Controlling the equilibrium of formic acid with hydrogen and carbon dioxide using ionic liquid

Article abstract of DOI:10.1021/jp908174s

The equilibrium for the reversible decomposition of formic acid into carbon dioxide and hydrogen is studied in the ionic liquid (IL) l,3-dipropyl-2- methylimidazolium formate. The equilibrium is strongly favored to the formic acid side because of the strong solvation of formic acid in the IL through the strong Coulombic solute-solvent interactions. The comparison of the equilibrium constants in the IL and water has shown that the pressures required to transform hydrogen and carbon dioxide into formic acid can be reduced by a factor of ～100 by using the IL instead of water. The hydrogen transformation in such mild conditions can be a chemical basis for the hydrogen storage and transportation using formic acid.

Full text of DOI:10.1021/jp908174s

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3510
J. Phys. Chem. A 2010, 114, 3510–3515
Controlling the Equilibrium of Formic Acid with Hydrogen and
Carbon Dioxide Using Ionic Liquid
Yoshiro Yasaka, Chihiro Wakai, Nobuyuki Matubayasi, and Masaru Nakahara*
Institute for Chemical Research, Kyoto UniVersity, Uji, Kyoto 611-0011, Japan
ReceiVed: August 25, 2009; ReVised Manuscript ReceiVed: January 28, 2010
The equilibrium for the reversible decomposition of formic acid into carbon dioxide and hydrogen is studied
in the ionic liquid (IL) 1,3-dipropyl-2-methylimidazolium formate. The equilibrium is strongly favored to the
formic acid side because of the strong solvation of formic acid in the IL through the strong Coulombic
solute-solvent interactions. The comparison of the equilibrium constants in the IL and water has shown that
the pressures required to transform hydrogen and carbon dioxide into formic acid can be reduced by a factor
of ∼100 by using the IL instead of water. The hydrogen transformation in such mild conditions can be a
chemical basis for the hydrogen storage and transportation using formic acid.
1. Introduction
Attempts to synthesize FA through the backward process of
reaction 1 have been made in weakly solvating environments
for the past 3 decades. Inoue et al. reported first FA synthesis
in benzene catalyzed by a Pd complex.5 Noyori et al. used
supercritical CO2 as the solvent and reactant in the presence of
amines as a base and Rh complexes as catalysts.6,7 These
syntheses would require considerable pressures unless stoichio-
metric amounts of bases are used, because the equilibrium of
eq 1 strongly favors the gaseous product side in nonpolar
molecular solvents. McCollom et al. observed FA formation in
the form of sodium salt in subcritical water in the presence of
sodium hydroxide; metal complexes were not necessary for
accelerating the reaction rate in this high-temperature reaction,
but a base was used to shift the equilibrium to the product side.8
The resort to bases can be avoided when some solvents with
more powerful solvation than benzene, supercritical carbon
dioxide, or hot water are used. In fact, Fukuzumi et al. reported
formation of FA in ambient water in acidic conditions,9,10
although relatively high pressures were still required. The
synthetic difficulties have been circumvented here by focusing
on the unique solvation of IL from the viewpoint of physical
chemistry.
One of the key roles played by solution physical chemistry
is to find a new method and principles for controlling reaction
pathways under suitable conditions. Such a challenge is made
here by focusing on solvent effect on the simple and useful
reaction related to reversible storage of hydrogen. The reversible
hydride-transferred reaction of formic acid (FA)
HCOOH a CO2 + H2
(1)
has been studied in relation to the water gas shift reaction1 via
FA intermediate,2–4 as well as a synthetic route to use carbon
dioxide as a carbon source of organic compounds.5–16
The free energy change accompanying reaction 1 is a very
large negative in the gas phase (∆G ) -43.4 kJ/mol, standard
state ) 1 bar17) so that the reaction is almost irreversible. In
ambient and hot water, on the other hand, the equilibrium can
be controlled by tuning the solvation of the reactants and
products.2–4 The computational free energy analysis4 predicted
that as the temperature is lowered to the ambient in water, the
reactant and product sides may be comparable in stability due
to the hydration effect characterized macroscopically by the high
dielectric constant. In thermally expanded subcritical water, the
CO2 + H2 side is more stable as a consequence of the decreased
hydration. Thus the proton-donating and accepting FA is more
stabilized microscopically through the hydrogen bonding strength-
ened at lower temperatures. Here we show that the stability
preference can be reversed in order to make milder the reaction
conditions of temperature and pressure when the reactant FA
is stabilized with the extremely strong solvation by full charges
of ionic liquid (IL) instead of the weaker one by partial charges
of neutral molecules of water. ILs are organic liquid electrolytes
with a low melting point near room temperature and attract much
attention as green solvent with a very low vapor pressure,18
strong solvation power,19,20 and multiple solvation interactions
useful for a variety of applications.21 In this paper we show
how to control the reversible FA reaction related to rather inert
hydrogen using IL.
