Dealumination of HZSM-5 zeolites: Effect of steaming on acidity and aromatization activity

Conversion of acetone/n-butanol mixtures (1:2 wt/wt) to hydrocarbons over HZSM-5 zeolites previously dealuminated by (a) steaming and (b) steaming with subsequent HCl leaching has been studied at different conditions. Steamed-treated catalysts largely enhance their deactivation behavior. Al species are dislodged during the dealumination process, thereby reducing the strength and density of the acid sites of the parent catalyst. Catalysts dealuminated by means of steam-HCl treatment were found to have the best aromatization stability (the loss in residual aromatization activity decreases from 70% in the parent to only 12% in treated catalysts after 5 h of reaction). This treatment produces a decrease in both acid strength as measured by tert-butylamine temperature programmed desorption (from T-d=280 degrees C to T-d=230 degrees C) and acid density (from 0.23 to 0.13 mmol TBA g(cat)(-1)). Medium/strong acid sites ratio close to unity and Bronsted/Lewis acid sites ratio lower than unity are characteristic in catalysts with the best deactivation behavior. Dealumination treatments generate a secondary pore structure that can contribute to enhance aromatization stability by reducing diffusional limitations.