A Sustainable Method for the Synthesis of Acetic Acid Based on Dehydrogenation of an Ethanol–Water Solution Catalyzed by an Iridium Complex Bearing a Functional Bipyridonate Ligand

Article abstract of DOI:10.1002/cctc.201800680

An efficient system for the synthesis of acetic acid via dehydrogenation of an ethanol–water solution using a new iridium catalyst bearing a functional bipyridonate ligand with N,N-dimethylamino substituents was developed. Quantitative production of acetic acid from an ethanol–water solution was achieved. In addition, recovery and reuse of the catalyst using a simple procedure was also accomplished. Moreover, this catalytic system can be applied in the dehydrogenative transformation of various primary alcohols to the corresponding carboxylic acids.

Full text of DOI:10.1002/cctc.201800680

DOI: 10.1002/cctc.201800680
Communications
Very Important Paper
A Sustainable Method for the Synthesis of Acetic Acid
Based on Dehydrogenation of an Ethanol–Water Solution
Catalyzed by an Iridium Complex Bearing a Functional
Bipyridonate Ligand
Masato Kuwahara, Masaaki Nishioka, Masato Yoshida, and Ken-ichi Fujita*[a]
An efficient system for the synthesis of acetic acid via dehydro-
genation of an ethanol–water solution using a new iridium cat-
alyst bearing a functional bipyridonate ligand with N,N-dime-
thylamino substituents was developed. Quantitative produc-
tion of acetic acid from an ethanol–water solution was ach-
ieved. In addition, recovery and reuse of the catalyst using a
simple procedure was also accomplished. Moreover, this cata-
lytic system can be applied in the dehydrogenative transforma-
tion of various primary alcohols to the corresponding carboxyl-
ic acids.
Acetic acid is an organic compound produced and consumed
on a large industrial scale. Global consumption of acetic acid is
growing year on year, with over 2.5 megatons consumed per
year in 2008 in the USA alone.[1] Acetic acid is primarily used
for the production of vinyl acetate, which is converted to poly-
vinyl acetate via polymerization. Furthermore, after conversion
into acetic anhydride via thermal dehydration, acetic acid is
used for the production of various esters. Among these esters,
acetyl cellulose is both a useful flame retardant and an envi-
ronmentally friendly resin. Acetic acid is also a renowned sol-
vent in the synthesis of terephthalic acid, which is one of the
raw materials in the production of polyethylene terephthalate
(PET).
Scheme 1. Catalytic methods for large-scale production of acetic acid.
from renewable plant resources.[4] Moreover, cutting-edge
technologies for ethanol production from inedible cellulose
have been recently reported.[5] Developing an efficient system
for the production of acetic acid from ethanol and water via
catalytic dehydrogenation (Scheme 1c) would have a large
impact on industrial organic chemistry. Notably, this method
would be of particular interest as it generates hydrogen as a
co-product; hydrogen is a promising energy carrier for the
future.
In the past, industrial production of acetic acid was primarily
performed using catalytic oxidation of acetaldehyde with
oxygen (Scheme 1a).[1,2] Currently, the production of acetic
acid is mainly accomplished via catalytic reactions from metha-
nol and carbon monoxide.[1–3] Well-known methods include the
rhodium-catalyzed Monsanto process and the iridium-catalyzed
Cativa process (Scheme 1b). In addition to these methods,
acetic acid is produced via oxidative decomposition of alkanes
and ethanol fermentation.
Recently, several catalytic systems for the synthesis of car-
boxylic acids from primary alcohols and water based on dehy-
drogenative transformation have been reported.[6] Most of
these approaches use primary alcohols with high carbon num-
bers (>3) as the starting material. However, there is only a
single report on the synthesis of acetic acid from ethanol on
the basis of dehydrogenative transformation using a homoge-
neous catalyst. In this report, Beller et al.,[7–9] achieved the pro-
duction of acetic acid from a mixture of ethanol and water
using a ruthenium catalyst. However, it was necessary to per-
form the reaction using a highly concentrated sodium hydrox-
ide solution (8m) in their catalytic system, and the yield of
acetic acid was lower than 70%.
