Direct Synthesis of Methyl Formate from CO2 With Phosphine-Based Polymer-Bound Ru Catalysts

Article abstract of DOI:10.1002/cssc.201900808

Methyl formate was produced in one pot through the hydrogenation of CO₂ to formic acid/formate followed by an esterification step. The route offers the possibility to integrate renewable energy into the fossil-based chemical value chain. In this work, a phosphine-polymer-anchored Ru complex was shown to be an efficient solid catalyst for the direct hydrogenation of CO₂ to methyl formate. The 1,2-bis(diphenylphosphino)ethane-like polymer presented the highest activity with a turnover number (TON) of up to 3401 at 160 °C. The reaction parameters were systemically investigated to optimize the reaction towards the formation of methyl formate. This catalyst could be reused seven times without a significant decrease in activity. Evolution of the catalytic Ru center during the reaction was revealed, and a possible reaction mechanism was proposed.

Full text of DOI:10.1002/cssc.201900808

Accepted Article
Title: Direct synthesis of methyl formate from CO2 using phosphine-
based polymer-bound Ru catalysts
Authors: Ruiyan Sun, Anna Kann, Heinrich Hartmann, Astrid Besmehn,
Peter J. C. Hausoul, and Regina Palkovits
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201900808
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ChemSusChem
10.1002/cssc.201900808
FULL PAPER
Direct synthesis of methyl formate from CO using phosphine-
2
based polymer-bound Ru catalysts
[
a]
[a]
[b]
[b]
[a]
Ruiyan Sun, Anna Kann, Heinrich Hartmann, Astrid Besmehn, Peter J. C. Hausoul* and
[
a]
Regina Palkovits*
Abstract: Methyl formate can be produced via the hydrogenation of
CO to formic acid/formate followed by an esterification step in one
A potential approach to circumvent this problem is using
alcohols such as methanol instead of water as the solvent to
2
pot. This route offers the possibility to integrate renewable energy
into the fossil-based chemical value chain. In this work, a phosphine
polymer-anchored Ru complex was shown to be an efficient solid
2
perform CO hydrogenation. The reaction equilibrium can be
shifted towards the product side via an in situ esterification of
formic acid with methanol to yield methyl formate (MF), a
[
4]
catalyst for the direct hydrogenation of CO
2
to methyl formate. The
platform chemical in C1 chemistry. Unlike formic acid, the
resulting methyl formate can be readily distilled from the product
1,2-bis(diphenylphosphino)ethane-like polymer presented the
o
highest activity with a turnover number (TON) of up to 3401 at 160
mixture owing to its very low boiling point (31.7 C) as well as
o
[5]
C. Reaction parameters were systemically investigated to optimize
the negligible bonding with base. Based on these properties,
the reaction towards the formation of methyl formate. This catalyst
could be reused 7 times without a significant decrease in activity.
Evolution of the catalytic Ru center during the reaction was revealed
and a possible reaction mechanism was proposed.
BASF designed a cascade process for the hydrogenation of CO
2
[6]
to methyl formate, in which the synthesis and separation of MF
proceeded simultaneously in a reactive distillation column. A
great number of catalytic materials have been developed for this
process, most of which focus on the rational combination of
homogeneous metal centers with appropriate ligands, such as
[
7]
[8]
[9]
RuCl
Fe(BF
Among them, RuCl
2
(PMe
3
)
4
,
RuCl
O/tris[(2-diphenylphosphino)phenyl]phosphine.
(dppe) exhibited the highest activity with a
2
(dppe)
2
,
RhCl(PPh
3
)
3
,
and
Introduction
[10]
4
)
2
·6H
2
2
2
The ever-increasing atmospheric CO
surge of interest in developing efficient catalytic technologies to
reduce CO emissions. By far, the available catalytic solutions
are mostly based on the utilization of the captured CO as an
alternative carbon source to produce fuels and value-added
2
levels have propelled a
TON of up to 12900, but it is inherently challenging to isolate
and recycle these organometallic complexes. This will also lead
to the decomposition of formate/MF during the following
distillation process. From the perspective of practical application,
solid catalysts offer more possibilities in achieving catalyst
recovery, continuous operation and process intensification.
Accordingly, an efficient solid catalyst for MF synthesis is highly
desirable. Nevertheless, only few efforts have been made to
address the heterogeneously catalyzed synthesis of MF, mainly
including the design of nano-architectures and immobilization of
molecular complexes. In 1998, Krocher et al. reported a TON of
2
2
[
1]
chemicals.
undertaken for the direct reduction of CO
promising energy carrier for renewable H storage. The main
In this respect, intensive efforts have been
2
to formic acid, a
[
2]
2
challenge of this process lies in the unfavorable thermodynamics.
