Catalytic cracking of isobutane over HZSM-5, FeHZSM-5 and CrHZSM-5 catalysts with different SiO2/Al2O3 ratios

Several systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with different ratios of SiO2/Al2O3 (25,38,50,80, and 150) were prepared and they were characterized by means of X-ray diffraction (XRD), UV-Vis, NH3-TPD and BET techniques. The results indicated that, compared with uncalcined HZSM-5 zeolites, the total acid amounts, acidic site density and acidic strength of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts obviously decreased, while those of weak acid amounts obviously enhanced with the decrease of SiO2/Al2O3 molar ratio. When the ratio of SiO2/Al2O3 is less than 50, the three systems of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolite catalysts with same ratio of SiO2/Al2O3 gave similar and high isobutane conversions. However, when the ratio of SiO2/Al2O3 was equal to or greater than 80, these three systems of catalysts possessed different altering tendencies of isobutane conversions, thus their isobutene conversions were different. High yields of light olefins were obtained over the FeHZSM-5 and CrHZSM-5 zeolite catalysts with high ratio of SiO2/Al2O3 (>= 80). The ratio of SiO2/Al2O3 has large effects on the surface area, and acidic characteristics of HZSM-5, FeHZSM-5 and CrHZSM-5 zeolites catalysts, and thus further affect their catalytic performances for isobutane cracking. That is the nature of SiO2/Al2O3 ratio effect on the catalytic performances.