Homogeneous Catalytic Hydrogenation of CO2 to Methanol – Improvements with Tailored Ligands

Article abstract of DOI:10.1002/adsc.201801314

Improved molecularly-defined cobalt catalysts for the hydrogenation of carbon dioxide to methanol have been developed. A key factor for increased productivity (up to twofold compared to previous state-of-the-art-system) is the specific nature of substitue

Full text of DOI:10.1002/adsc.201801314

Title: Homogeneous catalytic hydrogenation of CO₂ to
methanol – improvements with tailored ligands
Authors: Florian Scharnagl, Maximilian Hertrich, Gordon Neitzel, Ralf
Jackstell, and matthias Beller
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To be cited as: Adv. Synth. Catal. 10.1002/adsc.201801314
Link to VoR: http://dx.doi.org/10.1002/adsc.201801314

10.1002/adsc.201801314
Advanced Synthesis & Catalysis
UPDATE
DOI: 10.1002/adsc.2018
Homogeneous catalytic hydrogenation of CO2 to methanol –
improvements with tailored ligands
Florian Korbinian Scharnagl,a+ Maximilian Franz Hertrich,a+ Gordon Neitzel,a
Ralf Jackstella and Matthias Bellera*
a
Leibniz-Institut für Katalyse e.V. an der Universität Rostock, Albert-Einstein-Straße 29A, 18059 Rostock, Germany
*
corresponding author: Matthias Beller (e-mail: matthias.beller@catalysis.de)
+ These authors contribute equally to this work.
Received: ((will be filled in by the editorial staff))
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201######.
Abstract.
Improved
molecularly-defined
cobalt
sustainable methanol production on a larger scale
necessitates hydrogenation of (captured) CO2 with H2,
which in turn comes from electrolysis of water.[5b, 9]
Advantageously, such a concept offers the possibility
for a decentralized supply of methanol from CO2.
Although several heterogeneous materials are
known for the direct conversion of CO2 to methanol,
similar homogeneous approaches with organometallic
complexes are still at an early stage.[10] The majority
of these molecularly-defined systems work under
basic conditions. For instance, basic pre-activation of
CO2 with amines and subsequent hydrogenation of
the intermediate to methanol in a second step was
demonstrated by the groups of Milstein,[11]
Sanford,[12] Olah/Prakash,[1b, 13] and Wass,[14] as well
as Martins and Pombeiro.[15]
On the other hand, only few systems are known for
direct CO2 hydrogenation to methanol, which
requires acidic conditions. As a step in this direction,
Huff and Sanford reported a Ru-based cascade
catalysis process, which involves three different
catalysts at once.[16] In this system, CO2 was
hydrogenated to formic acid, which was subsequently
esterificated in the presence of acid. Eventually, the
ester was hydrogenated to methanol with an overall
TON of 2.5. Furthermore, the CO2 reduction to
methanol at ambient, aqueous, acidic conditions was
enabled by a disproportionation strategy. The groups
of Himeda and Laurenczy reported Ir-based, sulfuric
acid co-catalysed transformation of CO2 to formic
acid, followed by complete and selective
decomposition to methanol by regenerating CO2,
again.[17]
catalysts for the hydrogenation of carbon dioxide to
methanol have been developed. A key factor for
increased productivity (up to twofold compared to
previous state-of-the-art-system) is the specific nature of
substituents on the triphos ligand. In addition, the effect
of metal precursors, and variations of additives have
been investigated.
