Green chemistry is the design of chemical products and processes that reduce or eliminate the use and generation of hazardous substances (Anastas et al., 2000). The concept emerged 20 years ago with the introduction by Paul T. Anastas and J. C. Warner of the 12 principles of green chemistry (see Table 1.1). The subject continues to develop strongly around these principles (Anastas and Warner, 1998). Green chemistry aims to achieve (Clark and Macquarrie, 2002):

• maximum conversion of reactants into a determined product,

• minimum waste production through enhanced reaction design,

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Table 1.1 The 12 green chemistry principles

1. Prevention

It is better to prevent waste than to treat or clean up waste after it has been created.

2. Atom economy

Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product.

3. Less hazardous chemical syntheses

Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Designing safer chemicals

Chemical products should be designed to effect their desired function while minimizing their toxicity.

5. Safer solvents and auxiliaries

The use of auxiliary substances (e. g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.

6. Design for energy efficiency

Energy requirements of chemical processes should be recognized for their environmental and economic impacts and should be minimized.

If possible, synthetic methods should be conducted at ambient temperature and pressure.

7. Use of renewable feedstocks

A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

8. Reduce derivatives

Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.

9. Catalysis

Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.

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.

11. Real-time analysis for pollution prevention

Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

12. Inherently safer chemistry for accident prevention

Substances and the form of a substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.

• the use and production of non-hazardous raw materials and products,

• safer and more energy efficient processes, and

• the use of renewable feedstocks.

Efficiency is the key, and green chemistry has continued developing around the principles, which guide both academia and industry in their pursuit of more sustainable processes. In an ideal case, according to these principles, a reaction would only produce useful material. Waste and pollutants would be prevented, improving the reaction yield and reducing losses, thus improving the overall economics of a process. Since our society and industries are governed by increasing efficiency and profit, green chemistry therefore theoretically fits the agendas of most manufacturing companies these days, not only appealing to chemical producers.

Today, 20 years after their publication, the 12 principles of green chemistry are as meaningful as ever in the light of the increasing interest the area attracts due to concerns over sustainability (Anastas and Kirchoff, 2002). Misunderstandings have arisen due to the attractiveness of the area to sectors dealing directly with public demands for ‘greener and more environmentally friendly’ products. It is therefore of vital importance that the message is not distorted by common misconceptions over what is or is not ‘green’, thus altering their original goal: to aim towards safer and cleaner chemistry.

The implementation of REACH (Registration, Evaluation, Authori­zation and Restriction of Chemicals), or Directive (EC 1907/2006), ROHS (Restriction of the Use of Certain Hazardous Substances in Electrical and Electronic Equipment) or Directive 2003/108/EC, and other initiatives highlighting the hazardous character of some chemicals used in day-to-day consumer products, such as the SIN list (n. d.), are pushing hard for their replacement to avoid further risks to human and/or environmental health. However, we should make sure the substitutes used are genuinely safer across the whole life cycle and as effective as what they are replacing. Investing in R&D focused on finding truly greener alternatives, thus eliminating rushed and weak substitutions that can even increase the number of components present in formulations when ingredients are added to compensate for a lack of performance in the ‘greener’ formulation, is important. The same applies to the substitution of fossil-derived chemi­cals with more sustainable bio-derived chemicals: when using renewable feedstocks such as biomass, we have to use clean and efficient synthetic routes, minimizing the amount of unwanted by-products and the use of scarce resources (i. e., scarce metals).

Scarce metals are increasingly used in clean alternative energy-producing technologies. Their reserves are sometimes only estimated to last another 50 years, or even less for key elements such as indium (a key component in solar panels) (Dodson et al., 2012) and we must take this into account when modifying our energy and manufacturing infrastructure, taking advantage of the whole periodic table. This is especially relevant to the area of catalysis: re-usable catalytic metals are seen as better reagents than hazardous reagents such as AlCl3. But many of the most interesting catalytic metals are also becoming scarce and their production process can be resource intensive and wasteful, making their recovery and reuse essential. Water is increasingly seen as a scarce resource too in certain areas of our planet, but its use as a green solvent is increasingly envisaged due to its non-toxicity compared to hydrocarbon-based solvents (Simon and Li, 2012). Nevertheless, contaminated water is difficult and expensive to treat and re-use. Another alternative to VOC solvents are involatile solvents such as ionic liquids designed to eliminate air-borne emissions. Ionic liquids are used in phase transfer catalysis for example (Welton, 2004), but their non-emissions are counteracted by their toxicity and their environmental impact when prepared, used and separated for end-use.

Biodegradability is an important sought-after characteristic for ‘greener’ products, but increasing the life-time of a molecule to promote its re-use could be another strategy. Heavily halogenated compounds are poorly degradable and there are some large volume halogenated compounds that need to be phased out (e. g., the solvent dichloromethane). But we must not bundle all halogenated compounds in the same ‘red’ basket. Nature turns over enormous quantities of organohalogen compounds and we need to learn from nature and avoid, as much as possible, those compounds that it cannot deal with (e. g., perhalogenated compounds).

Food waste is a feedstock rich in functionalized molecules, and although it is biodegradable, it should be valorized for new applications as a raw material for renewable chemicals, materials and bio-fuels, leading us towards waste minimization and waste valorization. Wasting resources should be avoided in any optimized process. However, waste can also represent an opportunity as we can no longer afford the luxury of waste.

This past paragraph shows you how tightly knit these issues are, illustrating how important it is to assess the greenness of a process through each of its steps, from the use of raw materials to end-use through manufacturing and use. One change can affect several steps and it is important to assess a process through its full life cycle even though it is time-consuming and its quality is dependent on the data used. Such a tool can help us assess the use of bio-processes versus chemo-processes, for example. Many believe bio-processes are preferable to chemo-processes as they are superior in terms of environmental impact, since they use non-toxic components to selectively yield the targeted product. But as they are time-consuming and expensive, it is unrealistic to believe that chemo-processes will be entirely replaced by natural organism catalysed processes in the foreseeable future.