The Study On Mass Transfer Of One-dimensional Pore Structure Zeolite

In recent years,mordenite and EU-1 zeolites have been widely used in the field of catalysis due to their unique structural advantages and excellent hydrothermal stability.For example,mordenite has been used in the field of alkane and aromatics isomerization,and the EU-1 zeolite is used as a xylene isomerization catalyst.These two zeolites have great potential in the fields of fine chemicals,petrochemicals and other fields.What's more,mordenite and EU-1 are zeolites of one-dimensional pore structure.This structural feature allows their shape selectivity,but restricts their further application.As a result,mordenite and EU-1 zeolites with hierarchical pore structure have been developed.For a catalytic reaction,the adsorption and diffusion of reactant and product molecules in zeolite play a very important role.Therefore,the study of adsorption and diffusion properties of adsorbate in zeolite can help to optimize the catalytic reaction process.Some thermodynamic and kinetic data calculations and analyzes have some guiding significance for the design,preparation and better application of zeolite molecular sieve catalysts.The current research on mordenite and EU-1 zeolites mainly focus on the preparation and catalysis.However,the adsorption and diffusion properties of zeolites with such distinctive structures have seldom been reported.In order to explore the adsorption and diffusion mechanism on the zeolite with one-dimensional pore structure and investigate the effect of the introduction of mesopores on the adsorption and diffusion behaviors of these zeolites,a series of adsorption and diffusion experiments on various mordenite and EU-1 zeolites were carried out applying toluene and mesitylene as probe molecules.It was expected to lay the theoretical foundation for the evaluation,application and preparation of hierarchical pore mordenite and EU-1 zeolites.The main contents and conclusions of this paper are as follows:1.Adsorption and diffusion of toluene and mesitylene in hierarchical pore mordenite zeolitesToluene can be adsorbed into the microspores of the mordenite,but the desorption of toluene is quite difficult and requires high temperature.The mesitylene molecule can not enter the microspores of the mordenite,but it can be adsorbed in the hierarchical pore zeolite samples,which indicates that the introduction of mesopores can improve the accessibility of macro-molecules.The adsorption process of toluene and mesitylene in microporous mordenite is mainly endothermic,while in the hierarchical pore zeolite samples,it is exothermic.The TPD experiment of toluene proves that there are two kinds of adsorption centers in the mordenite,one represents the physical adsorption with relatively weaker force,and the other maybe the chemical adsorption with strong force.The dual site Toth model can describe the adsorption behavior of toluene in hierarchical mordenite well.The fitting parameters indicate that mesopores are favorable for promoting adsorption and weakening the interaction between toluene and adsorbent.The results of adsorption kinetic experiments show that the presence of mesopores can shorten the diffusion path and improve the mass transfer rate of the molecules.2.Adsorption and diffusion of toluene and mesitylene in hierarchical pore EU-1 zeoliteThe adsorption isotherm of toluene in the microporous EU-1 zeolite is type-I isotherms.However,the adsorption isotherms in the corresponding hierarchical EU-1 zeolites are the multi type isotherms of ? and ?.The mesitylene can not enter the microspores of EU-1 zeolite,but it can enter the mesopores of hierarchical EU-1,which shows that the introduction of mesopores can improve the accessibility of macromolecules.When the adsorbate molecules are adsorbed only on the surface of the EU-1 zeolite,the adsorption process is mainly exothermic,while in the microspores of the zeolite,it will be converted into endothermic adsorption because the probe molecule will suffer a strong diffusion limiting.The adsorption isotherms of toluene in hierarchical EU-1 zeolites were fitted by dual site Toth model.The results show that the introduction of mesopores is beneficial to increase the adsorption capacity.For the EU-1-3 sample,the introduction of the mesopores makes the side bag structure of EU-1 zeolite open,so that the adsorption capacities both of toluene in the microspores and mesopores are significantly increased.The study of adsorption kinetic shows that the presence of mesopores is beneficial to improve the mass transfer process of the molecules in EU-1zeolite.