Kinetic Studies on Formic Acid Dehydrogenation Catalyzed by an Iridium Complex towards Insights into the Catalytic Mechanism of High-Pressure Hydrogen Gas Production

Article abstract of DOI:10.1002/chem.201702969

Kinetic studies on the catalytic reaction mechanism of formic acid (FA) dehydrogenation were performed in the presence of a water-soluble iridium complex bearing a 4,4′-dihydroxy-2,2′-bipyridine ligand. Determination of kinetic isotope effects revealed that a shift of the rate-limiting step at low and high concentrations of FA can be caused by the pH dependence of the reaction steps. The proposed equation for the reaction rate corresponds well with the experimental results concerning the shift phenomena. Towards industrial application in future hydrogen fueling stations, this will able the design of a dehydrogenation system catalyzed by the iridium complex.

Full text of DOI:10.1002/chem.201702969

A Journal of
Accepted Article
Title: Kinetic studies of formic acid dehydrogenation catalyzed by an
iridium complex towards insights into the catalytic mechanism of
high-pressure hydrogen gas production
Authors: Masayuki Iguchi, Heng Zhong, Yuichiro Himeda, and Hajime
Kawanami
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To be cited as: Chem. Eur. J. 10.1002/chem.201702969
Link to VoR: http://dx.doi.org/10.1002/chem.201702969
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Chemistry - A European Journal
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ARTICLE
Kinetic studies of formic acid dehydrogenation catalyzed by an
iridium complex towards insights into the catalytic mechanism of
high-pressure hydrogen gas production
[a]
[a]
[b]
[a]
Masayuki Iguchi, Heng Zhong, Yuichiro Himeda , and Hajime Kawanami*
Abstract: The kinetic studies of the catalytic reaction mechanism of
the formic acid (FA) dehydrogenation were investigated in presence
of the water-soluble iridium complex having 4,4’-dihydroxy-2,2’-
bipyridine ligand. Kinetic isotope effect experiments revealed the shift
of the rate-limiting step at the low and high concentrations of FA can
be caused by the pH dependence of the reaction steps. The proposed
equation of reaction rate corresponds well the experimental results
about the shift phenomena. Towards an industrial application in future
hydrogen fuel filling stations, we will be able to design the
dehydrogenation system catalyzed by the iridium complex.
of H
2
after the reaction using a vapor-liquid phase separation
technique.[ However, most of the studies focused on producing
7]
H
2
at atmospheric pressure despite the advantage of generating
the high-pressure from FA decomposition under mild
temperatures. Few studies have reported the H
H
2
2
production from
FA under high-pressure conditions, even above 10 MPa,[ in
8]
which the reaction proceeds under severe conditions that the
catalyst is exposed to high concentrations of H
proton (H+).
2
2
, CO , FA and
Recently, we demonstrated the high-pressure gas (H
2
:CO =
2
1
:1) generation from FA decomposition over 100 MPa at 80 ºC
with Cp*Ir complexes (Cp* = pentamethylcyclopentadienyl).[7, 9]
The Ir complex with 4,4’-dihydroxy-2,2’-bipyridine (Figure 1)
catalyzed the dehydrogenation of FA with high selectivity at
pressures as high as 123 MPa.[ In our previous work, we
III
Introduction
7]
Molecular hydrogen (H
2
) plays a crucial role as an energy carrier
is a clean energy
reported the FA concentration dependence on the reaction rate at
atmospheric pressure,[10] but the reaction kinetics of catalytic
dehydrogenation of FA has not been studied in detail. The study
of reaction kinetics is crucial to understand the reaction
mechanism for its practical applications. Towards an application
for establishing a sustainable economy.[1]
H
2
and possesses higher energy density on a mass basis compared
to fossil-fuels, but its low-volumetric energy density forces to
compress for portable use such as fuel cell vehicles (≥ 35 MPa).[2]
Due to the gaseous nature, H
on suitable materials or using easy hydrogen generated materials
for transportation of large amount of H . Currently, formic acid
FA) is a stable liquid under ambient conditions, moderately
2
should be stored by the adsorption
2
of H production from FA, some reports are presented about the
development of continuous long-term production system that is
2
[8b, 11]