The reversible reaction represented by eq 1 is interesting and
useful also for application. The hydride-transferred decomposi-
tion of FA (eq 1) is one of the elementary processes of the water
gas shift reaction via FA intermediate
CO + H2O a HCOOH a CO2 + H2
(2)
which have played a historical role in hydrogen preparation from
water in industries.1–3 The formation and decomposition of FA
through each of the two reversible paths can be controlled by
the solvent, thermodynamic conditions, and/or catalysts, and this
is important for industrial application. By using IL and Ru
catalyst, we can selectively operate the hydride-shift pathway,
which is related to the chemical storage of hydrogen as FA.
Hydrogen, which is abundant on Earth and in the universe,
is a source of water and an ultimately clean fuel for the next
generation without carbon dioxide emission. Hydrogen is
renewable as it can be regenerated by employing more or less
costly chemical processes, such as electrolysis or photolysis of
* To whom correspondence should be addressed, nakahara@
scl.kyoto-u.ac.jp.
10.1021/jp908174s  2010 American Chemical Society
Published on Web 02/18/2010

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Controlling the Equilibrium of Formic Acid
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3511
SCHEME 1: Structure and Preparation of the IL
1,3-Dipropyl-2-methylimidazolium Formate
([ppmIm+][HCOO-])
Figure 1. Sample configuration for the in situ NMR measurements
for the gas phase.
stability of the IL against the reaction studied here. Also it was
found that the methyl group is undesirable as the alkyl on the
nitrogen atom of the imidazole ring, since it was more easily
detached than longer alkyl groups at high temperatures.
Therefore we used the n-propyl group for both of the nitrogen
sites.
2.2. NMR Measurements. The formic acid (FA) decompo-
sition reaction was monitored in the following procedure. The
reaction solution was prepared by mixing FA and 0.5-1 wt %
ruthenium catalyst (RuCl2(PPh3)4) in the IL. The complex was
very slowly (∼12 h) dissolved in the IL at room temperature,
generating a pale yellow solution.25 The solution was sealed in
a quartz tube (2.5 mm inner diameter and 4 cm long, 25% filled
in space by the solution) and heated at each constant temperature
using a standard 5 mm NMR probe loaded on JEOL ECA500w
water22 and biomass reforming,23 and is further obtained as
byproduct in iron and soda industries. However, the low
liquefaction temperature of hydrogen has been a burden to
hydrogen technology; compare the liquefaction temperatures of
hydrogen with those of popular fuels: -253 °C for hydrogen,
-161 °C for methane (natural gas), -25 °C for dimethyl ether,
1 °C for butane, and 30-200 °C for gasoline. Hydrogen is often
compressed by using heavy containers, but the high pressure is
not suitable for handling a highly flammable gas. Therefore,
other clean fuels, such as methanol and dimethyl ether, are now
considered more realistic alternatives to fossil fuels as advocated
by Olah et al.24
Here we show that hydrogen can be chemically stored in the
form of FA very efficiently by using IL. The formation of FA
can be achieved at pressures not far away from atmospheric.
The “transformed hydrogen”, FA, can be a candidate to
overcome the above-mentioned drawbacks inherent in the
hydrogen energy technology. FA liquefies at such a convenient
temperature as 101 °C and thus it serves as a hydrogen stock
in the liquid state.
1
(wide bore). The H and 13C NMR spectra were taken in situ
for the gas phase to quantify H2 and CO2 evolved without
breaking the quartz reactor as can be seen in Figure 1.
