A comparison of the esterification of acetic acid with methanol using heterogeneous versus homogeneous acid catalysis

To investigate the similarities and differences between heterogeneous and homogeneous catalyzed esterification, the kinetics of acetic acid esterification with methanol were investigated using a commercial Nafion/silica nanocomposite catalyst (SAC-13) and H2SO4, respectively. Reactions were carried out in an isothermal well-mixed batch reactor at 60 °C. Organic base titration, TGA, and elemental sulfur analysis were carried out to estimate the acid site density of SAC-13, permitting the assessment of its catalytic performance for esterification on a per site basis. It was found that SAC-13 has comparable site activity for acetic acid esterification as H2SO4. SAC-13 and H2SO4 also exhibited similar reaction inhibition due to the presence of water. The similar response to the presence of water and the results of pyridine poisoning experiments suggest that esterification with these catalysts occurs through a common reaction mechanism. Consistent with the hypothesized homogeneous-type reaction mechanism, reaction on SAC-13 appears to involve single-site catalysis with a probable rate-controlling Eley-Rideal surface reaction. A mechanistically derived kinetic model successfully predicts the esterification rate of SAC-13 as reaction progresses. The dissimilarities between homogeneous and heterogeneous acid-catalyzed esterification are also discussed.