controlled by the concentration of FA in reaction solution.
(
Herein, the kinetic studies on the FA dehydrogenation are
performed at various concentrations of FA and the catalyst,
temperatures, and pressures. We propose the rate law of catalytic
FA dehydrogenation from the experimental results. Our proposed
equation and kinetic isotope effects present the shift of rate-
limiting step in the reaction depending on the FA concentration.
hazardous and a major product from renewable resources
obtained by fermentation and processing,[3] and is regarded as a
promising hydrogen storage compound.[4] Especially, recent
progresses in the direct production of FA from H and CO without
2 2
any additives or by-products promises the use of FA as an
environmentally benign hydrogen storage compound.[4c, 5]
2
Furthermore, H can be easily released from FA under milder
temperature and higher pressure than other hydrogen storage
chemicals since the FA dehydrogenation has the lower reaction
enthalpy and thermodynamically favorable.
During the compression of H
is consumed to generate the pressure.[6] The pressurization of FA
in its liquid state and subsequent reaction to produce H avoids
the large amount of energy for compressing H . Furthermore,
high-pressure conditions provide easy separation and purification
2
, about 10-15 % of energy content
2
2
Figure 1. Structure of the iridium complex catalyst.
Results and Discussion
[
[
a]
b]
Dr. M. Iguchi, Dr. H. Zhong, Prof. Dr. H. Kawanami,
Research Institute of Chemical Process Technology
National Institute of Advanced Industrial Science and Technology
We carried out the dehydrogenation of FA with the variation of
substrate and catalyst concentrations to examine the effect on the
reaction rate at atmospheric pressure (Figures 2a, 2b and 3). In
presence of the catalyst, the volume of generated gas by the FA
dehydrogenation was increased with time, and then finished when
FA was completely decomposed (Figure 2a). The rate of FA
dehydrogenation calculated in terms of turnover frequency (TOF)
gradually decreased with time from 2440 h-1 (1.21 L/h) and 770 h-
4-2-1 Niagatake, Miyagino-ku, Sendai 983-8551 Japan
E-mail: h-kawanami@aist.go.jp
Dr. Y. Himeda
Research Institute of Energy Frontier
National Institute of Advanced Industrial Science and Technology
1-1-1 Higashi, Tsukuba, Ibaraki 305-8563 Japan
Supporting information for this article is given via a link at the end of
the document.
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Chemistry - A European Journal
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ARTICLE
1
(a)
(
0.38 L/h), respectively, while using 1 and 0.1 mol/L initial FA
1
200
solutions. The initial reaction rate based on the FA concentration
revealed an increasing reaction rate with the initial concentration
up to around 4 mol/L, and then became slower at high
concentration (Figure 2b). The reaction order obtained as less
than 1 from the slope of log-log plot of different FA concentrations
ranging from 0.05 to 0.5 mol/L (Table S1 in the Supporting
Information), whereas, the initial reaction rate was proportional to
the initial concentration of formate that is calculated from the
dissociation equilibrium of FA (Figure S1 in the Supporting
Information). At the high concentrations of FA (≥ 2 mol/L) and
formate (≥ 19 mmol/L), the reaction rate was almost identical on
its concentration. In order to identify the reaction order in terms of
catalyst concentration, the reaction was performed at different
catalyst concentrations from 0.05 to 0.4 mmol/L. The reaction rate
follows a first order with respect to the initial concentration of
catalyst at various FA concentrations (Figure 3 and Table S2 in
the Supporting Information). This suggests that the monomeric
species of catalyst involved in the reaction, and it scarcely
associate to form the dimeric species of catalyst once it formed.
Therefore, the FA dehydrogenation follows second order kinetics
in terms of substrate and catalyst concentrations for the low
concentration of FA (≤ 0.5 mol/L). This observation corresponds
well with the our reported mechanism based on density functional
theory calculations.[12] At the high FA concentration (≥ 2 mol/L), a
first order reaction was observed for the catalyst concentration.
The kinetic studies on the FA dehydrogenation at high
900
600
300
0
0
20
40
60
80
100
t /min
(b)
1
000
1
00
1
0
0.01
0.1
