Sustainable Catalytic Synthesis of Diethyl Carbonate

Article abstract of DOI:10.1002/cssc.202002471

New sustainable approaches should be developed to overcome equilibrium limitation of dialkyl carbonate synthesis from CO₂ and alcohols. Using tetraethyl orthosilicate (TEOS) and CO₂ with Zr catalysts, we report the first example of sustainable catalytic synthesis of diethyl carbonate (DEC). The disiloxane byproduct can be reverted to TEOS. Under the same conditions, DEC can be synthesized using a wide range of alkoxysilane substrates by investigating the effects of the number of ethoxy substituent in alkoxysilane substrates, alkyl chain, and unsaturated moiety on the fundamental property of this reaction. Mechanistic insights obtained by kinetic studies, labeling experiments, and spectroscopic investigations reveal that DEC is generated via nucleophilic ethoxylation of a CO₂-inserted Zr catalyst and catalyst regeneration by TEOS. The unprecedented transformation offers a new approach toward a cleaner route for DEC synthesis using recyclable alkoxysilane.

Full text of DOI:10.1002/cssc.202002471

Chemistry–Sustainability–Energy–Materials
Accepted Article
Title: Sustainable Catalytic Synthesis of Diethyl Carbonate
Authors: Wahyu S. Putro, Akira Ikeda, Shinji Shigeyasu, Satoshi
Hamura, Seiji Matsumoto, Vladimir Ya. Lee, Jun-Chul Choi,
and Norihisa Fukaya
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
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content of this Accepted Article.
To be cited as: ChemSusChem 10.1002/cssc.202002471
Link to VoR: https://doi.org/10.1002/cssc.202002471
01/2020

10.1002/cssc.202002471
ChemSusChem
COMMUNICATION
Sustainable Catalytic Synthesis of Diethyl Carbonate
Wahyu S. Putro,[a] Akira Ikeda,[a] Shinji Shigeyasu,[b] Satoshi Hamura,[b] Seiji Matsumoto,[b] Vladimir Ya.
Lee,[c] Jun-Chul Choi,*[a] and Norihisa Fukaya*[a]
[a]
Dr. W. S. Putro, A. Ikeda, Dr. J. C. Choi and Dr. N. Fukaya
Interdisciplinary Research Center for Catalytic Chemistry (IRC3),
National Institute of Advanced Industrial Science and Technology
(AIST), Tsukuba Central 5, 1-1-1 Higashi, Japan
E-mail: n.fukaya@aist.go.jp
[b]
[c]
S. Shigeyasu, Dr. S. Hamura, and S. Matsumoto
Tosoh Corporation
Minato-ku, Tokyo, 3-8-2 Shiba, Japan
Dr. V. Ya. Lee
Department of Chemistry, Faculty of Pure and Applied Sciences, University of Tsukuba
Tsukuba, Ibaraki, 305-8571, Japan
Supporting information for this article is given via a link at the end of the document.
Abstract: New sustainable approaches should be developed to
overcome equilibrium limitation of dialkyl carbonate synthesis from
CO2 and alcohols. Using tetraethyl orthosilicate (TEOS) and CO2 with
Zr catalysts, we report the first example of sustainable catalytic
synthesis of diethyl carbonate (DEC). The disiloxane byproduct can
be reverted to TEOS. Under the same conditions, DEC can be
synthesized using a wide range of alkoxysilane substrates by
investigating the effects of the number of ethoxy substituent in
alkoxysilane substrates, alkyl chain, and unsaturated moiety on the
fundamental property of this reaction. Mechanistic insights obtained
by kinetic studies, labeling experiments, and spectroscopic
investigations reveal that DEC is generated via nucleophilic
ethoxylation of a CO2-inserted Zr catalyst and catalyst regeneration
by TEOS. The unprecedented transformation offers a new approach
have been reported to be effective in producing DMC or DEC in
high yields. Regeneration of the dehydrating agent is another
problem that should be addressed to realize sustainability.
toward
a cleaner route for DEC synthesis using recyclable
alkoxysilane.