A widely used strategy to shift the reaction equilibrium presents
the introduction of a base such as triethylamine (NEt
3
) or
(
bi)carbonate to neutralize the in situ formed formic acid to
3
180 for the hydrogenation of supercritical CO
presence of NEt using an immobilized Ru complex
RuCl [P(CH ) on a porous silica substrate. The group of
2
to MF in the
obtain the formate salts. Water is preferred as the solvent for
this reaction, given its low price and availability on a large scale.
Moreover, the strong polarity of water can also mitigate the
3
[11]
(
2
3
)
3
]
4
Tsang described the use of Pd/Cu/ZnO nanocatalyst in a batch
reactor without base, affording a turnover frequency (TOF) of
[
3]
thermodynamic limitation by solvation effects. Nevertheless,
the prevailing use of NEt within aqueous systems poses
challenges for the following distillation process, the most energy-
intensive step, due to the formation of a formic acid/NEt adduct
3
–1
o
[12]
3
8.2 h at 150 C accompanied by the formation of CO. In a
follow-up study, they found that the addition of an appropriate
3
–1 [13]
amount of NEt
et al. reported an impressive TOF of up to 534 h by using ZrO
supported Au nanoparticles in the absence of base under harsh
3
can increase the TOF to 43.6 h . In 2015, Wu
[
3]
and a formic acid/water azeotrope.
–1
2
-
o
[14]
conditions (200 C, 160 bar).
Urakawa demonstrated the continuous hydrogenation of CO
MF in a fixed bed reactor catalyzed by a SiO -supported Ag
Most recently, the group of
[
[
a] Dr. R. Sun, A. Kann, Dr. P. J. C. Hausoul, Prof. Dr. R. Palkovits
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University, Worringerweg 2, 52074 Aachen (Germany)
E-mail: hausoul@itmc.rwth-aachen.de
palkovits@itmc.rwth-aachen.de
b] Dr. H. Hartmann, Dr. A. Besmehn
2
to
2
catalyst. 100% selectivity towards MF at a methanol conversion
o
of 4% was obtained at 280 C and 300 bar with a gas hourly
–1 [15]
space velocity (GHSV) of 9000 h .
Zentralinstitut fer Engineering, Elektronik und Analytik ZEA-3: Analytik
Forschungszentrum Jülich GmbH, 52425 Jülich (Germany)
Porous organic polymers, tailored by selecting appropriate
ligands as the basic unit to create binding sites, have been
widely studied as a versatile support material to design efficient
heterogeneous catalysts, in particular for the heterogenization of
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
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ChemSusChem
10.1002/cssc.201900808
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[
16]
metal complexes.
polymeric analogues of 1,2-bis(diphenylphosphino)ethane
pDPPE), 1,3,5-tris(4-bromophenyl)benzene (pTPPB), and
triphenylphosphine (pTPP) in combination with a Ru complex as
highly efficient solid Ru catalysts for CO hydrogenation to
formate (TON up to 13170) as well as formic acid decomposition
Recently, our group reported the use of
excess of
concentration of the soluble H
high CO conversions. The mass of Ru was maintained at 0.3
H
2
(H
2
/CO
2
=
3) was used to increase the
2
species in methanol to achieve
(
2
mg for each catalytic test. The catalytic activities were presented
in terms of TON, calculated based on the overall moles of Ru
atoms (Supplementary Equation 1). In order to compare the
intrinsic activity of Ru, a short reaction time of 2 h was applied
for the following studies. MF and formate were clearly
2
[
17]
in aqueous phase.
polymers hold great potential as heterogeneous catalysts for
CO hydrogenation in methanol to synthesize MF. In this work,
we explore the catalytic behaviors of the Ru complex supported
on phosphine-based polymers for the hydrogenation of CO to
This implies that phosphine-based
1
2
differentiated and quantified in H NMR spectra (Figure S1). No
MF or formate was produced in the experiment with the polymer
2
pDPPE only. The use of homogeneous Ru complexes [RuCl
cymene)] and [RuCl(p-cymene)(PPh )] led to TONs of 83 and
151 towards MF, respectively (Table 1). The catalytic activity of
[RuCl (p-cymene)] was significantly improved when immobilized
2
(p-
methyl formate. To maximize the production of MF, reaction
parameters involving temperature, reaction time, base
2
3
concentration,
systemically
(
H
2
/CO
2
ratio and catalyst recyclability were
for pDPPE-supported Ru
2
2
investigated
onto the solid phosphorus ligands of pTPP, pTPPB and pDPPE,
affording TONs of 271, 310 and 333, respectively, confirming the
beneficial effect of phosphine ligands even used in solid state.
