14 Green Chemistry 14.1 Introduction 14.2 Principles of Green Chemistry for Promoting Sustainable Development 14.3 Application of Green Chemistry in Practices 14.4 Feasibility of Green Chemistry for Daily Life Applications of Chemistry

Chemistry and sustainable development

Sustainable development (1987 UNCED) 14.1 Introduction Sustainable development (1987 UNCED) Meeting the needs of the present without compromising the ability of future generations to meet their own needs.

Green chemistry as a reduction process

14.2 Principles of Green Chemistry The principles cover such concepts as: the design of processes to maximize the amount of raw material that ends up in the product; the use of safe, environment-benign substances, including solvents, whenever possible; the design of energy efficient processes; the best form of waste disposal: not to create it in the first place.

14.2 Principles of Green Chemistry Prevention Atom Economy Less Hazardous Chemical Synthesis Designing Safer Chemicals Safer Solvents and Auxiliaries Design for Energy Efficiency

14.2 Principles of Green Chemistry 7. Use of Renewable Feedstocks 8. Reduce Derivatives 9. Catalysis 10. Design for Degradation 11. Real-time analysis for Pollution Prevention 12. Inherently Safer Chemistry for Accident Prevention

Yield is not enough! As an example, a typical school textbook procedure for an oxidation reaction to produce an alcohol gives a yield of 55 %, and the weight of product is 15 g. This reaction has actually involved 900 g of inputs and produced over 800 g of waste.

Atom Economy X + Y  P + U 100% yield 100% selectivity but < 100% AE % Atom Economy = 100 x (RMM of product/RMM of X + Y)

Atom economic reactions Atom non-economic reactions

14.3 Application of Green Chemistry in Practices Supercritical Carbon Dioxide as Solvents in Decaffeination Caffeine occurs naturally in tea, coffee, cocoa and chocolate products. It is also added to soft drinks and a variety of both prescription and over the counter drugs.

A supercritical fluid is any substance at a temperature and pressure above its critical point. It can diffuse through solids like a gas, and dissolve materials like a liquid.

In the past, caffeine was removed from coffee using solvents such as dichloromethane (CH2Cl2). At high concentrations, CH2Cl2 can cause health problems. Green coffee beans are soaked with pure water. The coffee beans expand; their pores get opened and the coffee molecules inside become mobile. Supercritical CO2 acts like a powerful non-polar solvent that extracts all the movable caffeine molecules out of a bean like a magnet.

Schematic diagram of CO2 decaffeination method for coffee Schematic diagram of CO2 decaffeination method for coffee. This is a continuous process in which the CO2 is recycled continuously.

The benefits of this method include: extracts caffeine effectively. It is a direct contact method but does not use toxic chemicals. does not chemically affect proteins or carbohydrates of the coffee beans, and hence their flavour and smell. Relatively accessible Tc and Pc (31oC, 74 atm) CO2 is available as a side product of many industrial process CO2 can be removed simply by reducing pressure Supercritical CO2 is environmentally benign. Supercritical CO2 is also inexpensive. The CO2 is recycled after the extraction process.

Use of H2O2 in presence of Manganese Based Catalyst as Bleaching Agent Chlorine gas was widely used as a bleach in paper and pulp industry. Unreacted chlorine in the effluent can react with naturally occurring compounds in water and turn them into toxics. Such as polychlorinated dibenzo-dioxins and furans. Eventually they are found in food products like dairy products, pork, beef and fish.

Trends in uses of bleaching agents in pulp industry in Western Europe between 1990 and 2002. Together ClO2 (elemental chlorine – free) and H2O2 are replacing Cl2 in this use, as both comply with environmental legislations.

Hydrogen peroxide is a cheap, relative safe and environmentally friendly oxidising agent that is converted to water in the course of the reaction. Manganese based co-ordination complexes are often added as catalysts to assist the heterolytic cleavage of the – O – O – bond. Another catalyst system utilises the TAML iron(III) activators which promotes the conversion of hydrogen peroxide into hydroxyl radicals. They break down lignin(木質素) in a shorter time and at a lower temperature.

14.4 Feasibility of Green Chemistry for Daily Life Applications of Chemistry. A no. of difficulties involves: chemical (what technologies are available?), economic (who pays, who benefits?), social (who are affected?) and political (who’s responsible?) Other difficulties: An environmentally mobile, persistent pollutant can move from one compartment of our Earth’s ecosphere to another compartment, causing unforeseen interactions and perturbations that could be catastrophic, such as the ozone hole.

However, pollution control and green chemistry have become growth industries and the jobs and income generated by these industries must be fitted into the overall economic equation when considering the costs and benefits of adopting less polluting habits or processes. Some successful examples of green chemistry in practices.

Possible contribution of Green chemistry can contribute to achieving sustainability in the 21st century in three key areas. Renewable energy technologies. (a) (b) Arrays of solar cells supply electric power to (a) space shuttle and (b) pleasure boat. 

The second area lies in reducing our dependence on the dwindling fossilized carbon. The third area goes back to the replacement of existing polluting technologies by benign alternatives,

To achieve such sustainable chemistry-The principles of green chemistry must become an integral part of chemical education and practice. We have to comprehensively incorporate environmental considerations into the reactions and technologies. We must not distort scientific data in order to protect our profits. Government, universities and industry must learn to value and support these research programs of reasonable promise, in stead of just chasing after short term goals of immediate profit.