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 [IrIII(Cp*)(4-(1H-pyrazol-1-yl-κN2)benzoic acid-κC3)(H2O)]2SO4 (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 IrIII-OH2 complex 1-H+ reacts with H2 in an aqueous phosphate buffer solution (pH 6.0) to produce the iridium(III) hydride complex 2 (λmax=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).
Complex 2 can efficiently reduce FMN (λmax=373 and 445 nm) to the 1,5-dihydroflavin (FMNH2: λmax=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 (λmax) and extinction coefficients at λmax for the absorption spectrum of FMNH2 obtained in this work (ε=3.6×103 M−1 cm−1 at λmax=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 λmax=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 (kB) 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).
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 λmax=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 (kobs) was obtained, and the second-order rate constant (kC) for the reaction of FMNH2 with O2 was determined to be 2.0×104 M−1 s−1.
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 λmax=445 nm, and this increase obeyed first-order kinetics (Figure S6 a in the Supporting Information). The pseudo-first-order rate constant (kobs) 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 kC 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).
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 IrIII-H complex 2, which reduces FMN to FMNH2, followed by oxidation by O2 to produce H2O2, accompanied by the regeneration of FMN.
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).
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).
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 IrIII-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.