Towards a Sustainable Synthesis of Oxymethylene Dimethyl Ether by Homogeneous Catalysis and Uptake of Molecular Formaldehyde

Article abstract of DOI:10.1002/anie.201802247

Oxymethylene dimethyl ethers (OME_n; CH₃(-OCH₂-)_nO-CH₃, n=3–5) are a novel class of sustainable synthetic fuels, which are of increasing interest due to their soot-free combustion. Herein a novel anhyd

Full text of DOI:10.1002/anie.201802247

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Accepted Article
Title: Towards a Sustainable Synthesis of Oxymethylene Dimethyl
Ether by Homogeneous Catalysis and Uptake of Molecular
Formaldehyde
Authors: Andreas Peter, Samuel M. Fehr, Valentin Dybbert, Daniel
Himmel, Ines Lindner, Eberhard Jacob, Mohamed Ouda,
Achim Schaadt, Robin J. White, Harald Scherer, and Ingo
Krossing
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802247
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http://dx.doi.org/10.1002/ange.201802247

Angewandte Chemie International Edition
10.1002/anie.201802247
Towards a Sustainable Synthesis of Oxymethylene Dimethyl Ether
by Homogeneous Catalysis and Uptake of Molecular Formaldehyde
a)
a)
a)
a)
a)
Andreas Peter, Samuel M. Fehr, Valentin Dybbert, Daniel Himmel, Ines Lindner, Eberhard
b)
c)
c)
c)
a)
Jacob, Mohamed Ouda, Achim Schaadt, Robin J. White, Harald Scherer and Ingo Krossing, *
a)
Andreas Peter, Samuel M. Fehr, Valentin Dybbert, Daniel Himmel, Ines Lindner, Harald Scherer and
Prof. Dr. I. Krossing, Department of Inorganic and Analytical Chemistry and Materials Research Center
Freiburg, University of Freiburg, Albertstr. 21, 79104 Freiburg, Germany.
b)
Eberhard Jacob, Motors Emissions Concepts UG, Karwendelstr. 25, 82152 Krailling, Germany.
c)
Mohamed Ouda, Achim Schaadt, Robin J. White, Sustainable Catalytic Materials Group, Division
Hydrogen Technologies, Fraunhofer Institute for Solar Energy Systems, Heidenhofstr. 2, 79110
Freiburg, Germany.
*
Correspondence to krossing@uni-freiburg.de.
TOC Graphic and Short Text
OME
n
goes green? A novel oxymethylene
synthesis from
+
n
dimethyl ether (OME
n
)
dimethoxymethane and monomeric gaseous
formaldehyde is reported. This anhydrous
synthesis route catalyzed homogenously by
trimethyloxonium salts represents a new
approach towards a sustainable synthesis of
oxymethylene dimethyl ethers from green
starting materials.
OME1
Formaldehyde
+
-
cat. OMe [X]
3
F
(
X = BF , Al(OR ) )
4
4
n+1
OMEn
Keywords: Oxymethylene dimethyl ethers – Homogeneous catalysis – Molecular formaldehyde –
Weakly coordinating anion – NMR spectroscopy.
1
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Angewandte Chemie International Edition
10.1002/anie.201802247
Abstract: Oxymethylene dimethyl ethers (OME
n 3 2
; CH (-OCH -)
n
3
O-CH , n = 3-5) are a novel class of
sustainable synthetic fuels, which are of increasing interest due to their soot-free combustion. Herein
a novel anhydrous OME synthesis route is presented. Catalyzed by trimethyloxonium salts,
dimethoxymethane takes up monomeric gaseous formaldehyde instantaneously and forms high
purity OME at temperatures of 25 - 30 °C. This new anhydrous approach using molecular
formaldehyde and catalytic amounts of highly active trimethyloxonium salts represents a promising
new step towards a sustainable formation of OME emanating from CO and H
n
n
n
2
2
.