Exponential increase in atmospheric CO2 levels has forced
the research community to seek sustainable ways to reduce and
transform CO2 into useful chemicals and/or fuels.[1] Among the
various carbon capture technologies available,[2] the use of CO2
as a C-1 feedstock for chemical synthesis such as dialkyl
carbonates could be an important method to mitigate CO2
emissions. Owing to increasing use of dialkyl carbonates growing
in chemical synthesis (as solvents and reagents), batteries (as
electrolytes), and polymer industries,[3] the search for newer and
safer synthesis methods is highly desired to substitute the harmful
phosgene route (Scheme 1.A.(1)). Conversion of CO2 into dialkyl
carbonates such as dimethyl carbonate (DMC) and diethyl
carbonate (DEC) is one of the most attractive methods from the
viewpoint of green chemistry.[4] Recently, Asahi Kasei Chemical,
Japan has constructed a plant for synthesizing dialkyl carbonates
via an indirect route, which is cycloaddition of epoxides followed
by trans-esterification with alcohols (Scheme 1.A.(2)).[3a] Direct
route for dialkyl carbonate synthesis seems promising because of
its nontoxicity and the use of abundantly available reactants, i.e.
alcohols; however, the reaction is limited by chemical equilibrium
(Scheme 1.A.(3)). An effective combination of the catalyst and
dehydrating agent is necessary to counteract the equilibrium
limitation.[5] For example, Bu2SnO‒acetal,[6] Bu2Sn(OMe)2‒
molecular sieves (MS),[7] CeO2‒butylene oxide,[8] CeO2‒2-
cyanopyridine,[9] Nb(OR)5‒dicylohexylcarbodiimide (DCC)[10]
To address this issue, we envisioned an alternative system
where water is not generated as a byproduct because it inhibits
the reaction progress. We report here a solution to this challenge,
a sustainable synthesis of DEC from CO2 and ethoxysilane
derivatives using Zr(OEt)4. We also discuss previous studies that
shed light on the mechanisms of DEC formation. Regeneration of
TEOS from the disiloxane byproduct is also demonstrated
(Scheme 1.B)
1
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10.1002/cssc.202002471
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Table 1. Reaction optimization
reactor (Fig. S8) indicated that 58% yield of DEC was achieved
with TON of 3.8 (Table 1, entry 11).
T
P
t
W
(g)b
0.3
0.3
0.3
0.3
0.3
0.3
0.3
0.1
0.01
0.3
Yield
(%)c
22
No
Catalyst
TONd
(C) (MPa)a
(h)
15
15
15
15
15
15
40
15
72
15
24
1
Ti(OEt)4
Zr(OEt)4
Hf(OEt)4
Zr(OEt)4
Zr(OEt)4
Zr(OEt)4
Zr(OEt)4
Zr(OEt)4
Zr(OEt)4
Zr(OEt)4
Zr(OEt)4
160
160
160
180
200
180
180
180
180
180
180
5
5
5
5
5
4
5
5
5
5
5
1.3
3.0
2.8
3.5
2.7
2.9
3.4
8.7
43.0
3.0
3.8
2
42
3
38
4
48
5
38
6
40
7
46
8
41
9
21
10e
11f
41
6.0
58
a
Reaction condition: TEOS, 3.5 mL (15.7 mmol); CO2 pressure at room
temperature, b Catalyst weight (g), c DEC yield was calculated by GC using
mesitylene as an internal standard based on TEOS conversion into DEC, d
TON (turnover number) = mole DEC / mole catalyst, e 1.5 mL of ethanol was
added; f TEOS, 70 mL (313 mmol).
