Catalytic hydrogen production from paraformaldehyde and water using an organoiridium complex

Article abstract of DOI:10.1039/c4cc06581f

Paraformaldehyde was decomposed using an organoiridium complex (1, [Ir^III(Cp?)(4-(1H-pyrazol-1-yl-κN²)benzoic acid-κC³)(H₂O)]₂SO₄) as a catalyst in water to produce H₂ and CO₂ in a 2:1 molar ratio at room temperature. The catalytic cycle is composed of the reduction of 1 by paraformaldehyde under basic conditions to produce formic acid and the hydride complex, which reacts with protons to produce H₂. Formic acid further decomposed to H₂ and CO₂ with 1.

Full text of DOI:10.1039/c4cc06581f

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Catalytic hydrogen production from
paraformaldehyde and water using an
Cite this:DOI: 10.1039/c4cc06581f
organoiridium complex†
Received 21st August 2014,
Tomoyoshi Suenobu,* Yusuke Isaka, Satoshi Shibata and Shunichi Fukuzumi*
Accepted 5th December 2014
DOI: 10.1039/c4cc06581f
www.rsc.org/chemcomm
Paraformaldehyde was decomposed using an organoiridium complex solid H2 carrier, which has a higher energy density (6.7%) than
(1, [IrIII(Cp*)(4-(1H-pyrazol-1-yl-jN2)benzoic acid-jC3)(H2O)]2SO4) as HCOOH (4.4%). Prechtl and coworkers recently reported selective
a catalyst in water to produce H2 and CO2 in a 2 :1 molar ratio at hydrogen production from paraformaldehyde and formaldehyde
room temperature. The catalytic cycle is composed of the reduction using a Ru catalyst [(Ru(p-cymene))2(m-Cl)2Cl2].15 However, a
of 1 by paraformaldehyde under basic conditions to produce formic relatively high temperature (95 1C) was required for the efficient
acid and the hydride complex, which reacts with protons to produce hydrogen production from paraformaldehyde.15 To the best of
H2. Formic acid further decomposed to H2 and CO2 with 1.
our knowledge, this is the only example of hydrogen production
from paraformaldehyde.
Paraformaldehyde is the polymerisation product of formalde-
We report herein the catalytic decomposition of paraform-
hyde with a molecular formula of HO(CH2O)nH (n = 8–100).1 aldehyde to H2 and CO2 (eqn (4)) with a water-soluble iridium
Paraformaldehyde forms slowly in aqueous formaldehyde aqua complex [IrIII(Cp*)(4-(1H-pyrazol-1-yl-kN2)benzoic acid-kC3)-
solutions as a white precipitate. Formalin actually contains (H2O)]2SO4 ([1]2ÁSO4, Cp* = Z5-pentamethylcyclopentadienyl) at
very little monomeric formaldehyde, most of which forms short room temperature.
chains of polyformaldehyde.2 Paraformaldehyde has been
Synthesis and characterisation of 1 were performed as
reported to be synthesised by electrocatalytic reduction of reported previously (see ESI†).14,16 The carboxylic acid form 1
CO2 and water.3–5 Paraformaldehyde can be depolymerised to is deprotonated to give the carboxylate form 2 with pKa = 4.0
formaldehyde [eqn (1)], which is subsequently hydrated to form and the aqua ligand of 2 is further deprotonated to the hydroxo
a geminal diol, i.e., methanediol [eqn (2)] in water under basic complex (3) as shown in Scheme 1.14,16 Under an N2 atmosphere
conditions.6,7
at pH 11 in the presence of a catalytic amount of 2, paraformalde-
hyde decomposed to produce H2 and CO2 with a 2 : 1 molar ratio
as shown in Fig. 1 as expected from eqn (4). The turnover number
(TON) based on the Ir catalyst (5.0 mM) at 14 h was 21 at 298 K.
When the catalyst concentration was decreased to be one-fifth,
i.e., 1.0 mM, the TON remains almost unchanged (24) at 298 K as
shown in Fig. S1a in ESI.† The TON increased to 51 at 333 K as
shown in Fig. S1b (ESI†). The detailed experimental procedure is
described in the Experimental section in ESI.†
HO(CH2O)nH - nHCHO + H2O
HCHO + H2O - H2C(OH)2
(1)
(2)
Formic acid, which is the two-electron oxidation product of
formaldehyde, has been regarded as a liquid H2 carrier because
of efficient generation of H2 from HCOOH [eqn (3)] using an
HCOOH - H2 + CO2
(3)
It should be noted that 1 is converted to 3 at pH 11
(Scheme 1). The rate of production of H2 decreases with decreasing
pH as shown in Fig. 2. No production of H2 from paraformaldehyde
appropriate catalyst under normal pressure at ambient tem-
perature.8–14 If H2 can be generated from paraformaldehyde
(eqn (4)), paraformaldehyde would be regarded as a convenient
HO(CH2O)nH + (n À 1)H2O - 2nH2 + nCO2
(4)
Department of Material and Life Science Graduate School of Engineering,
Osaka University and ALCA, Japan Science and Technology Agency (JST),
