A group contribution approach for predicting the environmental impacts of imidazolium-based ionic liquids

Abstract

In this work, the environmental impacts of imidazolium-based ionic liquids (ILs) are estimated and predicted by combining life cycle assessment (LCA) and a linear group contribution (GC) model to make this information readily available and accessible at early stages of ILs design and selection. This study has resulted in the development of predictive models for global warming, human toxicity and eco-toxicity impacts of imidazolium-based ILs with high accuracy. The validated models are based on the environmental impact data of a sample of 30 ILs with the same cation head group, which was estimated as part of this study using the LCA methodology. The sample considered variations in the cation's hydrocarbon chains, e.g. alkyl, aromatic, multiple side chain and chain length variations, and the commonly used anions chloride (Cl⁻), tetrafluoroborate ([BF₄]⁻) and hexafluorophosphate ([PF₆]⁻), and it was assumed that the ILs were synthesised via alkylation and salt metathesis reactions. Both LCA and GC models results show a clear distinction between the environmental impacts of imidazolium chloride, imidazolium tetrafluoroborate, and imidazolium hexafluorophosphate ILs in the order Cl⁻ < BF₄⁻ < PF₆⁻, thus indicating the large influence of the anion when it comes to their environmental sustainability performance. For example, the global warming potential of the ILs per kg are 2 and 3 times higher when substituting the chloride anion with [BF₄]⁻ and [PF₆]⁻ anions, respectively. It is recommended that the sample be expanded to include ILs from other head groups, compare alternative synthesis routes and include purification steps to extend the analysis.