By Taniya Adak
With mounting concerns over the state of our planet, a challenge that all scientists are struggling to address is how to increase quality of life while minimizing detrimental effects to human health, the environment and the biosphere. If Green Chemistry is not a panacea, it is certainly close to being one, as it has a range of tools, disguised as principles, at its disposal aimed at ameliorating chemical industries’ health and environmental impacts. Scientists involved in active chemical research hold immense responsibility towards the future, and it is high time to make Green Chemistry, just Chemistry.
In order to lay claim that our chemistry is “green”, it must adhere to twelve principles of Green Chemistry that ensure chemical products and processes being developed minimize the use and generation of hazardous substances, reduce waste and demand on diminishing resources through the adoption of simple chemical approaches and use renewable feedstocks. These set of principles, or philosophy of chemistry, were formulated by Prof. Paul Anastas and Dr. John Warner during the late 90s.1 Fast forward twenty years, how successful have we been in adopting these principles in our research culture? 101st Canadian Chemistry Conference and Exhibition in Edmonton this year.
Chemical industries are the largest producers of chemicals and hence have considerable impact on the environment. It is reasonable to state that having industries follow the twelve principles will have positive consequences. It is worth noting that abiding by certain environment-benefiting green principles such as atom economy, reagent optimization, in-situ analysis, energy reduction etc. also leads to economic benefits to industries. However, it has been found that on an average, only three out of twelve green chemistry principles are applied in industry, viz. prevention of waste, designing safer chemicals and using substances that minimize potential for accidents. Despite of the research advancements in green chemistry and engineering, the technology has not been fully embraced yet.
Solvents alone make up for 80-90% of waste by mass and less than 50% of solvents are recycled. A potential strategy to eliminate this issue is solvent reduction by removing unnecessary work-ups and purifications. If that is not feasible, solvents could either be recovered/reused, or a better solvent could be selected. Sanofi, a global biopharmaceutical company, provides the latest solvent selection guide (based on environmental, health and safety concerns), to help one choose an environmentally benign solvent.2.3 Water is one of the recommended solvents in the list, but choosing water has its own set of practical difficulties, such as solubility issues, it being unfriendly to catalysts and the difficulty to treat waste water.
Figure 1. Some laboratory solvent replacement options
Ideally, it is desirable to eliminate the requirement of solvents. Some of the alternatives are using liquid reactants, melt phases or carrying out mechanochemistry (such as, mortar-pestle and ball mills) in solid state reactants, wherever possible.4
There are a handful of entrepreneurial companies that have inculcated one or more principles of green chemistry as their guiding principle. For example, a US based company Resinate, converts landfill waste into high performance polyester polyols to be used for floor coatings, adhesives, fire-retardant foams etc., thereby preventing waste from being futile. Another UK based company Green Biologics uses microbial fermentation to convert a wide range of sustainable feedstocks into high value green chemicals including n-butanol and acetone.
But is it really ‘green’?
In order to determine which chemical process is greener, chemists have tried to develop ways of measuring greenness and sustainability. The earliest concept of mass metrics (tools used to measure the mass efficiency of a chemical process) deals with just the percentage yield. It does not consider the by-products and hence have become obsolete. More recently, the concept of atom economy (Figure 2) was introduced in our green chemistry education. This calculation method is a better surrogate for ‘greenness’ yet does not consider yield, solvents utilized in reaction, work ups and purification, energy expenditure and even the nature (toxic/benign) of waste stream. Eventually, an improved concept by the name of Reaction Mass Efficiency (RME) was introduced. Although the yield was considered, the losses in work ups and purification were still not accounted for. One of the latest concept introduced was Process Mass Intensity (PMI), which is currently the key metric for evaluating progress towards more sustainable manufacturing utilized by most pharmaceutical industries.
Figure 2. Formulae for various mass metrics
Most pre-clinical entries have a PMI ~ 400 and the commercial candidates have a PMI ~ 70. A popular example would be the top selling “little blue pill”, Viagra, which has a current PMI < 6, coming down from PMI = 1300 after decades of optimization. Thus, PMI is one key tool that is being used across the chemical industries to label their processes as “greener”, if not fully green. However, mass-based metrics need to be augmented by metrics which measure the environmental impact of waste, such as life cycle assessment (LCA).
As small actions create far-reaching ripples, researchers can also help by making their research green by avoiding potentially unnecessary methodologies involving purification steps, drying agents, work ups, solvents in general, excess reagents and protecting groups. One could recycle materials, especially solvents, or reduce solvent usage, and could even design atom economical reactions. Conversations on green chemistry need to happen through various channels including departmental clubs, seminar series and hands-on activities. Green Chemistry and Commerce Council (GC3) is a business-to-business forum that advances the application of green chemistry and design for environment across supply chains. They have a policy statement linked with multiple companies (such as, Nike, Dell, Dow, Hewlett Packard, Johnson & Johnson, Herman Miller etc.) that provides hiring preference for students with working knowledge of green chemistry and sustainability principles. Thus, with all the cumulative efforts, the goal is not to ‘heal’ damages done to the environment in the past, but to continue carrying out research and development in a responsible and sustainable approach that will benefit human health, environment, economy and business.
“Green chemistry is not just a catchphrase. It is an indispensable principle of chemical research that will sustain our civilized society in the twenty-first century and further into the future.” – Ryoji Noyori, 2001 Chemistry Nobel Laureate.
- Anastas, P. T.; Warner, J. C. Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998, p.30.
- Prat, D. et al. Process Res. Dev. 2013, 17, 1517.
- Clarke, C. et al. Rev. 2018, 118, 747.
- Do J-L, Friščić T. Mechanochemistry: A Force of Synthesis. ACS Central Science. 2017, 3, 13-19.