Structure And Properties Of Modified Y Zeolites

Y zeolite is the main component of the catalyst in the fluid catalytic cracking（FCC）reaction at the petroleum refining process.Due to the poor hydrothermal stability and poor carbon resistance of microporous Y zeolite,the modified Y zeolites are usually used as a catalyst in the industry.At present,the modified Y zeolites are mainly prepared by post-treatment in industry.The preparation method of modified Y zeolite samples selected in this article are:Modified Y zeolites were prepared by acid alkali modification,hydrothermal treatment and ammonium fluosilicate modification from NH₄Y,respectively,labeled YS,YR and YF.The modified Y zeolite of YF samples was marked as YFS by acid-alkali modification.The samples were labeled as YFR-530 and YFR-570 by hydrothermal treatment of YF samples at 530℃and 570℃.The structural properties,adsorption properties and diffusion properties of six modified Y zeolites（YS,YR,YF,YFS,YFR-530 and YFR-570）were studied to compare the effects of different modification methods on the production of Y zeolites.The specific experiment content is divided into the following three parts:1.The effects of different modification methods of the relative crystallinity,appearance,secondary pore structure,silicon and aluminum element distribution and acidity of zeolite were investigated by X ray diffraction（XRD）,scanning electron microscopy（SEM）,gas physical adsorption,solid NMR and acid characterization（including NH₃-TPD,Pyridine-IR）.The experiment found that six kinds of modification methods can effectively increase the Si/Al ratio of Y zeolite and increase the porosity of mesoporous.The effective mesoporous volume of YS and YFS samples is the largest and the acid concentration is the largest.Due to the high framework aluminum content of YS and YFS,the relative content of Br?nsted acid is relatively large.YR sample has a large effective mesoporous volume and weak acid concentration,and the Lewis acid content is relatively large.The YF,YFR-530 and YFR-570 samples have similar properties,with the highest Si/Al ratio,and the mesoporous volume and acid concentration of the samples is all small.2.Selecting toluene as a probe molecule,the adsorption isotherm and adsorption kinetics curves of the modified Y zeolites were measured.The adsorption capacity,adsorption force,Henry constant,adsorption heat and adsorption rate constant on modified Y zeolites were calculated by Dual-Toth model and first order kinetic equation to explore the influence of the properties change of Y zeolites on the adsorbability of modified Y zeolites.It was found that the adsorption amount of toluene on the modified Y zeolites was related to the mesoporous volume,and the interaction force and adsorption rate between toluene and zeolites were related to the concentration of zeolites acid.The saturated adsorption capacity,adsorption force,and adsorption rate of YS and YFS samples for toluene are the largest of the six modified Y zeolites.The YR sample has larger saturated adsorption capacity for toluene,but the adsorption force and adsorption rate are weaker.YF,YFR-530 and YFR-570 have the weakest adsorption properties.3.The diffusion curves of toluene on four samples of YS,YR,YF and YFS were measured by the zero length column method（ZLC）.The effective diffusion constants and diffusion activation energy of toluene in modified Y zeolites were calculated to explore the influence of the structural properties of the modified Y zeolites on their diffusion properties.The increase of effective mesoporous volume in zeolite can enhance the diffusivity of zeolite,but the acid of zeolites surface will impede the diffusion of adsorbents in zeolite pores.Although the YS and YFS samples have the largest effective mesoporous volume,the diffusion performance are worst due to the acid concentration are much higher than that of other samples.The toluene diffused the fastest in the YR sample,but the diffusion activation energy was larger.The diffusivity of YF samples is between YR and YS samples.