Phosphorus Modified ZSM-5 Zeolites And Its Application In FCC To Increase Propylene Production

Propylene is one of the basic raw materials of the chemical industry and occupies an important position in domestic industrial production.At present,domestic propylene resources are in short supply,and the process with the largest domestic propylene production capacity is fluid catalytic cracking（FCC）.Therefore,the FCC technology can be improved to achieve the purpose of increasing propylene production.Compared with the improvement and innovation of FCC in terms of reactor structure design,process operation optimization,etc.,adding additive to the catalyst has little impact on the production process and is an effective way for FCC to increase propylene production.In this paper,ZSM-5 zeolites,the key components of FCC additive,was modified to achieve the purpose of increasing propylene production,which was of great significance for alleviating the domestic shortage of propylene and adjusting the distribution of chemical products.First,ZSM-5 zeolites were treated with different concentrations of oxalic acid and sodium hydroxide solutions respectively,and their crystallinity,surface morphology,pore structure and acid properties were characterized,and they were used in FCC to evaluate their increased propylene production performance.The results showed that when oxalic acid was used for modification,it had little effect on the properties of ZSM-5 zeolites.When sodium hydroxide was used for modification,the zeolites exhibited a significant modification effect,and the acid strength of zeolites was reduced while the pore volume was increased.It was conducive to the subsequent element modification of zeolites.Then,on the basis of sodium hydroxide modification,the ZSM-5 zeolites was modified with phosphorus by hydrothermal dispersion method.The phosphorus modified zeolites was analyzed by XRD,TEM,SEM,BET,NH₃-TPD,Py-IR,³¹P NMR and XPS.And it was applied to FCC to evaluate its performance of increasing propylene production.The results showed that phosphorus modification not caused significant effects on the crystal structure and surface shape of the zeolites,and the pore volume and the acid strength of the zeolites were reduced.With the increase of the load of phosphorus,the propylene yield in the FCC evaluation gradually increased.When the phosphorus load was 8%,the yield of propylene was up to 8.62%,which was 1.44%higher than the non-modified zeolites.This may be the zeolites have a suitable pore structure,acid content and the B/L ratio.Finally,the new method and process of element modification were explored.ZSM-5zeolites were modified with phosphorus by incipient-wetness impregnation method,hydrothermal dispersion method and high-temperature hydrothermal dispersion respectively.Through the characterization of the modified zeolites,the effects of different modification methods were investigated.The results showed that the modification effects of different methods on zeolites are significantly different.With the increase of temperature of different methods,the degree of Al removal intensified,the pore volume increased,the acid strength increased and the B/L ratio decreased.At the same time,the³¹P NMR characterization showed that the high-temperature hydrothermal dispersion method and hydrothermal dispersion method have higher interaction strength between phosphorus and ZSM-5 zeolites due to higher modification temperature.The FCC evaluation results showed that the propylene yield of the additive prepared by the hydrothermal dispersion method was the highest,which was 12.43%.The order of propylene yield after phosphorus modification by different methods is as follows:Hydrothermal dispersion method is the highest,followed by high-temperature hydrothermal dispersion method,and incipient-wetness impregnation method is the lowest.