Synthesis Notes - Industrial Processes

Syllabus

-Haber Process (NH3)

-Contact Process (H2SO4)

-Soaps and detergents

-Plastics (covered during the organic chemistry topic)

-Ethanol (from ethene and by fermentation)

-Biodiesel (base catalysed and enzyme catalysed)

-Ethyl ethanoate

Haber Process

• The Haber process is a method of synthesising ammonia developed by the German Chemist Fritz Haber

• The process is immensely important in the production of fertilisers

N2(g) + 3H2(g) ← → 2NH3(g) ΔH = -92.4 kJmol-1

Rate considerations - haber

The rate of the reaction will be increased by the use of:

• High temperatures

  • (­ ^ velocity of particles and hence rate of collisions)

  • ( ^ proportion of collisions where EK > EA)

• High pressures

  • (­ ^ rate of collisions)

• Catalyst

  • (Fe/FeO/Fe3O4/K2O/Al2O3)

  • (Decrease EA, ­ increase proportion of collisions where EK > EA)

Yield considerations - haber

The yield of the reaction will be increased by the use of

• High pressures

  • (Favour reaction producing the least gaseous particles eg forward)

• Low temperatures

  • (Favour the exothermic forward reaction)

Cost considerations - haber

The cost can be minimised by the use of low pressures and temperatures

Actual reaction conditions - haber

The reaction conditions used to provide the best balance of rate, yield and economic considerations are:

• Moderate temperature of 400-500 °C

  • (balance of rate, yield and cost)

• Relatively high pressure of 200 atm

  • (High pressure ­increase rate and yield, but even higher pressures would be too expensive to maintain)

• Catalyst

  • (Fe/FeO/Fe3O4/K2O/Al2O3)

  • (^ ­ rate without affecting yield)

Contact processes

• The contact process is a method for producing sulfuric acid

• The majority (~75 %) of the sulfuric acid produced is used in fertiliser production

Step 1: S(s) + O2(g) → SO2(g) OR 4FeS2(s) +11O2(s) → 2Fe2O3(s) + 8SO2(g)

Step 2: 2SO2(g) + O2(g) ← → 2SO3(g)

Step 3: SO3(g) + H2SO4(l) → H2S2O7(l)

H2S2O7(l) +H2O(l) → 2H2SO4(l)

• They key to the efficiency of the process is the reversible reaction carried out at step 2

2SO2(g) + O2(g) ← → 2SO3(g) ΔH = -196 kJmol-1

Contact process: Rate considerations

The rate of the reaction will be increased by the use of:

• High temperatures

  • (­ increase velocity of particles and hence rate of collisions)

  • (­ increase proportion of collisions where EK > EA)

• High pressures

  • (increase rate of collisions )

• Catalyst (V2O5)

  • ( decrease EA, ­ proportion of collisions where EK > EA)

Contact process: yield considerations

The yield of the reaction will be increased by the use of:

• High pressures

  • (Favour reaction producing the least gaseous particles eg forward)

• Low temperatures

  • (Favour the exothermic forward reaction)

Contact process: cost considerations

The cost can be minimised by the use of low pressures and temperatures

Contact process: actual reaction conditions

The reaction conditions used to provide the best balance of rate, yield and economic considerations are:

• Moderate temperature of 400-450 °C

  • (balance of rate, yield and cost)

• Low pressure of 1-2 atm

  • Decrease cost more than makes up for decrease rate and yield)

• Catalyst (V2O5)

  • ­ ^ rate without affecting yield

Green Chemistry

In addition to the economic efficiency of a chemical synthesis process, synthesis reactions should also be designed to minimise the impact on the environment

Green chemistry aims to reduce the impact of chemical industry on the environment by:

1. Preventing pollution (preventing the production of harmful chemicals)

2. Where this is not possible, treating chemicals to make them safe prior to disposal

3. Where this is not possible, disposing of untreated chemicals safely

The 12 principles of green chemistry were developed to assist in designing chemical synthesis processes that are as environmentally friendly as possible

Atom economy

• One method of measuring the environmental efficiency of a chemical process is its atom economy

• It measures the percentage of the reactants effectively utilised to produce products

• atom economy = mass of atoms in desired product x 100/ mass of atoms in the reactant

Fats and oils

• Fats and oils are both examples of lipids, a class of compound produced by living organisms

• They are both large non-polar molecules called triglycerides

• Fats are solid at room temperature, while oils are liquids at room temperature

• Fats are generally animal derived saturated triglycerides, while oils are generally plant derived unsaturated triglycerides

