(22a) Novel Solvents for Extractive Separations
Extractions can frequently be done more advantageously with novel solvents. Supercritical fluids (SCFs) are a classic example ? they offer vastly increased transport properties, and the limited solvent power can to some extent be overcomed with pressure or cosolvents. There are many practical applications based on SCF technology ranging from natural product extraction to renewal of old books. Recently, another tool that has been developed is gas-expanded liquids (GXLs) ? most often these are miscible mixtures of organics with CO2. The solvent power of GXLs can be continuously tuned by CO2 pressure over a wide range, and they offer much better solubility at lower pressures than supercritical fluids, coupled with transport slightly better than liquids. We show two examples from our labs: extraction of vanillin from black liquor and the recovery of phase transfer catalysts.
Furthermore, we are now exploring a wholly new class of ?smart? solvents ? liquids capable of large, abrupt changes in property due to certain external stimulus, such as heat, light, or pH. We have developed a thermally switchable DMSO-like solvent which, after separation, can be easily removed and subsequently recycled by splitting into two gas molecules. We have also developed a series of reversible ionic liquids where a moderately polar organic compound (amidine, guanidine) can be readily turned into an ionic liquid by contact with CO2, then reversed by removal of the CO2. Such solvents offer a host of new extraction possibilities.