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RESEARCH

Research 1　Development of highly efficient and novel molecular transformations

Although liquid-phase organic reactions have been widely utilized from laboratory-level chemical synthesis to industrial production of medicines, agricultural chemicals, and fine chemicals, there is still room for improvement in terms of their greenness, resource utilization efficiency, and energy efficiency. For example, conversions of various organic substrates to oxygen-containing compounds and oxidative functional group transformations represented by dehydrogenation reactions still use superstoichiometric amounts of oxidants such as high valent chromium or manganese reagents in some cases. In addition, these antiquated methods also have serious problems such as the processing of metal salt wastes.
We have been engaged in research on the development of high-performance heterogeneous catalysts for highly efficient liquid-phase organic reactions, especially for catalytic oxidation reactions. We have introduced new molecular design concepts in the field of heterogeneous catalysts and created original nanostructured catalysts (including molecular and crystalline nano oxides, nano hydroxides, metal nanoparticles, and alloys) from our unique viewpoints and methods (Fig. 1.1).


Fig.1.1 “Nanostructured catalyst” our tools for developing new reactions

We have succeeded in developing several environmentally conscious highly efficient liquid-phase functional group transformations (including one-pot synthesis) using our nanostructured catalysts; for example, oxygenation (epoxidation) with high efficiencies of hydrogen peroxide utilization (>99%) by polyoxometalate-based catalysts[1.1], ammoxidation utilizing concerted alcohol activation by Bronsted base and Lewis acid in nano ruthenium hydroxide [1.2] (Fig. 1.2, top), amidation of alcohols, alkylarenes, and amines using the oxidation and water activation abilities of crystalline nanomanganese oxide[1.3] (Fig. 1.2, middle). In addition, we have newly discovered the specific dehydrogenation abilities of palladium, gold or their alloy nanoparticles. Utilizing their specific dehydrogenation abilities, various new aromatic ring formation reactions (dehydrogenative aromatization), such as phenol synthesis from cyclohexanols[1.4], selective arylamine synthesis (anilines vs diarylamines)[1.5], aniline synthesis from oximes[1.6] (Fig. 1.2, bottom), have been realized.


Fig.1.2 Examples of new reactions realized using our nanostructured catalysts

Furthermore, by designing a functionally integrated heterogeneous catalyst in which gold nanoparticles are supported on a basic hydroxide support LDH, a one-pot flavone synthesis from hydroxyacetophenones and aldehydes has been realized for the first time[1.7] (Fig. 1.3, top). Furthermore, it has been revealed that by designing a gold core-palladium oxide shell structure, aurones, which are very rare flavonoids, could be synthesized with high selectivities from chalcones[1.8] (Fig. 1.3, bottom). Such selectivity switches are unique to our precisely designed heterogeneous catalysts. Furthermore, it has succeeded in completely controlling the regioselectivity of amine oxygenation[1.9].


Fig.1.3 Example of selectivity switch by precise surface designed of heterogenous catalysts

The above-mentioned reactions are unique to heterogeneous catalysts that have been realized for the first time by our precisely designed nanostructured catalysts. These reactions possess the following advantages; inexpensive and readily available raw materials, molecular oxygen as the oxidant, and/or ammonia as the nitrogen source can be used, and by-products are only water and/or molecular hydrogen. So far, investigations of heterogeneous catalysts have been dominated by simply immobilizing homogeneous catalysts for existing reactions and discussing their activities and reusabilities. On the other hand, our research is focused on creating and integrating active species unique to heterogeneous catalysts for developing new reactions that have not been realized even with well-designed homogeneous catalysts.

