The catalytic performances of POM–ILs were investigated in the esterification of oleic acid with methanol (Table 1). The typical GC-MS chromatograms of oleic acid sample before and after the esterification reaction are shown in Fig. 2. The composition of the sample was oleic acid (82.54 wt.%), myristic acid (3.54 wt.%), palmitic acid (6.17 wt.%),palmitoleic acid (5.83 wt.%) and stearic acid (1.92 wt.%). The catalytic activities were expressed as yields of esters. [MIMPS]PW and [QPS]PW catalysts are more active than the others (Table 1).
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 esteification reaction (please see Table 1).