Environmentally-Benign Ionic Liquid Production: Mechanistic Understanding and Novel Synthesis Methods.
Project Summary: Ionic liquids are environmentally friendly solvents due to their lack of vapor pressure and molecularly tunable properties. Reports of their synthesis almost always include the very solvents that they will purportedly replace. Moreover, ionic liquids are currently too costly for utilization as alternative solvents in large-scale industrial processes. This is primarily due to small batch production and little kinetic and thermodynamic data of their synthesis. For ionic liquids to be truly green and to be used ubiquitously, they must be made in a corresponding benign way in potentially large quantities and for low cost. This project addresses these problems by first optimizing conventional synthesis routes down to the molecular level and then utilizing novel compressed or supercritical CO2 production as an environmentally benign alternative for a continuous reaction and separation scheme.
Intellectual Merit: Ionic organic compounds are usually formed by quaternization of nucleophilic N- and P-containing compound (e.g. imidazoles, etc.) with an electrophilic reactant (e.g. haloalkane, etc.). These new organic salts are used as an IL solvent or undergo an additional anion exchange reaction. Each of these steps has its own kinetic value and has its own separation issues. From preliminary data, the quaternization step is very solvent dependent and appears to have non-2nd order kinetic rates. This project will explore the effect of liquid solvent structure on the reaction rates and mechanism to optimize its production rate. Synthesis in compressed or supercritical fluids may be the optimal method for production and separation with clear advantages over current technology. The reaction rate can be tuned and optimized in compressed CO2 by changes in temperature, pressure, concentration and number of phases present (i.e. phase equilibria). A one-vessel reactor/separator may be possible as ionic liquids are immeasurably insoluble in compressed CO2, but the reactants can be made miscible by understanding their phase behavior. A CO2 anion exchange process is planned using CO2 to perform the difficult separation of the desired ILs from an aqueous anion exchange solution. Compressed CO2 may also be capable of removing residual organics and metal halide salt impurities with the proper choice of CO2-soluble metal-complexing agents. This research will extend these batch reactions to continuous reactor/separator scenarios.
Broader Impact: A thorough understanding of the fundamentals of ionic liquid synthesis and processing and subsequent reactor/process engineering will lead to a less expensive IL solvent that is produced in an environmentally-benign manner. The increased use of ionic liquids in a variety of industries could lead to a decrease in the exposure of both humans and the environment to conventional volatile solvents. Compressed CO2 or, in some cases, a wise choice of conventional solvents may lead to a time where the full environmental advantage of ionic liquids may be realized. This project will introduce students to the necessary research and analysis skills to develop an environmentally benign alternative and critically compare it to conventional technology. Moreover, the entire process will lead to incorporation of case studies in current courses at KU: Environmentally-Benign Reaction Engineering, and Environmental Assessment of Chemical Processing.
