Mesoporous Pore Structure And Adsorption And Diffusion Of Hydrocarbons In Hierarchical Zeolite

Zeolites have been applied widely on many industrial processes such as adsorption and catalysis due to their high surface area, adsorption capacity, ion-exchange capacity and regular arrays of channels and cavities. However, the small pore size of zeolite limits its application on the reactions involving large molecules resulting from the restriction of the intracrystalline accessibility and diffusion of reactants and products. Apparently, these drawbacks will lead to undesirable impacts on the usage of zeolites. A promising way to overcome above shortages is to introduce a secondary pore system in zeolites to form hierarchical (or mesoporous) materials. with materials containing both micro-and mesopores, allowing for promoting the rate of molecular transport by the presence of mesopores and maintaining simultaneously the functionality of the material owing to their contents of micropores. In these aspects, hierarchical zeolites have attracted considerable attentions because of their potential advantages in catalysis.Currently, studies of the hierarchical porous zeolite mainly focus on the preparation, characterization, and evaluation of its catalytic performance. However, based on the process of the specific catalytic application of the hierarchical porous zeolite, it is clear to see that adsorption and diffusion of the adsorbate in the crystal are the main factors affecting the process performance. 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 zeolite. Adsorption isotherms and desorption curves were measured by using gravimetric method and ZLC method respectively to investigate the mechanism of adsorption and diffusion in order to offer theoretical basis for the usage of hierarchical porous zeolite. The main research contents and conclusions of this paper are as follows:1. Adsorption and diffusion of xylene isomers in hierarchical porous Beta zeolitesDifferences in kinetic diameter, configuration and polarity of xylene isomers result in different adsorption capacities and KH. As p-xylene is a non-polar molecule and its kinetic diameter is small, makes it the smallest Henry constant value. While in the mesopores, the aperture size is so large that for these three molecules is not limiting, due to the minimum molecular volume of m-xylene, that the same volume filling makes it slightly larger than the amount of adsorption of m-xylene and p-xylene. 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. Moreover, the values of xylene isoners increase with the increasing mesoporosity of Beta zeolites in the same trend.2. Adsorption and diffusion of mesitylene on hierarchical ZSM-5 microspheresThe adsorption amounts of mesitylene on ZSM-5(a) are nearly zero but on ZSM-5(n) the adsorption amounts increase to a high level. In addition to this, by contrast the relationship between the theoretical monolayer adsorption capacity of mesitylene via the external surface area and real monolayer adsorption capacity on the ZSM-5 sample, it can be found, with the increase in the amount of mesopore holes introduced, more and more the mesopore holes can be reached, connectivity between the holes is also enhanced, effective diffusion time constants of mesitylne also increases by a large margin as the introduced mesopore hole is growing.