Simultaneous determination of adsorption coefficients and surface reaction mechanisms using initial reaction rates

Initial reaction rate data for the direct esterification of 1-tetradecene with heptanoic acid on dry Amberlyst (R) 15 catalyst permitted the identification of the surface reaction mechanism, as well as the adsorption coefficients of all reactants and products. Three different surface reaction mechanisms were considered, a two-site Langmuir-Hinshelwood mechanism and two single-site Eley-Rideal mechanisms. All the mechanisms considered competitive adsorption by all reactants and products. For each model, an error surface was calculated above the two-dimensional space of possible reactant adsorption coefficients. The error surface identified the best adsorption coefficients for each model and revealed the most likely mechanism for the surface reaction. The best mechanism was Eley-Rideal with adsorbed tetradecene reacting with heptanoic acid in the bulk phase. To explain initial rate responses to changes in initial reactant concentrations, both tetradecene and heptanoic acid must adsorb strongly to the active sites, and heptanoic acid must rapidly equilibrate between monomer and dimer forms. Data also revealed that adsorption and desorption rates were sometimes competitive with the rate of the surface reaction; equilibrium between adsorbed species and those in solution was not always maintained during short reaction times. Proper distinction between Langmuir-Hinshelwood and Eley-Rideal mechanisms required initial rate data for extreme ranges of initial reactant concentrations.

Automated summary

The study identified the best mechanism for the direct esterification reaction as the Eley-Rideal mechanism and revealed the optimal adsorption coefficients for reactants on the surface. Additionally, it was found that adsorption and desorption rates sometimes compete with the surface reaction rate, and heptanoic acid rapidly equilibrates between monomer and dimer forms.