Esterification of oleic acid for biodiesel production catalyzed by 4-dodecylbenzenesulfonic acid

The esterification of oleic acid with methanol by homogeneous acid catalysis was investigated, using 4-dodecylbenzenesulfonic acid as catalyst. First, the catalytic activity of 4-dodecylbenzenesulfonic acid in this reaction was compared with that of p-toluenesulfonic and sulfuric acid, and it was found that the reaction rate clearly increased with increasing hydrophobicity of the catalyst. Second, the effects of the catalyst/acid molar ratio, the methanol/oleic acid molar ratio, the water content, temperature, the stirring speed and the presence of triglycerides on the kinetics and the equilibrium of this esterification were studied. The main observations were that temperature and the proportion of catalyst had a positive effect on the kinetics, i.e., an increase in any of these factors led to an increase in the reaction rate. Methanol had an almost negligible effect. By contrast, water had a negative effect due to the formation of an aqueous phase, which removed part of the catalyst, thus reducing its concentration in the organic phase and hence the reaction rate. Third, the kinetics of esterification was studied in a wide range of operating conditions and a reversible second-order model was obtained that included the effect of the separation of the aqueous phase from the reaction mixture and hence the progressive decrease in the concentration of catalyst, methanol and the volume of the reaction mixture. This kinetic model adequately predicts the experimental data within the range of operation conditions investigated. The activation energy for the forward reaction was 58.5 kJ/mol, and 63.4 kJ/mol for the reverse reaction. (C) 2015 Elsevier B.V. All rights reserved.