Typical 1H and 13C NMR spectra in the gas phase are shown
in Figure 2. As shown, H2 (panel a) and CO2 (panel b) are the
only products detected. The time evolution of the H2 and CO2
concentrations (pressure) in the gas phase is shown in Figure
2c. Although the amounts of H2 and CO2 produced appear to
be unequal, it is due to the difference of the gas solubility in
the IL; in the gas phase more H2 is present than CO2, and vice
versa in the liquid phase. After ∼5 days of reaction time, H2
and CO2 no longer increased, and thus the reaction equilibrium
was perfectly attained. Since the reaction was carried out in a
sealed reactor and the measurements were done in a noninvasive
way by NMR, the concentrations of the chemical species in
the liquid phase, although not directly observed, could be
estimated based on the mass conservation law and Henry’s law
as described below.
2.3. Equilibrium Analysis. The equilibrium constant KIL for
the decarboxylation of FA is defined as
2. Experimental Section
2.1. Materials. The ionic liquid (IL), 1,3-dipropyl-2-meth-
ylimidazolium formate ([ppmIm+][HCOO-]) was synthesized
according to Scheme 1 and Supporting Information. Materials
employed for the synthesis were supplied by Aldrich (2-
methylimidazole), Nacalai (2-bromopropane, sodium hydride,
2-propanol, barium hydroxide octahydrate, acetonitrile), and
Wako (1,2-dimethoxyethane, silver sulfate, formic acid, acetone,
(liq)
c(Hliq)
c
CO2
2
KIL
)
(3)
(liq)
HCOOH
c
where c is the equilibrium molar concentration (mol/L) of the
species specified as the subscript in the phase specified as the
superscript. It is shown below how to determine the liquid-
phase concentrations from the gas-phase NMR observation.
Since the reaction proceeded in a sealed reactor, the mass
balances for carbon and hydrogen were maintained as
methanol) and used without further purification. Intermediate
products synthesized as [ppmIm+][Br-] and [ppmIm+]2[SO4
]
2-
were purified as much as possible by recrystallization using
acetone and acetonitrile, respectively, since the final product
[ppmIm+][HCOO-] could not be recrystallized due to the low
melting point and instability in solvents with carbonyl group
like acetone. 13C-enriched formic acid (99 atom %, 95% solution
in water) was obtained from ISOTEC and used after drying by
boron oxide. A ruthenium complex, RuCl2(PPh3)4 (PPh3 )
triphenylphosphine) was obtained from Aldrich.
1 - F
F
(liq)
HCOOH
(liq)
CO2
(gas)
CO2
(liq)
HCOOH
C0 ) c
+ c
+
c
) c
+
1 - F
The substitution of the hydrogen at position 2 of the imidazole
ring by a methyl group was necessary to improve the thermal
c(Hliq)
+
c(Hgas)
(4)
2
2
F

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3512 J. Phys. Chem. A, Vol. 114, No. 10, 2010
Yasaka et al.
TABLE 1: Equilibrium Conditions for the Formic Acid
Formation and Decomposition*
T (°C) C0 (mol/L) pH (bar) pCO (bar) cHCOOH (mol/L)
(liq)
2
2
40
60
80
2.43
2.24
2.48
3.79
4.2
8.9
11
23
3.5
7.1
7.4
1.9
1.3
1.4
1.4
105
22
* The partial pressures of H2 (pH ) and CO2 (pCO ) in equilibrium
with the formic acid solution in the2IL as measured2by in situ NMR
measurements. The concentrations of formic acid before the reaction
(liq)
and at the equilibrium are denoted by C0 and cHCOOH
,
respectively. pCO tends to be smaller than pH because CO2
2
2
dissolves better in the IL than H2.
40-105 °C together with the concentration of formic acid (FA)
in the liquid phase before the reaction and after the equilibria-
tion. The equilibrium pressures are in the range of 3-23 bar
(0.3-2.3 MPa) and not far from atmospheric. The temperature
effect is positive, and the equilibrium pressures increase by a
factor of ∼2 as the temperature is increased by 40 °C. The
results indicate in turn that we can synthesize FA at such mild
pressures of H2 and CO2 because reaction 1 is reversible. At
room temperature, even atmospheric pressure can induce
formation of FA in the IL. Thus the reaction conditions in the
IL are made dramatically milder than the hydrothermal one.
As shown below, the IL is the most efficient solvent for shifting
the equilibrium of reaction 1 to the FA side.
1
Figure 2. NMR spectra and the time evolution. (a) and (b) H and
13C NMR spectra obtained for the gas phase, respectively. No carbon
monoxide was detected around 185 ppm in 13C NMR spectrum. (c)
Time evolution of the H2 and CO2 pressures as observed by in situ
NMR at 60 °C. The reaction solution was the H13COOH solution in
[ppmIm+][H12COO-] (mole ratio 0.64:1) with 0.75 wt % Ru catalyst.
