Japanese scientists have successfully applied mechanical forces generated in a ball mill to initiate industrially important chemical reactions. Such so-called mechanoredox reactions may offer an attractive alternative to photoredox-based reactions now in use, they believe.
Led by chemistry professors Hajime Ito and Koji Kubota of the Institute for Chemical Reaction Design and Discovery (WPI-ICReDD) at Hokkaido University, Hokkaido, Japan, the team showed that piezoelectric materials such as barium titanate (BaTiO3), when subjected to mechanical pressure, generate electric potentials that can activate chemical reactions.
The scientists used BaTiO3 to activate aryl diazonium salts. The highly reactive radicals produced undergo high-efficiency bond-forming reactions such as arylation and borylation. The team also demonstrated that the borylation reaction could occur by striking the mixture in a plastic bag with a hammer. Details appear in a recent article in Science.
“This is the first example of arylation and borylation reactions using mechanically induced piezoelectricity. Our solvent-free system using a ball mill has enabled us to eliminate organic solvents, making the reactions easier to handle, more environmentally friendly, and applicable even to reactants that cannot be dissolved in the reaction solvent,” says Kubota.
Figure 1. Initial components are placed in steel jar (left) to which a steel ball is added, and then undergo vigorous shaking, resulting in reaction products (right). Source: Kubota, K. et al. Science, December 19, 2019.
The process offers an alternative route to the one employed by the photoredox catalysts developed over the last ten years that use visible light to enable highly specific and efficient chemical reactions, the team notes.
Attention at the WPI-ICReDD now focuses on tuning the mechanically generated electric potentials. “The performance of our piezoelectric catalysis is still very low as an excess amount of the catalyst is required to achieve high reactivity. Design and creation of new piezoelectric catalysts should improve the reactivity as well as the tuneability of the mechanoredox reactions. We are currently working on this,” explains Kubota.
If this work succeeds, it could usher in new, low-cost, high-performance piezoelectric-catalysis-driven processes, he reckons.
Because of the mechanism of the reaction, these processes could replace most photoredox reactions currently in use, he adds. “We have already found other redox reactions using mechanoredox catalysis and will report in due course.”
However, another issue that still needs addressing is how to generate enough mechanical impact to activate piezoelectric catalysis on an industrial scale. “This is a potential challenge and careful optimization studies on mechanochemical parameters must be performed,” admits Kubota.
Nevertheless, the team already has started collaborative research with several companies to consider future industrial applications, he reveals.