Direct Synthesis of Hydrogen Peroxide from Hydrogen and Oxygen by Using a Water-Soluble Iridium Complex and Flavin Mononucleotide

Hydrogen peroxide (H₂O₂) is known to be a highly selective and environmentally friendly oxidant and is used in the chemical industry for the manufacture of numerous organic and inorganic compounds.[1,2] However, the current industrial process for H₂O₂ production by sequential hydrogenation and oxidation of an alkyl anthraquinone is not environmentally benign as a result of a number of disadvantages, such as the requirement for toxic solvents, high energy consumption, and multiple steps.[3,4] To provide an alternative, extensive efforts have been devoted to achieving the direct synthesis of H₂O₂ from hydrogen (H₂) and oxygen (O₂) by using heterogeneous precious-metal catalysts (mainly Pd, Au, or Au–Pd).[3–12] This direct process suffers from serious problems such as unfavorable high-pressure conditions and relatively low yields owing to undesired side reactions such as formation of H₂O (H₂+1/2O₂→H₂O), decomposition of H₂O₂ (H₂O₂→H₂O+1/2O₂), and hydrogenation of H₂O₂ (H₂O₂+H₂→2 H₂O).[3–12] For this reason, selectivity in the direct synthesis of H₂O₂ from H₂ and O₂ has been limited. In addition, it has been quite difficult to elucidate the heterogeneous catalytic mechanism as compared with the homogeneous catalytic mechanism, in which intermediates can be detected. However, there has to date been no report of a homogeneous catalytic system for the direct synthesis of H₂O₂ from H₂ and O₂.[13]

We report herein the direct synthesis of H₂O₂ from H₂ and O₂ in water by using a water-soluble iridium aqua complex [Ir^III(Cp*)(4-(1H-pyrazol-1-yl-κN²)benzoic acid-κC³)(H₂O)]₂SO₄ ([1]₂SO₄), which can react with H₂ to produce an iridium hydride complex (2),[14,15] and flavin mononucleotide (FMN) under normal pressure and at room temperature. The synthesis and characterization of 1 were carried out as reported and are briefly described in the Experimental Section.[14]

At pH 6.0, the carboxylic acid group in 1 is deprotonated to give the carboxylate form 1-H⁺ [Eq. (1)].[14] The Ir^III-OH₂ complex 1-H⁺ reacts with H₂ in an aqueous phosphate buffer solution (pH 6.0) to produce the iridium(III) hydride complex 2 (λ_max=336 nm; Equation (2) and Figure 1).[14] The reaction of 1-H⁺ with H₂ proceeds rapidly to completion to form 2 within 3 s (Figure S1 in the Supporting Information). For the reaction of 1-H⁺ with H₂, the formation of 2 in the presence of O₂ was confirmed by changes in the UV/Vis absorption spectrum, which indicates the slower reaction of 2 with O₂ (Figure S2 in the Supporting Information).

Figure 1.

Changes in the UV/Vis absorption spectrum during the reaction of 1 (25 μm) with H₂ (0.82 mm) in an aqueous phosphate buffer (pH 6.0) at 298 K. An argon-saturated buffer solution of 1 (black line) was bubbled with H₂ (1.0×10⁻¹ MPa) for 5 min thus resulting in the formation of 2 (red line).

(1)

(2)

Complex 2 can efficiently reduce FMN (λ_max=373 and 445 nm) to the 1,5-dihydroflavin (FMNH₂: λ_max=291 nm and 390 nm) in aqueous phosphate buffer solution (pH 6.0) under N₂ [Equation (3) and Figure 2].[16] FMNH₂ was also generated by catalytic reduction of FMN with H₂ in the presence of 1 (Figure S3 in the Supporting Information). A similar spectral change has been reported in the reaction of FMN with sodium dithionite in water.[15] The wavelengths (λ_max) and extinction coefficients at λ_max for the absorption spectrum of FMNH₂ obtained in this work (ε=3.6×10³ m⁻¹ cm⁻¹ at λ_max=390 nm) are consistent with those in the literature,[14] thus indicating the stoichiometric formation of FMNH₂. The decay of the absorption band of FMN at λ_max=445 nm obeyed second-order kinetics (Figure S4 a in the Supporting Information). Irrespective of the ratio of the initial amounts of 2 to FMN, the observed second-order rate constants (k_B) agree well with each other (1.5×10⁵ m⁻¹ s⁻¹ and 1.2×10⁵ m⁻¹ s⁻¹, Figure S4 a and b in the Supporting Information, respectively).

Figure 2.

Changes in the UV/Vis absorption spectrum during the reduction of FMN (25 μm) by 2 (0.55 mm) under an H₂ (5.0×10⁻² MPa)/N₂ (5.0×10⁻² MPa) gas mixture after mixing a buffer solution of FMN (50 μm) under N₂ with that of 2. Complex 2 was formed by the reduction of 1 (1.1 mm) by H₂ (bubbling for 10 min) under normal pressure in an aqueous phosphate buffer (pH 6.0) at 298 K. The black and blue solid lines correspond to FMN and FMNH₂, respectively.

