Green Chemistry: The Twelve Principles of Green Chemistry

Green chemistry is the process of designing of chemical products and chemical processes that reduces or eliminate the use and/or generation of hazardous substances. This process involves definition of the feed materials, reaction pathways, products, and reactor conditions to minimize the impact on the environment, including workers.

Green chemistry uses all the chemical principles and techniques at its disposal to prevent pollution at its source. Green Chemical Engineering does likewise but also includes the design and operation of processes as well as the design and manufacture of products to minimize pollution and the risks to human health and the environment. Progress has been made in the search for processes using fewer toxic chemicals and producing less waste while requiring less energy (Dewulf et al. 2000; Matlack 2003).

Green Chemistry and Green Chemical Engineering promote the goal of achieving the so‐called triple bottom line of economic prosperity and continuity for corporations, social well‐being for communities and employees, and environmental protection and resource conservation. Economic, financial, and environmental accounting and risk management tools are being developed to assess green technologies to include LCA (see Chapter 6), total cost assessment (TCA; see Chapter 5), and ecological risk assessment (ERA; see Chapter 4). The goal of achieving green and sustainable chemical manufacturing has been embraced by a number of government and nongovernmental organizations (Mathew 2003).

The Principles of Green Chemistry

The design and implementation of completely green products and processes is an enormous challenge. Because the number of chemical synthesis pathways is enormous, there is no one systematic, fail‐safe method for ensuring that the chemistry being implemented is green. Indeed, it is more nearly correct to inquire if a proposed chemical manufacturing process is simply “greener” than other alternatives. Thus, green chemistry recognizes the importance of incremental improvements. Anastas and Warner (1998) formulated have a set of principles to define and guide the scope of green chemistry.