2-1 Yamada-oka, Suita, Osaka 565-0871, Japan.
E-mail: fukuzumi@chem.eng.osaka-u.ac.jp; Fax: +81-6-6879-7370; Tel: +81-6-6879-7368
† Electronic supplementary information (ESI) available: Experimental and kinetic
details. See DOI: 10.1039/c4cc06581f
Scheme 1 Deprotonation equilibrium of 1 to 2 and 3.
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Fig. 1 Time course of catalytic production of H2 (black line) and CO2
(red line) from paraformaldehyde (2.0 mg, 66.7 mmol) with 3 (5.0 mM) in an
aqueous solution (1.0 mL at pH 11) at 298 K.
Fig. 3 UV-visible absorption spectra of (a) an aqueous solution of 2
(25 mM, 2.0 mL, pH 7) before (black line) and after (red line) addition of
paraformaldehyde (31 mM) and (b) the resulting solution before (black line)
and after (blue line) addition of an aliquot of conc. NaOH solution (1 M) for
the adjustment of pH at 11.
Fig. 2 pH dependence of catalytic production of H2 from paraformaldehyde
(2.0 mg, 66.7 mmol) with 2 or 3 (5.0 mM) in an aqueous solution (1.0 mL) at
pH 6.0 (black line), 8.0 (blue line) and 10 (red line) at 298 K.
with 3 was observed at pH 3. Thus, the hydroxo form 3 rather
than 1 or 2 is the actual catalyst for the production of H2 and
CO2. This is confirmed by no spectral change of 2 with
paraformaldehyde at pH 7 (Fig. 3a).17
At pH 11, however, 3 reacted with paraformaldehyde to produce
the hydride complex (lmax = 340 nm)16 as shown in Fig. 3b.
It was confirmed that no hydrogen evolution was observed
from methanol with 1 at pH 3–11 in water.16b Thus, hydrogen
evolution occurs from either paraformaldehyde or its mono-
merised as well as hydrated equivalent, methanediol rather
than via disproportionation of formaldehyde to methanol and
formic acid.
The catalytic cycle is shown in Scheme 2. At pH 11, 1 is
converted to the hydroxo complex 3, which reacts with para-
formaldehyde HO(CH2O)nH to produce the methanediol adduct
([Ir-OCH2OH]À) and HO(CH2O)nÀ1H. The b-hydrogen elimina-
tion from [Ir-OCH2OH]À occurs to produce the hydride complex
(4) and formic acid. The hydride complex (4) reacts with H2O
to produce H2, accompanied by regeneration of 3 (upper-side
catalytic cycle in Scheme 2). The hydroxo complex 3 also reacts
with formate to produce the hydride complex (4) and CO2 by
b-hydrogen elimination. The characteristic visible absorption
Scheme 2 Catalytic cycles for decomposition of paraformaldehyde to H2
bands at lmax = 420 nm appeared due to formation of a formate and formate that is further decomposed to H2 and CO2 with 3.
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complex16b in the reaction between 3 and paraformaldehyde at and stability of 3 should be further improved, this study pro-
pH 7 as shown in Fig. S3 in ESI.† The hydride complex also reacts vides a convenient way to produce hydrogen from paraform-
with H2O to produce H2, accompanied by regeneration of 3 (lower- aldehyde as a solid hydrogen carrier at ambient temperature.
side catalytic cycle in Scheme 2).14 The formation of the methane-
This work was supported by an Advanced Low Carbon
diol adduct, the formate complex as well as hydride species in Technology Research and Development (ALCA) program from
Scheme 2 has been supported by 1H-NMR and ESI-MS analyses as Japan Science Technology Agency (JST) (to S. F.) and a Grant-in-Aid
shown in Fig. S4 and S5 (ESI†), respectively. The IR bands as well as (No. 24550077 to T. S.) from the Ministry of Education, Culture,
NMR peaks of the hydride species in the steady state of the catalytic Sports, Science and Technology, Japan. Mr Kengo Tachikawa, Prof
reaction would be too weak to be assigned well. Thus, the overall Akira Onoda and Prof Takashi Hayashi in Osaka University are
stoichiometry is given by eqn (4), where H2 and CO2 are produced gratefully acknowledged for their technical support in ESI-MS
with a 2 : 1 molar ratio as observed in Fig. 1.18
measurements.
When formalin without a stabilizer, i.e., methanol was used
instead of paraformaldehyde, HCHO that exists in the form of
methanediol [eqn (2)] in water under basic conditions also
decomposed to produce H2 and CO2 with a 2 : 1 molar ratio
[eqn (5)]
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Fig. 4 Time courses of catalytic production of H2 (black line) and CO2
(red line) from formalin (66.7 mmol) with 3 (5.0 mM) in an aqueous solution
(1.0 mL at pH 11) at 298 K.
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