Catalytic synthesis of DEC using TEOS under CO2 was
initially chosen as a model system. DEC was confirmed by GC
and NMR (Figure S1). A series of metal ethoxide catalysts listed
in Table S1, showed that group 4 metals, i.e. Ti, Zr, and Hf
ethoxides are the most prominent catalysts for DEC synthesis
(Table 1, entries 1-3). Among them, Zr(OEt)4 was selected for
further studies because of its superior activity (42%) (Table 1,
entry 2). Heating at 180 ºC improved the yield to 48% (Table 1,
entry 4); however, when the temperature was increased to 200 ºC,
the DEC yield decreased to 38% (Table 1, entry 5). The DEC yield
also reduced on decreasing the initial CO2 pressure (Table S3;
Table 1, entry 6).The reaction time was prolonged to 40 h, no yield
improvement was observed, which remained at 46% (Table 1,
entry 7), presumably because of the deactivation of the catalyst
Scheme 2. Scope and limitations. Reaction conditions: substrate 15.7 mmol,
Zr(OEt)4 7 mol%, CO2 5 MPa, 180 C, 15 h. DEC yield was calculated by GC
using mesitylene as an internal standard based on substrate conversion into
a
DEC. DEC yield from 3 was determined by 1H NMR using mesitylene as an
internal standard.
Next, we focused on the scope and limitations of the
Zr(OEt)4 catalysts to gain fundamental information about DEC
synthesis under the optimized reaction conditions. Herein, we
classified the scope into four categories: effect of the number of
ethoxy groups, length of the R1-substituent, presence of
unsaturation in R1 and miscellaneous (Scheme 2). When the
number of ethoxy groups on the substrate is increased the DEC
yield gradually improved, with the order 4 (12%) < 3 (30%) < 2
(41%) < 1 (48%). Time-on-stream results of these substrates
showed the same tendency (Figure S9), indicating the importance
of the ethoxy groups for DEC synthesis. Substrates bearing
longer alkyl chains decreased the DEC yield (2 > 6 > 7), probably
owing to steric hindrance. In contrast, hydrosilane 5 afforded a
low DEC yield of 7% because it underwent disproportionation
during the catalytic reaction (Scheme S1).[12] Substrates with
unsaturated moieties, i.e. vinyl 8, allyl 9, and phenyl 10 showed
no considerable effect on DEC formation (Figure S11). The alkyl
length of alkyl halide substrates (11 and 12) strongly influenced
the DEC yield and decreased it to 7% and 38%, respectively.
When 11 was used, the yield of DEC was 20% in the initial
reaction (6 h); however, when the reaction time was prolonged to
more than 15 h, the DEC yield decreased to 7% (Figure S12). The
1,2-Cl rearrangement might probably take place due to thermal
decomposition during the reaction (Scheme S2).[13] However, the
decomposition of the chloro-propyl substrate (12) was not
observed; therefore, the DEC yield gradually increased to 50%
when the reaction time was extended to 40 h. Substrate-bearing
nitrile (13) facilely synthesized DEC, 48% at 15 h, with no
decomposition.
or equilibrium. Decreasing the catalyst loading to 0.1
g
suppressed the DEC yield to 41%, but the TON number increased
to 8.7 (Table 1, entry 8). The TON number reached 43 when 0.01
g of the Zr(OEt)4 catalyst was used (Table 1, entry 9). To the best
of our knowledge, this is the highest TON number for DEC
synthesis ever reported in the literature (Table S6). This result
suggests a small extent of catalyst deactivation. To study the
possibility of the equilibrium process, the probability of the reverse
reaction using disiloxane and DEC as reactants under CO2 or Ar
atmosphere was investigated (Table S7). The results showed that
the disiloxane reverted to TEOS in 78% and 84% yields under
CO2 and Ar, respectively. This clearly implies that the equilibrium
led to a steady increase in the DEC yield after 15 h (Table S4).
