Structure Stabilization of Zeolite Y by Rare Earth and Its Influence on RFCC Catalyst Performance

Author:Liu Zuo Sheng

Supervisor:yu ji hong


Degree Year:2019





Resid fluid catalytic cracking(RFCC)is one of major approaches to process crude oil,and is also a key process for refineries to produce high-value products such as gasoline.How to improve the activity and selectivity of RFCC catalysts,especially in the case of heavy metal vanadium(V)contamination,has attracted wide attention given that the device design,the properties of feedstock and process parameters are basically unchanged.In the case of V contamination,the crystal structure of zeolite Y,the key active component of RFCC catalyst,will be subjected to serious damage by V,resulting in a rapid deterioration of its activity and selectivity.Hydrothermal dealumination and lanthanum(La)-exchange modification could improve the activity and selectivity of zeolite Y to a certain extent,but can not solve the problem thoroughly.At the same time,it is of very importance to decrease the coke selectivity of zeolite Y.Aiming at solving these problems,the destructive effect of heavy metal V on the active components of RFCC catalysts and the trapping performance of rare earth oxides on heavy metal V are studied in the thesis.On this basis,the yttrium-exchanged modification,the introduction of mesopores into zeolite Y and the effect of ZSM-5 additives on yield distribution and coke selectivety are investigated.The main research contents and results are as follows:1.The destructive effect of heavy metal V on zeolite Y and the vanadium passivation by RE2O3.Vanadium-contaminated NaHY zeolite samples were prepared by incipient wetness impregnation method,and then treated at elevated temperature in the presence of steam,a similar condition to severe RFCC environment.The results showed that the higher the sodium content of zeolite,the more serious the damage of V to its crystal structure.The relative crystallinity and specific surface area of NaHY zeolite were only 14 wt.%and 247 m2/g respectively when Na content was 1.07 wt.%and V content was 6100 ppm.The introduction of La2O3 using physical mixing could preserve the zeolite structure to a large extent without altering the properties of active components of RFCC catalyst to the greatest extent.Under the condition of high vanadium content,when the La/V ratio was 3,La2O3-based RFCC catalyst showed superior V-trapping performance.2.Preparation and catalytic cracking performance of yttrim-exchanged zeolite Y.Yttrium-exchanged zeolite Y zeolites were prepared by ion-exchange method.Rietveld refinement and hydroxyl stretch infrared spectra analyses showed that the distribution of Y3+was similar to that of La3+in lanthanum-exchanged zeolite Y,with Y3+mainly located at the site SI‘in the sod cage.The decrease in the bond distance between Y3+and framework O and the shrinkage of unit cell might result form the smaller ionic ridus and higher charge density of cation Y3+.The retention of specific surface area and Br?nsted acid sites for yttrium-exchanged zeolite Y after steam-treatment was higher than that for lanthanum-exchangd zeolite.Catalytic cracking of n-docosane showed that yttrium-exchanged zeolite Y had higher selectivity for liquefied petroleum gas and gasoline,lower selectivity for dry gas and coke,which was mainly related to the strong stabilization effect of Y3+to the framework and the higher retention for Br?nsted acid sites.3.The effects of hierarchical zeolite Y and ZSM-5 additives on the selectivity of RFCC catalyst.Meso-structured zeolite Y was prepared from NaY zeolite by citric acid dealumination and template self-assembly.After the treatment of parent zeolite by citric acid,the relative crystallinity of NaY zeolite decreased,the silica/alumina ratio increased obviously,and a lot of defective structures and irregular mesopores appeared on the surface of NaY zeolite,simultaneously.After self-assembly synthesis with CTAB template,the relative crystallinity of zeolite Y was healed and a large number of mesopore with the pore size of 25nm were formed on the external surface.The results of resid catalytic cracking showed that meso-structured Y zeolite exhibited outstanding selectivity for liquified petroleum gas and gasoline,and low selectivity for dry gas and coke under the conditions of iso-catalyst/oil and iso-conversion.The improvement of coke selectivity was mainly attributed to the rapid diffusion and efficient conversion of big hydrocarbon molecules in mesoporoes.In addition,the low acid density of meso-structured zeolites also effectively inhibited the formation of coke.In addition,ZSM-5 zeolites with varying SAR were used as additives to investigate their effects on the selectivity of RFCC catalysts.The results showed that the yield of LPG can be greatly increased by using ZSM-5 additive,and the selectivity of RFCC catalyst can be adjusted flexibly.This work provided a new sight for the design of RFCC catalysts with low coke selectivity and high yields for high value-added products.