Keywords: carbon dioxide; methanol; homogeneous
catalysis; triphos; hydrogenation
Introduction
The hydrogenation of mixtures of carbon
monoxide, carbon dioxide and hydrogen to methanol
is one of the most important catalytic processes in the
area of industrial bulk chemicals.[1] In fact, there was
an annual demand of more than 90 million tons in
2016.[2] In addition to current applications, methanol
is considered as potential energy carrier, within the
concept of “methanol economy”, which has been
proposed by Asinger[3] and Olah.[4] In this vision,
either methanol or its derivative dimethylether is used
for fuel cells or engines, respectively.[5]
Today, the present route to methanol uses synthesis
gas as main feedstock, which is transformed with
water to carbon dioxide in situ. The majority of these
processes make use of Cu/ZnO/Al2O3 catalyst
systems at 190-270 °C and 15-90 bar pressure.[6] In
recent years, an increasing interest exists for
methanol production directly by carbon dioxide
hydrogenation.[7] As an example, besides pilot
applications, the so-called “George Olah CO2 to
Renewable Methanol Plant” exists in Iceland with a
capacity of about 4’000 tons per year.[8] Obviously,
the particular conditions for low cost energy are
crucial for this demonstration unit. For the future a
However, all these systems involve either pre-
activation of CO2 or multiple catalysts for the
formation of methanol. For a direct process, the
tridentate phosphorous-ligand triphos (1,1,1-
tris(diphenylphosphinomethyl)ethane) plays a crucial
1
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10.1002/adsc.201801314
Advanced Synthesis & Catalysis
role. First, the groups of Klankermayer and Leitner
expanded their previously published studies on the
hydrogenation of carboxylic acids with a system
based on this very ligand and ruthenium to the
hydrogenation of CO2.[18] Here, methanol was formed
in the presence of HNTf2 (bis(trifluoromethane)
sulfimide) at 140 °C, 20 bar CO2 and 60 bar H2 with a
TON of up to 221.[19] The TON was later doubled to
442 by reducing the catalyst loading by half, along
Hence, ligands L1-L6 were tested for the CO2-
hydrogenation to methanol in the presence of cobalt
salts and HNTf2 as an additive. Using sublimed
Co(acac)2 together with the parent ligand L1 gave a
slightly more active catalyst system (TON: 60) than
Co(acac)3 (TON: 50). As shown in Scheme 2, using
the ortho-substituted derivative L2 instead of L1
inhibited the reaction completely. This indicates the
strong influence of sterically demanding groups,
with an extensive mechanistic investigation via NMR, which likely disturb the coordination of the P-atom to
MS and computational studies.[20] The group of de
Bruin transferred this concept to cobalt, which was
able to hydrogenate carboxylic esters and acids with
high conversion at 100 °C.[21] In continuation of this
work, the first homogeneous, base-metal catalyst for
the direct methanol production from CO2 was
reported by our group in 2017.[22] The combination of
triphos and Co(acac)3 with HNTf2 as an additive in a
solvent mixture of THF/ethanol gave methanol with a
TON of 50. The presence of ethanol was found to be
important for this system, even though ethyl formate
seems not to be an intermediate in this reaction. Here,
we report an improved cobalt catalyst system which
allows for the synthesis of methanol at low
temperature (90-100°C) with improved turnover
numbers and demonstrates the possibility to run such
a process without additives.
the metal centre. On the other hand, methyl-
substitution on the aryl substituents in meta- or para-
position improved the TONs (L3, L4) by a factor of
two with respect to unmodified triphos. This might
have two reasons; first the electron density on
phosphorus is significantly increased. Therefore, also
the metal centre is electron-rich, which accelerates
the H2-activation. In addition, a substitution in meta-
or para-position might prevent the formation of a µ2-
dihydrido-bridged Co-dimer. The generation of an
analogous Ru-dimer was determined as deactivation
pathway in the system reported by Klankermayer and
25]
Leitner.[20,
Unexpectedly, the stronger electron-
donating methoxy-group in para-position (L5) gave a
lower TON of 79.
Results and Discussion
Basically all the known homogeneous catalysts for
direct CO2 to methanol conversion make use of the
ligand triphos or its derivatives. In this respect,
modifications of the ligand skeleton, but also
variation of the phosphorus substituents are of
general interest.[23] Already in 1994, Huttner and co-
workers developed a convenient methodology for the
preparation of aryl-modified triphos derivatives.[24]
Starting from secondary phosphines and threefold
chloro-substituted
neopentane
(1,1,1-
tris(chloromethyl) ethane) in presence of potassium
hydroxide a simple metathesis reaction in DMSO
gave the corresponding tripod ligand, water and
potassium chloride as products. Based on a slightly
modified procedure,[25] we have recently reported the
preparation of a small library of such ligands.[23]
Scheme 2: Investigation of ligands in the homogeneous
hydrogenation of CO2 to methanol with cobalt.
Reaction conditions: Co(acac)2, ligand, HNTf2 (1:2:3),
THF:EtOH (8:3), 20 bar CO2, 70 bar H2, 100 °C, 24 h. Mass of
methanol was determined via GC using hexadecane as internal
Cl
standard. TON = nproduct/ncatalyst
.
R
R
R
R
DMSO
Cl Cl
- KCl
KOtBu
4 eq.
PR2
PR2 PR2
+
H
P
K P
4 eq.
Nonetheless, it is still more active than the parent
triphos ligand. In addition to these ligands, few other
tridentate phosphorus (L6) and scorpionate-type
nitrogen (L7, L8) containing ligands were
investigated. However, no product formation was
Scheme 1: General synthesis of triphos-derivatives.