Variation of the polymer configurations did not impose
pronounced effects on the TON of MF. The highest activity was
Ru@pDPPE). A possible reaction mechanism was suggested
based on the characterization results of the fresh and used
catalyst, respectively.
obtained over Ru@pDPPE, corresponding to
a
MF
concentration of 183 mmol/L. Interestingly, commercially
available Ru/C (0.5 wt%) provided a comparable TON of 140
Results and Discussion
3
with that of [RuCl(p-cymene)(PPh )], whereas the formation of
To start with, the solid phosphine polymers including pTPP,
pTPPB and pDPPE were synthesized via sequential lithiation
CO was much more pronounced (up to 439 ppm) than for other
Ru catalysts. CO is generally considered to be produced via the
(
with n-Butyllithium) and coupling reactions of the corresponding
[
3]
decarbonylation/dehydration of formic acid. For all Ru-based
catalysts, it should be noted that the concomitant formation of
linkers and phosphine centers according to the procedures
[
17b]
reported previously by our group (Figure 1).
2
[RuCl (p-
formate presented
a lower TON than that of MF, and
cymene)] was incorporated into the phosphine-based polymers
2
formate/formic acid probably acts as an intermediate for the
production of MF, which will be discussed later.
by a simple wet impregnation method using methanol as solvent.
Structures and chemo-physical properties of these polymers as
well as the polymer-anchored Ru catalysts have been well
[17-18]
characterized in our previous reports.
Accordingly, this work
Table 1. Catalytic hydrogenation of CO
catalysts in methanol.
2
over various Ru-based
will focus on investigating the catalytic behaviors of these well-
[
a]
defined materials for the catalytic hydrogenation of CO
methanol.
2
in
TON
Formate
Catalyst
CO (ppm)
MF
83
[
[
RuCl
2
(p-cymene)]
2
24
66
12
24
25
36
439
RuCl(p-cymene)(PPh
3
)]
47
151
333
310
271
140
1 wt% Ru@pDPPE
1 wt% Ru@pTPPB
1 wt% Ru@pTPP
0.5 wt% Ru/C
155
143
83
58
[
(
a] Reaction conditions: methanol (5 mL), NEt
3
(0.5 mL), mesitylene
o
0.5 mL), 0.3 mg of Ru, 160 C, 60 bar H
2
, 20 bar CO , 2 h, 750 rpm.
2
In order to improve the TONs of MF, reaction conditions
were further examined with the best catalyst Ru@pDPPE (1
wt%). We first looked into the influence of base, known as a
Figure 1. Structures of the synthesized porous phosphine-based polymers
and their corresponding linkers and phosphine centers.
crucial additive in capturing and delivering CO
shifting the equilibrium. Considering the fact that NEt
widely demonstrated to be an effective base in CO
2
, in addition to
3
has been
2
We initially conducted a catalyst screening for this reaction in
hydrogenation as well as the good miscibility with methanol, it
was therefore employed to investigate the effect of base
concentration. As listed in Table 2, MF and formate were
a batch reactor under a total pressure of 80 bar (room
o
temperature, H
2
/CO
2
= 3) at 160 C in the presence of NEt
3
. An
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ChemSusChem
10.1002/cssc.201900808
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produced with a same TON of only 149 in the absence of base.
1.5
0.7
0.7
0.4
1.5
794
1997
1364
129
2154
0
2791
1798
391
With the increase in the concentration of NEt
mol/L, the TON of MF first increased to the highest value of
042 (at 1.2 mol/L) and then slightly decreased to 1997. The
3
from 0.3 to 1.5
Ethylpiperidine
Piperidine
DBU
434
262
2
2503
5773
4657
5773
reduced activity is possibly caused by the inhibiting effect of
excess base on the esterification of methanol with formic
acid/formate, which is a typically acid-catalyzed reaction. This
esterification can proceed very slowly without acidic catalysts
2 3
K CO
[a] Reaction conditions: 1 wt% Ru@pDPPE (0.005 g), 5.4 mL of a prepared
methanolic solution with a different concentration of NEt , mesitylene (0.5 mL),
60 bar H , 20 bar CO , 160 C, 2 h.
