Deng et al. firstly reported the synthesis of cyclic carbonate over
ionic liquids 1-butyl-3-imidazolium tetrafluoroborate ([C4-
mim]BF4) [99]. They also found that either cation or anion affects
the activity of the ionic liquid. The activity for propylene carbonate
synthesis decreased in the order of imidazolium > pyridinium and
of BF4 > Cl > PF6. Kawanami et al. [100] investigated the cyclic
carbonates synthesis over various 1-alkyl-3-methylimidazolium
salts ([Cn-mim]X) under scCO2 and found that both anion and the
length of alkyl chain in the cation were important for the catalytic
performance. The carbonate yield increased with the increasing of
the alkyl chain length (from C2 to C8) which might be attributed to
the higher solubility of both the epoxide and CO2 in the ionic liquid
having longer alkyl chain under the same pressure and temperature.
Moreover, influence of CO2 pressure on the yield also
depended on the alkyl chain length, e.g. for [C8-mim]BF4, with the
CO2 pressure reduced from 14 MPa to a subcritical pressure of
6 MPa, a remarkable decrease of the yield was also observed.
It was also found that the presence of Lewis acidic compounds
as cocatalysts greatly enhanced the activity of ionic liquid for the
cyclic carbonate synthesis. Kim et al. [101] showed that with the
addition of ZnBr2, the catalytic activities of ionic liquid [C4-mimi]Cl or [C4-mim]Br for the reactions of CO2 with ethylene
oxide and propylene oxide can surprisingly be improved, although
zinc bromide alone showed no activity for the reactions. It might be
due to the in situ formation of bis(1-butyl-3-methylimidazolium)
zinc tetrahalides from ZnBr2 and [C4-mim]Cl or [C4-mim]Br.