Environmental Efficiency of Chemical Processes

Dr. Anuj Kumar Mittal, Head-R&D

PI Industries Ltd.

IGCW Convention - December 2013

Prelude

WHAT IS GREEN CHEMISTRY?

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“Green Chemistry is essentially a way of thinking rather than a new branch of chemistry and is about utilizing a set of principles that seek to reduce the environmental impact of chemical processes and products”

- Royal Society of Chemistry

‘Green Chemistry’ aims to improve the way that chemicals are both produced and used in chemical processes in order to reduce any impact on man and the environment.

It is not just about industrial production. The principles involved apply equally to the use of chemicals in for example laboratories and education.

Promotion of ‘Green Chemistry’ is one of the most important ways in which chemistry and chemists can contribute to

sustainable development

WHAT CONSTITUTE

ENVIRONMENTAL EFFICIENCY?

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By improving resource efficiency, ‘Green Chemistry’ provides financial benefits from lower material usage, energy and capital

expenditure costs in addition to the environmental benefits.

Atom Efficiency designing processes to maximize the amount of raw material that is converted into the product

Energy Conservation designing more energy efficient processes

Waste Minimization recognizing that the best form of waste disposal is not to create waste in the first place

Substitution using safer, more environmentally benign raw materials and solvents or solvent free processes.

Efficiency of a Chemical Process

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Is it high yielding ?

No. Yield alone does not characterize process efficiency

Then – what else does?

Let us look at the example below:

(NH3)2PtI2 + Ag2SO4 + 2 KCl → (NH3)2PtCl2 + 2 AgI + K2SO4

If one started with 100mg and isolated 50 mg of the main product:

the mass yield is 50% based on the base raw material

the theory yield is 80.5% based on the base raw material

WHAT IS AN ‘EFFICIENT’ PROCESS?

One may call this a “good” process…

Let us look closely at the “balanced equation”:

(NH3)2PtI2 + Ag2SO4 + 2 KCl → (NH3)2PtCl2 + K2SO4 + 2 AgI

Which of the reactants is the “limiting reactant”? Look at the table below (actual process quantities)

Used a large excess of KCl (only two equivalents were required by the reaction stoichiometry). Should have used a bit more silver sulfate. The silver sulfate thus serves as the limiting reagent ; theoretical yield of cisplatin was actually only 0.202 millimoles, or 60.6 mg, and our actual yield then is (50 mg/60.6 mg)*100% = 82.5% of the theoretically possible amount of product

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PROCESS EFFICIENCY

Is the process “efficient”?

Compound molecular wt milligrams millimoles

(NH3)2PtI2 482.960 100 0.207

Ag2SO4 311.794 63 0.202

KCl 74.551 330 4.43

(NH3)2PtCl2 300.057

82.5% yield based on the limiting raw material – that is good! Or is it?

ATOM IS TOO PRECIOUS A RESOURCE TO WASTE – BARRY TROST

Professor Barry Trost, a chemist at Stanford University, felt that reliance on “yield” as a measure of reaction efficiency represented an inappropriate measure

Reliance on yield as a measure of efficiency suggests that we are better at carrying out chemical transformations than we in fact are.

He developed a concept called “atom economy,” looking at a chemical reaction from the perspective of how many input atoms are incorporated in the desired product, vs. how many are discarded as waste.

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WHAT IS ATOM EFFICIENCY?

One should design a process to maximize the amount of raw material that is converted into the product

Look at the table below

We now see less than a third of the atoms in the starting materials is converted to the product

Atom Economy for this process = 300.057/(300.057 + 643.800)*100% = 31.8%

Considering the excess KCl used in actual experiment and lower than 100% theoretical yield, actual atom economy = (actual yield/mass of all reactants)*100% = [50 mg/(100 mg + 63 mg + 330 mg)]*100% = 10.1%.

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ATOM EFFICIENCY = ATOM ECONOMY

Conclusion: 89.9% of atoms used in the process are converted to waste!!!