2
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10.1002/adsc.201801314
Advanced Synthesis & Catalysis
observed for any of these ligands, which highlights
the importance of the triphos-scaffold, again. Apart
from the ligand, also non-coordinating acidic
additives, e.g. HNTf2 (A1) were found to be essential
for hydrogenation to methanol, previously.[22a] It was
hypothesised that this additive acts as a weakly
coordinating counter-anion to stabilise the cationic
catalytically active species. Also the group of de
Bruin found that non-coordinating anions are crucial
for their cobalt/triphos-based system.[21] To further
investigate this effect, additional additives were
tested (Scheme 3). While TMSOTf (A4) and
Fe(OTf)2 (A5) hardly showed any activity,
Brookhart’s acid (A3) and the cyclic triflimide A2
gave comparable results as A1. These results
demonstrate the importance of the additive and
besides highlights that the presence of nitrogen is not
mandatory for catalytic activity. Notably, also acidity
seems not to be decisive, as A2 (pKa: -13.1 in DCE)
is about 10-fold more acidic than A1 (pKa: -11.9 in
DCE).[26]
Interestingly, when either an aged or refluxed
(10 minutes) stock-solution of Co(NTf2)2 and L1 in
THF was used, an increased productivity (TON: 45-
46) could be observed compared to a freshly prepared
catalyst-solution (TON: 33). Noteworthy, a TON of
33 was also observed for the refluxed catalyst-
solution already at 90 °C. Notably, the previous
system based on Co(acac)3 did not show any activity
below 100 °C. Possibly the necessary cleavage of the
ligand affords higher temperatures, and as this step is
not needed in the case of Co(NTf2)2, it is active at
even lower temperatures. In addition, the simplified
system is still active after 67 h, as was shown via
detection of pressure consumption (see SI). Contrary
to our previous additive-containing system, a ligand
to metal ratio of 1:1 gives an increased TON of 70.
Table 1: Investigation of metals and metal-precursors
on the homogeneous hydrogenation of CO2 to
methanol.
entry
1
metal precursor
Mn(acac)2
Fe(acac)3
Ni(acac)2
Cu(acac)2
CoCO3
TON (MeOH)
0
traces
traces
traces
43
2
3
4
5
6
Co2(CO)8
Co(NTf2)2
Co(NTf2)2
Co(NTf2)2
Co(NTf2)2
0
7a
52
8b
9b,c
10b,d
45
33
70
Scheme 3: Investigation of additives in the homogeneous
hydrogenation of CO2 to methanol with cobalt.
Reaction conditions: [M], ligand, HNTf2 (1:2:3), THF:EtOH (8:3),
20 bar CO2, 70 bar H2, 100 °C, 24 h. Mass of methanol was
determined via GC using hexadecane as internal standard. TON =
nproduct/ncatalyst
.
a 1.0 eq. of A1 was used. b No additive was used.
Reaction conditions: Co(acac)2, ligand, HNTf2 (1:2:3),
THF:EtOH (8:3), 20 bar CO2, 70 bar H2, 100 °C, 24 h. Mass of
methanol was determined via GC using hexadecane as internal
c
Reaction was carried out at 90 °C. d 1.0 eq. of L1 was used.
standard. TON = nproduct/ncatalyst
.
All these further investigations underline that the
formation and stabilisation of the supposed catalytic
active species [Co(Triphos)(L)n]m+ is required for this
system. Clearly, only if the combination of metal-
precursor, ligand and additive enables this formation,
catalytic activity can be observed. For instance the
strong Brønsted-acids A1, A2, and A3 can potentially
protonate the acetylacetonate-ligands of Co(acac)2.
The resulting free coordination sites of cobalt can
rapidly be occupied by triphos. The conjugated base
of the additive acts as weakly coordinating anion and
stabilizes the cationic species. In the case of CoCO3,
Apart from ligands and additives, also different
metals (e.g. Fe, Mn, Cu, Ni) and cobalt-precursors
have been investigated for the title reaction (Table 1).
Unfortunately, none of the other 3d-metal
acetylacetonate-complexes tested showed significant
activity in this system. However, it was found that
Co(acac)2 can be replaced by either CoCO3 or
Co(NTf2)2. In addition, the latter precursor is also
active in the absence of any additive.
3
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10.1002/adsc.201801314
Advanced Synthesis & Catalysis
carbon dioxide is released in strong acidic media and
again cobalt can be ligated and form a cationic
species. Co(NTf2)2 does not need an additive, as
Tf2N- is weakly coordinating and triphos can
therefore easily coordinate to cobalt to give
[Co(Triphos)(L)n]m+, again.
