Friedel-Crafts acylation of pyrrole over ultrasonic-assisted phosphoric acid-modified Hβ zeolite

Friedel-Crafts acylation of pyrrole with acetic anhydride was studied over different zeolites and an ultrasonic-assisted phosphoric acid-modified H beta (P-H beta-US) zeolite was found to have the best catalytic performance among all the zeolites examined. The conversion of acetic anhydride reached 98.8% and the selectivity to acetylpyrroles was 97.0% under the optimized conditions. The high activity of the P-H beta-US zeolite is attributed to the increase of weak acid sites, caused by the synergistic effect between the phosphoric acid modification and ultrasound. Moreover, the carbonaceous deposits, mainly due to the adsorption of reactants and products, on the surface acid sites and the blockage of the pores is believed to be the reason for the deactivation of the reused P-H beta-US zeolite, confirmed from the Brunauer-Emmett-Teller (BET) method, scanning electron microscopy, and X-ray photoelectron spectroscopy. Furthermore, the catalyst activity can be recovered effectively by a subsequent calcination.