Molecular Dynamics Simulations on the Entrance of Methane and p‑Xylene into ZSM‑5 Zeolite

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.

界面阻力对整体传质性能的显著影响，尤其是在沸石催化体系中，已受到学界广泛关注；但相较于晶内扩散过程，学界对分子入孔过程的认知仍相当有限。当前评估分子入孔概率的方法，仍常依赖于基于硬球（hard sphere, HS）模拟推导得到的表达式，而这类模拟存在大量近似假设。本研究通过分子动力学模拟，阐明了甲烷与对二甲苯（p-xylene, PX）分子进入ZSM-5沸石过程中涉及的界面能垒，据此推导了两类分子的入孔概率，同时得到了入射分子的占位分布与驻留时间分布，并将所得结果与文献中的硬球模拟结果进行了对比。研究发现，两类分子的占位分布沿入孔路径呈现峰值，反映了吸附作用带来的表观阻力——这一现象无法通过硬球模拟观测到；同时还观测到了双峰分布，这可归因于独立吸附过程与入孔过程的共存。随着温度升高，这类峰值逐渐趋于平缓，结果趋近于硬球模拟的预测值，两类分子的入孔概率在高温条件下也同样趋近于硬球模拟的结果。由于两类分子的入孔概率随温度下降的斜率存在差异，在低温条件下，对二甲苯分子的入孔概率甚至高于甲烷分子；但由于对二甲苯分子的驻留时间更长，其入孔速率始终远低于甲烷分子。上述现象可基于熵效应与能量效应进行解释。本研究基于拟反应模型对入孔概率的结果进行了良好拟合，并据此提出了一个可应用于大规模模型的表达式；由于该表达式的推导基础相较于硬球模拟更为完善，其应用有望获得更优的预测结果。