Decomposition of ionic liquids with chloride anions: A combined study of the gas-phase dissociation of the isolated cations and pyrolysis of the bulk

Abstract

As the synthesis of diverse ionic liquids (ILs) proliferates and as (proposed) applications increase, there is growing concern about the possibility of finding IL components and degradation products in the environment, possibly as persistent and/or hazardous contaminants. Understanding IL stability and understanding what decomposition products arise when ILs do degrade will help us better understand potential environmental threats. While stability raises concern in terms of persistent environmental pollution, it is also one of the major strengths of ILs toward their many applications. From understanding degradation mechanisms that could be at play during use under extreme conditions to understanding the products that may form during incomplete incineration, a molecular-level understanding of IL thermal transformations is also desirable beyond the environmental concern. Ideally, such a molecular-level understanding could eventually lead to better predictions of thermal stability as a function of structure prior to synthesis and experimental characterization. In this work, the pyrolysis products of nine ILs, each with the chloride anion and various N-heterocyclic cations, were studied by gas chromatography-mass spectrometry and these results were compared to the unimolecular gas-phase dissociation behavior of the respective isolated cations. By comparing these two experimental approaches, differences between unimolecular decomposition pathways and bimolecular decomposition or transformation pathways could be explored. Further, these comparisons shed light on whether gas-phase dissociation of the isolated cation, which is a very straightforward experiment, could be used to provide any insights into bulk pyrolysis pathways. Overall trends, class-based trends, and behaviors specific to only certain species are identified and discussed. This work provides new molecular insights into the pyrolysis of ILs by studying an array of cations, including those with functionalized R-groups, and by integrating results from bulk pyrolysis with those from collision-induced dissociation of the isolated cation.