A Study Of Molecular Diffusion Behavior In MFI-Type Zeolite Catalyst

Since the zeolite catalyst ZSM-5 was firstly synthesized by Mobile, this catalyst is widely used in oil catalytical refining industries. The studies of modification and research about ZSM-5 are conducted by lots of researchers. Because of the limitation of experimental conditions, we can not observe the movement of guest-molecules in ZSM-5 directly. So we should find the mechanism of molecular diffusion inside the ZSM-5, and the mechanism of guest-molecules diffusion will be useful for improving the performance of ZSM-5 for catalyzing. The Molecular Dynamics (MD) method is one of the popular ways to study the movement of molecules and the conditions of system balance. So we will discuss the movement behavior of the guest-molecules and explore the diffusion mechanism inside the ZSM-5 zeolite by using MD method. The contents of this study consisted of follows:(1) Build the molecular structure of ZSM-5 by using the Buckingham potential. The molecular structure of ethylene, propylene, methanol, cumene was built by using the harmonic potential. And we use L-J 12-6 to describe the interaction between zeolite and guest-molecules. There are two different guest-molecules models available for building the guest-molecules structure, which are full-atom model and united-atom model. We discuss the influence to the self-diffusion coefficient between two different molecular models, which we have brought to build the structure of guest-molecules. Based on the structure of zeolite, we studied the thermostability of ZSM-5 molecules. The geometric structure of host-molecular and guest-molecules was stable, and these molecules had well performance on thermostability.(2) We have studied the changes of simulation system energy during the relaxation process. We also discussed the initialization location of guest-molecules inside ZSM-5. Based on the result, we simulated the single-molecule diffusion of these four kinds of guest-molecules in ZSM-5. The temperature of diffusion systems ranged from 100° to 500°. The guest-molecules are traced and the program output the track of guest-molecules inside zeolite, so we can calculate their self-diffusion coefficient. The result showed that the self-diffusion coefficient increased with increasing temperature. There are difference between the four kinds of guest-molecules on diffusion. The main factors of this phenomenon are the functional group in guest-molecule and the size of them. Besides, we studied the diffusion behavior by different channels in ZSM-5. The size of Straight channel is spacious and the construction of this pore is simple, so that the guest-molecules move actively; On the contrary, the movement of guest-molecules in Zigzag channel is negatively, because of the size of Zigzag channel is smaller and its structure is more complex.(3) We conducted the multi-molecules simulation systems by adding more molecules inside the unit ZSM-5 zeolite in order to discover the changing laws of self-diffusion coefficient. The result showed that self-diffusion coefficient decreases with increasing temperature. The rising of concentration influences methanol and cumene more heavily, but lightly on alkene molecules. The influence of temperature become more slightly with increasing concentration.