Green chemistry is an attempt to make chemical processes more environmentally responsible and to produce less waste.
In 1998, Anastas and Warner published 12 principles which outlined their idea of what makes a chemical process or product greener. Over the past 20 years, there have been many more contributions to define green chemistry, but no single definition or unified framework has been agreed upon . The most promising efforts stem from green chemistry metrics. Leaving value-based conceptions behind, green chemistry metrics aim at quantifying different aspects of a chemical process to determine whether or not it is environmentally friendly.
One of the biggest challenges in green chemistry is in defining what makes a chemical process or product green. Various metrics - each representing a different approach to quantifying the ‘greenness’ of a process - have been proposed over time but no unified framework has yet been agreed upon. The complexity of chemical processes is partly to blame because it makes it difficult to formulate clear and simple-to-use metrics. Metrics that have already been formulated encompass the environmental impacts of mass and energy, hazardous substance, and life cycles. The most notable metrics:
In 1991, Barry Trost introduced the concept of atom economy. In the atom economy analysis, you calculate how many atoms from the reactants remain in the final product. As a standalone metric, the atom economy calculation does not offer a complete picture of the greenness of a process.. However, it is a historically important metric that helped spread awareness of green engineering principles.
Reaction mass efficiency uses atom economy, yield, and stoichiometry (the ratios of products and reactants in a chemical process) to calculate the proportion of the mass of the reactants that remain in the final product. Reaction mass efficiency focuses on the impact of the use of materials rather than on the impact of waste. Like the atom economy, it fails to include other materials than reactants such as reagents, solvents, and catalysts. This is why process mass intensity was introduced.
Process mass intensity calculates the mass intensity of a reaction and takes all materials (reactants, reagents, catalysts, solvents) into account. It is the ratio of the total mass used in the reaction to the mass of the final product. Development of this metric was spurred by recognition of the general lack of efficiency of pharmaceutical processes.
The E-factor has a similar approach. This metric is the ratio of total waste generated by a chemical process to the mass of the product created. It is a simple and intuitive metric that can be used in all industries. Its main weakness is that it is not always clear what constitutes waste.
Effective mass yield is defined as the percentage of the mass of the chemical product relative to the mass of all "non-benign" materials used for its synthesis. This metric gives the proportion of the final mass of the product that is made from non-toxic materials. Hence it factors in the toxicity of all components (reactants and reagents). As with the E-Factor calculation, the definition of “non-benign” is imprecise.
Molar efficiency is the ratio of the moles of final product and moles of all reaction materials (reactants, additives, solvents, catalysts). This metric is mostly useful in evaluating the efficiency in discovery medicinal chemistry.
Introduced in 1997, step economy establishes the direct connection between the number of steps in a chemical process and its greenness. Fewer steps is better, all other things being equal. While seemingly obvious, this metric was important in engendering holistic thinking in about process and facility design.
Following the same logic, pot economy states that the fewer pots (or reaction vessels) that are used for a multicomponent reaction, the greener the process will be.
Each metric quantifies the greenness of a chemical process and develops unique green chemistry methodologies and approaches. Each metric is valid in its own limited scope of definition. They are most valuable when combined together to form a systematic approach to the evaluation of the greenness of a chemical process or product. While industry has not agreed on any unified framework, when used together, metrics are complementary and help paint a picture of an operation and point to opportunities for improvement.
The US Environmental Protection Agency has made available a summary of the textbook: Green Engineering: Environmentally Conscious Design of Chemical Processes Click here