Novel catalyst has right structure for producing biochemicals

Scientists at the Tokyo Institute of Technology (Tokyo Tech) in Japan have developed and analyzed a novel catalyst for oxidizing 5-hydroxymethyl furfural (HMF), which is crucial for generating biochemicals that can replace the classic non-renewable chemicals used for making many plastics. They describe the catalyst in a paper in the Journal of the American Chemical Society.

Finding an alternative to non-renewable natural resources is a key topic in current research. Much of the raw materials required for manufacturing many of today's plastics come from non-renewable fossil resources such as coal and natural gas, and so a lot of effort has been devoted to finding sustainable alternatives.

One option involves 2,5-furandicarboxylic acid (FDCA), which is an attractive raw material that can be used to create polyethylene furanoate, a bio-polyester with many applications. FDCA can be produced via the oxidation of HMF, which can be synthesized from the cellulose in plants. The necessary oxidation reactions for converting HMF to FDCA require the presence of a catalyst, which helps with the intermediate steps of the reactions.

Many of the catalysts studied for oxidizing HMF contain precious metals, which is clearly a drawback because these metals are expensive and not widely available. Other researchers have discovered that manganese oxides combined with certain metals (such as iron and copper) can also be used as catalysts. Building on this work, a team of scientists from Tokyo Tech has now found that manganese dioxide (MnO₂) can make an effective catalyst on its own, as long as it possesses the appropriate crystal structure.

The team, which includes Keigo Kamata and Michikazu Hara, worked to determine which MnO₂ crystal structure possesses the best catalytic activity for making FDCA and why. By taking advantage of computational analyses and current theory, they were able to infer that the structure of the crystals was crucial because of the steps involved in the oxidation of HMF.

First, MnO₂ transfers a certain amount of oxygen atoms to the substrate (HMF or other by-products), becoming MnO_2-δ. Then, because the reaction is carried out under an oxygen atmosphere, MnO_2-δ quickly oxidizes, reverting 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. The team calculated that active oxygen sites had a lower (and thus better) vacancy formation energy.

To verify this, the scientists synthesized various types of MnO₂ crystals and then compared their performance. Of these crystals, β-MnO₂ proved to be the most promising, because of its active planar oxygen sites. Not only was its vacancy formation energy lower than that of the other structures, but the material also turned out to be very stable, even after taking part in oxidation reactions with HMF.

The team did not stop there, though, as they also 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," says Kamata.

"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," adds Hara.

This story is adapted from material from the Tokyo Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.