Oxymethylene dimethyl ethers (OME ) of the general formula CH (-OCH -) O-CH
non-toxic compounds, which attract increasing interest due to their versatile applications. With their
diesel-like properties in combination with soot-free combustion, OME are considered as diesel
additives or even as a novel class of synthetic fuels (for n = 3-5). In addition, OME are discussed as
formation, two structural building units are
necessary: On the one hand molecules are required, which extend the oxymethylene chain by
providing CH O-units (e.g. para-formaldeyhde (para-FA), 1,3,5-trioxane (TRI)). On the other hand,
molecules are needed, which contain methoxy groups for the chain termination (e.g. methanol,
dimethoxymethane, dimethyl ether). Currently, established OME syntheses are either based on
n
3
2
n
3
are a novel class of
n
[
1]
n
[
2]
solvents or for the absorption of CO
2
.
For OME
n
2
n
aqueous syntheses routes via the reaction of methanol (MeOH) and a FA-source, or anhydrous by the
3–12]
reaction of dimethoxymethane (OME
1
) and TRI.[
However, the synthesis of dry TRI is complex and
syntheses initiates the formation
very energy demanding. In addition, the presence of water in OME
conversion decreases with increasing water content.
The product separation for aqueous syntheses routes is evolving, but remains a very energy-
n
of large amounts of side-products[13] and OME
n
*
intensive key step largely increasing the CO
2
footprint of the thereby produced OME
n
(ca. 7 -
–
1
−1
1
0 MJ kg ; this constitutes ⅓ to ½ of the lower heating value of OME1-6 of ca. 19-22 MJ kg ). To
counteract these disadvantages, we set out to evolve a sustainable and anhydrous synthesis route
starting from CO and H . In previous work, we established highly reliable thermodynamic data for
the OME
formation in gas and condensed phase (Table 1).[14]
2
2
n
Table 1: Thermodynamic data for the OME
n
formation starting with CO
2
and H .
. Data extracted from [14]
2

/
r
H°(gas)

r
G°(gas)
 G°(liquid)
/ kJ mol
r
Reaction
kJ mol–
1
/ kJ mol
–1
–1
CO
2
+ 3 H
2
 H
3
C-OH + H
2
O
–49.3
+92.4
–23.9
–55.5
–53.4
–53.9
–53.7
–53.8
+3.5
+59.7
–16.6
–
–
–
–
–
–
–
–
H
2
3
C-OH  H
H
2
C=O + H
2
a)
3
C-OH  DME + H
2
O
a)
DME + H
2
C=O  OME
1
–21.3
–15.3
–16.3
–16.1
–16.0
OME
OME
OME
1
2
3
4
+ H
+ H
+ H
+ H
2
2
2
2
C=O  OME
C=O  OME
C=O  OME
C=O  OME
2
3
4
5
OME
a)
3 2
DME = O(CH ) – dimethyl ether.
From Table 1, OME
n
synthesis, starting from DME or OME and n molecular monomeric FA (best from
1
–
1
dehydrogenation of methanol), is thermodynamically viable ( G°(liquid) = −21.3 / −15.3 kJ mol ).
r
*
Prof. Dr.-Ing. J. Burger, "OME Technology made in Germany, 25.01.2017", can be found under http://www.ome-
technologies.de/fileadmin/omet/OMETechnologiesGmbH_Jan2017.pdf.
2
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Angewandte Chemie International Edition
10.1002/anie.201802247
According to our previously published thermodynamic evaluation,[5,14] this represents energetically
the most efficient route to OME . Overall, starting from green CO / H , it follows the path CH OH,
DME, followed by the reaction with monomeric FA to give the desired liquids in an anhydrous route
n
2
2
3
[
5]
with 22 % less H used than on the conventional route via trioxane. Herein we demonstrate such an
2
OME
n
synthesis based on OME and FA (Eq. 1). In contrast to previously reported literature and
1
known syntheses,[
3–11]
where FA-sources (e.g. TRI, para-FA, FA (aq)) are used, we employ monomeric
+
–
F
F
FA. Catalyzed by trimethyloxonium salts OMe
3
[X] (X = BF
4
; Al(OR )
4
with OR = OC(CF ) ), these
3
3
reactions proceed at ambient temperatures, instantaneously take up the continuously introduced FA
and form OME (n = 2-6; ca. 99 %) with only about 1 % of TRI-impurities (Eq. 1).
n
(Eq. 1)
Depending on the OME
mixtures of the starting materials (OME
According to the law of mass action, the ratio OME
FA. In established anhydrous synthesis routes, this partial pressure is determined by TRI exclusively.
