Tailor-made Molecular Cobalt Catalyst System for the Selective Transformation of Carbon Dioxide to Dialkoxymethane Ethers

Article abstract of DOI:10.1002/anie.201702905

Herein a non-precious transition-metal catalyst system for the selective synthesis of dialkoxymethane ethers from carbon dioxide and molecular hydrogen is presented. The development of a tailored catalyst system based on cobalt salts in combination with selected Triphos ligands and acidic co-catalysts enabled a synthetic pathway, avoiding the oxidation of methanol to attain the formaldehyde level of the central CH₂ unit. This unprecedented productivity based on the molecular cobalt catalyst is the first example of a non-precious transition-metal system for this transformation utilizing renewable carbon dioxide sources.

Full text of DOI:10.1002/anie.201702905

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Accepted Article
Title: Tailor-made Molecular Cobalt Catalyst System for the Selective
Transformation of Carbon Dioxide to Dialkoxymethane Ethers
Authors: Jürgen Klankermayer
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201702905
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10.1002/anie.201702905
Angewandte Chemie International Edition
COMMUNICATION
Tailor-made Molecular Cobalt Catalyst System for the Selective
Transformation of Carbon Dioxide to Dialkoxymethane Ethers
Benjamin G. Schieweck and Jürgen Klankermayer*
Abstract: Herein a non-precious transition metal catalyst system for
the selective synthesis of dialkoxymethane ethers from carbon
dioxide and molecular hydrogen is presented. The development of a
tailored catalyst system based on cobalt-salts in combination with
selected Triphos ligands and acidic co-catalysts enabled a synthetic
pathway, avoiding the oxidation of methanol to attain the
formaldehyde-level of the central CH2-unit. This unprecedented
productivity based on the molecular cobalt catalyst is the first
the first homogeneous hydrogenation of carbon dioxide using a
related Triphos/cobalt system with acidic additives.[9] Based on
these findings and our own results regarding the formation of
dimethoxymethane,
a cobalt catalyst system with tailored
ligands and co-catalysts was envisaged for the formation of
DMM starting from CO2 and molecular hydrogen.
example of
a non-precious transition metal system for this
transformation utilizing renewable carbon dioxide sources.
The catalytic utilization of carbon dioxide (CO2) as renewable
carbon resource in combination with molecular hydrogen
enables the sustainable creation of important chemical building
blocks for high-value products.[1] Especially, the reduction of CO2
using molecular catalysts has been of great importance,
facilitating the transformation to formic acid[2], formates[3] and
methanol.[4] Recently, a new catalytic reaction provided the first
example for the selective conversion of CO2 and hydrogen to the
formaldehyde oxidation level, thus opening a novel synthetic
pathway to dialkoxymethane ethers, avoiding oxidation of
methanol to attain the formaldehyde-level of the central CH2-
unit.[5] These compounds are becoming increasingly important
as fuel or fuel additive, due to their ability to lower soot and NOx
formation in existing combustion engines.[6] In detail, this
Scheme 1: Reaction pathway for the catalytic formation of DMM utilizing
CO2/H2 and methanol.
The tailoring process for the synthesis of DMM was initiated with
investigations on the transformation of methyl formate, to
evaluate the consecutive hydrogenation and acetalization step.
For this two-step transformation, selected cobalt-salts in
combination with the Triphos ligand were tested, using a mixture
of THF and MeOH as solvent (Table 1). In the first experiments
the catalyst system was simply prepared by mixing the
respective cobalt salt with the Triphos ligand. At low
temperatures of 80 °C, methyl formate was reacted at a
hydrogen atmosphere of 80 bar with Co(BF4)2·6H2O/Triphos and
under these reaction conditions a formation of DMM with a low
TON of 15 could be observed (Table 1, entry 1). To facilitate the
acetalization, the subsequent experiments were performed with
the addition of HNTf2 and already an enhanced formation of
DMM with a TON of 145 could be detected using Co(acac)2 as
precursor (Table 1, entry 2). In the subsequent steps, further
cobalt(II) precursors were evaluated and with Co(OAc)2 a TON
of 262 was observed (Table 1, entry 3). In this series of
experiments, Co(BF4)2·6H2O was identified as the most active
cobalt precursor, producing a highly active system with a TON of
373 (Table 1, entry 4), resembling a yield of 56% based on the
starting amount of MF. Moreover, increasing the reaction
temperature to 100 °C resulted in a slight decrease in activity
with a lower TON of 321 (Table 1, entry 5). Co(acac)3, the most
active precursor in the hydrogenation of carbon dioxide to
methanol,[9] led to a reduced activity and only a TON of 99 for
the formation of DMM could be realized (Table 1, entry 6).
