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Transcript of Green Chemistry....
Non-renewable raw materials like fossil-fuels and metal ores, have become more expensive, encouraging companies to reduce the amounts they use and to look for other raw materials, especially materials that are renewable.
1) BIO-OIL FROM WHEAT.....
Wheat straw that is renewable as a fresh supply is grown each year, can be converted into a valuable oil for fuel by pyrolysis.
Pyrolysis involves heating the straw in a limited oxygen supply so it does not burn (sometimes heating is done by steam). In fast pyrolysis, straw is heated rapidly for only a few seconds, and this breaks it down into a large number of relatively small molecules. The resulting mixture is then cooled rapidly to prevent further reaction. It produces a dark, oily liquid called bio-oil or pyrolysis oil.
Used : as an alternative to fuel oil from crude oil.
Other products of pyrolysis: Char (carbon solids) + mixture of gases.
2) ETHANOL FROM ORGANIC WASTE
By fermentation of plant material.
With yeast: does not convert all sugars to ethanol.
KO11 is a genetically modified bacteria (would normally produce acids, but the modifications produces ethanol instead) : It converts all sugars to ethanol.
Advantage over yeast:
A wider range of sugars can be processed. Enables biomass waste (which would be thrown away) and other organic waste can be used for ethanol production.
There is an increasing tendency for farmers to grow crops for use in making bio-fuels and biochemicals. It is important that in doing this, the amount of food grown for the increasing population is not forgotten.
3) STARCH AND CELLULOSE:
Starch is the plant's main energy store produced as a product of photosynthesis from carbon dioxide and water. It can be extracted from plants as industrial starch.
The increasing price of crude oil and the realization that starch is renewable is making market starch-based products more acceptable. e.g: supermarkets plastic bags made from starch are biodegradable whereas the oil-based ones are not.
Starch is also an important source of glucose (from which many other materials can be synthesized).
Zorbix, a starch polymer which absorbs water readily and is useful for packaging.
Cellulose's source is wood pulp. Used to make polymers:
Acetate -for fibers in fabrics and photographic films.
Rayon -as substitute for silk.
4-oxopentanoic acid [levulinic acid] -making pharmaceuticals, plastics and rubbers
Lyocell [tencel]- in wide range of clothing
Advantages of cellulose
-Made using wood pulp managed from forests.
-It uses a non-toxic, biodegradable solvent in its manufacture and the solvent is re-used.
-Biodegradable and can be recycled.
Finding alternatives to very hazardous chemicals
Improving process design and efficiency to minimize the use of chemicals and reduce associated waste or emissions by:
• Identifying opportunities to use less hazardous chemicals and ensuring any residual risks
are appropriately managed
• Exploring opportunities for the use of renewable resources and bio-transformations
• Using inherently safer chemistry
• Minimizing energy- and water-intensive manufacturing processes
• Exploring and optimizing recycling and reuse opportunities
It is the utilization of a set of principles that reduces or eliminates the use or generation of hazardous substances in the design, manufacture and application of chemical products.
So wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to people or the environment!
Less Hazardous Chemical Synthesis
Designing safer chemicals
Chemical products should be designed to effect their desired function while minimizing their toxicity.
Toxics in the Environment
Substances that are toxic to humans, the biosphere and all that sustains it are currently still being released at a cost of life, health, and sustainability.
One of the green chemistry's greatest strengths is the ability to design for reduced hazard.
Using catalysts for reactions with higher atom economies, e.g: development of methods used to produce ethanoic acid based on catalysts of cobalt, rhodium and iridium.
A catalyst is used in the chemical industry to speed up the manufacturing process. In an equilibrium process it allows the same yield to be formed more quickly.
It allows the chemical engineer to use lower temperatures and so save energy.
In theory catalysts and not used up in the reaction,
In practice they often do have to be replaced due to mechanical wear and tear.
1) It can be produced in industry by oxidation of ethanol. Bur this has a low atom economy as there are other products to the reaction.
2) Direct oxidation of butane and naphtha (both from fossil fuels). It has a very poor atom economy of about 35%.
Methanol and carbon monoxide (by Monsanto).
(Rhodium as catalyst) + iodide ions
150-200 C and 30-60 atm pressure.