The CO2 ()12CO2 + 13CO2) concentration plotted here was obtained
by dividing the observed 13CO2 gas-phase concentrations by 0.47 (the
overall 13C enrichment of formic acid determined for the initial reaction
mixture).
The solvent effect on the FA synthesis can be understood by
comparing the equilibrium constant K in eq 3 in various
chemical environments. The equilibrium constant is related to
the free-energy change of the reaction ∆G as
∆G ) -RT ln K
(8)
where C0 is the initial concentration of FA and F is the filling
factor (the volume ratio of the liquid phase to the total reactor,
equal to 0.25).26 The value of F was measured before the reaction
at room temperature, and its change in the course of the reaction
was neglected. The concentrations in the liquid phase (c(liq)) and
the gas phase (c(gas)) are related by the Ostwald constant γ. It is
defined for hydrogen as
where the reference state of the concentration is taken as 1 mol/
L. We can shift the thermodynamic equilibrium constant in a
much wider range by changing the solvent than by changing
temperature only. The small K or large positive ∆G is an
indication of the good solvent for FA synthesis (backward
process of reaction 1). Such good solvents stabilize the product
(HCOOH) and destabilize the reactants (H2 + CO2) to favor
the FA synthesis. When the solvent is an IL, the chemical
species involved in the reaction are solvated by the ions and
subject to various types of solute-solvent interactions.21 In
particular, formic acid is so small and polar that its solvation
by formate ions in the IL is dominated by the Coulombic and/
or hydrogen-bonding interactions. For such apolar solutes as
hydrogen and carbon dioxide, on the other hand, the interactions
are driven mainly by weaker dispersive ones.
γH ) c(Hgas)/cH(liq)
(5)
2
2
2
The value of γH in the IL ([ppmIm+][HCOO]-) was
2
determined in this work, and the results are shown in the
Appendix. By combining eqs 4 and 5, we obtain
c(Hliq) ) c(Hgas)/γH
(5′)
2
2
2
The values of KIL (the equilibrium constant in the IL) are
plotted in Figure 3a. They are compared with those obtained in
the hydrogen-bonding molecular solvent water (Kwater)4 and in
the gas phase (K0) without solvent.17 The chemical potential
of the polar solute FA is considered to decrease (stabilized) in
the sequence
1 - F
F
(liq)
HCOOH
c
) C0 -
c(Hgas) + c(Hliq)
(6)
(
)
2
2
1 - F
(liq)
CO2
(liq)
HCOOH
(gas)
CO2
c
) C0 - c
-
c
(7)
F
vacuum > water > IL
as the solvation power increases in the sequence
vacuum < water < IL
(gas)
which can be evaluated directly from C0, cH (gas), and cCO
.
2
2
3. Results and Discussion
The observed partial pressures of H2 (pH ) and CO2 (pCO ) at
2
2
equilibrium are shown in Table 1 in the temperature range of

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Controlling the Equilibrium of Formic Acid
J. Phys. Chem. A, Vol. 114, No. 10, 2010 3513
be controlled by tuning the temperature as shown in Figure 3b.
It is to be noted that this cyclic process is carbon neutral despite
the emission of CO2 in the second process. The hydrogen storage
efficiency (energy density per weight or volume) can be
drastically improved when FA formed in the IL phase is distilled
and separated in the pure form. The distillation from the IL is
very convenient because the vapor pressure of the IL is
negligibly low. One more important merit of using IL instead
of hot water is that the CO + H2O pathway can be effectively
hindered by the overwhelmingly disfavored stabilization of the
CO2 + H2 side of eq 2; no CO peak in Figure 2b.
