through choice of the milling media. Milling balls made from denser materials impart greater kinetic energy during the milling process. An alternative approach to controlling the mechanochemical reactions is through the use of small controlled amounts of liquid or solid additives. Called Liquid Assisted Grinding (LAG), this offers advantages like shorter reaction time and better product selectivity.
Mechanochemical reactions are not limited to solids. Vigorous mixing or sonication of liquids can result in molecular forces that strain and break the bonds.
Mechanochemistry is no longer a laboratory curiosity. It is used to produce useful products with unique properties.
Photovoltaic cells using Perovskites produced by mechanochemical reaction have shown significant improvements in efficiency. Named in honour of Leo Perovski, the Russian geologist, Perovskites are a large group of materials like Calcium Titanate, finding application in photovoltaic cells. Solvent-based process for production of Perovskites leave behind residues affecting its crystalline structure. Mechanochemically synthesised Perovskites eliminate these structural defects leading to the higher efficiency of photovoltaic cells.
Production of noble metal salts, often used as catalysts and in electronics, require aggressive conditions. Their chloride salts, for example, are made by reacting the metals with aqua regia. Mechanochemistry offers a convenient and benign alternative. Vigorous shaking of the noble metal powders in a zirconia jar with simple halides like potassium chlo- ride or ammonium chloride results in the production of noble metal halides. By adding ligands to the mill, various gold and palladium complexes can also be produced.
Mechanochemistry holds out good promise in waste management. Difficult to recycle wastes like plastics and rubber have been successfully treated by mechanochemical processes. Lead from spent Cathode Ray Tubes has been recovered by co-grinding the glass with elemental sulphur to yield lead sulphide. Similarly, mechanochemical methods have been successful in immobilising heavy metals from fly ash.
Mechanochemistry is used for synthesis of pharmaceutical cocrystals, which are emerging as an alternative solid drug form with tailored physicochemical properties. Some cocrystals, in fact, are possible only through the mechanochemical route and not through the solvent process.
Many conventional chemical reactions are carried out in solvents, which are often hazardous or toxic. But solvents can sometimes suppress or slow down the reactions. Also, solvent-based chemistry presupposes good solubility of the reactants, which can sometimes act as a deterrent in the choice of raw materials. A case in point is the synthesis of nanographenes. Assembling large organic aromatic structures require either harsh conditions or modification of the starting materials to make them more soluble in solvents. Mechanochemistry offers a solvent-free reaction environment which makes hitherto challenging reactions simpler and more accessible. Scientists in Germany have recently synthesized nanographenes and large polycyclic ar- omatic hydrocarbons in ball mills. Mechanochemistry thus overcomes the hindrance of solvents in chemical synthesis.
Mechanochemical reactions are also energy efficient. Ball mills are energy intensive when used for comminution because the high lattice energies have to be overcome. But mechanochemical reaction do not require particle size reduction to nanometer scale as the reaction depends on particle mixing and surface activation. Energy considerations of mechanochemical processes are yet to be fully understood. The current understanding is that mechanochemical reactions consume less energy than equivalent solvent chemistry.
Mechanochemistry is rapidly emerging not only as a cleaner alternative to conventional chemical transformations, but also a novel tool to make unique molecules and materials. The main drawback of mechanochemistry is that we know very little about it currently. Even as the library of mechanochemical reactions is rapidly expanding, the knowledge base of this chemistry is still in its infancy. The atomic and molecular-level mechanisms underlying mechanochemical reactions are yet to be fully understood. In comparison, conventional chemistry has a head start of 200 years. The qualitative benefits of Mechanochemistry have been fully demonstrated. Systematic studies and precise theoretical models are required to advance this science to the next level.