Elucidating the Role of Tetraethylammonium in the Silicate Condensation Reaction from Ab Initio Molecular Dynamics Simulations.

Ngoc Lan Mai,Evert Jan Meijer,Thuat T Trinh,Ha T Do,Nguyen Hieu Hoang,Anh H Nguyen,Khanh-Quang Tran

doi:10.1021/acs.jpcb.0c06607

Abstract

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, ab initio 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.

Highlights

Zeolites are nanoporous aluminosilicate materials widely used in various industrial applications making use of their catalytic and separation properties.[1]
Zeolites are typically synthesized from aqueous gel solutions containing various heteroatomic compounds, with inorganic and/or organic cations acting as directing agents of the structure and mobilizing agents
Numerous experimental[2−10] studies have focused on the nature and structure of the silicate oligomers in solution, as understanding the formation of silicate oligomers in the initial stage is key to zeolite tsiyonnthweseirse.11e,x12teTnshiveeelylemstuedniteadryinstecposmfpour tSati(ioOnHal)s4tuodliigeosm11,e1r3i−z2a7using a continuum or explicit model of water

Summary

Introduction

Zeolites are nanoporous aluminosilicate materials widely used in various industrial applications making use of their catalytic and separation properties.[1]. Numerous experimental[2−10] studies have focused on the nature and structure of the silicate oligomers in solution, as understanding the formation of silicate oligomers in the initial stage is key to zeolite tsiyonnthweseirse.11e,x12teTnshiveeelylemstuedniteadryinstecposmfpour tSati(ioOnHal)s4tuodliigeosm11,e1r3i−z2a7using a continuum or explicit model of water. A common pathway of the oligomerization reaction is a two-step mechanism with an initial formation of a penta-coordinated intermediate, followed by a water removal stage.[20,21,28−31] Earlier studies (e.g., refs20, 21, 32) have shown that it is crucial to include the effect of thermal motion and the presence of explicit water molecules when modeling aqueous chemical reactions that involve solvent molecules that strongly bind to the reagents, or actively participate in the reaction mechanism. In the very first stage of silicate oligomer formation in solution, double 3-ring (D3R) and double 4-ring (D4R) structures were observed.[8,38,44] With excess of organic cation, the structures of D4R.8TMA+ and D3R.6TEA+ are the most stable species in solution.[34,35]

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Results

Conclusion