1
10
Initial [FA] /(mol L-1
)
pressures were performed in the presence of product gas (H
2 2
:CO
=
1:1). Previously, the same complex used in this work, catalyzed
Figure 2. (a) Time course of generated gas volume by the FA dehydrogenation
under different FA concentrations: (blue circle; ○) 0.1 mol/L FA (50 mL), (red
square; □) 1 mol/L FA (20 mL), and (b) concentration dependence of initial rate
of the dehydrogenation of FA varied from 0.05 mol/L to 8 mol/L: (○) this work,
(●) previous work,[10] (solid line) Eqs.1 and 2, (dash line) observed rate law.
Reaction conditions: 60 ºC, 0.1 MPa (atmospheric pressure), catalyst (10 mol
for (a) and 0.2 mmol/L for (b)). The reaction rate is averaged over initial several
minutes.
the production of methanol by the disproportionation of FA at high
pressures.[13] The reaction rate of methanol production was much
slower than that of FA decomposition in the absence of strong
acids at high temperatures, therefore, this work focused on the
dehydrogenation of FA. At the constant pressure of 10 MPa, the
gas generation rate strongly depends on the concentration of FA.
For instance, the reaction rate decreased with time from the TOF
-1
value of 4420 h (3.76 L/h) using 4 mol/L initial FA solution (Figure
S2a in the Supporting Information). When 8 mol/L of initial FA
solution was used under the same reaction condition, the reaction
rate remains almost constant with the TOF value of 5130 h-1 (4.36
L/h) for 1 hour and then gradually decreased with time (Figure
S2b in the Supporting Information). The similar behavior observed
for the initial FA concentration of 16 mol/L. Hence, the change of
reaction rate with time can be related to the initial FA
concentration. To obtain 10 MPa of gas pressure, FA was
consumed, and the estimated FA concentrations were 7 and 3
mol/L using the initial FA concentrations of 8 and 4 mol/L,
respectively. The reaction rate was dropped after reaching the FA
concentration of around 3 mol/L, while it remains constant at the
high FA concentration. Consequently, the reaction rate at high
pressure has similar dependence on the FA concentration at
atmospheric pressure as shown in Figure 2.
1
0000
1000
100
0.01
0.1
1
Initial [catalyst] /(mmol L-1
)
Subsequently, we examined the effect of catalyst concentration
to the rate of FA dehydrogenation in the presence of 10 MPa
product gas at different temperatures (Figure 3). The reaction rate
linearly increased with the initial concentrations of catalyst from
Figure 3. Catalyst concentration versus rate of the FA dehydrogenation: (circle:
) 0.1 MPa and 80 ºC, (triangle: ■) 10 MPa and 80 ºC, (triangle: ▲) 10 MPa and
0 ºC, (lines) observed rate law. Reaction conditions: FA (8 mol/L, 40 mL),
catalyst (0.05-1.6 mmol/L). The reaction rate is averaged over several hours.
○
6
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Chemistry - A European Journal
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ARTICLE
0
.1 to 1.6 mmol/L and follows a first order kinetics with respect to
Information). Hence, the gas pressure does not affect the reaction
rate significantly, especially at the low-pressure range.
The reaction rate largely decreased above the gas pressure of
10 MPa. Since the pressure barely affect the reaction mechanism,
the decrease in the reaction rate with increasing pressure might
be attributed to that collisions of reactants were prevented at the
high pressure.
Based on our reported mechanism (Scheme 1),[12] we derived
the rate law for the catalytic dehydrogenation of FA at
atmospheric pressure. The reported pathway gives the reaction
rate (r) as follows (Derivations of rate law in the Supporting
Information):
the initial concentration of catalyst at temperatures of 60 and 80
ºC, corresponds to the atmospheric pressure. Under the applied
conditions, we determine the activation energy (E ) of the reaction
a
by the Arrhenius plot over the range of temperature from 50 to 80
ºC (Figure 4 and Table S3 in the Supporting Information).
Interestingly, the calculated E
reaction at 10 MPa (E =70.1 kJ/mol) and at atmospheric pressure
= 71.7 kJ/mol). This implies the gas pressure have negligible
a
values are almost same for the
a
(E
a
effect on the reaction mechanism of catalytic dehydrogenation of
FA.
When the gas pressure was applied over the range from 0.1 to
4
0 MPa, the rate of FA dehydrogenation decreased with an