The DEC or DMC can reportedly convert SiO2 into the
corresponding alkoxysilane.[11] Therefore, the subsequent
reaction of DEC with disiloxane to regenerate TEOS hampered
the DEC production. The addition of ethanol did not increase the
DEC yield (41%) because the formed water might inhibit DEC
formation (Table 1, entry 10). The addition of acetonitrile,
benzonitrile, and 2-cyanopyridine enhanced the DEC yield, but
not more than 50% (Table S8). Prolonging the reaction time using
acetonitrile showed the same tendency with the maximum yield of
50% (Figure S7). The large-scale experiment using 200-mL
2
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yield mediated by TEOS was significantly higher than that
obtained using other reagents (Table S11), revealing that TEOS
probably acts as a catalyst regenerator. This is validated by the
fact that DEC could be formed with a TON of 3 using [ZrO(OEt)2]
(Scheme S4). Accordingly, the [ZrO(OEt)2] species was
reconverted by TEOS to the initial Zr(OEt)4 catalyst species,
thereby closing the catalytic cycle of DEC synthesis.
Scheme 4. Proposed mechanism for DEC synthesis from TEOS
Scheme 3. Control experiments. aYield was determined by GC using internal
standard method based on Zr(OEt)4
Next, isotope-labeling experiments were performed using
both 13CO2 and C18O2 to determine the fate of the CO2 atoms
during the catalytic reaction. The mass number (m/z) of unlabeled
DEC is 119 (Figure S16), whereas that of DEC resulting from
13CO2 was 120 (Figure S17), revealing that the C atom of the DEC
carbonyl moiety was derived from CO2. In addition, the 13C NMR
results showed that 13CO2 was initially inserted into Zr(OEt)4-
13CO2 prior to DEC (with the 13C=O moiety) formation owing to
heat treatment (Figure S18). Previously, alkyl carbonate
synthesis from metal alkoxides has been reported to be formed
through two possible pathways: (i) nucleophilic alkoxylation or (ii)
electrophilic alkylation (Scheme S5).[9a,15] To confirm which
pathway is followed during the reaction, C18O2 was employed. If
electrophilic ethylation is the main route, the intensity of m/z = 123
would be greater than that of 121. In contrast, the nucleophilic
ethoxylation process should give the opposite result. The GC-MS
data showed that the m/z of DEC generated by C18O2 was 121
(Figure S19). A similar result was obtained when TEOS was used
for DEC synthesis (Figure S21). Hence, we can claim that DEC
was generated via a nucleophilic attack of the ethoxy group on
the hemicarbonate moieties of the [Zr(OEt)4‒CO2] intermediate.
Interestingly, the m/z of disiloxane was mainly 344 (Figure S22),
which is two units higher than that of disiloxane resulting from
unlabeled CO2 (342) (Figure S20). This result indicates that TEOS
incorporates an O atom from CO2 during catalyst regeneration.
Therefore, it is spectroscopically confirmed that the two O atoms
of CO2 are distributed between DEC and disiloxane.
