Molecular Dynamics Simulations on the Entrance of Methane and p-Xylene into ZSM-5 Zeolite,Industrial & Engineering Chemistry Research

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Molecular Dynamics Simulations on the Entrance of Methane and p-Xylene into ZSM-5 Zeolite
Industrial & Engineering Chemistry Research ( IF 3.8 ) Pub Date : 2021-08-26 , DOI: 10.1021/acs.iecr.1c01611
Mingcan Zhao _{1,

2} , Wen Lai Huang ₁ , Wei Ge _{1,

2,

3}

Affiliation

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.

中文翻译：

甲烷和对二甲苯进入 ZSM-5 沸石的分子动力学模拟

界面阻力对整体传质性能的显着影响，特别是对于沸石催化，引起了很多关注，但与晶内扩散过程相比，对进入孔过程的理解仍然非常有限。评估进入概率仍然经常依赖于从硬球 (HS) 模拟得出的表达式，其中存在大量近似值。在这项工作中，分子动力学模拟已经进行阐明参与甲烷和界面障碍p-二甲苯 (PX) 分子进入 ZSM-5 沸石。相应地推导出进入概率，以及入射分子的占据分布和停留时间分布。结果也与文献中的 HS 模拟的结果进行了比较。观察到这两种物质的占有率分布沿着进入路径呈现峰值，反映了由于吸附引起的表观阻力，这无法通过 HS 模拟揭示。还观察到双峰分布，这可以归因于单独的吸附和进入孔隙过程的共存。随着温度的升高，这种峰值趋于平稳，接近 HS 模拟的结果。这两个物种的进入概率接近 HS 模拟以及高温下的结果。由于两种物质的进入概率随温度呈不同的递减斜率，在低温下，PX分子的进入概率可高于甲烷分子。然而，PX分子的进入速度总是比甲烷分子低得多，因为它的停留时间要长得多。此类现象已根据熵和能量效应进行了解释。基于伪反应模型，进入概率的结果已经很好地拟合，并相应地提出了一个表达式，有望应用于大规模模型。因为这样的表达式是在比 HS 模拟更复杂的基础上开发的，

更新日期：2021-09-15

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