8 Commercial Production of Ionic Liquids
199
Ionic Liquids Formed by Brønsted Acids and Bases
Historically, Brønsted acid–base reactions were the first type of reactions leading
to ionic liquids, such as Walden’s “red oil” ethylammonium nitrate. In this case,
ionic liquids can be generated by a simple proton transfer from a Brønsted acid to a
Brønsted base. To form stable ionic liquids, the difference in the pK a values of the
acid and base should be above 10, otherwise the temperature-dependent equilibrium
between the starting materials and product will shift more and more to the side of the
starting materials. By this type of reaction, commercially relevant ammonium-based
ionic liquids, such as ethylammonium nitrate and formate, are prepared on a larger
scale [2].
Ionic Liquids Formed by Lewis Acids and Bases
In addition to Brønsted-type reactions, Lewis acid–base reactions are also of technical
relevance for the synthesis of tetrachloroaluminate-based ionic liquids, which are
used in catalysis [3] as well as for electrodeposition of metals [4]. In terms of safety,
these reactions are extremely challenging. The addition of a Lewis acid, such as
AlCl 3 , must be made in small portions under adequate stirring to avoid hot spots
or overheating. Furthermore, it is important to exclude even traces of water, since
AlCl 3 reacts violently and produces toxic and corrosive HCl. This can be avoided by
performing the complete process under an inert atmosphere.
Creating Anions via Reactive Intermediates
Some ionic liquids can be produced by reactions of reactive ionic intermediates,
which form neutral, nonionic side products during the reaction. One of the major
advantages of this route is that it is halogen-free. A disadvantage is the fact that
the solvent (often water or methanol) has to be removed by using time-consuming
vacuum technology.
Quaternary phosphonium, ammonium, numerous types of N-heterocycles, and
R,R
,R
-imidazolium hydroxides (R,R
,R
is H or -alkyl) can be converted into
numerous types of ionic liquids with alternative anions just by neutralization with
acetates, triflates, or hydrogen sulfates, just to name a few. Some less important
approaches involve hydroxide-based ionic liquids being synthesized from halidebased ionic liquids via ion-exchange resins (Sect. 8.2.2.2). In this context, it is worth
noting that an aqueous solution of 1,3-dialkylimidazolium hydroxide, which is of
particular commercial interest, is only stable up to concentrations of approximately
5–10 wt%. At higher concentrations, the C2 position is deprotonated to form the corresponding carbenes, which typically undergo further reactions, such as dimerization
(Scheme 8.1).
An elegant route is to synthesize (methyl) carbonate-based ionic liquids
(CBILS©), which are typically provided as 30 wt% solutions in methanol. They
199
Ionic Liquids Formed by Brønsted Acids and Bases
Historically, Brønsted acid–base reactions were the first type of reactions leading
to ionic liquids, such as Walden’s “red oil” ethylammonium nitrate. In this case,
ionic liquids can be generated by a simple proton transfer from a Brønsted acid to a
Brønsted base. To form stable ionic liquids, the difference in the pK a values of the
acid and base should be above 10, otherwise the temperature-dependent equilibrium
between the starting materials and product will shift more and more to the side of the
starting materials. By this type of reaction, commercially relevant ammonium-based
ionic liquids, such as ethylammonium nitrate and formate, are prepared on a larger
scale [2].
Ionic Liquids Formed by Lewis Acids and Bases
In addition to Brønsted-type reactions, Lewis acid–base reactions are also of technical
relevance for the synthesis of tetrachloroaluminate-based ionic liquids, which are
used in catalysis [3] as well as for electrodeposition of metals [4]. In terms of safety,
these reactions are extremely challenging. The addition of a Lewis acid, such as
AlCl 3 , must be made in small portions under adequate stirring to avoid hot spots
or overheating. Furthermore, it is important to exclude even traces of water, since
AlCl 3 reacts violently and produces toxic and corrosive HCl. This can be avoided by
performing the complete process under an inert atmosphere.
Creating Anions via Reactive Intermediates
Some ionic liquids can be produced by reactions of reactive ionic intermediates,
which form neutral, nonionic side products during the reaction. One of the major
advantages of this route is that it is halogen-free. A disadvantage is the fact that
the solvent (often water or methanol) has to be removed by using time-consuming
vacuum technology.
Quaternary phosphonium, ammonium, numerous types of N-heterocycles, and
R,R
,R
-imidazolium hydroxides (R,R
,R
is H or -alkyl) can be converted into
numerous types of ionic liquids with alternative anions just by neutralization with
acetates, triflates, or hydrogen sulfates, just to name a few. Some less important
approaches involve hydroxide-based ionic liquids being synthesized from halidebased ionic liquids via ion-exchange resins (Sect. 8.2.2.2). In this context, it is worth
noting that an aqueous solution of 1,3-dialkylimidazolium hydroxide, which is of
particular commercial interest, is only stable up to concentrations of approximately
5–10 wt%. At higher concentrations, the C2 position is deprotonated to form the corresponding carbenes, which typically undergo further reactions, such as dimerization
(Scheme 8.1).
An elegant route is to synthesize (methyl) carbonate-based ionic liquids
(CBILS©), which are typically provided as 30 wt% solutions in methanol. They