Acid site requirements for the synthesis of o-hydroxyacetophenone by acylation of phenol with acetic acid

The acid site requirements for the synthesis of o-hydroxyacetophenone (o-HAP) by acylation of phenol with acetic acid were studied on samples containing only strong Brönsted (HPA/C) or Lewis (NaY) acid sites, and also on catalysts containing both Lewis and Brönsted acid sites of either strong (zeolite ZSM5) or moderate (Al-MCM-41) strength. The nature, density, and strength of surface acid sites were probed by temperature-programmed desorption of NH3 coupled with infrared spectra of adsorbed pyridine. The direct synthesis of o-HAP by C-acylation of phenol was promoted only on samples containing Lewis sites. The initial o-HAP formation rate followed the order ZSM5 ≫ Al-MCM-41 ≅ NaY, probably because ZSM5 promoted at higher rates the generation of acylating agent CH3CO+ from acetic acid. o-HAP was also produced via a two-step reaction pathway involving the initial O-acylation of phenol to phenyl acetate and the consecutive transformation of phenyl acetate to o-HAP. This latter step greatly depended on the nature and strength of surface acid sites, and it was specifically investigated by feeding phenyl acetate alone or together with phenol. It was found that phenyl acetate is converted to o-HAP via a Fries rearrangement mechanism on very strong Brönsted acid sites, and through an intermolecular phenol/phenyl acetate acylation on samples containing both Brönsted and Lewis acid sites, such as Al-MCM-41 and ZSM5.