Energy

Eact uncatalysed

Eact catalysed

reactants

products

Module 4I10: Green Chemistry Imperial College

London

Lecture 3: Catalysis

4.I10 Green Chemistry Lecture 3 Slide 1

Before we begin, a correction to last week’s slide 24 Imperial College

London

E-factor = 462 / 40

= 11.6

E-factor = mass of waste produced

mass of desired product

Mass of waste = [37g + 60g + 250g + 100g + 25g + 25g + 5g] – 40g

= 504g – 40g = 462g4.I10-3-2

Imperial College

LondonAnswers to the question from lecture 2

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, while the but-1-

ene route only gives yields of 55 %.

(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 not toxic.

(b) Which route would you recommend to industry? Outline the factors which

might influence your decision.

4.I10-3-3

Imperial College

LondonAnswer (a), part (i) atom economies

Benzene Oxidation

But-1-ene Oxidation

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

RMM of desired product = 98

∴ Atom economy = 64 %

∴ Atom economy = 44 %

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

RMM of desired product = 98

4.I10-3-4

Imperial College

LondonAnswer (a), part (ii) effective mass yields

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 %

4.I10-3-5

Imperial College

LondonAnswer (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 probably better -

BUT ONLY IF raw material costs are acceptable.

4.I10-3-6

Imperial College

LondonLecture 3 - Learning Outcomes Imperial College

London

By the end of this lecture you should be able to

(i) explain why catalysis is central to Green Chemistry

(ii) understand the difference between heterogeneous and

homogeneous catalysis

(iii) describe three examples of processes which use green

heterogeneous catalysis

4.I10-3-7

Imperial College

LondonWhy is Catalysis green?

Using catalysts should reduce:

• energy required (e.g. heat)

• the use of stoichiometric reagents

• by-products

• waste.

Recall the 12 principles of green chemistry (lecture 2):

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.

4.I10-3-8

Imperial College

LondonPotential disadvantages of catalysis

Many catalysts are based on heavy metals and may be toxic. 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.I10-3-9

Imperial College

LondonCase study: Boots synthesis of Ibuprofen

AcOH, HCl,

Al waste HCl

AcOH

NH3

4.I10-3-10

Imperial College

LondonCase study: Hoechst synthesis of Ibuprofen

AcOH

All three steps

are catalytic

99 % conversion

96 % selectivity

Less waste is generated as a result of using catalysed reactions

4.I10-3-11

Homogeneous catalysis

Reagents 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).

Biocatalysis

Using enzymes to catalyse a reaction (Lecture 7).

Imperial College

LondonSome definitions

4.I10-3-12

Imperial College

LondonHeterogeneous 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

Heterogeneous catalysts are used in refining / bulk chemical syntheses

much more than in fine chemicals and pharmaceuticals (which tend to use

homogeneous catalysis).

4.I10-3-13

Imperial College

LondonHomogeneous catalysis - principles

Well-defined active site allows rational catalyst development.

Typical single-site catalyst:

Ln M

X

sterically bulky ligand(s)

controls stereochemistry

substrate approaches

vacant coordination site

and may then react with X

e.g. Cp2ZrMe+ for the

polymerisation of ethene

4.I10-3-14

Imperial College

LondonHomogeneous 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.

L* =

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

4.I10-3-15

Imperial College

LondonHeterogeneous 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)

which adhere 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

of product(s) away from the surface.

4.I10-3-16

Imperial College

LondonHeterogeneous Catalysis: AB + C2 AC + BC

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 College

LondonHeterogeneous Catalysts

Surface

M

Active sites

are in pores

Imperial College

LondonHeterogeneous Catalysts

Active sites

are in pores...

...and every pore may contain lots of active sites

Imperial College

LondonHeterogeneous Catalysts

Typical features:

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

Three dimensional highly porous structure with a very high surface area.

A B

C C

1. Diffusion to surface

2. Physisorption

3. Chemisorption

M 4. Surface diffusion

5. Reaction

6. Desorption

7. Diffusion out of pore

A C

B C

6,7

porous support

4,5

1-3

Reactants

Products

1-3

4.I10-3-17

Imperial College

LondonHeterogeneous 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.

4.I10-3-18

Imperial College

LondonZeolites - crystalline, hydrated aluminosilicates

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

4.I10-3-19

Imperial College

Londone.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!

Td

4.I10-3-20

Imperial College

LondonZeolites - Asahi Cyclohexanol process

Traditional synthesis

225 °C

10 atm

For selectivity reasons, the reaction is run at low conversions (approx 6%

per tank) and the hot cyclohexane stream is continuously recycled.

Zeolite catalysed process:

100 °C

98 % selectivity

4.I10-3-21

Imperial College

LondonWhy is the Asahi process important?

Tanks 1, 2 and 3

Temporary pipework between

tanks 4 and 6 ruptured and

cyclohexane cloud exploded

2 3 4 61Tank 5 removed

for repairs

225 °C

10 atm

Flixborough 1974 - 28 deaths

4.I10-3-22

Tank 4

Imperial College

LondonZeolites - 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)

This process is important because only para-xylene is required for PET:

poly(ethylene terephthlate) - PET

4.I10-3-23

Imperial College

LondonA 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 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 component

of EPDM rubber

4.I10-3-24

Imperial College

LondonConclusions

The learning objectives of lecture 3 were:

• explain how catalysis may be considered green

• identify the characteristics of heterogeneous and homogeneous

catalysis

• describe three examples of green heterogeneous catalysis

Catalysis may reduce materials, waste and energy

Heterogeneous are easily recycled and long-lived but ill-defined

Homogeneous are more active and selective but expensive and hard to recover

Asahi Cyclohexanol process

H-ZSM-5 alkylation of toluene/ isomerisation of xylene

Sumitomo base-catalysed isomerisation of vinylnorbornene

4.I10-3-25

Imperial College

LondonAnother 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?

4.I10-3-26