Towards a sustainable synthesis of formate salts: Combined catalytic methanol dehydrogenation and bicarbonate hydrogenation

Article abstract of DOI:10.1002/anie.201400456

Formate salts are important chemicals widely used in everyday products. The current industrial-scale manufacture of formates requires CO at high pressure and harsh reaction conditions. Herein, we describe a new process for these products without the utilization of hazardous gases and chemicals. By application of ruthenium pincer complexes, a simultaneous methanol dehydrogenation and bicarbonate hydrogenation reaction proceeds, which provides a green synthesis of formate salts with excellent TON (>18 000), TOF (>1300 h^-1), and yield (>90 %). Get rid of CO and H ₂: An efficient route for the industrial synthesis of formate salts without the utilization of carbon monoxide is highly desirable. A catalytic reaction combining methanol dehydrogenation and bicarbonate hydrogenation has been developed, which provides a green and cost-efficient process for the synthesis of formate salts with excellent turnover numbers and yields.

Full text of DOI:10.1002/anie.201400456

Angewandte
Chemie
DOI: 10.1002/anie.201400456
Homogeneous Catalysis Very Important Paper
Towards a Sustainable Synthesis of Formate Salts: Combined Catalytic
Methanol Dehydrogenation and Bicarbonate Hydrogenation**
Qiang Liu, Lipeng Wu, Samet Gꢀlak, Nils Rockstroh, Ralf Jackstell, and Matthias Beller*
Abstract: Formate salts are important chemicals widely used in
everyday products. The current industrial-scale manufacture of
formates requires CO at high pressure and harsh reaction
conditions. Herein, we describe a new process for these
products without the utilization of hazardous gases and
chemicals. By application of ruthenium pincer complexes,
a simultaneous methanol dehydrogenation and bicarbonate
hydrogenation reaction proceeds, which provides a green
synthesis of formate salts with excellent TON (> 18000),
transport on a bulk scale. On a small scale, sodium formate
has been prepared by reacting chloroform or chloral hydrate
with sodium hydroxide in alcoholic solution [Scheme 1,
Eq. (2)]. Salt formation and the use of toxic substrates restrict
the further application of this process on a large scale. More
recently, in the context of the development of hydrogen-
[
4]
storage materials, formate salts have been also obtained
[
5]
through the hydrogenation of bicarbonates or carbon
[
6]
dioxide in hydroxide solution [Scheme 1, Eqs. (3) and (4)].
In principle, these protocols represent a green process for the
synthesis of formate salts, in which only water is generated as
the by-product. Notably, excellent catalyst turnover numbers
(TONs) were achieved for the hydrogenation of carbon
À1
TOF (> 1300 h ), and yield (> 90%).
F
ormate salts are widely applied in various industrial
processes and represent important products of our daily life.
For example, sodium formate is frequently used in fabric
dyeing and printing processes. It is also employed as a buffer-
ing reagent for strong mineral acids, as a drilling fluid, and as
[
6]
dioxide in hydroxide solution as well. However, the yields of
these reactions are usually below 80%. As a result, it is
difficult to isolate pure formate salts from the reaction
mixtures. In addition, hydrogen (and carbon dioxide in some
cases) at high pressures is needed to facilitate the desired
reaction. Hence, the development of novel and more efficient
processes for formate synthesis remains highly desired.
Very recently, we developed the first ruthenium-catalyzed
alkoxycarbonylation of alkenes with carbon dioxide and
alcohols. In this reaction carbon dioxide is reduced by the
alcohols through transfer hydrogenation to carbon monox-
[
1]
a food additive as well. Moreover, potassium formate is
considered to be a more environmentally benign de-icing
salt. Currently, the industrial-scale production of sodium
formate proceeds by absorbtion of carbon monoxide under
high pressure in solid sodium hydroxide at 1608C [Scheme 1,
[2]
[
3]
Eq. (1)]. Clearly, this process relies on the use of hazardous
and flammable carbon monoxide, which is also difficult to
[
7]
ide. Moreover, ruthenium-pincer-complex-catalyzed dehy-
[
8]
drogenation of methanol and dehydrogenative transforma-
[
9]
tion of alcohols to carboxylic acid salts in basic aqueous
solution were developed by our group and Milsteinꢀs group,
respectively. Inspired by these achievements, we envisioned
that the transfer hydrogenation of bicarbonate with methanol
should be feasible. In our concept bicarbonate constitutes an
ideal hydrogen acceptor for the methanol dehydrogenation to
formate since the same product (formate) can be formed from
bicarbonate hydrogenation and methanol dehydrogenation.
