Paul Anastas, called the ‘Father of Green Chemistry’, provided us with the 12 principles of Green Chemistry, which now form the basis of all green education programs around the world. I do not wish to jot down the 12 principles, instead we can elucidate each of them with examples related to the chemical industry.
‘Dirty Dozens’ and the ‘Clean Dozens’, the former are a list of 12 deadly chemicals and the latter will consider a dozen of examples that can clear your concept of the dozen principles.
- Polychlorinated biphenyls
- Prevention: By alternate technology, Revamp, Material reduction by design strategy.
Atom Economy: Did you know, SN2 reactions are not atom economical? Is there any reaction that is a 100 percent atom economical? Uh huh. Polymerisation of ethylene is one such example. How is atom economy different from mole balance? Here’s how.
If A and B gives C then, Atom Economy = (Molecular weight of C)/(Molecular weight of A + Molecular weight of B)
Hydrogenation: 100 percent atom economical, it uses heterogeneous catalysis.
Carbonylation: 100 percent atom economical, it uses homogeneous catalysis.
Oxidation: Around 87 percent atom economical. Why? Let’s take a look.
In this reaction, the products also could have been CO2 and CO if not taken care of. So, here, the catalyst and the conditions will determine the efficiency.
- Less hazardous chemical synthesis: Phosgene is hazardous, therefore we should replace it with a safer one.
Read more: Sigma Aldrich Learning center
- Designing safer chemicals: Certain functional groups are hazardous by nature. Let’s try to compare these isosteres (substances with similar molecular and electronic characteristics): Benzene, Pyridine and Thiophene. They are all structurally same, but have similar properties. So, we can replace one with another, yes? To reduce pollution and risk at source is to design safer chemicals. The advances in molecular toxicology and the relationship between chemical structure and important properties exploited for industrial applications are the basic tools. Synthesis using catalysis, renewable resources, atom economy, solvent-less conditions will be the hallmark.
SPINOSAD: A natural product for insect control. It is produced by Saccharopolyspore spinosa. It is isolated from Caribbean soil sample. It demonstrates high selectivity and low toxicity.
Thermal Polyaspartic acid (TPA): It is a substitute for non-biodegradable polyacrylic acid (PAC).
- Safer solvents: Now, there are hardly any reactions that do not use solvents. But, people are working on it and it is the new trend: Solventless reactions. Cool? To first understand the use of safer solvents, let us know why we need solvents in the first place. We use solvents to bring all the molecules together in a single environment. We use solvents to form a single phase. They are used to make sure the temperature is under control, i.e. they make it easy for us to control the temperature of reactions. They take care of heat of reactions. Aren’t there other ways to control the reaction? Yes, there are. Reflux is one way to do it. Another way is to have a negative order reaction, wherein the concentrate is low and the rate is high. Solvents can be of two types, those that react and those that are inert. Now we can say that solvent-less reactions are the best, although not possible every time. So what’s second best? Reactions that use water as a solvent. Water is benign, simple as that. Next in the row are alcohol-based reactions. The newer category include Supercritical fluids, which are greener substitutes for organic solvents.
A green solvent should therefore:
- Be inert to the reaction condition
Chlorinated solvents are highly corrosive and must be avoided.
- Dissolve reactants and reagents
- Have an appropriate boiling point
VOCs must be avoided
- Be easily removed at the end of the reaction
The choice of solvents depends on the following:
- Characteristics: Dipole moment, Dielectric constant, Miscibility with water
As dielectric constant decreases, the miscibility with water decreases.
- Classification: Polar protic, Dipolar aprotic, Non-polar
Here’s a list of eco-friendly solvents:
- Water and aqueous blends
- N-methyl-2-pyrrolidone ( It is used to recover hydrocarbons while processing petrochemicals and desulfurisation of gases. It is also used as a solvent in polymer field, surface treatment of textiles)
- Propylene glycol for ethylene glycol
- Design for Energy efficiency: How can one make an energy intensive process into an energy efficient process? One way is to think of alternative energy systems. Ultrasound, Microwave, Solar are a few of the alternative energy sources. Did you know, sulphuric acid releases enough heat to run a power plant? Oh, yes, it is true. Economics is at the heart of any process, therefore design for energy efficiency makes a process economical.
Use of renewable feedstock: Not many believe in this, because economy doesn’t favor it, for now. But did you know, that before use of crude oil and coal, the industry was based on ethanol which is now called ‘bioethanol’.
Reduce derivatives: We should not derivatize unnecessarily. For example, there are ways to produce phenol without producing derivatives. Let me introduce you to a concept called as ‘Retrosynthesis’. The most famous of all the restrosynthesis processes is the production of Ibuprofen, wherein 6 steps were reduced to just 3.
- Catalysis: One could write books and books on this. So it would be unfair to talk about it in a few sentences. I will talk more about it in the subject called as ‘Industrial Catalysis’. Meanwhile, I can say that if you can make a process without using a catalyst, it is way to go, my friend. The areas which will be majorly impacted by catalysis are: Fuel production, Commodities, Food, Agriculture, Health, Materials production and Environmental protection.
Design for degradation: Biodegradibility, everything should be biodegradable because our Earth is one giant organism and it will sustain only when things are biodegradable. Refractory materials are not biodegradable, but can they be made biodegradable?
Real-time analysis for pollution prevention: This well ensure that quick actions are taken towards formation of hazardous products.
Inherently safer chemistry for accident prevention: For example, we can replace di-ethylether in the lab with safer crystallizing agents.
ACS Green Chemistry Institute® (Starting in January, 2013, a “Green Chemistry Principle of the Month” will be published, consecutively illuminating each principle through the perspective of an expert contributor.)