Rhodium-BiPhePhos catalyzed hydroformylation studied by operando FTIR spectroscopy: Catalyst activation and rate determining step

Article abstract of DOI:10.1016/j.molcata.2016.10.028

The homogeneously rhodium catalyzed hydroformylation of 1-decene was studied using operando FTIR spectroscopy. The bulky chelating diphosphite ligand BiPhePhos was used for catalyst modification. Special emphasis was given to the transformation of the Rh-precursor Rh(acac)(CO)₂ to the activated HRh(BiPhePhos)(CO)₂ catalyst. Under hydroformylation conditions, this complex was found to be the most abundant catalyst species over a wide range of olefin conversion. Other inactive or non-selective rhodium species were not detectable. Analysis of the turnover frequency revealed a first order dependence of the hydroformylation rate with respect to the concentration of 1-decene. These findings indicate that the coordination of the olefin to the Rh-BiPhePhos catalyst is determining the hydroformylation rate of 1-decene.

Full text of DOI:10.1016/j.molcata.2016.10.028

Accepted Manuscript
Title: Rhodium-BiPhePhos catalyzed hydroformylation
studied by operando FTIR spectroscopy: Catalyst activation
and rate determining step
Author: Andreas Jo¨rke Andreas Seidel-Morgenstern Christof
Hamel
PII:
S1381-1169(16)30450-2
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.10.028
MOLCAA 10090
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
23-9-2016
21-10-2016
23-10-2016
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Christof Hamel, Rhodium-BiPhePhos catalyzed hydroformylation studied
by operando FTIR spectroscopy: Catalyst activation and rate determining
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http://dx.doi.org/10.1016/j.molcata.2016.10.028
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Rhodium-BiPhePhos catalyzed hydroformylation studied by operando
FTIR spectroscopy: Catalyst activation and rate determining step
Andreas Jörkea,∗, Andreas Seidel-Morgensterna,b, Christof Hamela,c
aOtto von Guericke University, Institute of Process Engineering, Magdeburg, Germany
bMax Planck Institute for Dynamics of Complex Technical Systems, Physical and Chemical Foundations of Process Engineering, Magdeburg,
Germany
cAnhalt University of Applied Sciences, Department of Bioscience and Process Engineering, Köthen, Germany
Abstract
The homogeneously rhodium catalyzed hydroformylation of 1-decene was studied using operando FTIR
spectroscopy. The bulky chelating diphosphite ligand BiPhePhos was used for catalyst modification.
Special emphasis was given to the transformation of the Rh-precursor Rh(acac)(CO)2 to the activated
HRh(BiPhePhos)(CO)2 catalyst. Under hydroformylation conditions, this complex was found to
be the most abundant catalyst species over a wide range of olefin conversion. Other inactive or
non-selective rhodium species were not detectable. Analysis of the turnover frequency revealed a first
order dependence of the hydroformylation rate with respect to the concentration of 1-decene. These
findings indicate that the coordination of the olefin to the Rh-BiPhePhos catalyst is determining the
hydroformylation rate of 1-decene.
Keywords: BiPhePhos, 1-decene, hydroformylation, rhodium
1. Introduction
The reaction of olefins with synthesis gas (CO, H2) in presence of a transition metal complex as catalyst
has been known for decades as hydroformylation. It is one of the most important industrial applications
of homogeneous catalysis [1]. The Rh-complex catalyzed hydroformylation of long-chain olefins using the
bulky chelating diphosphite ligand BiPhePhos [2] was investigated intensively in the literature [3–6].
Usage of this catalyst enables the double bond isomerization of straight-chain olefins [7, 8] which can be
exploited in tandem isomerization-hydroformylation reactions to produce linear aldehydes from mixtures
of internal olefins [9–11] or renewable feedstocks [12, 13]. However, desired mechanistic kinetic models of
these reactions for efficient reactor design and operation are not available in the literature.
∗Corresponding author, Tel.: +49-391-67-54655, Email: andreas.joerke@ovgu.de
Preprint submitted to Journal of Molecular Catalysis A: Chemical
21. Oktober 2016
Page 1 of 15

