Journal of the Japan Petroleum Institute, 60, (4), 159-169 (2017) 159 [Review Paper] Metal-catalyzed Hydrogen Peroxide Oxidation for Synthesis of Value-added Chemicals Yoshihiro KON＊ Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, Higashi 1-1-1, Tsukuba, Ibaraki 305-8565, JAPAN (Received December 26, 2016) Hydrogen peroxide is an ideal oxidant for industrial processes that produce useful chemicals such as propylene oxide, oxime and catechol because this environmentally benign oxidant has excellent oxygen atom efficiency and low cost. However, hydrogen peroxide is a weak oxidant with low selectivity, so application of hydrogen peroxide oxidation technology to the synthesis of complex organic compounds containing various functional groups remains challenging. We have found that various methods of hydrogen peroxide oxidation of olefins can be used to synthesize a wide range of fine chemicals. Here we report our recent investigations of a practical synthetic method that uses common metals and reusable catalysts to expand our previous catalytic system using tungsten- based catalysts using three components. For example, our new method employs high-speed styrene oxide syn- thesis using an iron picolinate catalyst, bulky sulfide oxidation employing a reusable titanosilicate zeolite catalyst, and high-conversion synthesis of nitroxide radical polymer. The desired compounds are formed in over 90 % selectivity despite their complex structures, under safe reaction conditions and with the high efficiency of hydrogen peroxide. Keywords Oxidation catalyst, Hydrogen peroxide, Fine chemicals, Iron complex, Titanosilicate zeolite, Nitroxyl radical polymer 1. Introduction 1. 1. History of H2O2 Oxidation Hydrogen peroxide (H2O2) is one of the best candi- Oxidation of organic compounds is an important dates for generating high-value chemicals under envi- reaction for the production of value-added chemicals. ronmentally benign reaction conditions3). In particular, Generally, the selection of oxidants as well as that of the only byproduct of the reaction is water. Another 1) reactants is important to promote a specific reaction . important feature is that H2O2 is generally used as an For example, adipic acid, an important precursor to approximately 30-40 % aqueous solution (H2O2 aq.), in make 6,6-nylon, is synthesized by oxidation from contrast with oxygen gas. For example, H2O2 aq. can mixed cyclohexanol and cyclohexanone compounds be used for the generation of chemicals with high boil- using nitric acid as the oxidant, and epoxy resins, fre- ing points, by mixing with a liquid or dissolved solid quently used as insulating resins in electronic devices, precursor. are produced by oxidation of the corresponding olefin However, H2O2 has low potential, and identification using peracetic acid. However, these reactions also of suitable catalytic systems compatible with the reac- produce harmful waste chemicals such as nitrous oxide tivity of specific precursors has been very difficult. (N2O) and acetic acid. N2O is known as a greenhouse H2O2 oxidation using catalysts was first reported around gas, and the worldwide production of 2.2 million tons/ 1950, and catalytic reactions with high selectivity for year of adipic acid generates 400,000 tons/year of N2O. target compounds were frequently reported after 1980. Acetic acid causes water pollution, and much effort is For example, H2O2 oxidation using a tungsten complex required to separate it from the crude products. was reported in 1983, and H2O2 oxidation with tungsto- Therefore, future industrial systems will require less phosphoric acid and quaternary ammonium salt was environmentally damaging systems of oxidation1),2). reported in 19884)～6). The concept of sustainable chemistry was proposed worldwide in 1990, and halide- and solvent-free H2O2 oxidation using sodium tung- DOI: doi.org/10.1627/jpi.60.159 state, quaternary ammonium salt, and phosphonic acid ＊ E-mail: [email protected] was reported in 1996 as an example of an environmen- J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 160 tally benign reaction7). Our research group reported tungsten-based catalysts that can oxidize reactants to give target products, such as terpene oxides, in good to excellent yields8). Our research into H2O2 oxidation recognized the importance of practical H2O2 oxidation for the synthesis of value- added chemicals with high boiling points that are usually synthesized in organic solution. In addition, further development of basic techniques and concepts easily applicable to industrial synthesis is required to achieve Fig. 1● Concept of Hydrogen Peroxide Oxidation Catalyzed by Metal Compounds the goals of high-speed synthesis, use of recyclable cat- alysts, and applicability to large-scale synthesis while maintaining the excellent yield percentages achieved on 2. Result and Discussion a laboratory scale. Here we report three types of catalytic reactions that 2. 