References

  1. K. Kamata, K. Yonehara, Y. Sumida, K. Yamaguchi, S. Hikichi, N. Mizuno, Science 2003, 300, 964-966.
  2. T. Oishi, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2009, 48, 6286-6288.
  3. (a) K. Yamaguchi, H. Kobayashi, T. Oishi, N. Mizuno, Angew. Chem. Int. Ed. 2012, 51, 544-547; (b) Y. Wang, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2012, 51, 7250-7253; (c) Y. Wang, H. Kobayashi, K. Yamaguchi, N. Mizuno, Chem. Commun. 2012, 48, 2642-2644; (d) Y. Miyamoto, Y. Kuroda, T. Uematsu, H. Oshikawa, N. Shibata, Y. Ikuhara, K. Suzuki, M. Hibino, K. Yamaguchi, N. Mizuno, Sci. Rep. 2015, 5, 15011.
  4. (a) X. Jin, K. Taniguchi, K. Yamaguchi, N. Mizuno, Chem. Sci. 2016, 7, 5371-5383; (b) X. Jin, K. Taniguchi, K. Yamaguchi, K. Nozaki, N. Mizuno, Chem. Commun. 2017, 53, 5267-5270.
  5. (a) K. Taniguchi, X. Jin, K. Yamaguchi, K. Nozaki, N. Mizuno, Chem. Sci. 2017, 8, 2131-2142; (b) Y. Koizumi, K. Taniguchi, X. Jin, K. Yamaguchi, K. Nozaki, N. Mizuno, Chem. Commun. 2017, 53, 10827-10830.
  6. X. Jin, Y. Koizumi, K. Yamaguchi, K. Nozaki, N. Mizuno, J. Am. Chem. Soc. 2017, 139, 13821-13829.
  7. T. Yatabe, X. Jin, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2015, 54, 13302-13306.
  8. T. Yatabe, X. Jin, N. Mizuno, K. Yamaguchi, ACS Catal. 2018, 8, 4969-4978.
  9. (a) X. Jin, K. Kataoka, T. Yatabe, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2016, 55, 7212-7217; (b) S. Nakai, T. Yatabe, K. Suzuki, Y. Sasano, Y. Iwabuchi, J. Hasegawa, N. Mizuno, K. Yamaguchi, Angew. Chem. Int. Ed. 2019, 58, 16651-16659.

 

Research 2 Precise design of metal oxide nanoclusters

Metal oxides are widely used functional materials because they exhibit unique properties depending on the types, compositions, and oxidation states of the constituent metals as well as the structures. They are utilized in a wide range of fields such as catalysis, magnetic materials, electronic materials, and optical materials, and therefore, are important materials in both academic and industrial fields. If we can synthesize metal oxides with precise control over the numbers, compositions, arrangements of metals, it will be possible to explore for unknown substances with new structures and functions, which that cannot be realized by the conventional material design methods. We have designed various molecular metal oxides (lacunary polyoxometalates, Fig. 2.1) that function as inorganic ligands, and have developed unique metal oxide design methods by use them as "molecular templates" in organic solvents[2.1].


Fig.2.1 Conceptual diagram of precise design of metal oxide nanoclusters

By utilizing the lacunary polyoxometalates as templates, in addition to a one-step synthesis method that arranges the same kind of metals, we have developed a sequential synthesis method that can selectively and precisely arrange multiple types of metals. Based on these methods, we can design various functional metal oxide materials that show unique catalytic and magnetic properties (Fig. 2.2)[2.2]. In addition, not only using molecular metal oxides as simple "molecular templates", but also utilizing the concerted functions with the introduced metals, we have developed molecular catalysts that exhibits high catalytic activity and new reactivity that surpasses existing catalysts. Furthermore, we have pioneered new methodologies in metal oxide design, such as the development of visible-light-responsive photoredox catalysis using intermolecular charge transfers (Fig. 2.3)[2.3]. We have also developed new reactions using well-known heteropoly acids[2.4].


Fig.2.2 Synthesis of metal oxide clusters by one-step or sequential synthesis methods


Fig.2.3 Development of visible light-responsive metal oxide clusters and their photocatalytic properties

In this way, we aim to develop our own design technology for metal oxide materials and pioneer new methodologies for the creation of highly functional materials. Recently, we have succeeded in synthesis of metal nanoclusters[2.5] and organic-inorganic hybrid nanomaterials (Fig. 2.4)[2.1a] using molecular metal oxides. By using these catalysts, we are working on the development of new reactions that cannot be realized with existing catalytic systems.