Previously, it was shown that the activity of the
cobalt-based system decreases during time.[22a] For
this reason, studies on the deactivation of the catalyst
in the presence of potential poisons have been
conducted (Scheme 4). It was found that both
products of the reaction, water and methanol, inhibit
the catalyst system. With increasing amounts of them,
the TON for the production of methanol decreased to
a TON of 10 (1.1 ml MeOH) or 1 (1 ml of water),
respectively. Hence, we tried to remove the in-situ
produced water by addition of triethyl orthoester.
Unfortunately, with a TON of 52 the catalyst
performance could not be improved. Besides of
product-inhibition, a major poisoning effect has
60
50
40
30
20
10
0
0.0
0.5
p(CO)/p(CO2)
1.0
Scheme 4: Investigations on the deactivation of the
catalyst in the presence of potential poisons: water (blue),
methanol (red) and CO (green).
Conclusion
In conclusion, the performance of the cobalt-
catalysed reduction of CO2 to methanol has been
improved. Using modified triphos ligands led to
higher TON up to 125 and by replacing Co(acac)2
with Co(NTf2)2, an additive free system was
developed, which is also active below 100 °C. Apart,
the role of the additive as a weakly coordinating
anion was highlighted and catalyst deactivation
pathways were identified. For further optimisations,
CO-, methanol- and water-resistant catalyst systems
have to be developed. Currently, further mechanistic
investigations are ongoing in our laboratory, which
should shed more light on this interesting topic.
carbon
monoxide.
Even
traces
of
CO
(CO2:CO = 80:1) quenched the reaction completely
and no methanol was detected. Hence, it is clear that
catalyst improvements in the future need to take these
points into account, for instance by removing the
products from the product phase.
60
50
40
30
20
10
0
Experimental Section
All chemicals were purchased from commercial
sources and were used as received without additional
purification, if not stated otherwise. Gases were
purchased by Linde. All experiments were carried out
under argon atmosphere by using a glovebox or
standard Schlenk-techniques, unless stated otherwise.
Solvents were stored over molecular sieves 4Å. THF
was dried over sodium and benzophenone. Ethanol
was dried over magnesium. The ligands L2,[27] L3,[25]
L4,[25] L5[23] and L6[28] were synthesised according to
reported procedures. Catalytic experiments were
conducted in high pressure Parr autoclaves and stirred
either mechanically, or with a cross-shaped stirring
0.0
0.5
1.0
V(H2O) [mL]
60
50
40
30
20
10
0
0.0
0.5
1.0
1
bar. H, 31P and 13C NMR spectra were recorded on
V(MeOH) [mL]
Bruker AV-300, Bruker Fourier 300 or Bruker AV-
400 spectrometers. Chemicals shifts (δ) are reported
in ppm downfield of tetramethylsilane. The NMR
chemical shifts are reported relative to the centre of
solvent resonance [CD2Cl2: 5.32 (1H), 53.8 (13C),
CDCl3: 7.26 (1H), 77.0 (13C)]. Gas chromatography
4
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10.1002/adsc.201801314
Advanced Synthesis & Catalysis
analysis was performed on an Agilent HP-6890
chromatograph with a FID detector and an Agilent
HP Ultra 1 column (19091A-105, 50 m, 0.20 mm i.d.,
0.33 μm film thickness, 100% dimethylpolysiloxane)
using hydrogen as carrier gas.
with an ice-bath to quench the reaction. 20 µL
hexadecane were added as internal standard and after
proper stirring of the solution it was analysed by GC.
Experiments with a stock-solution
0.7 mmol (433.46 mg, 1.0 eq) Co(NTf2)2 and
1.4 mmol (874.54 mg, 2.0 eq) L1 were weighed in a
100 mL Schleck-flask inside the glovebox. Outside,
40 mL THF were added and the reddish solution was
refluxed for 10 minutes, during which the colour
changed to dark red/brownish (see SI). Alternatively,
the fresh stock-solution was stored at room-
temperature for several days, during which the same
colour-change took place. For the catalytic testing,
8 mL of the stock-solution was injected in the
autoclave under argon, followed by 3 mL ethanol.
The autoclave was loaded with 20 bar CO2 at room
temperature, followed by 70 bar H2. It was heated by
an aluminium-block for 24 h. Finally, it was cooled
with an ice-bath to quench the reaction. 20 µL
hexadecane were added as internal standard and after
proper stirring of the solution it was analysed by GC.
Synthesis of Brookhart’s acid
According to a modified literature procedure of
Brookhart et al.[29] 1.14 mmol (1012.6 mg) Na[BArF ]
4
were dissolved in 14 mL Et2O at -78 °C. To the pale
yellow solution, 4.57 mL of a 1M solution of HCl in
Et2O were added. A white solid precipitated out and
after 2 h stirring, the cold suspension was filtered.
