Molecular Dynamics Simulations on the Entrance of Methane and p-Xylene into ZSM-5 Zeolite
MC Zhao and WL Huang and W Ge, INDUSTRIAL & ENGINEERING CHEMISTRY RESEARCH, 60, 13358-13367 (2021).
The significant influence of interfacial resistance on the overall mass transfer performance, especially for catalysis with zeolites, has attracted a lot of attention, but the understanding on entering-pore processes is still very limited in comparison with that on intracrystalline diffusion processes. Evaluating entering probabilities still depends frequently on the expressions derived from hard sphere (HS) simulations where substantial approximations exist. In this work, molecular dynamics simulations have been performed to elucidate the interfacial barriers involved in methane and p-xylene (PX) molecules entering the ZSM-5 zeolite. The entering probabilities have been derived accordingly, along with the occupancy distributions and the residence time distributions of the incident molecules. The results have been compared with those of HS simulations in the literature as well. It is observed that the occupancy distribution of these two species exhibits peaks along the entering paths, reflecting the apparent resistance due to adsorption, which cannot be revealed through HS simulations. Bimodal distributions have also been observed, which can be attributed to the coexistence of the separate adsorption and entering-pore processes. With the increase in temperature, such peaks tend to level off, approaching the results of HS simulations. The entering probabilities of these two species approach the results from HS simulations as well at high temperatures. Because the entering probabilities of the two species show different decreasing slopes with temperature, at low temperatures, the entering probability of PX molecules can be higher than that of methane molecules. However, the entering rate of the PX molecules is always quite lower than that of the methane molecules because of its much longer residence time. Such phenomena have been explained on the basis of entropic and energetic effects. Based on a pseudo-reaction model, the results of the entering probabilities have been well fitted, and an expression has been proposed accordingly, which is expected to be applied to large-scale models. Because such an expression is developed on a more sophisticated ground than HS simulations, its applications might lead to more satisfactory results.
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