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Module 4I10: Green Chemistry
Lecture 3: Catalysis
Energy
Eact uncatalysed
Eact catalysed
reactants
products
4.I10 Green Chemistry Lecture 3 Slide 1
Before we begin, a correction to last week’s slide 24
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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 = 462g
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Answers to the question from lecture 2
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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-1ene 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.
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Answer (a), part (i) atom economies
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Benzene Oxidation
RMM of reactants = 78 + (4.5 x 32) = 222
RMM of desired product = 98
∴ Atom economy = 44 %
But-1-ene Oxidation
RMM of reactants = 56 + (3 x 32) = 152
RMM of desired product = 98
∴ Atom economy = 64 %
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Answer (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 maleic anhydride
x 100 %
mass of non-benign reagents
= [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
=
mass of maleic anhydride
x 100 %
mass of non-benign reagents
= [96.3/ 100] x 100 %
= 96.3 %
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Answer (b), recommendation to industry
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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.
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Lecture 3 - Learning Outcomes
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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
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Why is Catalysis green?
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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.
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Potential disadvantages of catalysis
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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
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Case study: Boots synthesis of Ibuprofen
AcOH, HCl,
Al waste
HCl
AcOH
NH3
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Case study: Hoechst synthesis of Ibuprofen
All three steps
are catalytic
AcOH
99 % conversion
96 % selectivity
Less waste is generated as a result of using catalysed reactions
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Some definitions
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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).
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Heterogeneous versus Homogeneous
General features:
Heterogeneous
Readily separated 
Readily recycled / regenerated 
Long-lived 
Cheap 
Lower rates (diffusion limited) 
Sensitive to poisons 
Lower selectivity 
High energy process 
Poor mechanistic understanding 
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).
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Homogeneous catalysis - principles
Well-defined active site allows rational catalyst development.
Typical single-site catalyst:
X
Ln
sterically bulky ligand(s)
controls stereochemistry
M
e.g. Cp2ZrMe+ for the
polymerisation of ethene
substrate approaches
vacant coordination site
and may then react with X
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Homogeneous 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):
L* =
28 % e.e.
60 % e.e.
85 % e.e.
95 % e.e.
0.1% catalyst loading; Rh readily recovered (some L* is lost)
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Heterogeneous Catalysis
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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.
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Heterogeneous Catalysis: AB + C2
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AC + BC
Stage
Stage
Stage
Stage
Stage
Stage
4:3:
2:Surface
6:
Chemisorption
Physisorption
1:
5:
7:Desorption
Diffusion
Reaction
Diffusion
diffusion
A
B
C
A
B
C
M
Surface
C
C
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Heterogeneous Catalysts
Active sites
are in pores
M
Surface
Heterogeneous Catalysts
Active sites
are in pores...
...and every pore may contain lots of active sites
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Heterogeneous 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
C
Products
A
B
B
C
Reactants
C
1.
2.
3.
C
Diffusion to surface
Physisorption
Chemisorption
1-3
1-3
6,7
4,5
6. Desorption
7. Diffusion out of pore
M
4. Surface diffusion
5. Reaction
porous support
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Heterogeneous acid-base catalysis
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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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Zeolites - 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
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e.g. ZSM-5
Td
Channels cross in three dimensions
- a highly porous material
Top-view
5.5 Å
Side-view
● = Si / Al
●=O
NB: Cations
not shown!
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Zeolites - Asahi Cyclohexanol process
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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:
98 % selectivity
100 °C
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Why is the Asahi process important?
Flixborough 1974 - 28 deaths
225 °C
10 atm
1
2
3
4
6
Tank 5 removed
for repairs
Tanks 1, 2 and 3
Tank 4
Temporary pipework between
tanks 4 and 6 ruptured and
cyclohexane cloud exploded
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Zeolites - shape selective alkylation of toluene
H-ZSM-5 catalyses:
• toluene alkylation
• xylene isomerisation
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H-ZSM-5
(acidic ZSM-5)
Channel size only allows para-xylene to emerge
This process is important because only para-xylene is required for PET:
poly(ethylene terephthlate) - PET
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A rare example of solid base catalysis
Traditional synthesis of 5-ethylidene-2-norbornene (ENB) via VNB:
VNB
ENB
key component
of EPDM rubber
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 
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Conclusions
The learning objectives of lecture 3 were:
• explain how catalysis may be considered green
Catalysis may reduce materials, waste and energy
• identify the characteristics of heterogeneous and homogeneous
catalysis
Heterogeneous are easily recycled and long-lived but ill-defined
Homogeneous are more active and selective but expensive and hard to recover
• describe three examples of green heterogeneous catalysis
Asahi Cyclohexanol process
H-ZSM-5 alkylation of toluene/ isomerisation of xylene
Sumitomo base-catalysed isomerisation of vinylnorbornene
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Another exam-style question
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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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