Mechanochemistry offers a green alternative to conventional chemical processes, and in fact this was one of the main triggers for its revival. The second, more exciting, benefit of mechano-chemistry is that it achieves reactions previously considered impossible, and in the process yields unique molecules and compounds.
This month, our columnist takes a peep into the intriguing world of Mechanochemistry, where chemical reactions are initiated by mechanical forces. Mechanochemical reactions are almost solventfree and yield novel materials and microstructures. Their exact mechanisms need to be thoroughly understood before they come into the mainstream in a big way.
To initiate a chemical reaction, reactants usually need heat; sometimes light. But there are chemical reactions that can be triggered by mechanical energy. Welcome to the fascinating world of Mechanochemistry. Mechanochemistry is that branch of chemistry which is concerned with chemical and physicochemical changes of substances due to the influence of mechanical energy. Mechanochemistry holds out much promise and has been gaining considerable traction in recent years.
Greek philosopher Theoprastus (circa 4th C.B.C.) crushing mercury sulfide and vinegar in a copper mortar and pestle to produce elemental mercury is probably the first chronicled account of a mechanochemical reaction. However, Carey Lea is widely regarded as the Father of mechanochemistry based on his work in the late 19th Century. Lea was the first to demonstrate that reactions initiated by mechanical energy are distinctly different from those initiated by heat. Lea’s work on the decomposition of silver and mercury halides are frequently cited as historic milestones of mechanochemistry. They are also the clearest examples of the uniqueness of mechanochemical reactions. In the 1960s, mechanochemistry was widely used to make metal alloys. Now there is a renewed interest in mechanochemistry as a sustainable “green” route to synthesis of chemical compounds.
Mechanochemistry has two notable advantages. First one is environmental. Unlike conventional chemistry, mechanochemical reactions do not require the reactants to be dissolved in a solvent. Thus, mechanochemistry offers a green alternative to conventional chemical processes, and in fact this was one of the main triggers for its revival.
The second, more exciting benefit of mechanochemistry is that it achieves reactions previously considered impossible, and in the pro- cess, yields unique molecules and compounds. There are many reports of mechanochemical transformations that either do not happen or happen only with great difficulty in conventional solution-based chemistry. Mechanochemical reactions produce compounds and microstructures that are essentially different from conventional chemical reactions. This has huge implications for pharmaceutical and solar energy industries.
Forces used in mechano-chemistry are compression, shear and friction. Mechanochemical reactions are carried out in ball mills. Shaker and Planetary mills are the most preferred designs. In the shaker design, the jars swing back and forth with a frequency that determines the milling intensity. In the planetary design, the jar rotates around a central axis while spinning around its own axis. Jars and
milling balls are made of stain
less steel, zirconia, tungsten
carbide or even PTFE. Different
types of mechanical motions have been found to yield different types of products with varying kinetics. Energy input is adjusted by varying milling
time and frequency and also
K Sahasranaman Independent Consultant - Process Engineering, Energy, Utilities and Safety