Under these circumstances, developing a new method for
the production of acetic acid using ethanol as a raw material
would be beneficial, given that ethanol can be easily produced
[a] M. Kuwahara, M. Nishioka, M. Yoshida, Prof. Dr. K. Fujita
Graduate School of Human and Environmental Studies
Kyoto University
Sakyo-ku Kyoto 606-8501 (Japan)
E-mail: fujita.kenichi.6a@kyoto-u.ac.jp
Supporting information and the ORCID identification number(s) for the
author(s) of this article can be found under:
https://doi.org/10.1002/cctc.201800680.
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We have developed a series of systems for dehydrogenative
transformation of alcohols[10] which lead to carbonyl products,
such as aldehydes, ketones, and carboxylic acids, using iridium
catalysts that feature cooperative catalysis of the metal-center
with a functional ligand.[11] The iridium-catalyzed dehydrogena-
tive oxidation of alcohols in aqueous media[11c,f,h] and hydrogen
production from a methanol–water solution was recently re-
ported.[11e,12] We have also developed a method for the catalyt-
ic dehydrogenation of saturated cyclic amines to aromatic
amines.[13] In this paper, we report an efficient and sustainable
system for the synthesis of acetic acid via dehydrogenation of
an ethanol–water solution using an iridium catalyst which
bears a functional bipyridonate ligand. It should be noted that
the present system achieved selective production of acetic
acid from sustainable resources (ethanol and water).
Scheme 3. Preparation of the new iridium catalyst 3 bearing an electron-rich
functional bipyridonate ligand.
We started our investigation by performing the reaction of
an ethanol–water solution under basic conditions using iridium
catalysts 1[11d] and 2[13c] bearing a functional bipyridonate
ligand, those exhibited high catalytic performance for the de-
hydrogenation of alcohols and cyclic amines, expecting the
formation of the acetate salt (CH3COONa) along with evolution
of hydrogen (Scheme 2).[14,15] When the solution of ethanol
With catalyst 3 in hand, we again investigated the dehydro-
genation of an ethanol–water solution under basic conditions
with the aim of obtaining the acetate salt. Results are shown
in Table 1.[14,15] Entries 1 and 2 are a re-posting of the results
Table 1. Optimization of the reaction conditions for dehydrogenative oxi-
dation of an ethanol–water solution to produce acetic acid.
Entry
Catalyst
Base
Yield [%][a]
1
2
3
4
1
2
3
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
Na2CO3
KOH
34
8
99 (99)[b]
0
0
0
94
[Cp*IrCl2]2
[Cp*Ir(H2O)3][OTf]2
[Cp*Ir(bpy)(H2O)][OTf]2
3
5
6[c]
7[d]
8
Scheme 2. Dehydrogenation of an ethanol–water solution using our previ-
ous catalysts 1 and 2.
3
85
9
3
LiOH
72
10
11
12[e]
none
3
3
NaOH
none
NaOH
0
0
(3.0 mmol) and sodium hydroxide (3.6 mmol) in water (6.0 mL)
was refluxed (oil bath temperature: 1358C) for 18 h in the pres-
ence of catalyst 1 (0.25 mol%), sodium acetate was obtained
in the yield of 34%. In contrast, the yield of sodium acetate
obviously decreased (8%) by the employment of catalyst 2
bearing an electron-deficient functional bipyridonate ligand
with trifluoromethyl substituents under the similar reaction
conditions. From these experimental results, we hypothesized
that the catalyst bearing an electron-rich functional bipyrido-
nate ligand might exhibit better performance for the dehydro-
genation of an ethanol–water solution.
100 (98)[b]
1
[a] Yield of acetate salt determined by H NMR using an internal standard.
[b] Yield of acetic acid after acidic work-up determined by 1H NMR using
an internal standard. [c] The abbreviation “bpy” means 2,2’-bipyridine.
[d] Na2CO3 (1.8 mmol) was used. [e] Reaction was performed in 30 mmol
scale.
shown in Scheme 2. When the solution of ethanol (3.0 mmol)
and sodium hydroxide (3.6 mmol) in water (6.0 mL) was re-
fluxed for 18 h in the presence of catalyst 3 (0.25 mol%),[18]
sodium acetate was formed in the yield of 99% (entry 3).