Mechanistic kinetic models of complex catalytic reaction cycles often include a large number of unknown
kinetic parameters which are difficult to estimate [14]. Therefore, it is necessary to reduce these models to
allow parameter estimation. Significant model reduction can be achieved by assuming one step in the
catalytic reaction mechanism as rate determining step (RDS) [14]. A possibility to find out which step in a
catalytic sequence is rate determining is the detection of intermediates. If one step is much slower than the
others and therefore determining the rate, the intermediate before this step would accumulate as a
consequence of Bodenstein’s principle. In our case, most of the hydroformylation catalyst would therefore
be present as Rh-carbonyl intermediate before a possible rate determining step.
Operando FTIR spectroscopy is a very useful tool to investigate homogeneously catalyzed reactions
regarding the detection of reactants, intermediates and products [15, 16]. It is especially well suited for
the detection and distinction of metal-carbonyls [17]. Therefore, this technique was applied in this
contribution to the Rh-BiPhePhos catalyzed hydroformylation of 1-decene to undecanal to investigate
which Rh-carbonyl species is the most abundant intermediate under reaction conditions and which step
in the reaction mechanism can be regarded as rate determining. This information directly affects the
mechanistic kinetic modeling of the mentioned reaction system, which is our goal in a subsequent
publication. Furthermore, it is crucial to accurately balance the active, inactive or non-selective catalyst
material present under reaction conditions for a correct description of the reaction rate and selectivities.
Hence, the catalyst formation was also studied to evaluate, if possible inactive or non-selective
Rh-complexes are formed from the Rh-precursor using BiPhePhos that do not take part in the desired
hydroformylation reaction.
2. Reaction mechanism
A widely accepted hydroformylation reaction mechanism is the "Wilkinson cycle"[18] (See Figure 1 for
chelating P-ligands including postulated catalyst pre-equilibrium). It is assumed in our case (1) that the
oxidative hydrogen addition and the reductive elimination of the Rh-acyl complex VIII to release the
aldehyde are irreversible [19–21] and (2) that under catalytic conditions the formation of non-selective
unmodified Rh-tetracarbonyl complexes [22] and inactive Rh-dimers with bridging ligand is possible [3].
2
Page 2 of 15

O
O
2 CO
O
O
O
O
CO
CO
P
P
Rh
Rh
O
O
O
O
P
P O
O
BiPhePhos
CO/H2
acac-H
BiPhePhos
2 CO
BiPhePhos
OC
OC
OC
CO
CO
CO
H
H
P
P
P
OC
OC
P
P
CO
Rh
Rh
CO
Rh
CO
Rh
CO
P
I
H2
2 CO
Rh-tetracarbonyl
Rh-dimer
O
CO
R
H
R
H
R
P
P
Rh
CO
H
P
P
C
O
H
II
P
P
Rh
Rh
H
VIII
CO
R
CO
III
R
R
R
H2
IV
P
C
O
C
O
P
P
VI
Rh
Rh CO
CO
Rh
R
P
P
P
CO
CO
V
CO
VII
P
CO
Rh
CO
P
CO
Figure 1: Rh-BiPhePhos catalyzed hydroformylation mechanism according to Evans and Wilkinson [18].
In the literature, mainly two possible RDS are discussed for the hydroformylation of cyclic, branched and
straight-chain olefins: (1) the oxidative addition of hydrogen to the acyl complex VI which was observed for
unmodified Rh-carbonyl complexes [19, 23, 24], monophosphines [25–28] and also monophosphites [20, 29]
and (2) the coordination of the olefin double bond to the 16 valence electron (ve) Rh-hydridocarbonyl
complex II which was also observed for monophosphines [30], diphosphines [31, 32], monophosphites
[33–35] and diphosphites [36–38]. Two effects are responsible for the differing rate observations: (1) the
ligand type (σ-donor: phosphines, π-acceptor: phosphites) influences the electron density in the partially
occupied d-orbitals of Rh which affects the π-backbonding ability of the metal with respect to substrates or
CO ligands and (2) the steric interaction between substrate and ligand can slow down the coordination of
the double bond to the metal center [1, 15, 37]. The coupling of both effects explains why the same catalyst
shows different RDS with different substrates [20, 27]. Therefore, no general predictions regarding the RDS
of a specific hydroformylation system can be made and experimental investigations are required.
3
Page 3 of 15

