Preparation And Performance Optimization Of Composites Consisting Of Y Zeolite And ZrO₂

Catalytic cracking（FCC）is one of the most important processes of heavy oil refining in petrochemical industry,and it is also a secondary processing process of crude oil with the largest amount of catalysts.In recent years,the quality of worldwide crude oil is becoming heavier and poorer,and the proportion of high-basic nitrogen feedstock is also increasing.Enhancing the basic nitrogen resistance of FCC catalyst is the most efficient way to improve the ability of FCC unit to process high-basic nitrogen feedstock.As the chemical properties of catalyst materials are mainly determined by their chemical composition,the modification of the chemical composition of the catalyst is mainly reflected in its support under the premise that the active component of FCC catalyst remains Y zeolite and unchanges.Based on the above considerations,this paper proposes to improve the ability of FCC catalyst to resist basic nitrogen by changing the type of support and preparing composites consisting of Y zeolite and Zr O₂.The synthesis methods of dispersive-type Y/Zr O₂ composite were screened and the experimental conditions were optimized.Y/Zr O₂ composites were prepared by sol,dry-gel and in-situ hydrothermal crystallization methods with solid-state reaction respectively.Results show that the crystallinities and specific surface areas of samples obtained by sol and dry-gel methods are obviously lower than those of in-situ hydrothermal crystallization method.Unfortunately,the content of Si-O,double six-membered rings and T-O tetrahedrons in the hydrothermal crystallization system of Y zeolite are significantly reduced by adding Zr O₂.The formation ofβcages and octahedral zeolite cages consequently is restricted,which causes that the crystallinity of Y/Zr O₂ prepared by in-situ hydrothermal crystallization method is still significantly lower than that of pure Y zeolite.Therefore,guided by the growth theory of zeolites,a two-stage hydrothermal crystallization method was developed to effectively weaken the influence of zirconia precursor on the crystal growth process of Y zeolite.The crystallinity and specific surface area of as-prepared Y/Zr O₂ composite are higher than 50%and 300 m²/g respectively which are higher than those of other methods.In additon,the Y/Zr O₂composite prepared by two-stage hydrothermal crystallization method not only has higher total acid content and acid strength,but also has good thermal and hydrothermal stability.Besides,the addition of appropriate amount of aluminum complex into the synthesis system can promote the crystallization of Y/Zr O₂ and reduce the particle size of the composite.The core-shell composite consisting of Y zeolite and Zr O₂（Y@Zr O₂）was prepared by hydrothermal crystallization coupling with electrostatic deposition method.In order to improve the ability of dispersive-type Y/Zr O₂ composites to capture basic nitrogen species,a core-shell Y@Zr O₂ composite with Zr O₂ as shell and Y zeolite as core was designed.The key factor to obtain the core-shell structure is that the p H value during electrostatic deposition must be between the isoelectric points of Y zeolite and Zr O₂.The as-synthesized core-shell Y@Zr O₂ composite shows high crystallinity（>50%）and large specific surface area（>400 m²/g）,and the total acid content,strong acid content and Lewis/Br?nsted ratio are slightly higher than those of the dispersed Y/Zr O₂ composite.In addition,Y@Zr O₂ also has good hydrothermal stability and can keep the core-shell structure well after high-temperature hydrothermal treatment.It is worth mentioning that the types of zirconium species of Y@Zr O₂are different from those of Y/Zr O₂.There are two forms of zirconium in Y@Zr O₂:Zr O₂ oxides which are uniformly dispersed in the shell,and zirconium which is in the form of Zr-O-Si bond contacting with the surface of Y-zeolite.As for Y/Zr O₂,except the two kinds of zirconium species which are the same as Y@Zr O₂,there is also zirconium which enters the framework of Y zeolite and exists in the form of Zr-O tetrahedron.The FCC performences of high-basic nitrogen feedstock of composites consisting of Y zeolite and Zr O₂with different structure were investigated in a fixed bed micro-reactor.The results showed that the dispersive-type Y/Zr O₂ catalyst exhibited better catalytic performance than the mechanically mixed catalyst.Compared with Y/Zr O₂,the core-shell Y@Zr O₂ has a better ability of efficiently cracking high-basic nitrogen feedstock.In addition,the cracking process of high-basic nitrogen VGO on Y@Zr O₂ catalyst was investigated by using mixed bed packing.The results confirmed that the reactant molecules in the feedstock trend to be pre-cracked firstly on the Zr O₂ support shell.Moreover,the experimental results also confirmed that the synergistic effect between Zr O₂ and Y zeolite in composite not only promotes the conversion of high-basic feedstock,but also optimizes the product distribution:with the increase of LPG and gasoline yield,the yield of heavy oil decreased,and the coke yield increased slightly.In order to reveal the functional advantages of the composites,the synergy of Y zeolite and Zr O₂ was studied.Compared with the mechanical mixture of Zr O₂ and Y zeolite,the composite achieved relatively uniform mixing between Zr O₂ and Y zeolite through Zr-O-Si chemical bonds.Zr O₂ supports with abundant Lewis acid sites can selectively adsorb basic nitrogen compounds in feedstock,thus protecting the active sites of Y zeolites,which improves the resistance of the composite to basic nitrogen compounds.As for Y@Zr O₂,the Zr O₂shell with abundant Lewis acid sites completely covers the of Y zeolite core,which forms an effective protective layer for the active centers of FCC catalyst.Thus the high-basic nitrogen feedstock inevitably contacts with the Zr O₂support shell firstly,so as to maximize the protection of Zr O₂ support for the active centers of FCC catalyst.Therefore,as for the core-shell Y@Zr O₂ composite,the advantage of resistance to basic nitrogen is more significant.In addition,the coupling of microporous structure in Y zeolite with the mesoporous pore in Zr O₂ not only realized the efficient conversion of reaction species in different scales in feedstock,but also reduced the diffusion resistance of reactants and products,which endow the FCC catalyst higher ability of heavy oil conversion and carbon deposition resistance.