= Rhodium metal is very expensive -more expensive than gold.
= Rhodium and iodide ions form insoluble salts, so the water content in the reaction vessel has to be kept relatively high to prevent this.
= A final distillation step is needed to remove water, adding cost and energy demands.
= Any precipitation forming removes the catalyst from the reaction mixture, which must be recovered and removed to the main reactor.
= Rhodium catalyses side reactions.
Cobalt as catalyst + an iodide based co-catalyst
300 C and 700 atm pressure
These conditions require a lot of energy input to achieve and this process can be hazardous.
This has 100% atom economy.
When Iridium is used as a catalyst, it is less active than Rhodium
With Ruthenium, the reaction becomes more active as it is a more specific catalyst than the rhodium compound.
It is called the Cativa Process:
Less water is used in the reaction mixer
Reduces the number of necessary drying columns
Decreases the formation of by-products
Less propanoic acid by-product is produced
Methanol comes from the synthesis gas which is made from oil, can also be produced from wood, household waste, sewage.
So whats a catalyst??...
Ways of making more use of efficient energy:
Chemical manufacturers use various methods to reduce their use of energy, for both financial and environmental reasons.
In chemical factories, leaking taps and pipes, insufficient insulation and poor maintainance cause energy to be wasted.
Heat energy is extracted from waste gases by a heat exchanger; it works like a refrigerator , transforming energy from the flue gases to where it can be useful. It can provide hot water and heating in the plant, as well as pre-heat reactants.
Make adjustments in pressure, catalysts, etc so that the reaction can be carried out at a a lower temperature (which will save energy). It is more economical to use pure oxygen rather than air for an oxidation process to prevent the use of energy to heat nitrogen (which is 80% of air) which is useless in the reaction.
Factories generate their own electricity, which is cheaper and avoids transmission costs.
The use of microwaves in pharmaceutical industry for heating reactions:
It allows a process that would require 48 hrs in a conventional over to produce a 84% yield in a very short time.
Allows reactions which require heavy metal catalyst to proceed quickly even without the catalyst.
Environmental benefits to not using toxic heavy metals.
Mechanical chemists are able to carry out reactions that would have needed overnight in a conventional oven.
It uses continous and much more powerful radiation than domestic microwaves.
Microwaves create an electric field which causes polar molecules like water to line up with the field. Before they could do that the field switches to the opposite directions and the molecules swivel round and line up. The microwave energy is effectively converted into thermal energy. The polar molecules then heat up the molecules around them.
If a mixture is heated in a closed vessel using microwaves it is possible to carry out the reaction at approaching twice the boiling temperatures of the solvent.
Reducing waste and preventing pollution of the environment
It is better to prevent waste than to treat or clean up waste after it has been created.
Unnecessary derivatization (use of blocking groups, protection/de-protection, and temporary modification of physical/ chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.
Design for Degradation
Chemical products should be designed so taht at the end of their function they break down into innocuous degradation products and do not persist in the environment.
Atom economy in laboratory and industrial processes
A reaction with a very good atom economy is very efficient at turning reactants into desired products with little waste.
Atom economy: Mr of desired product
Mr of reactants
Addition reaction of bromine to propene
Mr of 1,2 dibromopropane= (12x3)+(6)+(79.9x2) = 201.8
Mr of reactants= (12x3)+(6)+(79.9x2) = 201.8
Atom Economy= 201.8
= 100% atom economy.
Atom economy tells you how much of the reactants are turned into product.
Greenhouse gases and Global Warming
Relative Greenhouse factor:
It compares the effects that different gases have in absorbing of infrared radiation compared with carbon dioxide.
Carbon dioxide is not the only culprit; other greenhouse gases have even greater effects than CO .
Water-vapor is also a green house gas but the amount in the atmosphere varies widely.
Global Warming Potential:
It is the measure of how much a given mass of a greenhouse gas is estimated to contribute to global warming.
It is determined by:
Their ability to absorb infrared radiation (their relative greenhouse factor).
Their half-life in the atmosphere - A measure of how long they last in the atmosphere before reacting and being broken down.
Difference between Anthropogenic and Natural Climate Change
It is the climate change that is due to the activities of human beings.