• Fats and oils are important reactants for organic synthesis processes such as soap and biodiesel production, in addition to their uses in the food industry

Structure and synthesis of triglycerides

• Triglycerides are triesters that are synthesised in a condensation (esterification reaction) between a glycerol molecule and three fatty acids

• Glycerol is the common name for propan-1,2,3-triol

• Fatty acids are long chain (~C8-C20) carboxylic acids

• They are classified as saturated or unsaturated (mono-or poly-) based on the presence/absence of double bonds within the carbon chain

Fats and oils are synthesised in the condensation reaction shown below, which involves the formation of three ester links

Fats and oils can be broken down in a hydrolysis reaction, which is the reverse of the above reaction

synthesis of soaps and detergents

• Soaps and detergents are both cleaning agents known as surfactants or emulsifying agents

• Emulsifying agents are compounds that able to keep two normally immiscible substances together in the same phase

• They can be used to clean non-polar substances such as fats and oils from surfaces with water

Saponification

• Soaps are the salts of long chain fatty acids

• They are produced by the hydrolysis of triglycerides in the presence of a base to form soap and glycerol (the process is called saponification)

• Animal fats are boiled in sodium hydroxide solution and salt is added, causing the soap to precipitate from the solution

• The glycerol can be recovered by distillation

• The soap produced in the reaction has a long non-polar hydrocarbon “tail” that can interact with non-polar compounds by dispersion forces and a charged carboxylate “head” that can interact with polar compounds (like water) by ion-dipole forces

Limitations of soaps (hard water)

• The water in some areas (particularly in WA) has a high concentration of metal ions such as Ca2+, Mg2+ or Fe2

• This is called hard water

• Soaps are ineffective in hard water as the metal cations are able to combine with soap ions to form a precipitate known as soap scum in the reaction:

M2+(aq) + 2R-COO-(aq) → M(R-COO)2(s)

• Soap scum can be hard to remove from surfaces, leading to stains around sinks and basins and blocked drains

• When washing with hard water, more soap is required as many soap molecules are removed as a precipitate

Detergents

• Detergents are designed to have a similar structure and to work in the same way as soaps, without forming insoluble precipitates

• In detergents, the carboxylate (COO-) group is replaced with a sulfonate (SO3-)

• While still an effective surfactant, sulfonate ions will not form scum in hard water

• Detergents can be produced by reacting an alkylbenzene with concentrated sulfuric acid, then with sodium hydroxide

• The non-polar hydrocarbon tail of the molecule can interact with non-polar compounds by dispersion forces and the charged sulfonate “head” that can interact with polar compounds (like water) by ion-dipole forces

Cleaning action of soaps and detergents

• Unless a surfactant is added, water is extremely ineffective at cleaning non-polar substances such as fats and oils from surfaces

• This is because water is immiscible with fats and oils due to the differences in polarity

• Adding soap or a detergent to water allows the fats/oils and water to be present in the same phase

• Both soaps and detergents work through the formation of micelles

• When soaps and detergents are added to water, their non-polar hydrocarbon tails are able to dissolve non-polar oils and grease (due to dispersion forces), while the ionised head is able to dissolve in water (due to ion-dipole forces)

• This results in the formation of “micelles”, with a blob of oil/grease surrounded by soap/detergent molecules, orientated with their tails facing inwards and their heads outwards

• The formation of micelles is aided by agitation and the use of hot water

Biofuels

• Biofuels are fuels that are produced from biomass

• They are produced as a renewable, less environmentally harmful, low sulfur alternative to fossil fuels

• Common biofuels include ethanol and biodiesel

• Ethanol can be used in special engines designed for ethanol only, or it can be blended with petrol and used in petrol engines

• Biodiesel can be used in diesel engines, either alone or blended with diesel from crude oil

Ethanol- synthesis by fermentation

• Ethanol from fermentation is considered a biofuel, as it is produced from biomass

• Approximately 93% of ethanol (including all ethanol for alcoholic beverages) is produced in this manner

• Sources of biomass for the production of alcoholic beverages include grains (wheat, rye, barley, corn etc), fruits (apples, pears, grapes) and sugar cane

• Ethanol for biofuels is typically produced form corn, wheat or sugar cane

• The exact process varies, depending on the source of biomass

• The use of starch products (corn/wheat etc) requires the use of enzymes to convert the starch into simpler sugars such as sucrose

