In a society increasingly aware of its environmental impact, virtually any activity is subject to scrutiny—and rightly so. The chemical industry is no exception. According to data from the Europen Environment Agency, total greenhouse gas emissions from the chemical industry in 2021 amounted to 155,495 kilotonnes of CO₂ equivalent, representing 5% of the European Union’s total net greenhouse gas emissions. The industrial sector as a whole is responsible for around 17% of EU emissions. This is a significant slice of the emissions pie. In addition to this, there are other sustainability challenges for the chemical industry: most notably waste production and its management. Indeed, around half of all waste generated by the sector is categorised as hazardous.
Against this backdrop, green chemistry has gained widespread recognition within the scientific community as a framework for making chemistry safer, cleaner and more efficient.
But how do we know whether a chemical practice truly deserves the label of “green”? To help answer this question, chemists Paul Anastas and John Warner proposed in 1998a framework in their book Green Chemistry: Theory and Practice, setting out a list of guiding principles. We are referring to the 12 Principles of Green Chemistry, which are closely connected to mechanochemistry.
Anastas and Warner described green chemistry as “the utilisation of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products”. Avoiding solvents wherever possible, as mechanochemistry does, already fulfils a substantial part of this objective.
However, these principles extend far beyond this. Let’s examine each of them and consider how mechanochemistry meets many of these criteria.
Waste prevention
The first of the 12 principles of green chemistry may well be most important. And it’s something that seems self-evident: the greenest person is not the one who cleans up the most, but the one who produces the least waste. The idea is to design chemical syntheses that avoid creating waste in the first place —rather than cleaning it up afterwards.
Solvents, usually the base of traditional chemistry, are often responsible for generating large quantities of toxic waste. The solvent-free nature of mechanochemical synthesis therefore makes waste prevention one of its key strengths.
Atom economy
Chemists must not only aim for the highest possible yield, but also maximise the incorporation of reactant atoms into the desired product. In other words, synthetic methods should be designed so that the final product contains the maximum proportion of the starting materials. This is also an indicator of efficiency and waste reduction.
Avoiding solvents is an interesting approach to improve the overall atom economy of chemical transformations. Mechanochemistry has provided alternatives that enhance the atom economy index of established reactions, such as the synthesis of metal-organic frameworks (MOFs). Masing molecules also allows amines and carbonyls to react and form amides without the need of a base, which is typically required in solution.
Less hazardous synthesis
Wherever practicable, synthetic methods should be designed to use and generate substances with little or no toxicity to human health and the environment. Avoiding toxic substances in chemistry is challenging, as reactive chemicals allow kinetically and thermodynamically favourable reactions. Once again, mechanochemistry offers valuable alternatives.
For example, the mechanochemical activation of calcium carbide (CaC₂) in organic synthesis and materials science has permitted to circumvent the need for gaseous acetylene. This avoids the difficulties associated with acetylene’s gaseous nature, its explosive character, and the difficulties to handle it.
Designing safer chemicals
The idea of this principle is to design chemical products that preserve their functional efficacy while reducing toxicity. This is a difficult balance to achieve and one of the most challenging aspects of developing safer products and processes.
Chemists usually look for highly reactive chemicals: they are quite valuable at driving molecular transformations. However, being highly reactive also means they are more likely to interact with unintended biological targets, both human and ecological. This leads to undesirable adverse effects. But even if we avoid them and, instead, we use safer reagents or even a solvent-free mechanochemical reaction, the final product might still be harmful. Predictive tools that assess environmental and biological impact before synthesis can play a key role in developing safer chemicals.
Safer solvents and auxiliaries
Solvents are major contributors to the overall toxicity profile of a chemical process. On average, they pose the greatest process-safety issues because they are flammable, volatile or even explosive. That’s why this principle encourages the avoidance of auxiliary substances, such as solvents and separation agents, wherever possible. And, if their use is a must, then it’s better to go for safer options.
Once again, this principle is central to mechanochemistry. It tries to minimisesolvents by design. Mechanochemistry has also enabled the synthesis of products otherwise impossible to obtain in solution due to solubility issues, providing alternative routes for access them.