1. H2O2 Oxidation of Styrene Using Iron use H2O2 oxidation and meet the criteria of simplicity, Complex Catalyst ease of handling, and high selectivity to give useful Iron is one of the most abundant metals and is easy compounds. The reactions are selective production of to handle9). Iron is known to be an active catalyst for styrene oxide from styrene using an iron picolinate cat- H2O2 oxidation, as iron complexes are involved in cata- 10) alyst, oxidation of sulfides through tunable reactivity lytic H2O2 activation processes in biological systems . using an interlayer-modified titanosilicate zeolite cata- Since the beginning of this century, many efforts have lyst, which is easily separated from the products, and been made to construct artificial systems with bio- 9) synthesis of radical polymers on a kilogram scale by mimetic iron complexes for H2O2 oxidation . In addi- using tungsten-catalyzed peroxide species under low tion, many useful methods for high-yield synthesis have oxygen concentration conditions with appropriate reac- been reported. However, some critical problems must tion times. These three reactions achieve environmen- be solved to achieve practical synthesis, such as the use tally benign oxidation and low-cost synthesis and are of environmentally benign reagents that are chloride- good candidates for practical synthesis using H2O2. free, reduction of H2O2 as a side reaction, and develop- 1. 2. Concept for Making Metal Peroxide Active ment of an easy handling process. Species from Precatalyst and H2O2 We have demonstrated high yield, high rate syntheses Catalytic H2O2 oxidation can be achieved through the of styrene oxides using an iron complex coordinated by 11) formation of a metal peroxide species from H2O2 and two types of picolinates in the presence of H2O2 . metal catalyst, because the metal peroxide active spe- The developed iron catalyst was effective for the oxida- cies can oxidize the reactant with good selectivity. tion of various styrenes to give the corresponding epox- Selectivity for the desired product through H2O2 oxida- ides in over 90 % yields at 25 ℃ under chloride-free tion requires use of a suitable metal oxide precursor to conditions. The reaction must be easy to handle to form a metal_oxygen_oxygen (M_O_O_) bond with obtain the target styrene oxides. The iron complex superior characteristics to the simple decomposition of catalyst was prepared with simple mixing of 0.02 equiv- H2O2. The balance between the stability of the oxida- alent per substrate (eq.) of iron(II) acetate, 0.02 eq. of tion state and the redox potential of the precatalyst is 2-picolinic acid (picH), and 0.02 eq. of 6-methyl-2-pic- critical for targeted oxidation, as production of the olinic acid (Me-picH) in MeCN. Styrene (1.0 mmol) required metal peroxide in smooth and good yields was poured into the prepared iron catalyst solution at facilitates the specified reaction without the formation 25 ℃, followed by the addition of 1.40 eq. of 35 % of byproducts. Formation of a metal peroxide species H2O2 aq. for 10 min. After extra stirring for 5 min, the seems to be the rate-determining step, and the reaction styrene was fully converted to styrene oxide in 95 % proceeds smoothly if the active species is successfully yield (Table 1, entry 4). Benzaldehyde and 2-phenyl- formed. However, the effects of the solvent, the rela- 1-ethanol were produced as byproducts as 2 % of each tive thermal stabilities of the products and reactants, yield. We also carried out a 100 g-scale H2O2 oxida- and the size of the contact area between the active site tion of styrene using 1.25 eq. of 35 % H2O2 aq. with the and the reactant should all be considered in the condi- same iron catalyst solution used in entry 4 to produce tions of a highly selective reaction. The formed styrene oxide in 80 % isolated yield, as shown in M_OOH species can be converted to various configura- Scheme 1. tions such as an M_O_O triangle shape, M＝O double Oxidation of styrene was not observed in the absence bonded shape, and its dimer. The specific shape of iron metal (Table 1, entry 1). Using another formed depends on the characteristics of the metal and 0.02 eq. of picH instead of Me-picH also catalyzed the of the reaction environment (Fig. 1). epoxidation, but the reactivity was lower than that of J. Jpn. Petrol. Inst., Vol. 60, No. 4, 2017 161 a),11) Table 1 Oxidation of Styrene by 35 % H2O2 aq. with Iron Catalysts b) c) Entry Fe(OAc)2 [eq.] picH [eq.] Me-picH [eq.] Yield [%] Selec. [%] 1 0 0.02 0.02 0 0 2 0.02 0.04 0 42 74 3 0.02 0.03 0.01 78 85 4 0.02 0.02 0.02 95 95 5 0.02 0.01 0.03 52 81 6 0.02 0 0.04 37 71 7 0.01 0.01 0.01 69 93 8 0.04 0.04 0.04 92 92 a) Styrene : Fe(OAc)2 : ligand molar ratios were selected as shown here. MeCN solution, 25 ℃, dropwise addition of 1.4 eq.