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The last few decades of the 20th century witnessed growing concerns over the impact of industry on the global environment. These concerns included acid rain, increasing levels of greenhouse gases, fertilizers in streams and rivers, polluted city atmospheres, and a hole in the ozone layer. Some of these are directly attributable to power generation and transport, which have caused major problems and lie outside the direct influence of the chemical industry.

However, in spite of an enormous investment over the last 50 years to ensure that the production of chemicals does not have a malign effect on the environment, many of the public, the very consumers of the products, still associate the chemical industry with the worst sorts of pollution.

There is no doubt that there are still mistakes and these are generally well publicised in the media but overall there have been significant changes in the operation of the chemical industry that are designed to reduce the impact on the environment.

 

Figure 1 A photograph of Widnes in the north-west of England, a centre of industry in the late 19th century.

Figure 2 Widnes, 120 years later. The chemical plant (1) is due for relocation as a new bridge is planned to span the Mersey. The residential area (2) now has very clean air. The Manchester Ship canal (3) is used for recreation.
By kind permission of Cyril J Wood (canalscape.net)

 

The acceptance that the chemical industry must not adversely affect the environment for future generations has been the driving force behind the development of green chemistry. This is not a separate branch of chemistry, but an approach that permeates every stage of process development.

The aspiration can be summed up in one word: sustainability. Sustainable development and manufacture meets the needs of the present without compromising the ability of future generations to meet their own needs. The problems it aims to address are:

  • the depletion of finite oil, gas and mineral resources
  • the production of waste, some of it harmful to living organisms
  • reagents and processes that present a risk to human health and the environment
  • products, which when disposed of, do not degrade easily.

The principles of green chemistry

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 Synthesis
Wherever practical, synthetic methods should be designed to use and generate substances that possess little or no toxicity to people or 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 or separation agents) should be made unnecessary whenever possible and innocuous when used.

6. Design for Energy Efficiency
Energy requirements of chemical processes should be recognised 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/de-protection, and 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 Safer Prevention
Substances and the form of the substance used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions and fires.

Table 1 The twelve principles of green chemistry.
By kind permission of the U.S. Environmental Protection Agency.

In the early 1990s, the term green chemistry was introduced by the Environmental Protection Agency, an agency of the US Government. The EPA produced a set of 12 principles to guide the chemical industry (Table 1) and in this unit some of these principles will be explained, using wherever possible examples taken from subsequent units dealing with the manufacture of chemicals. These examples illustrate how the search for ways of meeting these principles is a continuing activity. In many instances, changes which reduce the environmental impact of a process also lead to an increase in the profitability of the process. For example, if a new catalyst is developed that reduces the operating temperature and pressure for the process, less energy is consumed which is good both for the environment and for the company.

Prevention

Manufacturers try to generate as little waste as possible, through reaction choice, process design and recycling. Industry aims to use chemical reactions and processes that make the most effective use of available resources and generate the smallest possible amount of waste material. But can prevention be assessed quantitatively?

One way of measuring the efficiency of a process is to calculate the yield, which compares the expected product quantity with the actual amount obtained (although some potential product may be 'lost' as a result of competing reactions).

An example is the manufacture of phenol. It used to be made from benzene using sulfuric acid and sodium hydroxide in a multi-stage process, which, overall, can be expressed as:

 

The chemical equation shows that 1 mol of benzene (78 g) should yield 1 mol of phenol (94 g). In practice, the quantity of phenol produced is found to be about 77 g, giving a yield of 82%, which may be regarded as quite good.
(Yield % = mass produced / mass expected x 100 %)
However, the calculation obscures the fact that the reaction also generates 1 mol (126 g) of sodium sulfite for each mole of phenol produced. This may be acceptable if there is enough demand for sodium sulfite, but if not, it presents a serious problem of waste management and adds significantly to costs, meaning that this may not be the most suitable reaction for manufacturing phenol.

Atom economy

As an alt