Effect of Structure of Room Temperature Ionic Liquids on Organic Reactions Involving Electrochemically Generated Superoxide Ions

University South Carolina Research Foundation, Columbia SC

Investigators

Abstract

ABSTRACT PI: John W. Weidner, Michael A. Matthews and John Monnier Institution: University of South Carolina Proposal Number: 0500032 Title: Effect of Structure of Room Temperature Ionic Liquids on Organic Reactions Involving Electrochemically Generated Superoxide Ion Project Summary: Intellectual merit: This project is aimed at providing the scientific basis for a novel and sustainable approach for the selective, partial oxidation of organic substrates. The approach utilizes electrochemical means to generate the superoxide ion (O2o-) in highly conducting, non-volatile room-temperature ionic liquid (RTIL) solvents, and then utilizes O2o- to carry out subsequent homogeneous reactions. The premise of this work is that the rational design of RTIL structures will allow selective control of O2o- formation and subsequent partial oxidation. Therefore, the scientific objectives of this project are to identify RTILs that are stable in the presence of electrochemically generated O2o-, conduct certain organic oxidation reactions in broad classes of RTILs, and relate the structure of the anion and cation of the RTIL to the rate and yield of these reactions. This will involve identifying reaction products and measuring intrinsic kinetics of selected organic reactions involving O2o-. The three classes of organic reactions of interest are: (1) carboxylic acids and ketones, produced by oxidation of primary and secondary alcohols, respectively; (2) carbonates and carbamates, produced from alcohols and amines, respectively, in the presence of carbon dioxide; and (3) oxidation of polyhalogenated aromatics, (e.g. polychlorinated biphenyls). The first step in all these reactions is the electrochemical generation of O2o- in RTILs. With prior support from an exploratory NSF grant, the PIs showed that a stable O2o- species can be generated in RTILs as long as the structure and purity of the RTIL are controlled. Further, O2o- reacts with benzyl alcohol, benzhydrol, carbon dioxide, and hexachlorobenzene to form the desired products. They also showed that small changes in the structure of the RTIL cation dramatically affect product yield. In addition, the preferred cation depends on the reaction. For example, adding a methyl group in the position 2 of the imidazolium ring increased the average yield of benzhydrol to benzophenone from 50% to over 98%. Equally significant was that no degradation of the RTIL was detected along with these high yields. In contrast, the reaction of O2o- with benzyl alcohol to form benzoic acid decreased the yield from 23% to 0.0%, indicating reaction inhibition caused by the RTIL. There is also a problem with the long-term stability of the PF6 anion used to date since it is subject to electrolysis and hydrolysis, producing hydrofluoric acid that reacts with O2o-. While the results to date are very promising, there is no a priori way to know which RTIL is appropriate, much less optimal, for a given reaction. To match a given reaction with an appropriate RTIL, there is a need to understand the effect of RTIL structure on homogeneous reaction rates and yield. It is desired to discover the fundamental knowledge that will identify RTILs that have both long-term stability in the presence of O2o- and favorable solvent catalytic properties. Therefore, the PIs plan to relate anion and cation structure to their role in product yield and intrinsic reaction rates, for representative reactions within three important classes of organic reactions in this project. This will enable a rational choice of an RTIL for a given reaction. Broader impacts: This work will accelerate the use of novel and potentially environmentally friendly strategies for electro-organic chemical syntheses. Not only are RTILs promising green solvents, but superoxide electrochemistry utilizes air or oxygen and electricity at room temperature. Thus the combination of RTIL technology with electrochemistry promotes the development of an environmentally friendly technology for either the manufacturing of organic intermediates, or the remediation of chlorinated aromatics. The project will be linked with a program to recruit minority Ph.D. students through Sloan Foundation Minority Doctoral Fellowships that have been in place for several years. Minority Ph.D. students at the University of South Carolina (USC) benefit from the USC African American Professors Program (AAPP), which pairs minority students and faculty mentors. USC undergraduates will participate in the NSF-Sponsored Research Communications Studio (NSF EEC 0212244, PI Dr. Michael Matthews), which provides instruction in technical publishing and presenting. Undergraduates from other universities will participate through the ongoing NSF Research Experience for Undergraduate (REU) program in the Department of Chemical Engineering in the area of pollution prevention (NSF-EEC-0097695, PI Dr. John Weidner).