To elucidate the possible reaction pathway, several control
experiments were performed. No DEC was obtained in the
absence of Zr(OEt)4 catalysts (Scheme 3(A)); however, DEC was
stoichiometrically produced with Zr(OEt)4 (Scheme 3(B)),
indicating the importance of Zr(OEt)4. Under Ar, the Zr(OEt)4
catalyst was inactive (Table S10), but the CO2-inserted Zr(OEt)4
catalyst was active for DEC synthesis and the yield of DEC
catalyzed by Zr(OEt)4-CO2 (42%) was slightly higher than that
catalyzed by Zr(OEt)4 (37%) (Scheme 3(C)). It is likely that the
CO2-inserted Zr(OEt)4 species is the first reaction intermediate,
i.e., CO2 insertion into the Zr‒OEt bond proceed prior to DEC
formation. Therefore, DEC formation presumably occurs via an
intramolecular pathway involving CO2-inserted Zr(OEt)4, similar to
Sn complex catalysis.[6a, 6c] We observed that CO2 insertion is fast
and reversible,[14c] because CO2 uptake takes place under
ambient conditions and CO2 is completely released under vacuum,
as confirmed by attenuated total reflection (ATR) analysis (Figure
S13). New bands were observed at 1600 and 1327 cm-1, which
were assigned to the asymmetric (as(C=O)) and symmetric
stretching s(C=O) of the carbonyl group, respectively, indicating
that CO2 was inserted via 2-coordination, because the value of
∆ of 273 cm-1 matches those reported for bridging or chelating
the OCOO moieties (Scheme S3).[14] The 13C NMR results also
confirmed two new peaks, at 161.6 and 166.8 ppm, which might
be assigned to the hemicarbonate moieties (Figure S14). Heating
the CO2-loaded Zr(OEt)4 catalyst at 180 ºC led to the appearance
of a new peak at 155.6 ppm as the C=O bond of DEC, with the
disappearance of the peak at 166.8 ppm (Figure S15). The DEC
The spectroscopic results from the control experiments
demonstrated that the reaction initially underwent a reversible
insertion of CO2 into Zr(OEt)4 (Step 1), followed by DEC formation
via nucleophilic ethoxylation of [Zr(OEt)4‒CO2] (Step 2) (Scheme
3
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4). The [ZrO(OEt)2] intermediate formed after DEC elimination
was converted by TEOS to the starting Zr(OEt)4 and also
produced disiloxane (Step 3). To verify the validity of this
proposed mechanism, kinetic studies of DEC synthesis from
TEOS and CO2 with the Zr catalysts were also performed. At the
beginning of reaction, the rate of DEC was independent of the
TEOS concentration, which confirms the pseudo-zero-order on
TEOS under this experimental condition (Figure S23). In addition,
the slope values for the straight line over various CO2 (Figure S24)
and Zr(OEt)4 (Figure S25) concentration reveal that DEC
synthesis follows the first-order reaction kinetics for both CO2 and
Zr(OEt)4. Overall, the DEC synthesis follows second-order
reaction kinetics. These experimental kinetic results are
consistent with the kinetic model derived from the proposed
mechanism (Scheme S6). Activation energy of 72.6 kJ/mol was
calculated from the Arrhenius plot for DEC formation (Figure S27).
DEC Synthesis: In a typical procedure, 3.5 mL of TEOS (15.7
mmol) and 0.3 g of Zr(OEt)4 were loaded into an 10 mL autoclave.
At ambient condition, the reactor was pressurized with 5 MPa CO2
and heated to 180 C for 24 h. After reaction, mesitylene was
added to the reaction mixture for a quantitative analysis by gas
chromatography (GC) or nuclear magnetic resonance (NMR).
TEOS regeneration: The disiloxane (1.8 mmol), KOH (5mol%)
and excess amount of ethanol (8 mL, 137 mmol) were added to
the 10 mL autoclave. 8 g of MS 3A was added to the joint and
connected with the autoclave and condenser so that the reaction
mixture can be circulated. The autoclave was subsequently
heated to 240 ºC for 3 h. After reaction, mesitylene was added to
the liquid phase for GC analysis.
Acknowledgements
We thank Dr. Tadahiro Nitta for his assistance with the ex-
periments. of waste-free synthesis of DEC.
Keywords: Sustainable catalytic synthesis • CO2 • alkoxysilane
• diethyl carbonate • Zr catalysts
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5
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Entry for the Table of Contents
COMMUNICATION
The first example of
a
sustainable
Wahyu S. Putro, Akira Ikeda, Shinji
Shigeyasu, Satoshi Hamura, Seiji
Matsumoto, Vladimir Ya. Lee, Jun-Chul
Choi,* and Norihisa Fukaya*
catalytic synthesis of diethyl carbonate
(DEC) from CO2 and alkoxysilane
substrate with Zr(OEt)4 catalysts is herein
reported. The result that the disiloxane
byproduct was regenerable offers a new
promising direction for the development
of waste-free synthesis of DEC.
Page 1 – Page 4
Sustainable Catalytic Synthesis of
Diethyl Carbonate
6
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