The greatest advantage of heterogeneous catalysis is the ease of separation, while the disadvantages are often limited activity and selectivity. We report solvents that use tunable phase behavior to achieve homogeneous catalysis with ease of separation. Tunable solvents are homogeneous mixtures of water or polyethylene glycol with organics such as acetonitrile, dioxane, and THF that can be used for homogeneously catalyzed reactions. Modest pressures of a soluble gas, generally CO2, achieve facile post-reaction heterogeneous separation of products from the catalyst. Examples shown here are rhodium-catalyzed hydroformylation of 1-octene and p-methylstyrene and palladium catalyzed C-O coupling to produce o-tolyl-3,5-xylyl ether and 3,5-di-tert-butylphenol. Both were successfully carried out in homogeneous tunable solvents followed by separation efficiencies of up to 99% with CO2 pressures of 3 MPa. Further examples in tunable solvents are enzyme catalyzed reactions such as kinetic resolution of rac-1-phenylethyl acetate and hydrolysis of 2-phenylethyl acetate (2PEA) to 2-phenylethanol (2PE). Another tunable solvent is nearcritical water (NCW), whose unique properties offer advantages for developing sustainable alternatives to traditional processes. Some examples discussed are Friedel-Crafts alkylation and acylation, hydrolysis of benzoate esters, and water-catalyzed deprotection of N-Boc-protected amine compounds.

Tunable solvent systems couple homogeneous catalytic reactions to heterogeneous separations, thereby combining multiple unit operations into a single step and subsequently reducing waste generation and improving process economics. In addition, tunable solvents can require less energy than traditional separations, such as distillation. We extend the impact of such solvents by reporting on the application of two previously described carbon dioxide tunable solvent systems: polyethylene glycol (PEG)/organic tunable solvents (POTS) and organic/aqueous tunable solvents (OATS). In particular, we studied: (1) the palladium catalyzed carbon-oxygen coupling of 1-bromo-3,5-dimethylbenzene and o-cresol to potassium hydroxide to produce o-tolyl-3,5-xylyl ether and 1-bromo-3,5-di-tert-butylbenzene to potassium hydroxide to produce 3,5-di-tert-butylphenol in PEG400/1,4-dioxane/water and (2) the rhodium-catalyzed hydroformylation of p-methylstyrene in water/acetonitrile to form 2-(p-tolyl) propanal. In addition, we introduce a novel tunable solvent system based on a modified OATS where propane replaces carbon dioxide. This represents the first use of propane in a tunable solvent system.

Reversible systems such as silylamines are neutral amines that can react reversibly with CO2 to form the corresponding ammonium carbamate ionic pair. The ionic-to-neutral ‘switch’ capability provides an advantageous means for efficient synthesis and facile deposition of nanoparticles onto a solid support. Herein, we first illustrate the surface active/non-surface active duality of the silylamine systems with the reversible solubilization-precipitation of methyl orange from a hexane solution of the 3-(aminopropyl)tripropylsilane (TPSA) system (0·275 M). The authors then demonstrate the application of this system to the controlled reduction of an ionic gold salt to yield gold nanoparticles using two reversible silylamine systems: TPSA and 3-(aminopropyl)trihexylsilane (THSA). The post-synthesis deposition of the THSA-stabilized gold nanoparticles from solution onto a support using the surface active/non-surface active duality of these systems was also demonstrated.

John Prausnitz was never content to measure new data solely because the data were new. He always had an application in mind-a separation or even a process that required the data for implementation. We present here an advance in using designed changes in phase equilibria to enable the facile recovery and recycle of homogeneous catalysts. We show a new application of organic-aqueous tunable solvents (OATS) to run homogeneous reactions (fast rates and high selectivities) followed by facile and efficient heterogeneous separations and the recycle of the homogeneous catalysts. This is done by using CO 2 to manipulate the phase behavior of monophasic organic-water mixtures to form heterogeneous organic-rich and aqueous-rich phases. The example shown is the hydroformylation of hydrophobic p-methylstyrene catalyzed by rhodium catalyst to which is attached a hydrophilic phosphorus ligand. The OATS method increases the conversion rate of styrene to the aldehyde products by an order of magnitude compared to heterogeneously reported reactions. Also, the selectivity toward the branched aldehyde (the desired product) increases by 30 %. The hydrophobic product partitions into the organic-rich phase with more than 99 % removal efficiency, and the hydrophilic catalyst is retained in the aqueous-rich phase with 99.9 % efficiency. In addition, we recycle the catalyst for five consecutive reactions without significant loss of catalytic activity.

We discuss novel solvents that improve the sustainability of various chemical reactions and processes. These alternative solvents include organic-aqueous tunable solvents; near-critical water; switchable piperylene sulfone, a volatile dimethylsulfoxide substitute; and reversible ionic liquids. These solvents are advantageous to a wide variety of reactions because they reduce waste and energy demand by coupling homogeneous reactions with heterogeneous separations, acting as in situ acid or base catalysts, and providing simple and efficient postreaction separations.