Highly efficient dehydrogenation of formic acid in aqueous solution catalysed by an easily available water-soluble iridium(III) dihydride

Article abstract of DOI:10.1039/c6dt01695b

The new water-soluble cis-mer-[IrH₂Cl(mtppms)₃] (mtppms = monosulfonated triphenylphosphine) was employed as a catalyst for selective decomposition of formic acid to H₂ + CO₂ in aqueous solution at T = 30-100 °C. The easily synthesized compound showed high catalytic activity (TOF up to 298 000 h^-1) and could be reused several times with no loss of activity (total TON = 67 650). A sharp maximum in the reaction rate was observed at pH = 3.75; its coincidence with the pK_a of formic acid shows that both H⁺ or HCOOH and HCOO^- play important roles in the reaction mechanism.

Full text of DOI:10.1039/c6dt01695b

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DOI: 10.1039/C6DT01695B
Highly efficient dehydrogenation of formic acid in aqueous solution
catalysed by an easily available water-soluble iridium(III) dihydride.
a
b
a
b
a,b
G. Papp, *G. Ölveti, H. Horváth, Á. Kathó and F. Joó *
a
Hungarian Academy of Sciences, Research Group of Homogeneous Catalysis and Reaction
Mechanisms, P.O.Box 400, Debrecen, H-4002 Hungary
b
University of Debrecen, Department of Physical Chemistry, P.O.Box 400, Debrecen, H-4002
Hungary
Corresponding authors:
Gábor Papp, e-mail: papp.gabor@science.unideb.hu
Ferenc Joó, e-mail:joo.ferenc@science.unideb.hu

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DOI: 10.1039/C6DT01695B
Abstract
2 3
The new water-soluble cis-mer-[IrH Cl(mtppms) ] (mtppms = monosulfonated
triphenylphosphine) was employed as catalyst for selective decomposition of formic acid to
+ CO in aqueous solutions at T=30-100°C. The easily synthesized compound showed
H
2
2
-1
high catalytic activity (up to TOF=298 000 h ) and could be reused several times with no loss
of activity (total TON=67 650). A sharp maximum in the reaction rate was observed at
+
pH=3.75; its coincidence with the pK
a
of formic acid shows that both H or HCOOH and
-
HCOO play important role in the reaction mechanism.

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DOI: 10.1039/C6DT01695B
Introduction
Formic acid (FA) is regarded as a potential hydrogen storage material since its selective
dehydrogenation yields a 1:1 mixture of H and CO (eq. 1). Hydrogen can be used for
generation of electricity in fuel cells where it combines with O and produces water as the
sole product with no harm to the environment. Ideally, FA can be produced by hydrogenation
2
2
2
of CO
2
using hydrogen from renewable sources such as e.g. from water via photochemical
§
water splitting, and this allows carbon neutral utilization of renewable H
2
. However,
hydrogen generation should be highly selective in order to prevent formation of CO (eq. 2)
and to avoid poisoning the fuel cell electrodes.
HCOOH ⇌ H
2
+ CO
2
(1)
(2)
HCOOH ⇌ H
2
O + CO
1
Selective dehydrogenation of formic acid has been in the focus of intensive research and was
2
achieved by both heterogeneous and homogeneous catalysis. High catalytic activities were
3
4
5
observed with soluble complexes of ruthenium, rhodium and iridium and many of these
catalysts proved active for long periods of time leading to extremely high turnover numbers
(TON=total number of catalytic cycles for one molecule of catalyst). Catalysts based on
6
7
abundant and cheap metals such as Fe and Ni have also been discovered. For practical
applications it is important to avoid the use of any additives such as (volatile) amines or
organic co-solvents.
5
Iridium complexes play aprominent role in catalytic dehydrogenation of FA. Himeda
and coworkers investigated a family of Ir-complexes with various hydroxybipyridine and
8
9
hydroxybipyrimidine as well as with various azole and imidazoline ligands. For example,
2
+
with [Cp*Ir(H
2
O)L] (Cp*=pentamethylcyclopentadienyl, L=6-(4,5-Dihydro-1H-imidazol-2-
generation (TOF=TON/reaction time) was
22 000 h , while with the [Cp*Ir(4DHBP)(H O)][SO ] catalyst (4DHBP=4,4’-dihydroxy-
yl)pyrimidine-2,4-diol) the rate of H
2
-1 10
3
2
4

