Weitkamp, Angew. Chem., Int. Ed. Engl. 40 (2001)
2954.
[15] L. Schmid, M.S. Schneider, D. Engel, A. Baiker, Catal. Lett. 88 (2003)
105.
[16] C.-C. Tai, J. Pitts, J.C. Linehan, A.D. Main, P. Munshi, P.G. Jessop,
Inorg. Chem. 41 (2002) 1606.
[17] O. Kröcher, R.A. Köppel, A. Baiker, J. Chem. Soc., Chem. Commun.
(1996) 1497.
[18] L. Schmid, M. Rohr, A. Baiker, Chem. Commun. (1999) 2303.
[19] T. Mallat, A. Baiker, Appl. Catal. A: Gen. 200 (2000) 3.
[20] T. Mallat, C. Brönnimann, A. Baiker, Appl. Catal. A: Gen. 149 (1997)
103.
[21] R. Mason, D.W. Meek, G.R. Scollary, Inorg. Chim. Acta 16 (1976)
L11.
[22] O. Kröcher, R.A. Köppel, A. Baiker, J. Mol. Catal. A: Chem. 140
(1999) 185.
[23] K. Okamoto, T. Takahashi, K. Kohdate, H. Kondoh, T. Yokoyama, T.
Ohta, J. Synchrotron Rad. 8 (2001) 689.
[24] T. Ressler, J. Synchrotron Rad. 5 (1998) 118.
[43] D. Bazin, H. Dexpert, J. Lynch, in: Y. Iwasawa (Ed.), X-ray Absorp-
tion Fine Structure for Catalysts and Surfaces, vol. 2, World Scientific,
Singapore, 1996.
[44] G. Sankar, J.M. Thomas, Top. Catal. 8 (1999) 1.
[45] J.-D. Grunwaldt, B.S. Clausen, Top. Catal. 18 (2002) 37.
[46] J.-D. Grunwaldt, R. Wandeler, A. Baiker, Catal. Rev.-Sci. Eng. 45
(2003) 1.
[47] J.-D. Grunwaldt, M. Caravati, M. Ramin, A. Baiker, Catal. Lett. 90
(2003) 221.
[48] S.L. Wallen, D.M. Pfund, J.L. Fulton, C.R. Yonker, M. Newville, Y.
Ma, Rev. Sci. Instrum. 67 (1996) 2843.
[49] S.G. Fiddy, J. Evans, T. Neisius, X.Z. Sun, Z. Jie, M.W. George, Chem.
Commun. 6 (2004) 676.
[50] R.G. Heidenreich, J.G.E. Krauter, J. Pietsch, K. Köhler, J. Mol. Catal.
A: Chem. 182–183 (2002) 499.
[51] L. Costella, A. Delzotto, A. Mezzetti, E. Zangrando, P. Rigo, J. Chem.
Soc., Dalton Trans. (1993) 3001.