However, upon cooling, [QPS]PW and [PyPS]PW were precipitated
from the reaction mixture and their solubilities were gradually decreased
due to their relatively low solubilities in methanol (Table 2,
entries 3 and 4). The precipitates were separated from the reaction
mixture by centrifugation, dried and weighed. The recyclabilities of
the catalysts based on wt.% of precipitate were reported in Table 2.
As can be seen, [QPS]PW showed desired temperature-dependence
solubility (Table 2, entry 4). This catalyst was soluble in the reaction
mixture at 65 °C (as a homogeneous catalyst) and precipitated quantitatively
at the end of the reaction upon cooling. Therefore, it can be
separated easily as a heterogeneous catalyst. This catalyst was used
as the best one in further investigations.