Deposition Of Si,Al Species And Zeoiltization To Synthesize Core-shell Structured Zeolite-zeolite Composite FAU@MFI

As a highly versatile member of the faujasite family,because of its highly acid properties,thermal stability and shape-selectivity,Y zeolite is widely used as a catalyst to the catalytic reactions such as catalytic cracking,hydrocracking and alkylation.However,the intrinsically tiny micropores of Y zeolie limit the mass transfer of reagents and products during the catalytic reaction.Furthermore,the vast majority of acid sites exist in the micropores of Y zeolite.Although the occupation rate of acid sites on the external surface of Y zeolite accounts for 3-5%of total amount of acid,the existence of acid sites on the external surface,on the one hand,can impair the shape selectivity characteristics of zeolite catalyst;on the other hand,it facilitates the deposition of carbon on the external surface,which may reduce the effective diameter of pore openings and even block the pores that would increase the diffusion resistance of reagents and products and ultimately gives rise to an untimely deactivation of the catalyst during the catalytic reaction.Overgrowth of another high silica zeolite on the external surface of Y zeolite,not only retains the adequate acid sites in the channel system of Y zeolite,but reduces the acid density on the external surface as as to improve the acidic property of zeolite surface,which makes the catalyst have a more gentle performance in the catalytic reaction and thus enhance the activity and stability of catalyst.In this thesis,we used a method of composite modification to adjust and modify the external surface acid properties of Y zeolite by overgrowth of high silica MFI-type zeolite.Core-shell structured composite FAU@SiO2 was synthesized via a surfactant-directed sol-gel coating strategy by silicon source as a precursor,and subsequently for the synthesis of core-shell structured zeolite-zeolite composite FAU@ZSM-5.The incompatibility between the core crystals and the shell precursor was circumvented by amorphous silica deposited on core zeolite surface,which upon a subsequent hydrothermal treatment induced the formation of a well overgrown shell.The structural,textural and acid properties of the as-synthesized composites were characterized using X-ray diffraction,SEM,TEM(SAED),N2 adsorption-desorption,FT-IR and NH3-TPD.The catalytic performances of the as-synthesized samples were investigated by craking of n-heptane and isopropylbenzene,respectively.The results displayed that a hierarchical pore system was created in the as-synthesized core-shell structured zeolite-zeolite composites possessing a FAU core and a shell of overgrowth ZSM-5 crystals.In the process of n-heptane cracking,the zeolite composite showed excellent catalytic performance with higher conversion of n-heptane and better selectivity to ethylene and ethane than that of pure FAU zeolite and sediment sample FAU@Si02;as compared with the corresponding mechanical mixture of FAU and ZSM-5 zeolite,the composites had a higher isopropylbenzene conversion and a longer catalytic life due to the introduced hierarchical pores system during cracking of isopropylbenzene.