Chemistry thing

12 Principles of Green Chemistry

1. Prevention
   1. It is better to prevent waste than treat or clean up waste after it has been created
   2. Minimise waste
2. Atom economy
   1. Synthetic materials should be designed to maximise incorporation of all materials used in the process into the final product
   2. Maximise efficiency of the reaction
   3. What atoms of the reaction are incorporated into the final design of the product(s), and what atoms are wasted?
3. Less Hazardous Chemical Syntheses
   1. Where practicable, synthetic materials should be designed to use and generate substances that possess little or no toxicity to human health and the environment
   2. Limit hazards of products and materials
   3. Note: this implies it may not be possible/practical to avoid certain toxic materials
4. Designing Safer Chemicals
   1. Chemical products should be designed to preserve efficacy of function while reducing toxicity
   2. Minimising toxicity while simultaneously maintaining function and efficacy
   3. Efficacy: ability to produce a desired/intended product
5. Safer Solvents and Auxiliaries
   1. The use of auxiliary substances (e.g. solvents, separation agents, etc) should be made unnecessary wherever possible, and innocuous when used
   2. Minimise use of auxiliary substances, and wherever used, use non-harmful substances
   3. Solvents and separation agents do matter, we just want to limit hazards/risks
   4. Solvents account for 50-80% of the mass in a standard batch chemical operation
6. Design for Energy Efficiency
   1. Energy requirements should be recognised for their environmental and economic impacts and should be minimised. Synthetic methods should be conducted at ambient temperatures and pressures
   2. Use ideal conditions for synthesis
   3. Maximise energy efficiency
   4. Usually, no consideration is given to temperature/pressure for energy requirements
   5. Thus, we need to consider the energy requirements and try to maximise efficiency
7. Use of Renewable Feedstocks
   1. A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable
   2. Use sustainable materials for synthesis
8. Reduce Derivatives
   1. Unnecessary derivatization(use of blocking groups, protection/deprotection, temporary modification of physical/chemical processes) should be minimised and avoided if possible, because such steps require additional reagents and can generate waste
   2. i.e. keep process as simple as possible
   3. Minimise/prevent unnecessary reactions to reduce waste/use of reagents
   4. Reduce use of derivatives and protect groups in the synthesis of target molecules
   5. Derivatization: A chemical compound is converted into a product of similar chemical structure, called a derivative
9. Catalysis
   1. Catalytic reagents (as selective as possible) are superior to stoichiometric reagents
   2. Concept of efficiency shifts from maximising yield to minimising waste
   3. Prioritise waste over yield
   4. Use catalysts to speed up reaction, instead of adding more reagents
10. Design for Degradation
    1. Chemical products should be designed so that at the end of their function, they break down into innocuous degradation products and do not persist in the environment
    2. Optimise the commercial function of a product while minimising its hazard and risk
11. Real-time Analysis for Pollution Prevention
    1. Analytical methodologies need to be further developed to allow for real-time, in-process monitoring, and control prior to the formation of hazardous substances
    2. Real-time feedback/process analysis for the safe and efficient operations of chemical plants/reactions
12. Inherently Safer Chemistry for Accident Prevention
    1. Substances and the form of a substance used in a chemical process should be chosen to minimise the potential for chemical accidents, including releases, explosions, and fires
    2. Laboratory safety
    3. Minimise potential safety hazards
    4. Substances and the form of a substance used in a chemical process should be chosen to mimise the potential for chemical accidents

Atom Economy:

$\frac{\text{M[desired product]}\times 100\%}{\text{M[reactants]}}$

Ethanol Production

Both processes are exothermic

Fermentation

• Metabolic process where organisms convert carbohydrates (sugars, starch) into alcohols/acids
• Starts with glucose and ends with ethanol and carbon dioxide
• Conditions
  • Room pressure and ambient temperature (~37˚ C)
  • 3-5 pH
  • Enzymes as catalysts
    • Enzyme: biological catalyst
  • Oxygen is excluded from this to prevent the anaerobic respiration of sugars, and encourage the production of ethanol

1. Hydrolysis of sucrose to glucose and fructose, (isomers of $C_6{H}_{12}{O}_6$). This reaction is catalysed by the yeast enzyme invertase or sucrase
2. Fermentation of the glucose/fructose mixture to ethanol and carbon dioxide. This is catalysed by the yeast enzyme zymase

$\text{Hydrolysis of sucrose via invertase/sucrase: }{C}_{12}{H}_{22}{O}_{11(aq)}+H_2{O}_{(l)}\to 2C_6{H}_{12}{O}_{6(aq)}$

