Ceramic catalyst uses sodium and boron to drive sustainable industrial reactions

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Heterogeneous catalysts speed up chemical reactions by being in a different state than the reactants. They are efficient and stable, even under challenging conditions such as high temperature or pressure. Traditionally, metals like iron, platinum, and palladium have been widely used in industries like petrochemicals and agriculture for important reactions such as hydrogenation and Haber's process.

However, these metals are rare and can have problems like buildup from coking. Scientists are increasingly exploring common elements as catalysts for more sustainable and cost-effective industrial applications.

In the mid-2000s, the introduction of the frustrated Lewis pair (FLP) concept marked a major advancement in catalysis, particularly in small molecule activation. An FLP is made up of a combination of two components—one acting as a Lewis acid and the other as a Lewis base—that are unable to fully react with each other due to spatial or electronic hindrance. This "frustration" leaves them in a highly reactive state, allowing them to activate stable molecules like hydrogen, carbon dioxide, or ammonia, which are normally quite hard to break apart.

FLPs stand out because they have multiple active sites, making them more reactive and selective compared to traditional catalysts, which typically have just one active site. There are two main types of FLPs: heterogeneous defect-regulated FLPs and molecular-based homogeneous FLPs.

The first type controls the number of active sites through surface defects; it can be tedious to accurately tune its reactivity and control its stability. The second type involves small molecules where the acid-base pair exists within the same molecular structure, making it easier to adjust their reactivity by simply changing the surrounding components.

A study has broken new ground by adapting molecular-based FLPs for use in solid-state systems. The researchers achieved this by leveraging the chemical versatility of pre-ceramic polymers through the Polymer-Derived Ceramic (PDC) process.

This collaborative effort brought together experts from around the globe, including Professor Yuji Iwamoto and Dr. Shotaro Tada from Nagoya Institute of Technology, Japan; Dr. Samuel Bernard from the University of Limoges, France; and Professor Ravi Kumar from the Indian Institute of Technology Madras, India. Their findings were published in Angewandte Chemie International Edition on November 11, 2024.

Professor Yuji Iwamoto, the lead researcher, explains, "We used a nitrogen-containing organosilicon polymer, known as polysilazane, as a precursor for Lewis base sites as well as for the amorphous silicon nitride (a-SiN) matrix. By converting it through a thermochemical process, we created the a-SiN scaffold with precisely controlled pore sizes that act as nanoconfined reaction fields."

In this study, the research team chemically modified polysilazane with boron (B)—a naturally abundant and less toxic Lewis acid—and sodium (Na). The modified material was then exposed to flowing ammonia at 1000 °C, which resulted in sodium-doped amorphous silicon-boron-nitride (Na-doped SiBN).

Using cutting-edge spectroscopic techniques, the researchers uncovered how the sodium-doped SiBN material interacted with hydrogen at a molecular level. They found that the unique structure of this material enhanced the reactivity of boron and nitrogen sites when exposed to hydrogen. Specifically, hydrogen molecules interacted with both the boron sites and the sodium ions, transforming the 3-fold-coordinated boron-nitrogen moiety into a more distorted and polar structure to form a 4-fold-coordinated geometry with small molecules, acting as frustrated Lewis acid (FLA) sites.

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When hydrogen was introduced at certain temperatures, it triggered changes in the nitrogen-hydrogen (N-H) bonds, leading to the formation of frustrated Lewis base (FLB) sites. These sites created a dynamic interaction pattern of the FLP that enabled reversible hydrogen adsorption and desorption, confirmed through thermodynamic experiments. The high activation energy for hydrogen release suggested strong interactions, making the material a promising catalyst for efficient and sustainable hydrogen-based reactions.

This newly developed amorphous sodium-doped SiBN material stands out for its exceptional thermal stability, surpassing other molecular FLPs and making it an ideal candidate for catalytic processes under harsh conditions. Additionally, its flexible ceramic-based structure offers immense potential for practical applications, particularly in hydrogenation reactions, which are essential processes in industries like energy and chemicals.

"This approach holds promise for advancing main-group-mediated solid-gas phase interactions in heterogeneous catalysis, offering valuable insights and promising significant impacts in this domain," explains Professor Iwamoto.

The pioneering findings of this study highlight the potential of this innovative material to revolutionize sustainable catalysis.

More information: Shotaro Tada et al, Novel Lewis Acid‐Base Interactions in Polymer‐Derived Sodium‐Doped Amorphous Si−B−N Ceramic: Towards Main‐Group‐Mediated Hydrogen Activation, Angewandte Chemie International Edition (2024). DOI: 10.1002/anie.202410961

Journal information: Angewandte Chemie International Edition

Provided by Nagoya Institute of Technology