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2
,2’-bipyridine) the pressure reached as high as123 MPa. Another important achievement
DOI: 10.1039/C6DT01695B
was made by Li and coworkers, who reported unprecedently active catalysts based on Ir(I)
and N,N’-diimine ligands. For example, the [Cp*IrCl(2,2’-bi-1,4,5,6-tetrahydropyrimidine)]Cl
-1
complex showed an initial TOF of 487 500 h at 90°C (the highest presently known) and a
1
1
TON of 2 400 000 (at 80°C). Both the Himeda and Li groups found a maximum in FA
10-11
a
dehydrogenation rate at about pH 3.7 corresponding to the pK of HCOOH in water.
Nozaki et al reported the use of an Ir-PNP complex as an efficient catalyst for
1
2
dehydrogenation of FA in the presence of base.
Homogeneous catalysis of dehydrogenation of FA by several Pt(II), Ru(II), Rh(I),
1
3
Ir(I), and Ir(III) complexes in refluxing acetic acid was first reported by Coffey in 1967. The
-
1
most active catalysts were [IrH
3
(PPh
TOF=1187 h ). Despite these findings, no other studies of tertiary monophosphine
complexes of Ir(I) or Ir(III) have been reported for FA dehydrogenation.
3 3 2 3 3
) ] (TOF=8890 h , TON˃11 000) and [IrH Cl(PPh ) ]
-
1
(
14
In the course of our research on aqueous organometallic catalysis and on chemical
1
5
hydrogen storage we have discovered that the easily synthesized water-soluble cis-mer-
[
IrH Cl(mtppms) ] (1) (mtppms=monosulfonated triphenylphosphine Na-salt or 3-
2
3
diphenylphosphinobenzenesulfonic acid Na-salt) catalyzed the dehydrogenation of aqueous
FA with exceedingly high activity and selectivity in the 30-100°C temperature range. The
catalyst is stable for prolonged use and pressures up to 140 bar (H
short reaction times. Details are described below.
2 2
+CO ) can be produced in
Results and discussion
cis-mer-[IrH
IrCl ×3H O and excess mtppms in refluxing ethanol.The complex was characterized by
elemental analysis and its structure was assigned on the basis of H-, C-, and P-NMR, ESI-
2 3
Cl(mtppms) ] (1) was obtained as a pale yellow solid in the reaction of
3
2
1
13
31

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DOI: 10.1039/C6DT01695B
MS and IR spectroscopies. (For details see ESI.) Specifically, H-NMR showed two triplets of
3
1
doublets of doublets in the hydride region at δ= -27.71 ppm and δ= -11.10 ppm, while the P
NMR spectrum showed a doublet at δ= 8.21 ppm and a triplet at δ= -0.76 ppm with the
appropriate coupling constants (Fig. S1-S3). Terminal hydride stretching frequencies in the IR
-1
spectrum were observed at 2249 and 2145 cm (Fig. S4). These features are in accord with
the suggested cis-mer configuration. A related complex, [IrH Cl(PPh ](water-insoluble) was
first synthesized by Vaska, and the effect of the mtppms vs PPh ligand is shown by the ν(Ir-
H) values which are larger by 34 and 35 cm , respectively, than those reported for
IrH Cl(PPh ]. 1 is freely soluble in methanol, water and in aqueous FA solutions, poorly
soluble in ethanol and insoluble in apolar organic solvents.
2
3 3
)
1
6
3
-1
[
2
3 3
)
2 3
Scheme 1. cis-mer-[IrH Cl(mtppms) ] (1)
cis-mer-[IrH
+ CO = 1:1 with high activity (the reaction mixtures contained 0.098-0.500 M HCOONa).
Gas chromatographic measurements showed the CO content of the resulting gas ˂ 10 ppm
allowing the use of the produced H in fuel cells. Gas evolution could be conveniently
2 3
Cl(mtppms) ] (1) catalysed the dehydrogenation of aqueous FA selectively to
H
2
2
2
followed in the 30-70°C temperature range by using a thermostated gas burette. The reactions
started with no induction period and proceeded with a steady rate (Fig. 1) until 100%
decomposition of FA (established by HPLC); the final solutions contained only the
-1
unchanged HCOONa. Initial turnover frequencies of 437-17813 h were determined in this
temperature range (Fig. S5) and an Arrhenius-plot of these data yielded an apparent activation

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a
energy, E =79.6 kJ/mol (Fig. S6), in good agreement with the 72.1 kJ/mol determined by Li et
DOI: 10.1039/C6DT01695B
1
1
al for a tetrahydropyrimidine-ligated Ir catalyst which points to a similar rate determining
step in the reaction mechanism, probably the C-H bond rupture in FA.
2 3
Figure 1. Catalytic dehydrogenation of formic acid catalysed by cis-mer-[IrH Cl(mtppms) ]
-6
-3
-4
(
1) at various temperatures. nIr = 4.9×10 mol; nHCOOH = 3.58×10 mol; nHCOONa = 4.9×10
mol; V = 5.0 mL.
The rate of FA dehydrogenation varied linearly with catalyst concentration (Fig. S7) and
showed saturation with increasing FA concentration (Fig. S8). Addition of mtppms decreased
the reaction rate exponentially (Fig. S9). Variation of the pH of the reaction mixture in the
2.0-8.5 range led to a sharp maximum of the rate at pH=3.75 (Fig. 2). Similar pH effect was
1
0-11
observed by Himeda and Li, too.
Figure 2 also shows the molar distribution of HCOOH
-
17
and HCOO as a function of pH (calculated with pK
a
=3.797 at 60°C ). At the rate maximum
-
HCOOH and HCOO are present in equal concentrations showing that both formate anion and
+
undissociated HCOOH and/or H play important role in the reaction.

