This illustration shows the ß-MnO2 nanomaterial catalyzing the oxidation of HMF to FDCA. Image: Keiko Kamata, Tokyo Institute of Technology.
This illustration shows the ß-MnO2 nanomaterial catalyzing the oxidation of HMF to FDCA. Image: Keiko Kamata, Tokyo Institute of Technology.

Scientists at the Tokyo Institute of Technology in Japan have investigated a novel method for synthesizing manganese dioxide with a specific crystalline structure called β-MnO2. Their research sheds light on how different synthesis conditions can produce manganese dioxide with distinct porous structures, hinting at a strategy for the development of highly tuned MnO2 nanomaterials that could serve as catalysts for fabricating bioplastics.

Materials engineering has advanced to a point where scientists are not only concerned about the chemical composition of a material, but also about its nanoscale structure. Nanostructured materials have recently drawn the attention of researchers from a variety of fields, as their physical, optical, and electrical characteristics can be tuned and pushed to the limit by tailoring their nanostructures.

MnO2 can form many different crystalline structures with applications across various engineering fields, including as catalysts for chemical reactions. One particular crystalline structure of MnO2, called β-MnO2, is exceptional at catalyzing the oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA), which can be used to produce environmentally friendly bioplastics. Thus, finding ways to tune the nanostructure of β-MnO2 to maximize its catalytic performance is crucial.

However, producing β-MnO2 is difficult compared with other MnO2 crystalline structures. Existing methods are complicated and involve the use of template materials onto which β-MnO2 ‘grows’, resulting in the desired structure after several steps. Now, scientists from Tokyo Institute of Technology led by Keigo Kamata have developed a template-free approach for the synthesis of different types of porous β-MnO2 nanoparticles.

Their method, reported in a paper in ACS Applied Materials & Interfaces, is very simple and convenient. First, manganese precursors are obtained by mixing aqueous solutions and letting the solids precipitate. After filtration and drying, the collected solids are subjected to a temperature of 400°C in a normal air atmosphere, a process known as calcination. During this step, the material crystallizes to produce a black powder that is more than 97% porous β-MnO2.

The scientists found this porous β-MnO2 to be much more efficient as a catalyst for synthesizing FDCA than the β-MnO2 produced using a more widespread approach called the ‘hydrothermal method’. To understand why, the scientists analyzed the chemical, microscopic and spectral characteristics of β-MnO2 nanoparticles produced under different synthesis conditions.

They found that β-MnO2 can take on markedly different morphologies according to certain parameters. In particular, by adjusting the acidity (pH) of the solution in which the precursors are mixed, β-MnO2 nanoparticles with large spherical pores can be obtained. This porous structure gives the nanoparticles a higher surface area, thus providing better catalytic performance.

"Our porous β-MnO2 nanoparticles could efficiently catalyze the oxidation of HMF into FDCA in sharp contrast with β-MnO2 nanoparticles obtained via the hydrothermal method," Kamata explained. "Further fine control of the crystallinity and/or porous structure of β-MnO2 could lead to the development of even more efficient oxidative reactions."

This study also provided much insight into how porous and tunnel structures are formed in MnO2, which could be key to extending its applications. "Our approach, which involves the transformation of Mn precursors into MnO2 not in the liquid-phase (hydrothermal method) but under an air atmosphere, is a promising strategy for the synthesis of various MnO2 nanoparticles with tunnel structures," said Kamata. "These could be applicable as versatile functional materials for catalysts, chemical sensors, lithium-ion batteries and supercapacitors."

This story is adapted from material from 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.