(1)
increase in the gas pressure (Figure 5). A reaction order of -0.1
was obtained from the slope of a log-log plot of pressure versus
the reaction rate for less than 5 MPa (Table S4 in the Supporting
k1[HCO2 ][catalyst]0
r 
k1
k2

k1
K
a

1

[HCO2 ] 

k3 [H ]
2


 K  K  4K ([HCO H]  [H ]
a a a 2 0 2
(2)
[
HCO2 ]  [H ] 
2
1
1
9
7
where k
is the dissociation constant for FA, and the subscript of 0 is the
initial value. The initial reaction rates could be represented by the
i
is the reaction rate constant for step i in the Scheme 1,
K
a
8
.
equations 1 and 2 with parameters k
.726×106 h-1, and k
1
= 8.554×10 L/(mol h), k
2
=
.
3
3
= 1.407×107 L/(mol h) within an average
relative deviation of 2.3 % (Figure 2b). Furthermore, the proposed
equation well-described the measured results of time course of
the gas generation volume using numerical integrations and the
pH dependence on the reaction rate (Figure 2a and Figure S3 in
5
0
the Supporting Information). The k
reaction rate at high and low concentrations of FA, respectively
Figure S4 in the Supporting Information). The proposed equation
2 3
and k values affected the
.0027
0.0029
0.0031
/T /(K-1
)
0.0033
1
(
shows the shift of rate-limiting step in the reaction depending on
the FA concentration.
To give further insight into the reaction mechanism, we
performed the kinetic isotope effect (KIE) measurements (Table
Figure 4. Arrhenius plot for the FA dehydrogenation: (○) 0.1 MPa, (▲) 10 MPa;
line) Arrhenius equation. Reaction conditions: 40-80 ºC, FA (8-16 mol/L, 40 mL),
catalyst (0.4 mmol/L). The TOF value of 10 MPa is averaged over several hours,
and that of 0.1 MPa is taken from previous work.[10]
(
1
). The reaction rate was compared with deuterated (DCO
non-deuterated (HCO H) substrates in water (H O) and in D
The KIE value with DCO D in high concentration of 8.0 mol/L,
replacing FA was higher than that with D O as a replacement of
water under the high pressure conditions of 10 MPa. Thus,
DCO D is involved in the rate-limiting step rather than D O when
the high concentration of substrate was used (8 mol/L). The
release of CO by -hydrogen elimination from the formate
intermediate (2, in step 2 of Scheme 1) determines the reaction
rate rather than the H generation from the hydride intermediate
3, in step 3 of Scheme 1).
The KIE value of atmospheric pressure was similar to that of
high pressures, but the value with DCO D in replace of FA at
2
D) and
2
2
2
O.
2
1
0000
2
2
2
2
1000
2
(
2
1
00
atmospheric pressure was much higher than that at high
pressures. The decrease in the KIE value at high pressures might
0.1
1
10
100
p /MPa
be attributed to the H/D exchange between DCO
2
D and water
(H
2
O). These results indicate that at the high concentration of FA
Figure 5. Pressure effect on rate of the FA dehydrogenation using different
catalyst concentrations: (■) 0.2 mmol/L catalyst, (▲) 0.4 mmol/L catalyst, (lines)
the rate-limiting step is the release of CO2 from the formate
intermediate (2), and which has independent on the gas pressure.
observed rate law. Reaction conditions: 80 ºC, 0.1-40 MPa (H
8-16 mol/L, 40 mL), catalyst (0.4 mmol/L).
2 2
:CO = 1:1), FA
In contrast, D
2
O is involved in the rate-limiting step rather than
(
DCO
2
H at the low concentration of FA (1 mol/L).[14] The release of
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Chemistry - A European Journal
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ARTICLE
Table 1. Kinetic isotope effect on the catalytic dehydrogenation of FA.[a]
Entry
Substrate & Solvent
10 MPa, 8 mol/L
0.1 MPa, 8 mol/L
10 MPa, 1 mol/L
0.1 MPa, 1
mol/L[
d]
Initial TOF
/
KIE[c]
Initial TOF
/(1/h)
KIE[c]
Initial TOF
/(1/h)
KIE[c]
KIE[c]
(1/h)[
b]
[b]
[b]
1
2
HCO H
H
2
O
5950
-
10830
-
3620
-
-
2
3
HCO
2
2
H
D
H
2
O
O
3530
2370
1.7
2.5
8020
1380
1.4
7.8
1480
2270
2.4
1.6
2.1
1.4
DCO
D
2
[
3
a] Reaction conditions: 80 ºC, catalyst (0.2 mmol/L, 40 mL). [b] Average TOF value in the initial 30 min. [c] KIE = TOF (entry 1) / TOF (entry n) where n = 2 and
. [d] Data is taken from Wang et al.[
14]
H
2
from the hydride intermediate (3) was proposed for the rate-
We investigated the rate law and catalytic mechanism of the FA
dehydrogenation using the water-soluble iridium complex with the
variations of FA and catalyst concentrations, temperature and
pressure. The proposed equation of reaction rate can be applied
to other catalysts, and will help to get insights into the
complicating behavior of catalytic activity on the pH dependence
in reaction solution. This work can be applied for FA as the
limiting step (step 3).[12, 14] Thus, the shift of rate-limiting step in
the reaction depending on the FA concentration was observed
from the KIE measurements, which is consistent with the
