Gas-phase synthesis of hydroxyacetophenones by acylation of phenol with acetic acid

The gas-phase synthesis of aromatic ketones via acylation of phenol with acetic acid was studied on SiO2–Al2O3, Al-MCM-41, zeolites HY and ZSM5, and tungstophosphoric acid (HPA) supported on MCM-41 and carbon. At contact times of 146 g h/mol, the initial conversion of phenol varied between 12.5% (HY) and 19.1% (HPA/MCM-41), and the initial selectivity to ortho-hydroxyacetophenone (o-HAP) between 37.1% (HPA/C) and 69.1% (HY). In all the cases, the formation of o-HAP was clearly favored in comparison to that of para-isomer. Zeolites HY and ZSM5, which contained strong Brønsted and Lewis acid sites, produced efficiently o-HAP via both the direct C-acylation of phenol and the acylation of phenyl acetate intermediate formed from O-acylation of phenol. HPA-based catalysts contained only Brønsted acid sites and formed o-HAP exclusively as a secondary product from phenyl acetate. The o-HAP yield remained constant with time on stream on ZSM5 but drastically decreased on the other samples because of coke formation. The superior stability of zeolite ZSM5 was interpreted by considering that coke precursor formation is avoided into the microporous structure of this zeolite.