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T. J. S. Schubert
8.2.3 Purification of Ionic Liquids
As mentioned in Sect. 8.2.1, once an ionic liquid is synthesized, its purification is
often challenging. Nevertheless, there are methods available, but they may require a
significant effort.
8.2.3.1 Removing Typical Ionic Impurities
Extraction
For hydrophobic ionic liquids, it is relatively easy to remove ionic impurities, such as
inorganic salts, using the anion-exchange process and washing with purified water.
The halide content of the water determines the quality of each ionic liquid. Depending
on the specific water uptake of each hydrophobic ionic liquid at a given temperature,
water must be removed at reduced pressure after the washing procedure (see also
Sect. 8.2.3.2).
Removing ionic impurities from hydrophilic ionic liquids is of enormous interest,
but it is generally difficult to achieve since washing with water is of course not
an option. In our company (IOLITEC), many different approaches are currently
under investigation at the lab scale, but until they are evaluated in terms of potential
submission of intellectual rights, it is not possible to report on them.
On a larger scale, extraction processes that can be continuously operated are of
interest. In our own labs, we investigated the potential of counterflow extraction for
hydrophobic ionic liquids with the result that it becomes feasible on larger scale
when compared to batch extraction as shown in Fig. 8.2, which is in principle just a
larger version of a separatory funnel.
Another option, which can be operated continuously, is mixer–settler systems,
which are commonly used in mineral processing.
Recrystallization
If a product is not a room-temperature ionic liquid or an ionic liquid intermediate,
recrystallization and zone melting are useful purifications methods. For any kind of
recrystallization, a suitable solvent must be identified for each ionic liquid. In terms
of costs, this method should be avoided if possible, since it suffers from a significant
yield loss and extra cost associated with solvents and drying.
Zone Melting
Zone melting is a process that is typically applied in purifications of semiconductors
to produce ultrapure silicon. It is an alternative to recrystallization and can thus
T. J. S. Schubert
8.2.3 Purification of Ionic Liquids
As mentioned in Sect. 8.2.1, once an ionic liquid is synthesized, its purification is
often challenging. Nevertheless, there are methods available, but they may require a
significant effort.
8.2.3.1 Removing Typical Ionic Impurities
Extraction
For hydrophobic ionic liquids, it is relatively easy to remove ionic impurities, such as
inorganic salts, using the anion-exchange process and washing with purified water.
The halide content of the water determines the quality of each ionic liquid. Depending
on the specific water uptake of each hydrophobic ionic liquid at a given temperature,
water must be removed at reduced pressure after the washing procedure (see also
Sect. 8.2.3.2).
Removing ionic impurities from hydrophilic ionic liquids is of enormous interest,
but it is generally difficult to achieve since washing with water is of course not
an option. In our company (IOLITEC), many different approaches are currently
under investigation at the lab scale, but until they are evaluated in terms of potential
submission of intellectual rights, it is not possible to report on them.
On a larger scale, extraction processes that can be continuously operated are of
interest. In our own labs, we investigated the potential of counterflow extraction for
hydrophobic ionic liquids with the result that it becomes feasible on larger scale
when compared to batch extraction as shown in Fig. 8.2, which is in principle just a
larger version of a separatory funnel.
Another option, which can be operated continuously, is mixer–settler systems,
which are commonly used in mineral processing.
Recrystallization
If a product is not a room-temperature ionic liquid or an ionic liquid intermediate,
recrystallization and zone melting are useful purifications methods. For any kind of
recrystallization, a suitable solvent must be identified for each ionic liquid. In terms
of costs, this method should be avoided if possible, since it suffers from a significant
yield loss and extra cost associated with solvents and drying.
Zone Melting
Zone melting is a process that is typically applied in purifications of semiconductors
to produce ultrapure silicon. It is an alternative to recrystallization and can thus