Grinding chemicals together in an effort to be greener

The timer started, and a middle school student, Tony Mack, began his first chemistry experiment. As he weighed chemicals under a graduate student’s supervision, his father, James, a chemist at the University of Cincinnati, USA assembled glassware next to him, engrossed in his own experiment.

The two were racing to prepare a mix of stilbene molecules used to make dyes, but were employing different methods. For James Mack, the ingredients were simmered in a stirred solution in a heated flask. But for Tony, they were crushed with balls that tumbled and hit them as a machine, called a ball mill, shook them vigorously. Tony crossed the finish line while James was still two hours away, and did so with about 30% more stilbene.

“He was so happy he beat me,” James said, laughing. Conducted at James’ laboratory in 2014, the race was meant to prove a point: that milling or grinding chemicals together without a solvent, could outperform established methods and yet be safe and simple enough for an eighth-grader to do.

The technique, based on so-called mechanochemistry, or chemistry driven by mechanical force, is radically different from the traditional way of dissolving, heating and stirring chemicals in a solution. Removing solvents could help make many chemical processes used by industry more environment-friendly. “Chemists typically aren’t as concerned about solvents as they should be,” said David Constable, director of the American Chemical Society’s Green Chemistry Institute. Many commonly used solvents, like chloroform, acetone and hexane, are harmful and volatile, posing risks to people, who inhale them, as well as the environment.

Solvents also make up the vast majority of chemical waste, and consume most of the energy used in a chemical reaction. But solvents have been the norm for centuries, so many chemists tend to doubt the viability of mechanochemistry. “We learn that solution is just the way to go,” James said, “and never think about alternatives.” The green chemistry movement has motivated some chemists to ditch the old methods in favour of mechanochemistry.

“Green chemistry is a recognition that chemistry could do better through design,” said Paul Anastas, a chemistry professor at Yale University, USA.

Milling has traditionally been used to reduce particle sizes. Milling has traditionally been used to reduce particle sizes, for example, by geologists to grind rock into powder. It is also a common technique for making metal alloys and facilitating chemical reactions that involve insoluble materials like fullerenes, a molecular form of carbon with tubular or spherical structures known as carbon nanotubes.

But persuading scientists to extend mechanochemistry to the vast majority of chemical reactions normally conducted in solutions was a hard sell. In solutions, particles get the energy they need to react from heat: Chemists carefully choose the temperature for their experiments and ensure that heat is evenly distributed. But in a ball mill, energy is imparted through mechanical force when the balls strike the particles as they are tossed around. That energy is very difficult to quantify, James said.

In 2011, mechanochemist Tomislav Friscic and his team used mechanochemical methods to make bismuth subsalicylate, the active ingredient of Pepto-Bismol, by grinding together bismuth oxide and salicylic acid. The method not only does away with solvents, but also uses bismuth oxide, a safe reagent, in lieu of toxic bismuth salts. “There is really an explosion of what mechanochemistry can do,” Tomislav said. “It is clearly becoming a new reaction environment.”

Despite the progress, it is unlikely that mechanochemistry will completely eliminate solvents. “Mechanochemistry, like many other technologies, is very promising,” David said, “but it will not solve all the problems.” But the method has fostered an awareness that chemistry can be practical and environment-friendly, its proponents say. “When we focus on making things environmentally friendly, most people think of it as restrictions,” Anastas said. “But this has been an opportunity and discovery.”