2 2
3
o
[19]
even in the presence of a base, as described in literature. The
same negative effect was also observed for the homogeneous
[
7, 19b]
catalytic system RuCl
appropriate NEt concentration of 1.2 mol/L was needed to
promote CO hydrogenation without severely hampering the
subsequent esterification step. When 0.6 mol/L NEt was added,
the MF/formate molar ratio, an indicator of product selectivity,
increased from 1 (without NEt ) to 2.8. This suggests that the
addition of NEt not only enhances catalytic performance but
also improves MF selectivity. The positive results of adding NEt
2
(PMe
3
4
) /NEt
3
/CH
3
OH.
Hence, an
3
2
3
3
3
3
encouraged us to test other types of bases, such as
ethylpiperidine, piperidine, 1,8-diazabicyclo[5.4.0]undec-7-ene
(
DBU), and K
2 3
CO (Table 2). The selection of organic bases is
based on the reported rule that a medium basicity (pKa between
[
20]
8
and 12) usually leads to high yields of formic acid/formate.
A
base concentration of around 0.6 mol/L was applied for all
organic bases. The use of ethylpiperidine, bearing a similar
Figure 2. Dependence of the concentrations of MF and formate on reaction
basicity (pKa = 10.4) to that of NEt
comparable TON of 1364 with that of NEt
TONs for both MF and formate were lowered to 129 and 262 in
the presence of piperidine (pKa = 11.22), respectively. It was
reported that the reaction of primary or secondary amines with
3
(pKa = 10.65), led to a
(1454) for MF. The
time. The inset shows the concentrations between 0 and 150 min. Reaction
conditions: 1 wt% Ru@pDPPE (0.005 g), methanol (5 mL), NEt (0.5 mL),
3
3
o
2 2
mesitylene (0.5 mL), 60 bar H , 20 bar CO , 160 C.
[
21]
Figure
2
illustrates the time dependence of the
2
CO can yield a stable ionic carbamate species, which might
concentrations of MF and formate. With the reaction time
increasing from 0 to 28 h, the concentration of MF first increased
and then declined, finally approaching the equilibrium. The
maximum concentration of MF (322 mmol/L) was obtained at 12
h, corresponding to a TON of 3401. The inset in Figure 2
illustrates a short induction period of 0 to 10 minutes for MF with
a very low concentration (about 2 mmol/L), whereas the
concentration of formate increased significantly from 0 to 164
mmol/L during this period. After 10 minutes, the concentration of
MF started to increase at the expense of the concentration of
formate. The kinetic behavior of MF and formate clearly
be the reason for the poor catalytic performance of piperidine.
The production of MF and formate was greatly accelerated by
the addition of the strong base DBU (pKa = 12), giving TONs up
to 2154 and 2503 after 2 h, respectively. The superior catalytic
activity of DBU can be attributed to its outstanding Lewis basicity
in capturing CO
heterolytic splitting of H
but a large amount of potassium formate was formed with a
TON of up to 5773 when using an inorganic base K CO . The
formation of MF seems to be completely blocked by the addition
of K CO , likely due to its strong suppressive effect on the
2
as well as the potential role in assisting the
[
22]
2
.
Interestingly, no MF was detected
2
3
2
3
indicates that the hydrogenation of CO
consecutive reaction mode, involving first the hydrogenation of
CO to formate followed by an esterification with methanol to
yield MF. Formate actually plays the role of reaction
2
to MF follows a
esterifaction step, as mentioned above.
2
a
2
Table 2. Impact of base on the catalytic hydrogenation of CO to MF and
[
a]
intermediate in MF synthesis, which can account for the
observed changing trend of formate concentration at the very
beginning (inset in Figure 2). A similar trend was also observed
for the MF/formate molar ratio versus reaction time (Figure S2),
which reached the highest value (3.6) at 4 h and then declined
to a stable value around 1. This implies that the formation of MF
appears to be kinetically controlled in the beginning and
reaching a thermodynamic control after long reaction times.
Notably, it could be seen that the first hydrogenation step
proceeded much faster than the following esterification step
within the first ten minutes (inset in Figure 2). We therefore
speculate that esterification presents the rate-determining step
formate.
TON
MF
Conc.
mol/L)
Base
(
Formate
Sum
None
0
0
0
149
521
522
149
298
.3
.6
1098
1454
1619
1976
NEt
3
0
.9
.2
637
809
1854
2042
2491
2851
1
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ChemSusChem
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FULL PAPER
for MF synthesis over Ru@pDPPE, in agreement with previous
[7, 19b]
reports.
