Effect of carbon chain length on esterification of carboxylic acids with methanol using acid catalysis

This paper reports on an investigation into the impact of carboxylic acid chain length on the kinetics of liquid-phase acid-catalyzed esterification. Using sulfuric acid and a commercial Nafion/silica composite solid acid catalyst (SAC-13), initial kinetics were measured for the reactions of a series of linear chain carboxylic acids (acetic, propionic, butyric, hexanoic, and caprylic acid) with methanol at 60 degrees C. It was found that reaction rate decreased as the number of carbons in the linear alkyl chain increased for both H2SO4 and SAC-13. This trend is discussed in terms of the polar and steric effects of the alpha-substituent to the carboxylic group and evaluated by a Taft-type correlation. Using a mechanistically based kinetic model, the reaction kinetic parameters of SAC-13 catalysis were determined and compared for different carboxylic acids. Moreover, important parameters, such as water deactivation, catalyst reusability, and regeneration, were also affected by the size of the carboxylic acid used. SAC-13 underwent significantly more activity loss with subsequent reaction cycles as the size of the alkyl tail on the carboxylic acid increased. Characterization of the catalyst after reaction suggested that the deactivation of SAC-13 is likely caused by the entrapment of bulky reaction intermediates in or on Nafion polymeric nanodomains blocking catalytic acid sites. (c) 2006 Elsevier Inc. All rights reserved.