MD模拟研究MFI沸石晶体双模板剂择优生长机理

doi:10.6023/A1203041

摘要/Abstract

以分子动力学(MD)方法模拟研究双模板剂在全硅MFI型沸石b轴择优取向晶体定向生长过程中的作用机制. 分别以全硅晶格模型和切面模型, 模拟两种模板剂分子(正丙胺、六亚甲基四胺)各自在孔道交叉笼内和孔道开口处的动力学行为, 考察其动力学吸附位置以及与沸石孔壁的尺寸匹配作用. 计算表明, 正丙胺分子对MFI晶体的形成和生长起主要导向作用, 而六亚甲基四胺分子有利于晶体沿a轴方向的晶面生长, 却阻碍沿b轴方向的晶面生长.

关键词: MD模拟, MFI沸石, 定向生长, 模板剂

Co-templating role in the growth of b-axis oriented siliceous MFI type zeolite crystals was studied by molecular dynamic (MD) simulation. A siliceous supercell consisting of 2×2×2 unit cell is adopted as the inner part of the simulated structure for the zeolite. The crystal structures, adopted as simulated growing part of the zeolite, are four cleavage surface models along a or b axis individually, which cleaved from a supercell consisting of 1×1×2 unit cell and the Si-O hanging bonds are saturated by H atoms. The dynamic behaviors of two templates (n-propylamine and hexamethylene tetramine) in the cages of channel intersections and the opening channel windows were investigated with NVT MD simulations in 450 K for 5×10⁵ fs each run, using pcff forcefields. The simulations were performed by Discover module from Accelrys Material Studio. The interaction energies between the templates and the zeolite, and the average values of coordinates imaging the trajectories of the templates in times of whole MD simulation, are used as two indicators for investigating the dynamic adsorption sites and the size match with the channel wall for the templates. The results indicate: (1) hexamethylene tetramine (HMTA) molecule is hold at the opening window of the channel along a axis (site I) and extruded out of the opening window of channel along b axis (site III) to nearby crystal-growing site (site IV); (2) mean-squared-displacement (MSD) analysis reveals that strong interaction exists between n-propylamine molecule and the zeolite because of the fact that N-atom is sticking to the adsorption site while other atoms of the molecule are rotating and moving with freedom. On the other hand, all atoms of HMTA molecule are almost frozen in the constrained space of the channel cage. As an evidently deduction, n-propylamine molecules play a major directing role in the formation and the growth of the zeolite crystals, while HMTA molecules promote the growth of the crystal surface along a axis and impede the growth of the crystal surface along b axis.

Key words: MD simulation, MFI zeolite, oriented growth, template

引用此文

余辉, 郭娟, 汪靖, 龙英才. MD模拟研究MFI沸石晶体双模板剂择优生长机理[J]. 化学学报, 2012, 70(14): 1543-1550.

Yu Hui, Guo Juan, Wang Jing, Long Yingcai. Growth Mechanism Study on b-axis Oriented MFI Zeolite with MD Simulation[J]. Acta Chimica Sinica, 2012, 70(14): 1543-1550.

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