Figure 3. Temperature dependence of the TON of MF and formate. Reaction
conditions: 1 wt% Ru@pDPPE (0.005 g), methanol (5 mL), NEt (0.5 mL),
mesitylene (0.5 mL), 60 bar H , 20 bar CO , 1 h.
3
Figure 4. Impact of catalyst loading on CO hydrogenation to MF and formate.
3
Reaction conditions: methanol (5 mL), NEt (0.5 mL), mesitylene (0.5 mL), 60
bar H , 20 bar CO , 160 C, 2 h.
2 2
2
2
2
o
Reaction temperature was found to possess an enormous
influence on the TONs of MF and formate (Figure 3). As the
o
reaction temperature increased from 100 to 180 C, the TON of
MF first underwent a gradual increase to a maximum of 1079 at
o
1
60 C and then a slight decrease. In contrast, a monotonously
decreasing trend from 4083 to 295 was found for the TON of
formate. As mentioned above, the applied basic conditions as
well as the lack of an acidic catalyst for the esterification step
could be the reason for the required high reaction temperatures
for MF production, while the use of Ru@pDPPE enabled the
o
efficient synthesis of formate at a temperature as low as 100 C.
The influence of H
partial pressure from 10 to 80 bar under a fixed CO
pressure of 20 bar (Figure. S3a). The TON of MF was nearly
linearly dependent on H partial pressure, reaching the highest
value of 1950 at 80 bar H . As CO partial pressure was varied
from 10 to 50 bar under a fixed H pressure of 60 bar, an
appreciable increase in the TON of MF was observed only below
0 bar (Figure. S3b). In contrast, the TON of formate increased
slightly with varying H and CO partial pressures.
In addition, the dependence of catalytic performance on the
catalyst loading was studied (Figure. 4). With the catalyst
loading increasing from 2.9 to 9.7 mg, a linear increase in the
concentration of formate (30–92 mmol/L) with unchanged TONs
2
pressure was studied by varying H
2
2
partial
Figure 5. Recycling of Ru@pDPPE for CO
Reaction conditions: 0.08 of
methanol/NEt /mesitylene at a volume ratio of 10/1/1 for the first test run; for
the following runs, an appropriate amount of the prepared solution was added
2
hydrogenation to MF and formate.
1 wt% Ru@pDPPE and 15 mL of
g
2
3
2
2
o
2
2 2
to keep a same catalyst conc. (5.3 mg/mL); 160 C, 80 bar H , 30 bar CO , 3 h.
3
2
2
Recycling studies were conducted with 1 wt% Ru@pDPPE
o
2 2
at 160 C and a total pressure of 110 bar (CO /H = 3/8) for 3 h
(Figure. 5). After each test, the filtrated and dried catalyst, along
with an appropriate amount of the fresh stock solution containing
methanol/NEt /mesitylene with a volume ratio of 10/1/1, was
3
(
around 500) was observed. Further increasing the catalyst
directly used for the next runs. The overall TON of MF and
formate reached 622 in the first run, and dropped to 450 in the
second run. The overall TON stabilized at around 450 with a bit
of fluctuation for the subsequent five runs. In contrast to the
stable TONs of formate over the seven catalytic runs, the
reduced formation of MF made a major contribution to the initial
decrease in the overall TON. A negligible amount of Ru species
loading did not alter the concentration of formate, but led to a
decrease in TON, indicating that this reaction is likely mass-
transfer controlled. For the concentration of MF, only a moderate
growth was observed. Correspondingly, the TON of MF dropped
continuously from 1999 to 333.
(
below 0.5 ppm) was detected in the filtrate by inductively
coupled plasma optical emission spectroscopy (ICP-OES) for all
catalytic runs (Table S1), corresponding to a catalyst leaching
below 0.8 % of the introduced Ru. TEM image of the fresh 1
wt% Ru@pDPPE catalyst revealed no nanoparticles present on
the polymer, emphasizing the atomic dispersion of the Ru
complex (Figure. 6a). In the STEM image of the used catalysts
after 7 runs (Figure. 6b), most Ru complexes still remained
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ChemSusChem
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invisible with a high dispersion but a few Ru nanoparticles
appeared in some regions of the polymer matrix, which might be
one reason for the slightly decreased activity.
uncoordinated
and
coordinated
phosphorus
species,
respectively. Compared to the coordinated phosphorus in fresh
Ru@pDPPE usually resonating at 36.1 ppm reported in our
previous work,
[
17b]
the chemical shift in the used catalyst shifted
downfield to 42.6 ppm. This shift probably stems from the
change of the coordination mode of the Ru complex during the
reaction.
