Adaptation of catalysts for solid state reactions

Chemists at Hokkaido University and the Institute for Chemical Reaction Planning and Discovery (WPI-ICReDD) have developed the first high-performance catalyst specifically designed and optimized for solid-state mechanochemical synthesis.

The team discovered that by attaching long polymer molecules to a metal catalyst, they could trap the catalyst in a liquid phase, which enabled efficient reactivity at near room temperature. This approach, referred to in Journal of the American Chemical Societycould yield cost and energy savings if adapted for widespread application in chemical research and industry.

Chemical synthesis reactions are usually carried out in solution, where the solutes can mix and react freely. In recent years, however, chemists have developed a process called mechanochemical synthesis, in which crystals and solid-state 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 perform well in solid-state mechanochemical reactions and that high reaction temperatures were required. The use of the unmodified palladium catalyst for solid state reactions resulted in limited yield due to the tendency of the palladium to aggregate in an inactive state. The team chose to start in a new direction, designing a catalyst to overcome this mechanochemical problem of aggregation.

“We developed an innovative solution by linking palladium via a specially designed phosphine linker to a large polymer molecule called polyethylene glycol,” explains researcher Hajime Ito.

The polyethylene glycol molecules form a region between the solid materials that behaves like a molecular-level fluid phase, where the mechanochemical Suzuki-Miyaura cross-coupling reactions proceed much more efficiently and without the problematic palladium agglomeration. In addition to achieving significantly higher product yields, the reaction proceeded efficiently near room temperature—the previously best alternative requiring heating to 120°C. Similar cross-coupling reactions are widely used in research and the chemical industry.

“This is the first demonstration of a system specifically modified to exploit the potential of palladium complex catalysts in the unique environment of a mechanochemical reaction,” says researcher Koji Kubota.

They believe it could be adapted for many other reactions, as well as for catalysts that use elements other than the transition metals on the periodic table.

Wider adoption of the process, and others like it, could ultimately yield significant cost and energy savings in commercial chemical processes, while enabling more environmentally friendly, large-scale production of many useful chemicals.