Transition metals have been traditionally utilized as catalysts to facilitate chemical reactions, converting smaller molecules into valuable products. Due to their cost and limited natural availability, researchers are searching for more readily available alternatives. A recent study has introduced a catalyst that does not rely on transition metals, utilizing a concept known as ‘frustrated Lewis pairs’ to activate hydrogen. This advancement could pave the way for more sustainable, economical, and effective chemical processes.
Heterogeneous catalysts enhance chemical reactions by being in a different state than the reactants. They exhibit high efficiency and stability, even in tough conditions like high pressure or temperature. Historically, metals such as iron, platinum, and palladium have been commonly used in industries like petrochemicals and agriculture for crucial reactions including hydrogenation and the Haber process. Nonetheless, these metals are rare and can suffer from issues such as coking buildup. As a result, researchers are increasingly investigating more abundant elements as catalysts to promote sustainable and cost-effective industrial processes.
In the mid-2000s, the concept of frustrated Lewis pairs (FLPs) emerged, representing a significant leap forward in catalysis, especially for activating small molecules. An FLP consists of two components: one serves as a Lewis acid and the other as a Lewis base, but they can’t react completely due to spatial or electronic obstacles. This “frustration” keeps them in a highly reactive state, enabling them to activate stable molecules such as hydrogen, carbon dioxide, or ammonia, which are typically difficult to decompose. FLPs are distinct because they possess multiple active sites, making them more reactive and selective compared to conventional catalysts, which usually feature only one active site. FLPs can be classified into two main types: heterogeneous defect-regulated FLPs, which control active site numbers via surface defects, and molecular-based homogeneous FLPs, where the acid-base pair exists within a single molecular entity, allowing easier adjustment of reactivity by altering surrounding components.
A recent study has made strides by adapting molecular-based FLPs for use in solid-state systems. Researchers accomplished this by utilizing the chemical versatility of pre-ceramic polymers through the Polymer-Derived Ceramic (PDC) approach. This collaborative project involved experts from various countries, including Professor Yuji Iwamoto and Dr. Shotaro Tada from Nagoya Institute of Technology in Japan, Dr. Samuel Bernard from the University of Limoges in France, and Professor Ravi Kumar from the Indian Institute of Technology Madras in India. Their research findings were published online on August 9, 2024, and recognized as a “Hot Paper” on October 2, 2024, appearing in Volume 63, Issue 46 of the journal Angewandte Chemie International Edition on November 11, 2024.
Lead researcher Professor Yuji Iwamoto states, “We utilized a nitrogen-containing organosilicon polymer known as polysilazane as a precursor for Lewis base sites and the amorphous silicon nitride (a-SiN) matrix. By converting it through a thermochemical process, we produced an a-SiN scaffold with precisely controlled pore sizes that function as confined reaction fields at the nanoscale.”
In this investigation, the research team chemically modified polysilazane with boron (B)—which is abundant and less toxic—and sodium (Na). The modified compound was then treated with flowing ammonia at 1000 °C, resulting in sodium-doped amorphous silicon-boron-nitride (Na-doped SiBN).
Using advanced spectroscopic methods, the researchers examined how the sodium-doped SiBN material interacted with hydrogen at a molecular level. They discovered 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 three-fold coordinated boron-nitrogen configuration into a more distorted and polar state, forming a four-fold coordinated geometry with small molecules, thereby acting as frustrated Lewis acid (FLA) sites. When hydrogen was introduced at certain temperatures, it caused changes in the nitrogen-hydrogen (N-H) bonds, leading to the emergence of frustrated Lewis base (FLB) sites. This created a dynamic interaction pattern within the FLP that facilitated reversible hydrogen adsorption and desorption, as confirmed through thermodynamic tests. The substantial activation energy required for hydrogen release indicated strong interactions, marking the material as a promising catalyst for efficient and sustainable reactions involving hydrogen.
This newly created amorphous sodium-doped SiBN material is distinguished by its superior thermal stability, exceeding that of other molecular FLPs, making it an ideal candidate for catalytic processes in harsh environments. Additionally, its adaptable ceramic-based structure presents significant opportunities for practical applications, particularly in hydrogenation reactions, which are critical in industries like energy and chemicals.
Professor Iwamoto explains, “This method shows potential for enhancing main-group-mediated solid-gas phase interactions in heterogeneous catalysis, providing valuable insights and the prospect of substantial advancements in this area.”
The groundbreaking results of this study underscore the potential of this novel material to transform sustainable catalysis.
