The properties and some key applications of solvents such as (1) supercritical fluids (SCFs), (2) gas-expanded liquids (GXLs) and organic-aqueous tunable solvents (OATS), (3) water at elevated temperature (WET), and (4) switchable solvents like reversible ionic liquids (RevILs) are discussed in this review. Each system offers a unique set of properties to enable alternative strategies for varied applications. These applications range from chemical transformations, product separation, catalyst recycling, nanomaterial processing, and CO₂ capture. For each application, however, the common thrust is to enable greener and sustainable solutions for chemical processes.

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.

Switchable solvents are a unique class of solvents that were developed to facilitate both reaction and subsequent product separation. This mini review discusses two classes of switchable solvents: 1) piperylene sulfone and 2) one and two-component reversible ionic liquids.

A new class of one-component, thermally reversible, neutral to ionic liquid solvents derived from siloxylated amines is presented and characterized.