by Andryj M. Borys & John De Backere
The 12 principles of green chemistry provide a framework towards a sustainable future in the chemical sciences. Examples and suggestions on how these principles can be implemented into routine Schlenk line practices are provided.
1. Waste Prevention
It is better to prevent waste than to treat or clean up waste after it has been created.
Schlenk line example: One major source of waste is the liquid nitrogen used to cool the cryogenic trap. Make sure to use Dewars that are in good condition and to insulate the trap to minimise boil off. When shutting down the Schlenk line, recover the liquid nitrogen for future use.
Liquid nitrogen production is an energy-intensive process, and a conventional cryogenic air separation unit consumes 0.357 kWh of power per kg of liquid nitrogen.
2. Atom Economy
Synthetic methods should be designed to maximise the incorporation of all materials used in the process into the final product.
Schlenk line example: Consider titrating organometallic reagents (e.g. nBuLi or Grignard reagents) prior to use to ensure that the correct stoichiometry is employed in a reaction.
3. Less Hazardous Chemical Syntheses
Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.
Schlenk line example: Whilst Schlenk lines facilitate the safe manipulation of highly hazardous reagents (e.g. pyrophoric organolithiums), alternative synthetic routes that avoid their use should be considered where possible.
4. Designing Safer Chemicals
Chemical products should be designed to affect their desired function while minimising their toxicity.
Schlenk line example: Researching and designing catalytic transformations based on earth-abundant and environmentally benign metals such as calcium, aluminium and iron.
5. Safer Solvents and Auxiliaries
The use of auxiliary substances (e.g. solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.
Schlenk line example: Even though exposure is usually limited when using Schlenk line techniques, the use of greener solvents, particularly from renewable feedstocks (see principle 7), is recommended when possible.
6. Design for Energy Efficiency
Energy requirements of chemical processes should be recognised for their environmental and economic impacts and should be minimised. If possible, synthetic methods should be conducted at ambient temperature and pressure.
Schlenk line example: When possible, turn off vacuum pumps to minimise energy consumption. Furthermore, it may be suitable to share Schlenk lines amongst numerous individuals.
7. Use of Renewable Feedstocks
A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.
Schlenk line example: Using 2-methyltetrahydrofuran (2-MeTHF) or cyclopentylmethyl ether (CPME) as an alternative ethereal solvent to tetrahydrofuran (THF). When cleaning Schlenk lines or removing grease from glassware, consider using ethyl acetate (EtOAc) instead of toluene or dichloromethane (DCM).
8. Reduce Derivatives
Unnecessary derivatisation (use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimised or avoided if possible, because such steps require additional reagents and can generate waste.
Schlenk line example: Consider alternative synthetic methods that avoid protecting group strategies (e.g. the synthesis of substituted phosphines directly from elemental phosphorus).
9. Catalysis
Catalytic reagents (as selective as possible) are superior to stoichiometric reagents.
Schlenk line example: Employing established catalytic methods when possible or designing new catalysts and sustainable transformations.
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.
Schlenk line example: Investigating novel systems based on earth-abundant metals for the degradation and/or recycling of polymers, fluorocarbons, SF6 and other pollutants.
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.
Schlenk line example: The use of real-time analytical techniques such as NMR spectroscopy can be employed to test and monitor reactions which can miminise waste and energy consumption.
12. Inherently Safer Chemistry for Accident Prevention
Substances and the form of a substance used in a chemical process should be chosen to minimise the potential for chemical accidents, including releases, explosions, and fires.
Schlenk line example: Avoid the use of mercury bubblers and instead consider using an oil-bubbler or over-pressure bubbler.
Resources & Further Reading
Paul T. Anastas, John C. Warner – Green Chemistry: Theory and Practice
ACS 12 Principles of Green Chemistry
Sigma Aldrich Green Chemistry Principles
Green Chemistry Initiative (GCI) at the University of Toronto
Beyond Benign
The Schlenk Line Survival Guide 