Supercritical used with ionic liquids

Supercritical fluids (e.g. supercritical carbon dioxide, scCCb) are regarded as benign alternatives to organic solvents and there are many examples of their use in chemical synthesis, but usually under homogeneous conditions without the need for other solvents. However, SCCO2 has been combined with ionic liquids for the hydroformylation of 1-octene [16]. Since ionic liquids have no vapour pressure and are essentially insoluble in SCCO2, the product can be extracted from the reaction using CO2 virtually uncontaminated by the rhodium catalyst. This process is not a true biphasic process, as the reaction is carried out in the ionic liquid and the supercritical phase is only added once reaction is complete. 

In their test system, the researchers used the ionic liquid l-butyl-3-methylimidazol-ium hexafluorophosphate (bmim)(PF6), which is stable in the presence of oxygen and water, with naphthalene as a low-volatility model solute. Spectroscopic analysis revealed quantitative recovery of the solute in the supercritical CO2 extract with no contamination from the ionic liquid. They found that CO2 is highly soluble in (bmim)(PF6) reaching a mole fraction of 0.6 at 8 MPa, yet the two phases are not completely miscible. The phase behavior of the ionic liquid-C02 system resembles that of a cross-linked polymer-solvent system (Moerkerke et al., 1998), even though... 

Recently, applications of lion-aqueous solutions in the field of modern electrochemical technologies are increasing. Books [1] and review articles [2] that deal with the technological aspects of non-aqueous electrochemistry have appeared. In this chapter, examples of such applications of non-aqueous solutions are outlined. In the last section, the electrochemical use of supercritical fluids and ionic liquids as environmentally benign media is also discussed. 

Unfortunately, most polymers are insoluble in supercritical C02, and hence extraction from the ionic liquid by this method is difficult. However, if C02-soluble polymers were synthesized (for example, fluoropolymers and polysi-loxanes), then this method has the potential to be a very useful approach. Moreover, supercritical fluid-swollen ionic liquids offer a new solvent system that combines the viscosity-lowering properties of the supercritical fluid with the good solubilizing properties of the ionic liquid and may be a hybrid exotic solvent of the future. 

This graph gives a selection of 14 (out of approximately 360) of the usual solvents above the baseline and seven more exotic solvents (supercritical CO2 and ionic liquids included) below. The 14 compormds, from left to the right with increasing solvent polarity, include apolar, aprotic (such as TMS, cyclohexene, or benzene), bipolar (such as acetone or DMF), and eventually bipolar, protic solvents (cyclo-hexanol, ethanol, phenol). Using the values, numerous solvent-dependent processes may be correlated with each other. Other measures that can be used for the estimation of miscibility/solvent power are the cohesive pressures, solubility parameters, dispersive forces, Kamlet-Taft parameters, etc. [6a,b]. Solvent combinations of exotic members and systems with more than two members are known and have been recommended, but their application has been concentrated in the lab because of economic disdavantages with their handling and recyclability/ separability [6b-e]. 

Another approach to the isolation of the product is to use supercritical CO2. It was observed that organic molecules can be easily extracted from ionic liquids to SCCO2, with no trace of the ionic solvents being detected in the supercritical fluid. Recently, procedures involving the use of ionic liquids/scC02 have been studied extensively they will be described in Section 4.4.2. ... 

The ionic liquids show excellent extraction capabilities and allow catalysts to be used in a biphasic system for convenient recycling. For example, the hydrovinylation of styrene with ethene can be carried out successfully using an ionic liquid and supercritical CO2 as solvent (Eq. 10-15). The ionic liquid dissolves the metal organic complex catalyst and SC-CO2 facilitates mass transfer and continuous processing. 

Use of supercritical fluid extraction (SFE), for a more effective separation of products from ILs Continuous bicycUc process of extraction of sulphur-containing aromatic compounds (SAs) from diesel fuel and gasoline with ionic liquids and reextraction of SAs from ILs with supercritical carbon dioxide (scCOj)... 

Palladium-catalyzed carbonylation of aryl halides with nucleophiles such as alcohols, amines, and water can be performed in ionic liquid media. Several systems have been designed so that the ionic phase can be isolated and recycled. Once carbonylation substrates/products form homogeneous mixtures with ionic liquids, the experimental protocols for catalyst/ionic liquid mixture recycling involve separation of the product by either distillation or extraction procedures using organic solvents or supercritical CO2. 

When ionic liquids are used as replacements for organic solvents in processes with nonvolatile products, downstream processing may become complicated. This may apply to many biotransformations in which the better selectivity of the biocatalyst is used to transform more complex molecules. In such cases, product isolation can be achieved by, for example, extraction with supercritical CO2 [50]. Recently, membrane processes such as pervaporation and nanofiltration have been used. The use of pervaporation for less volatile compounds such as phenylethanol has been reported by Crespo and co-workers [51]. We have developed a separation process based on nanofiltration [52, 53] which is especially well suited for isolation of nonvolatile compounds such as carbohydrates or charged compounds. It may also be used for easy recovery and/or purification of ionic liquids. 

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