Imperial College London. Module 4I10: Green Chemistry. Lecture 4: Solvents. 4.I10 Green Chemistry Lecture 4 Slide 1

Transcription

1 Module 4I10: Green Chemistry Lecture 4: Solvents 4.I10 Green Chemistry Lecture 4 Slide 1

2 Lecture 4 - Learning Outcomes By the end of this lecture you should be able to describe the advantages and disadvantages of traditional organic solvents list the characteristics of 4 different types of green solvent and for each one describe an example of a suitable process suggest alternative solvent choices for reactions. 4.I10-4-2

3 The big problem with organic solvents VOCs - volatile organic compounds form street-level ozone and smog via free radical air oxidation processes. All solvent waste must be contained and treated (e.g. incineration) According to GlaxoSmithKline, solvents make up ca. 85 % of all their nonaqueous waste. Typical recovery efficiencies are %. The main alternatives to organic solvents are: solvent-free processes water-based chemistry supercritical fluids (particularly water and CO 2 ) ionic liquids fluorous biphasic systems 4.I10-4-3

4 Replacing organic solvents is not always green! Organic solvents good heat and mass transfer low viscosities (good for kinetics) Replacing organics may incur an increased energy input Also, not all organic solvents are harmful, e.g. isopropanol, ethyl acetate ethanol 2-butanone limonene (extracted from citrus fruit peel) Therefore industry has concentrated on eliminating the most toxic solvents first: e.g. chlorocarbons, benzene, toluene, hexane, dioxane, pyridine, methanol 4.I10-4-4

5 Alternative 1. Solvent-free systems Many high-volume chemicals are already produced without solvents e.g. polymerisation of propene: Catalyst is soluble in liquid propene e.g. synthesis of MTBE: (fuel additive in USA) Liquid phase reaction (90 C, 8 atm) Main disadvantages of solvent-free syntheses: solvents are often still required during work-up (e.g. extraction) poor heat transfer in the solid state (although this may be overcome using microwaves) Fewer solvent-free examples exist for fine chemicals / pharmaceuticals. 4.I10-4-5

6 A rare example - a terpyridine synthesis 1 2 Step 1: Aldol - no solvent. 3 Step 2: Michael Addition - no solvent. Both steps 1 and 2 are fast and quantitative, (in EtOH, yields of both steps are ca. 50%) Step 3: the only stage that requires solvent, but no purification of the dione precursor is required 4.I10-4-6

7 2. Water-based chemistry - the ultimate green solvent? Advantages Disadvantages Non-toxic Cheap Biorenewable Non-flammable High specific heat capacity Removal requires distillation energy intensive Waste streams may be difficult to treat Many reagents are water-sensitive Generally a poor solvent for organics Despite the disadvantages, water-based organic synthesis is a very popular area of research 4.I10-4-7

8 2. Water-based chemistry e.g. Diels-Alder Solvent Relative rate octane 1 MeOH 12.5 water 740 LiCl (aq) 1800 Why do you think the rate is so much faster in (i) water? Hydrophobic effect (ii) aqueous lithium chloride solution? Salting in effect 4.I10-4-8

9 High temperature water At high temperatures water becomes less dense and less polar becomes more like an organic solvent (due to reduction in H-bonding). At 300 C water behaves similarly to acetone At high temperatures water also becomes more ionic becomes more acidic and more basic (increased [H 3 O + ] and [OH - ]). e.g. geraniol isomerisation - a source of fragrances without organic solvents. 220 C a-terpinol linalol 4.I10-4-9

10 Water can also be used in biphasic systems Traditional hydroformylation chemistry: Separation of catalyst from medium chain ( C 8 ) aldehydes is difficult Biphasic approach (phase transfer catalysis): alkene water-soluble catalyst organic solvent water aldehyde Catalysis occurs at the interface Rh-catalyst bears water-solubilising P(Ar) 3 ligands: 4.I

11 3. Supercritical solvents ("sc-fluids") Phase diagram: Pressure, P Solid P c Liquid supercritical fluid Above T c and P c sc-fluids have densities of liquids, but viscosities of gases Gas critical point triple point T c Temperature, T H 2 O CO 2 NH 3 C 2 H 4 C 2 H 6 C 3 H 8 CHF 3 T c / C P c / bar T c often quite low, but P c is usually high 4.I

12 e.g. Supercritical CO 2 - T c = 31.1 C, P c = 73.8 bar, ρ c = g cm -3 Sub-critical Approaching critical At, or above, critical point Liquid CO 2 CO 2 vapour meniscus poorly defined homogeneous supercritical CO 2 A major advantage of sc-solvents is their ease of removal - decrease the pressure and vent off the gas 4.I

