194
T. J. S. Schubert
8.2 Methods for the Commercial Production of Ionic
Liquids
From the thousands of ionic liquids reported in the literature over the past two
decades, only a few percent (less than 50) have become commercially relevant. In
addition, because of their complexity, it is not possible to include all methods within
this chapter.
8.2.1 Purification of Starting Materials
As mentioned in Sect. 8.1.1, to achieve sufficient quality for ionic liquids, it is necessary to work with the highest quality starting materials. Once a reaction is completed,
it is typically difficult and cost-intensive to remove all nonvolatile impurities from the
ionic liquid because of its ultra-low vapor pressure. As a consequence, it is essential
to work with purified starting materials with a purity greater than 98%.
8.2.1.1 Purifications of Amines and N-Heterocycles
By far, most ionic liquids are based on nitrogen-containing cations, such as ammonium, pyrrolidinium, pyridinium, and imidazolium being the most prominent. Most
of these materials show a tendency to react with CO 2 in the air to form carbonates.
Thus, to achieve the best results, those impurities can be removed by distillation
over potassium hydroxide. Since amines and in particular N-heterocycles often have
high boiling points, the distillation should typically be operated under reduced pressure. To avoid any further contamination with CO 2 , the use of an inert atmosphere
is beneficial.
8.2.1.2 Purifications of Alkyl Halides
Many alkyl halides typically contain stabilizers, such as hydroquinone, that can be
removed easily by stirring with 5–10 vol.% concentrated sulfuric acid. After settling,
a dark liquid can be separated.
8.2.1.3 Purification of Inorganic Salts
Most of the inorganic salts (sodium, potassium, and ammonium cations and hexafluorophosphate, tetrafluoroborate, and bis(trifluoromethylsulfonyl)imide anions) can be
used without any further purification. If higher purities are needed, recrystallization
or zone melting are the methods of choice.
T. J. S. Schubert
8.2 Methods for the Commercial Production of Ionic
Liquids
From the thousands of ionic liquids reported in the literature over the past two
decades, only a few percent (less than 50) have become commercially relevant. In
addition, because of their complexity, it is not possible to include all methods within
this chapter.
8.2.1 Purification of Starting Materials
As mentioned in Sect. 8.1.1, to achieve sufficient quality for ionic liquids, it is necessary to work with the highest quality starting materials. Once a reaction is completed,
it is typically difficult and cost-intensive to remove all nonvolatile impurities from the
ionic liquid because of its ultra-low vapor pressure. As a consequence, it is essential
to work with purified starting materials with a purity greater than 98%.
8.2.1.1 Purifications of Amines and N-Heterocycles
By far, most ionic liquids are based on nitrogen-containing cations, such as ammonium, pyrrolidinium, pyridinium, and imidazolium being the most prominent. Most
of these materials show a tendency to react with CO 2 in the air to form carbonates.
Thus, to achieve the best results, those impurities can be removed by distillation
over potassium hydroxide. Since amines and in particular N-heterocycles often have
high boiling points, the distillation should typically be operated under reduced pressure. To avoid any further contamination with CO 2 , the use of an inert atmosphere
is beneficial.
8.2.1.2 Purifications of Alkyl Halides
Many alkyl halides typically contain stabilizers, such as hydroquinone, that can be
removed easily by stirring with 5–10 vol.% concentrated sulfuric acid. After settling,
a dark liquid can be separated.
8.2.1.3 Purification of Inorganic Salts
Most of the inorganic salts (sodium, potassium, and ammonium cations and hexafluorophosphate, tetrafluoroborate, and bis(trifluoromethylsulfonyl)imide anions) can be
used without any further purification. If higher purities are needed, recrystallization
or zone melting are the methods of choice.