transformation could be accomplished using
a molecular
ruthenium-Triphos catalyst [Ru(triphos)(tmm)] (Triphos=(1,1,1-
tri(diphenylphosphinomethyl)ethane, tmm = trimethylene-
methane) and aluminium triflate (Al(OTf)3) as Lewis acidic co-
catalyst, via the formation of methyl formate (MF) and methoxy
methanol intermediates (MM, Scheme 1).[5] Consequently, the
tailored catalyst has to facilitate three consecutive reaction steps,
a formylation, a hydrogenation and an acetalization reaction.
With the developed system consisting of two adapted catalyst
parts, TONs up to 214 could be obtained for the formation of
dimethoxy methane (DMM) and linear or branched aliphatic
alcohols resulted in slightly lower TONs. As this system requires
the use of ruthenium transition metal catalysts, alternatives
based on non-precious metals are of great interest for future
applications. Catalysts built on cheap and abundant 3d-metals
are therefore an alternative to established systems and recent
developments with non-precious transition-metal compounds in
hydrogenation reactions paved the way to tailored catalysts for
the valorization of CO2.[7]
Recently, Elsevier and de Bruin established a cobalt-Triphos
catalyst system with a remarkable activity in the hydrogenation
of esters and acids.[8] Furthermore, the group of Beller presented
[*]
B. Schieweck, Prof. Dr. J. Klankermayer
Institut für Technische und Makromolekulare Chemie
RWTH Aachen University
Worringerweg 2, 52074 Aachen (Germany)
E-mail: jklankermayer@itmc.rwth-aachen.de
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201702905
Angewandte Chemie International Edition
COMMUNICATION
Table 1: Hydrogenation of MF to DMM using selected Co-salts and Triphos.[a]
Table 2: Cobalt-catalyzed transformation of CO2/H2 to DMM with the
Co2+/Triphos/acid system.[a]
Yield (DMM)
Entry
Co-salt
T [°C]
TON (DMM)[b]
[%][b]
Entry T [°C] p(H2/CO2) [bar/bar][b]
TON (MF)[c]
TON (DMM)[c]
1[c]
2
Co(BF4)2 · 6 H2O 80
Co(acac)2 80
15
2
1
2
3
4
5
6
60
60/20
60/20
60/20
60/20
90/30
30/10
2
0
148
262
373
321
99
22
39
56
48
15
80
11
32
39
16
6
7
3
Co(OAc)2 · 4 H2O 80
Co(BF4)2 · 6 H2O 80
Co(BF4)2 · 6 H2O 100
100
120
100
100
92
79
55
0
4
5
6
Co(acac)3
80
[a] n(Co) = 15 µmol, n(Triphos) = 16.5 µmol (1.1 eq.), n(HNTf2) = 45 µmol (3.0
eq.), n(MF) = 10 mmol, p(H2) = 80 bar at r.t., VTHF = 1 mL, VMeOH = 2 mL, t =
22 h; [b] TON and Yield of DMM were determined by 1H-NMR-spectroscopy
with mesitylene as internal standard; [c] without acid co-catalyst.
[a] n(Co(BF4)2 ∙ 6 H2O) = 15 µmol, 1.1 eq. of Triphos, 3.0 eq. of HNTf2, VTHF
1 mL, VMeOH = 2 mL, t = 22 h; [b] at r.t.; [c] TONs determined by 1H-NMR-
spectroscopy with mesitylene as internal standard.
