利用分子動力學研究沸石初始階段鋁矽酸鹽網絡的形成;Studying the Formation of Aluminosilicate Networks during Initial Stages in Zeolites Using Molecular Dynamics

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题名:	利用分子動力學研究沸石初始階段鋁矽酸鹽網絡的形成;Studying the Formation of Aluminosilicate Networks during Initial Stages in Zeolites Using Molecular Dynamics
作者:	許芸禎;Hsu, Yun-Chen
贡献者:	化學工程與材料工程學系
关键词:	分子動力學;沸石結晶;鋁矽酸鹽縮合;結構導向劑;金屬陽離子;Molecular Dynamics;Zeolite Crystallization;Aluminosilicate Polymerization;Structure-Directing Agents;Metal Cations
日期:	2025-07-24
上传时间:	2025-10-17 11:26:48 (UTC+8)
出版者:	國立中央大學
摘要:	沸石是一類具微孔結構的材料，因其優秀的吸附能力、形狀選擇性以及離子交換能力，被廣泛利用於石化與化學產業中的固體催化劑。在沸石的合成過程中，常添加結構導向劑（structure-directing agents, SDAs），其被認為在調控孔洞形貌與尺寸方面扮演關鍵角色。因此，深入理解結構導向劑在沸石形成中的作用，對於設計具特定孔結構的沸石材料至關重要。然而，礙於原子尺度表徵技術的侷限，目前對其形成機制的理解仍尚未被完全揭示。尤其是結構導向劑與鋁矽酸鹽網絡在孔洞結構形成過程中的具體作用與相互作用仍有待釐清。相較之下，分子模擬提供了可於原子層級探討結構導向劑對孔洞幾何結構影響的有效途徑。因此，本研究利用分子動力學模擬（MolecularDynamics）中的反應性力場（ReaxFF Force Field），以探討在沸石形成初期階段，結構導向劑與純矽酸鹽跟鋁矽酸鹽之間的相互作用行為。本研究依據實驗設置改變系統中金屬陽離子的組成，其中金屬陽離子也就是結構導向劑的種類包括鈣離子、鈉離子、鉀離子，模擬不同結構導向劑對純矽酸鹽與鋁矽酸鹽系統之結構演化的影響。首先，在保持陽離子數量一致的條件下，模擬結果顯示，縮合程度介於 0.2 至 0.4，顯示系統處於沸石結晶初始階段，且無論陽離子種類為何，只要 Si/Al 比與陽離子數量相同，各系統最終的縮合程度與聚集行為相近，鋁矽酸鹽的初期縮合反應速率普遍快於純矽酸鹽系統。其次，在比較不同 SDA 濃度下，純矽酸鹽與鋁矽酸鹽系統中顯示金屬陽離子在高濃度下可促進純矽酸鹽縮合反應，而配對距離分析進一步指出，鋁矽酸鹽有傾向發展成單一的巨大團簇結構，而純矽酸鹽則偏好形成多個且分散的微小團簇。此外，當在保持相同 Si/Al 比的條件下擴大模擬系統規模時，最大團簇尺寸顯示尺寸效應對最終的拓撲結構影響較大，隨著組成單體數量的增加，系統中更容易形成尺寸更大、結構更連續的團簇，其中團簇尺寸最多可增加近兩倍。然而，對於 Qn 分布變化不明顯，說明尺寸效應對縮合反應的速率影響有限。另外，在模擬過程中也發現在縮合反應進行時鈉離子有穩定鋁矽酸鹽結構的作用。綜上所述，本研究結果可望增進對結構導向劑與純矽酸鹽與鋁矽酸鹽間交互作用的理解，並為設計特定結構之沸石材料提供理論依據。;Zeolites are microporous materials extensively utilized as solid catalysts in petroleum and chemical industries due to their high adsorption capacity, shape selectivity and ion-exchange ability. During zeolite synthesis, structure-directing agents (SDAs) are usually added and is believed to play a pivotal role in controlling pore shape and dimensions. Therefore, the comprehensive understanding in the role of the SDAs is essential for tailoring zeolites with desired pore structures. However, understanding of their formation remains incomplete due to the limitations of characterization techniques at an atomistic scale. Specifically, the precise roles and interactions between aluminosilicate networks and SDAs in determining pore structure formation is still unclear. In contrast, molecular simulations offer an approach to investigate how SDAs influence structure geometry at the atomic level. Therefore, this study utilizes molecular dynamics (MD) with the reactive force field (ReaxFF) to investigate the interplay between aluminosilicate and SDA during the formation of zeolites at the initial stages. A series of MD calculations are performed to explore the effects of different SDAs on pure silicate and aluminosilicate polymerization by varying the types and counts of metal cations on the basis of experimental data. First, under the condition of keeping the number of cations the same, the simulation showed that the degree of condensation (c) ranges from 0.2 to 0.4, indicating that the systems are in the initial stages of crystallization, and regardless of the type of cations, as long as the Si/Al ratio and the number of cations were the same, the degree of condensation and the aggregation behavior of each system were similar, and the condensation rate of aluminosilicate systems are generally faster than that of pure silicate systems. Next, in the comparison of aluminosilicate and pure silicate systems with the different SDA concentrations, it was shown that metal ions can promote condensation reaction at high SDA concentration. Pairwise distributions revealed that aluminosilicates tend to develop into one dominant and large cluster structure, while pure silicates prefer to form multiple and dispersed clusters. In addition, when the simulation system is expanded while maintaining the same Si/Al ratio, the maximum cluster size showed that the box size effect has a greater impact on the final topological structure. As the number of monomers increases, it is easier to form larger and more continuous clusters in the system, and the cluster size can increase by nearly two times at most. However, the Qn distribution did not change much, implying that the box size effect had a limited effect on the rate of the condensation reaction. Furthermore, it was found that Na ions have a stabilizing effect on the aluminosilicate structure during the condensation reaction. In summary, it is anticipated that the outcome from this study will enhance the understanding of the interaction between SDA and pure silicates/aluminosilicates, and provide a theoretical foundation for the design of zeolite materials with tailored structures.
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