Hydrothermally ultrastablized USY zeolites were further modified with citric acid in unbuffered systems. The crystal structure, surface area and pore distribution, acid properties and stability in catalytic cracking of modified samples were determined by XRD, N~2- adsorption, FT-IR, DTA techniques respectively. The micro-activity performance of the modified samples wasevaluated on a standard MAT apparatus. The results show that the Si/Al ratio, crystallinity, lattice constant, and particle size of the modified samples are all significantly improved The pore system is developed, moreover, the mesopores of the modified zeolites are much great than those of the parent zeolites. The number of acidic sites and the acid strength are consisten with those reported in the literature. The present work also shows that modified zeolites are promising deep cracking catalyst for producing lighter olefins (C~3^=～C~6^= hydrocarbons) with good yield and selectivity.

Key words: CITRIC ACID, MOLECULAR SIEVE, MODIFICATION, X-RAY DIFFRACTION ANALYSIS, DIFFERENTIAL THERMAL ANALYSIS, FOURIER TRANSFORM, INFRARED SPECTROPHOTOMETRY