1
/TRI-ratio and temperature, classical OME
, TRI) and higher OME (n ≥ 2) (Flory-Schulz-distribution).
/OMEn-1 is proportional to the partial pressure of
n
syntheses produce equilibrium
1
n
n
[
15]
Based on the similar equilibrium constants for the reaction OMEn-1(l) + FA(g) ⇌ OMEn(l) (where n > 2, )
and the low FA-partial pressure of TRI at the reaction temperature (e.g. 4.2 mbar at 25 °C, calculated
using thermodynamic data from Ref. [16]), the contribution of higher OME
However, the use of higher FA-partial pressures (ca. 8 mbar) would be sufficient to make higher
OME -chains thermodynamically more favorable than TRI.
n
declines exponentially.
n
To run the reaction using monomeric FA and thus to eliminate the Flory-Schulz product distribution,
we developed a FA generator, which provides a continuous flow of monomeric gaseous FA (5 –
2
6 Vol.% in N
2
and 7 – 36 mmol h–1 respectively; ESI Section 1). This monomeric FA-stream was
introduced into liquid OME
1
at 25 to 30 °C. OME
n
formation was catalyzed by 0.8 to 3.0 mol.% of
+
+
–
–
dissolved Me -donor OMe
3
[X] . To understand the influence of [X] counterion, and to demonstrate
+
–
–
3
the use of a commercially available catalyst, OMe
coordinating counterion was used as a benchmark catalyst.
characterized and tested novel OMe
3
[BF ] with a smaller (V = 0.079 nm ) more
4
[
17]
In addition, we prepared, fully
+
F
–
–
3
3
[Al(OR ) ] with a very large (V = 0.755 nm ) and very weakly
4
[
18]
F
–
coordinating anion (WCA; see ESI - Section 2). The [Al(OR )
in low polarity solvents.[
4
] WCA typically induces solubility also
19]
–
Catalysis with OMe
3
+[BF
4
] : Initial experiments were performed by using catalytic amounts of
+
–
+
–
OMe
preferentially polymerizes to a para-FA precipitate, instead of extending the OME
by adding acetonitrile (OME :acetonitrile = 0.5 v/v), this problem is overcome and the preferential
formation of higher OME (n = 2-6; Exp. 1a, 1b) was observed. The product mixtures were analyzed
by 2D-NMR-spectroscopy and OME selectivities (n ≥ 2) were determined by integration of the
proton-NMR-signals (ESI - Section 3). A complete NMR characterization of the pure OME
provided in the ESI (Section 4) and an exemplary OME mixture analysis by NMR is compared with
gas chromatographic measurements leading to consistent results in OME selectivity (ESI - Section 5).
3
4
[BF ] in OME
1
at 25 °C. Due to the low solubility of OMe
3
4
[BF ] in OME
1
the introduced FA
n
chain. However,
1
n
n
n
(OME1-4) is
n
n
Compatibility of OMe + ]– and MeCN: To elucidate the question, if OMe +
[BF
[BF
] is the active Me -
–
+
3
4
3
4
+
–
donor in acetonitrile, both were reacted in the absence of OME . NMR spectra of OMe
1
3
[BF
4
] in
+
–
acetonitrile-d
3
, demonstrated that acetonitrile was quantitatively methylated to [Me-CN-Me] [BF
4
] .
3
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Angewandte Chemie International Edition
10.1002/anie.201802247
In addition, this known compound[20] was prepared and fully characterized (ESI - Section 6). Probably
+
this compound forms the catalytically active species that acts as a Me donor and catalyzes the OME
formation in the presence of acetonitrile (see below for mechanistic investigations).
n
Catalysis with OMe +
[Al(ORF)
]–: Experiments with pure OME
and catalytic amounts of OMe +
[BF
]–
3
4
1
3
4
led to FA-polymerization instead of FA-uptake. Yet, catalysis without the addition of MeCN solvent
+
would be very desirable. To increase the catalytic efficiency of the OMe
3
source, the synthesized
+
F
–
OMe
3
[Al(OR )
4
] was used. With this perfluorinated anion, an enhanced solubility is achieved and
+
the catalytic activity of a more “naked” OMe
catalyst (with respect to OME
3
cation is increased. The addition of only 0.8 mol.%
) resulted in an instantaneous uptake of the introduced FA (ca.
