Kinetics and Mechanism of Integrated Catalytic Ammonolysis and Dehydration from Methyl Salicylate over ZnAl2O4 Spinel

A kinetic and mechanistic study of direct catalytic nitrilation from methyl salicylate and ammonia is conducted by using an amphoteric ZnAl2O4 spinel as a model catalyst. This overall process integrates the catalytic ammonolysis of esters with the dehydration of amides, proceeding stepwise over the concerted Lewis acid–base pairs of Zn–O–Al linkages. The chemisorption and activation of C–O bonds of the ester over Lewis acid–base pairs facilitate the leaving of the methoxy group, while Lewis basic oxygen (Zn–O*–Al) serves as the main hub station for multistep proton transportation, thus leading to the decreased apparent activation energy of nitrilation and ammonolysis. The combined experimental and computational evidence confirms that this direct nitrilation process follows a monomolecular surface adsorption model, i.e., the Eley–Rideal mechanism, involving eight elementary reaction steps in which chemisorbed surface species of methyl salicylate react with gaseous NH3 molecules via nucleophilic addition–elimination and multistep proton transfer to generate amides and nitriles in sequence. Microkinetic model discrimination and DFT calculations reveal that the formation of chemisorbed imine (C═N–H) via proton transfer from the Lewis basic oxygen atom (Zn–O*–Al) to the carbonyl oxygen (C═O*) is the rate-determining step, thereby providing a potential consideration of protonation and deprotonation ability to rationally design an improved catalyst.