Imperial CollegeLondonAnswers to the question from lecture 3

Maleic anhydride may be prepared using two routes:

Oxidation of benzene:

Oxidation of but-1-ene:

The benzene oxidation route typically occurs in 65 % yield and produces 35 g non-benign waste for every 100 g benzene used, while the but-1-ene route only gives yields of 55 %, and produces 45 g waste per 100 g but-1-ene.

(a) Assuming that each reaction is performed in the gas phase only, and that no additional chemicals are required, calculate (i) the atom economy and (ii) the effective mass yield of both reactions. You should assume that O2, CO2 and H2O

are benign chemicals.

(b) Which route would you recommend to industry? Outline the factors which might influence your decision.

Imperial CollegeLondonAnswer (a), part (i) atom economies

Benzene Oxidation

But-1-ene Oxidation

RMM of reactants = 78 + (4.5 x 32) = 222RMM of desired product = 98

∴ Atom economy = 64 %

∴ Atom economy = 44 %

RMM of reactants = 56 + (3 x 32) = 152RMM of desired product = 98

Imperial CollegeLondonAnswer (a), part (ii) effective mass yields

There are several ways of tackling this question - this is one way…

Benzene Oxidation

100 g benzene (1.28 mol) would give 81.5 g maleic anhydride (0.83 mol, 65 %).

EMY = mass of non-benign reagents

mass of maleic anhydridex 100 %

= [81.5 / 100] x 100 %

= 81.5 %

But-1-ene Oxidation

100 g but-1-ene (1.79 mol) would give 96.3 g maleic anhydride (0.98 mol, 55 %).

EMY =

= [96.3/ 100] x 100 %

= 96.3 %

mass of non-benign reagents

mass of maleic anhydridex 100 %

Imperial CollegeLondonAnswer (b), recommendation to industry

The butene oxidation route would appear to be slightly greener (higher atom economy and a higher effective mass yield). It also avoids the use of the toxic reagent benzene (we would therefore expect its wastestream to be less hazardous). However, the percentage yield is higher for the benzene oxidation route.

However, without a full life cycle analysis (which would take into account the environmental impact of producing both benzene and butene) a definitive answer is clearly not possible.

Recommendation: Butene route is possibly better - but only if raw material costs are acceptable.

Imperial CollegeLondon

Lecture 4: Catalysis

4.I6 Green Chemistry Lecture 4 Slide 1

4.I6 Green Chemistry

Energy

Eact uncatalysed

Eact catalysed

reactants

products

Imperial CollegeLondonRevised course timetable

Lecture 4 - 15th February - Catalysis

Lecture 5 - 22nd February - Solvents

Lecture 6 - 1st March - Biotechnology

Lecture 7 - 8th March - Waste

Lecture 8 - 15th March - Energy and the Environment

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Imperial CollegeLondonLecture 4 - Learning Outcomes Imperial CollegeLondonLecture 4 - Learning Outcomes

By the end of this lecture you should be able to

• explain why catalysis is 'green'

• differentiate between the characteristics of heterogeneous and homogeneous catalysis

• describe three examples of processes which use green heterogeneous catalysis

• describe one example of a process which uses green homogenous catalysis

4.I6 4 - 3

Imperial CollegeLondon

4.I6 4 - 4

Why is Catalysis green?

Using catalysts should reduce:

• energy

• the use of stoichiometric reagents

• by-products (particularly if the catalyst is highly selective)

• waste.Recall the 12 principles of green chemistry (lecture 1):

1. It is better to prevent waste than to treat or clean up waste after it is formed.

6. Energy requirements should be minimized. Synthetic methods should be conducted at ambient temperature and pressure.

9. Catalytic reagents are superior to stoichiometric ones.

Materialsincluding

plant

Energy Waste

Risk andHazards

Toxicity

Impact on theenvironment

Cost

Imperial CollegeLondonPotential disadvantages of catalysis

Many catalysts are based on heavy metals and may be toxic (e.g. the Cr(VI) oxidation catalyst mentioned in lecture 2). Therefore the following factors should also be considered when assessing a catalyst:

• separation of catalyst residues from product

• recycling of the catalyst

• degradation of the catalyst

• toxicity of the catalyst, of the catalyst residues and of catalyst degradation products.

