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 Prevention Atom economy Less hazardous chemical synthesis Designing safer chemicals Safer solvents and auxiliaries Design for energy efficiency Use of renewable feedstocks Reduce derivatives Catalysis Design for degradation Real-time analysis for pollution prevention 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 Reactants from renewable sources (e.g. plants are preferable). Processes should rely on renewable energy resources, rather than fossil fuels. Reactions that use fewer reactants, particularly ones that aren’t hazardous, are better. Materials produced by chemists should be biodegradable so they don’t persist in the environment after they’ve been used. Solvent use should be minimised, & solvents should be benign in their impact on the environment.

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!

sum of RFMs of all products Yield vs Atom economy Atom 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 % RFM desired product sum of RFMs of all products 85 85 + (2 x 36.5)

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

Controlling air pollution Emissions of nitrogen oxides A three-way catalytic converter

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 2008 This document may have been altered from the original

POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES POLLUTANTS POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES Carbon monoxide CO Origin • incomplete combustion of hydrocarbons in petrol because 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)

POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES POLLUTANTS POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES Oxides of nitrogen NOx - NO, N2O and NO2 Origin • combination of atmospheric nitrogen and oxygen 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 + O ozone is produced O + O2 —> O3

POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES POLLUTANTS POLLUTANT GASES FROM INTERNAL COMBUSTION ENGINES Unburnt hydrocarbons CxHy Origin • hydrocarbons that have not undergone combustion Effect • toxic and carcinogenic (cause cancer)

POLLUTANTS POLLUTANT FORMATION Nitrogen combines with oxygen N2(g) + O2(g) —> 2NO(g) Nitrogen monoxide is oxidised 2NO(g) + O2(g) —> 2NO2(g) Incomplete hydrocarbon combustion C8H18(g) + 8½O2(g) —> 8CO(g) + 9H2O(l)

POLLUTANTS POLLUTANT REMOVAL Oxidation of carbon monoxide 2CO(g) + O2(g) —> 2CO2(g) Removal of NO and CO 2CO(g) + 2NO(g) —> N2(g) + 2CO2(g) Aiding complete hydrocarbon combustion C8H18(g) + 12½O2(g) —> 8CO2(g) + 9H2O(l)

CATALYTIC CONVERTERS REMOVAL OF NOx and CO • CO is converted to CO2 • NOx are converted to N2 2NO(g) + 2CO(g) —> N2(g) + 2CO2(g)

CATALYTIC CONVERTERS REMOVAL 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 CONVERTERS REMOVAL 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 CONVERTERS STAGES OF OPERATION

CATALYTIC CONVERTERS STAGES 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 CONVERTERS STAGES OF OPERATION Reaction • being held on the surface increases chance of favourable collisions • bonds break and re-arrange

CATALYTIC CONVERTERS STAGES OF OPERATION Desorption • products are released from the active sites

CATALYTIC CONVERTERS STAGES OF OPERATION Adsorption Reaction Desorption

CATALYTIC CONVERTERS STAGES 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