Regulation Of Pore Structure And Acidity Of Industrial Zeolite Y And Its Performance In Hydrocracking

Hydrocracking is a process in which the feedstock is converted in the presence of hydrogen and catalysts at high temperature and high pressure.It is also one of the most important means to convert various heavy and inferior feeds into high-quality fuel,lube base oil and chemical raw material.The core of this technology is hydrocracking catalyst,which consists of active metal and support material.Among the support materials,zeolite Y with unique acid properties,pore structure and thermal stability has become the most popular one.However,properties such as smaller pore size and inferior hydrothermal stability make it unsuitable as catalyst support directly.In industrial applications,zeolite Y is always modified to increase the hydrothermal stability and secondary mesoporosity.However,the severely damaged framework structure,the reduced acid density,and the poor connectivity of the second mesopores of conversional modified zeolite Y reduce the efficiency of heavy oil conversion.Therefore,it is necessary to develop an efficient modification method to increase the strong acid content with better availability,to introduce more mesopores with good connectivity,and to improve the hydrothermal stability of zeolite Y.Based on the existing modification methods,three series of zeolite Y were prepared by optimizing the combination of various post-treatment methods and adjusting the parameters of the treatment process.The crystal structure and the form of the silica/alumina species of the zeolite framework were analyzed with XRD and NMR.The morphology,pore structure and pore connectivity of the zeolite were investigated with SEM and N₂/Ar physical adsorption.The NH₃-TPD,Py-IR and OH-IR were used to measure the total acid content,acid sites distribution and the accessibility.The physicochemical properties of the zeolite were verified by the toluene adsorption and diffusion process.The catalytic performances of the post-treated zeolite Y and the corresponding hydrocracking catalysts were investigated with the hydrocracking reaction of1,3,5-triisopropylbenzene and VGO.The following results were obtained:The framework structure characterization of the post-treated zeolite Y obtained with hydrothermal treatment at different temperature and multiple acid treatments shows that the post-treatment combination method is an effective way to maintain the complete crystal structure of zeolite Y,and the framework Si/Al of the zeolite is adjustable by changing the hydrothermal treatment temperature and the number of acid treatment.The series of zeolites obtained with this method contain all the tetra-coordinated,penta-coordinated and hexa-coordinate aluminum species.Pore structure tests show that these modified zeolites have similar pore size distribution,and the increase of hydrothermal treatment temperature and acid treatment time is beneficial to improve the mesoporous structure performance of zeolite and the content of effective mesopores?<12 nm?.The corresponding SEM indicates that the surface of the modified zeolites becomes rough and the larger size mesopores are obviously observed.Characterization of acid performance shows that a large amount of framework aluminum is removed during the post-treatment combination process,which leads to a decrease in the total acid amount of the zeolite,and the L acid sites decrease more significantly,so that the zeolite shows a higher B/L value.The adsorption and diffusion process of zeolite proved that the weak interaction between acid center and toluene and the introduction of mesopores are conducive to improve the diffusion performance of toluene in the zeolites.The probe reaction and hydrocracking experiments show that both the pore structure and acid content affect the catalytic activity of zeolite Y,and when the mesopore content reaches a certain level,the acid site content significantly influences catalytic activity.Therefore,the zeolite obtained with hydrothermal treatment at 550?and acid treatment exhibits the best hydrocracking catalytic activity and the product properties are superior to other modified catalysts.After 2800 hours of operation evaluation,the reaction temperature of the cracking section increased by 2?,which means that the stability of the catalyst is good.SNY is obtained with a combination of steaming-acid-alkali treatment.It exhibits a framework silica-aluminum ratio of 6.2 and maintains a good Y zeolite crystal structure.During the alkali treatment process,a large amount of framework silicon is removed and the re-aluminization reaction of some aluminum species happens at the framework defect sites.Only the tetra-coordinated framework aluminum exists in the obtained zeolite SNY.The pore structure characterization shows that the zeolite SNY not only retains the microporous structure of the untreated zeolite,but also has more mesopores concentrated in 3²0 nm,so that the mesoporous volume is greatly improved,and the effective mesoporous volume?<12 nm?is unchanged.SEM characterization shows that the particles of zeolite SNY sustained serious damage and the surface exposed a large number of large size pore structures.The acidity test demonstrates that the total acid amount of zeolite SNY is similar to that of reference zeolite,which is more than twice of hydrothermal-acid treatment zeolite,and the ratio of B acid sites is about 85%,indicating that the post-treatment is effective to increase the content of B acid sites.Infrared characterization of hydroxyl groups attests that the increase in acidity is mainly due to the release of the acid center in the supercage and the improvement of the acid site accessibility in the sodalite cages.Moreover,the results of adsorption and diffusion demonstrate that the adsorption of toluene mostly occurs in the B acid site of zeolite,and the performance of B acid enhances the interaction,which leads to the increase of diffusion activation energy,but the introduction of a large number of mesoporous structures is more beneficial to improve the diffusion efficiency.1,3,5-triisopropylbenzene cracking experiments also show that the increased mesoporous channels in zeolite SNY are more conducive to the rapid diffusion of intermediates,so the product contains more diisopropylbenzene.Under the same conditions,the reaction temperature of zeolite SNY in the hydrocracking of VGO is 7?lower than that of hydrothermal-acid treated catalyst,and the paraffin content in UCO is greatly improved,which is benefited from the improved performance of B acid sites and the mesopore structure.However,after 2100 hours of operation evaluation,the reaction temperature of the cracking section increased by 32?,indicating that the catalyst has poor catalytic stability and cannot meet the requirements of long-term operation of the apparatus.SNFY obtained with the ammonium fluorosilicate pretreatment and the steaming-acid-base treatment of zeolite Y shows the same crystal structure and silica-alumina ratio with SNY.SNFY contains not only the tetra-coordinated framework aluminum,but also penta-coordinated and octahedrally coordinated aluminum species.It can be seen from the pore structure results that the zeolite has much moremesopores concentrated in about 7 nm without destroying the microporous structure of the zeolite,the mesoporous pore volume is up to 0.27cm³/g,and the volume of the effective mesopores?<12 nm?reaches to 0.079cm³/g.SEM characterization showed that the pores with larger sizes are still distributed on the surface of the modified zeolite.According to the acidity tests,the acid performance and B acid and L acid distribution of the zeolite SNFY are similar to SNY.The results of adsorption,diffusion and probe cracking reaction prove that the two zeolites have similar adsorption and diffusion properties as well as similar catalytic activity and product distribution.In the hydrocracking reaction of VGO,the reaction temperature of zeolite SNFY is 9°C lower than that of hydrothermal-acid treated zeolite Y under the same conversion depth,and the product properties are improved,mainly due to the increasing concentration of B acid and more mesopores in the zeolite.After 2600 hours of operation evaluation,the reaction temperature of the cracking section increased by 2°C,which corresponds to a lower temperature increase rate,indicating that the catalyst has good stability and can achieve the aim of the long-term operation.The physicochemical properties and catalytic results of the three series of zeolites testify that the increase of B acid content,acid strength and mesoporous structure are beneficial to improve the catalytic activity of the catalyst,meanwhile the presence of various aluminum species is more conducive to enhance the structural and catalytic stability.The catalytic activity of the hydrocracking catalyst containing zeolite SNFY is better than that of the reference industrial catalyst,and the high-efficiency conversion of VGO is obtained under different reaction conditions.The properties of the corresponding products are excellent,and the cracked UCO shows lower content of polycyclic hydrocarbons and high viscosity index.When processing different feedstocks,the catalyst can produce high-quality distillate products,indicating that the catalyst has good applicability.