Decaffeinating coffee with scCO 2. Green Chemistry What is it? Why do we need it?
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Transcript of Decaffeinating coffee with scCO 2. Green Chemistry What is it? Why do we need it?
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