In general, it is greener to use catalysts than to not use them

4.I6 4 - 5

Imperial CollegeLondonCase study: Boots synthesis of Ibuprofen

AcOH, HCl,Al waste HCl

AcOH

NH3

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Imperial CollegeLondonCase study: Hoechst synthesis of Ibuprofen

AcOH

All three stepsare catalytic

99 % conversion96 % selectivity

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Less wastegenerated

Homogeneous catalysisReagents and catalyst are all in the same phase (typically all are in solution).

Heterogeneous catalysis ('surface catalysis')Reagents are in a different phase from the catalyst - usually the reagents are gases (or liquids) and are passed over a solid catalyst (e.g. catalytic convertors in car exhausts).

BiocatalysisUsing enzymes to catalyse a reaction (see lecture 6).

Imperial CollegeLondonSome definitions

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Imperial CollegeLondonHeterogeneous versus Homogeneous

Heterogeneous

Readily separated Readily recycled / regenerated

Long-lived Cheap

Lower rates (diffusion limited) Sensitive to poisons

Lower selectivity High energy process

Poor mechanistic understanding

General features:

Homogeneous

Difficult to separate Difficult to recover Short service life

Expensive Very high rates

Robust to poisons Highly selective Mild conditions

Mechanisms often known

Ultimate goal: to combine the fast rates and high selectivities of homogeneous catalysts with the ease

of recovery /recycle of heterogeneous catalysts

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Imperial CollegeLondonHeterogeneous Catalysis

Used in refining / bulk chemical syntheses much more than in fine chemicals and pharmaceuticals (which tend to use homogeneous catalysis).

Seven stages of surface catalysis:

1. Diffusion of the substrate(s) towards the surface.2. Physisorption - i.e. physical absorption via weak interactions, e.g. van der

Waals, adhering the substrate(s) to the surface.3. Chemisorption - formation of chemical bonds between the surface and the

substrate(s).4. Migration of the bound substrate(s) to the active catalytic site - also known as

surface diffusion.5. Reaction.6. Desorption of product(s) from the surface.7. Diffusion away from the surface.

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Imperial CollegeLondonHeterogeneous Catalysts

Surface

A B C C

Stage 1: DiffusionStage 2: PhysisorptionStage 3: ChemisorptionStage 4: Surface diffusionStage 5: Reaction

A C B C

Stage 6: DesorptionStage 7: Diffusion

M

Imperial CollegeLondonHeterogeneous Catalysts

Surface

M

Active sitesare in pores

Imperial CollegeLondonHeterogeneous Catalysts

Typical features:

Metal or metal oxide impregnated onto a support (typically silica and / or alumina).

Three dimensional highly porous structure with very high surface area

A B

C C

1. Diffusion to surface2. Physisorption3. Chemisorption

M 4. Surface diffusion5. Reaction

6. Desorption7. Diffusion out of pore

A C

B C

6,7

porous support

4,5

1-3

Reactants

Products

1-3

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Imperial CollegeLondonHeterogeneous acid-base catalysis

ca. 130 industrial process use solid acid-base catalysts

• Mainly found in bulk/ petrochemicals production e.g. dehydration, condensation, alkylation, esterification etc.

• Most are acid-catalysed processes.

ca. 180 different catalysts employed

• 74 of these are zeolites, ZSM-5 is the largest group.

• Second largest group are oxides of Al , Si , Ti , Zr.

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Imperial CollegeLondonZeolites - crystalline, hydrated aluminosilicates

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Crystalline inorganic polymer comprising SiO4 and AlO4- tetrahedra (formally

derived from Si(OH)4 and Al(OH)4- with metal ions balancing the negative charge).

