Acylating Capacity of the Phosphotungstic Wells-Dawson Heteropoly Acid: Intermediate Reactive Species

The mechanism of adsorption and decomposition of acetic anhydride over phosphotungstic Wells-Dawson heteropoly acid (HPA) was investigated through infrared spectroscopy with especially dedicated in situ cells. Both gaseous and liquid acetic anhydride dissociatively chemisorbs on the heteropoly acid, producing the intermediate acyl CH3CO+ species, adsorbed acetate species, and acetic acid. The former is known as the key intermediate species in the Friedel-Crafts acylation reaction. Moreover, the consumption of the Bronsted acid sites observed during the adsorption and decomposition of acetic anhydride over the HPA indicates that those sites are the active ones of the genesis of the acyl intermediate species. Surface acetate species and acetic acid strongly adsorb on the active acid sites deactivating the heteropoly acid.