The chemical industry is one of the largest on the planet, essential for supplying us with pharmaceuticals, agrochemicals, materials and more. Great care is taken to optimize reagents and conditions for each reaction, striving for efficiency and, increasingly, sustainability. A growing field of synthesis is mechanochemistry, in which reagents are mixed using mechanical force, making for greener reactions with less solvent usage and enabling access to a wide array of essential chemicals.
In a typical mechanochemistry setup, the reagents are placed in a jar filled with balls, which are shaken at high frequency to enable mixing and grinding. Often, solid, insoluble additives such as metal oxides or piezoelectric materials (materials which become electrically polarized under mechanical stress) are included in the reaction mixture, as mechanochemists have commonly believed these additives help activate catalysts or reagents. However, one important aspect of such methodology has often remained overlooked: mixing solid reagents in such a setup inevitably wears away at the grinding balls, which are often made of stainless steel. Now, new research published in Angewandte Chemie reveals the importance of this abrasion within mechanochemical catalysis processes, prompting researchers to reexamine their assumed reaction mechanisms.
The new study from the Okinawa Institute of Science and Technology (OIST) shows that abrasion from a range of common additives can lead to efficient reactions under mechanochemical conditions. Surprisingly, even abrasives typically considered chemically unreactive, such as tungsten carbide or diamond powder, led to efficient catalyst activation and coupling reactions.
Professor Julia Khusnutdinova, head of OIST's Coordination Chemistry and Catalysis Unit and author on the study, says, "This research changes the way we think about mechanochemical catalysts. It highlights the importance of both the additives we use and the equipment itself in driving mechanochemical reactions, helping us to consider the hidden influences that may play a part in our reaction mechanisms".
Getting to the grind
The team used cross-coupling reactions as a model for this study, given their wide application to make many different common chemicals. They showed how a high-yielding reaction in a stainless-steel container with stainless steel balls, using the same materials and conditions, was unsuccessful in a ceramic container with ceramic balls. Using a range of analysis techniques, they identified that the steel was being abraded, releasing iron, chromium and other metals into the reaction mixture. The abraded metals then activated a stable nickel pre-catalyst present in the reaction mixture, transforming it into a catalytically active species. Microscopic analysis of the abrasive additives used to activate the catalyst showed that fine particles of the abrasive additive were covered locally with a thin layer of stainless steel, scraped from the grinding balls. "This study should prompt researchers to think about their reaction equipment, and to take into consideration the impact of abrasion on chemical reactions," says first author Thomas Hasiweder. "We have proven that abrasion can play an important role in mechanochemical catalysis."
New opportunities in catalysis
Whilst the research warns of potential hidden complexities to mechanochemical mechanisms, it also provides an opportunity for cheap and convenient catalysis for a host of chemical reactions. When discussing the researchers' future plans, Prof. Khusnutdinova notes, "We want to explore the extent of reactions where such abrasion matters. With this knowledge, we can create simple cheap protocols for a wide range of mechanochemical syntheses, opening up new avenues to essential agrochemicals, drugs and beyond."