3. Experimental
To perform the operando FTIR studies, a 75 ml autoclave (Parr Instrument Co.) was connected to an FTIR
spectrometer (Mettler Toledo, ReactIR iC 10 with MCT detector cooled by liquid nitrogen, recorded
wavenumber range = 700 - 2280 cm−1, wavenumber resolution = 8 cm−1) using a high pressure Si-ATR
probe. The reactor was evacuated and flushed with nitrogen several times before toluene (Merck 99.9 %,
used as received) was added with Schlenk technique through a capillary as solvent. After heating the
solvent to 40 ◦C reaction temperature, the toluene background spectrum was recorded and removed. The
Rh-precursor Rh(acac)(CO)2 (Umicore) and the ligand BiPhePhos (Molisa) were dissolved in toluene in
separate Schlenk tubes, inertized and added to the reactor successively under constant stirring of 1200
rpm (Rh:Ligand = 1:1.5 (molar), Rh-concentration = 250 ppm). After pressurization of the reactor with 5
bar H2 and and subsequently 4 bar CO (total pressure = 9 bar), the activation of the Rh-catalyst started.
Afterwards, 28.3 mmol of 1-decene (Sigma-Aldrich, 94.0 %, used as received) was added with Schlenk
technique through a pressure lock with 10 bar CO against the 9 bar atmosphere in the reactor, causing the
start of the hydroformylation reaction (final gas composition CO:H2 = 1:1, total pressure = 10 bar, initial
1-decene concentration = 1.63 mol l−1, Rh:Olefin = 1:1000 (molar)). At low temperatures of 40 ◦C, no
significant double bond isomerization, hydrogenation or production of branched aldehydes could be
detected with gas chromatography or FTIR spectroscopy (see discussion below). Hence, it was possible to
study the hydroformylation reaction exclusively without the disturbing influence of side reactions.
4. Results and discussion
The experimentally observed operando FTIR spectra in the metal-carbonyl vibration region (1900 - 2200
cm−1) are shown in Figure 2 to Figure 3. The second derivative of the absorbance with respect to the
wavenumber is used because it is almost insensitive to baseline shifts and allows better visual inspection of
bands that correspond to highly diluted metal carbonyls. The catalyst pretreatment is presented in Figure 2
while Figure 3 shows the metal-carbonyl vibration region during hydroformylation until 75 % conversion
of 1-decene was reached (see Figure 8). Characteristic steady-state spectra were numbered in Figure 2 to
Figure 3 and summarized in Figure 4.
4
Page 4 of 15

4
3
2
1
Figure 2: Operando FTIR spectra of the Rh-CO vibration region. Transformation of the Rh-precursor Rh(acac)(CO)2 to the activated
Rh-hydridodicarbonyl complex I (Rh:Ligand = 1:1.5 (molar), T = 40 ◦C, solvent: toluene). Numbers indicate representative steady-
state spectra summarized in Figure 4. The 2nd derivative of the absorbance with respect to the wavenumber is shown to improve
visual inspection.
After adding the Rh-precursor to the reactor, characteristic Rh-CO vibrations at α = 2085 cm−1 and β = 2014
cm−1 were observed (spectrum 1) in accordance to the literature [36, 39]. Adding the ligand causes the Rh-
CO vibrations to disappear rapidly, indicating an almost complete replacement of CO with BiPhePhos at
the Rh-center (see Figure 1). A weakly intensive band remains at γ = 2025 cm−1 which could be assigned
to a complex where the diphosphite is coordinated with one P-atom and one CO molecule remains at
the Rh-center (spectrum 2) [36]. After adding 5 bar H2, the two characteristic Rh-CO vibrations of the
Rh-hydridodicarbonyl complex I appear at δ = 2077 cm−1 and e = 2017 cm−1 (spectrum 3), which is in
accordance to the literature [3, 4, 37]. It was proven by independent DFT calculations that the observed
Rh-CO vibrations for complex I correspond to an e,e-coordinated BiPhePhos ligand [4, 40]. This reaction is
possible without adding additional CO because the CO that was replaced by BiPhePhos is most probably
still dissolved in the solvent. Adding 4 bar CO increases the Rh-CO vibrations even further and complete
conversion to complex I is reached instantly (spectra 4 and 5). No formation of other possibly inactive or
non-selective Rh-species was observable. Therefore, the description of the catalyst pre-equilibrium can be
simplified by neglecting the Rh-tetracarbonyl complex and the bridging Rh-dimer for the Rh-BiPhePhos
system shown in Figure 1 under the applied reaction conditions.
5
Page 5 of 15

