Incorporation of Cr in ZSM-5 zeolite framework as bifunctional catalysts for n-butane catalytic cracking

The Cr-doped ZSM-5 zeolites were synthesized by in-situ hydrothermal method and were used in studying Cr effect on catalytic cracking of n-butane to produce light olefins. Zeolite structure, chemical state of Cr species and acidity of zeolites were characterized by several techniques, including X-ray diffraction, nitrogen adsorption and desorption, ultraviolet visible diffuse reflectance spectroscopy, Raman spectroscopy, ammonia temperature-programmed desorption and pyrrolidine (Py)-FTIR. Cr species were found to partially incorporate in the ZSM-5 framework and present in the form of monodispersion. No significant difference in the weak acidity of zeolites at low Cr content. However, a small amount of Cr species in zeolites could significantly enhance conversion of n-butane and improve yield of ethylene and propylene. When the molar ratio of Cr/Al was 0.04, the conversion of n-butane was 99.6% and the yield of ethylene and propylene was 53.8% at reaction temperature of 650℃, which was 15.5% and 5.0% higher than those of unmodified ZSM-5, respectively. The superior catalytic performance could be attributed to synergistic effects of dehydrogenation of active Cr species and cracking function of the acidic zeolite, which promoted highly efficient conversion of n-butane.