Catalysis of Organic..

Hallett, Pollet, Eckert and Liotta 39544. Recycling HomogeneousCatalysts for Sustainable TechnologyAbstractJason P. Hallett, Pamela Pollet, Charles A. Eckert and Charles L. LiottaSchool of Chemistry and BiochemistrySchool of Chemical & Biomolecular EngineeringSpecialty Separations CenterGeorgia Institute of TechnologyAtlanta, GA 30332-0325charles.liotta@carnegie.gatech.eduHomogeneous catalysts possess many advantages over heterogeneous catalysts, suchas higher activities and selectivities. However, recovery of homogeneous catalysts isoften complicated by difficulties in separating these complexes from the reactionproducts. The expense of these catalysts (particularly asymmetric catalysts) makestheir recovery and re-use imperative.We have developed several techniques using CO 2 as a “miscibility switch” toturn homogeneity “on” and “off”. The goal is to create a medium for performinghomogeneous reactions while maintaining the facile separation of a heterogeneoussystem. Our approach represents an interdisciplinary effort aimed at designingsolvent and catalytic systems whereby a reversible stimulus induces a phase changeenabling easy recover of homogeneous catalysts. The purpose is to preserve the highactivity of homogeneous catalysts while taking advantage of simple separationtechniques, such as filtering and extraction, normally applied to heterogeneous orbiphasic catalytic systems. Specific examples include the application of gaseousCO 2 as a benign agent in gas-expanded liquids to induce organometallic catalyticrecycle of water/organic, fluorous/organic biphasic systems. Additional applicationsinvolve the enhancement of solid-liquid phase transfer catalysis with supercriticalsolvents and improved recovery of phase transfer catalysts from biphasic liquidmixtures using gas-expanded liquids. Specific reaction systems include thehydroformylation of hydrophobic olefins using water-soluble catalysts, thehydrogenation of pro-chiral and achiral substrates using fluorous-modified catalystscaptured in a fluorinated solvent or on a fluorous surface phase, the hydrolysis ofhydrophobic esters using enzymatic biocatalysts in mixed aqueous/organic media,and nucleophilic substitutions using novel phase transfer catalyzed systems.IntroductionCatalytic synthesis can be achieved by a variety of methods, including homogeneousand heterogeneous organometallic complexes, homogeneous enzymatic biocatalysts,phase transfer catalysts, and acid and base catalysts. However, each of these