Designed Synthesis And Performance Study Of Hierarchical Zeolitic Catalysts For Cracking Of Heavy Oil

With the development of the social economy,the demand of energy is increasing gradually.However,the present situation of worldwide reserves of crude oils is challenging:production of light crude oils is declining and production of heavy and extra-heavy oils is increasing.Heavy crude oil brings great difficulties for the present refining technologies,especially the fluid catalytic cracking?FCC?unit which can convert heavy oil fractions into the lighter ones.In FCC unit,the catalyst technology is the key technology.Thus,designing and developing more effective FCC catalysts so as to processing heavy oil efficiently is an important mean to solve the energy crisis,which is vital for the economic development and the energy security at this stage.Zeolite is the most important catalytic material used in oil refining at present.The FCC process mainly involves FAU-type Y-zeolite and MFI-type ZSM-5.Because the critical molecular diameters of heavy oil are larger than the inlet diameters of the zeolites,the desired FCC catalysts for processing heavy oil need to have improved diffusion abilities and enhanced acid sites accessibilities.This paper focuses on designing and developing the zeolite materials with more reasonable pore structures and acidities relative to the present materials and then studying the related performances in order to increase the conversion of heavy oil,improve the distribution of products and upgrade the product quality.First,a hierarchical ZSM-5 zeolite catalyst is prepared by one-step method using the raw materials of traditional microporous zeolite without any secondary templates.The simple and cost-effective method overcomes most of the limitations associated with the existing methods,such as complex operations and high costs.The prepared hierarchical ZSM-5 zeolite is studied by XRD,FT-IR,SEM,EDS,TEM,N₂ adsorption-desorption measurement,MAS NMR and so on.The results show the hierarchical ZSM-5 has a core-shell structure.The shell is thin and consist of intergrown strip and block crystals,the core is composed by loosely accumulational nano crystals smaller than 100 nm in diameters.The abundant meso-and macropores exist among these nano crystals.Moreover,the introduction of meso-and macropores has no effect on the intrinsic crystalline and microporous properties of zeolite.During the crystallization of the hierarchical ZSM-5 zeolite,the adding amount of NaOH in the gel is the determining factor for producing hierarchical pore structure and influences the crystallization route.Performance studies show that the introduction of meso-and macropores in ZSM-5 zeolite causes the diffusion abilities improved and the acid sites accessibilities enhanced significantly.In the catalytic cracking of model molecule of heavy oil,the prepared hierarchical ZSM-5 exhibits excellent properties.The conversion of bulky molecule is increased more than tripled,compared with the commercial ZSM-5 and the microporous ZSM-5.Then,a new core-shell hierarchical pore catalyst?i.e.a core-shell zeolite composite?with employing a polycrystalline nano ZSM-5 aggregate as meso-and macroposous shell and a commercial Y-zeolite as core is successfully synthesized.The core-shell zeolite composite has a high thermal and hydrothermal stability because of high crystallinity,which can meet the industrial requirements.Moreover,the abundant Br?nsted sites in the crystalline meso-and macroposous shell can improve the pre-cracking of bulky molecules.More importantly,using the designed core-shell catalyst,the bulky molecules can be cracked step-by-step into the required products.Specifically,the meso-and macroporous shell facilitates pre-cracking the large molecules in heavy oil into moderate ones which can directly diffuse into the micropores of the Y-zeolite to be cracked selectively into the desired products.That is,the core-shell structure has better controllability of cracking reaction than the mechanical mixing structure and can improve the distribution of products.In the practical catalytic cracking of VGO heavy oil,the conversion of heavy oil is increased by 1.8%on the core-shell composite catalyst,compared with a commercial FCC catalyst.What should be highlighted is that the distribution of products is improved greatly by the core-shell composite catalyst.The yield of LPG is decreased by 6.9%,gasoline is increased by 8.9%,iso-paraffins and olefins in gasoline fraction are increased by 5.21%and 6.31%respectively.At last,a hollow ZSM-5 microsphere with a hollow core smaller than 1?m in diameter and a complete crystal shell with the thickness of about 300 nm is constructed using a sustainable biomass-derived template.The involved simple,cost-effective and efficient method and the unique“surface to core”crystallization process are valuable for the development of hollow microsphere material.This material can be used in the selective hydrodesulfurization of FCC gasoline as excellent catalyst carrier,and then can upgrade the quality of gasoline product.In short,three zeolite catalytic materials with new structures are designed and developed in order to solve the problems existing in the present techniques for processing heavy oil.Moreover,a preliminary study on the application of these catalytic materials is made and the results are outstanding.