Herein, we describe the first ruthenium-based PNP-pincer-
complex-catalyzed reaction combining methanol dehydro-
genation and bicarbonate hydrogenation [Scheme 1, Eq. (5)],
which provides a green process for the sustainable synthesis of
formate salts with excellent TON (> 18000), TOF
Scheme 1. Various synthetic routes to formate salts.
[*] Dr. Q. Liu, L. Wu, Dr. S. Gꢀlak, Dr. N. Rockstroh, Dr. R. Jackstell,
À1
Prof. Dr. M. Beller
(> 1300 h ), and yield (> 90%).
[
10]
Leibniz-Institut fꢀr Katalyse an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: matthias.beller@catalysis.de
The Ru-MACHO complex 1 (the structure is shown in
Table 2, entry 1) catalyzes the dehydrogenation of methanol
under basic conditions. In an open reaction system three
molecules of hydrogen and one molecule of carbonate salt are
[
**] This research was funded by the State of Mecklenburg-Vorpom-
mern, the BMBF, and the DFG (Leibniz Prize). We thank Dr. W.
Baumann, Dr. C. Fischer, S. Buchholz, S. Schareina, A. Koch, and S.
Rossmeisl (all at LIKAT) for their excellent technical and analytical
support. Q.L. thanks the Alexander von Humboldt Foundation for
financial support.
[8]
generated. For the preparative synthesis of formates the
dehydrogenation sequence has to be selectively stopped at
[11]
this intermediate and carefully controlled. Thus, at the start
of our investigations, the dehydrogenation reaction of meth-
anol was studied in the presence of sodium hydroxide at
1008C in a closed system (100 mL autoclave). Under such
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400456.
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1
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conditions the dehydrogenation of formate should be more
difficult due to the in situ generated hydrogen pressure.
Indeed, the desired product, sodium formate, was produced
with a TON of 130 (Table 1, entry 1). The TON was further
Table 2: Synthesis of sodium formate from sodium bicarbonate, sodium
hydroxide, and methanol: Variation of ruthenium catalysts.
[
a]
[
b]
Entry
1
Ru catalyst
TON
Table 1: Synthesis of sodium formate through the dehydrogenation of
[
a]
methanol.
1
2
746
[
b]
Entry
n (NaHCO3)
n (NaOH)
V (MeOH/H O)
TON
2
2
3
190
183
[
mmol]
[mmol]
[mL]
1
2
3
0
20
20
30
30
30
30/0
30/0
25/5
130
746
769
3
4
[a] Reaction conditions: 100 mL autoclave, Ru-MACHO complex
(
10 mmol), 1008C, 20 h. [b] TON=mmol HCOONa/mmol Ru catalyst.
4
302
increased to 746 when sodium bicarbonate was introduced as
a hydrogen acceptor (Table 1, entry 2). These results demon-
strated the feasibility of our concept [Scheme 1, Eq. (5)]. To
our delight, the catalytic activity is retained after addition of
water to the reaction system (Table 1, entry 3). From a prac-
tical point of view it is important to note that the tolerance of
water will not only improve the solubility of larger amounts of
substrates, but also ensure the possibility of methanol
recycling without a drying process to remove the water
generated in this reaction.
5
6
7
8
[RuCl (benzene)] /dppm
0
103
0
2
2
[RuCl
(benzene)]
/dppe
2
2
Ruh(OAc)(PPh3)
3
no catalyst
0
[a] Reaction conditions: 100 mL autoclave, Ru-MACHO complex (10
mmol), NaHCO (20 mmol), NaOH (30 mmol), MeOH (30 mL), 1008C,
2
3
0 h. [b] TON=mmol HCOONa/mmol Ru catalyst.
To further prove that sodium bicarbonate can act as
a hydrogen acceptor and generate sodium formate in this
reaction, the Ru-MACHO-catalyzed hydrogenation of
sodium bicarbonate in methanol under 10 bar of hydrogen
at 1008C was performed [Scheme S1 in the Supporting
Information, Eq. (1)]. A TON of 1420 is obtained after 20 h.
Additionally, under the same conditions, no sodium formate is
Table 3: Synthesis of sodium formate from sodium bicarbonate, sodium
hydroxide and methanol: Variation of reaction parameters.
[a]
[
b]
Entry
n (NaHCO3)
mmol]
n (NaOH)
[mmol]
TON
Yield
[%]
[c]
[
1
2
3
4
20
60
20
60
20
20
30
30
60
60
60
60
880
1647
2568
1498
2611
3266
9
9
16
6
16
20
formed in the absence of H [Scheme S1, Eq. (2)]. When THF
2
is used as the solvent instead of methanol, a TON of 758 is
acquired as well [Scheme S1, Eq. (3)]. These results indicate
that the hydrogenation of sodium bicarbonate proceeds
smoothly in our designed sodium formate synthesis by
methanol reforming.