• Producing ethanol from sucrose is a two-step process catalysed by enzymes produced by yeast, with both stages occur in a single reaction vessel

Step 1. Hydrolysis of sucrose

• In the first reaction, the disaccharide sucrose is hydrolysed into the monosaccharides glucose and fructose (both are isomers of C6H12O6), catalysed by the enzyme invertase

Step 2: fermentation

• In the second reaction, glucose and fructose are converted to ethanol and carbon dioxide by the enzyme zymase

Ethanol-synthesis by fermentation: reaction conditions

• in enzyme catalysed reactions, the reaction conditions must be chosen to keep the enzyme within its working temperature and pH range, to prevent the enzymes from becoming denatured and losing their activity

  
  

• Temperature: 25-37°C:

  • (Lower temperatures reduce reaction rate)

  • (Higher temperatures cause enzymes to denature)

• pH: 3-5:

  • (Yeast enzymes are active in this range)

  • (Low pH inhibits bacterial growth)

• Low O2 concentration:

  • (Higher O2 concentration causes yeast to respire aerobically)

  • (This produces CO2 and H2O instead of CH3CH2OH)

Ethanol-synthesis by hydration of ethene

• Ethanol can be synthesised either by fermentation, or by the hydration of ethene

• Ethanol produced by hydrating ethene is not technically a biofuel

• Hydration of ethene is the quickest method of producing ethanol for industrial use

Ethanol-synthesis by hydration of ethene: reaction condiitons

• Moderate temperature (~300°C):

  • low temp ^ yield and decrease cost

  • High temp ^ rate

• Moderate/High pressure (60-70 atm):

  • high pressure ^ rate and yield

  • Low pressure decrease cost

• Catalyst (H3PO4):

  • ^ rate without affecting yield

Ethyl ethanoate (not a biofuel)

• Ethanol produced by the hydration of ethene is often used to produce ethyl ethanoate

• Ethyl ethanoate is industrially important as a highly useful solvent

• It is used in nail polish remover, adhesives, varnishes and in many industrial processes

• It has a low toxicity for an organic solvent

• It is produced by the acid catalysed esterification of ethanol and ethanoic acid (called Fischer esterification)

• The ethanol is produced by the hydration of ethene (which is itself produced by cracking hydrocarbons)

Reaction Conditions (for Fischer esterification)

• H2SO4 catalyst used:

  • (­ ^ rate without affecting yield)

• Ethanol added in excess:

  • (­ ^ yield and decrease cost (ethanol is cheaper than ethanoic acid)

Biodiesel

• Diesel from crude oil consists of long hydrocarbon chains (C8-C20)

• Biodiesel molecules are a similar length, but consist of the methyl or ethyl esters of fatty acids

• It is produced by the transesterification of triglycerides (TGs)

• Waste vegetable oil from restaurants is the most common source of TGs for the process

• The reaction can be catalysed by a strong base or with an enzyme

• The reaction of trigylcerides with methanol (the most common alcohol used) to produce Biodiesel in the form of fatty acid methyl esters (FAMEs) is shown below

Base catalysed method

The base catalysed method is used for almost all commercially produced biodiesel

Base catalysed method: reaction conditions

• Water is removed from oils to prevent the hydrolysis of triglycerides into glycerol and free fatty acids (FFAs)

• If this occurs, the FFA can react with the base catalyst to form soaps

• If FFAs are present above ~4%, they must be converted to FAMEs in an acid catalysed reaction, to prevent saponification

• The reaction uses a low initial temperature, but eventually produces high temperatures due to its exothermic nature

• It is performed in a sealed reaction vessel to prevent the loss of methanol by boiling

• Alcohol is added in excess to increase yield (yield can be up to 98%)

• Following the reaction, the biodiesel top layer can be separated from the glycerol bottom layer, washed and dried

Enzyme catalysed method

• Enzyme catalysed methods use lipases, a class of enzymes responsible for breaking down fats to catalyse the reaction

• While the process is considered greener, it has a number of disadvantages that make it economically uncompetitive, preventing its use industrially

Advantages of enzyme catalysed transesterification

• Prevents saponification, thereby increasing yield

• Converts FFAs into FAMEs, removing need for pretreatment

• Can be performed at milder temperatures and pH

• Uses less energy, due to lower temperatures and simpler refining process

Disadvantages of enzyme catalysed transesterification

• Lower reaction rate than base catalysed reaction

• High cost of lipases compared to bases

• Difficulty recovering lipases following reaction