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-6
Figure 2. Effect of pH on FA dehydrogenation catalysed by 1 (red dots; nIr = 9.9×10 mol;
-3
n
HCOOH+ HCOONa = 1.00×10 mol; V = 5.0 mL; T = 60°C), and the calculated molar distribution
-
of HCOOH (full line) and HCOO (broken line), respectively.
Catalysis of FA dehydrogenation by 1 was also studied in a 100 mL Parr-reactor by following
the resulting increase in pressure. Time course of FA dehydrogenation is shown on Fig S10.
2 2
After 12 min, HPLC determination showed full conversion of FA to H and CO resulting in
7
0.9 bar total pressure in the reactor, meaning that the reaction proceeded with a
-1
TOF = 298 000 h . Accordingly, cis-mer-[IrH
2 3
Cl(mtppms) ] is one of the most active
1
0
catalysts for FA dehydrogenation, together with the catalysts reported by Himeda et al.
1
1
-1
(
TOF = 332 000 h-1) and by Li et al. (TOF = 487 500 h ). The final pressures were
proportional to the HCOOH concentration and a pressure of 140 bar was observed after 100%
conversion of 0.25 mol FA (Fig. S11). However, with increasing [HCOOH] (but constant
[
HCOONa]) the reaction rate decreased considerably (Fig. S12), accompanied by a
calculated) pH changed from 2.9 to 2.4. In agreement with the data of Figure 2, this pH
(
change accounts fully for the observed decrease of the reaction rate (Fig. S13) and,
consequently, no self-inhibition by HCOOH should be considered.

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Figure 3. Repeated use of the same solution of 1 for catalysis of FA dehydrogenation (nIr
=
-6
-2
-1
9
.8x10 mol; nHCOONa = 5x10 mol; nHCOOH = 1.33x10 mol/cycle; V = 20.0 mL; T = 100°C).
Repeated use of the catalyst is demonstrated by Figure 3. In these experiments a new batch of
FA was added to the reaction mixture after the previous one was dehydrogenated with full
conversion. At 100°C, 80-81 bar pressure was produced in each cycle; these conditions did
not lead to decomposition of the catalyst as shown by the constant initial TOFs (121 000 ±
-1
-8
6
100 h ) in the five subsequent cycles. Furthermore, in a closed system 8.81xl0 mol 1
-2
dehydrogenated 5.94x10 mol FA (total TON 674 000) in 40 h at 115°C, demonstrating the
2 3
long-term stability of cis-mer-[IrH Cl(mtppms) ] (see ESI for details).
Conclusion
We reported here that cis-mer-[IrH
2
Cl(mtppms)
TOF=298 000 h at 100°C) for selective dehydrogenation of aqueous formic acid to H
CO in the presence of small amounts of HCOONa. The highly water-soluble complex is
easily sythesized from IrCl ×3H O and monosulfonated triphenylphosphine. Final pressures
3
] (1) is an exceedingly active catalyst
-1
(
2
and
2
3
2
up to 140 bar were achieved in short reaction times and the high catalyst activity did not
change in five consecutive cycles. These findings encourage further studies on the use of 1
and related complexes in chemical hydrogen storage.

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Acknowledgement
The authors are grateful to COST Action CM1205, Catalytic Routines for Small Molecule
Activation (CARISMA) for travel grants to Action Meetings. This research was supported by
the National Research, Development and Innovation Office of Hungary through the grants
OTKA-NKFIH K101372 and NKFI-1PD115535. Support by the Gábor Baross program,
grant REG_EA_09-1-2009-0028 (LCMS studies) is also acknowledged.
Notes and references
§
Distinction should be made between the use of formic acid as a replenishable fuel (H
2
source) and as a hydrogen storage material in a hydrogen battery. While in both devices H
2
is
generated by dehydrogenation of FA, a hydrogen battery also requires hydrogenation of CO
2
1
8
back to formic acid in the same device. Presently available methods for additive-free
hydrogenation of CO to HCOOH are either slow, and/or yield low HCOOH concentrations
and/or require organic solvents. Therefore the known hydrogen batteries use aqueous
2
1
5,19
20
solutions of formate salts
(instead of HCOOH) or other hydrogen storage materials.
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Highly efficient dehydrogenation of formic acid in aqueous solution catalysed by an
easily available water-soluble iridium(III) dihydride
Gábor Papp, Gábor Ölveti, Henrietta Horváth, Ágnes Kathó, and Ferenc Joó
Water-soluble cis-mer-[IrH
2
3
Cl(mtppms) ] selectively dehydrogenated formic acid with TOF
-1
2
98000 h , final pressure of 140 bar, and a TONmax of 674 000.