Decaffeinating coffee with

scCO2

Green Chemistry

What is it? Why do we need it?

Learning outcomes

• Describe principles and discuss issues of chemical sustainability

• Understand the importance of establishing international cooperation to promote the reduction of pollution levels.

Green Chemistry• Means different things to different

people.

• It’s not just one thing – there are many aspects to Green Chemistry.

• Lets consider some of the ‘Principles of Green Chemistry’.

The 12 principles

1. Prevention2. Atom economy3. Less hazardous chemical synthesis4. Designing safer chemicals5. Safer solvents and auxiliaries6. Design for energy efficiency7. Use of renewable feedstocks8. Reduce derivatives9. Catalysis10. Design for degradation11. Real-time analysis for pollution prevention12. Inherently safer chemistry for accident

prevention

Principles of Green Chemistry

It’s better to develop reactions with fewer waste products than to have to clean up the waste.

i.e. high atom economy

Reactions that use fewer reactants, particularly ones that

aren’t hazardous, are better.

Reactants from renewable sources (e.g. plants are preferable).

Processes should rely on renewable energy resources,

rather than fossil fuels.

Solvent use should be minimised, & solvents should be benign in

their impact on the environment.

Materials produced by chemists should be biodegradable so they don’t persist in the environment

after they’ve been used.

Yield vs Atom economy

Yield can be calculated as:

% yield = mass (g) of product obtained x 100 theoretical yield (g)

The yield tells us how efficient a reaction is in terms of the amount of product we obtained relative to the maximum we could get from the amounts of reactants we used.But it doesn’t take account of waste products!

Yield vs Atom economyAtom economy can be calculated as:

% AE = x 100

A reaction may have a high % yield but a low atom economy.

RFM desired product sum of RFMs of all

products

Atom economy – some examples

Calculate the % atom economy of CH2Cl2:

CH4 + 2Cl2 → CH2Cl2 + 2HCl

RFM: CH2Cl2 = 85, HCl = 36.5

% AE = x 100

AE = x 100 = 53.8 % 85 85 + (2 x 36.5)

RFM desired product sum of RFMs of all

products

Atom economy – some examples

CH4 + 2Cl2 → CH2Cl2 + 2HCl

An atom economy of 53.8% may be considered to be quite low. How could a chemical company maximise their profits from this chemical process?

The by-product is hydrogen chloride, which can be sold as a gas or made into hydrochloric acid. These can then be sold.

Atom economy – some examples

Calculate the % atom economy of ethylene oxide:

RFM: C2H4O = 44, CaCl2 = 111, H2O = 18

AE = x 100 = 37.4 %

(2 x 44)

(2 x 44) + 111 + (2 x 18)

Atom economy – some examples

Ethylene oxide – A case of Green Chemistry

An atom economy of 37.4% is particularly poor, and this is a very wasteful process.

Nonetheless, this was the preferred method for synthesising ethylene oxide for many years.

Atom economy – some examples

Ethylene oxide – A case of Green Chemistry

Recently, a method of synthesising ethylene oxide from ethene and oxygen using a silver catalyst was developed.

What’s the atom economy of this reaction?

100 %

The role of catalysts• Catalysts have a crucial role to play in the

future of Green Chemistry.

• They allow the development of new reactions which require fewer starting materials and produce fewer waste products.

• They can be recovered and re-used time and time again.

• They allow reactions to run at lower temperatures, cutting the amount of energy required.

Catalysts in Action

Animation credit: Robert Raja / University of Southampton

The future of chemistry• We need to reconsider

the way we go about all aspects of our lives.

• The planet is feeling a burden.

• Science has the potential to solve our problems.

• Green Chemistry can play a significant role in a sustainable future.

Question

1. How does green chemistry enable chemicals and resources to be preserved?

Controlling air pollution

A three-way catalytic converter

Emissions of nitrogen oxides

Learning outcomes

• Explain the formation of carbon monoxide, oxides of nitrogen and unburnt hydrocarbons from the internal combustion engine.

• State environmental concerns relating to the toxicity of these molecules and their contribution to low-level ozone and photochemical smog.

• Outline how a catalytic converter decreases toxic emissions via adsorption, chemical reaction and desorption.