[d]
5
[
e]
6
[a] Reaction conditions: 100 mL autoclave, Ru-MACHO complex
In recent years, a variety of ruthenium pincer complexes
have been proven to be efficient catalysts for the dehydrogen-
(
5 mmol), MeOH/H O (25/5 mL), 1008C, 20 h. [b] TON=mmol
2
HCOONa/mmol Ru-catalyst. [c] Yield=mmol HCOONa/mmol (NaH-
CO +NaOH). [d] The reaction temperature is 1308C. [e] The reaction
[
8,12]
ation of alcohols.
Therefore, a selection of different
3
temperature is 1508C.
ruthenium pincer complexes was tested for their activities in
iPr
[12d]
the target reaction. We found that HPNP /Ru complex 2,
[
13]
[14]
Milsteinꢀs catalyst 3, and Barattaꢀs catalyst 4 also showed
activity, albeit somewhat lower than that of the Ru-MACHO
complex 1 (Table 2, entries 1–4). Other ruthenium catalysts
known to be efficient for the hydrogenation of bicarbonates
and carbon dioxide had much lower activity (Table 2,
entries 5–7). The blank reaction without ruthenium catalyst
did not lead to any conversion under the given reaction
conditions (Table 2, entry 8).
resulted in an increased TON of 880, however only with 9%
yield (Table 3, entry 1). Better TONs were observed with
higher amounts of sodium bicarbonate and/or sodium hy-
droxide (Table 3, entries 2–4). The best catalyst TON was
obtained at a NaOH/NaHCO ratio of 1:3 (Table 3, entry 3).
3
Additional sodium bicarbonate prevents the generation of
desired product because the methanol dehydrogenation is less
effective at lower pH (Table 3, entry 4). Additionally, increas-
ing the reaction temperature resulted in improved TONs
(Table 3, entries 5 and 6). A TON of 3266 with 20% yield was
achieved at 1508C (Table 3, entry 6).
Based on the reaction conditions from entry 3 in Table 1,
we screened other reaction parameters to improve the
efficiency of this transformation to a more practical level
(
Table 3). Reducing the catalyst loading from 10 to 5 mmol
2
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Performing the reaction for the potassium formate syn-
thesis indicated that the catalyst TON was dramatically
increased to 12380 with 77% yield (Table 4, entry 1) under
the same reaction conditions as in entry 6 of Table 3. In the
early stages of the reaction, a TOF of 1346 h (after 4 h) is
reached (Table 4, entry 2). As shown in entry 3 of Table 4,
Table 5: Synthesis of potassium formate from carbon dioxide, potas-
sium hydroxide, and methanol.
[
a]
À1
[b]
Entry
CO pressure
n (KOH)
TON
Yield
2
[
c]
[bar]
[mmol]
[%]
1
2
3
4
0
5
10
20
80
80
80
80
10882
12308
8024
0
68
77
50
0
Table 4: Synthesis of potassium formate from potassium bicarbonate,
potassium hydroxide, and methanol: Variation of reaction parameters.
[
a]
[a] Reaction conditions: 100 mL autoclave, Ru-MACHO complex
(
5 mmol), MeOH/H O (25/5 mL), 1508C, 20 h. [b] TON=mmol
2
[
b]
Entry
n (KHCO3)
mmol]
n (KOH)
[mmol]
TON
Yield
[%]
HCOOK/mmol Ru catalyst. [c] Yield=mmol HCOOK/mmol (KOH).
[
c]
[
1
2
20
20
20
0
10
40
60
20
60
60
60
60
60
60
60
80
12380
5384 (4 h)
4481
8108
10185
9523
77
33
28
68
73
48
39
92
[
d]
3
4
5
6
7
8
9428
18422
[
e]
[a] Reaction conditions: 100 mL autoclave, Ru-MACHO complex
(
5 mmol), MeOH/H O (25/5 mL), 1508C, 20 h. [b] TON=mmol
2
Scheme 2. Proposed reaction pathway.
