4.1. Principles of Green Chemistry

4.1. Principles of Green Chemistry

Green chemistry is the approach in chemical sciences that efficiently uses renewable raw materials, eliminating waste and avoiding the use of toxic and hazardous reagents and solvents in the manufacture and application of chemical products. Green chemistry takes into account the environmental impact and seeks to prevent or lessen that impact through several key principles outlined below.

Here are the 12 key principles of green chemistry as formulated by P.T. Anastas and J.C. Warner, in Green Chemistry: Theory and Practice, 1998. 

1. Prevention. It is better to prevent waste formation than to treat it after it is formed.
2. Atom economy. Design synthetic methods to maximize incorporation of all material used into final product.
3. Less hazard. Synthetic methods should, where practicable, use or generate materials of low human toxicity and environmental impact.
4. Safer chemicals. Chemical product design should preserve efficacy whilst reducing toxicity.
5. Safer solvents. Avoid auxiliary materials - solvents, extractants - if possible, or otherwise make them innocuous.
6. Energy efficiency. Energy requirements should be minimized: conduct synthesis at ambient temperature and pressure.
7. Renewable feedstocks. Raw materials should, where practicable, be renewable.
8. Reduce derivatives. Unnecessary derivatization should be avoided where possible.
9. Smart catalysis. Selectively catalyzed processes are superior to stoichiometric processes.
10. Degradable design. Chemical products should be designed to be degradable to innocuous products when disposed of and not be environmentally persistent.
11. Real-time analysis for pollution prevention. Monitor processes in real time to avoid excursions leading to the formation of hazardous materials.
12. Hazard and accident prevention. Materials used in a chemical process should be chosen to minimize hazard and risk for chemical accidents, such as releases, explosions, and fires.

Chemists are guided to use these 12 principles as a checklist for evaluation of a specific process or chemical technology at the stage of design and scale-up.

Initiation and development of the above-listed principles was closely tied to the Pollution Prevention Act enacted in the USA in 1990. This document was a turning point in environmental policy by putting a particular focus not on environmental remediation and clean-up (i.e., fixing the damage at the end of the pipe) but rather on waste minimization and elimination of pollutants at the point of origin. This strategy of pollution prevention is also referred to as source reduction and is viewed as the first-choice measure to reduce risk to human health and the environment. Some of the attractive features of the source reduction are cost effectiveness, reduction in raw material use, pollution control savings, reduced risk to workers and the environment.

During the following year (1991), the Environmental Protection Agency (EPA) and National Science Foundation (NSF) initiated the Green Chemistry Program. A number of other similar initiatives were formed in the UK and some other countries. US Presidential Green Chemistry Challenge Award was founded in 1996. All these actions mark stepping stones in the green chemistry movement and philosophy, which gained more momentum in the following decades.

The green chemistry philosophy seeks to respond to public perceptions that chemistry and its applications via chemical technology have been primarily responsible for many of the ways the world degrades the environment. To reverse this stereotype, one of the central goals of green chemistry is to reduce risk to humans and environment from chemical synthesis, manufacturing, and application of chemical products through design of clean and closed-loop procedures.

Mitigating Risk and Hazard

Is there difference? Formally, in chemical fields, risk can be defined as a function of hazard and exposure:

Risk = f (hazard, exposure)

Traditionally, in industry and society, the reduction of risk is achieved through the reduction of exposure. By characterization of hazards (toxicity data) and knowing the effectiveness of the exposure controls ('containing the hazard'), risk can be manipulated or dissipated, especially at the early stages of the chemical chain, when it is easy to identify and measure. However, exposure controls may be not as useful downstream. The farther the hazard is from its source, the less the awareness of the potential hazard. With uncertainties in chronic effects, bio-accumulation, synergistic effects of chemicals, there is an uncertainty in risk mitigation.

The Green chemistry approach, in contrast with traditional practice, targets risk reduction through reduction of hazard. This is a safer approach because, if hazard is eliminated in the first place, there is no way risk can increase through any unpredicted spontaneous exposure increase anywhere downstream (Anastas and Warner, 1998).

Green Chemistry Control Keys

So, what are possible avenues for changing the existing practices towards the minimum-risk alternatives? There are several controls that can be manipulated at different stages of a chemical manufacturing process.

Using alternative feedstock or starting materials: Selection of the starting materials has a major effect through the whole synthetic pathway. It determines what hazards will be faced by the workers extracting the substance, shippers transporting the substance, chemists handling the substance. It also predetermines possible future risks from the end-products and wastes. Using more environmentally benign alternative feedstock may improve the environmental profile of the whole process (this links to green chemistry principle #7). One of the examples of this step is choosing between the petroleum feedstock and biological feedstock. Currently, 98% of all organic chemicals in the USA are produced from petroleum. Petroleum refining is extremely energy-consuming (15% of total national energy use) and contains high-pollution oxygenation processes. Agricultural feedstocks can be a great alternative that eliminate much of that hazard. Research has shown that many agricultural products (e.g., corn, soy, molasses) can be transformed via a variety of processes into textile, nylon, etc. (Anastas and Warner, 1998).

Using alternative reagents: Reagents are needed to transform the starting molecules into a target substance. Reagents are not necessarily consumed and are often recycled, but can still bear harm to people and environment exposed to the process. At this point, a chemist must balance the criteria of chemical efficiency and availability with potential hazards. This practice taps into green chemistry principles #2, 4, and 5.

Using alternative solvents: Solvents are a very common focusing point because a wide range of syntheses are performed in the liquid media. Many of the currently used solvents are volatile organic compounds. Many of those are responsible for air quality problems (smog, etc.) when released to air. While the traditional organic solvents are easily available, well characterized, and regulated, there is a push for alternative systems that are more environmentally benign in the long run – aqueous solvents, ionic liquids, immobilized solvents, supercritical fluids, etc. (Principle #5) The choice of an alternative solvent requires careful and specific analysis, which determines if the new process would be as efficient or as cost-effective. How such trade-offs are resolved is discussed later in this lesson.

Changing target product: Chemistry is function oriented – the target chemical is needed to perform a certain function or possess certain properties. This avenue is related to the search of the alternative final product, which may require radical change in the way synthesis is done (Principle #3). Through chemical research, it is possible to identify those parts of a molecule that provide the chemical with a desired function as well as those parts that provide toxicity. Maximizing the former and minimizing the latter is a worthy challenge for chemical design.

Process monitoring: Real time measurements (sensing) of process parameters and concentrations sometimes provide valuable information and hints how the process should be tuned to avoid adverse effects or risk (Principle #11). Also, process monitoring may open avenues for making the process more cost-effective.

Alternative catalysis: Catalysis bears enormous benefits, not only from the standpoint of technical efficiency. Environmental benefit results from the use of a much smaller amount of reagents in catalyzed reactions, which otherwise would contribute to the waste stream. Using less chemicals is also economically profitable. It should be noted, though, that many classes of catalysis (e.g., heavy metals) are very toxic. Hence, the challenge of alternative catalysis is to develop environmentally benign options (Principle #9).

As you can see, most of these measures are oriented towards reducing hazard in the first place. Eliminating, minimizing, or neutralizing toxic components at earlier stages of the process allows for more relaxed exposure control at later stages. Item 5 is more universal, as sensing can help monitor and control both toxicity and exposure at both inlet and outlet of the chemical system.

The green chemistry principles are also important as guidance for designing metrics for chemical technology evaluation. Some examples of those metrics are discussed further in section 4.3.