Compound atoms used in pdt wt used atoms discarded wt discarded

(NH3)2PtI2 (NH3)2Pt 229.151 I2 253.809

Ag2SO4 - - Ag2SO4 311.794

2 KCl Cl2 70.906 K2 78.197

Totals (NH3)2PtCl2 300.057 I2 K2 Ag2SO4 643.800

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Waste is increasingly expensive to dispose of and is a major source of pollution arising from the chemical industry.

Maximizing atom efficiency is linked to waste reduction. This means designing chemical reactions so that as many atoms of starting material as possible end up in useful product.

In an ideal process, all reactant atoms end up within the useful product molecule. Hence, no waste is produced!

Inefficient, wasteful reactions have low atom economy

Efficient processes have high atom economy and are important for sustainable development. They conserve natural resources and create less waste

TAKEAWAY

Better Atom Economy = Lower Product Cost

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Freidel-Craft reactions using Lewis acid catalyst such as AlCl3 are very Atom Uneconomic

New catalysts for this process such as zeolites change these reactions to more atom economic and environmentally friendly

EXAMPLE

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E-Factor is the number of kilograms of waste generated for making one kg of

the product

It is a direct indicator of the environmental efficiency of a chemical process

Obviously, E-factor calculation takes into account all the inputs and outputs in a

process – including solvents and catalysts

Solvents contribute negatively to E-factor if they are not efficiently recovered

and recycled. Hydrocarbon solvents are very high on the negative list.

WHAT IS E-FACTOR?

Efficient Process: High Atom Economy + Low E-Factor

Environmental Efficiency

ENERGY CONSERVATION

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Our focus should not only be on using alternative, environmentally friendly chemicals in synthetic routes but also to increase reaction rates and lower reaction temperatures to save energy.

The environmental footprint is more to do with energy consumption, the climate crisis and depleting natural resources.

Chemists must recognize that until now there was very little thought to energy requirements in chemical synthetic chemical processes.

Designing more efficient methods is a necessity and if possible, synthetic methods should be conducted at room temperature and pressure to reduce energy requirements.

There is an urgent need to design more energy efficient processes

WASTE MINIMIZATION

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In the last decades, 600-700 million tones of chemical materials are produced every year (excluding fossil fuels, fertilizers and medicines) from the chemical industries of the world. One can imagine the amount of waste that was generated due to this.

Green Chemistry looks very carefully on reaction efficiency, use of less toxic solvents, minimizing the hazards of feedstocks & products and reduction of waste.

In order to achieve Waste Minimization and Prevention, we may

make use of catalysts instead of stoichiometric quantities,

reduce the use of chemical derivatives and

use renewable feedstocks

It is better to prevent than to clean or to treat afterwards

SUBSTITUTION

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Green Chemistry must strive, wherever practical, to design safer synthetic methods by using less toxic substances as well as the products of the synthesis.

Less toxic materials mean lower hazards to workers in industry and research laboratories and less pollution to the environment.

Solvents, separation agents and auxiliary chemicals used in synthetic chemistry must be replaced or reduced with less toxic chemicals.

We must strive to use safer, more environmentally benign raw materials and solvents or solvent free processes.

Replace more toxic raw materials and solvents with eco-friendly chemicals

SUMMARY

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Atom economy and E-Factor of a process are important criteria for process chemists

Atom Economic processes are more efficient than simply “high yielding” processes

Such processes are also more environment friendly as they generate less waste

Wherever possible, process chemists should evaluate atom economy of a reaction and use the best option rather than look only at yields.

Once the process is established at commercial scale it is important to continuously review and look into possibilities of improvement w.r.t. 3R principle.

Environmentally efficient processes are cost efficient and energy efficient too.

Atom Efficiency, Energy Conservation, Waste Minimization and Substitution are key to achieve Environmental Efficiency in a

Chemical Process

Thank you