Are ionic liquids eco-friendly?

Ionic liquids are molten salts that have excellent chemical and thermal stability. Because they have qualities inherent to some precepts of green chemistry, they are identified as a potential substitute for traditional organic solvents. These useful physical and chemical properties have led to several promising applications in renewable energy technologies. Despite being classified as green solvents, these neoteric solvents present a number of problems related to their synthesis, toxicity and biodegradability, making their use questionable environment-wise. In this review, we analyzed the processes of synthesis, recovery and recycling, toxicity and biodegradability of different ionic liquids. A comparative analysis with some fossil-based solvents and organic-based green solvents was also undertaken. Through the construction of synthesis trees, it was found that all ionic liquids analyzed presented some problematic stage, mainly due to the use of volatile compounds containing C, N, S and halogens. On the other hand, several eco-friendly methodologies have been used for recovery and recycling, such as ultrafiltration and water extraction. One of the main critical points is related to toxicity and biodegradability, as most ionic liquids currently used are toxic and poorly biodegradable or non-biodegradable. Due to their infinite combination, computational modeling studies combined with life cycle assessment studies may, in the future, design new eco-friendly ionic liquids that largely comply with the 12 principles of green chemistry.