ATR-IR spectroscopy was applied to further understand the
change of Ru species at the molecular level (Figure. 8). Two
–
1
new bands at 1975 and 2032 cm were found in the spectrum
of the used catalyst, which agree with the characteristic
wavenumber of carbonyl and hydride species, respectively,
[
24]
according to literature. These new bands indicate a most likely
change of the Ru complex to the coordination of CO and hydride
species to the Ru center, which is possibly the main reason for
the deactivation of Ru@pDPPE. In addition, despite using
methanol as the solvent, it is very unlikely that methanol or
methoxy group bind to the Ru center, since signals
corresponding to methanol or methoxy group were not found in
ATR-IR and SS NMR spectra.
Figure 6. (a) TEM image of the fresh 1 wt% Ru@pDPPE catalyst; (b) HAADF-
STEM image of the used 1 wt% Ru@pDPPE catalyst after 7 catalytic runs.
To gain more insights into the changes in the polymer as
well as the catalytically active center (Ru complex) before and
after reaction, Ru@pDPPE with higher Ru loadings (10 and 24
wt%) were synthesized and then applied in the hydrogenation of
2
CO to MF under identical conditions. The spent catalysts with
high loadings (after one catalytic test) were further characterized
by a series of techniques including solid-state magic angle
spinning NMR (SS MAS NMR), X-ray photoelectron
spectroscopy (XPS) and attenuated total reflectance infrared
spectroscopy (ATR-IR). As shown in Figure. 7, the spent
catalyst displayed the same binding energies for Ru 3d core
levels of 285.6 (Ru 3d3/2) and 281.5 eV (Ru 3d5/2) as that in the
fresh catalyst, implying the unchanged oxidation state of Ru
[
23]
(
+2). A small peak with a binding energy of 286.6 eV (C 1s),
assigned to the C–O species, appeared in the spent catalyst,
which was probably due to the residual MF on the polymer.
Moreover, the elimination of chlorine ligands in the spent
catalyst was clearly observed in the XPS survey spectra (Figure.
S4).
1
In the SS MAS H NMR spectrum of the spent catalyst, two
intense peaks at 3.3 and 6.4 ppm were observed (Figure. S5),
which can be assigned to the aliphatic and aromatic protons of
1
3
the polymer, respectively. The SS MAS C NMR spectrum
showed three overlapping peaks at 156.5, 146.0 and 142.3 ppm
(
Figure. S6), which correlate to the benzene rings in the polymer.
These findings indicate the unchanged polymer backbone after
1
3
reaction. Compared to the reported C NMR signals for the
[
17b]
fresh Ru@pDPPE,
the disappearance of the signals at
around 100 ppm revealed the cleavage of the p-cymene ligand
in the spent catalyst. The broad signals, centering at 38.3 ppm
(
Figure. S6), seemingly involved a set of peaks, which can be
ascribed to the aliphatic carbon atoms in the polymer. Two main
Figure 7. C 1s and Ru 3d XPS spectra of the fresh (a) and spent (b) 10 wt%
Ru@pDPPE catalyst.
3
1
peaks at –9 and 42.6 ppm were observed in the SS MAS
P
NMR spectra (Figure. S7), which can be assigned to the
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Figure 8. ATR-IR spectra of the fresh and used 24 wt% Ru@pDPPE catalyst.
2
Scheme 1. Proposed reaction mechanism for the hydrogenation of CO to MF over Ru@pDPPE.
[
19b, 25]
Taken the above findings and literature together,
a
with methanol to finally yield methyl formate. At the same time,
free NEt is liberated from the adduct.
plausible reaction mechanism is proposed for the synthesis of
MF over Ru@pDPPE. As shown in Scheme 1, the immobilized
Ru complex (1) first undergoes the substitution of chlorine
3
ligands by two hydride species supplied by H
2
to form a Ru- Conclusions
hydride intermediate 2. The formation of Ru- hydride species is
generally considered as a prerequisite for Ru complex-catalyzed
This work demonstrated a straightforward hydrogenation of CO
to MF in methanol catalyzed by an immobilized Ru complex on
phosphine polymers. The incorporation of [RuCl (p-cymene)]
into pDPPE was uncovered as the best catalyst for MF synthesis,
offering a TON of 3401 at 160 C and 80 bar total pressure
2
[
19b, 26]
CO
2
hydrogenation.