13 Supercritical CO 2 Advantages Disadvantages Non-toxic Readily removed (and recyclable) Non-flammable Low viscosity (fast diffusion) CO 2 is cheap Good solvent of gases (e.g. H 2 ) High pressure equipment is expensive and potentially dangerous CO 2 is a relatively poor solvent Reacts with strong nucleophiles (e.g. amines) Uses of sc-co 2 : extraction of caffeine from coffee (traditional method uses CH 2 Cl 2 ) extraction of fatty acid triglycerides from crisps (low-fat crisps) dry-cleaning (traditional method uses C 2 Cl 4 ) spray-painting 4.I

14 conversion (%) sc-co 2 as a reaction solvent One area where scco 2 has found particular use is in hydrogenation chemistry (mainly because it is very miscible with H 2 gas) e.g. imine hydrogenation (20 x faster in sc-co 2 than CH 2 Cl 2 ) sc-co 2 CH 2 Cl 2 time (hr) 4.I

15 4. Ionic liquids Liquids at room temperature (large non-coordinating ions pack poorly) Common examples: Advantages Disadvantages Readily prepared Very low vapour pressure Can act as catalysts Tuneable viscosity (via anion) Stable at high temperature Highly solvating Recyclable Non-biodegradable Concerns over toxicity Synthesis often requires haloalkanes Product isolation often requires distillation or extraction into an organic solvent 4.I

16 Ionic liquids - e.g. Pd-catalysed Heck arylation yield > 95 % Work-up procedure: (i) add cyclohexane and water (ii) physically separate into three components (iii) distill off cyclohexane to obtain Heck product (iv) recycle catalyst without the need to extract from ionic liquid cyclohexane ionic liquid water product Pd catalyst HNEt 3 I 4.I

17 5. Fluorous biphasics Fully fluorinated solvents (e.g. C 6 F 14 ) are non-polar and immiscible with organic solvents. Ideal if reactants are non-polar, but products are polar: e.g. hydroformylation: alkene + catalyst organic fluorinated solvent aldehyde Catalyst: Rh(H)(CO){P(CH 2 CH 2 (CF 2 ) 5 CF 3 ) 3 } 2 Disadvantages: Fluorinated solvents are expensive and concerns exist for their long-term environmental impact. 4.I

18 Conclusions There are several ways in which organic solvents may be replaced, and a good argument can often be made for doing so on green chemistry grounds. However, it is important to remember that changing solvents may require additional energy (e.g. stronger heating), and organics may still be needed for work-up / purification steps. The choice of green solvent depends upon the reaction, upon the catalyst(s) and upon the method of product separation. 4.I

19 Learning outcomes Learning outcomes (i) describe the advantages and disadvantages of traditional organic solvents Good heat transfer and good diffusion of reactants. Potential source of VOCs & waste; not always easy to recover / recycle. (ii) list the characteristics of 4 different types of green solvent and for each one describe an example of a suitable process No solvent (terpyridine synthesis); water (Diels-Alder, Hydroformylation) sc-fluids (Hydrogenation in sc-co 2 ); ionic liquids (Heck coupling) (iii) suggest alternative solvent choices for a previously unseen reaction. Try the practice exam question on the next slide! 4.I

20 Practice exam question The following is an experiment from an undergraduate laboratory course: "In a 50 cm 3 round-bottom flask, dissolve 3.0 g of 2-methylcyclohexanone in 12.5 ml of methanol. Cool in an ice bath and carefully add 0.5 g of sodium borohydride. When the vigorous reaction has subsided, remove the flask from the ice bath and allow it to stand at room temperature for 10 minutes. Then add 12.5 ml of 3 M NaOH and to the resulting cloudy solution, add 10 ml of water. The product will separate as a clear layer. Remove as much of this as possible. Then extract the remaining product from the reaction mixture with 2 x 5 ml portions of dichloromethane. Dry the combined organic layers with sodium sulfate. Filter the drying agent off. Remove the solvent by warming under a stream of nitrogen. Be careful when boiling off the dichloromethane and methanol to avoid boiling off the product. Methanol and dichloromethane have low boiling points and will boil out of solution rather easily." Suggest ways in which the method could be modified in accord with the principles of Green Chemistry. 4.I

21 And the answer to last week s 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? Possible answers: Reduced waste (NB: Friedel-Crafts acylations require excess AlCl 3 ); Reduced energy (catalysis); Improved recovery and reuse of catalyst; No solvent required. 4.I