Although FA synthesis from H2 and CO2 has been attempted
in many molecular solvents, such as benzene,5 toluene,11
tetrahydrofuran,12,13 acetone,14 alcohols,15 water,8–10,16 and su-
percritical CO2,6,7 bases have often been used to enhance
hydrogen transformation, because the reaction equilibrium is
otherwise in favor of the starting materials (H2 and CO2) in
these solvents. The added base can shift the equilibrium to the
FA side in the following way. When basic aqueous media are
used, for example, carbon dioxide is converted into the
corresponding hydrogen carbonate salt by 1 equiv of base and
then reacts with hydrogen to form the formate salt. Since FA is
by far stronger as an acid than carbon dioxide, the formation of
the formate salt in basic conditions becomes much easier than
the formation of FA in acidic conditions as seen in the free
energy diagram in Figure 4a. The equilibrium control by base,
however, suffers from a drawback; the formate salt thus
synthesized must be treated with 1 equiv of acid to recover FA
in the acidic form. Overall, 1 equiv of base and acid is to be
consumed. This is not advantageous from the environmental
point of view when industrial application is considered. The IL
studied here can shift the FA formation equilibrium as ef-
fectively as a base but, more importantly, without chemical
wastes and energy loss as seen in Figure 4b. This is characteristic
of the equilibrium control by solvation.
As a summary, we have examined the equilibrium of FA with
hydrogen and carbon dioxide in ILs. By comparison of the
equilibrium constants obtained in ILs with those in water and
vacuum (gas phase), it has been revealed that the Coulombic
solvation power of the IL plays a key role in shifting the reaction
equilibrium to the FA side. As a medium for the simple and
important reaction, the IL investigated here is quite different
from polar molecular solvents, whereas their similarity and
difference are conducted by solvatochromic studies using
fluorescent molecules much larger than formic acid. The
formation of FA from hydrogen and carbon dioxide can be a
basis of chemical storage of hydrogen, and in this work we have
shown that FA formation can be achieved most efficiently in
ILs. The use of mild operating temperatures and pressures
without resorting to bases in FA synthesis in ILs is an advantage
for industrial applications. One may consider that FA formation
presented here is a very slow thermal reaction when compared
to the sophisticated laser-induced chemical dynamics on pico-
second or femtosecond time scales, but nevertheless it is
sufficiently faster than the fossil fuel formation in nature
occurring on the geological time scale.
Figure 3. Equilibrium analysis of the formic acid formation and
decomposition. (a) The equilibrium constant is compared in the gas
phase (open circle, data from ref 17), water (open square, ref 4), and
the IL (filled circle, this work) in the temperature range of 0-120 °C.
The standard state is taken by 1 mol/L in each case. (b) The H2 (pH )
2
and CO2 (pCO ) gas pressures in equilibrium with 1 mol/L of formic
2
acid are compared for water and the IL. Also shown are the pH and
pCO in equilibrium with 1 bar (30-40 mmol/L) of formic acid. 2
2
As a consequence, it is expected that the equilibrium shifts to
the FA side and the equilibrium constant in eq 3 becomes
smaller in this sequence. As seen in Figure 3a, the equilibrium
constant is in the order
K0 > Kwater > KIL
The smallest equilibrium constant in the IL implies that the
backward process of reaction 1 (FA formation) can be achieved
most efficiently.
The H2 (pH ) and CO2 gas pressures (pCO ) required for
2
synthesizing 12mol/L of FA through the backward process of
reaction 1 in the IL and water are compared in Figure 3b.27
Also shown are the pressures of H2 and CO2 required for
synthesizing 1 bar (30-40 mmol/L) of gaseous FA by the gas-
phase reaction. We can see the marked difference in the pressure
conditions required for FA synthesis. By using the IL we can
reduce the gas pressure by a factor of 102 in comparison to
water. The factor is 103 when compared to the gas-phase
reaction. The decrease of the reaction temperature in the IL
allows us to decrease the pressure. As can be seen in Figure
3b, FA can be synthesized under ambient temperature and
pressure (pH ,pCO ∼ 1 bar) by using IL.
Acknowledgment. This work was supported by Grants-in-
Aid for Scientific Research (Nos. 18350004, 20550014, and
21300111) from the Japan Society for the Promotion of Science
and by the Grants-in-Aid for Scientific Research on Priority
Areas and the Grants-in-Aid for Scientific Research on Innova-
tive Areas (Nos. 20031015, 20038034, and 20118002), the
Nanoscience Program of the Next-Generation Supercomputing
Project, and Global COE Program, “International Center for
2
2
Interestingly, FA can serve as the chemical tank more
efficiently in the IL than in hot water.2 To store hydrogen, we
pressurize H2 with CO2 around or above the equilibrium pressure
and allow them to be absorbed in the IL.28 When hydrogen is
in need, H2 can be released back from FA by simply removing
the applied pressure. The output hydrogen pressure (pH ) can
2