Abstract:

In this study, phosphorus modification by trimethyl phosphate impregnation was employed to enhance the hydrothermal stability of nano-sized HZSM-5 zeolites. A parallel modification was studied by ammonium dihydrogen phosphate impregnation. The modified zeolites were subjected to steam treatment at 800℃ for 4 h (100% steam) and employed as catalysts for olefin catalytic cracking (OCC) of full-range fluid catalytic cracking (FCC) gasoline. X-ray diffraction, N₂ physical adsorption and NH₃ temperature-programmed desorption analysis indicated that, although signifi-cant improvements to the hydrothermal stability of nano-sized HZSM-5 zeolites can be observed when adopting both phosphorus modification strategies, impregnation with trimethyl phosphate displays further enhancement of the hydrothermal stability. This is because higher structural crys-tallinity is retained, larger specific surface areas/micropore volumes form, and there are greater numbers of surface acid sites. Reaction experiments conducted using a fixed-bed micro-reactor (catalyst/oil ratio=4, time on stream=4 s) showed OCC of full-range FCC gasoline-under a fluid-ized-bed reaction mode configuration-to be a viable solution for the olefin problem of FCC gaso-line. This reaction significantly decreased the olefin content in the full-range FCC gasoline feed, and specifically heavy-end olefins, by converting the olefins into value-added C₂-C₄ olefins and aromat-ics. At the same time, sulfide content of the gasoline decreased via a non-hydrodesulfurization pro-cess. Nano-sized HZSM-5 zeolites modified with trimethyl phosphate exhibited enhanced catalytic performance for OCC of full-range FCC gasoline.

Key words: Hydrothermal stability, Nano-sized HZSM-5, Phosphorus modification, Olefin catalytic cracking, FCC gasoline