Kinetics of the conversion of methyl benzoate to benzamide by the alumina catalysed reaction with liquid ammonia at 120 °C

Abstract

The direct conversion of unactivated esters to the corresponding primary amide is a difficult process. Using methyl benzoate as an example, we have shown that, in liquid ammonia, chromatographic alumina is a useful catalyst for this conversion. The kinetics are complex, mainly because inhibition by the product benzamide is a significant factor. The rate of reaction reaches a plateau with increasing methyl benzoate concentration, which is consistent with Langmuir adsorption of methyl benzoate followed by reaction with ammonia in solution (Eley–Rideal mechanism), rather than a Langmuir–Hinshelwood model in which adsorbed methyl benzoate reacts with adsorbed ammonia. The kinetics have been interpreted in terms of the adsorption coefficients of methyl benzoate and benzamide determined from the kinetic analysis; the kinetic data could not be fitted using adsorption coefficients determined from chromatographic retention time data. This is attributed to the reaction proceeding on active sites that are a small proportion of the surface sites available for adsorption.