Two ball mill chambers mixing chemicals during a solid-state, mechanochemical reaction. Credit: WPI-ICReDD
Chemists at Hokkaido University and the Institute for Chemical Reaction Design and Discovery (WPI-ICReDD) have developed the first high-performance catalyst specifically designed and optimized for solid-state, mechanochemical synthesis.
The team found that by attaching long polymer molecules to a metal catalyst, they could trap the catalyst in a fluid-phase, which enabled efficient reactivity at near room temperature. This approach, reported in the Journal of the American Chemical Society, could bring cost and energy savings if adapted for wide application in chemical research and industry.
Chemical synthetic reactions are usually performed in solution, where dissolved molecules can intermingle and react freely. In recent years, however, chemists have developed a process called mechanochemical synthesis, in which solid state crystals and powders are ground together. This approach is advantageous because it reduces the use of hazardous solvents and can allow reactions to proceed faster and at lower temperatures, saving energy costs. It can also be used for reactions between compounds that are difficult to dissolve in available solvents.
However, solid-state reactions occur in a very different environment than solution-based reactions. Previous studies found that palladium complex catalysts originally designed for use in solution often did not work sufficiently in solid-state mechanochemical reactions, and that high reaction temperatures were required. Using the unmodified palladium catalyst for solid-state reactions resulted in limited efficiency due to the tendency of palladium to aggregate into an inactive state. The team chose to embark in a new direction, designing a catalyst to overcome this mechanochemical problem of aggregation.
“We developed an innovative solution, linking palladium through a specially designed phosphine ligand to a large polymer molecule called polyethylene glycol,” researcher Hajime Ito explains.
The polyethylene glycol molecules form a region between the solid materials that behaves like a molecular-level fluid phase, where mechanochemical Suzuki-Miyaura cross-coupling reactions proceed much more efficiently and without the problematic aggregation of palladium. In addition to achieving significantly higher product yields, the reaction proceeded effectively near room temperature—the previously best-performing alternative required heating to 120°C. Similar cross-coupling reactions are widely used in research and the chemical industry.
2023-03-09 22:00:04
Source from phys.org
Tailoring Catalysts for Solid-State Reactions
Solid-state reactions are an essential part of modern industrial chemistry, with numerous applications in the synthesis of advanced materials, the fabrication of electronic devices, and the production of fine chemicals. These reactions occur between solid-phase reactants and products, without the intervention of a solvent or a gaseous atmosphere, thus enabling the direct conversion of bulk solids into desired products. However, due to the high activation energies and slow diffusion rates inherent in solid-state reactions, their kinetics and selectivity can be greatly improved by the use of suitable catalysts. In this article, we will discuss the principles and strategies of tailoring catalysts for solid-state reactions, with emphasis on recent advances in the field.
The role of catalysts in solid-state reactions
Catalysts are substances that increase the rate of a chemical reaction without being consumed or drastically altered in the process. In the case of solid-state reactions, catalysts can enhance the reaction kinetics by several mechanisms, such as:
– Lowering the activation energy of the reaction by providing a lower-energy pathway for the formation of intermediate species or the breaking of chemical bonds.
– Increasing the surface area and reactivity of the solid reactants by creating defects, vacancies, and active sites that facilitate the adsorption and diffusion of reactant molecules.
– Altering the thermodynamics of the reaction by inducing or suppressing certain reaction pathways, or by changing the equilibrium constants of the involved species.
Depending on the nature of the reaction and the desired products, various types of catalysts can be used in solid-state chemistry, such as metals, metal oxides, metal sulfides, metal halides, zeolites, clays, and organic molecules. The choice of catalyst depends on factors such as the reactant properties, the desired reaction mechanism, the reaction conditions (temperature, pressure, atmosphere), and the scale of the process.
Tailoring catalysts for solid-state reactions
Tailoring catalysts for solid-state reactions involves designing and optimizing their properties to maximize their effectiveness and selectivity towards the desired products. This can be achieved by several approaches, such as:
– Modifying the chemical composition, crystal structure, and morphology of the catalysts to enhance their activity, stability, and specificity.
– Controlling the size, shape, and dispersion of the catalyst particles to optimize their surface area and accessibility to the reactants.
– Introducing dopants or promoters to the catalysts to enhance their performance or to tailor their selectivity towards specific products.
– Using hybrid or multifunctional catalysts that combine different catalytic functionalities, such as acid-base, redox, or Lewis acid-base.
Recent advances in tailoring catalysts for solid-state reactions
In recent years, several new approaches and materials have been developed for tailoring catalysts for solid-state reactions. Some examples include:
– Metal-organic frameworks (MOFs), which are porous materials consisting of metal ions or clusters linked by organic ligands. MOFs have been explored as catalysts for various solid-state reactions, such as oxidation, hydrogenation, and carbon dioxide conversion. Their high surface area, tunable pore size, and diverse functionalities make them promising candidates for tailoring catalytic performance.
– Nanostructured catalysts, which are catalyst particles with sizes below 100 nanometers. Nanostructured catalysts have been shown to exhibit enhanced activity and selectivity towards certain solid-state reactions, due to their high surface energy and unique physicochemical properties. For example, nanosized gold particles have been used as catalysts for the partial oxidation of methane to methanol, with high selectivity and efficiency.
– Single-atom catalysts (SACs), which are catalysts consisting of individual metal atoms anchored on support materials. SACs have attracted attention as highly efficient and selective catalysts for various reactions, including solid-state reactions. Their unique electronic and geometric structures can enable precise control of the catalytic performance and product selectivity.
Conclusion
Tailoring catalysts for solid-state reactions is a vital aspect of modern chemical synthesis and engineering. By understanding the principles of catalyst design and optimization, researchers can create novel materials and strategies for enhancing the efficiency, selectivity, and sustainability of solid-state reactions. This can lead to the development of new functional materials, improved manufacturing processes, and greener approaches to industrial chemistry. Further research and innovation in this field are essential for meeting the growing demands of modern society for advanced materials and sustainable chemistry.