.5 mmol min ) for the formation of OME . The increase in catalytic efficiency was confirmed by
1
–
1
0
n
NMR-analyses of the reaction mixture before and after quenching (Figure 1).
Reaction mixture of OME & FA (1.8 : 1)
1
+
F
-
0
.8 mol-% OMe [Al(OR ) ]
3 4
CH3
Before quenching
After quenching
CH2
5
.0
4.6
4.2
3.8
3.4
3.0
δ1H ← [ppm]
1
Figure 1: H NMR spectra (400.17 MHz) of the reaction mixture of OME
1
and FA (molar ratio = 1.8:1) with
+
F
−
0
.8 mol.% OMe
5 °C.
3
[Al(OR ) ] before and after quenching with Na[OMe]. Measurements were performed at T =
4
2
1
The H NMR-spectrum before quenching with Na[OMe] shows two broad resonances at δ = 3.65 ppm
and in the range of δ = 4.2 - 5.0 ppm, which relate to the methoxy and oxymethylene groups of the
different OME
blocks below the millisecond NMR-timescale. In contrast, the proton spectrum after quenching with
Na[OMe] shows sharp singlet peaks, which can be assigned to the protons of OME (n = 1-6).
n
. This demonstrates effective intermolecular exchange of all the molecular building
n
Analysis of the OME Yields and Selectivity: The molar ratio of the generated OME (n = 2-6) for the
n
n
+
–
+
F
–
syntheses with OMe
3
[BF
4
] in acetonitrile and OMe
3
[Al(OR )
4
] in pure OME
1
are summarized in
+
F
–
Figure 2. Interestingly, the selectivity for OME
despite of the lower molar ratio (OME
:FA = 1.0; 1.5). This illustrates a higher selectivity for OMEn>2 with OMe
By-products are methyl formate (MF; δ = 3.68 ppm, d, JHH=0.8 Hz; 8.02 ppm, q, JHH=0.8 Hz),
dimethyl ether (δ = 3.22 ppm, s) and traces of TRI (δ = 5.07 ppm, s). However, the TRI content was
n
(n > 2) with catalytic amounts of OMe
:FA = 1.8), larger than the ratios obtained with OMe
3
[Al(OR )
4
] is,
+
–
1
3
[BF
4
] in
+
F
–
acetonitrile (OME
1
3
4
[Al(OR ) ] .
4
4
<
1 mol.% and the formation of MF presumably arises due to the side reaction of OME upon
1
+
F
–
prolonged exposure to OMe
results partially from the quenching of OMe
3
[Al(OR )
4
] (ESI Section 7). The existence of small amounts of DME
+
F
–
3
4
[Al(OR ) ] with Na[OMe] or additionally from a partial
4
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Angewandte Chemie International Edition
10.1002/anie.201802247
OME
1
dismutation.
In addition, the expected OME
n
selectivity was calculated based on a vapor-liquid-equilibrium model
described in Ref. [5]. The simulation confirms the experimentally obtained product ratios (Figure 2).
7
6
5
4
3
2
1
0
0
0
0
0
0
0
0
+
-
OME :FA=1.0; OMe [BF ] , MeCN
1
3
4
OME :FA=1.0, simulation
1
+
-
OME :FA=1.5; OMe [BF ] , MeCN
1
3
4
OME :FA=1.5, simulation
1
+
F
]-
OME :FA=1.8; OMe [Al(OR
)4
1
3
OME :FA=1.8, simulation
1
OME2
OME3
OME4-6
Figure 2: Experimental and simulated OME
n
-selectivity for the different OME :FA-ratios and catalysts.
1
DFT-Investigations towards a Hypothetic Reaction Mechanism: To obtain a deeper understanding of
the occurring reactions, a cycle for the OME formation catalyzed by the trimethyloxonium cation
was calculated with dispersion corrected density functional theory (DFT) methods (BP86 (D3BJ)/def-
TZVP; ESI - Section 8). The methylation of OME can be expected to initiate the formation of the
catalytically active species 1 in an exergonic reaction (Δ
n
1
0
–1
r
G = –24 kJ mol , A; Figure 3). After cleavage
of DME, the generated oxocarbeniumion is stabilized by resonance (see box in Figure 3).
