Activity, selectivity and stability of Zn-exchanged NaY and ZSM5 zeolites for the synthesis of o-hydroxyacetophenone by phenol acylation

The gas-phase synthesis of o-hydroxyacetophenone (o-HAP) by phenol acylation with acetic acid was studied on Zn-exchanged NaY and ZSM5 zeolites. The density, nature and strength of surface acid sites were determined by temperature programmed desorption of NH3 coupled with infrared spectra of adsorbed pyridine. NaY zeolite contained only surface Lewis acid sites. The exchange of Na+ with Zn2+ increased both the density and strength of Lewis acid sites of parent NaY zeolite. The Lewis/Brønsted acid sites ratio on ZSM5 zeolite was about 1. The addition of Zn increased the density of Lewis acid sites but partially eliminated the protonic sites of parent ZSM5 zeolite. Thus, the Lewis/Brønsted acid sites ratio was 3.2 on a ZnZSM5 containing 1.19% Zn. The incorporation of Zn2+ to zeolite NaY increased the initial selectivity to o-HAP because o-HAP was preferentially formed through the attack of the acylium ion to the phenol molecule adsorbed in vertical orientation on Lewis acid sites. Nevertheless, the activity decay rate on ZnNaY samples increased with the Zn content. ZnZSM5 zeolites were more active but less selective to the o-HAP formation than ZnNaY zeolites. However, ZnZSM5 zeolites formed only small quantities of coke and exhibited a remarkable stability on stream.

Initial o-HAP selectivities of up to 83% are obtained on zeolites ZnNaY containing about 9.3% Zn.Figure optionsDownload as PowerPoint slideHighlights
► Zeolites with high density of Lewis acid sites are generated by incorporation of Zn.
► Lewis acid sites favor the o-HAP formation by direct C-acylation of phenol.
► o-HAP initial selectivities of up to 83% are obtained on zeolites ZnNaY.
► The activity decay rate on ZnNaY samples increased with the Zn content.
► ZnZSM5 catalysts are stable by avoiding the formation of bulky intermediate of coke.