calculations by our proposed rate law. The shift concentration of
FA was obtained from our equation as 0.26 mol/L, which is too
low as expected from the KIE experiments. The underestimated
value obtained from our equation might be associated with the
calculation of formate concentration. The rate-limiting step shift
for low and high concentrations of FA is probably due to the pH
dependence (pH is 1.8 at 1.0 mol/L, and pH is 0.8 at 8 mol/L of
hydrogen storage compound to feed the high-pressure H
applications of fuel cell vehicles.
2
in
[
12,
FA, respectively) of the reaction steps of 2 and 3 in Scheme 1.
Experimental Section
1
5]
As an increase in the FA concentration, the pH value of solution
decreases, which might cause to increase the activation energy
for the step 2 and to decrease the activation energy for the step
General: The iridium complex catalyst, [Cp*IrIII(4,4’-dihydroxy-2,2’-
[
16]
bipyridine)(H
acid (FA, >99.0 %), deuterium oxide (D
deuterated formic acid (DCO D, >97.3 %, ≥99.5 atom % D) were used as
received from Wako Pure Chemical Industries, Ltd. for FA, and Merck
KGaA for D O and DCO D. Deionized water was purified through filtration
system (EMD Millipore Corp., ZFSQ240P4) and water distillation system
Toyo Roshi Kaisha, Ltd., GS-590). All reagents were degassed by
2
O)][SO
4
] was prepared according to the literature. Formic
3, but the total energy of each step hardly changes with the pH
2
O, ≥99.8 atom % D), and
2
value. Consequently, the rate-limiting step varies from the step 3
for the low FA concentration to the step 2 for the high FA
concentration.
2
2
(
bubbling helium gas before use.
Low HCO H
Ir－ OH2]2+
2
[
H2
conc.
1
HCO2-
A wet gas meter (Shinagawa Co., Ltd., W-NK-0.5A) was used to measure
the volume of generated gas. The measured volume was converted to the
value at standard ambient temperature and pressure using the ideal gas
law. A GC-TCD system (Agilent Technologies, 3000A Micro GC) was
HCO2-
Step 3
Ir =
Step 1
2 2
used to determine the concentrations of H , CO , and CO in the generated
HCO H
2
gas. The FA concentration in the reaction solution was measured by a
HPLC system on an ion exclusion column (Showa Denko K. K., KC-811)
with 20 mmol/L phosphoric acid aqueous solution and a UV detector at
210 nm. The pH value of reaction solution was determined with a glass
electrode (DKK-TOA Corp., HM-25R).
H O
2
H O
2
+
[
Ir－ H]+
3
[Ir－ O CH]
2
2
Step 2
High HCO H conc.
2
A detail description of the apparatus used for the dehydrogenation of FA
was presented in our previous work.[7] The catalyst and the aqueous
solution of FA (40 mL) were loaded into a reactor (50 mL) at room
temperature. The mixture was stirred and heated to the desired
temperature. Pressure of generated gas was controlled by a back-
pressure regulator (JASCO Corp., BP-2080) within 0.1 MPa. After
reaching the desired pressure, the gas exhausted from the regulator was
measured by the gas meter. After completing the reaction, the reactor was
cooled and then depressurized to atmospheric pressure. The FA
concentration was calculated from the total volume of generated gas after
reaction and the exhausted gas volume at desired pressure. A TOF value
was determined from the average rate of generated gas in consideration
CO2
Scheme 1. Reaction mechanism and the rate-limiting steps for low and high
concentrations of FA in the catalytic dehydrogenation of FA.
Conclusions
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Chemistry - A European Journal
10.1002/chem.201702969
ARTICLE
of the volume change with the reaction. The calculations of TOF value and
adjustable parameters in the equations were described in the Supporting
Information. The uncertainties were estimated to be below 5 % for the
reaction rate of above 500 mmol/L/h and 8 % for that of less than the value.
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Chemistry - A European Journal
10.1002/chem.201702969
ARTICLE
Entry for the Table of Contents
Layout 1:
ARTICLE
-
2
H O
HCO
2
Rate-limiting step shift:
The kinetic studies of the
catalytic reaction
Masayuki Iguchi, Heng Zhong,
Yuichiro Himeda, and Hajime
Kawanami*
mechanism of the formic
acid dehydrogenation by
the iridium complex were
investigated and the shift
of the rate-limiting step at
the low and high
Step 1
Page No. – Page No.
]2+
[Ir－ O CH]
+
[
Ir－ OH
2
2
Kinetic studies of formic acid
dehydrogenation catalyzed by an
iridium complex towards insights
into the catalytic mechanism of
high-pressure hydrogen gas
production
Rate-Limiting
Step
2
HCO H
Step 3
Shift
Step 2
2
H O
concentrations of formic
acid was observed.
Low
Conc. FA
High
Conc. FA
Ir =
H
2
Ir－ H]+
CO
[
2
-
HCO
2
This article is protected by copyright. All rights reserved.