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Study on the Production of High Value Added Raw Material Chemicals through a Carbon Dioxide Conversion Catalyst

Validity of the CODH reaction mechanism finally identified

The first achievement of Prof. Han’s team was modeling the binding reaction of nickel and CO2, the electron transfer reaction, and the reaction mechanism for the conversion to CO, all of which take place at the activated site of the CODH/ACS enzyme. In this study, the team successfully synthesized the dinuclear complexes Ni-CO2-Ni and Ni-CO2-Fe, as well as the mononuclear complex Ni-CO2 species, and analyzed their characteristics. In particular, they succeeded in modeling the enzyme active site by using a pincer-type PNP ligand system, and identified the reaction mechanism of CO2 binding to one nickel atom and a two-electron transfer for the first time in the world.

Furthermore, the team introduced ferrous metals to synthesize the (PNP) Ni-CO2-Fe (PNP) species, a new species not known to date. Also, they confirmed that the C-O bond dissociation occurred due to the reaction between an acid and the nickel-CO2 chemical species, resulting in a conversion to the nickel carbonyl compound {(PNP) Ni-CO}+, which well represents the importance of metal-carbon bonds in the conversion of CO2 to CO. This study is of great significance, in that CO2 activation using the dinuclear metal system utilized by the CODH enzyme was successfully modeled and that the validity of the CODH mechanism has finally been identified. In recognition of its excellence, the result of this study was introduced in Chemical Science in January 2017. It is expected that this study will provide key information in identifying the ultimate role of metals such as iron and nickel in the future.

Nickel-amide species synthesized using SiP2 ligand

Another big achievement of the team is that they synthesized nickel metal compounds and quaternary nickel amide species by using a SiP2 ligand and studied their reaction characteristics. In this study, it was observed that the new type of metal-ligand cooperation, which was also suggested initially by the team through a study on the PPP system, operated similarly in the SiP2 system as well. The most remarkable achievement is the identification of the mechanism where (MeSiP2) Ni-NHTrip, which is a nickel-amide species, reversibly substitutes Si-C bonds and Ni-N bonds to generate new Si-N and Ni-C bonds. This substitution reaction did not occur in a different nickel-amide compound, (MeSiP2) Ni-NHMes, where the successful production of carbamate was observed, by reaction with carbon dioxide. They also found that Ni-CONHPh can be synthesized by the carbonylation of nickel-amide. Because of this, the material will be undergoing a vigorous study for its potential to act as an important intermediate in the production of urea and isocyanate. Due to its excellent results, this study will be published in the special edition of “Inorganic Chemistry – The Next Generation” of Inorganica Chimica Acta in 2017.
Applying heterogeneous catalysts to homogeneous catalytic reactions

Metal nano particles used as heterogeneous catalysts are basically neutral in zero-valent metal states, so the catalytic reactions reported thus far tended to be limited to catalytic reactions that use low cost metals. In contrast, the transition metal compounds used in homogeneous catalysts can possess various oxidation states and therefore can be used for various reactions. Because of this merit, studies on the application of heterogeneous catalysts to homogeneous catalytic reactions have attracted a plethora of attention recently. Against this backdrop, the team synthesized a new core-shell nano catalyst by oxidizing the surface of rhodium nano particles. They then used this catalyst in reactions to synthesize the cyclic carbonate by combining carbon dioxide with epoxide, which has been shown to generally react with Lewis acid homogeneous catalysts.

Various X-ray spectroscopic methods such as XPS, EXAPS, and XANES have been actively used in the research process for analyzing solid surfaces. The results showed that the surface of the nanoparticles was oxidized to Rh (III) and the number Rh-Br and Rh-O bonds increased. In addition, the oxidized rhodium nano particles showed a higher reactivity than conventional rhodium salts, and also showed a high recovery rate in five recycling reactions, which is characteristic of heterogeneous catalysts. The TEM image illustrates that the catalyst’s structure is maintained well in the recycling reaction. The results of this study are of great academic significance in terms of proposing a new principle of applying heterogeneous catalysts to homogeneous catalytic reactions, and it is expected that various technologies applying the principle will be developed in the future.

Prof. Han, Sang Woo
2016 Annual Report

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