Organic Aqueous Tunable Solvents (OATS): A Vehicle for Coupling Reactions and Separations

摘要

Inn laboratory-based chemical synthesis, the choice of the solvent and the means of product purification are rarely determinednby cost or environmental impact considerations. When a reaction is scaled up for industrial applications, however, these choicesnare critical: the separation of product from the solvent, starting materials, and byproduct usually constitutes 60 80% of the over-nall cost of a process. In response, researchers have developed solvents and solvent-handling methods to optimize both the reac-ntion and the subsequent separation steps on the manufacturing scale. These include “switchable” solvents, which are designed sonthat their physical properties can be changed abruptly, as well as “tunable” solvents, wherein the solvent’s properties change con-ntinuously through the application of an external stimulus. In this Account, we describe the organic aqueous tunable solvent (OATS)nsystem, examining two instructive and successful areas of application of OATS as well as its clear potential for further refinement.nOATS systems address the limitations of biphasic processes by optimizing reactions and separations simultaneously. The reac-ntion is performed homogeneously in a miscible aqueous organic solvent mixture, such as water tetrahydrofuran (THF). The effi-ncient product separation is conducted heterogeneously by the simple addition of modest pressures of CO2 (50 60 bar) to thensystem. Under these conditions, the water THF phase splits into two relatively immiscible phases: the organic THF phase con-ntains the hydrophobic product, and the aqueous phase contains the hydrophilic catalyst. We take advantage of the unique prop-nerties of OATS to develop environmentally benign and cost-competitive processes relevant in industrial applications. Specifically,nwe describe the use of OATS for optimizing the reaction, separation, design, and recycling of (i) Rh-catalyzed hydroformylationnof olefins such as 1-octene and (ii) enzyme-catalyzed hydrolysis of 2-phenylacetate.nWe discuss the advantages of these OATS systems over more traditional processes. We also consider future directions thatncan be taken with these proven systems as well as related innovations that have recently been reported, including the use of poly-n(ethylene glycol) (PEG) as a tunable adjunct in the solvent and the substitution of propane for CO2 as the external stimulus. OATSnsystems in fact represent the ultimate goal for a sustainable process, because in an idealized setup there is only reactant comingnin and product going out; in principle, there is no waste stream.