Beginner’s guide to eco-friendly solvents

It’s hard to emphasize just how essential solvents are in our world today. Solvents account for 80% of chemicals used in chemical processes. It’s often impossible to find a way around them, despite rising safety and environmental concerns. In this article, we’ll discuss just why solvents are so important, what makes a green solvent, and how chemical industries can improve their carbon footprint going forward.

Not just for chemists: solvents in your everyday life

Think about your daily routine, and how it might involve solvents. Did you have coffee this morning? You used water as a solvent. Have you had your nails done? Acetone or ethyl acetate are the solvents used in nail polish and polish remover. Cleaned anything lately? You probably used a product containing a mixture of solvents to make sure everything got dissolved.

Solvents are substances that can dissolve materials (solutes). Which chemicals can act as solvents is determined by inter-molecular forces, or the forces that exist between molecules (rather than covalent or ionic forces within molecules). This is most commonly described as the principal of “like dissolves like.”

In chemical industries, solvents are ubiquitous, essential, and often used in copious quantities. This is especially true for reaction chemistry. Even if the solvent doesn’t take any form in the final product – for example in the purification stage of a reaction – a large volume is required. Often times, solvents are toxic and pose environmental risk. They may be sourced from petroleum, further contributing to environmental impact. But finding greener replacements is not always straight-forward, as we’ll see in this article.

Going solvent-free

It might seem obvious to say that the best solvent for the environment is no solvent at all – but that isn’t always the case. For example, if a process can be performed without a solvent (which usually isn’t the case, anyway), we still have to consider how this affects the overall energy demand or other hazards, like potential for overheating.

One area where there has been success with solvent-free processes is in polymerization. These solvent-free processes include melt polymerization, solid-state polymerization, and ball milling processing. This can avoid solvent use while also reducing overall energy consumption. There is also something called “solvent-free microwave extraction” which can effectively be used for the extraction of essential oils from plant materials.

Green is a relative term

Many conventional processes require solvents that are shown to be carcinogenic, ozone-depleting, or teratogenic. For example, amide solvents can cause reproductive harm, hydrocarbons can contribute to aquatic toxicity and bioaccumulation, benzene is carcinogenic, and carbon tetrachloride is damaging to the ozone layer.

In the rush to replace these solvents, laws are put in place that force the use of new – but similar – solvents. Unfortunately, the only “advantage” of these replacement solvents is often that they haven’t been studied enough yet to be conclusively dangerous.

There are plenty of examples of this. Chlorinated solvents like DCM and chloroform were used to replace carbon tetrachloride. But these “greener” solvents have since been shown to be likely carcinogens, and damaging to the ozone as well. Hence, “green” is never an absolute term – it is almost always a choice between two evils.

Hence, when you replace a solvent with a newer, “less dangerous” solvent that works in the same way…it is likely to eventually present the same issues. The very fact that it is similar enough to do the same job means that it is similar enough to cause the same environmental and health issues.

Nothing exists in isolation

Even if a solvent itself is green when considering the actual chemical process, we still have to account for all the other up-stream and down-stream processes involved.

When we define what is a green solvent, we have two main considerations: 1) health concerns and safety hazards and 2) energy demand, which includes the energy required to produce the solvent and the energy required to recycle or recover the solvent. Researchers have a way to calculate the overall “greenness” of a solvent.

How chemical industries can take responsibility

A 2016 review was published as a green solvent selection guide. The intent of this review was to help avoid the historical errors made in attempts to replace dangerous solvents, and come up with more sophisticated solutions.

The authors addressed hazards associated with different solvents, and provided analyses of the best production and waste-recovery routes. For example, they analyze whether it is better to attempt to recover a solvent, or to incinerate it.

They also provided data from the most influential pharmaceutical companies regarding solvent “greenness.” The greenest solvents are generally considered to be: methanol, ethanol, 1-propanol, butanol, ethyl acetate, propyl acetate, and acetone.

Conclusion

Solvents are critical to our everyday lives and to chemical processes that keep the world turning as we know it. Solvents account for 80% of chemicals used in chemical processes, and are often difficult or impossible to replace. Rising concerns about the safety and environmental impact of certain solvents are the driving force behind legislature to eliminate the use of some chemicals. However, this rushed and unsophisticated approach has historically resulted in replacing toxic solvents with other equally toxic – but less studied – solvents. Researchers strive to streamline a more sophisticated approach towards greener solvents.

References

Byrne, Fergal P., et al. “Tools and techniques for solvent selection: green solvent selection guides.” Sustainable Chemical Processes 4 (2016): 1-24.

Capello, Christian, Ulrich Fischer, and Konrad Hungerbühler. “What is a green solvent? A comprehensive framework for the environmental assessment of solvents.” Green Chemistry 9.9 (2007): 927-934.

Häckl, Katharina, and Werner Kunz. “Some aspects of green solvents.” Comptes Rendus Chimie 21.6 (2018): 572-580.

Winterton, Neil. “The green solvent: A critical perspective.” Clean technologies and environmental policy 23.9 (2021): 2499-2522.