Switchable solvent systems

Clearly this is the least mature field within the solvent alternatives arena however, this also means that, as with tailor-made ionic liquids, it is likely that tailor-made switchable solvent systems will continue to advance and become an increasingly important area of research during the coming decades. As with all areas of clean technology, synergies and overlaps with other areas of sustainable development will increase and lead to new advances. For example, in the area of gas expanded liquids, the focus has so far been on petroleum-sourced VOCs and therefore significant advances could be made by investigating other types of gas expanded media, whether they be renewably sourced VOCs or non-volatile alternatives. 

The recovery of the ionic liquid l-ethyl-3-methylimidazolium acetate (abbreviated as [C2mim][OAc]) with a solvent mixture containing acetone, 2-propanol, and a small amount of water was demonstrated. This formed a phase switchable solvent system, which could concentrate and separate oleophilic solutes, short chain carbohydrates, and lignin fragments from used ionic liquid in a two-stage extraction procedure. Figure 7.5 shows the whole process of ionic liquid recovery and biomass separation. The research was conducted by pre-treating 100 g of corn stover with 10 wt.% of ionic liquid. After the separation process, the recovered ionic liquid contained few residual solutes, so it could be reused for biomass dissolution and... 

A significant advantage that these switchable solvents have over many other media is that they can be tailor-made for a particular process and particular properties can be turned on and off as desired. Unfortunately, this means that in most cases they will be considerably more expensive than simple alternatives such as water. Often the switch in these systems is the introduction of a gas such as carbon dioxide, and although the pressures involved are typically lower than those used for supercritical conditions, many users would still be wary about using and containing these gases. Further information on the switching mechanism for several cases is provided below. 

Soluble In organic solvent Scheme 7.2 Switchable catalyst system based on the ligand Tris-SwitchPhos. 

Another approach to polymer/catalyst separation is to replace the use of costly wasteful solvents with a procedure that produces little to no waste, and requires no catalyst modification. These switchable polarity solvents (SPS), as developed by Jessop and coworkers, are able to utilize the reaction of secondary amines with C02 to reversibly form carbamate salt ionic liquids (Scheme 8.10) [66]. Upon the completion of the polymerization reaction, EtBuNH (Et = ethyl, Bu = n-butyl) is added to dissolve the polymer/catalyst mixture. The subsequent bubbling of C02 through this solution results in a precipitation of the polymer product, such that the purified polymer can be isolated by simple filtration. The catalyst can then be recovered by distilling the amine/carhamate mixture, and the entire system can (in theory) be recycled (Scheme 8.11). 

The host guest chemistry of crown ethers continues to be exploited for the development of new ionophores for the selective recovery of Hg(II). A novel crown ring system containing a redox switchable trithiadiazapentalene/trithiotriuret unit (17/18) allows control of Hg(II) in solvent extraction experiments between chloroform and water. The thiocar-bonyl sulfur donor sites outside the macrocycUc cavity of (18) are responsible for strong complexation and extractabiUty into chloroform. 

After the reaction two clear liquid phases occur (Figure 3) that can easily be separated. The upper phase is the pure product - no solvent is needed any more - and the lower is the pure IL [2,6]. HMIMCl as an IL has a great advantage over the classical dialkylated systems it can be switched on and off just by protonation and deprotonation. This is crucial when recycling and purification of the ILs are considered. To distinguish the switchable ILs - the HMIM - from the conventional ones, Freeman tie used the term smart ionic liquids in the context of the BAS 1L process [4]. 

Figure 24 Switchable helical structu res of laterally tethered nanorods, (a) Model system composed of a bilayer sheet of laterally tethered nanorods. (b)-(< ) Structural evolution of helical structures as a function of timeforthree different solvent conditions, noting the pitch and radius of each helix atthe final time. R, rod T, tether. Adapted from Nguyen, T. D. Glotzer, S. C. Sma//2009,5(18), 2092-2098. Reproduced by permission of The Royal Society of Chemistry.

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