(3)

FMNH₂ can readily be oxidized by O₂ to regenerate FMN, accompanied by the formation of H₂O₂ [Eq. (4)].[17] Upon mixing an O₂-saturated phosphate buffer solution with a buffer solution of FMNH₂ that was formed by the reduction of FMN by H₂ in the presence of 1, the characteristic absorption bands of FMN were completely regenerated (Figure 3). The rise of the absorption band of FMN at λ_max=445 nm obeyed first-order kinetics (Figure S5 in the Supporting Information). From the slope of the first-order plot (inset of Figure S5 in the Supporting Information), a pseudo-first-order rate constant (k_obs) was obtained, and the second-order rate constant (k_C) for the reaction of FMNH₂ with O₂ was determined to be 2.0×10⁴ m⁻¹ s⁻¹.

Figure 3.

Changes in the UV/Vis absorption spectrum during the oxidation of FMNH₂ (50 μm) by O₂ (0.68 mm) under an O₂ (5.0×10⁻² MPa)/N₂ (5.0×10⁻² MPa) gas mixture in an aqueous phosphate buffer (pH 6.0) after mixing an O₂-saturated buffer solution with a buffer solution of FMNH₂ under N₂. FMNH₂ was formed in the reduction of FMN (100 μm) by H₂ (bubbling for 10 min) with 1 (5.0 μm) under normal pressure at 298 K.

(4)

FMNH₂ can also be independently generated by the reduction of FMN with sodium hydrosulfite (Na₂S₂O₄).[16a] As the reaction of FMNH₂ with O₂ proceeds, the formation of FMN leads to an increase in the absorbance at λ_max=445 nm, and this increase obeyed first-order kinetics (Figure S6 a in the Supporting Information). The pseudo-first-order rate constant (k_obs) proportionally increased with concentration of O₂ (Figure S6 b in the Supporting Information). From the slope of the linear plot, the second-order rate constant for the reaction of FMNH₂ with O₂ was determined to be 2.8×10⁴ m⁻¹ s⁻¹. This value agrees well with k_C determined for the reaction in the presence of 1 and H₂. These values are more or less consistent with the value (5.8×10⁴ m⁻¹ s⁻¹) reported for the reaction of a reduced form of a flavoprotein oxidase with O₂ at pH 7.0.[18]

The amount of H₂O₂ produced was determined by titration with the oxo[5,10,15,20-tetra(4-pyridyl)porphyrinato]titanium(IV) complex.[19] In the reaction consisting of the stepwise reduction of FMN by H₂ to form FMNH₂ when using 1 followed by the oxidation of FMNH₂ by O₂ to generate H₂O₂, a stoichiometric amount of H₂O₂ was produced when the concentration of FMN was varied, thus demonstrating a linear relationship between the concentrations of H₂O₂ and FMN with a slope of 1.0 (Figure 4).

Figure 4.

Plot of the concentration of H₂O₂ produced by the catalytic reduction of O₂ (1.0×10⁻¹ MPa) by FMNH₂ (generated through the reduction of FMN by 2 under H₂) versus the concentration of FMN (25 μm–1.0 mm) loaded at the beginning of the reaction at 298 K. 2 was produced by the reduction of 1 (25 μm) by H₂ (1.0×10⁻¹ MPa) in an aqueous phosphate buffer solution (pH 6.0).

Thus, the overall catalytic cycle for the selective direct synthesis of H₂O₂ from H₂ and O₂ by using 1 and FMN is expected to proceed according to Scheme 1: the iridium(III) complex 1-H⁺ reacts with H₂ to produce the Ir^III-H complex 2, which reduces FMN to FMNH₂, followed by oxidation by O₂ to produce H₂O₂, accompanied by the regeneration of FMN.

Scheme 1.

When the catalytic reduction of FMN (5.0 μm) followed by the oxidation of FMNH₂ by O₂ was made possible by the presence of 1 (5.0 μm) in an aqueous phosphate buffer solution (pH 6.0), H₂O₂ was catalytically produced from H₂ and O₂. The formation of H₂O₂ stopped within a few minutes at pH 6.0 and 100 min at pH 2.8, when the turnover number (TON) of H₂O₂ production with respect to 1 and FMN reached 28 at pH 6.0 and 41 at pH 2.8 (Figure 5, black •). This limited TON stands in sharp contrast to the stepwise catalytic reduction of FMN and the oxidation of FMNH₂ by O₂ (see above).

Figure 5.

Time course of H₂O₂ production by the reaction of H₂ (5.0×10⁻² MPa) with O₂ (5.0×10⁻² MPa) catalyzed by 1 (5.0 μm) and FMN (5.0 μm) in water (pH 2.8) at 298 K in the absence (black •) or presence of Sc(NO₃)₃ (25, 50, and 100 mm; green ▴, blue ♦, and red ▪, respectively). Data shown by ○ were obtained in the absence of FMN but with 1 (5.0 μm) and Sc(NO₃)₃ (100 mm) under otherwise the same experimental conditions.

