Solvents are commonly used for performing photochemical reactions. Many of these solvents are toxic, and require additional downstream processing to purify the product and treat waste. Now, by combining light with mechanical energy in ball mills, researchers at Ruhr University Bochum (Germany; www.ruhr-uni-bochum.de) have succeeded in virtually eliminating the need for solvent, while significantly reducing the energy needed to perform a number of reactions. The study, performed by the research team of professor Lars Borchardt, Chair of Inorganic Chemistry, was reported in a recent issue of Angewandte Chemie International Edition.
The researchers in Bochum used ball mills as reactors. Here, the starting materials are placed in vessels together with milling balls and shaken at high frequencies. This creates high-energy impacts that provide the mechanical energy for the reaction and thoroughly mix the substances. In a photoreactor that was specially adapted to a ball mill, the researchers were able to synthesize triphenylene via two routes — the Mallory reaction and by cyclodehydrochlorination (CDHC), resulting in yields of 81 and 92%, respectively. The reaction was successfully scaled up to the gram scale. Also, the regioselective assembly of nanographenes by mechanochemistry was demonstrated.
“This new process enabled us to carry out specific reactions and synthesize chemical substances in a much more sustainable way,” says Borchardt. “We reduced reaction times by up to 56%, while using 98% less solvent than in equivalent syntheses done with conventional methods. Last but not least, the new photoreactor consumes almost 80% less energy than conventional equipment.”