Fig.2.4 Metal nanoclusters and organic-inorganic hybrid nanomaterials

References

  1. (a) C. Li, N. Mizuno, K. Yamaguchi, K. Suzuki, J. Am. Chem. Soc. 2019, 141, 7687; (b) T. Minato, K. Suzuki, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2016, 55, 9630; (c) K. Suzuki, T. Hanaya, R. Sato, T. Minato, K. Yamaguchi, N. Mizuno, Chem. Commun. 2016, 52, 10688; (d) K. Suzuki, R. Sato, N. Mizuno, Chem. Sci. 2013, 4, 596-600.
  2. (a) T. Minato, K. Suzuki, Y. Ohata, K. Yamaguchi, N. Mizuno, Chem. Commun. 2017, 53, 7533; (c) R. Sato, K. Suzuki, T. Minato, M. Shinoe, K. Yamaguchi, N. Mizuno, Chem. Commun. 2015, 51, 4081; (d) K. Suzuki, Y. Kikukawa, S. Uchida, H. Tokoro, K. Imoto, S. Ohkoshi, N. Mizuno, Angew. Chem. Int. Ed. 2012, 51, 1597-1601.
  3. (a) K. Suzuki, N. Mizuno, K. Yamaguchi, ACS Catal. 2018, 8, 10809 (review); (b) C. Li, K. Suzuki, N. Mizuno, K. Yamaguchi, Chem. Commun. 2018, 54, 7127; (c) K. Suzuki, F. Tang, Y. Kikukawa, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2014, 53, 5356; (d) K. Suzuki, J. Jeong, K. Yamaguchi, N. Mizuno, Chem. Asian J. 2015, 10, 144; (e) Y. Kikukawa, K. Suzuki, M. Sugawa, T. Hirano, K. Kamata, K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed. 2012, 51, 3686-3690.
  4. K. Yamaguchi, N. Xu, X. Jin, K. Suzuki, N. Mizuno, Chem. Commun. 2015, 51, 10034.
  5. K. Yonesato, H. Ito, H. Itakura, D. Yokogawa, T. Kikuchi, N. Mizuno, K. Yamaguchi, K. Suzuki, J. Am. Chem. Soc. in press, DOI: 10.1021/jacs.9b10569.

 

Research 3 Conversion of natural carbon resource

Methane and ethane, which are the main components of natural gas, are desired not only to use for combustion but for effective and high-value-added utilization because they are increased in supply thanks to the shale gas revolution and are clean fossil resources. (Fig. 3.1)[3.1]. If chemical products and energy can be efficiently obtained from natural carbon resources such as methane, it will lead to the establishment of a future industrial base that can utilize various natural carbon resources in a balanced manner. Now, we are paying attention to the chemical conversion (especially oxidation reaction) of abundant natural carbon resources (mainly C1-C4 alkanes) with heterogeneous catalysts. For example, direct methanol synthesis using methane is a process that uses oxygen to synthesize methanol in one step without using syngas, a mixed gas of carbon monoxide and hydrogen. However, it is very difficult to synthesize methanol in high yield and high selectivity because methanol is more reactive than methane and easily causes sequential oxidation. When synthesizing methanol via syngas, the production of the syngas in the previous stage is a large endothermic reaction and it becomes a large-scale plant that requires multiple stages of heat exchangers. Therefore, a green chemistry-oriented energy-saving process that is manufactured directly on demand is desired. There are few reports of direct synthesis processes for value-added oxygen-containing compounds such as acetic acid and acrylic acid from lower alkanes such as ethane and higher.
The difficulty of these lower alkane selective oxidation reactions is the cleavage of the C-H bond, which is the first stage of the elementary reaction. In the conventional catalytic process, the reaction has been carried out by the “hard-work” technique of high-temperature and high-pressure conditions. On the other hand, the product is often more reactive than the substrate, and under these severe conditions, it is difficult to suppress sequential oxidation. Therefore, reactive oxygen species (O- and O22-) with the ability of moderate hydrogen abstraction from hydrocarbons under lower temperature conditions (573 K or lower) that can suppress the sequential reaction in the gas phase as much as possible. One solution is to design an oxide catalyst that can be made from molecular oxygen without the use of expensive oxidants such as hydrogen peroxide and nitrous oxide. Herein, we design polyoxometalate catalysts that precisely define metal multinuclear active site structures (e.g. V5+, Fe3+, Cu2+, Co2+, Mn2+, Zn2+, Ga3+, etc.) targeted by controlling the structure in units of one atom for developing an oxidation reaction using various carbon resources (Fig. 3.2).


Fig 3.1　C1 chemistry using various carbon source as start material


Fig 3.2　Development for polyoxometalate-based catalysts