$\text{Fermentation via zymase: }{C}_6{H}_{12}{O}_{6(aq)}\to 2C_2{H}_{5}O{H}_{(aq)}+2C{O}_{2(g)}$ $\text{Overall process via yeast enzymes: }{C}_{12}{H}_{22}{O}_{11(aq)}+H_{2}O\to 4C_2{H}_{5}O{H}_{(aq)}+4C{O}_{2(g)}$

Extension

1. Glycosis

• Breakdown of glucose into 2 pyruvate molecules
• 11 chemical reactions, breaks down sugars and releases energy in the form of ATP
• Overall products are 2 pyruvate molecules, 2 NADH (enzyme), and 2 molecules of ATP
• The pyruvate molecules are further processed in the absence of oxygen to form ethanol

$C_6{H}_{12}{O}_6+2\text{ ADP}+2P_i\text{ (inorganic phosphate)}+2{\text{ NAD}}^{+}\to 2C{H}_{3}COCO{O}^{-}\text{ (pyruvate)}+2\text{ ATP}+2\text{ NADH}+2H_{2}O+2H^{+}$

2. Pyruvate to Ethanol Conversion

• Pyruvate molecules are converted into ethanol and carbon dioxide
• Step 1: carboxyl group is removed and released in the form of $C{O}_2$ , producing acetaldehyde. This reaction is catalysed by pyruvate decarboxylase enzyme
• Step 2: Acetaldehyde is reduced by NADH to form ethanol. The $NA{D}^{+}$ molecule/enzyme is regenerated during this. The catalyst for this is alcohol dehydrogenase

$\text{1. }C{H}_{3}COCO{O}^{-}+H^{+}\to C{H}_{3}CHO\text{ (acetaldehyde)}+C{O}_2$ $\text{2. }C{H}_{3}CHO+NADH+H^{+}\to C_2{H}_{5}OH\text{ (ethanol)}+NA{D}^{+}$

3. Overall:

$C_6{H}_{12}{O}_6\text{ (glucose)}\to 2C_2{H}_{5}OH\text{ (ethanol)}+2C{O}_2\text{ (carbon dioxide)}$

Chemical Synthesis

• Produced by reacting ethene with steam
• The reaction is reversible, and the forward reaction is exothermic
• Only 5% of the ethene is reacted for every pass through the reactor, but 95% can be achieved by removing the ethanol
• Ethene is used in excess, even though water is cheaper
  • This is because of the phosphoric(V) acid, which is coated on a solid silicon dioxide support. An excess of steam results in the dilution of the catalyst, and perhaps even removal of it, as it washes off
• As the forward reaction is exothermic, a low temperature is ideal, to maximise yield.
• However, as rate requires a high temperature, a compromise temperature of 300˚ C is used
  • Produces an acceptable amount of ethanol in a very short time (5% of ethanol reacts)
• High pressure is ideal, however only a moderate pressure is used due to safety, energy and economic concerns
  • Furthermore, at high pressures, ethene polymerises to make poly-ethene
  • This produces waste, and could clog up the plant
• A catalyst of phosphoric(V) acid coated on a solid silicon dioxide support is used
  • This is to increase the rate of reaction without decreasing yield
  • Without a catalyst, the reaction is slow, such that virtually no reaction happens in a realistic amount of time
  • Catalyst ensures dynamic equilibrium is achieved as fast as possible within the short time the gases spend in the reactor
• Conditions:
  • Phosphoric acid catalyst
  • 60-70 atm (moderate pressure)
  • ~300˚ C

