On the way to greener ionic liquids: identification of a fully mineralizable phenylalanine-based ionic liquid

Over the past few decades ionic liquids (ILs) are increasingly seen as an important building block of green chemistry because of their specific properties as solvents, such as their potential for high recyclability, low volatility, low flammability, low toxicity, and their potential for synthesis from renewable resources. However, avoiding persistent or toxic cation/anion fragments is also urgently needed. In the best case they should be fully mineralizable by microorganisms after their release into the aquatic environment. The fragments fostering this can be determined by biodegradation studies, and the employment of identified readily biodegradable building blocks presents an innovation in the targeted design of green environmentally friendly ILs. The aim of this study was to improve the data-platform for the design of completely mineralizable ILs. Therefore the ready biodegradability of seven phenylalanine-based ILs and three nonionic related compounds was investigated with a modified Closed Bottle test based on OECD guideline 301D. Liquid chromatography combined with high-resolution mass spectrometry (LC-HRMS) analysis was used to identify the chemical structures of products resulting from incomplete biodegradation and transformation. Two kinds of degradation pathways were observed: the hydrolysis of an ethyl ester group or the hydrolysis of an amide bond and biodegradation of the released phenylalanine ethyl ester. Both degradation pathways resulted in persistent transformation products (TPs) with the exception of IL (4), a pyridinium substituted phenylalanine derived IL and the non-ionic deanol derivative (2a). IL (4) was ultimately biodegraded in the CBT after 42 days without leaving any TP. The biodegradation of compound (2a) was 78% after 42 days but resulted in a TP, which was readily biodegradable in a further CBT after a lag phase of 3 weeks, respectively. Even if both compounds were not "readily biodegradable" in the sense of the OECD guideline, particularly IL (4) can be proposed as a basic structure for sustainable and green ILs (benign by design) with the aim of optimizing its degradation rate further.