Example: deforestation, melting of glaciers, flooding etc
1/3 of CO2 emissions burning of fossil fuels
2/3 of CO2 emissions land-use change (deforestation)
45% of this CO2 remains in this atmosphere
30% taken up by oceans
Remaining 25% taken up by trees & other plants
Half is removed over 30years
30% over few centuries
20% stay in the atmosphere for many thousands of years
Anthropogenic climate change
It is the climate change that is due to the natural processes occurring on the Earth.
Example: formation of carbonate rocks, volcanoes, plate tectonics etc.
Natural climate change
A fuel is carbon neutral if the amount of carbon dioxide absorbed when the raw material is grown, or the fuel is formed, equals the amount of carbon dioxide produced when it is manufactured and then burned (zero carbon footprint).
They are formed millions of years ago by microorganisms which captured carbon dioxide from the atmosphere.
They produce CO2 when they burn.
They are not carbon neutral; because the carbon dioxide absorbed cannot be set against the carbon dioxide produced within a reasonable time span, say 50 years.
Bio-ethanol is produced from wheat and the stages in the production of ethanol require energy.
So it is not carbon neutral, unless the energy is generated without the release of CO2 into the air
The basic steps for large scale production of ethanol are:
microbial (yeast) fermentation of sugars, distillation, dehydration, and denaturing. Prior to fermentation, some crops require saccharification or hydrolysis of carbohydrates such as cellulose and starch into sugars.
Hydrogen burns to form only water.
But it is not carbon neutral, because hydrogen has to be manufactured as it does not occur naturally.
It is made chemically from natural gas or the electrolysis of water. Either way there is some CO2 produced in the different manufacturing, construction and distribution steps.
A carbon footprint is a measure of the impact human activities have on the environment in terms of the amount of greenhouse gases produced, measured in units of mass of carbon dioxide.
It can be calculated using (LCA) Life Cycle Assessment
CFC 's and the depletion of the ozone layer
CFC's are chlorofluorocarbons - compounds containing carbon with chlorine and fluorine atoms attached.
CFC's are non-flammable and not very toxic. They therefore had many uses.
They were used as refrigerants, propellents, for aerosols, for generating foamed plastic like expanded polystyrene, and as solvents for dry cleaning and for general degreasing purposes.
Unfortunately, CFC's are largely responsible for destroying the ozone layer. In the high atmosphere, the carbon-chlorine bonds break to give chlorine free radicals. It is these radicals which destroy the ozone. CFC's are now being replaced by less environmentally harmful compounds.
CFC's can also cause global warming. One molecule of CFC-11, for example, has global warming potential 5000 times greater than a molecule of carbon dioxide.
On the other hand, there is far more carbon dioxide in the atmosphere than CFC's, so global warming isn't the major problem associated with them.
Replacement for CFC's
Hydro chlorofluorocarbons, H CFC's these are carbon compounds which contain hydrogen as well as halogen atoms. For example: HCFC -22
These have a shorter life in the atmosphere than CFC's, and much of them is destroyed in the low atmosphere and so doesn't reach the ozone layer. HCFC-22 has only about one-twentieth of the effect on the ozone layer as a typical CFC.
Aircraft Emissions and the depletion of ozone layer
High-flying jet aircraft produce frozen water vapor trails called contrails.
Those are not officially classified as ‘artificial clouds”; however, they can contribute to long-term changes in the Earth’s Climate.
Jet engines also produce nitrogen oxides, as being greenhouse gases, these destroy ozone in the upper atmosphere
How halogen radicals destroy ozone
Under natural conditions, ozone levels are relatively constant since they are both formed and destroyed by ultra-violet light.
So as ozone concentrations increase, the amount of ozone destroyed also increases.
However, chlorine radicals (Cl) react with ozone simply to destroy it. They are very efficient at removing ozone because they act as catalysts.
This means that they are not consumed by the reaction but are recycled and can continue to react with other ozone molecules to destroy them as well.
Cl + O3 → ClO + O2
ClO + O → Cl + O2
O3 + O → 2O2 : Overall reaction
The ozone hole is the region over Antarctica with total ozone of 220 Dobson Units or lower. This map shows the ozone hole on October 4, 2004. The data were acquired by the Ozone Monitoring Instrument on NASA’s Aura satellite.
Done by: Dana Dawod