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Tuesday, Green Solvents

• Why and where are solvents used? Why do we want to replace VOCs?

• Some ‘green’ alternatives:• No solvent• Supercritical carbon dioxide• Water• Ionic liquids• Some Pros & Cons of the alternatives

• Incl. examples to illustrate concepts

242nd Ed, 2013

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The five parts of a typical chemical process

1. Starting materials/feedstocks2. Reaction types3. Reagents4. Solvents and reaction conditions5. Chemical products/target molecules

• Reactors can influence these• Don’t forget analysis & monitoring

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Solvent roles

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Mass Transfer Cleaning

Heat Transfer Stabilization

Why?

Reminder, solvents are not only used in reactions but also processes, formulations (e.g. consumer products) and analyses

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What is the problem with VOCs?• Volatile organic solvents are the normal media for carrying out organic

syntheses and extractions - usage $8,000,000,000 p.a.• They cause considerable environmental concern! e.g. Climate

Change/Ozone Depletion• What are the adverse health effects caused by VOCs in the environment

that have been identified by the EPA? Hexane is still widely used to extract oil in the agricultural industry e.g. soybean oil. Consider the following Environmental Health and Safety factors, and discuss why alternatives to hexane need to be found. (You will need to use the safety sheet for hexane).

• Release potential; Fire/explosion; Reaction/decomposition; Acute toxicity; Irritation; Chronic toxicity; Persistency; Air hazard; Water hazard

• There are alternatives but first you need to think about how solvents function on the molecular level

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Thermodynamics of solvation (DG = DH – TDS)

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Polarity & Volatility of ‘Green’ Solvents

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Choosing solventsOften, like dissolves Like!Sometimes this is too simple

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Solvent-Free Systems & Mechanochemistry

• Misconception that reactions need solvents – most bulk chemicals are produced ‘solvent-free’

• BUT what does solvent-free mean?• Often the reactants/products are acting as the

solvents or the reaction is in the gas-phase.• Too quick scale-up leads to solvent use industrially,

more time needed at Process Development stage.• Organic (condensation reactions) and coordination

chemistry• Mortar and pestle• Mixer-mills and grinders

• Note: In mechanochemistry, the by-products or moisture from air can act as a ‘template’ or ‘solvent’ for the reaction

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Recent exciting results• Angew. Chem. Int. Ed. 2018, 57, 2621–2624

• “In summary, we demonstrated efficient, clean enzymatic hydrolysis of cellulose into glucose by switching from conventional solution media to mechanochemistry without bulk solvent. This mechanoenzymaticRAging process has so far enabled unprecedented, direct enzymatic conversion of 50% of MCC within 12 hours, without chemical pre-treatment … while also producing cellulose nanocrystals (CNCs), a value-added form of cellulose”

• reactive aging = RAging…. (5 min milling followed by 55 min of ‘aging’)32

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Water

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• All solvents will have advantages and disadvantagese.g. Advantages and disadvantages of using water as a solvent

Opportunity for replacing VOCs Distillation/Separation is energyintensive

Non-toxic, non-flammable,odourless, colourless, naturallyoccurring, inexpensive

Contaminated waste streamsmay be difficult to treat

High specific heat capacity(exothermic reactions can bemore safely controlled)

High specific heat capacity(difficult to heat or cool rapidly)

Advantages Disadvantages

Potential for easy catalystrecycling

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New methods can be developed with a little imagination…

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Org. Lett. 2015, 17, 2428−2430

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Hydrophilicity and hydrophobicitycan be tailored to facilitateseparations

Some phosphine ligands thatinduce water solubility of catalystsare shown here:

Hydrophilic groups includesulfonated salts, amino-aryls andhydroxy-alkyls are attached tootherwise hydrophobic ligands thatrender them water soluble. Sugarshave also been used

pH can sometimes be used to‘switch’ solubility on/off

P

SO3Na

Ph2P

SO3Na3

NaO3S SO3Na

PPh2Ph2P

NPh2P

H+

P P OH

OH

HO

HO

Sulfonated groups

Aminoaryl Hydroxylalkyl

TPPTSTPPMS

BINAS

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An industrial example

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Hoechst AG and Rhône-Poulenc use an aqueous-organic process thatconverts propene to n-butyraldehyde (hydroformylation reaction).

This biphasic process has replaced the old cobalt catalysed high pressureprocess that used organic solvents.

The substrates are water soluble whereas the products are insoluble in water and are therefore easily extracted.

Propene (propylene) + syn gas butyraldehyde (butanal)

+ CO + H2CHO +/or

CHO

n- iso-

Rh catalystH2O

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TPPTS

BINAS

activity98n/i

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• The best phosphine is BINAS (comparewith TPPTS in the graph) which giveshighest activity (i.e. most product per Rhcenter) and highest n/i ratio.

• The overall E factor has been calculated,i.e. the ratio of all by-products and wasterelative to the product. This is calculatedon a w/w basis and is 0.04 for this process.This value compares well with the previouscobalt-catalyzed process, E = 0.6.