8
7
6
Olefin
injection
5
Figure 3: Operando FTIR spectra of the Rh-CO vibration region under hydroformylation conditions until 75 % conversion of 1-decene
(Rh:Ligand 1:1.5 (molar), Rh:Olefin = 1:1000 (molar), pCO/H = 10 bar, T = 40 ◦C, solvent: toluene). Numbers indicate representative
2
steady-state spectra summarized in Figure 4. The 2nd derivative of the absorbance with respect to the wavenumber is shown to
improve visual inspection.
ε
δ
8
7
6
5
4
3
γ
2
β
α
1
2200 2150 2100 2050 2000 1950 1900
Wavenumber / cm-1
Figure 4: Selected operando FTIR spectra with characteristic Rh-CO bands (α = 2085 cm−1, β = 2014 cm−1, γ = 2025 cm−1, δ =
2077 cm−1 and e = 2017 cm−1). The 2nd derivative of the absorbance with respect to the wavenumber is shown to improve visual
inspection.
The detected bands in the metal-carbonyl region remain constant from the moment of olefin injection until
75 % olefin conversion (see Figure 3). No wavenumber shift of the Rh-CO bands was observed (see Figure 4,
6
Page 6 of 15

spectra 6-8). The slight intensity decrease is caused by a dilution due to the olefin injection. The mainly
observed Rh-hydridodicarbonyl complex I is outside the catalytic cycle but at equilibrium with the active
Rh-hydridocarbonyl complex II which is the starting point of the catalytic cycle and the Rh-intermediate
before olefin coordination. Observing complex II is difficult because the equilibrium between I and II is on
the side of the saturated 18ve complex I. Moreover, the Rh-CO vibration of the unsaturated 16ve complex
II will be shifted only slightly compared to complex I which is probably within the wavenumber resolution
of the IR spectrometer.
Possible catalyst or ligand degradation by oxidation during hydroformylation was rejected because the
reaction temperature was low, the reaction solution was clear with a light green color and no particulates
were observed after the experiment in the solution. Furthermore, the emergence of possible P=O bands of
oxidized phosphorus could not be detected in the typical P=O wavenumber region of 1100 - 1300 cm−1
(see Figure 5).
75% olefin
conversion
Olefin
injection
Figure 5: Operando FTIR spectra of the P=O vibration region of oxidized phosphorus under hydroformylation conditions until 75 %
conversion of 1-decene (Rh:Ligand = 1:1.5 (molar), T = 40 ◦C, solvent: toluene). The 2nd derivative of the absorbance with respect to
the wavenumber is shown to improve visual inspection.
Characteristic =C-H bending vibrations of the terminal double bond of 1-decene at 913 and 996 cm−1 can be
seen in Figure 6. Olefins with internal double bond in trans position have one characteristic =C-H bending
vibration at 970 cm−1. However, this band was not observable to a significant extent confirming that double
bond isomerization plays a minor role under the applied experimental conditions.
7
Page 7 of 15

The branched aldehyde 2-methylundecanal has a characteristic C-H bending vibration at 921 cm−1 that
does not appear in the spectrum of the corresponding terminal aldehyde dodecanal [41]. Hence, it is likely
that this is also the case for the hydroformylation products of 1-decene. However, a similar band could
not be observed (see Figure 6). Gas chromatographic analysis confirmed the high aldehyde regioselectivity
after 75 % conversion of 1-decene of n/iso ≈ 99:1. Thus, the production of branched aldehydes is negligible
under the applied experimental conditions supporting that the Rh-BiPhePhos catalyst was not damaged
significantly.
75% olefin
conversion
Olefin
injection
Figure 6: Operando FTIR spectra of the =C-H bending vibration region under hydroformylation conditions until 75 % conversion of
1-decene (Rh:Ligand = 1:1.5 (molar), T = 40 ◦C, solvent: toluene).
The strong C=O stretching band of undecanal at 1729 cm−1 can be seen in Figure 7 as well as the less
intensive C=C stretching band of 1-decene at 1643 cm−1. Normalizing those bands in accordance to the
stoichiometry of the hydroformylation reaction allowed to use them for the calculation of the conversion
X of 1-decene and the turnover frequency TOF (see Figure 8).
8
Page 8 of 15

