Study On The Structure-activity Relationship Of Methyclyclohexane Adsorption And Diffusion In Modified Y Zeolite

Y Zeolite has become the main active component of FCC catalysts in petroleum refining process due to its three-dimensional intercommunicating pore structure,large pore size and high activity.However,the directly synthesized Y zeolite is of low stability and easy deactivation,so it cannot achieve the desired catalytic effect.Therefore,modification is required.After modification,the Si/Al ratio,pore structure,acidity and other properties of Y zeolite will change to different degrees,which will further affect the adsorption and diffusion properties of Y zeolite.In order to explore the influence of changes in pore structure and surface properties of modified Y zeolite on its adsorption and diffusion properties,a series of adsorption and diffusion experiments on various USY zeolites were carried out applying methylcyclohexane as probe molecules,and compared with the experimental results using toluene as the probe molecule.The research content and main conclusions of this paper are as follows:1.Adsorption of methylcyclohexane on modified USY zeolitesThe adsorption isotherms of methylcyclohexane on USY zeolites was measured by weight method（IGA）,the results showed that the adsorption isotherm of methylcyclohexane in NH₄Y zeolite sample was type I isotherm,and those in other modified USY zeolites were type I and type IV composite isotherms.The adsorption amount of methylcyclohexane in USY zeolite samples is affected by two factors:acidity and pore volume.Among them,the adsorption amount of methylcyclohexane is mainly affected by the amount of medium and strong acid and pore volume.While the adsorption capacity is mainly related to the mesopore volume,the larger the pore volume is,the larger the adsorption capacity is.Q,and b as well as the Henry constant（K_H）and initial adsorption heat(Q_st),which represent the the interaction between adsorbate and adsorbent,all show an increasing trend with the increase of the amount of acid on the zeolite surface.2.Diffusion of methylcyclohexane on modified USY zeolitesThe diffusion properties of methylcyclohexane in USY zeolites were measured by zero length column method（ZLC）.It was found that the factors influencing the diffusion process of methylcyclohexane in USY zeolite were complex.The increase of the diffusion rate could be attributed not only to the increase of mesopore volume,but also to the weakening of the interaction between adsorbate and the adsorbent.While the activation energy is mainly affected by the interaction between adsorbate and the surface of zeolite.3.Comparison of adsorption and diffusion of methylcyclohexane and tolueneIn order to further explore the influence of different probe molecular properties on the adsorption and diffusion properties of zeolite,the experimental results of methylcyclohexane and toluene were compared.The results showed that the interaction force between toluene and the electrostatic field on the zeolite surface was stronger due to the larger polarity of toluene,and the interaction force between toluene molecules was also stronger,and they were more closely arranged in the zeolite pore channels.Therefore,the adsorption capacity of toluene in the samples of USY（1）,USY（S）,USY（FS）,USY（F）,USY（530）and USY（570）was larger than that of methylcyclohexane.However,other samples have lower preferential adsorption capacity for polar molecule toluene due to the different surface properties of zeolites.As for the diffusion,the effective diffusion constant of toluene in all USY zeolites is less than that of methylcyclohexane due to the stronger interaction between polar molecules and the surface of zeolite and the greater resistance during the diffusion.