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Supercritical carbon dioxide

• Non-toxic• Sometimes unreactive• Can be selective (as it

is generally a poor/weak solvent)

• Co-solvents can be used

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Predicting solubility in scCO2

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• 2 groups of polymer show good solubility

• Fluoropolymers• polar interactions

• Polysiloxane• flexible backbone

• Why?

CF2

F2C

n

SiMe2

O

n

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CO2 Extraction – Used industrially• Advantages

• low temperature extraction results in minimal degradation of volatile compounds

• higher product yields than with steam distillation• spent material undamaged unlike steam distill/solvent extraction

• Disadvantages• very high capital installation costs• high running costs• requires technically skilled operators• not suitable for wet raw materials• lower product yield than solvent extraction

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Ionic liquids• Ionic compounds

composed of organic cations and inorganic or organic counterions that have melting points below the boiling point of water (100 ˚C).

41http://www.basf.com

http://www.chemistry.wustl.edu

NaCl

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Cations

Anions

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§ Usually non-volatile and non-flammable

§ Thermally stable

§ Tunable properties depend on the cation and anion

§ Many of them are recyclable

§ Moderate to high viscosity

§ Many of them are expensive

§ Questionable toxicity

Properties of ionic liquids

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compound LC50[BMIm]PF6 250-300[BMIm]BF4 225-275Acetone 30642DMF 12220Dichloromethane 310Toluene 60-313Benzene 203Chlorobenzene 5-86Phenol 5Ammonia 0.53-4.94Chlorine 0.028

Degradability and toxicity of ionic liquids• BF4

-, PF6- anions hydrolyze in

aqueous solutions and cannot be used for long term applications involving water. Toxic and corrosive species such as HF or HCl might be liberated (use of halogen-free and relatively hydrolysis-stable anions is suggested).

• …bioderived anions are now more common

• LC50 levels were studied for twoimidazolium based ionic liquids,[BMIm]PF6 and [BMIm]BF4 withDaphnia magna.

• …should consider chronic toxicityindicators too e.g. mutagenicity

• Daphnia are filters feeders and are atthe base of the aquatic foodchain.

LC = lethal concentration (in mg/mL)

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Discussion: Are bio-solvents green?• Renewable organic solvents can be considered ‘greener’

than traditional petroleum sourced ones. D-Limonene is being used to replace the following solvents in cleaning fluids: methyl ethyl ketone, acetone, toluene, xylene and many chlorinated solvents.

• Find an MSDS sheet for limonene and one of these organic solvents. Compare (depending on the data available) their flammability (e.g. flashpoint) and hazards in general.

• In your opinion, what are the benefits and problems in using Bio-VOCs to tackle issues related to green chemistry (e.g. pollution, risk, economics)?

• Compare with advantages/disadvantages for water and scCO2 use

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Green Washing

•Green washing?

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1-BUTOXY-2-PROPANOL: COMBUSTIBLE LIQUID AND VAPOUR. EYE AND SKIN IRRITANT

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Group work/Discussion: Do I need a solvent? Could I use a greener one?

• Discuss which ‘green’ solvents might be best suited for (i) electrochemistry, (ii) coffee decaffeination, (iii) household paints and (iv) degreasers

• Discuss the use of alternative solvents such as (a) supercritical carbon dioxide, (b) ionic liquids or (c) water, as reaction media for homogeneously-catalysed reactions using simple metal salts and phosphine ligands e.g. RhCl3 + PPh3, (in particular specifics related to ‘ligand design’ and/or particular processes/reactions)

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Evaluating a paper for ‘greenness’

• Angew. Chem. Int. Ed. 2015, 54, 12112 –12115• …Is the reaction atom efficient?• Are the reagents hazardous?• Are they renewable etc?• Are any of the principles of green chemistry already being employed?

• Are we provided with all the information we need?• Were solvents screened? Could be organic solvent be replaced?

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In Research Labs• Based on Pfizer solvent-selection guide: Green Chem, 2008, 10, p. 31

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PREFERREDWater, Acetone, Ethanol,* 2-Propanol, 1-Propanol, Ethyl acetate,* Isopropyl acetateMethanol, Methyl ethyl ketone, 1-Butanol, t-Butanol

USABLECyclohexane, Heptane, Toluene, Methylcyclohexane, Methyl t-butyl ether, isooctane2-MethylTHF, Cyclopentyl methyl ether, Xylenes, DMSO, Acetic acid, Ethylene glycol

UNDESIRABLEPentane, Hexane(s), Di-isopropyl ether, Diethyl ether, Dichloromethane, DichloroethaneChloroform, DMF, NMP, Pyridine, DMA, Acetonitrile, THF, Dioxane, DME, Benzene Carbon tetrachloride

* Bio-sourced alcohols and esters could also be considered as they become available at competitive prices e.g. ethyl lactate

DME = dimethoxyethane

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‘Help’ – I don’t know what to include in my e-poster

• In most cases, you cannot copy images from published works because this breaks copyright laws….so• Use royalty free images. A good place to find them is pixalbay.org• Prepare your own images (take photos, use chemdraw or powerpoint or similar).

• Draft on a sheet of paper first or make a spider diagram• If you are not sure where to begin, chat with a friend to try and identify key points• Preparing images, tables, graphics/pictures is sometimes a good starting point if you have ‘writer’s block’• We are here to help – just ask

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