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Lecture 3: Catalysis

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Lecture 3: Catalysis Imperial College Module 4I10: Green Chemistry London Lecture 3: Catalysis Energy Eact uncatalysed Eact catalysed reactants products 4.I10 Green Chemistry Lecture 3 Slide 1 Imperial College Before we begin, a correction to last week’s slide 24 London E-factor = 462 / 40 = 11.6 mass of waste produced E-factor = mass of desired product Mass of waste = [37g + 60g + 250g + 100g + 25g + 25g + 5g] – 40g = 504g – 40g = 462g 4.I10-3-2 Imperial College Answers to the question from lecture 2 London 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 Answer (a), part (i) atom economies London 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 ∴ Atom economy = 64 % RMM of desired product = 98 4.I10-3-4 Imperial College Answer (a), part (ii) effective mass yields London Benzene Oxidation 100 g benzene (1.28 mol) would give 81.5 g maleic anhydride (0.83 mol, 65 %): mass of maleic anhydride EMY = 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 %): mass of maleic anhydride EMY = x 100 % mass of non-benign reagents = [96.3/ 100] x 100 % = 96.3 % 4.I10-3-5 Imperial College Answer (b), recommendation to industry London 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 Lecture 3 - Learning Outcomes 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 Why is Catalysis green? London 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 Potential disadvantages of catalysis London 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 Case study: Boots synthesis of Ibuprofen London AcOH, HCl, Al waste HCl AcOH NH3 4.I10-3-10 Imperial College Case study: Hoechst synthesis of Ibuprofen London All three steps are catalytic AcOH 99 % conversion 96 % selectivity Less waste is generated as a result of using catalysed reactions 4.I10-3-11 Imperial College Some definitions London 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). 4.I10-3-12 Imperial College Heterogeneous versus Homogeneous London General features: Heterogeneous Homogeneous Readily separated Difficult to separate Readily recycled / regenerated Difficult to recover Long-lived Short service life Cheap Expensive Lower rates (diffusion limited) Very high rates Sensitive to poisons Robust to poisons Lower selectivity Highly selective High energy process Mild conditions Poor mechanistic understanding 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 Homogeneous catalysis - principles London Well-defined active site allows rational catalyst development. Typical single-site catalyst: X Ln M + sterically bulky ligand(s) e.g. Cp2ZrMe for the controls stereochemistry polymerisation of ethene substrate approaches vacant coordination site and may then react with X 4.I10-3-14 Imperial College Homogeneous asymmetric catalysis London 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.I10-3-15 Imperial College Heterogeneous Catalysis London 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 Heterogeneous Catalysis: AB + C2 AC + BC London StageStageStageStageStageStageStage 4: 3:2: Surface 6:Chemisorption Physisorption1: 5:7: Desorption Diffusion ReactionDiffusion diffusion A B C C A C B C M Surface Imperial College Heterogeneous Catalysts London Active sites are in pores M Surface Imperial College Heterogeneous Catalysts London Active sites are in pores... ...and every pore may contain lots of active sites Imperial College Heterogeneous Catalysts London 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 Reactants Products C C A C 1. Diffusion to surface 2. Physisorption B C 3. Chemisorption 11--33 6,7 4,5 6. Desorption M 4. Surface diffusion 7. Diffusion out of pore 5. Reaction porous support 4.I10-3-17 Imperial College Heterogeneous acid-base catalysis London 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 Zeolites - crystalline, hydrated aluminosilicates London - 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 e.g. ZSM-5 London Td Channels cross in three dimensions - a highly porous material Top-view Side-view ● = Si / Al 5.5 Å ● = O NB: Cations not shown! 4.I10-3-20 Imperial College Zeolites - Asahi Cyclohexanol process London 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.I10-3-21 Imperial College Why is the Asahi process important? London Flixborough 1974 - 28 deaths 225 °C Tank 5 removed 1 2 3 4 6 10 atm for repairs Tanks 1, 2 and 3 Tank 4 Temporary pipework between tanks 4 and 6 ruptured and cyclohexane cloud exploded 4.I10-3-22 Imperial College Zeolites - shape selective alkylation of toluene London H-ZSM-5 catalyses: H-ZSM-5 • toluene alkylation (acidic ZSM-5) • xylene isomerisation Channel size only allows para-xylene to emerge This process is important because only para-xylene is required for PET: poly(ethylene terephthlate) - PET 4.I10-3-23 Imperial College A rare example of solid base catalysis London 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
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