Acidity of POM–ILs is a major factor that should be considered in
their catalytic application. The Brönsted acidity was determined
using Hammett method. H0 values of the catalysts are listed in
Table 2. We could summarize the acidity order of the catalysts:
[MIMPS]PW N [QPS]PW N [PyPS]PW N [TMAPS]PW. This order could
be found consistent with the order of the catalytic activity in the esterification
reaction (please see Table 1).