75% olefin
conversion
Olefin
injection
Figure 7: Operando FTIR spectra of the C=O and C=C stretching vibration region under hydroformylation conditions until 75 %
conversion of 1-decene (Rh:Ligand = 1:1.5 (molar), T = 40 ◦C, solvent: toluene).
The TOF is defined as the rate of aldehyde formation rald divided by the total Rh concentration (1). The
aldehyde production rate was calculated by finite differences (2) and the conversion X of 1-decene (3).
r
ald(t)
TOF =
(1)
cRh,total
dcald
dt
∆cald
∆t
rald
=
≈
(2)
(3)
cald = X · c1-decene(t = 0)
It can be seen in Figure 8 that the TOF decreases from 800 h−1 to 200 h−1 for the studied system with
ongoing reaction time. To check the relation between the concentration of 1-decene and the TOF, the latter
is plotted in Figure 9 against (1 − X) which is equal to the substrate concentration divided by its initial
value (normalized substrate concentration).
9
Page 9 of 15

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
1200
1000
800
600
400
200
0
1-decene
undecanal
TOF
0
20
40
60
80
100
Time t / min
Figure 8: Normalized FTIR absorbance of the substrate 1-decene (C=C stretching vibration at 1643 cm−1) and the hydroformylation
product undecanal (C=O stretching vibration at 1729 cm−1) as a function of reaction time with corresponding time resolved turnover
frequency TOF.
The regression in Figure 9 shows a strong linear correlation (R2 = 0.9923) between the normalized
concentration of 1-decene and the TOF. Hence, the hydroformylation is a first-order reaction with respect
to the substrate concentration. Combining this finding with the operando FTIR results leads to the
conclusion that the olefin coordination to the Rh-catalyst is most probably the rate determining step in the
Rh-BiPhePhos catalyzed hydroformylation of 1-decene.
900
TOF
800
700
600
500
400
300
200
100
0
Linear (TOF)
R² = 0.9923
0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
Norm. substrate concentration (1 - X) / -
Figure 9: Turnover frequency TOF vs. normalized substrate concentration (1 − X).
10
Page 10 of 15

5. Summary
In summary, operando FTIR spectroscopy was successfully applied to study mechanistic features of the
Rh-BiPhePhos catalyzed hydroformylation of 1-decene. The presented methodical approach was able to
show experimentally that the transformation of the Rh-precursor Rh(acac)(CO)2 to the activated
Rh-hydridodicarbonyl complex I proceeds fast, and that the diphosphite ligand BiPhePhos is coordinated
in e,e-position at the Rh-center. It was not possible to detect any other inactive or non-selective
Rh-complexes after catalyst activation and during hydroformylation which simplifies the description of
the catalyst pre-equilibrium drastically. Under hydroformylation conditions, complex I was detected to be
the most abundant Rh-intermediate over a wide range of olefin conversion. Possible catalyst or ligand
degradation by oxidation during hydroformylation was rejected because no characteristic P=O bands for
oxidized phosphorus or particulates in the reaction solution were found. Analysis of the TOF revealed
that the hydroformylation is of first order with respect to 1-decene. It was concluded that the olefin
coordination is the rate determining step of the Rh-BiPhePhos catalyzed hydroformylation of 1-decene.
Acknowledgment
The financial support of the German Science Foundation (Project SFB/TRR 63: „InPROMPT - Integrated
Chemical Processes in Multi-Phase Fluid Systems“) is gratefully acknowledged. We are also very grateful
to Umicore AG and Co. KG for the supply of the catalyst precursor Rh(acac)(CO)2.
11
Page 11 of 15

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Highlights



No significant formation of inactive or non-selective Rh-species using BiPhePhos
First order dependence of the hydroformylation with respect to olefin concentration
Olefin coordination to the Rh-catalyst is determining the hydroformylation rate
Page 14 of 15

*Graphical Abstract
H
P
P
CO
Rh
CO
O
CO
R
R
H
H
P
P
R
Rh
CO
H
P
C
O
P
P
H
Rh
Rh
P
H
CO
R
CO
R
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Page 15 of 15
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