Design for energy efficiency
Energy concerns also extend to chemistry. In this sense, energy requirements should be minimised. Temperature and pressure are the main factors to take into account here as many chemical reactions require external heating and the addition of extra pressure, which increases energy consumption.
But mechanochemistry comes to the rescue once again. Most mechanochemical reactions avoid high temperatures and pressures. An additional energetic advantage is that some mechanochemical reactions achieve the corresponding products in shorter times than solution-based protocols.
Use of renewable feedstocks
Wouldn’t it be better to produce all our fuels, chemicals and materials from resources that will never run out? Green chemistry pursues also this goal, encouraging the use of renewable feedstocks rather than depletable ones.
Fossil fuels —including petroleum, natural gas and coal— are among the materials that should be avoided, and there is a broad effort under way to reduce our dependence on them. One example of this more sustainable approach is the production of biofuels from organic waste materials. In mechanochemistry, there are various studies that demonstrate that valorising biomass offers a great number of advantages.
Reduce derivatives
One fundamental aspect of green chemistry is the minimisation of derivatives and protecting groups when synthesising target molecules. Derivatization steps involve the use of additional reagents and, consequently, generate extra waste. Enzymes play a crucial role in achieving this goal, as their high specificity allows them to react selectively with certain parts of a molecule, thereby eliminating the need for additional protective measures.
Catalysis
This principle states that it is better to use catalytic reagents rather than stoichiometric ones. Catalysts lower the activation energy of reactions without being consumed. This means they can, in principle, be used in small amounts and recycled indefinitely, generating minimal waste. In contrast, stoichiometric inorganic reagents produce enormous amounts of waste as they are used in amounts directly proportional to the quantities of reactants.
Design for degradation
Green chemistry pursues the design of products that break down to innocuous substances after they’ve served their function. By ensuring that these products do not persist in the environment, green chemistry aims to reduce the overall risk or probability of harm. Risk depends on both a molecule’s inherent hazard and exposure—the interaction between a chemical and a species. Effective degradation can significantly reduce exposure levels, thereby minimizing risk regardless of the hazard of the chemical involved.
Exposure to long-lasting chemicals can be high because these substances can spread widely around the world. This happens due to their natural properties, like easily turning into gas (volatility), sticking to particles, or building up in organisms’ fat. To manage this, regulators use have stablished criteria like their half-lives in water, soil, air to decide if it is persistent. Chemicals that are persistent, build up in living things, and are toxic are classified as PBT (Persistent, Bioaccumulative, Toxic).
Real-time pollution prevention
Real-time feedback is essential for the proper functioning of chemical processes. Thanks to it, chemists can detect unwanted by-products or hazardous conditions before they escalate and create bigger problems.
Over the past few years, the number of analytical techniques enabling in-process monitoring of mechanochemical reactions has increased. In-situ X-ray diffraction and Raman spectroscopy allow the study of solid-state changes in crystalline materials, as well as structural changes at the molecular level. Similarly, real-time temperature monitoring has improved our understanding of the development of thermal effects during ball-milling reactions and the detection of reaction onsets in mechanochemical transformations.
Chemistry for accident prevention
Working with chemicals inherently carries some risk. However, it can be minimised. The last principle of green chemistry is closely connected to several other principles that address the use of hazardous products or reagents. Wherever possible, exposure to hazards should be eliminated, and processes should be designed to minimise risks.
Mechanochemistry has enabled the simplification of standard reaction conditions, achieving a level of operational simplicity that can, in turn, reduce the potential for accidents. Additionally, the use of room-temperature protocols provides a safer way to carry out exothermic mechanochemical reactions.
By aligning closely with many of the 12 principles of green chemistry, mechanochemistry offers a powerful platform for transforming the chemical industry. Its ability to reduce waste, lower energy consumption, eliminate hazardous solvents and improve safety makes it a key contributor to the future of sustainable chemistry.
As environmental regulations tighten and sustainability becomes a defining criterion for innovation, mechanochemistry is poised to play an increasingly central role in reshaping how chemicals are designed, produced and used.