Green Chemistry Principle #10: Design for Degradation

By Shira Joudan, Chair of the Education Subcommittee for the GCI

10. Design for degradation: Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.

In video #10, Matt and I discuss designing chemicals that break down once their desired function is completed. Essentially, we want chemicals to degrade to molecules that are not harmful to humans, animals or the environment.

A lot of the chemicals we use in our day-to-day lives need to be stable to perform their function. For example, if your coffee mug dissolved when you poured your coffee into it, it wouldn’t be very helpful! Similarly, if lubricants degraded under high temperature and pressure, they may not work well in the engines of our cars or planes.

Once chemicals are done providing their main function, they might end up in a landfill or wastewater treatment plant where they can enter the waters, soil and air of our environment, or be taken up by animals or humans. The biggest challenge is making chemicals that are stable during usage, but don’t persist in the environment – or in other words, chemicals that can be degraded. Another important thing – we want the breakdown products to also be non-toxic and not persistent! It’s important to remember that there are different reasons a chemical can break down. It can be due to reactions with light (photodegradation), water (hydrolysis) or biological species, often with enzymes (biodegradation).

A common example that we hear about is biodegradation, especially with the well-known “biodegradable soaps.” We can use this as a good example about how we can design soaps, or detergents, to break down more easily in the environment.

Sodium dodecylbenzenesulfonate

Figure 1 Sodium dodecylbenzenesulfonate, an example of a linear alkylbenzene sulfonate (LAS) which is biodegradable.

Sodium dodecylbenzenesulfonate (Figure 1) is a common detergent, and is often referred to as LAS, for linear alkylbenzene sulfonates. Looking at its structure, you can see that it has a linear alkyl chain with a benzylsulfonate attached to it. It is useful as a detergent because it has a polar headgroup (sulfonate) and a non-polar alkyl group, making it a surfactant.

LAS is used in many things, especially laundry detergent. It degrades quickly in the environment under aerobic conditions, or when oxygen is present, because microbes are able to use to the linear alkyl chain as energy, via a process called β-oxidation, a process which breaks down the carbon chain. Once the long chain is degraded, the rest of the molecule can be degraded as well.

Branched alkylbenzene sulfonate.

Figure 2 A branched alkylbenezene sulfonate (does not biodegrade).

If you compare LAS to a branched version (Figure 2), you can immediately see that the alkyl chain looks very different. This molecule was also used as a detergent just like the linear version, but because of the location of the branches, microbes cannot perform β-oxidation since there are no good sites for that reaction to be initiated. Therefore, these branched detergents have been phased out in most developed countries because they are too persistent – they do not biodegrade.

The main way these molecules are degraded is through microbes, when oxygen is present. So if these soaps end up directly in water, like straight into a lake, they will not be broken down very quickly (even the linear version!). This is because there are fewer microbes in water as compared to in soil. Interestingly, the branched version is 4 times less toxic than the linear version, but can cause more damage because of its persistence. This is one of the reasons that it is very important to consider persistence, or a molecule’s resistance against degradation, and not only its toxicity.

You can see how designing chemicals to break down can be very challenging, but many researchers around the world are working on this right now. Some examples are biodegradable polymers that are used in plastics, like compostable cutlery.

Principle 10 is currently one of the largest challenges in green chemistry. If scientist designing new chemicals understand more about the mechanisms that can degrade them, we may be able to make chemicals that are reliable and stable during their intended use, but break down in the environment!

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Green Chemistry: From the Bench Top to Industry, A Chemical Engineer’s Perspective

By Cynthia Cheung, Member-at-Large for the GCI

As a chemist, do you ever think about how to scale up your chemical reactions, or your chemical processes? For most of us, the answer is no. However, this idea of industrial scale is something that is constantly addressed in the Chemical Engineering and Applied Chemistry department. Consequently, the 12 Principles of Green Chemistry become fundamental to scale up a reaction from the bench top in a research lab to mass production in a chemical plant (Figure 1).

Cynthia_blog post figure 1

Figure 1. Example of a chemical plant design [1]

For me, the biggest difference I have found moving from the Chemistry department to the Chemical Engineering department is that the principles of Green Chemistry are not concepts that Chemical Engineers often have to think about or address, because these principles are integrated and engrained into their work as objectives and limitations. A process can be thrown out simply because it uses columns for purification (which cannot be done on a large scale, because you would need truckloads of solvent) or because one of the reagents is toxic.

As a chemist, we do what works and worry about alternatives after we have established what we targeted. So pause and think about it for a second: can you continue what you’re doing and upscale it from milligrams to tonnes? Even theoretically speaking, if you could produce that much product, would that process make sense? For example, would that Stille coupling reaction be safe when you’re using tonnes of tin2, or could you afford to be using or making catalysts when you need tens of kilograms of it? So how to do you go about engineering your process so that it would be suitable for industry? Rather, how do the engineers do it?! Well, broadly speaking (and from asking around) a few of the main considerations that seem to be in common are:

Cost Analysis

How much are all of the reagents and solvents going to cost, and where are there substitutes for cheaper alternatives?

Rate of Reaction

How long is this reaction going to take? Because time is money.

Waste

How much waste is produced, what type of waste and is it recyclable? Also to keep in mind is if the waste is hazardous, then what alternatives can be used from the beginning to avoid any hazardous waste generation? In addition, if there’s pollution, then that also has to be reduced or eliminated altogether.

Work up

This consideration is often tricky, as most organic labs usually use techniques that are not scalable (I’m looking at those purification columns). In addition to that, side products and by-products are also something that often can give engineers a headache. Atom economy is very important for industry, as it lessens the amount of waste produced, and also aids in producing a good product yield.

Energy Consumption

How much energy is required for a reactor and for a chemical plant to run is also part of the cost analysis for a process. It would definitely be preferential to be using less energy (so lower temperature reaction conditions and reactions that generate no heat). Essentially, if all reactions could be at room temperature, that would be perfect.

So with these considerations in mind, could you re-engineer your synthesis?

 

References:

[1] http://en.citizendium.org/wiki/File:FCC.png

[2] Stille, J. Palladium Catalyzed Coupling Of Organotin Reagents With Organic Electrophiles. Pure and Applied Chemistry 1985, 57.