September 23, 2020

Green Chemistry: criteria and applications

In recent decades, the chemical industry has evolved towards a different way of approaching the development of processes that takes into account problems such as pollution, the use of environmental resources or renewable sources. From this “new way” of seeing chemistry, what is now called Green Chemistry has developed.

Definition of Green Chemistry

This new concept of chemistry aims to redirect the chemical industry, on eco-sustainability paths. Sustainable development, which has become increasingly central in scientific and technological progress in the last century, requires chemistry to play a primary role in the conversion of old technologies into new “clean” processes and in the design of new products and new processes that are always more eco-friendly.

Since the problem of environmental pollution (especially air and water) goes beyond national borders, the adoption of international policies is increasingly required. The large government environmental agencies, the large industry and the world of chemistry in general, are developing and adopting a code of conduct that identifies precise strategies to prevent pollution.

In addition, given the progressive decrease in available fossil fuels, green chemistry aims to optimize consumption as much as possible, reducing energy waste in the execution of industrial processes and using renewable energy sources for the power supply of industrial plants.

Modern synthetic chemistry still depends largely on petrochemistry, which uses products derived from oil as its raw materials, which are now on the verge of exhaustion. A new line of research is therefore being developed aimed at the production of plastic materials and chemicals obtained from biological and renewable sources.

The 12 principles of Green Chemistry

Green chemistry is not a separate branch of chemistry, but rather a different way of approaching the work of the chemist. To do this, it uses 12 principles to keep in mind when designing a new synthesis or chemical plant:

Preventing waste: designing chemical syntheses to avoid waste. Do not leave waste to be treated or cleaned.
Maximizing the atomic economy: designing synthesis so that the final product contains the maximum percentage of the starting materials. Little or no atom is wasted.
Designing less dangerous chemical syntheses: designing syntheses that use or generate substances with minimal or no toxicity for humans or the environment.
Designing safer chemicals: designing chemicals that are completely effective but have minimal or no toxicity.
Use safer solvents and reaction conditions: avoid the use of solvents, separating agents or other auxiliary chemicals. If you need to use these chemicals, use the safer ones.
Increase energy efficiency: perform chemical reactions at room temperature and pressure when possible.
Use renewable raw materials: use starting materials that are renewable instead of exhaustible. Renewable raw material sources are often agricultural products or waste from other processes; the sources of exhaustible raw materials are often fossil fuels (oil, natural gas or coal) or mining activities.
Avoid chemical derivatives: avoid the use of protective groups or any temporary changes, if possible. The derivatives use additional reagents and generate waste.
Use catalysts, stoichiometric non-reagents: reduce waste to a minimum by using catalytic reactions. Catalysts are effective in small quantities and can perform a single reaction many times. They are preferable to stoichiometric reagents, which are used in excess and only react once.
Design of chemicals to degrade after use: design chemicals that reduce harmless substances after use so that they do not accumulate in the environment.
Real-time analysis to prevent pollution: include in-process monitoring and control, in real time during the synthesis to minimize or eliminate the formation of by-products.
Minimize the risk of accidents: design chemicals and their physical forms (solid, liquid or gaseous) to minimize the potential for chemical accidents including explosions, fires and releases to the environment.
Applications of Green Chemistry
Among the possible applications there is undoubtedly the optimization of existing chemical syntheses, modifying them to be more sustainable, replacing organic solvents with water, or developing catalysts that improve their efficiency.

The use of catalysts, in particular, proved to be a particularly profitable strategy: the traditional synthesis of ibuprofen, for example, one of the most important non-steroidal anti-inflammatory drugs in the pharmaceutical industry, was carried out using six stoichiometric steps incorporating less than 40% of the atoms used. The BHC (Boots / Hoechst Celanese) Company, a chemical multinational, has instead designed a catalytic synthesis that requires only three steps, with the atomic economy reaching 80% (99% also recovering acetic acid). Hydrofluoric acid HF, which serves both as a catalyst and as a solvent, is recovered and recycled with an efficiency greater than 99.9%.

The BHC process illustrates the compromises necessary in the prevention of pollution: while the atomic economy has doubled, the synthesis uses a dangerous catalyst / solvent that tops the HF. With HF recovery, however, the new process represents an improvement over the traditional synthesis of ibuprofen. and in fact this technology has been marketed since 1992 at the BHC facility in Bishop, Texas. Another significant example is the development of an economic synthesis to produce TPA (Thermal Polyaspartate), a biopolymer with properties similar to those of polyacrylate and which can therefore be used in its replacement in various areas, such as for example in female sanitary towels or in Baby diapers.

The synthesis of TPA starts from an amino acid, the L-aspartic acid which ends in just two steps results in a biodegradable polymer with multiple applications, including agriculture and waste management.

Future developments

With the emergency climate change increasingly prevalent also in public opinion, the development of a green chemistry, which is also attentive to the energy-environmental factor, appears to be increasingly central both economically and socially, so much so as to be included among the Objectives for Sustainable Development of the United Nations.

Although a first unanimous definition of Green Chemistry was introduced in 1991, there is still much work to be done to become a real systematic approach to research and development in all countries of the world, significantly increasing awareness of sustainability in chemistry. However, this requires a substratum of cooperation between economics, politics, interdisciplinary commitment, equity, education, regulation, metrics and awareness. Transforming traditional chemical companies into a sustainable key requires significant change. If the extraction, production, distribution and usage profiles were developed before there was an awareness of the consequences of sustainability, there is no reason to believe that they can be optimized, or even adjusted for today’s circumstances.

Of course, this would require a greater social awareness of the economic and political authorities regarding sustainability, research and future planning which cannot always be found in the current protagonists of the international political scene. Certainly universities and research centers are proponents of a flow that for years has been moving towards an increasingly greener chemistry that is starting to invest in various industrial realities. Hopefully a different awareness of environmental issues compared to the past bodes well but much remains to be done not only to carry out research, but also and above all to change the way we approach chemistry, what it can bring in terms of well-being and technological and social progress.

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