Resonance structure of 1
after DME cleavage:
0
Δ G = -24
r
A
0
Δ H = -20
r
1
0
0
D
Δ G = +10
Δ G = +38
B
r
r
0
0
Δ H = +9
Δ H = +41
r
r
0
3
Δ G = -52
r
2
0
Δ H = -104
r
C
+
Figure 3: Catalytic Cycle for the formation of OME
n
with OMe
3
-Catalysts starting from OME
1
and FA. Thermodynamic data
-1
was calculated with BP86 (D3BJ)/def-TZVP for n = 1. All values are given in kJ mol .
By nucleophilic attack of FA, the elongation of the OME chain takes place and an oxocarbenium-ion
n
0
–1
2
and DME are formed in an endergonic reaction (Δ G = +38 kJ mol , B). These two compounds
r
react exergonic by nucleophilic attack of the released DME (or an OME molecule nearby) and
n
5
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Angewandte Chemie International Edition
10.1002/anie.201802247
0
–1
formation of the dimethyloxonium cation 3 (Δ G = –52 kJ mol , C). By transferring a methyl group
r
from 3 to OME
1
the catalytically active species 1 is regenerated and the higher OME
n
is formed
0
–1
(
Δ
r
G = +10 kJ mol , D). The calculated energy profile for this process is viable and in very good
agreement with the experiment. In accordance with the rapid exchange on the NMR time scale
Figure 1), one expects that the barriers for the processes A to D to be low.
(
In summary, a novel anhydrous synthesis of oxymethylene dimethyl ethers (OME) based on the
reaction of monomeric FA and OME was established. By the use of the highly active salt
an instantaneous uptake of the introduced FA was observed
accompanied by the formation of OME (ca. 99 %) with marginal TRI impurities (ca. 1%). The
experiments were confirmed by comparison with simulations, and a catalytic cycle was proposed and
supported by DFT calculations. To use OME as sustainable synthetic fuels on a large scale, the high
catalytic activity in combination with the selective formation of higher OME is crucial. Here OME
1
+
F
–
OMe
3
[Al(OR )
4
] in pure OME
1
n
n
n
1
served as a model substance. Future investigations will be carried out on the reaction of DME and FA.
Moreover, catalyst recycling is essential and ongoing work is performed in our laboratories. In
addition, the process will be analyzed by mass spectroscopy for a full understanding of the catalytic
cycle. Yet, by using gaseous FA instead of TRI, an energy-intensive synthesis step for the preparation
of starting material can be avoided, with the concurrent OME
n
synthesis proceeding quickly with high
yield and minimal amounts of by-products. In addition, product work-up is facile and moves OME
n
production one step closer towards a sustainable OME
Experimental and Method Section: In a typical experiment the catalyst (OMe +
was dissolved in OME
n
synthesis starting from CO
2
and H .
2
–
3
[X] , 0.8 - 3.0 mol.%)
1
(2 - 5 mL; with or without addition of MeCN) and a cooler (-45 °C) was placed
on top of the reaction flask. Gaseous FA was introduced into the solution at 25 - 30 °C under
permanent stirring. After the FA-introduction was completed, the solution was stirred (typically for
3
0 min, in some cases also for 72 h), and NaOMe was added for quenching at 0 °C. The suspension
was filtered and the obtained OME
provided in ESI - Section 9.
n
filtrate was analyzed via NMR spectroscopy. Further details are
Supporting Information: Full experimental details, procedures, weights, NMR spectra of the
reactions, IR and Raman data are deposited. Details to the quantum chemical calculations are given
together with crystallographic details.
Acknowledgement: This work was supported by the Albert-Ludwigs-Universität Freiburg, by the
BMBF project Biomethanol and the Sustainability Center shared between the Albert-Ludwigs-
Universität Freiburg and the Freiburg Fraunhofer Institutes. Furthermore, we thank the magnetic
resonance portal “Magres” of the Albert-Ludwigs-Universität Freiburg and Dr. Klaus Kurz from
Celanese for the supply of trioxane. RJW is thankful for an “Attract” grant as awarded by the
Fraunhofer Society. We would like to thank Fadime Bitgül for the measurement of the NMR spectra
and B.Sc. Boumahdi Benkmil and M.Sc. Philippe Weis for their support regarding single X-Ray
crystallography.
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