Transcript Slide 1

Answers to the question from lecture 3
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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 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.
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
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 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
mass of non-benign reagents
= [96.3/ 100] x 100 %
= 96.3 %
x 100 %
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 possibly better but only if raw material costs are acceptable.
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4.I6 Green Chemistry
Lecture 4: Catalysis
Energy
Eact uncatalysed
Eact catalysed
reactants
products
4.I6 Green Chemistry Lecture 4 Slide 1
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Revised 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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Lecture 4 - Learning Outcomes
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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
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Why is Catalysis green?
Materials
including
plant
Using catalysts should reduce:
• energy
Energy
Waste
Cost
• the use of stoichiometric reagents
Risk and
Hazards
Impact on the
environment
• by-products (particularly if the catalyst is highly selective)
Toxicity
• 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.
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Potential disadvantages of catalysis
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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
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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
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All three steps
are catalytic
AcOH
Less waste
generated
99 % conversion
96 % selectivity
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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 (see lecture 6).
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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 
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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Heterogeneous Catalysis
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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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Heterogeneous Catalysts
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
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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 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
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
Only para-xylene is required for PET synthesis:
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 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 
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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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Conclusions
The learning objectives of lecture 4 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
•describe one example of green homogenous catalysis
Asymmetric hydrogenation - e.g. Monsanto L-DOPA synthesis
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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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