Following the initial
cation formation, the synthesis of ionic
liquids (ILs) often involves an anion-exchange or metathesis reaction.
For hydrophobic ILs, this is generally performed through several cross-current
contacts of the IL with a fresh salt solution of the desired anion.
However, if a large number of contacts is required to attain an adequate
conversion, this procedure is not economical because of the large
excess of the reagent that is consumed. In this study, the metathesis
of an IL, Aliquat 336 or [A336][Cl], to ILs with other anions ([A336][X]
with X = HSO
4
–
, Br
–
, NO
3
–
, I
–
, and SCN
–
) was studied in a continuous counter-current mixer-settler
setup. McCabe–Thiele diagrams were constructed to estimate
the required number of stages for quantitative conversion. Significantly
higher IL conversions were achieved, combined with reduced reagent
consumption and waste production. This improvement in efficiency was
most pronounced for anions placed low in the Hofmeister series, for
example, HSO
4
–
, Br
–
, and NO
3
–
, which are difficult to exchange.
The performance of the counter-current experiments was compared with
the conventional multistep cross-current batch process by calculating
the reaction mass efficiency (RME) and the environmental factor (E-factor).
The RMEs of the cross-current experiments were notably smaller, that
is, 38–78% of the values observed for the counter-current experiments.
The E-factors of the counter-current experiments were a factor of
2.0–6.8 smaller than those of the cross-current experiments.
These sustainability metrics indicate a highly efficient reagent use
and a considerable, simultaneous decrease in waste production for
the counter-current IL metathesis reactions.