Faculty of Applied Sciences, Ton Duc Thang University, 19 Nguyen Huu Tho Str., Tan Phong Ward, District 7, Ho Chi Minh City, Vietnam.
Department of Materials and Nanotechnology, SINTEF Industry, 7034 Trondheim, Norway.
J Phys Chem B. 2020 Nov 12;124(45):10210-10218. doi: 10.1021/acs.jpcb.0c06607. Epub 2020 Oct 29.
The understanding of the formation of silicate oligomers in the initial stage of zeolite synthesis is important. The use of organic structure-directing agents (OSDAs) is known to be a key factor in the formation of different silicate species and the final zeolite structure. For example, tetraethylammonium ion (TEA) is a commonly used organic template for zeolite synthesis. In this study, molecular dynamics (AIMD) simulation is used to provide an understanding of the role of TEA in the formation of various silicate oligomers, ranging from dimer to 4-ring. Calculated free-energy profiles of the reaction pathways show that the formation of a 4-ring structure has the highest energy barrier (97 kJ/mol). The formation of smaller oligomers such as dimer, trimer, and 3-ring has lower activation barriers. The TEA ion plays an important role in regulating the predominant species in solution via its coordination with silicate structures during the condensation process. The kinetics and thermodynamics of the oligomerization reaction indicate a more favorable formation of the 3-ring over the 4-ring structure. The results from AIMD simulations are in line with the experimental observation that TEA favors the 3-ring and double 3-ring in solution. The results of this study imply that the role of OSDAs is not only important for the host-guest interaction but also crucial for controlling the reactivity of different silicate oligomers during the initial stage of zeolite formation.
理解沸石合成初期硅酸盐齐聚物的形成过程非常重要。众所周知，有机结构导向剂（OSDA）的使用是形成不同硅酸盐物种和最终沸石结构的关键因素。例如，四乙铵离子（TEA）是沸石合成中常用的有机模板。在这项研究中，采用分子动力学（AIMD）模拟来深入了解 TEA 在各种硅酸盐齐聚物形成过程中的作用，包括二聚体到 4 元环。计算得到的反应途径自由能曲线表明，形成 4 元环结构的能垒最高（97 kJ/mol）。形成较小的齐聚物，如二聚体、三聚体和 3 元环的活化能垒较低。TEA 离子通过在缩合过程中与硅酸盐结构配位，在溶液中对主要物种的调节起着重要作用。齐聚物反应的动力学和热力学表明，3 元环的形成比 4 元环结构更有利。AIMD 模拟的结果与实验观察一致，即 TEA 有利于 3 元环和双 3 元环在溶液中的形成。这项研究的结果表明，OSDA 的作用不仅对主客体相互作用很重要，而且对控制沸石形成初期不同硅酸盐齐聚物的反应性也至关重要。
