Grinding Chemicals Together in an Effort to be Greener

The timer started, and a middle school student named 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, 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 Dr. Mack, the ingredients 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 Dr. Mack was still two hours away, and did so with about 30 percent more stilbene. “He was so happy he beat me,” Dr. Mack said, laughing. Conducted at Dr. Mack’s laboratory in 2014, the race was designed 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 inexperienced 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 environmentally friendly. “Chemists typically aren’t as concerned about solvents as they should be,” said David Constable, the 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 because solvents have been the norm for centuries, many chemists tend to doubt the viability of mechanochemistry. “We learn that solution is just the way to go,” Dr. Mack said, “and never think about alternatives.” The green chemistry movement has motivated some chemists to ditch the old solution 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 who developed the Environmental Protection Agency’s groundbreaking green chemistry program in 1991. 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 or “buckyballs.” But persuading scientists to extend mechanochemistry to the vast majority of chemical reactions normally conducted in solutions was a hard sell, in part because it was uncharted territory. 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, Dr. Mack said. Without a solvent, chemists could not be confident that all the particles would mix well enough to meet each other and react. Stuart James, a mechanochemist at Queen’s University Belfast in Northern Ireland, related the first time he brought a ball mill, which cost 3,000 pounds, or about $4,870, into his laboratory in 2003. “I told the group what it was, and my students thought I was crazy,” he said. “It’s really about taking a risk,” Dr. Mack said of those chemists who eventually adopted the newer method. They demonstrated that a ball mill could impart enough energy to break and form strong bonds like carbon-carbon and carbon-metal bonds. They also discovered that milling enabled reactions that could not be performed in solutions, opening up new possibilities. In 2011, the 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. In 2013, Dr. Friscic reported a new single-step process to prepare sulfonylguanidines, a family of chemicals used in herbicides and pharmaceuticals, through grinding. His research team at McGill University in Montreal tried to reproduce the process in solution, but failed. Last year, a research group from the Ames Laboratory at Iowa State University applied for a patent on a mechanochemical method to make pure alane, an aluminium-based material for hydrogen storage. Solution methods typically produce impure alane because solvent molecules tend to stick to it. “There is really an explosion of what mechanochemistry can do,” Dr. Friscic said. “It is clearly becoming a new reaction environment.” Figuring out the rules of the game has been a major research task. The mechanochemists slowly learned how to control the milling process by changing parameters like the shaking speed, number of balls, ball size and materials of the reaction vessel and the balls. Some chemists have started to apply the technique to industry. Dr. Friscic is introducing mechanochemistry to drug and mining companies. In 2012, Dr. James set up a company called MOF Technologies to produce a type of porous material with potential applications in storing hydrogen and natural gas, without using solvents. The company’s facilities can churn out several kilograms of product an hour, and recently received funding to build a pilot plant that would operate on a much larger scale. Despite the progress, it is unlikely that mechanochemistry will completely eliminate solvents. “Mechanochemistry, like many other technologies, is very promising,” Dr. Constable said, “but it will not solve all the problems.” But the method has fostered a growing awareness that chemistry can be both practical and environmentally friendly, its proponents say. At McGill, chemistry students are taught to measure the energy, water and materials used in each experiment they perform, to understand the environmental costs of chemistry. In addition to milling, other research areas include developing an energy efficient way of heating chemical reactions with microwaves and clean, nontoxic solvents like supercritical carbon dioxide — carbon dioxide held at such a high temperature and pressure that it behaves like a gas and a liquid at once. (It’s used to decaffeinate coffee, for example.) “When we focus on making things environmentally friendly, most people think of it as restrictions,” Dr. Anastas said. “But this has been opportunity and discovery.”

New York Times, 18 July 2016 ; ;