Three years ago, the International Union of Pure and Applied Chemistry (IUPAC) published a list of ten chemical innovations that will change our world. IUPAC nominated reactive extrusion, a mechanochemical process, as one of the emerging technologies in chemistry with the potential to make our planet more sustainable. It was mentioned along with other significant innovations such as Solid-state batteries, 3D bioprinting, and Metal-Organic Frameworks (MOF).
Despite its obvious sustainable benefits, mechanochemistry is still very much undervalued by the chemical industry.
We met more than 20 R&D managers and scientists working in the chemical industry in Europe and we realized that most of them had limited knowledge about mechanochemistry and certainly no experience with this growing field of chemistry.
But what is mechanochemistry all about and what makes it so interesting?
★ Mechanochemistry enables remarkable energy efficiency, waste reduction and better safety versus thermochemical processes.
★ Mechanochemistry has been applied for organic, inorganic, and polymer synthesis, and new applications are explored daily.
★ Mechanochemistry can be performed at a large scale in continuous process pieces of equipment that are available today.
★ Mechanochemistry is the future of chemistry, and it is time to realize it.
How it helps remove the use of solvents in chemical processes
Almost every type of chemical reaction in industrial chemistry requires a solvent. In the wet chemical industry up to 90% of the reacting masses consist solely of solvents. Because of the major impact of solvent on the reaction kinetics, yield, and selectivity, its selection criteria have become more critical.
Besides, many of the solvents are potentially toxic, so their production, storage, and disposal have been a serious concern for the environment and the health of living organisms. Solvents can also introduce many other unavoidable potential challenges, such as the generation of solvent waste, prolonged reaction time or tedious post-synthesis cleaning
Remarkably, with mechanochemistry, solvents become far less important as mechanochemical reactions involve solid-state grinding of materials performed under completely solvent-free conditions or a catalytic amount of liquid additives, such as water or green solvents which anticipates the modern technique of liquid-assisted process (LAG). The immediate consequences of this paradigm shift include:
♻️ Smaller size equipment, reducing the energy and time required to heat and cool down the batches
♻️ Cost reduction enabled by the reduction of raw material usage (i.e. solvent), energy consumption and process steps
♻️ Improvement of process safety by avoiding manipulation of a large quantities of solvents and high temperatures
♻️ Improvement of efficiency because of a higher concentration of reactives and therefore an increase in reaction speed and yield.
The above-mentioned features along with reduced waste production contribute to making mechanochemistry an environmentally friendly and sustainable process perfectly aligned with the 12 principles of green chemistry.
Considering those proven benefits, I am still wondering about the obstacles to the industrial development of mechanochemistry.
To know more from Mechanochemistry scientists and experts, the Center for Mechanical Control of Chemistry (CMCC) released a series of webinars on its youtube channel: here
Its pivotal role in emerging technologies
Over the past decade, the number of applications of mechanochemistry, demonstrating a real benefit over standard processes, has been growing drastically.
Mechanochemistry is very relevant when the existing process exhibits the following challenges:
- A need for high dilution because of poor solubility or high viscosity
- A very high temperature is required to perform the reaction
- A Prolonged reaction time from hours to days
- The relevant solvent for the reaction induces an adverse effect on outcomes
Here is a selection of 4 applications under investigation where mechanochemistry has been proven to be a relevant alternative to existing processes. This is a non-exhaustive list that focuses on a few applications that could contribute to tackling the world’s biggest problems today.
Reducing solvent use in Organic Synthesis: Despite the ability to reproduce most organic reactions without solvents, the mechanochemical process decreases reaction time and increases space-time yield. It includes C-C, C-Ni, and C-O bond reactions, and couplings as well as Cycloaddition, Oxidations, and Reductions.
Addressing Ocean pollution with biodegradable and Bio-sourced polymers: Extrusion is a well established technique to process polymers to form continuous profiles, but its uses in a reactive configuration are less common despite its capabilities such as bulk polymerization, grafting, compatibilization or chain extension. Recently, the combination of enzymes as biocatalysts with Reactive Extrusion enabled the bioconversion of polymeric materials.
Enabling Hydrogen storage with Metal-Organic Framework (MOF): Mechanosynthesis offers an alternative to conventional synthesis of MOFs providing a fast, economical, green, and versatile method for the synthesis of these advanced porous materials. It also ensures the delivery of dry MOFs products, ready-made for direct use in various applications without the need of any supplementary treatment.
Improving the performance of Active Pharmaceutical Ingredient (API) : Experiments on co-crystals growth conducted with mechanochemical methods showed better control over the polymorphic outcome, the yield and selectivity improvement, and the removal of the need for purification steps.
Toward the discovery of uncharted chemical reactions, beyond green chemistry: However, most mechanochemical processes today focus on chemical reactions already widely known in solution processes.But keep in mind thatthe benefits will emerge when the unique capabilities of mechanochemistry are exploited to access otherwise unreachable brand-new solutions.
From alchemy to mechanochemistry
Many centuries ago, alchemists were performing mechanochemical reactions using mortar and pestle to make the Philosopher’s Stone. A Greek philosopher Theophrastus, a contemporary of Aristotle, reported in the 4th century BC what is considered today the first mechanochemical reaction. He used a copper mortar and pestle as a cold extraction method to produce metallic mercury grinding cinnabar with vinegar. Scientists now understand that copper acts as a reagent to create copper sulfide and turn cinnabar, i.e. mercury sulfide, into mercury.
However, the real father of mechanochemistry seems to be Matthew Carey Lea, a chemist passionate about photography who developed silver colloids. His lifetime dedication to mechanochemistry started in 1886 with a 600-mile trip to his summer house.
He discovered, during that trip, that an off-gold paper treated with silver colloid which is supposed to turn white under light exposure would also turn white under the frictional force generated between the paper and its container.
Carey Lea’s research highlighted some of the fundamental characteristics of mechanochemistry:
- All forms of energy, including mechanical energy, can activate chemical reactions.
- Shear force is more effective than static pressure to enable chemical reactions.
- Chemical reaction outcomes can differ when mechanically induced or heat induced.
By the end of the 19th century, however, while the basic principles of mechanochemistry were just being discovered, thermochemistry had already developed to a massive scale and triggered the creation of companies that would become chemistry leaders (BASF, Dow Chemical, Shell, and Bayer). This led it to become the dominant chemical pathway in the chemical industry. Mechanochemistry faded away because of a lack of transfer to industrial-scale production and the lack of need for more sustainable processes.
The interest in mechanochemistry revived in the 1960s, with the creation of several research labs dedicated to the topic in Russia, Israel, and Japan. Petr Aleksandrovich Rebinder and Peter Adolf Thiessen were major contributors to the community, organizing the first conference and publishing the first book on Mechanochemistry and Tribochemistry. Today, INCOME (International Conference on Mechanochemistry and Mechanical Alloying) is the most important conference for the community.
Curious to know the full story of mechanochemistry?
For a virtual experience, have a look to “Mechanochemistry: the Science of Crush”
You can also look for Laszlo Takacs who authored several articles on the history of mechanochemistry and the life of some of its main protagonists such as Matthew Carey Lea and Walthére Spring.
Takacs, L. Quicksilver from cinnabar: The first documented mechanochemical reaction?, 2000, JOM 52, 12–13
L. Takacs, The historical development of mechanochemistry, Chem. Soc. Rev., 2013,42, 7649-7659
L. Takacs, Two important periods in the history of mechanochemistry, Journal of Materials Science, 2018, 53(19)