芳烃、环烷烃分子在MFI和FAU分子筛中扩散行为的分子模拟

 应用分子模拟技术计算了不同芳烃和环烷烃分子最低能量构象的分子尺寸(a×b×c)，并计算了其在 MFI、FAU 分子筛中的扩散能垒。计算结果表明，分子的最小截面尺寸(a×b)与扩散能垒相关。苯、萘、环戊烷、环己烷分子可以在 MFI 分子筛中扩散，其它多环芳烃、多环环烷烃分子均很难扩散。分子在 MFI 直孔道中扩散比在正弦孔道中容易。FAU 分子筛由于孔径尺寸较大，分子在其孔道内扩散比在 MFI 孔道中容易，而两超笼间的十二元环会限制芘、全氢芘及比之更大的分子在 FAU 孔道中扩散。从计算结果可以推断，催化裂化原料中的重油大分子只有先在分子筛或其他活性材料表面一次裂化成一定尺寸的小分子，才能扩散进入 MFI、FAU 分子筛晶内发生进一步反应。

Molecular Simulation for the Diffusion Characteristics of Aromatic and Naphthenic Hydrocarbons in the Channel of MFI and FAU Zeolites

Molecular simulation methods were applied to investigate the molecular dimensions (a×b×c) of different aromatic and naphthenic hydrocarbons in their minimum energy configurations and their diffusion energy barriers(ED) in MFI and FAU zeolites. According to the results, molecular diffusion energy barrier was sensitive to the minimum cross-sectional dimension (a×b) of molecule. Benzene, naphthalene, cyclopentane and cyclohexane molecules could diffuse in the channel of MFI, while other larger aromatic and naphthenic molecules could not. Hydrocarbon molecules diffused more easily in the straight channel than in the sinusoidal channel of MFI zeolite. Aromatic and naphthenic molecules diffused more easily in FAU than in MFI because of the larger pore size of FAU zeolite. The 12-ring openings between two adjacent super cages of FAU zeolite limited the diffusion of pyrene, perhydropyrene and other larger hydrocarbon molecules. It can be concluded that the heavy oil molecules could only first be cracked into small molecules on the surface of zeolites or other active materials, then diffuse into the channel of MFI and FAU zeolites for further conversion.