Structure-Directing Behaviors of Tetraethylammonium Cations toward Zeolite Beta Revealed by the Evolution of Aluminosilicate Species Formed during the Crystallization Process
Structure-Directing Behaviors of Tetraethylammonium Cations toward Zeolite Beta Revealed by the Evolution of Aluminosilicate Species Formed during the Crystallization Process
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DOI：
10.1021/jacs.5b11046
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发表时间：
2015-11-18
影响因子：
15
通讯作者：
Okubo, Tatsuya
中科院分区：
文献类型：
--
作者：
Ikuno, Takaaki;Chaikittisilp, Watcharop;Okubo, Tatsuya
有机结构导向剂（OSDAs）在沸石合成中得到了广泛的应用。在大多数情况下，osda被封闭在沸石中，作为一个孤立的阳离子或分子几何上适合于沸石腔。使用四乙基铵（TEA(+)）阳离子作为OSDA合成的沸石则不是这种情况，其中ca. 6个TEA（+）阳离子的簇/聚集体完整地封闭在沸石的腔中（即通道交叉点）。TEA（+）在这种非典型的簇状模式下的结构方向仍然难以捉摸。本研究在不含其他碱金属阳离子的情况下，利用TEA（+）水热合成了沸石β，目的是研究单独使用TEA（+）的结构导向行为。通过一系列表征技术，包括氩吸附和解吸、高能x射线全散射、拉曼和固体核磁共振波谱以及高分辨率透射电子显微镜，对水热合成过程中形成的固体产物进行了分析。结果表明，非晶态TEA(+)-铝硅酸盐复合材料的形成及其在诱导期向非晶态β -沸石样结构的结构、化学和织构演化对β -沸石的形成至关重要。考虑到TEA（+）的聚集行为，提出了一种全面的沸石β形成方案：(i)加热后形成TEA(+)-铝硅酸盐复合材料，（ii）铝硅酸盐的重组与TEA（+）的构象重排，形成具有β -类沸石结构的无定形TEA(+)-铝硅酸盐复合材料，（iii）通过这种β -类沸石，TEA(+)-铝硅酸盐复合材料的固态重组形成β -沸石核，以及（iv）随后的晶体生长。预计这些发现可以为更有效地扩大osda在结构方向簇化模式中的应用提供基础。
Organic structure-directing agents (OSDAs) have been widely used for the synthesis of zeolites. In most cases, OSDAs are occluded in zeolites as an isolated cation or molecule geometrically fitted within the zeolite cavities. This is not the case for zeolite beta synthesized by using tetraethylammonium (TEA(+)) cation as an OSDA, in which a cluster/aggregate of ca. six TEA(+) cations is occluded intact in the cavity (i.e., the channel intersection) of zeolite beta. The structure direction of TEA(+) in such a nontypical, clustered mode has remained elusive. Here, zeolite beta was hydrothermally synthesized using TEA(+) in the absence of other alkali metal cations in order to focus on the structure-directing behaviors of TEA(+) alone. The solid products formed throughout the hydrothermal synthesis were analyzed by an array of characterization techniques including argon adsorptiondesorption, high-energy X-ray total scattering, Raman and solid-state NMR spectroscopy, and high-resolution transmission electron microscopy. It was revealed that the formation of amorphous TEA(+)-aluminosilicate composites and their structural, chemical, and textural evolution toward the amorphous zeolite beta-like structure during the induction period is vital for the formation of zeolite beta. A comprehensive scheme of the formation of zeolite beta is proposed paying attention to the clustered behavior of TEA(+) as follows: (i) the formation of the TEA(+)-aluminosilicate composites after heating, (ii) the reorganization of aluminosilicates together with the conformational rearrangement of TEA(+), yielding the formation of the amorphous TEA(+)-aluminosilicate composites with the zeolite beta-like structure, (iii) the formation of zeolite beta nuclei by solid-state reorganization of such zeolite beta-like, TEA(+)-aluminosilicate composites, and (iv) the subsequent crystal growth. It is anticipated that these findings can provide a basis for broadening the utilization of OSDAs in the clustered mode of structure direction in more effective ways.