Reviewed by Lexie CornerJan 22 2025
A study published in Angewandte Chemie International Edition by researchers from Nagoya Institute of Technology, the University of Limoges, and the Indian Institute of Technology Madras explores the adaptation of molecular-based frustrated Lewis pairs (FLPs) for solid-state systems.
Development of a transition metal-free sodium-doped amorphous silicon-boron-nitride (Na-doped SiBN) for small molecule activation and catalysis. The Polymer-Derived Ceramic method was employed to design and synthesize a transition metal-free, sodium-doped amorphous SiBN ceramic made up of silicon (Si), boron (B), and nitrogen (N) atoms for small molecule activation and catalysis. The distribution of sodium (Na+) and B sites within the amorphous silicon nitride enhances the reactivity of both B and N sites, leading to the formation of a frustrated Lewis pair (FLP) motif upon exposure to hydrogen. Image Credit: Professor IWAMOTO Yuji from Nagoya Institute of Technology, Japan
Heterogeneous catalysts facilitate chemical reactions by existing in a different phase than the reactants, offering efficiency and stability under extreme conditions such as high temperatures or pressures. Metals like iron, platinum, and palladium have long been used in key industrial reactions, including hydrogenation and the Haber process, particularly in petrochemicals and agriculture.
However, these metals are scarce and present challenges such as coke accumulation, prompting researchers to explore more abundant and sustainable alternatives for industrial catalysis.
The frustrated Lewis pair (FLP) concept, introduced in the mid-2000s, has advanced catalysis by enabling efficient activation of small, stable molecules like ammonia, carbon dioxide, and hydrogen. An FLP consists of a Lewis acid and a Lewis base that cannot fully react with each other due to spatial or electronic constraints. This "frustration" creates a highly reactive state, allowing activation of otherwise inert molecules. Unlike traditional catalysts with a single active site, FLPs have multiple active sites, enhancing their reactivity and selectivity.
FLPs are classified into two main types: heterogeneous defect-regulated FLPs and molecular-based homogeneous FLPs. Heterogeneous FLPs rely on surface imperfections to control active site availability, but fine-tuning their stability and reactivity can be complex. Homogeneous FLPs, composed of small molecules with integrated acid-base pairs, offer greater flexibility, as their reactivity can be modified by altering surrounding chemical groups.
Using the Polymer-Derived Ceramic (PDC) technique, researchers used the chemical versatility of pre-ceramic polymers to advance FLP development. This work was achieved through a collaborative international effort.
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.
Yuji Iwamoto, Professor, Nagoya Institute of Technology
The research team chemically modified polysilazane by incorporating sodium (Na) and boron (B), a naturally occurring and less hazardous Lewis acid. The modified material was then exposed to flowing ammonia at 1000 °C, producing sodium-doped amorphous silicon-boron-nitride (Na-doped SiBN).
Using advanced spectroscopic techniques, the researchers investigated molecular interactions between Na-doped SiBN and hydrogen. They observed that the material’s unique structure enhanced the reactivity of nitrogen and boron sites upon exposure to hydrogen.
Specifically, the interaction of hydrogen molecules with sodium ions and boron sites transformed the 3-fold-coordinated boron-nitrogen moiety into a more distorted and polar structure. This resulted in the formation of a 4-fold-coordinated geometry that served as frustrated Lewis acid sites. Frustrated Lewis base sites were also generated through changes to nitrogen-hydrogen (N-H) bonds at specific temperatures, creating a dynamic interaction pattern enabling reversible hydrogen adsorption and desorption.
Thermodynamic analysis revealed strong hydrogen interactions, indicated by a high activation energy for hydrogen release. This characteristic makes the material a potential catalyst for efficient and durable hydrogen-based reactions.
The newly developed amorphous Na-doped SiBN material offers excellent thermal stability, distinguishing it from other molecular FLPs. Its adaptable ceramic-based structure provides a strong foundation for applications in hydrogenation reactions, which are critical in the chemical and energy industries.
“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,” explained Iwamoto.
The findings of this study highlight the potential of the novel material to advance sustainable catalysis.
JSPS KAKENHI, Grant Number JP20K05076, and CNRS via the International Research Project (IRP) 'Functional inorganic materials for global social challenges' labeled FRESH funded the study. N. Asakuma appreciates receiving financial support from JST SPRING under Grant Number JPMJSP2112.
Journal Reference:
Tada, S., et. al. (2024) 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. doi.org/10.1002/anie.202410961