烃分子尺寸及其与扩散能垒关系的初步研究

Molecular Dimensions of Hydrocarbons and a Primary Study on Its Relationship to Diffusion Energy Barriers in Zeolites

依次采用分子力学、分子动力学、量子力学方法计算得到烃分子的最低能量构象,获得了分子的三维尺寸,并与动力学直径进行了对比分析,并初步研究了烃分子尺寸与在MFI、FAU分子筛中扩散能垒的关系。结果表明,在分子筛催化研究中用三维尺寸描述烃分子大小比用动力学直径更合理。与环烷烃、芳烃分子相比,长链烷烃分子结构柔性较大,长链烷烃分子尺寸不能只考虑其最低能量构象的尺寸,还应考虑分子构象的动力学变化。在同一分子筛中,随烃分子最小截面尺寸的增加,扩散能垒增大;同一分子在MFI分子筛中的扩散能垒大于在FAU分子筛中的扩散能垒。研究结果对于探索分子筛择形催化机理具有一定的理论价值。

Molecular mechanics,molecular dynamics and quantum mechanics methods were used to calculate the lowest energy molecular structures of hydrocarbons.The molecular three-dimensional dimensions were obtained and compared with kinetic diameters.A primary study was made on the relationship between molecular dimensions and diffusion energy barriers in MFI and FAU zeolites.It was proposed that it was more reasonable to describe the size of a molecule by using molecular three-dimensional dimensions（a×b×c） than by using kinetic diameters in zeolite catalysis.The structures of long-chain alkanes were more relaxed than aromatic and naphthenic hydrocarbons.Not only the dimensions of the lowest energy molecular structures but also the dynamics factors should be considered when the dimensions of long-chain alkanes were discussed.It was found that the diffusion energy barriers increased with the increase of minimum cross-sectional dimensions in the same zeolite.The diffusion energy barrier of a molecule was higher in MFI zeolite than that in FAU zeolite.These research results can offer some useful theory basis to the shape-selective catalysis of zeolite.