=
The detailed optimization yielded the combination of the
precursor Co(BF4)2, the ligand Triphos and co-catalyst HNTf2 as
most effective catalysts system for the formation of DMM from
CO2 and H2. To further substantiate the findings and to evaluate
the substrate scope of the cascade transformation, selected
alcohol substrates were investigated towards the formation of
alkoxy formates (AF) and dialkoxymethane ethers (DAM). The
results of the variation in the alcohol reactants are presented in
Table 3. Starting from ethanol, a selective formation of diethoxy
methane (DEM) was observed with a remarkable TON of 109 at
80 °C (Table 3, entry 1). This reaction enables the conversion of
ethanol to DEM, a promising diesel fuel or additive, via the
combined catalytic transformation of both glucose fermentation
products ethanol and CO2 with molecular hydrogen. The
corresponding ethyl formate product was detected in a low
amount, resulting in a TON of only 19. Moreover, the additional
formation of methanol with a low TON of 14 opens the possibility
to generate in future transformations DAMs directly using only
CO2 and H2. The use of 1-butanol gave the corresponding
dibutoxymethane with a TON of 48 (Table 3, entry 3) and
comparable adverse effects with increasing alcohol chain length
were observed with the recently established Ru/Triphos
system.[5] The transformation could also be realised with tertiary
alcohols, e.g. with tert-butanol a TON of 70 for the formation of
the dialkoxymethane ethers could be established (Table 3, Entry
4). However, iso-propanol yielded only in a minor formation of
the corresponding di-iso-propoxymethane (TON = 16), but
surprisingly methanol was formed as major product in this
reaction with a TON of approximately 60 (Table 3, entry 4).
Increasing the temperature to 100 °C enabled the selective
formation of methanol with a TON of 98, thus allowing a
switching of this catalyst system from the formation of
dialkoxymethane ethers to a selective formation of methanol by
simple variation of the alcohol additive or solvent. To prove this
assumption, a reaction with the more acidic hexafluoro iso-
propanol was performed, resulting in the selective formation of
methanol with a TON of 130 (Table 3, entry 6).
After the successful formation of DMM from MF via
hydrogenation and acetalization, the direct synthesis utilizing
carbon dioxide and molecular hydrogen was targeted. In the first
set of experiments towards the formation of DMM, the
developed Co(BF4)2/Triphos/HNTf2 catalytic system was
investigated (Table 2). Starting the analysis at 60 °C resulted
only in a reduced formation of methyl formate (MF) with a TON
of 2 and no DMM could be detected (Table 2, entry 1).
Increasing the reaction temperature to 80 °C at a partial
pressure H2/CO2 of 60/20 bar yielded the formation of DMM with
a
TON of 7, thus corroborating the envisaged cascade
transformation of CO2 (Table 2, entry 2). Moreover, at 100 °C a
more active system could be observed with a TON of 32 for MF
and 92 for DMM (Table 2, entry 3). A further increase in the
reaction temperature to 120 °C led to a decrease in activity and
for DMM formation a TON of only 79 could be obtained (Table 2,
entry 4). However, at this reaction condition the TON for MF
formation was increasing to 39, indicating
temperature optimum for the formylation
a
different
and
hydrogenation/acetalization steps. This finding is in agreement
with the results from the transformation of MF, where at higher
temperatures also a decrease in activity could be observed.
Consequently, the results clearly support a temperature window
between 100 and 120 °C as optimum, as the TTON is
comparable for the two respective reactions. In the subsequent
experiments the influence of the partial pressure on activity was
investigated, while maintaining a ratio of 3:1 (H2:CO2) for the two
reactive gases. However, at higher as well as at lower partial
pressures, only a reduced activity could be achieved. In detail, at
an elevated pressure of 120 bar the TON drops to 55 for the
formation of DMM (Table 2, entry 5) and a lower pressure of
40 bar inhibited the formation of DMM (Table 2, entry 6).
Experiments with variations of the acidic additive (para-
toluenesulfonic acid, Al(OTf)3) did not lead to an enhanced
activity and details on these investigations are presented in the
supporting information.
