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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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