Lattice consists of interconnected cage-like structures featuring a mixture of pore (channel) sizes depending upon the Al : Si ratio, the counter-cation employed, the level of hydration, the synthetic conditions etc.

Hydrated nature of zeolites allows them to behave as Brønsted acids

Imperial CollegeLondone.g. ZSM-5

Top-view Side-view

Channels cross in three dimensions- a highly porous material

5.5 Å● = Si / Al

● = O

NB: Cations not shown!

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Td

Imperial CollegeLondonZeolites - Asahi Cyclohexanol process

Traditional synthesis

225 °C10 atm

For selectivity reasons, the reaction is run at low conversions (approx 6% per tank) and the hot cyclohexane stream is continuously recycled.

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Zeolite catalysed process:

100 °C

98 % selectivity

Imperial CollegeLondonWhy is the Asahi process important?

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Tanks 1, 2 and 3

Temporary pipework between tanks 4 and 6 ruptured and

cyclohexane cloud exploded

2 3 4 61Tank 5 removed

for repairs225 °C10 atm

Flixborough 1974 - 28 deaths

Imperial CollegeLondonZeolites - shape selective alkylation of toluene

Channel size only allows para-xylene to emerge

H-ZSM-5 catalyses: • toluene alkylation• xylene isomerisation

H-ZSM-5 (acidic ZSM-5)

Only para-xylene is required for PET synthesis:

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poly(ethylene terephthlate) - PET

Imperial CollegeLondonA rare example of solid base catalysis

Traditional synthesis of 5-ethylidene-2-norbornene (ENB) via VNB:

ENBVNB

The base used for the isomerisation is typically Na/K alloy in liquid ammonia:• ammonia easily recycled • metal recycle difficult • Na/K is very dangerous (much more reactive than either Na or K)

Sumitomo process:

Base is a heterogeneous catalyst composed of Na and NaOH on alumina. • High activity (isomerisation proceeds at room temperature) • Catalyst is readily recycled • Catalyst is much safer than Na/K

key componentof EPDM rubber

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Imperial CollegeLondonHomogeneous catalysis - principles

Well-defined active site allows rational catalyst development.

Typical single-site catalyst:

Ln M

X

sterically bulky ligand(s)controls stereochemistry

substrate approachesvacant coordination site

and may then react with X

e.g. Cp2ZrMe+ for thepolymerisation of ethene

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Imperial CollegeLondonHomogeneous asymmetric catalysis

Most of the industrially important homogeneous catalysed processes are found in asymmetric syntheses - e.g. pharmaceuticals.

e.g. Monsanto synthesis of L-DOPA (Parkinson's disease):

28 % e.e. 60 % e.e. 85 % e.e. 95 % e.e.

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L* =

0.1% catalyst loading; Rh readily recovered (some L* is lost)

Imperial CollegeLondonConclusions

The learning objectives of lecture 4 were:

• explain how catalysis may be considered green

• identify the characteristics of heterogeneous and homogeneous catalysis

• describe three examples of green heterogeneous catalysis

•describe one example of green homogenous catalysis

Catalysis may reduce materials, waste and energy

Heterogeneous are easily recycled and long-lived but ill-definedHomogeneous are more active and selective but expensive and hard to recover

Asahi Cyclohexanol processH-ZSM-5 alkylation of toluene/ isomerisation of xylene

Sumitomo base-catalysed isomerisation of vinylnorbornene

Asymmetric hydrogenation - e.g. Monsanto L-DOPA synthesis4.I6 4 - 21

Imperial CollegeLondonAnother exam-style question

The traditional synthesis of ethylbenzene is a Friedel-Crafts alkylation, such as that shown below:

The modern industrial synthesis involves mixing ethylene and benzene in the presence of a zeolite (ZSM-5). In what ways would you consider this method to be greener than the Friedel-Crafts reaction?

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