气相法磷改性对USY沸石酸性及其水热稳定性的调控

Improvement of the Acidity and Hydrothermal Stability of USY Zeolite by Phosphorus Modification Using Gas-phase Method

磷(P)改性是提高沸石催化剂稳定性的有效方法之一.我们考察了三氯化磷(PCl_(3))气相法和磷酸氢二铵浸渍法制备得到的不同P改性USY样品.研究发现,2种P改性方法制备的样品中P物种均能够与USY中的Si―OH发生相互作用,同时夺取附近的非骨架Al物种,将表现为L酸的Si―O(EFAl)―Al转变成显示强B酸的Si―OH―Al.非骨架Al的迁移改善了沸石的孔径分布.在600℃水热老化后发现,气相法PCl_(3)改性后样品的桥羟基没有被破坏,其原因是PCl_(3)改性后样品的P物种主要为低聚态P物种,能够更好地与附近的四配位骨架Al发生相互作用,从而保护沸石的桥羟基不被破坏.而浸渍法磷酸氢二铵改性后样品主要生成高聚态P物种,与附近的四配位骨架Al发生相互作用能力弱,保护沸石桥羟基的作用不强.因此,气相法PCl_(3)改性样品在600℃水热老化4 h后的强B酸酸量(41μmol·g^(-1))明显高于浸渍法P改性样品(21μmol·g^(-1)).在800和900℃水热老化4 h后,PCl_(3)改性样品较浸渍法P改性样品和未改性样品展现了更高的结晶保留度,结合正辛烷催化裂化反应数据,表明气相法PCl_(3)改性后的USY样品具有更高的催化活性稳定性和结构稳定性.

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 PCl_(3) 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 PCl_(3)-modified sample was not destroyed.The reason is that the P species of the PCl_(3)-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(41μmol·g^(-1))of the PCl_(3) modified sample by the gas phase method was significantly higher than that of the P-modified sample by the impregnation method(21μmol·g^(-1))after hydrothermal aging at 600℃ for 4 h.After hydrothermal aging at 800 and 900℃ for 4 h,the PCl_(3) modified samples exhibited higher crystallinity retention compared to the impregnation method modified samples and the unmodified samples,combined with the fact that the PCl_(3)-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 PCl_(3) has higher catalytic activity stability and structural stability.