$C_2{H}_{4(g)}+H_2{O}_{(g)}\rightleftharpoons C{H}_{3}C{H}_{2}O{H}_{(g)}$

Comparisons

• Rate
  • To maximise rate, a catalyst and moderate temperatures and pressures are utilised in the synthesis of ethanol
  • To maximise rate in fermentation, temperature of ~37 ˚C and ambient pressure is used
• Equilibrium
  • To shift equilibrium right, an excess of ethene is provided, and moderate pressures are used, in the synthesis of ethanol
• Biofuel
  • Biofuels: fuels made using living organisms or the wastes they produce
  • Thus, ethanol produced through fermentation is biofuel
  • Synthetic ethanol is not
• 12 Principles of Green Chemistry
  • Prevention
    • Synthesis: the excess ethene is reacted, to minimise waste, and thus prevention is upheld
    • Fermentation: the yeast dies after a few weeks, and thus waste is created
  • Atom Economy
    • Synthesis: The excess ethene is reacted, to maximise the amount of reactants reacted, and thus the Atom Economy is upheld
      • Atom economy calc is 100%
    • Fermentation: Not all of the feedstock is consumed, and thus the Atom Economy is not upheld
      • Atom economy calc is 51%
    • Synthesis is preferable
  • Less Hazardous Chemical Synthesis
    • Synthesis: only ethanol is produced, which is flammable, and could potentially be harmful. Furthermore, a phosphoric(V) acid catalyst is used, which is corrosive
    • Fermentation: only ethanol is produced, which is flammable, and could potentially be harmful
  • Designing Safer Chemicals
    • Ethanol is relatively safe, in that large doses must be taken for it to be permanently harmful to anyone
    • However, small doses can cause headaches, dizziness, and in extreme cases, irritation of the eye
  • Safer Solvents and Auxiliaries
    • Synthesis: no harmful, extra, unnecessary substances
    • Fermentation: no harmful, extra, unnecessary substances
  • Design for Energy Efficiency
    • Synthesis: Only moderate temperatures and pressures are used, to conserve energy
    • Fermentation: Large amounts of heat energy are required to isolate feedstock
  • Use of Renewable Feedstocks
    • Synthesis: Water and ethene are not renewable
    • Fermentation: Organic waste, such as corn and starch, are renewable
  • Reduce Derivatives
    • Synthesis: Only necessary reactants are converted into products
    • Fermentation: Some by-products are produced, through other reactions
  • Catalysis
    • Synthesis: A phosphoric (V) acid catalyst on a silicon dioxide support is used
    • Fermentation: No catalyst is used
  • Design for Degradation
    • Ethanol is biodegradable and nontoxic
  • Real-time Analysis for Pollution Prevention
    • Carbon dioxide is produced by fermentation
    • No greenhouse gases are produced in the synthesis by addition of ethanol
    • However, carbon dioxide is produced from the combustion of ethanol
  • Inherently Safer Chemistry for Accident Prevention
    • Workers have respirators, overalls, gloves, boots and safety goggles
    • Ethanol is kept in containers that are securely sealed to prevent leakage/evaporation, which are accurately labelled, and have features to prevent explosions. These are also kept in cool areas
    • When transporting ethanol, grounding/bonding procedures are present to prevent static charges
    • Ethanol should not be kept near incompatible materials
    • Regular risk assessments
    • Fire extinguishers ready in case of fire
    • Ethanol is absorbed in the case of a spill, so that it does not enter water supply
    • Monitoring/warning systems in place
    • Regular fire drills

Principles	Fermentation ✅	Addition ❌
Energy use	Fermentation is conducted at room pressure and ambient temperatures (~37 ˚C) so less energy is needed to heat reagents or create high pressures ✅	Considerable energy is needed to heat reagents (~300 ˚C) and raise pressure (60-70 atm) in order to achieve a satisfactory reaction rate and yield ❌
Renewable feedstock	Uses renewable biomass and biomass waste products ✅	Uses ethene which is a non-renewable fossil fuel based product ❌
Production or use of toxic substances	Neither sugar nor yeast are toxic reagents. While the product $C{O}_2$ occurs naturally in the environment. High concentrations of $C{O}_2$ are asphyxiating in confined spaces ✅	Ethene is a highly flammable hazardous gas. It is an asphyxiant (due to oxygen exclusion). The acid catalyst, pure phosphoric acid, is a corrosive and hazardous substance ❌
Inherent safety due to production method Ethanol is flammable	Fermentation occurs in enclosed vats at ambient temperature and pressure and may be considered to pose few safety risks ✅	Involves the use of high temperatures and pressures as well as the flammable gas ethene and the corrosive acid catalyst $H_{3}P{O}_4$. These reagents and conditions pose inherent safety risks ❌
Atom economy	Fermentation has a low atom economy (~51.1%) as a considerable amount of the reactant mass ends up as a waste, i.e. $C{O}_2$ ❌	Addition has a 100% atom economy as all of the products are incorporated into ethanol, i.e. there is no other product ✅

Thus, fermentation is preferred, as it obeys more of the 12 principles of green chemistry than addition.

Sources

https://www.acs.org/greenchemistry/principles/12-principles-of-green-chemistry.html https://microbenotes.com/alcohol-fermentation-ethanol/ https://letstalkscience.ca/educational-resources/backgrounders/how-ethanol-made https://www.chemguide.co.uk/physical/equilibria/ethanol.html https://www.energy.gov/eere/bioenergy/biofuel-basics https://www.gexcon.com/blog/ethanol-associated-hazards-and-safety-measures/ Honourable mentions: https://www.chemguide.co.uk/physical/catalysis/hydrate.html#top https://www.leaf.tv/articles/how-to-make-homemade-wine-using-welchs-grape-juice/ https://www.mdpi.com/2311-5637/7/4/268