Towards greener and sustainable ionic liquids using naturally occurring and nature-inspired pyridinium structures

Abstract

The present study investigated the ready biodegradability of naturally occurring and nature-inspired pyridinium cations for the design of greener or sustainable ionic liquids (ILs). Our results showed that trigonelline ([C₁COOHPy][Cl]) and 1-methylnicotinamide ([C₁CONH₂Py][I]) were completely mineralizable and non-toxic under the conditions of the Closed Bottle Test (CBT), while [C₁CONH₂Py][I] was readily biodegradable. In contrast, the nicotinium structures, 1-methylnicotinium and 1,1′-dimethylnicotinium, were biologically persistent but non-toxic to the inoculum, whereas S-nicotine itself was identified as readily biodegradable. The biodegradability of these pyridinium structures was compared with that of the commercial pyridinium ILs [C₁Py][PF₆] and [C₂Py][Br] having short alkyl residues. While [C₂Py][Br] demonstrated only partial but not ultimate biodegradation, [C₁Py][PF₆] was shown to be readily biodegradable in the CBT. HRMS confirmed the biodegradation results. The results also showed the negative influence of short alkyl residues on biodegradability. Based on the results, new highly biodegradable and non-toxic pyridinium ILs can be designed to limit the accumulation of persistent ILs in the aquatic environment. Moreover, since some of our pyridinium structures and all their pyridine precursors can be isolated either from renewable sources or waste materials, ILs based on [C₁COOHPy][Cl], [C₁CONH₂Py][I] and [C₁Py][PF₆] as cationic scaffolds can also be referred to as sustainable ILs if the anion fulfills the criteria too. Besides their physicochemical and environmental properties, the use of such methylated pyridinium blocks offers the opportunity for biotechnological synthesis using S-adenosyl methionine as the methyl donor catalyzed by N-methyltransferases and therefore the avoidance of commonly used carcinogenic methylating agents such as iodomethane or dimethyl sulfate. Thus, the synthesis of these ILs can be improved towards sustainability and the principles of Green Chemistry (e.g., 3^rd less hazardous chemical synthesis, 9^th catalysis, and 12^th inherently safer chemistry for accident prevention) and an inherently safer pathway to ILs can be created.