The Theoretical Research Of Hydrogen Storage Capacities Of Ion-Exchanged Zeolites Mazzite And Levyne

The microstructure, macroscopic thermodynamics and transport properties can be obtained by the theoretical calculation which plays a more and more important role in many fields, such as chemical, material, biological and pharmaceutical fields, etc. Hydrogen energy is regarded as the most potential clean energy source to the fuel, which attracts more and more attentions all over the world. The porous materials have potential application in hydrogen storage, especially zeolites, which have regular and uniform channels and cavities, large pore volume and specific surface area, high stability, low cost and a variety of porous structures. The objective of this paper is to research the hydrogen storage capacities of zeolites Mazzite and Levyne by molecular simulation of grand canonical Monte Carlo (GCMC) and molecular dynamic (MD) simulation respectively. And explore microscopic reasons of hydrogen adsorption sites in two zeolites by Density Functional Theory method (DFT). The main conclusions of this study are as followed:The models of zeolites Mazzite and Levyne were constructed according to the XRD date of inorganic chemistry structural database (ICSD). And exchanged different cations(H+, Li+, Na+, K+, Mg2+, Ca2+) in zeolites Mazzite and Levyne, which were used the locate option of Sorption in Materials Studio 4.1.The adsorption of hydrogen in zeolite was studied by GCMC. The zeolite Levyne has more cage structures and has higher hydrogen storage capacity, and Li-LEV zeolite has the highest hydrogen storage capacity (2.67 wt%). The zeolite Mazzite has larger 12-rings, so it could not encapsulate hydrogen and the hydrogen storage capacity is lower. Mg-MAZ has the higest hydrogen storage capacity (2.09 wt%) in all Mazzite zeolites. The heat of adsorption between hydrogen and Mazzite zeolites is higher than that of Levyne zeolites, which shows that the thermal effect of adsorption for hydrogen in zeolite will not be dominant effect on hydrogen adsorption capacities, and the main influential factor is the pore structure. Besides that, bivalent cations have a stronger influence on hydrogen adsorption than univalent cations.The diffusion of hydrogen in Mazzite and Levyne zeolites were studied by MD. It could be found that the self-diffusion coefficient of zeolite Mazzite is larger than that of zeolite Levyne in lower temperature. With the temperature increase, the self-diffusion coefficients of Levyne overpass that of zeolite Mazzite. The diffusion activation energy of hydorgen in zeolite Mazzite is larger than that in zeolite Levyne. So Levyne zeolite has a better diffusion character.Hydrogen adsorption sites in zeolites Mazzite and Levyne were investigated by using DFT method of quantum chemistry. The resultes showed that the interaction between H2 and zeolites was dominated by the action between the hydrogen and Na+ or Li+. Calculated adsorption energies indicated that hydrogen adsorption sites were mainly distributed around the cations which located in the 12-ring and 8-ring of zeolite Mazzite, and in the 8-ring and 6-ring of zeolite Levyne. The cation could not exist around the small ring stably due to the steric hindrance. The more exposed cation exists, the higher bonding energy is between hydrogen and cation. The trend of H-H bond distances and stretching frequency shifts are agreement with the adsorption energies, the lower adsorption energies, the longer H-H bond distances and the lower stretching frequency shifts they are.