Ionic Liquids– Classes and Properties: Ionic liquids recycling for reuse

Several procedures for the recycling of ILs have been reported in the literature. Depending on the IL used and the application performed on it, a variety of recycling methods were possible. By picking the right purification steps, an individually optimized work-up procedure can be obtained [Wasserscheid & Welton, 2008]. In this survey several procedures have been reported on how to recover ionic liquids from their solution. Among these methods are distillation/stripping at some suitable temperature (<300 oC) and under vacuum, liquid-liquid extraction (using objectionable VOCs!), supercritical fluids (using CO2 at room temperature and several MPa), and membrane separation (to separate nano size particles from ILs). It was clear that ionic liquids will not provide advantages in all systems, but improvements in reactivity or selectivity are observed in many cases when the appropriate combination of cation and anion are selected [Gordon, 2001]. Despite this, the two factors that will ultimately decide whether reaction systems, when ILs are used, are viable on a larger scale are likely to be the ability to reuse the catalyst without a decrease in its activity, and whether the products can be separated efficiently without contamination from the ionic liquid or the catalyst. The combination of scCO2 and ionic liquids seems to be a very promising approach to attain this goal. The simultaneous efficient nano-filtration recycling of ILs and homogeneous catalysts extends the possibilities of the practical application of these media in organic synthesis [Volkov et al. 2008]. Destruction of ILs and removal of extracted organic contaminants by photolytic degradation is possibly another method of recycling ILs [Yang & Dionysiou, 2004; Khodadoust et al., 2006]. Due to the current market size and the relatively high cost of ILs, the industrial production of ionic liquids is still small or limited to lab-scale (and sometimes pilot scale) applications and hence no industrial technology is yet available for ionic liquids recycling with the objective of reuse. Most of the published works on ionic liquids recycling are made at the bench scale where the investigators are trying to extract a solute from a mixture using some suitable ionic liquid then recover the ionic liquid for further reuse. The number of trial cycles in most cases was very few (3 to 5). Some other researchers were trying to recover a catalyst from a reaction mixture where some ionic liquid was used. In this case, both the catalyst and the ionic liquid have to be recovered and recycled to the process for further cycles of the reaction. Again, the number of cycles was, in most cases, between 3 and 5. Someone might say: If the customer does not feel comfortable with the task of recycling the IL, then why not rent or lease the IL rather than buy it? The customers, in this case, perform their application with the IL and send, the probably impure, IL back to the supplier, who has the expertise to recycle and clean it up [Wasserscheid & Welton, 2008]. This scenario could be interesting from an economic point of view for truly commercial applications on a large scale. But transferring of such possibly hazardous and contaminated materials from one place, or country, to another is not safe. Thus I think the recycling methods must be available and practicable for both parties; the customer and the supplier. In summary, many authors and researchers agree that we are only at the very beginning of understanding the recyclability of ILs based on the available literature in the various application fields. Understanding of ILs volatility, purity, stability, biodegradability and toxicity is necessary for their recovery, since this determines whether an IL can be sustainably developed. In other words, there is a long way to go before large-scale implementation of ILs. Hopefully, this review could provide some clues to support a great deal of future research on ILs recycling and reuse.