A study of esterification of caffeic acid with methanol using p-toluenesulfonic acid as a catalyst

Caffeic acid (CA) can be considered as an important natural antioxidant. However, the low solubility and stability of CA in various solvent systems is a major limiting factor governing the overall application in the lipid industry, so the synthesis of methyl caffeate (MC)using CA and methanol is a feasible way to improve its lipophilicity. Here, the reaction conditions and kinetic parameters for the synthesis of MC using p-toluenesulfonic acid (PTSA) as a catalyst were firstly investigated, and the product was confirmed byliquid chromatography-mass spectrometry (LC-MS),Fourier transform infrared spectroscopy (FTIR), nuclear magnetic resonance (NMR), and melting point analysis. The highest yield of MC catalyzed by PTSA reached 84.0% under the optimum conditions as follows: molar ratio of methanol to CA of 20:1, reaction temperature of 65°C, mass ratio of catalyst to substrate of 8 %, and reaction time of 4 h. The esterification kinetics of CA and methanol is described by the pseudo-homogeneous second order reversible model, the relationship between temperature and the forward rate constant is k1 = exp (358.7 - 2111/T), and the activation energy is 17.5 kJ mol-1. These results indicated that the PTSA possesses high catalytic activity in the synthesis of MC, which is an efficient catalyst suitable for MC production in the chemical industry