The filtrate was concentrated to give a white solid in
a pale yellow liquid. The mixture was stored
overnight and then the mother liquor was removed
with a syringe. The white product was dried in high
vacuum (801 mg, 69%)
General Procedure for the Hydrogenation of CO2
Experiments in a 100 mL autoclave
0.14 mmol of metal-precursor and 0.28 mmol
ligand were weighed in a Schlenk-tube inside the
glovebox. In a separate Schlenk-tube were weighed
in 0.42 mmol of additive, if used, inside the glovebox.
Outside, the metal-precursor and the ligand were
dissolved in 8 mL THF. The additive was dissolved
in 3 mL ethanol. The solutions were combined and
Deactivation studies for the CO2 hydrogenation
Inhibition with water or methanol
The catalyst-solution was prepared as usual, but
the indicated amounts of water or methanol,
respectively, were added additionally. The reaction
was then carried out and worked up as stated above.
stirred for further 5-10 minutes.
A
100 mL
(Hastelloy® C) autoclave was sealed, and evacuated
and purged with argon for three times. If a stainless-
steel autoclave was used, the reaction was carried out
in a glass insert. Afterwards, the catalyst-solution was
injected. The autoclave was loaded with 20 bar CO2
at room temperature, followed by 70 bar H2. It was
heated by an aluminum-block for 24 h. Finally, it was
cooled with an ice-bath to quench the reaction. 20 µL
hexadecane were added as internal standard and after
proper stirring of the solution it was analysed by GC.
Poisoning with CO
The catalyst-solution was prepared as usual and
injected in the autoclave. For a 1:1 mixture of CO2
and CO, the autoclave was first loaded with 10 bar
CO, followed by 10 bar CO2. Afterwards H2 was
introduced and the reaction was then carried out and
worked up as stated above. For a poisoning with
1.25 vol% CO, the autoclave was first loaded with
1 atm of CO. Then, CO2 was loaded to a total
pressure of 40 bars at room temperature. The pressure
was released to 10 bar, and CO2 was loaded to a total
pressure of 20 bars. Afterwards, a total pressure of
90 bar was adjusted with H2 and the reaction was
carried out and worked up as stated above.
Experiments in a 25 mL autoclave
0.035 mmol of metal-precursor and 0.07 mmol
ligand were weighed in a Schlenk-tube inside the
glovebox. In a separate Schlenk-tube were weighed
in 0.105 mmol of additive, if used, inside the
glovebox. Outside, the metal-precursor and the ligand
were dissolved in 2 mL THF. The additive was
dissolved in 0.75 mL ethanol. The solutions were
combined and stirred for further 5-10 minutes. A
25 mL Hastelloy® C autoclave was sealed, and
evacuated and purged with argon for three times.
Afterwards, the catalyst-solution was injected. The
autoclave was loaded with 20 bar CO2 at room
temperature, followed by 70 bar H2. It was heated by
an aluminium-block for 24 h. Finally, it was cooled
Acknowledgements
Financial support from the state of Mecklenburg-
West Pomerania, the Federal Ministry of Education
and Research (BMBF), the Danish National Research
Foundation CADIAC (Carbon Dioxide Activation
Centre), and MethaCycle Project from the Federal
Ministry for Economic Affairs and Energy (BMWi)
5
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10.1002/adsc.201801314
Advanced Synthesis & Catalysis
is gratefully acknowledged. We also thank Dr.
Weiping Liu (LIKAT) for providing L5, and
Veronica Papa, Jacob Schneidewind (both LIKAT)
and Prof. Jiwoong Lee (University of Copenhagen)
for helpful discussions. We thank the analytical
department of LIKAT for conducting measurements.
Kothandaraman, A. Goeppert, G. K. Surya
Prakash, J. CO2 Util. 2018, 23, 212-218.
J. R. Khusnutdinova, J. A. Garg, D. Milstein,
ACS Catal. 2015, 5, 2416-2422.
N. M. Rezayee, C. A. Huff, M. S. Sanford, J.
Am. Chem. Soc. 2015, 137, 1028-1031.
a) S. Kar, A. Goeppert, J. Kothandaraman, G.
K. S. Prakash, ACS Catal. 2017, 6347-6351;
b) S. Kar, R. Sen, A. Goeppert, G. K. S.
Prakash, J. Am. Chem. Soc. 2018, 140, 1580-
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D. Wass, M. Everett, Chem. Commun. 2017,
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A. P. C. Ribeiro, L. M. D. R. S. Martins, A. J.
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[11]
[12]
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Advanced Synthesis & Catalysis
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methanol – improvements with tailored ligands
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