Through acidic work-up, acetic acid was obtained in 99% yield
(shown as the value in parentheses). Other iridium complexes
without a functional ligand, such as [Cp*IrCl2]2, [Cp*Ir(H2O)3]
[OTf]2 or [Cp*Ir(bpy)(H2O)][OTf]2, showed no catalytic activity
for this reaction (entries 4–6). After comparing the results
shown in entries 1–6, it was clear that the iridium catalyst 3
bearing an electron-rich functional bipyridonate ligand was in-
dispensable for achieving high yield for the dehydrogenative
Thus, we planned to prepare a new iridium catalyst 3 bear-
ing an electron-rich functional bipyridonate ligand with N,N-di-
methylamino substituents at the 4- and 4’-positions. As illus-
trated in Scheme 3, the reaction of 6,6’-dihydroxy-4,4’-bis(N,N-
dimethylamino)-2,2’-bipyridine[16] with a half equivalent of
[Cp*IrCl2]2 (Cp*: h5-pentamethylcyclopentadienyl) in methanol
at 608C gave the cationic chloro complex A in 98% yield.[17]
Subsequent reaction with two equivalents of sodium t-butox-
ide in water gave catalyst 3 in 91% yield. The catalyst 3 was
air- and moisture-stable, therefore, it was very easy to handle.
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transformation of an ethanol–water solution. Optimal base was
sodium hydroxide, although the use of sodium carbonate, po-
tassium hydroxide, and lithium hydroxide resulted in 94, 85,
and 72% yield, respectively (entries 7–9). No product formation
was detected in the control experiments that did not contain
catalyst or base (entries 10 and 11).[19] We also performed the
similar reaction using 30 mmol of ethanol (entry 12), which re-
sulted in the formation of acetic acid in high yield (98%).
Quantitative and qualitative analysis of the evolved hydro-
gen as a co-product was performed as follows. The evolved hy-
drogen under the same reaction conditions as those of entry 3
in Table 1 was collected in a gas burette, and its volume was
measured intermittently. A time-resolved profile of the yield of
hydrogen is shown in Figure 1. After a reaction time of 18 h,
the yield reached up to 95% (5.7 mmol), clearly indicating that
the reaction proceeded via dehydrogenation processes. The
purity of the evolved hydrogen was confirmed by GC analysis.
Details are shown in the Supporting Information.[20]
Scheme 5. Possible mechanism for the dehydrogenation of an ethanol–
water solution catalyzed by 3.
basic conditions.[22,23] Then, the species B would be converted
to the ethoxo species C by the reaction with ethanol. b-Hydro-
gen elimination of C would occur to release acetaldehyde and
give the anionic hydrido species D. Protonation of the oxygen
atom in the pyridonate ligand followed by protonolysis of the
hydride on iridium through species E would occur. By these
processes, release of hydrogen and generation of a neutral un-
saturated species F would occur. Finally, either B or C would
be regenerated by the reaction of F with a hydroxide or ethox-
ide anion.
Figure 1. Time-resolved profile of the yield of hydrogen for the dehydrogen-
ation of an ethanol–water solution catalyzed by 3.
Interestingly, a facile procedure could be employed to make
the catalyst 3 reusable while still maintaining its high activity
(Scheme 4). After dehydrogenation of an ethanol–water solu-
tion using catalyst 3 (0.50 mol%) gave a 100% yield during the
first run, the volatile components were evaporated under
vacuum leaving a residual mixture. Extraction of the residual
mixture with dichloromethane resulted in the recovery of
active catalyst. The recovered catalyst was then subjected to
two additional runs, each of which resulted in a 100% yield.[21]
A possible catalytic cycle for the dehydrogenation of an eth-
anol–water solution is illustrated in Scheme 5. Firstly, the
anionic hydroxo species B would be formed from 3 under the
Acetaldehyde formed by b-hydrogen elimination would be
hydrated to give a gem-diol, which would be catalytically dehy-
drogenated through the similar mechanistic processes. Actual-
ly, as shown in Scheme 6, the reaction of acetaldehyde
(3.0 mmol) and sodium hydroxide (3.6 mmol) in water (6.0 mL)
under reflux for 18 h in the presence of catalyst 3 (0.25 mol%)
gave sodium acetate (88%) along with hydrogen (84%). This
result supported the proposed catalytic processes.[24]
As an expansion of the present catalytic system, dehydro-
genative oxidation of several primary alcohols to generate the
corresponding carboxylic acids was attempted. Results are
Scheme 4. Reuse of the catalyst in the dehydrogenation of an ethanol–
water solution.
