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A new type of zeolite La-USL (ultra stable zeolite L (zeolite USL) modified by La),which has superior activity,stability and selectivity in catalytic cracking of hydrocarbons and thus can be used as an active catalyst component,is reported in this paper.The zeolite L with relative crystallinity of above 90% was synthesized by the hydrothermal crystallization method under optimum conditions and characterized by means of XRD,NH3-TPD and isotherm adsorption techniques.The in-situ synthesized zeolite L with a SiO2/Al2O3 mole ratio of 5-6 was modified by cation ion exchange,hydrothermal dealumination and chemical modifications with La in order to prepare La-containing USL with a higher framework SiO2/Al2O3 mole ratio of 15-30.The modified zeolite La-USL was used as an active additive component of fluid catalytic cracking (FCC) catalyst and the resulting catalysts were evaluated by microactivity test (MAT) and fixed-fluidized bed (FFB) experiments using heavy oil as feedstock.The influence of La content in La-USL on cracking product distribution,gasoline group composition and research octane number (RON) was investigated.The results showed that when La content in La-USL was 0.8 wt%,the addition of the corresponding La-USL could result in a FCC catalyst that produced significant improvement in product distribution and gasoline quality.