72 If a hydrophilic product is obtained in a hydrophobic IL water may be used

72 if a hydrophilic product is obtained in a

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72 If a hydrophilic product is obtained in a hydrophobic IL, water may be used to remove the product from the IL. 44 The biggest problem is to recover nonvolatile or thermo-sensitive products from ILs. Even though organic solvents (such as diethyl ether, 73 hexane 73 and ethyl acetate 74 , 75 ) can be effective in extracting the product, this approach obviously compromises the ultimate goal of ‘green’ technologies. To overcome this limitation, another type of ‘green’ solvent, supercritical fluids (SCFs), has been adapted for product recovery from ILs. Among SCFs, supercritical CO 2 ( scCO 2 ) is readily available and has been a mature technology for decaffeination, extraction of natural products and dry cleaning. 76 The volatile and nonpolar scCO 2 forms interesting two-phase systems with nonvolatile and polar ILs. The principle of product recovery by these biphasic systems is based on the solubility of scCO 2 in the IL (controlled by pressure) to transfer organic products to the scCO 2 -rich phase, and the insolubility of the IL in scCO 2 . For example, after extracting naphthalene from [BMIM][PF 6 ] with scCO 2 , 77 the same research group successfully recovered a variety of aromatic and aliphatic compounds from the same IL with scCO 2 . 78 The phase behavior of IL/scCO 2 systems was further established. 79 It has also been demonstrated that by using low- pressure CO 2 (< 5 . 2MPa ) both hydrophobic and hydrophilic ILs can be separated from aqueous solution. 80 This finding is important for the effective separation of water from IL aqueous solutions after reactions or other processes complete. Even, fascinatingly, a three-phase system was formed by [BMIM][PF 6 ]–MeOH–scCO 2 , which allows the solute separation from ILs, even under dilute conditions (Fig 1). 81 The solubility of ILs in pure scCO 2 is normally very low. 82 , 83 However, when scCO 2 contained high contents ( > 10mol%) of polar solutes (eg ethanol, acetone), the solubility of [BMIM][PF 6 ] in scCO 2 was quite high. 82 This finding presented a challenge when trying to remove a high concentration of polar organics from ILs. Fortunately, organic solutes can also be recovered through the pervaporation using nonporous membranes with the selective layer consisting of hydrophilic or hydrophobic polymers. 84 , 85 More recently, the biphasic IL/scCO 2 systems have been used in metal-catalyzed and enzyme- catalyzed reactions. This unique combination of reaction and separation offers unique advantages: (a) organometallic and enzymatic catalysts are soluble and stable in ILs, but have low solubility in scCO 2 ; Figure 1. Phase behavior of IL/H 2 O/CO 2 mixture at room temperature. 81 1092 J Chem Technol Biotechnol 80 :1089–1096 (2005)
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Use of ionic liquids as ‘green’ solvents for extractions (b) many organic compounds are soluble in scCO 2 , enabling easy separation of products from ILs; (c) this process can be designed as batchwise or continuous operations. Examples of metal-catalyzed organic reac- tions include batchwise asymmetric hydrogenation of tiglic acid, 86 batchwise hydrogenation of alkenes and carbon dioxide, 87 continuous flow hydroformylation of oct-1-ene catalyzed by rhodium complexes, 88 , 89
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