American Chemical Society

Greener Solvents

Mary M. Kirchhoff

With special thanks to Philip Jessop

South African Chemical Institute

Conference

29 November 2015

Goal of Green Chemistry: Reduce Solvent Use

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http://www.caraet.com/Waste_solvent.htm

Mass utilization in fine chemical production

Solvent 80-90%

Other

10-20%

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Sertraline Synthesis

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Solvents

• Organic solvent concerns

• Flammable

• Carcinogenic

• Mutagens/teratogens

• High vapor pressure

• Toxic

Murphy’s Law of Solvents

“The best solvent for any

process step is bad for the

subsequent step.”

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An Imaginary Process

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Solving Murphy’s Law of Solvents:

1. A compromise solvent

2. A switchable solvent

Switchable Polarity Solvent

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CO2

N2 or Ar Nature (2005) 436, 1102

Ind Eng Chem Res (2008) 47, 539

N2

Polystyrene Synthesis

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75 ˚C

6 h

CO2

filter or centrifuge

+

Mn = 412,000

PDI = 1.57

N2

1.0 mL styrene

4.5 mL DBU/PrOH

45 mg initiator

Decreasing Solvent Impact

• Reduce volume of solvent

– Use higher concentrations

– Use solvent for more than one step

• Make the solvents greener

– Carefully choose conventional solvents

– Use new greener solvents

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Solvent Properties

• For greenness

– Boiling point/energy to distill

– Flash point

– Cumulative energy demand

• For utility

– Polarity

– Acidity/basicity

– Viscosity

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Pfizer Solvent Selection Guide: Conventional Solvents

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Which Solvent is Greener?

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solvent #1 solvent #2

General Comparison

• solvent impact

• solvent impact including manufacture

• energy to manufacture/cumulative energy demand

Application-Specific Comparison

• ISO LCA

The greenest solvent is the one that makes your

process the greenest

EHS Risks

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Capello et al., Green Chem (2007) 9, 927

Energy for Manufacture

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Capello et al., Green Chem (2007) 9, 927

Energy Requirements for a Solvent

How is hexane made? How is DMF made?

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oil distillation

hexane

Proposed Greener Organic Solvents

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γ-valerolactone

Low toxicity, biodegradable,

renewable

ethyl lactate

Low toxicity, biodegradable,

renewable

2-methyltetrahydrofuran

Renewable

cyclopentylmethylether

Doesn’t form peroxides,

low solubility in water

Non-Traditional Greener Solvents

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Supercritical CO2 Ionic liquid CO2-expanded liquid

Water Liquid polymer Switchable solvents

Why Not Water?

• Cheap, nontoxic, nonflammable

• Poor solubility of organic compounds

• Perceived incompatibility with organometallics

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Li, 1995

Switchable Water

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breaking emulsions breaking clay

suspensions

breaking latex

suspensions

switching viscositypost-reaction separations

product!&!

catalyst!

product!

catalyst!

purifying water

Ionic Liquids (ILs)

• Pure salts that are liquid around ambient

temperature

• Advantages

– Nonvolatile, nonflammable, no inhalation

hazards, chemical diversity (combinations of

cations and anions)

• Examples

– [bmim][BF4], [emim][Tf2N], [emim][EtSO4]

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ILs: Tunable Solvents

• Cations • Anions

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ARE IONIC LIQUIDS GREEN?

Zhang, Env. Sci. Tech. (2008) 42, 1724

Jessop, Green Chem (2011) 13, 1391

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Chemical Steps in Solvent Synthesis

Jessop, Green Chem (2011) 13, 1391

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Chemical Steps in Solvent Synthesis

Jessop, Green Chem (2011) 13, 1391

ILs for CO2 Separations

• Potential advantages

– High capacity with chemical complexation

– Reaction enthalpy tunable

– Can run “neat”, without added water

– Good thermal and oxidative stability

– Low volatility

• Potential disadvantages

– High viscosities

– High molecular weights

– Absorbent more expensive

than molecular amines Brennecke

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Carbon Capture and Sequestration

• Current technology impractical

• Monoethanolamine

• High energy load

• Corrosive

• Degrades at low temperatures

• Alternative approach: ionic liquids

• Testing in lab-scale unit at Notre Dame and

Babcocks & Wilcox Brennecke

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Supercritical CO2

• Characteristics

– Low polarity

– Variable solvent properties

– High mass transfer rates

– Miscibility with gases

– Nontoxic

– Nonflammable

• Disadvantages

– Poor solubility of some reagents

– Energy required for pressurization

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CO2 as a Solvent

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Natex CO2 decaffeination plant

DuPont fluoropolymer plant, N.C.

CO2 dry cleaning

Conclusions

• No solvent is perfectly green

• Nonconventional solvents are more exciting,

but the conventional ones are greening the

industry

• Is it really green?

– Consider the solvent’s synthesis and its effect

on the process

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