When Sc(NO₃)₃ (100 mm) was added to this system, the amount of H₂O₂ dramatically increased (Figure 5, red ▪). The TON with respect to 1 and FMN reached 201 at 4 h. When 1 (1.0 μm) and FMN (50 μm) were used, the TON based on 1 reached 847 at 4 h. The product yield of H₂O₂ based on the total amount of H₂ and O₂ supplied in the catalytic reaction system was 19.2 % at 10 min (Figure S7 in the Supporting Information). This value is more than three times larger than that obtained in the nanocolloidal Pd–Au system under normal pressure of a H₂/O₂ gas mixture (6.1 %).[11d] The rate of catalytic formation of H₂O₂ is accelerated with [Sc³⁺] (Figure 5) and reached a turnover frequency (TOF) of 50 h⁻¹, however, the rate remained unchanged on increasing the concentration of Sc³⁺ over 50 mm. The limited TOF might be caused by the loss of H₂O₂ through the catalytic reduction of H₂O₂ by H₂ with 1 to produce H₂O (Scheme 2). This was independently confirmed by the reduction of H₂O₂ by H₂ when using 1 in water in the absence and presence of Sc(NO₃)₃ (Figure S8 in the Supporting Information).

Scheme 2.

The reduction of H₂O₂ to H₂O by H₂ was catalyzed by 1 (Figure S8 in the Supporting Information, black •). However, this reaction was effectively retarded by the presence of Sc³⁺ (Figure S8 in the Supporting Information, blue ♦ and red ▴), thus indicating that the further hydrogenation of H₂O₂ as shown in Scheme 2 could be inhibited by the presence of strong acid; a result consistent with the fact that H₂O₂ is known to be stabilized under acidic conditions.[20] In the same manner as in heterogeneous catalytic systems, the decomposition of H₂O₂ directly synthesized from H₂ and O₂ in reactions catalyzed by carbon- or TiO₂- supported Au, Pd, or an Au–Pd alloy was strongly retarded by the pretreatment of the catalyst with acid.[5a,c]

The rate-determining step of the catalytic scheme shown in Scheme 1 was investigated by examining the dependence of the overall rate of catalytic H₂O₂ production on the concentration of 1. The rate of formation of H₂O₂ from H₂ and O₂ when using 1 in the presence of Sc(NO₃)₃ increased with increasing concentrations of 1 (Figure S9 in the Supporting Information), thus indicating that the rate-determining step is the reduction of FMN to FMNH₂ by 2 to regenerate 1.[21] The characteristic UV/Vis absorption bands of FMN in the presence of 1 under N₂ (Figure S10 a in the Supporting Information) remained unchanged under both H₂ and O₂ throughout the catalytic reaction (Figure S10 b), thus indicating that FMN reduction by 2 is the rate-determining step of the overall catalytic reaction at pH 6.0. Under these conditions, selective two-electron reduction of O₂ to H₂O₂ occurs without the further reduction of H₂O₂ to H₂O.

In conclusion, the water-soluble iridium(III) complex 1 can efficiently catalyze the direct synthesis of H₂O₂ from H₂ and O₂ when using a water-soluble flavin (FMN) under normal pressure in aqueous solution at 298 K. The catalytic cycle consists of the reduction of 1 by H₂ to form the Ir^III-H complex 2, followed by the reduction by 2 of FMN to FMNH₂, which then reacts with O₂ to produce H₂O₂, accompanied by the regeneration of FMN and 1. The addition of Sc(NO₃)₃ led to a high TON (847) and a reasonably high yield of H₂O₂ (19.2 %). This is because Sc(NO₃)₃ can inhibit the catalytic reduction of H₂O₂ by H₂ when using 1.

Experimental Section

All experiments were performed under an Ar or N₂ atmosphere by using standard Schlenk techniques unless otherwise noted. The limiting concentration of O₂ in aqueous solution was prepared by using a mixed gas flow of O₂ and N₂ under normal pressure (1.0×10⁻¹ MPa). The mixed gas was controlled by using a gas mixer (Kofloc GB-3C, KOJIMA Instrument Inc.), which can mix two or more gases at a defined partial pressure and mass flow rate. A defined concentration of H₂ in an aqueous solution was prepared by a mixed gas flow of H₂ and N₂ (or O₂) controlled by using a gas flow meter (KOJIMA Instrument Inc.) appropriate for each gas under normal pressure (1.0×10⁻¹ MPa). UV/Vis absorption spectra were recorded on a Hewlett–Packard 8453 diode array spectrophotometer with a quartz cuvette (light-path length=1 cm) at 298 K. Kinetic measurements for reactions with short half-lives (under 10 s) were performed on a UNISOKU RSP-601 stopped-flow spectrometer equipped with a MOS-type highly sensitive photodiode array at 298 K. Because the O₂/H₂ ratio is within the explosive range, dry catalyst should not be added to the O₂/H₂ gas mixture. Moreover, precautions should be taken to contain the system in the event of an explosion.

Details concerning chemicals, determination of the reaction yield, detection of H₂O₂, and pH adjustment are described in the Supporting Information.

Supporting Information

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