Subsequently, a carbonyl group is
bound to the Ru-hydrides via replacing the p-cymene ligand by
CO formed during the reaction, giving rise to complex 3. This in
situ formed complex 3 is considered as the real catalyst that
2
2
o
2
participates in the reaction. Within the catalytic cycle, CO is
(
H
2
/CO
2 3
= 3) after 12 h in the presence of NEt as a base. The
inserted into the Ru-hydride bond, leading to the formation of
formate bound to the Ru center (4). Next, formic acid is extruded
from the complex 4 by a classical reductive elimination step
involving the combination of the other hydride species with
TON of MF was greatly influenced by the concentration of NEt
3
,
reaching a maximum at an optimum concentration of 1.2 M. The
concentration/time profiles revealed the fast formation of formate
at the initial stage, which was further converted to MF via a slow
esterification step. Variation of the reaction temperature
formate. By the oxidative addition with H
regenerated. The released formic acid is immediately trapped by
NEt to form formic acid/NEt adduct followed by an esterification
2
, complex 3 is
disclosed that formate was produced as the main product at
o
3
3
1
00–140 C and the formation of MF was dominant only at 160–
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o
1
80 C. Strong CO
2
and H
2
partial pressure dependences were
low catalyst loading was
The filtrate was concentrated by a rotary evaporator and transferred to a
volumetric flask to determine the Ru content on polymer.
observed for MF formation.
A
preferable, since high catalyst loadings caused a diffusion-
controlled reaction. Furthermore, this catalyst showed good
stability and could be recycled up to seven runs, only with a
slight decrease of TON after the first run. The structure of
pDPPE was preserved after the reaction, whereas the
coordination environment of Ru changed and some Ru species
agglomerated to form nanoparticles during the reaction.
Catalyst characterization
Transmission electron microscopy (TEM) and scanning transmission
electron microscopy (STEM) were performed on a FEI Tecnai F20
operated at a voltage of 200 keV with high-angle annular dark field
(
HAADF) detector. X-ray photoelectron spectroscopy (XPS) were
performed using a Phi5000 VersaProbe II spectrometer (ULVAC-Phi Inc.,
USA) equipped with an Al K source with an excitation energy of 1.486
α
keV and operated at a voltage of 15 kV. All binding energies reported in
XPS spectra were referenced to the C1s peak at 285 eV. IR spectra were
recorded on a Vertex 70 equipped with an Attenuated Total Reflection
Experimental Section
–1
(
ATR) accessory in the wavenumber range of 4000–400 cm with a
Materials and catalyst synthesis
–1
resolution of
2 cm . Gaseous products were analyzed by gas
chromatography (GC) on an Agilent HP6890 system equipped with a 2 m
Shin carbon ST 100/120 mesh micropacked column and a TCD detector.
Helium was used as carrier gas at a flow rate of 24.6 mL/min.
Temperature-programmed method was applied to control the column
temperature with a heating rate of 8 C/min from 35 to 150 C. Ru
contents in the as-synthesized catalysts and the leached Ru species in
All synthetic and purification procedures of polymers and catalysts were
performed under argon atmosphere using Schlenk line techniques. 4,4’-
dibromobiphenyl (98%),
n-Butyllithium (1.6
M
in hexane),
dichloro(pcymene)ruthenium(II) dimer, 1,8-diazabicyclo[5.4.0]undec-7-
ene (>99%), potassium carbonate (>99%), ethylpiperidine (99%),
ruthenium on active carbon carbon (0.5 wt% Ru), piperidine (99%), and
o
o
reaction solution were measured by
a SPECTROBLUE inductively
1,3,5-tris(4-bromophenyl)benzene (97%) were purchased from Sigma
coupled plasma optical emission spectroscopy (ICP-OES). Liquid-state
Aldrich. 1,2-Bis(dichlorophosphino)ethane (96%), mesitylene (>98%) and
phenyllithium (1.8 M in dibutyl ether) were purchased from Alfa Aesar.
Phosphorus trichloride (>99%) was obtained from Merck. Trimethylamine
1
H NMR was measured on a Bruker Avance spectrometer (400 MHz).
13
31
1
Solid-state MAS C CP and P { H} spectra were measured on a Bruker
Avance III spectrometer (4 mm MAS rotor, 15/19 kHz spinning
frequencies, 500 MHz). Chemical shifts are reported in parts per million
(99.5%) was purchased from Carl Roth. All commercial chemicals were
used as received unless otherwise stated. Anhydrous methanol (99.8%)
and diethyl ether (99.8%) were purchased from ChemSolute and
AppliChem, respectively. Commercial THF and n-pentane were
pretreated by a solvent purification system to reach water- and air-free
conditions. Deionized water was provided by an ion-exchange system.
