A greener-still class of room temperature ionic liquids to replace toxic and flammable organic solvents has been devised by German chemists.
As has been reported previously many times, ionic liquids could make very useful replacements for volatile organic compounds because of their negligible vapour pressure and other factors, such as lack of flammability. However, most of them do have a hidden environmental cost that must be weighed into the equation when making the decision to utilise them for a particular reaction. They often rely on halide-based ions, [AlCl4]-, [PF6]-, [BF4]-, [CF3SO3]- or [(CF3SO2)2N]-, for instance.
Halides bring with them an inevitable waste problem later on when the ionic liquid is spent and beyond reaction recycling. “The presence of halogen atoms may cause serious concerns if the hydrolysis stability of the anion is poor (e.g., for [AlCl4]- and [PF6]–) or if a thermal treatment of spent ionic liquids is desired. In both cases additional effort is needed to avoid the liberation of toxic and corrosive HF or HCl into the environment,” the researchers explain.
Now, a new class of halogen-free ionic liquids has been developed by a German team at the Institute for Chemical Technology and Macromolecular Chemistry in Aachen. Their RTILs are based on the octylsulfate group as the anion rather than the conventional fluoro or chloro anions used by previous researchers. The researchers, Peter Wasserscheid, Roy van Hal, and Andreas Bösmann, point out that alkylsulfates are less expensive than halogen-based materials, they are also widely available and have a well-documented toxicology. But, perhaps more importantly, they will not lead to the ultimate release of toxic and ozone-eating halogens into the atmosphere at the end of the RTIL’s usable life. The team has demonstrated that their new class of room temperature ionic liquid has great promise as a novel solvent for example, transition metal catalyzed hydroformylation. The archetypal member of the group is 1-n-butyl-3-methylimidazolium ([BMIM]) octyl sulfate and represents a halogen-free and relatively hydrolysis stable RTIL. “In the hydroformylation of 1-octene using [BMIM][octylsulfate] a TOF of 892 1/h could be obtained in comparison to 276 1/h in the traditionally used ionic liquid [BMIM][PF6] under identical conditions,” Wasserscheid told Reactive Reports.
Wasserscheid, P., Hal, R., & B??smann, A. (2002). 1-n-Butyl-3-methylimidazolium ([bmim]) octylsulfate???an even ???greener??? ionic liquid Green Chemistry, 4 (4), 400-404 DOI: 10.1039/b205425f