Ceramic catalyst uses sodium and boron to drive sustainable industrial reactions

The polymer-derived ceramic method uses transition metal-free, sodium-doped, amorphous SiBN ceramics composed of silicon (Si), boron (B), and nitrogen (N) atoms for small molecule activation and catalysis. was adopted to design and synthesize. The distribution of sodium (Na+) and B sites within amorphous silicon nitride enhances the reactivity of both B and N sites, leading to the formation of frustrated Lewis pair (FLP) motifs upon exposure to hydrogen. Credit: Nagoya Institute of Technology Professor Yuji Iwamoto
Heterogeneous catalysts accelerate chemical reactions because they are in a different state than the reactants. It is efficient and stable even under harsh conditions such as high temperature and pressure. Traditionally, metals such as iron, platinum, and palladium have been widely used in industries such as petrochemicals and agriculture for important reactions such as hydrogenation and the Haber process.
However, these metals are rare and can cause problems such as build-up through coking. Scientists are increasingly exploring common elements as catalysts for more sustainable and cost-effective industrial applications.
In the mid-2000s, significant advances were made in catalysis, particularly in the activation of small molecules, with the introduction of the frustrated Lewis pair (FLP) concept. FLP consists of a combination of two components. One acts as a Lewis acid and the other acts as a Lewis base. They are unable to fully react with each other due to spatial or electronic obstacles. This “frustration” causes the molecules to become highly reactive and can activate stable molecules such as hydrogen, carbon dioxide, and ammonia that are normally very difficult to break down.
FLPs stand out because they have multiple active sites, resulting in higher reactivity and selectivity compared to traditional catalysts, which typically have only one active site. There are two main types of FLP. One is heterogeneous defect-controlled FLP and the other is molecular-based uniform FLP.
The first type controls the number of active sites through surface defects. Precisely tuning reactivity and controlling stability can be tedious. The second type targets small molecules in which the acid and base pairs are present within the same molecular structure, making it easy to tune the reactivity simply by changing the surrounding components.
One study broke new ground by adapting molecular-based FLPs for use in solid-state systems. The researchers achieved this by exploiting the chemical versatility of preceramic polymers through a polymer-derived ceramic (PDC) process.
The collaboration brought together experts from around the world, including Professor Yuji Iwamoto and Dr. Shotaro Tada from Nagoya Institute of Technology in Japan. Dr. Samuel Bernard of the University of Limoges, France. Professor Ravi Kumar of the Indian Institute of Technology Madras. Their findings were published in Angewandte Chemie International Edition on November 11, 2024.
Lead researcher Professor Yuji Iwamoto said, “We used a nitrogen-containing organosilicon polymer known as polysilazane as a precursor for Lewis base sites and an amorphous silicon nitride (a-SiN) matrix. We created an a-SiN scaffold with precisely controlled pore size that acts as a reaction field.”
In this study, the research team chemically modified polysilazane with boron (B) and sodium (Na), which are naturally abundant and less toxic Lewis acids. The modified material was then exposed to flowing ammonia at 1000 °C, resulting in sodium-doped amorphous silicon boron nitride (Na-doped SiBN).
Researchers used cutting-edge spectroscopic techniques to reveal how sodium-doped SiBN material interacts with hydrogen at the molecular level. They found that the material’s unique structure increases the reactivity of boron and nitrogen sites when exposed to hydrogen. Specifically, the hydrogen molecule interacts with both the boron site and the sodium ion, changing the triple-coordinated boron-nitrogen moiety into a more distorted and polar structure, and changing the quadruple-coordinated geometry in small molecules. Formed and acted like frustration Lewis. Acid (FLA) site.
Introduction of hydrogen at a certain temperature induces a change in the nitrogen-hydrogen (NH) bond, forming frustrated Lewis base (FLB) sites. These sites create a dynamic interaction pattern in FLP that allows reversible hydrogen adsorption and desorption, which was confirmed by thermodynamic experiments. The high activation energy for hydrogen release suggests strong interactions, making this material a promising catalyst for efficient and sustainable hydrogen-based reactions.
This newly developed amorphous sodium-doped SiBN material is distinguished by its exceptional thermal stability, which outperforms other molecular FLPs and makes it an ideal candidate for catalytic processes under harsh conditions. Moreover, its flexible ceramic-based structure offers immense potential for practical applications, especially in hydrogenation reactions, which are essential processes in industries such as energy and chemistry.
“This approach has the potential to develop main group-mediated solid-gas phase interactions in heterogeneous catalysis and is expected to provide valuable insights and have a major impact on this area. Professor Iwamoto explains.
The pioneering findings of this study highlight the potential of this innovative material to revolutionize sustainable catalysis.
Further information: Shotaro Tatsu et al., Novel Lewis acid-base interactions in polymer-derived sodium-doped amorphous Si-BN ceramics: Towards main group-mediated hydrogen activation, Angewandte Chemie International Edition (2024). DOI: 10.1002/anie.202410961
Provided by Nagoya Institute of Technology
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