Imperial College London

1. Imperial College London Answers 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. • 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 H2Oare benign chemicals. • Which route would you recommend to industry? Outline the factors which might influence your decision.

2. Imperial College London Answer (a), part (i) atom economies 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 %

3. Imperial College London 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 %). mass of maleic anhydride x 100 % EMY = 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 %). mass of maleic anhydride x 100 % EMY = mass of non-benign reagents = [96.3/ 100] x 100 % = 96.3 %

4. Imperial College London Answer (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 onlyif raw material costs are acceptable.

5. Energy Imperial College London 4.I6 Green Chemistry Lecture 4: Catalysis Eact uncatalysed Eact catalysed reactants products 4.I6 Green Chemistry Lecture 4 Slide 1

6. Imperial College London 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 4.I6 4 - 2

7. Imperial College London Imperial College London Lecture 4 - Learning Outcomes Lecture 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

8. Materials including plant Energy Waste Cost Impact on the environment Risk and Hazards Toxicity Imperial College London 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): • It is better to prevent waste than to treat or clean up waste after it is formed. • Energy requirements should be minimized. Synthetic methods should be conducted at ambient temperature and pressure. • 9. Catalytic reagents are superior to stoichiometric ones. 4.I6 4 - 4

9. Imperial College London Potential 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

10. Imperial College London Case study: Boots synthesis of Ibuprofen AcOH, HCl, Al waste HCl AcOH NH3 4.I6 4 - 6

11. Imperial College London Case study: Hoechst synthesis of Ibuprofen All three steps are catalytic AcOH Less waste generated 99 % conversion 96 % selectivity 4.I6 4 - 7

12. Imperial College London Some definitions 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). 4.I6 4 - 8

13. Imperial College London Heterogeneous versus Homogeneous General features: Heterogeneous Homogeneous Readily separated  Readily recycled / regenerated  Long-lived  Cheap  Lower rates (diffusion limited)  Sensitive to poisons  Lower selectivity  High energy process  Poor mechanistic understanding  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 4.I6 4 - 9

14. Imperial College London Heterogeneous 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: • Diffusion of the substrate(s) towards the surface. • Physisorption - i.e. physical absorption via weak interactions, e.g. van der Waals, adhering the substrate(s) to the surface. • Chemisorption - formation of chemical bonds between the surface and the substrate(s). • Migration of the bound substrate(s) to the active catalytic site - also known as surface diffusion. • Reaction. • Desorption of product(s) from the surface. • Diffusion away from the surface. 4.I6 4 - 10

15. A A B C C B C C Imperial College London Heterogeneous Catalysts Stage 4: Surface diffusion Stage 3: Chemisorption Stage 2: Physisorption Stage 6: Desorption Stage 1: Diffusion Stage 5: Reaction Stage 7: Diffusion M Surface

16. M Surface Imperial College London Heterogeneous Catalysts Active sites are in pores

17. A A B C B C C C Imperial College London 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 Reactants Products • 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 4.I6 4 - 11

18. Imperial College London Heterogeneous 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.I6 4 - 12

19. Imperial College London Zeolites - 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.I6 4 - 13

20. Imperial College London e.g. ZSM-5 Td Channels cross in three dimensions - a highly porous material Top-view Side-view ● = Si / Al ● = O NB: Cations not shown! 5.5 Å 4.I6 4 - 14

21. Imperial College London Zeolites - 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: 98 % selectivity 100 °C 4.I6 4 - 15

22. Imperial College London Why is the Asahi process important? Flixborough 1974 - 28 deaths 1 2 3 4 6 225 °C 10 atm Tank 5 removed for repairs Tanks 1, 2 and 3 Temporary pipework between tanks 4 and 6 ruptured and cyclohexane cloud exploded 4.I6 4 - 16

23. Imperial College London Zeolites - shape selective alkylation of toluene H-ZSM-5 (acidic ZSM-5) • H-ZSM-5 catalyses: • toluene alkylation • xylene isomerisation Channel size only allows para-xylene to emerge Only para-xylene is required for PET synthesis: poly(ethylene terephthlate) - PET 4.I6 4 - 17

24. Imperial College London 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  4.I6 4 - 18

25. Imperial College London Homogeneous catalysis - principles Well-defined active site allows rational catalyst development. Typical single-site catalyst: Ln X M e.g. Cp2ZrMe+ for the polymerisation of ethene sterically bulky ligand(s) controls stereochemistry substrate approaches vacant coordination site and may then react with X 4.I6 4 - 19

26. Imperial College London 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) 4.I6 4 - 20

27. Imperial College London Conclusions • 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-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 Asymmetric hydrogenation - e.g. Monsanto L-DOPA synthesis 4.I6 4 - 21

28. Imperial College London Another 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.I6 4 - 22