All solvents except deionized water were degassed and stored in
Schlenk bottle under argon atmosphere.
31
1
(ppm) relative to calibration standards. Spinning side-bands in P{ H}
spectra were identified by varying the spinning frequencies.
Catalytic test
2 2
The hydrogenation of CO with H in methanol was performed in a
stainless steel autoclave (50 mL) with a teflon liner and a magnetic
stirring bar inside. In a typical test, a certain amount of catalyst
corresponding to a Ru amount of 0.3 mg, 5 mL of methanol, 0.5 mL of
The syntheses of pTPP, pDPPE and pTPPB were performed according
[
17]
to the method reported previously by Hausoul et al.
Typically, the
NEt
the reactor. The closed reactor was first flushed 4 times and pressurized
to 20 bar with CO , and then H was added up to an overall pressure of
0 bar in a H /CO molar ratio of 3. The reactor was finally placed in a
3
and 0.5 mL of mesitylene as an internal standard were charged into
dropwise addition of n-Butyllithium (80.0 mmol, 50 mL, 1.6 M in hexane)
to a stirring solution of aryl bromides (4,4’-dibromobiphenyl for pTPP and
pDPPE, 1,3,5-tris(4-bromophenyl)benzene for pTPPB) followed by
stirring for 1 h led to the formation of suspension, which was filtrated by a
glass filter. 200 mL of diethyl ether was added to the obtained solid, and
the resulting suspension was sequentially subjected to filtration, washing
with hexane (200 mL) and filtration. The isolated solid was suspended in
THF (120 mL). To this suspension, a solution of chlorophosphine (1,2-
Bis(dichlorophosphino)ethane for pDPPE and phosphorus trichloride for
pTPP and pTPPB) in THF (60 mL) was added dropwise over 1 h using a
syringe pump. Then, phenyllithium (19.0 mmol, 10 mL, 1.8 M in dibutyl
ether) was added and left overnight for a sufficient coupling reaction.
Methanol (100 mL) was added and left to react for 15 min to quench the
2
2
8
2
2
o
preheated heating plate at 160 C and a stirring speed of 750 rpm, which
was counted as the starting point of the reaction. The reaction was
quenched by placing the autoclave in an ice bath. After cooling down,
gas-phase product (CO) was collected by a gas bag for GC analysis. For
the products in liquid phase, a certain amount of reaction solution was
1
filtered with a syringe filter and diluted with deuterated DMSO for H NMR
quantitative analysis.
The recycling experiments were carried out by using a solution-change
[
17a]
strategy described by Hausoul et al.
The same procedure as
reaction. The resulting suspension was filtrated with a glass filter. The
o
described above was applied for all recycling tests except using different
amounts of catalyst and substrates. 1%Ru@pDPPE (0.08 g) and 15 mL
of stock solution (methanol/NEt /mesitylene at a volume ratio of 10/1/1)
3
obtained solid was first washed with methanol (200 mL, stirring at 55
C
for 3 h) and then washed with diethyl ether (200 mL) followed by filtration.
The polymers were obtained after a final drying procedure under vacuum
o
were used for the first run. For the following runs, the added volume of
fresh stock solution depends on the mass of the recovered catalyst in
order to keep a same catalyst concentration of 0.0053 g/mL for all runs.
After each run, the reaction solution containing catalyst was transferred
to a round bottom flask and then filtrated by a glass filter. The recovered
catalyst was washed with methanol (5 mL) and finally dried under
at 60 C.
The immobilization of Ru complexes onto the obtained phosphine-based
polymers was realized by a wet impregnation method. Typically, a certain
amount of dichloro(p-cymene)ruthenium(II) (e.g., 30.3 mg for 1 wt% Ru)
and polymer (e.g., 1000 mg for pDPPE) were dissolved in methanol (100
o
o
vacuum at 60 C for 1h. All operations were performed under argon
mL) and left to stir for 3 days at 60 C. Subsequently, the resulting
o
atmosphere.
catalyst (Ru@pDPPE) was filtrated and dried under vacuum at 60 C.
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ChemSusChem
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FULL PAPER
Heterogenized Ru complex on solid
phosphine polymers exhibits high
selectivities and activities towards
the one-pot synthesis of methyl
formate via a tandem
Ruiyan Sun, Anna Kann,
Heinrich Hartmann, Astrid
Besmehn,Peter J. C.
Hausoul*, Regina Palkovits*
hydrogenation/esterification reaction
of CO
2
.
Page No. – Page No.
Direct synthesis of methyl formate
2
from CO using phosphine-based
polymer-bound Ru catalysts
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