• Outline the use of infrared spectroscopy in monitoring air pollution.

© Pearson Education Ltd 2008This document may have been altered from the original

POLLUTANTSPOLLUTANTS

POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINESPOLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES

Carbon monoxide CO

Origin • incomplete combustion of hydrocarbons in petrolbecause not enough oxygen was present

Effect • poisonous • combines with haemoglobin in blood • prevents oxygen being carried to cells

Process C8H18(g) + 8½O2(g) —> 8CO(g) + 9H2O(l)

POLLUTANTSPOLLUTANTS

POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINESPOLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES

Oxides of nitrogen NOx - NO, N2O and NO2

Origin • combination of atmospheric nitrogen andoxygen under high temperature

Effect • aids formation of photochemical smog which is irritating to eyes, nose, throat

• aids formation of low level ozone which affects plants and is irritating to eyes, nose and throat

Process sunlight breaks oxides NO2 —> NO + Oozone is produced O + O2 —> O3

POLLUTANTSPOLLUTANTS

POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINESPOLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES

Unburnt hydrocarbons CxHy

Origin • hydrocarbons that have not undergone combustion

Effect • toxic and carcinogenic (cause cancer)

POLLUTANTSPOLLUTANTS

POLLUTANT FORMATIONPOLLUTANT FORMATION

Nitrogen combines with oxygenN2(g) + O2(g) —> 2NO(g)

Nitrogen monoxide is oxidised2NO(g) + O2(g) —> 2NO2(g)

Incomplete hydrocarbon combustionC8H18(g) + 8½O2(g) —> 8CO(g) + 9H2O(l)

POLLUTANTSPOLLUTANTS

POLLUTANT REMOVALPOLLUTANT REMOVAL

Oxidation of carbon monoxide2CO(g) + O2(g) —> 2CO2(g)

Removal of NO and CO2CO(g) + 2NO(g) —> N2(g) + 2CO2(g)

Aiding complete hydrocarbon combustionC8H18(g) + 12½O2(g) —> 8CO2(g) + 9H2O(l)

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

REMOVAL OF NOx and COREMOVAL OF NOx and CO

• CO is converted to CO2

• NOx are converted to N2

2NO(g) + 2CO(g) —> N2(g) + 2CO2(g)

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

REMOVAL OF NOx and COREMOVAL OF NOx and CO

• CO is converted to CO2

• NOx are converted to N2

2NO(g) + 2CO(g) —> N2(g) + 2CO2(g)

• Unburnt hydrocarbons converted to CO2 and H2O

C8H18(g) + 12½O2(g) —> 8CO2(g) + 9H2O(l)

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

REMOVAL OF NOx and COREMOVAL OF NOx and CO

• CO is converted to CO2

• NOx are converted to N2

2NO(g) + 2CO(g) —> N2(g) + 2CO2(g)

• Unburnt hydrocarbons converted to CO2 and H2O

C8H18(g) + 12½O2(g) —> 8CO2(g) + 9H2O(l)

• catalysts are rare metals - RHODIUM, PALLADIUM

• metals are finely divided for a greater surface area - this provides more active sites

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

STAGES OF OPERATIONSTAGES OF OPERATION

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

STAGES OF OPERATIONSTAGES OF OPERATION

Adsorption • NO and CO seek out active sites on the surface• they bond with surface

• weakens the bonds in the gas molecules

• makes a subsequent reaction easier

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

STAGES OF OPERATIONSTAGES OF OPERATION

Reaction • being held on the surface increases chance of favourable collisions

• bonds break and re-arrange

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

STAGES OF OPERATIONSTAGES OF OPERATION

Desorption • products are released from the active sites

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

STAGES OF OPERATIONSTAGES OF OPERATION

Adsorption Reaction Desorption

CATALYTIC CONVERTERSCATALYTIC CONVERTERS

STAGES OF OPERATIONSTAGES OF OPERATION

Adsorption • NO and CO seek out active sites on the surface• they bond with surface

• weakens the bonds in the gas molecules

• makes a subsequent reaction easier

Reaction • being held on the surface increases chance of favourable collisions

• bonds break and re-arrange

Desorption • products are released from the active sites