1. Prevent wasteIt is better to prevent waste than to treat or clean it up waste after it has been created. In the past, the Earth and its rivers, oceans, atmosphere, and soil have been considered infinite sinks for the discharge of pollution and waste. The command and control philosophy implies that it is acceptable to generate waste, as long as the waste is safely treated and disposed or stored. Green Chemistry’s highest principle is that waste should not be created at all. From an economic standpoint, waste is a cost that requires paying twice: the first cost is for purchase of raw materials and the second is for management or disposal of the waste.
2. Employ the atom economy when practicalSynthetic methods should be designed to maximize the incorporation of all materials used in the process within the final product. Thus, it is not sufficient to achieve even a 100% yield of the desired product if the synthesis also produces by‐products. Rather, it is desirable to incorporate all of the atoms of the raw materials into the products. The atom economy, as discussed by Trost (1991), and atom utilization, a term coined by Sheldon (2000), embody the same concept with slightly different definitions. The atom economy is defined as the formula weight of the desired product divided by the stoichiometrically weighted formula weights of all reactants. The degree of atom utilization is the quotient of the formula weight of the desired product and the formula weight of all products and by‐products. In situations where the exact composition of all by‐products is difficult to determine, the atom economy is the more practical one to use.Sheldon (2000) also defines the E factor, which is the mass ratio of waste to desired product. E factors for bulk commodity chemicals are typically less than 5, and often less than 1. By contrast, fine chemicals and pharmaceuticals typically generate large amounts of waste per unit of product, with E factors perhaps exceeding 100. Neither the atom efficiency nor the E factor provides any quantitative measure of the potential for human harm of the waste products.
3. Use the least hazardous chemical synthesesWherever practical, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment. This principle promotes the use of less toxic reagents and intermediates, and the generation of less toxic by‐products. The use of less toxic feedstocks drives the development of, for instance, renewable raw materials. Better catalysts and chemical reactor designs are also critical to developing less hazardous syntheses.
4. Design safer chemicalsChemical products should be designed to achieve their desired function while minimizing their toxicity. This principle requires matching the desired function of a given chemical compound with its chemical structure. It also requires the ability to predict beforehand the possible toxic effects of a given chemical substance. Thus, the research and development of methods for establishing structure–property relationships promote this principle. In making the decision to develop and market a particular product, industry should go beyond considerations of the traditional performance properties (vapor pressure, color, stability, etc.) and add as performance metric such favorable environmental properties as reduction in toxicity and carcinogenicity.
5. Use safer solvents and auxiliariesThe use of auxiliary substances, solvents, separation agents, and other additives should be made unnecessary wherever possible and innocuous if they must be used. Safer solvents such as water, alcohols, supercritical carbon dioxide, or biodegradable surfactant solutions are preferable to chlorinated solvents. Separation processes like liquid extraction require the use of mass separation agents, but such alternatives as reactive separations through membrane reactors (see Chapter 8) may provide a more environmentally acceptable alternative.
6. Design for energy efficiencyThe energy requirements of the various chemical processes should be recognized for their environmental and economic impacts, which should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure. The chemical industry has substantially reduced its use of energy in manufacturing, starting with the oil embargo and energy shortages of the 1970s. Heat integration and other engineering tools are well developed in the chemical industry and can be applied within a given process. However, it is still true that a substantial portion of energy usage in manufacturing can be attributed to the chemical industry. Thus, advances that can fundamentally alter the required energy usage in manufacturing a product are a key aspect of green chemistry.
7. Design for water optimization and integrationAmong the various species in a chemical and allied manufacturing process, water is critical and is widely used across several industries. Water is a relatively inexpensive and useful solvent which is easy to handle and presents minimal toxicity and safety concerns. In a typical chemical process, water may play several roles including the following:
   • It may act as a solvent for reaction (such as in single or multiple phase reactions).
   • It may be a solvent for separations (such as in decanters and liquid–liquid extraction systems).
   • It may be used for heat removal operations (such as a noncontact stream in condensers).
   • It may be used in postproduction operations (such as in washing operations wherein, the final product may be washed to remove undesirable solvents, by‐products, and unreacted raw materials).
   • It may be used in environmental systems (such as in scrubbers that handle gaseous emissions from vents and process units).
   • It may be used in safety systems (such as to provide seals, heels, and in pump seal systems).
   • It may be used to generate utilities in the process (such as steam generation).
   Wastewater reduction and water conservation are becoming increasingly more important issues in process industries. More stringent environmental regulations, concerns over long‐term health effects on humans and nature, and the future availability of “clean” water resources are just a few of the factors that are driving efforts toward improvements in water conservation and wastewater reduction in manufacturing processes. As these issues continue to receive intense government scrutiny and to raise concern among community‐organized environmental groups, industries that fail to address the problems to the satisfaction of such outside stakeholders may find present operations threatened and the sustainability of future operations in doubt. These critical concerns have refocused efforts over the past decade toward identifying cost‐effective wastewater reduction and water conservation process designs, involving direct recycle and reuse of water that can be implemented within a variety of manufacturing process industries. Unsustainable strain on freshwater sources globally could be reduced if industries reuse their wastewater after advanced physico‐chemical‐biological treatments.
8. Use renewable feedstocksRaw material, or feedstock, should be renewable rather than depletable, whenever technically and economically practical. The process of manufacturing chemicals from agricultural feedstocks is known as Chemurgy. Fuels derived from biomass may include such cultivated ones as ethanol from corn or fuels from waste biomass. Natural polymers such as chitin and cellulose are receiving attention for their use in manufacturing engineered materials. Of particular interest is the use of CO₂ as a raw material for C₁ chemistry.
9. Reduce or avoid the use of derivativesThe unnecessary use of derivatives in blocking groups, for protection–deprotection, and the temporary modification of physical–chemical processes should be minimized or avoided when possible, because such steps require additional materials, increased solvent usage, and greater energy requirements. This principle is related to principles 2 and 5.
10. Use catalytic reagentsCatalytic reagents, employed as selectively as possible, are superior to stoichiometric reagents. The use of catalysts, whether heterogeneous or heterogeneous, is essential to realizing principles 2 and 3. Anastas et al. (2001) referred to the use of catalysis as a foundational pillar of green chemistry because catalysts can help achieve a number of green chemistry’s goals, including decreased material usage, increased atom efficiency, and reduced energy demands.
11. Design for degradationChemical products should be designed so that at the end of their function they are broken down into innocuous products that degrade and do not persist in the environment. This principle clearly intersects with the concept of DfE (see Section 10.1.7). Product design involves communication along a diverse chain of stakeholders: customers, sales representatives, engineers and materials scientists, and synthetic chemists and others. Green chemistry therefore requires close collaboration of chemists with these other stakeholders.
    1.  Use real‐time analysis to pollution prevention Analytical methodologies need to be developed further to allow for real‐time, in‐process monitoring and control prior to the formation of hazardous substances. Waste production is not always the result of synthetic chemistry; sometimes it is simply the result of poor process monitoring and control. The analytical methods currently in use frequently rely on sampling the product at the end of the production line, which may be accomplished in minutes but also may last for hours. In such cases, by the time the product analysis has been completed it is typically too late to adjust the production process for optimum performance. For this reason, real‐time process monitoring and control of temperature, pressure, feed rate, feed composition, and product composition are vital to green chemical engineering.
    2. Choose inherently safer chemistry for accident preventionSubstances and the forms of them used in a chemical process should be chosen to minimize the potential for chemical accidents, including releases, explosions, and fires. There is some overlap of this principle with principle 5 regarding safer solvents and auxiliaries: for instance, supercritical carbon dioxide is an inherently nontoxic substitute for chlorinated solvents. Substitutes for lead compounds in paint and gasoline are also examples of principles 5 and 12(a). This principle also applies to strategies such as eliminating the bulk transport and storage of hazardous intermediates in favor of generating these intermediates on‐site. While the ultimate goal of green chemistry would be to eliminate the hazardous material entirely, the possibility of a massive release of the hazardous compound is reduced or eliminated by generating it on‐site instead of relying on bulk storage.

These principles address an important criterion necessary for achieving the goal of a green technology‐preventing pollution on all fronts. This includes conserving materials (feedstocks, reagents, and solvent) and energy (decreased byproduct formation and increased conversion, which, in turn, minimizes the number of process steps), using improved catalysts or catalytic processes in place of non‐catalytic ones, and designing safer chemicals and chemical reactions.

As a new technology surpasses initial bench‐level development, the researcher should address the 12 additional principles of green chemistry, introduced by Winterton (2001), as follows:

1. Identify byproducts; quantify if possible
2. Report conversions, selectivities, and productivities
3. Establish a full mass balance for the process
4. Quantify catalysis and solvent losses
5. Investigate basic thermochemistry to identify exotherms (safety)
6. Anticipate other potential mass and energy transfer, limitations
7. Consult a chemical or process engineer
8. Consider the effect of the overall process on choice of chemistry
9. Help develop and apply sustainable measures
10. Quantify and minimize use of utilities and other inputs
11. Recognize where operator safety and waste minimization may be incompatible
12. Monitor, report, and minimize wastes emitted to air, water, and solids from experiments or process.