C8芳烃在BaX分子筛中的扩散机理

通过分子动力学方法，模拟对二甲苯、邻二甲苯、间二甲苯和乙苯4种分子的混合物在纯硅FAU分子筛(SiFAU)和钡离子交换的X型分子筛(BaX)中的扩散过程；通过密度泛函理论和波函数方法计算和分析C₈芳烃分子的电子密度等值面及其确定的分子尺寸，同时考虑了乙苯侧链柔顺性导致的构象多样性。将C₈芳烃分子的横截面积、分子筛的孔口尺寸变化与扩散能力进行关联，来探究C₈芳烃分子在BaX分子筛孔道内动态扩散的微观机理。结果表明：相比SiFAU分子筛，BaX分子筛中阳离子的引入减小了孔口尺寸，4种C₈芳烃分子在BaX分子筛中的扩散能力由大到小的顺序为对二甲苯、乙苯、邻二甲苯、间二甲苯，这与其最小横截面积大小(乙苯平面构象)的顺序相一致。

Diffusion Mechanism of C8 Aromatics in BaX Zeolite

The method of molecular dynamics was used to simulate the diffusion process of the mixture of four molecules, i.e., p-xylene (PX), o-xylene (OX), m-xylene (MX) and ethylbenzene (EB), in the pure silicon FAU zeolite (SiFAU) and barium ion-exchanged X-type zeolite (BaX). The density functional theory (DFT) and wave function method were employed to calculate and analyze the electron density isosurface of C₈ aromatics and its molecular dimensions, and the conformational diversity due to ethylbenzene side chain flexibility was also considered. Moreover, the changes in the cross sectional area of C₈ aromatics and pore mouth size of zeolite were correlated to the diffusion ability, so as to explore the micromechanism for the dynamic diffusion of C₈ aromatics in the pore channel of BaX zeolite. The results shows that compared with SiFAU zeolite, the pore mouth size is decreased due to the introduction of cation into BaX zeolite, and the diffusion abilities of four C₈ aromatic molecules in BaX zeolite are ordered as follows: PX>EB>OX>MX, which is consistent with the order of their minimum cross sectional sizes (planar conformation of ethylbenzete).