Radiation Stability of Cations in Ionic Liquids. 1. Alkyl and Benzyl Derivatives of 5-Membered Ring Heterocycles

Abstract

In order to use hydrophobic room temperature ionic liquids (ILs) as diluents in nuclear separations for advanced fuel cycles, it is desirable to reduce the breakdown of the constituent ions caused by ionizing radiation. In this series, we survey radiation stability for different classes of organic cations used to formulate ILs. While radiolysis of 1-alkyl-3-methylimidazolium cations has been extensively studied, there have not been complementary studies of 1-benzyl derivatives of these cations nor organic cations that are derived from 5-membered ring heterocycles other than imidazole, such as 1,2,4-triazole and thiazole. In part 1, we establish the fragmentation pathways for such cations and quantify product yields for 2.5 MeV electron beam radiolysis of these aromatic cations. Radiolytic reduction of 1-benzyl cations derived from imidazole and 1,2,4-triazole is shown to cause the elimination of benzyl radicals from their electron adducts, whereas this elimination does not occur in the thiazole derivatives due to stabilization of the excess electron as a dimer radical cation. No such elimination occurs in the corresponding 1-alkyl derivatives, but there is significant C-N and C-C bond fragmentation in the aliphatic arms. As such bond dissociation reactions are irreversible, there is significant loss of 1-alkyl cations during the radiolysis. For 1-benzyl derivatives, this electronic excitation causes fragmentation of the C-N bonds in the benzyl arms with the release of the corresponding base and the benzyl carbocation that can subsequently attack this base or add to another cation. Such systems exhibit more predictable fragmentation patterns and yield well-defined products; some of the systems also exhibit increased radiation resistance. The C-N bond fragmentation in the reduced cations can be further suppressed through the use of appropriate electron scavengers, including acids and aromatic imide anions. The observed trends are rationalized using density functional theory calculations, and the implications of these results for the design of IL diluents are examined.