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

Abstract

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 NH 3 coupled with infrared spectra of adsorbed pyridine. NaY zeolite contained only surface Lewis acid sites. The exchange of Na + with Zn 2+ 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 Zn 2+ 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.