Catalytic hydrogen production from paraformaldehyde and water using an organoiridium complex

Abstract

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.

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Paraformaldehyde is the polymerisation product of formaldehyde with a molecular formula of HO(CH₂O)_nH (n = 8–100).¹ Paraformaldehyde forms slowly in aqueous formaldehyde solutions as a white precipitate. Formalin actually contains very little monomeric formaldehyde, most of which forms short chains of polyformaldehyde.² Paraformaldehyde has been reported to be synthesised by electrocatalytic reduction of CO₂ and water.^3–5 Paraformaldehyde can be depolymerised to formaldehyde [eqn (1)], which is subsequently hydrated to form a geminal diol, i.e., methanediol [eqn (2)] in water under basic conditions.^6,7

HO(CH₂O)_nH → nHCHO + H₂O (1)

HCHO + H₂O → H₂C(OH)₂ (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)

appropriate catalyst under normal pressure at ambient temperature.^8–14 If H₂ can be generated from paraformaldehyde (eqn (4)), paraformaldehyde would be regarded as a convenient

HO(CH₂O)_nH + (n − 1)H₂O → 2nH₂ + nCO₂ (4)

solid H₂ carrier, which has a higher energy density (6.7%) than HCOOH (4.4%). Prechtl and coworkers recently reported selective hydrogen production from paraformaldehyde and formaldehyde using a Ru catalyst [(Ru(p-cymene))₂(μ-Cl)₂Cl₂].¹⁵ However, a relatively high temperature (95 °C) was required for the efficient hydrogen production from paraformaldehyde.¹⁵ To the best of our knowledge, this is the only example of hydrogen production from paraformaldehyde.

We report herein the catalytic decomposition of paraformaldehyde to H₂ and CO₂ (eqn (4)) with a water-soluble iridium aqua complex [Ir^III(Cp*)(4-(1H-pyrazol-1-yl-κN²)benzoic acid-κC³)(H₂O)]₂SO₄ ([1]₂·SO₄, Cp* = η⁵-pentamethylcyclopentadienyl) at room temperature.

Synthesis and characterisation of 1 were performed as reported previously (see ESI†).^14,16 The carboxylic acid form 1 is deprotonated to give the carboxylate form 2 with pK_a = 4.0 and the aqua ligand of 2 is further deprotonated to the hydroxo complex (3) as shown in Scheme 1.^14,16 Under an N₂ atmosphere at pH 11 in the presence of a catalytic amount of 2, paraformaldehyde 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 μM) at 14 h was 21 at 298 K. When the catalyst concentration was decreased to be one-fifth, i.e., 1.0 μM, 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.†

Scheme 1 Deprotonation equilibrium of 1 to 2 and 3.

Fig. 1 Time course of catalytic production of H₂ (black line) and CO₂ (red line) from paraformaldehyde (2.0 mg, 66.7 μmol) with 3 (5.0 μM) in an aqueous solution (1.0 mL at pH 11) at 298 K.

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 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

Fig. 2 pH dependence of catalytic production of H₂ from paraformaldehyde (2.0 mg, 66.7 μmol) with 2 or 3 (5.0 μM) in an aqueous solution (1.0 mL) at pH 6.0 (black line), 8.0 (blue line) and 10 (red line) at 298 K.

Fig. 3 UV-visible absorption spectra of (a) an aqueous solution of 2 (25 μM, 2.0 mL, pH 7) before (black line) and after (red line) addition of paraformaldehyde (31 μM) 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.

At pH 11, however, 3 reacted with paraformaldehyde to produce the hydride complex (λ_max = 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 monomerised 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 paraformaldehyde HO(CH₂O)_nH to produce the methanediol adduct ([Ir-OCH₂OH]⁻) and HO(CH₂O)_n−1H. The β-hydrogen elimination from [Ir-OCH₂OH]⁻ 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 β-hydrogen elimination. The characteristic visible absorption bands at λ_max = 420 nm appeared due to formation of a formate complex^16b in the reaction between 3 and paraformaldehyde at pH 7 as shown in Fig. S3 in ESI.† The hydride complex also reacts with H₂O to produce H₂, accompanied by regeneration of 3 (lower-side catalytic cycle in Scheme 2).¹⁴ The formation of the methanediol adduct, the formate complex as well as hydride species in Scheme 2 has been supported by ¹H-NMR and ESI-MS analyses as shown in Fig. S4 and S5 (ESI†), respectively. The IR bands as well as NMR peaks of the hydride species in the steady state of the catalytic reaction would be too weak to be assigned well. Thus, the overall stoichiometry is given by eqn (4), where H₂ and CO₂ are produced with a 2 : 1 molar ratio as observed in Fig. 1.¹⁸

Scheme 2 Catalytic cycles for decomposition of paraformaldehyde to H₂ and formate that is further decomposed to H₂ and CO₂ with 3.

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)]

HCHO + H₂O → H₂C(OH)₂ → 2H₂ + CO₂ (5)

as shown in Fig. 4. However, the rate of formation of H₂ and CO₂ from formalin (Fig. 4) is much slower as compared with that from paraformaldehyde (Fig. 1). The formation of the methanediol adduct from paraformaldehyde may be faster than that from formalin because of partial polymerization of HCHO in formalin without a stabilizer. When formaldehyde was replaced by propanal, butanal or 2-methylpropanal (4.2 M) at pH 11.8, no reaction occurred with 3. Because only formaldehyde can be converted in water to the hydrated form as methanediol,⁷ methanediol may act as a hydride source as well as a proton source for the hydrogen production as suggested by Prechtl and coworkers.¹⁵ On the other hand, the catalytic transformation of primary alcohols to the corresponding carboxylic acid salts and H₂ has recently been reported by using a ruthenium complex at high temperature under reflux conditions.¹⁹ In the same manner, methanol can directly be converted to carbon dioxide with evolution of H₂ in the presence of a transition metal complex as a catalyst in aqueous solution at temperatures higher than 65 °C.^20,21

Fig. 4 Time courses of catalytic production of H₂ (black line) and CO₂ (red line) from formalin (66.7 μmol) with 3 (5.0 μM) in an aqueous solution (1.0 mL at pH 11) at 298 K.

In conclusion, a water-soluble iridium(III)–hydroxo complex 3 catalyses production of H₂ from paraformaldehyde in water under basic conditions at 298 K. Although the catalytic activity and stability of 3 should be further improved, this study provides a convenient way to produce hydrogen from paraformaldehyde as a solid hydrogen carrier at ambient temperature.

This work was supported by an Advanced Low Carbon Technology Research and Development (ALCA) program from Japan Science Technology Agency (JST) (to S. F.) and a Grant-in-Aid (No. 24550077 to T. S.) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. Mr Kengo Tachikawa, Prof Akira Onoda and Prof Takashi Hayashi in Osaka University are gratefully acknowledged for their technical support in ESI-MS measurements.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental and kinetic details. See DOI: 10.1039/c4cc06581f

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