Scheme 6. Dehydrogenation of an acetaldehyde-water solution catalyzed by
3.
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shown in Table 2. Aliphatic primary alcohols, such as 1-propa-
nol, 1-butanol, and 1-octanol, were converted to the corre-
sponding carboxylic acids in good to excellent yields by using
the 0.25 mol% of catalyst 3 under reflux in water (entries 1–3).
Benzyl alcohols with methyl, methoxy, trifluoromethyl, fluoro,
chloro, and carboxyl substituents were also good substrates
for this catalytic system, giving the corresponding benzoic acid
derivatives in high yields (entries 4–9).
Conflict of interest
The authors declare no conflict of interest.
Keywords: acetic acid · carboxylic acid · dehydrogenation ·
ethanol · iridium
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Table 2. Dehydrogenative oxidation of primary alcohols to generate the
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[2] K. Weissermel, H.-J. Alpe in Industrial Organic Chemistry, 4th ed., Wiley-
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[3] P. M. Maitlis, A. Haynes, G. J. Sunley, M. J. Howard, J. Chem. Soc. Dalton
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Substrate
Product
Yield [%][a]
1
2
3
96
71
83 (71)
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4
5
94 (87)
96 (92)
6
7
8
9
98 (90)
94 (94)
100 (93)
98 (97)
[7] P. Sponholz, D. Mellmann, C. Cordes, P. G. Alsabeh, B. Li, Y. Li, M. Nielsen,
H. Junge, P. Dixneuf, M. Beller, ChemSusChem 2014, 7, 2419.
[8] Dehydrogenative conversion of ethanol into ethyl acetate has been
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2012, 51, 2772; Angew. Chem. 2012, 124, 2826; b) M. Nielsen, H. Junge,
A. Kammer, M. Beller, Angew. Chem. Int. Ed. 2012, 51, 5711; Angew.
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2012, 31, 5239; d) D. Spasyuk, C. Vicent, D. G. Gusev, J. Am. Chem. Soc.
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M. F. Haddow, J. Lee, R. L. Wingad, D. F. Wass, Angew. Chem. Int. Ed.
2013, 52, 9005; Angew. Chem. 2013, 125, 9175.
[a] Yields were determined by 1H NMR using an internal standard. Isolated
yields are indicated in parentheses.
[10] Pioneering report for dehydrogenative transformation of alcohols: A.
Dobson, S. D. Robinson, Inorg. Chem. 1977, 16, 137.
In summary, we have developed an efficient system for the
synthesis of acetic acid via dehydrogenation of an ethanol–
water solution using a new iridium catalyst 3 bearing a func-
tional bipyridonate ligand with N,N-dimethylamino substitu-
ents. This catalytic system could be operated under weakly
basic conditions (ca. 0.6m sodium hydroxide solution). More-
over, this catalytic system was applicable to the dehydrogena-
tive transformation of various primary alcohols to their corre-
sponding carboxylic acids.
[11] a) K. Fujita, N. Tanino, R. Yamaguchi, Org. Lett. 2007, 9, 109; b) K. Fujita,
T. Yoshida, Y. Imori, R. Yamaguchi, Org. Lett. 2011, 13, 2278; c) R. Kawa-
hara, K. Fujita, R. Yamaguchi, J. Am. Chem. Soc. 2012, 134, 3643; d) R. Ka-
wahara, K. Fujita, R. Yamaguchi, Angew. Chem. Int. Ed. 2012, 51, 12790;
Angew. Chem. 2012, 124, 12962; e) K. Fujita, R. Kawahara, T. Aikawa, R.
Yamaguchi, Angew. Chem. Int. Ed. 2015, 54, 9057; Angew. Chem. 2015,
127, 9185; f) K. Toyomura, K. Fujita, Chem. Lett. 2017, 46, 808; g) R. Ya-
maguchi, D. Kobayashi, M. Shimizu, K. Fujita, J. Organomet. Chem. 2017,
843, 14; h) K. Fujita, R. Tamura, Y. Tanaka, M. Yoshida, M. Onoda, R. Ya-
maguchi, ACS Catal. 2017, 7, 7226.
[12] Catalytic hydrogen production from a mixture of methanol and water
was also reported by other researchers: a) M. Nielsen, E. Alberico, W.
Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495,
85; b) R. E. Rodrꢃguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-
Quinones, H. Grꢄtzmacher, Nat. Chem. 2013, 5, 342.
[13] a) R. Yamaguchi, C. Ikeda, Y. Takahashi, K. Fujita, J. Am. Chem. Soc. 2009,
131, 8410; b) K. Fujita, Y. Tanaka, M. Kobayashi, R. Yamaguchi, J. Am.
Chem. Soc. 2014, 136, 4829; c) K. Fujita, T. Wada, T. Shiraishi, Angew.
Chem. Int. Ed. 2017, 56, 10886; Angew. Chem. 2017, 129, 11026.
[14] In this work, the yield of acetate salt (e.g. sodium acetate) can be re-
garded as the yield of acetic acid, which was confirmed by the treat-
ment of sodium acetate with HCl aq. See entry 3 of Table 1.
Acknowledgements
This work was financially supported by JSPS KAKENHI Grant
Number JP16H01018 and JP18H04255 in Precisely Designed Cata-
lysts with Customized Scaffolding.
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[15] These reactions were performed with an open system. The illustration
of the reaction setup was added as Figure S1 in the Supporting Infor-
mation.
ure S4 in the Supporting Information. The results indicated that the
slight decomposition of the catalyst occurred during the recycling ex-
periments.
[16] Procedure for the preparation of this compound is described in the
Supporting Information.
[17] Single-crystal X-ray analysis of the complex A was performed. See the
Supporting Information for details.
[22] To obtain information about mechanism, the reaction of 3 with base
was conducted. As a result, an anionic species which would be corre-
sponding to B was observed in 1H and 13C{1H} NMR analysis. See the
Supporting Information for details.
[18] In this case, the concentration of sodium hydroxide is ca. 0.6m, which
is clearly a milder condition than that of reference 7.
[19] More detailed optimization of the reaction conditions is shown in the
Supporting Information.
[20] We performed the reaction using [D6]ethanol as a substrate. Time-
course of the reaction was monitored by measuring the yield of hydro-
gen like as Figure 1. In the case of the reaction using CH3CH2OH, the
yield of H2 after 1 h was 21%. On the other hand, in the case of the re-
action using CD3CD2OD, the yield of H2 (probably including D2 and HD)
was 9%. From these results, kinetic isotope effect of the present reac-
tion was determined to be 2.3, indicating that the CÀH bond cleavage
step would be a rate limiting step.
[23] We performed the measurement of pKa of the Ir-OH2 in the complex 3
and found that its value was 11.8. This result also supports the pro-
posed mechanism.
[24] As shown in Scheme 6, the yield of acetic acid was 88% using acetalde-
hyde as a starting material, although similar reaction starting with etha-
nol resulted in 99% yield (Table 1, entry 3). This deference of the yield
would be caused by the boiling points of the starting materials (acetal-
dehyde: 20.28C, ethanol: 78.48C). In the case of the reaction starting
with acetaldehyde, some amount of it might be lost because of its low
boiling point. Therefore, the yield might be lowered. Additionally, aldol-
type reaction might occur under basic conditions in the case of high
concentration of acetaldehyde, which lead to low yield of acetic acid.
[21] We investigated the kinetic profile of the recycling experiments. Time-
courses of the reaction for 4 h using a fresh catalyst, 1st time-reused
catalyst, and 2nd time-reused catalyst were monitored by measuring
the yield of hydrogen like as Figure 1. These results are shown in Fig-
Manuscript received: April 24, 2018
Version of record online: && &&, 0000
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M. Kuwahara, M. Nishioka, M. Yoshida,
K. Fujita*
&& – &&
A Sustainable Method for the
Synthesis of Acetic Acid Based on
Dehydrogenation of an Ethanol–Water
Solution Catalyzed by an Iridium
Complex Bearing a Functional
Bipyridonate Ligand
The iridium life: An efficient system for
the synthesis of acetic acid via dehydro-
genation of an ethanol–water solution
using a new iridium catalyst bearing a
functional bipyridonate ligand with N,N-
dimethylamino substituents has been
developed. Quantitative production of
acetic acid from an ethanol–water solu-
tion was achieved. This catalytic system
can be also applied in the dehydrogena-
tive transformation of various primary
alcohols to their corresponding carbox-
ylic acids.
&
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ꢀ 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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