2.3. Determination of the Hammett acidity function of catalyst
Hammett method was used for determination of the catalysts acidities [27]. The POM–IL catalyst and 4-nitroaniline were dissolved in high purity water at concentrations of 0.1 and 2.0 mmol/L respectively. The solution was shaken vigorously and left to stand for 6 h.Then UV–vis spectra were recorded. Hammett acidity function (H0) can be calculated by the equation: H0 = pKa + log[I]/[IH+]. Here,“I” represents the indicator base, and [IH+] and [I] are the molar concentrations of the protonated and unprotonated forms of the indicator, respectively. The pKa value of 4-nitroaniline is 0.99.