Pore Structure And Adsorption And Diffusion Of Hierarchical Zeolite

Zeolites are a unique class of crystalline aluminosilicates with very high surface area and ordered micropores of molecular dimensions(typically 0.3-1.0 nm).It has become one of the most important nanostructured catalysts and been used widely in the refining and the petrochemistry industries due to its good acidity,shape-selectivity and hydrothermal stability.For traditional zeolitic materials,the pore diameter of microporous zeolites is close to or even smaller than the kinetic diameter of many important hydrocarbon molecules,which imposes the accessibility to active sites and mass transfer of relatively larger hydrocarbons in catalytic reaction,rendering low utilization of the zeolite active sites in catalytic reactions.To circumvent the limitations imposed by the micropores in zeolite,numerous efforts have been made to improve the catalytic performance of zeolite.Inserting secondary mesopores within the microporous zeolite crystals is one of the most frequently employed ways and more attention has been attracted.The micro-/mesoporous hierarchical structures provide distinct benefits of utilizing two different pore size regimes;micropores in the zeolite framework provide sites for adsorption or catalysis of guest molecules,while mesopores provide the highway and adsorption/reaction space for large molecules.As catalyst,the adsorption and diffusion properties of zeolite are the two factors that affect its catalytic performance,and the promotion of mesopores to the adsorption and diffusion properties is closely related to the pore structure of hierarchical zeolite.However,the introduced mesopores are unordered and its pore size is nonuniform,as a result,the pore structure of hierarchical zeolite is complex and unpredictable.Even so,the adsorption and diffusion properties of hierarchical zeolite can be used to explore its pore structure.For this,this paper conducts a systematic and detailed analysis of the adsorption and diffusion properties of various hydrocarbons over two different kinds of hierarchical porous Beta zeolites and mesoporous EU-1 zeolites.Adsorption isotherms and desorption curves were measured by using gravimetric method and ZLC method respectively to investigate the adsorption and diffusion so as to explore the pore structure.The main research contents and conclusions of this paper are as follows:1.Pore structure and adsorption and diffusion of hierarchical Beta zeoliteAdsorption isotherms of both mesitylene and tetrahydronaphthalene over microporous Beta zeolite display a typical type-I adsorption isotherm and the isotherms of mesitylene can be successfully described by the Toth model.The dual-site Toth model fitted the isotherms of mesitylene over mesoporous Beta zeolites very well,and the isotherms of mesitylene and tetrahydronaphthalene show a combination adsorption behaviour of type I and type IV isotherms.The introduction of mesopores into zeolite increases the external surface are,enhances the accessiblity of pore in zeolite which leads to a higher adsorption amount.For diffusion process,as the diffusion path can be shortened and the diffusion resistance can be reduced via introducing mesopores into the Beta zeolite,the Deff/R2 are enhanced greatly while the activation energies for diffusion are reduced.However,the degree of the improvement depends on the interconnectivity of the mesopores.In mesoporous samples,the diffusion of mesitylene is considered to be controlled by micropore diffusion and mesopore diffusion is a dominant mechanism for tetrahydronaphthalene and 1,3,5-triisopropylbenzene transport.2.Evaluation of the main diffusion path in mesoprous EU-1 zeoliteThe diffusion path pattern of n-heptane in microporous EU-1 zeolite belonged to one-dimensional(anisotropic)diffusion and the mesoporous EU-1 behaved approximately as three-dimensional(isotropic)diffusion system as the introduced mesopores interconnected to the inherent micropores in EU-1.The diffusion path did not change within the measured temperature range.This work confirmed that this effective and relatively inexpensive method can be used as an additional tool for the characterization of porous materials.The Deff/R2 of EU-1(meso)is approximately 7 times than that of EU-1(micro),indicating the introduction of the mesopores has greatly promoted the diffusion of adsobates molecules.