Improvement of the Acidity and Hydrothermal Stability of USY Zeolite by Phosphorus Modification Using Gas-phase Method
P modification is an effective method to improve the stability of zeolite catalysts.This article discusses the differences between different P-modified USY zeolites prepared by the PCl3 gas-solid reaction method and the conventional(NH4)2HPO4 impregnation method.The study found that the P species of the two P modification methods can interact with the Si―OH in USY,and at the same time capture the nearby non-framework Al species,converting the L acidic(Si―O(EFAl)―Al)into a strong B acid Si―OH―Al.The migration of non-framework Al improves the pore size distribution of zeolite.After hydrothermal aging at 600 ℃,it was found that the zeolite bridge hydroxyl group of the PCl3-modified sample was not destroyed.The reason is that the P species of the PCl3-modified sample was mainly an oligomeric P species,which can better interact with nearby the four-coordinated framework Al interacts to protect the bridge hydroxyl groups of the zeolite.The sample prepared by the impregnation method mainly generates higher polymer P species,which weakly interacts with the nearby four-coordinated framework Al,resulting in little protection for the bridge hydroxyl groups of the zeolite.Therefore,the amount of strong B acid(41 μmol·g-1)of the PCl3 modified sample by the gas phase method was significantly higher than that of the P-modified sample by the impregnation method(21 μmol·g-1)after hydrothermal aging at 600 ℃ for 4 h.After hydrothermal aging at 800 and 900 ℃ for 4 h,the PCl3 modified samples exhibited higher crystallinity retention compared to the impregnation method modified samples and the unmodified samples,combined with the fact that the PCl3-modified sample has the highest n-octane catalytic cracking reaction activity after hydrothermal aging at 600 ℃ for 4 h,all results show that the USY sample modified by gas phase PCl3 has higher catalytic activity stability and structural stability.
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