甲苯歧化反应物与产物在ZSM系分子筛孔道中扩散特性的分子模拟

 应用分子模拟技术计算了甲苯歧化反应中的反应物甲苯、产物苯和二甲苯同分异构体与ZSM系分子筛(MFI (ZSM-5)，MEL (ZSM-11)，MTW (ZSM-12))的相互作用能，得到了扩散特性等信息。计算结果表明，分子扩散的难易程度受分子筛孔径和孔道结构的共同影响。MFI分子筛对于对二甲苯有很好的选择性；芳烃分子在其弯孔中的扩散能垒较直孔中的高，因此，倾向于在MFI分子筛的直孔中扩散。MEL分子筛孔径略小于MFI分子筛，各种芳烃分子的扩散能垒比在MFI分子筛孔道中的高；由于对二甲苯的扩散能垒远小于间二甲苯和邻二甲苯，因此MEL分子筛也表现出对于对二甲苯的较好选择性。MTW分子筛的孔径在三者中最大，各种产物分子的扩散能垒相差较小，对甲苯歧化反应产物的选择性不高。从3种分子筛对于对二甲苯的选择性而言，MFI与MEL相当，MTW较低。当分子筛孔径与扩散分子的动力学直径接近时，分子筛将表现出较好的择形催化性能。

MOLECULAR SIMULATION ON THE DIFFUSION CHARACTERISTICS OF TOLUENE DISPROPORTIONATION REACTANTS AND PRODUCTS IN THE CHANNEL OF ZSM SERIES ZEOLITES

Molecular simulation methods were applied to investigate the diffusion characteristics of toluene disproportionation reactants and products in ZSM series zeolites,such as MFI(ZSM-5),MEL(ZSM-11) and MTW(ZSM-12).The diffusion characteristics were sensitive to the pore dimensions and architectures of zeolites.MFI zeolite showed significant selectivity for p-xylene.The diffusion energy barriers for all the aromatic molecules in the sinusoidal channel of MFI zeolite were higher than that in the straight channel,thus the molecules prefered to diffuse through the straight channel.The calculated diffusion energy barriers for the molecules in MEL zeolite were uniformly higher than that in the channel of MFI zeolite due to the smaller pore dimension of MEL.Because the energy barrier of p-xylene was much lower than that of m-xylene and o-xylene in MEL,MEL zeolite showed efficient shape-selectivity for p-xylene.The pore dimension of MTW zeolite was the largest among the three zeolites and the difference of diffusion energy barriers between every two aromatic molecules was small,so MTW zeolite was not expected to be efficient shape-selective zeolites for toluene disproportionation products.For the shape-selectivity of p-xylene over these three zeolites MFI and MEL were almost the same,MTW was the lowest as predicted from their diffusion energy barriers.The zeolites with channels whose kinetic diameters were close to the dimensions of the aromatic molecules were the shape-selective catalysts.