Compelling Chemistry

 

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 solvent-free and yield novel materials and microstructures. Their exact mechanisms need to be thoroughly understood before they come into the mainstream in a big way.

 

Compelling Chemistry

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.

History

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 the synthesis of chemical compounds.

Advantages

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 process, 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.

Equipment

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 stainless 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 through the 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.

Applications

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 the 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 chloride 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 the 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 the 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.

Solvent-free Chemistry

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 aromatic hydrocarbons in ball mills. Mechanochemistry thus overcomes the hindrance of solvents in chemical synthesis.

Energy Efficiency

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 does 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.

Current Status

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.

Readers’ responses may be sent to k.sahasranaman@gmail.com or chemindigest@gmail.com

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