Mechanism of acetic acid esterification over sulfonic acid-functionalized mesoporous silica

The kinetics of acetic acid esterification with methanol using a propylsulfonic acid-functionalized SBA-15 catalyst were investigated. To determine whether a different mechanism was applicable for heterogeneous or homogeneous catalyzed esterification, propane sulfonic acid was also examined as this homogeneous acid has the same structure as the functional groups tethered onto SBA-15. In isothermal experiments at 323 K, the apparent reaction orders using the heterogeneous catalyst were determined to be 0.72 for methanol and 0.87 for acetic acid. Reactant adsorption studies showed that pre-adsorption of acetic acid hindered the reaction rate, while pre-adsorption of methanol or acetic acid with methanol increased the reaction rate, indicating that acetic acid adsorbs more strongly than methanol over the heterogeneous acid catalyst. The experimental results demonstrated that acetic acid esterification with methanol followed a dual-site Langmuir-Hinshelwood type reaction mechanism, which required both the adsorption of acetic acid and methanol over propylsulfonic acid-functionalized SBA-15. In contrast, esterification reaction with the homogeneous catalyst followed an Eley-Rideal mechanism. The kinetic data were successfully fit with a model in which the surface reaction was the rate-limiting step. (C) 2011 Elsevier Inc. All rights reserved.