Simulation of the effect of coke deposition on the diffusion of methane in zeolite ZSM-5

Abstract

Coke deposition is one of the main reasons for catalyst deactivation, which is often associated with coverage of the active sites and diffusion restrictions. In this article, the self-diffusion coefficient of methane in ZSM-5 (MFI) with different amounts of coke is studied through molecular dynamics simulation combining hard-sphere and pseudo-particle modeling at a gas loading of 4 molecules per unit cell at 723K. The T12 tetrahedral sites are considered as the possible coke deposition sites at which the pore is blocked. It is shown that the self-diffusion coefficient of methane in ZSM-5 without coke (D0) is approximately 7.0×10−9m2/s. Two coke distribution models are proposed to simulate the actual process of coke formation. At the initial stage of coke deposition, the coke with uniform distribution has little influence on the diffusion process. Once the coke deposits are randomly concentrated on some sites and a few pores are therefore completely blocked, the coke has a significant influence on the self-diffusion coefficient, which decreases to 37.0% of D0 almost linearly with increasing coke amount to 20%. The distribution of coke deposition is the result of the coupling of reaction and diffusion processes. These results may be useful for understanding the reaction–diffusion coupling mechanism of the catalyst deactivation.