A comparative investigation of palmitic acid esterification over p-sulfonic acid calix[4]arene and sulfuric acid catalysts via 1H NMR spectroscopy

This work reports the novel results of p -sulfonic acid calix[4]arene-catalyzed esterification reactions of palmitic acid with deuterated methanol. The p- sulfonic acid calix[4]arene organocatalyst is easily obtained and handled, and is significantly less corrosive than mineral acids commonly used in esterification reactions. Although used in homogeneous conditions, p- sulfonic acid calix[4]arene catalyst can be recovered and reused without losing its activity and may therefore be potentially useful in biodiesel production. A comparative study by use of in situ 1 H NMR spectroscopy between the kinetic and thermodynamic parameters obtained from the palmitic acid esterification reaction, showed that at 0.5 mol% concentration the activation energy of the p -sulfonic acid calix[4]arene-catalyzed reaction is lower than that measured for sulfuric acid. However, a higher dependence in relation to sulfuric acid concentration was observed. Conversely, ΔG values obtained suggested that this reaction becomes more spontaneous with an increase in temperature than calix[4]arene catalyzed esterification reactions.