Highly Effective F-Modified HZSM-5 Catalysts for the Cracking of Naphtha To Produce Light Olefins

A series of fluorinated HZSM-5 zeolites (F/HZSM-5) were prepared by immersing the zeolites with different concentrations of NH4F solution, and their performances for the catalytic cracking of naphtha to produce light olefins were investigated. The results indicated that F-modified HZSM-5 zeolites are effective catalysts for the cracking of naphtha to light olefins. At the temperature of 600 degrees C, the yields of propene and ethene were achieved at 36.4 and 20.2%, which were 7.3 and 4.3% higher than those over parent HZSM-5 zeolite, respectively. The physicochemical features of F/HZSM-5 catalysts were characterized by means of X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), temperature-programmed desorption of ammonia (NH3-TPD), Fourier transform infrared (FTIR) spectra of adsorbed pyridine, etc. The results indicated that fluorine (F) modification not only regulates the pore characteristics of the HZSM-5 zeolites but also modulates the amount of acid sites, especially the amount of Bronsted (B) acid. Consequently, F modification with suitable content was favorable for increasing the conversion of naphtha and enhancing the selectivity to light olefins.