Designing Sustainable Synthesis

Tom Squire
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Designing sustainable synthesis is an important part of a modern chemists work whether that is in a research or industrial setting. 

Green chemistry and the 12 principles of green chemistry are important concepts chemists are following to make their chemistry more sustainable. To learn more about the 12 principles of green chemistry see the link to the chemBAM page

Some of the most important design factors when considering sustainability of a chemical process are detailed below;

Atom economy / E factor

Atom economy / E factor are important concepts when designing chemical reactions to ensure sustainability, with atom economy in particular a common measure of how “green” a reaction is. For example a reaction may have 100% yield, but still generate more waste than product. To learn more about atom economy / E factor and how to calculate them see the link to the chemBAM page

Catalysts

Catalysts are useful tools for chemists which allows them to speed up a chemical reaction, while also being recoverable and therefore reusable. They also allow reactions to be carried out in less harsh conditions, for example lower temperature, therefore reducing the energy needed to be input into the system. Thus the addition of a suitable catalyst could be described as making a reaction “more green”.

Is that a catalyst? – Friedel Crafts acylation

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Alternate energy sources

Alternative sources of energy can also be useful when catalysts are not an appropriate means of lessening the harsh conditions of a reaction. These alternatives can be utilising photochemistry to produce free radicals in organic chemistry or using electrochemistry in inorganic chemistry. Microwave reactors are also a relatively new technique utilising microwave radiation as the energy input into a reaction. See link to recent research review to learn more about how microwave reactors.

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Reduction of materials/Recycling

 Many materials used by chemists come from non-renewable sources such as mined minerals and oil. Therefore it is important to make the most of these materials especially in industrial scale synthesis/manufacturing. It is also important to recover and recycle some materials such as rare earth metals used both in chemistry labs but also electronic devices. 

A real-world example of this can be seen in the recycling of mobile phones. Small amounts of rare metals such as Gold, Palladium, Yttrium and Indium are used to manufacture the electronics and touch screens on our mobile devices.

These metals are expensive to mine/extract from there ores and in some cases can only be found in a few places world-wide. For example, the metal Indium is used in a unique compound called indium tin oxide, which is vital for touch screens, because it conducts electricity and is transparent. There’s not a lot of it in the Earth and you would need 1 kilo of ore to extract just a few milligrams of Indium. Scientists estimate that Indium mines, among others, could run out within 1 century even though our demand for new technology continues to increase.

Therefore, there is a renewed effort in recovering these metals through recycling this so called e-waste. This is usually done on a big scale due to the small amounts of these metals in 1 phone e.g. 1 million mobile phones could deliver nearly 16 tonnes of copper, 350kg of silver, 34kg of gold and 15kg of palladium.

Another similar example is the recycling of lithium-ion batteries from electric vehicles. There is a rapidly growing demand for electric vehicles, but the lithium-ion batteries present a serious waste-management challenge for recyclers at end-of-life. To learn more about this issue and to see how researches at the University of Birmingham are trying to tackle this issue click here.

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Solvents

Solvents are receiving much greater attention now to try and make chemistry “more green”. This is because of the large volumes of solvents are used routinely by chemists, most of which are toxic, flammable, or environmentally damaging. To learn more about how to make the use of solvents safer/greener, visit the chemBAM page.

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Risk and hazards

A major consideration in green chemistry is safety where Risk = hazard * exposure. Two options to reduce risk:    

  1. Reduce hazard – through designing experiments that minimise the need for hazardous chemicals/dangerous conditions. For example, traditionally hydrogenation reactions require a highly pressurised environment of hydrogen which has a heightened risk of explosion. It is possible however now to do hydrogenation reactions through electrolysis which has a lot less risk attached.
Image of glovebox (attributed: Rune.welsh).

2. Reduce exposure – through following correct procedures and practises as well as having correct access to safety equipment. An example of this is preparing a reaction in a glovebox, a sealed container that allows a scientist to manipulate equipment in an isolated unit (see right). 

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This work is licensed under a Creative Commons Attribution 4.0 International License.