Presidential Green Chemistry Challenge: 2011 Academic Award
Professor Bruce H. Lipshutz of the University of California, Santa Barbara
Towards Ending Our Dependence on Organic Solvents
Innovation and Benefits: Most chemical manufacturing processes rely on organic solvents, which tend to be volatile, toxic, and flammable. Chemical manufacturers use billions of pounds of organic solvents each year, much of which becomes waste. Water itself cannot replace organic solvents as the medium for chemical reactions because many chemicals do not dissolve and do not react in water. Professor Lipshutz has designed a safe surfactant that forms tiny droplets in water. Organic chemicals dissolve in these droplets and react efficiently, allowing water to replace organic solvents.
Summary of Technology: Organic solvents are routinely used as the medium for organic reactions and constitute a large percentage of the world's chemical production waste. Most organic solvents are derived from petroleum and are volatile, flammable, and toxic. Typically, organic reactions cannot be done in water because the reactants themselves are insoluble. Surfactants can be used to increase the solubility of organic reactants in water, but they often disperse the reactants, slowing the reactions.
Professor Lipshutz has designed a novel, second-generation surfactant called TPGS-750-M. It is a "designer" surfactant composed of safe, inexpensive ingredients: tocopherol (vitamin E), succinic acid (an intermediate in cellular respiration), and methoxy poly(ethylene glycol) (a common, degradable hydrophilic group also called MPEG-750). TPGS-750-M forms "nanomicelles" in water that are lipophilic on the inside and hydrophilic on the outside. A small amount of TPGS-750-M is all that is required to spontaneously form 50–100 nm diameter micelles in water to serve as nanoreactors. TPGS-750-M is engineered to be the right size to facilitate broadly used organic reactions, such as cross-couplings. Reactants and catalysts dissolve in the micelles, resulting in high concentrations that lead to dramatically increased reaction rates at ambient temperature. No additional energy is required.
Several very common organic reactions that are catalyzed by transition metals can take place within TPGS-750-M micelles in water at room temperature and in high isolated yields. These reactions include ruthenium-catalyzed olefin metatheses (Grubbs), palladium-catalyzed cross-couplings (Suzuki, Heck, and Sonogashira), unsymmetrical aminations, allylic aminations and silylations, and aryl borylations. Even palladium-catalyzed aromatic carbon–hydrogen bond activation to make new carbon–carbon bonds can be done at room temperature, an extraordinary achievement. Product isolation is straightforward; complications such as frothing and foaming associated with other surfactants are not observed. Recycling the surfactant after use is also very efficient: the insoluble product can be recovered by extraction, and the aqueous surfactant is simply reused with negligible loss of activity. Future generations of surfactants may include a catalyst tethered to a surfactant to provide both the "reaction vessel" (the inside of the micelle) and the catalyst to enable the reaction. Tethering catalysts in this way may reduce one-time use of rare-earth minerals as catalysts.
In all, this technology offers opportunities for industrial processes to replace large amounts of organic solvents with very small amounts of a benign surfactant nanodispersed in water only. High-quality water is not needed: these reactions can even be run in seawater. Sigma-Aldrich is currently offering TPGS-750-M for sale, making it broadly available to research laboratories.
Podcast on the technology:(MP3, 1 MB, 1:06 minutes), Narrator: Dr. Richard Engler, US EPA.
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