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10.1002/anie.201702905
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COMMUNICATION
Table 3: Hydrogenation of CO2 with the developed Co2+/Triphos system in the
be achieved for the synthesis of DMM. Furthermore, the present
catalytic system opens a sustainable synthetic pathway to
dialkoxymethane ethers as liquid energy carriers via the
combined utilization of bio-based feedstock and CO2 as carbon
sources together with “green hydrogen” from water electrolysis
(“bio-hybrid fuels”).
presence of selected alcohols.[a]
Entry
ROH
TON (AF)[b]
TON (DAM)[b] TON (MeOH)[b]
1
EtOH
19
16
3
109
48
70
16
0
14
25
16
65
98
131
2
nBuOH
tBuOH
3
4
iPrOH
20
18
0
Experimental Section
5[c]
6
iPrOH
(CF3)2CHOH
0
General procedure for the homogeneous transformation of carbon
dioxide to dimethoxymethane using Co(BF4)2 ∙ 6 H2O and a Triphos
based ligand system: Under an argon atmosphere, a Schlenk tube was
charged with a freshly prepared stock solution (cCo = 15 µmol/mL) of
Co(BF4)2 ∙ 6 H2O, the Triphos ligand (1.1 equiv.) and HNTf2 (3 equiv.) in
THF. The solution (1 mL) was transferred under argon to a stainless-
steel autoclave equipped with a glass inlet and a magnetic stirring bar.
Methanol (2 mL) was added and the autoclave was pressurized at room
temperature with CO2, followed by hydrogen to the indicated total
pressure. The reaction mixture was stirred and heated to the respective
reaction temperature in an aluminium heating cone. After the reaction
time, the autoclave was cooled to room temperature and then carefully
vented. Mesitylene was added as internal standard and the resulting
solution analysed by 1H-NMR-spectroscopy. TONs were found to be
reproducible within ΔTON = ±5% in at least two independent runs.
[a] n(Co(BF4)2 ∙ 6 H2O) = 15 µmol, 1.1 eq. of Triphos, 3.0 eq. of HNTf2, VTHF
=
1 mL, VROH = 2 mL, p(H2/CO2) [bar/bar] = 60/20 at r. t., T = 80 °C, t = 22 h [b]
TON determined by 1H-NMR-spectroscopy with mesitylene as internal
standard; [c] T = 100 °C.
In a recent study the possibility to tailor the ligand sphere in
Triphos based catalysts could be described and the design
strategy enabled the development of a molecular catalyst with
increased activity, stability and productivity. Consequently, in the
present cobalt system the effect of systematic ligand variation
should be evaluated with a focus on catalyst activity.[10]
Therefore, selected substituents on the phenyl groups of the
Triphos ligand were altered and correlated to the effect on the
transformation of CO2 to dimethoxymethane. Selected results of
this initial investigation are summarized in Scheme 4.
Acknowledgements
This work was supported in part by the Cluster of Excellence
“Tailor-Made Fuels from Biomass”, which is funded by the
Excellence Initiative by the German Federal and State
Governments to promote science and research at German
universities, and by the German Federal Ministry of Education
and Research (BMBF) within the Kopernikus Project P2X:
Flexible use of renewable resources – exploration, validation
and implementation of ‘Power-to-X’ concepts.
Scheme 4: Influence of Triphos-derivatives on the transformation of CO2/H2 to
DMM.
Keywords: molecular catalysis • hydrogenation • carbon dioxide
• cobalt • methanol • dimethoxymethane
As already described, the unaltered Triphos ligand in
combination with Co(BF4)2 resulted in a TON of 92 for the
formation of DMM. The sterically more demanding and electron-
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dimethoxymethane. The application of the Triphos ligand in
combination with selected cobalt-salts and acidic co-catalysts
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COMMUNICATION
Benjamin Schieweck,
Jürgen Klankermayer*
Page No. – Page No.
Title
Stand by me! Dialkoxymethane and oxymethylene ethers are becoming increasingly important
as fuel or fuel additive, due to their ability to lower soot and NOx formation in existing combustion
engines. The present non-precious metal based catalytic system opens a sustainable synthetic
pathway to these liquid energy carriers via the combined utilization of bio-based feedstock and
CO2 as carbon sources together with “green hydrogen” from water electrolysis (“bio-hybrid
fuels”).
This article is protected by copyright. All rights reserved.