Tokyo researchers develop novel green catalyst for oxidizing HMF


Scientists at Tokyo Tech have developed and analyzed a novel catalyst for the oxidation of 5-hydroxymethyl furfural, a renewable chemical building block for, among others, the plastics industry.


An interesting molecule that can be derived from HMF is 2,5-Furandicarboxylic acid (FDCA), which can be used to create the biopolyester polyethylene furanoate, or PEF.
One way of making FDCA is through the oxidation of 5-hydroxymethyl furfural (HMF), a compound that can be synthesized from cellulose. However, the necessary oxidation reactions require the presence of a catalyst, which helps in the intermediate steps of the reaction so that the final product can be achieved.

Many of the catalysts studied for use in the oxidation of HMF involve precious metals; this is clearly a drawback because these metals are not widely available. Other researchers have found that manganese oxides combined with certain metals (such as iron and copper) can be used as catalysts. And now, a team of scientists from Tokyo Tech report that manganese dioxide (MnO2) can be used by itself as an effective catalyst - if the crystals made with it have the appropriate structure.

The team, which includes associate professor Keigo Kamata and professor Michikazu Hara, worked to determine which MnO2 crystal structure would have the best catalytic activity for making FDCA and why. They inferred through computational analyses and the available theory that the structure of the crystals was crucial because of the steps involved in the oxidation of HMF. First, MnO2 transfers a certain number of oxygen atoms to the substrate (HMF or other by-products) and becomes MnO2–δ. Then, because the reaction is carried out under an oxygen atmosphere, MnO2–δ quickly oxidizes back to MnO2. The energy required for this process is related to the energy required for the formation of oxygen vacancies, which varies greatly with the crystal structure. In fact, the team calculated that active oxygen sites had a lower (and thus better) vacancy formation energy.
To verify this, they synthesized various types of MnO2 crystals and then compared their performance through numerous analyses. Of these crystals, β-MnO2 was the most promising because of its active planar oxygen sites. Not only was its vacancy formation energy lower than that of other structures, but the material itself proved to be very stable even after being used for oxidation reactions on HMF.

The team did not stop there, though, as they proposed a new synthesis method to yield highly pure β-MnO2 with a large surface area in order to improve the FDCA yield and accelerate the oxidation process even further.

"The synthesis of high-surface-area β-MnO2 is a promising strategy for the highly efficient oxidation of HMF with MnO2 catalysts," said Keigo Kamata.

With the methodological approach taken by the team, the future development of MnO2 catalysts has been kickstarted.

"Further functionalization of β-MnO2 will open up a new avenue for the development of highly efficient catalysts for the oxidation of various biomass-derived compounds," concluded Hara.


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