HCOOK/mmol Ru catalyst. [c] Yield=mmol HCOOK/mmol
KHCO +KOH). [d] The reaction temperature is 1308C. [e] The reaction
(
3
time is 36 h.
ing the initial dehydrogenation of methanol to hydrogen and
formaldehyde, the base-promoted dehydrogenation of form-
aldehyde to formate and hydrogen, and the final dehydrogen-
[
8]
a lower reaction temperature resulted in considerably lower
yield and TON (28% and 4481, respectively). Decreased
TONs and yields were gained with lower amounts of
potassium bicarbonate, which illustrates its important role
as a hydrogen acceptor to promote this transformation
ation of formate to hydrogen and carbonate. The last step in
this reaction sequence is rate limiting when the Ru-MACHO
complex is used. With increased hydrogen pressure this step is
further slowed down. Actually, the pressure of the in situ
generated hydrogen under the optimized conditions (Table 4,
entry 8) for the potassium formate synthesis was > 30 bar
after the reaction had been cooled to room temperature. In
addition, an analysis of the gas phase revealed that only
hydrogen was produced in the reaction. In this case, the
formate dehydrogenation step was completely prevented.
Meanwhile, bicarbonate acts as a hydrogen acceptor or
(
Table 4, entries 4 and 5). However, a greater amount of
potassium bicarbonate decreased the reaction efficiency as
well due to the lower pH value of the reaction system as
mentioned above (Table 4, entries 6 and 7). Most signifi-
cantly, a combination of 20 mmol potassium bicarbonate and
8
0 mmol potassium hydroxide afforded a TON of 18422 with
[15]
up to 92% yield in aqueous methanol (Table 4, entry 5). The
undergoes transfer hydrogenation
with methanol to
1
3
C NMR spectrum of the final product indicates that there is
afford the formate product as well, which also facilitates the
methanol dehydrogenation process.
no potassium bicarbonate remaining and no potassium
carbonate generated after reaction (Figure S3). To the best
of our knowledge, this result is superior to any reported
example for the direct catalytic hydrogenation of bicarbon-
ates to formates even under high pressure of hydrogen gas.
Interestingly, carbon dioxide serves also as a suitable
hydrogen acceptor for this transformation (Table 5). Hence,
In summary, we have developed the first combined
catalytic process consisting of methanol dehydrogenation
and bicarbonate hydrogenation to give formate salts. This
industrially relevant reaction allows for a straightforward and
cost-efficient synthesis of formate salts, which also produces
hydrogen as an additional valuable product. When the
ph
8
2
0 mmol of potassium hydroxide reacted with 5 bar (about
0 mmol) carbon dioxide affording high TON and yield
commercially available HPNP /Ru pincer complex was
applied, a high product yield (> 90%) of potassium formate
with a TON exceeding 18000 was realized. Studies on the
detailed reaction mechanism and application of non-noble-
metal catalysts for this transformation are currently underway
in our laboratory.
(
Table 5, entries 1 and 2). Under otherwise identical condi-
tions, additional carbon dioxide suppressed this transforma-
tion (Table 5, entry 3). The reaction was completely shut
down by almost equal amounts of carbon dioxide and
potassium hydroxide (Table 5, entry 4).
Based on these observations, we propose the reaction
pathway of our transformation as shown in Scheme 2.
Previous studies on methanol dehydrogenation at ambient
pressure indicated the following sequential processes, includ-
Experimental Section
Synthesis of potassium formate: KHCO3 (20 mmol), KOH
(60 mmol), and Ru-MACHO complex 1 (5 mmol) were placed in an
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Angewandte
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autoclave (100 mL) and MeOH (25 mL)/H O (5 mL) was added
under an argon atmosphere. The reaction mixture was stirred
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fully evaporated in a rotary evaporator and the potassium formate
content (white in color) was measured through H NMR spectroscopy
1
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Received: January 16, 2014
Revised: March 3, 2014
Published online: && &&, &&&&
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Keywords: dehydrogenation · formates · hydrogen acceptors ·
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4
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Angew. Chem. Int. Ed. 2014, 53, 1 – 5
These are not the final page numbers!

Angewandte
Chemie
Communications
Homogeneous Catalysis
Get rid of CO and H : An efficient route
2
for the industrial synthesis of formate
Q. Liu, L. Wu, S. Gꢁlak, N. Rockstroh,
salts without the utilization of carbon
monoxide is highly desirable. A catalytic
reaction combining methanol dehydro-
genation and bicarbonate hydrogenation
has been developed, which provides
a green and cost-efficient process for the
synthesis of formate salts with excellent
turnover numbers and yields.
R. Jackstell, M. Beller*
&&&& — &&&&
Towards a Sustainable Synthesis of
Formate Salts: Combined Catalytic
Methanol Dehydrogenation and
Bicarbonate Hydrogenation
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ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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These are not the final page numbers!