无有机模板剂体系水热合成Beta沸石中晶种的导向机理

摘要

在无有机模板剂体系中研究了不同硅铝比和晶粒度Beta沸石晶种的结构导向行为，采用X射线衍射、X射线荧光光谱、扫描电镜、透射电镜、紫外-拉曼光谱、红外光谱和N2物理吸附等方法对不同晶化时间固相产物和Beta沸石产物进行了表征。结果表明，不同Beta沸石晶种，包括全硅晶种，均能够导向合成Beta沸石，而且晶种在晶化诱导期都发生溶解。但是，晶种的硅铝比、晶粒度、预处理(焙烧)以及晶种加入的时间对晶种的溶解行为、Beta沸石晶化过程和产物都有重要影响。形貌研究还发现，含铝晶种不仅溶解后的残体通过提供晶核聚集的“固载化”表面导向了新生Beta沸石小晶体的密集生长，而且溶解下来的结构片段也提供了分散的晶核导向形成相对分散的Beta沸石小晶体；全硅晶种则仅通过溶解下来的结构片段提供分散的晶核。在无模板体系中，使用适当高硅铝比、小晶粒和经过焙烧处理的Beta沸石作为晶种有利于合成得到高结晶度的Beta沸石纯相。%The organic structure-directing agent-free synthesis of zeolite Beta was carried out using several zeolite Beta seeds that differed in SiO2/Al2O3 ratio and crystal size. The synthesis was studied using X-ray diffraction, X-ray fluorescence, scanning electron microscopy, transmission electron micros-copy, ultraviolet-Raman spectroscopy, infrared spectroscopy, and N2 physisorption. Synthesis was successful using different zeolite Beta seeds including pure silica seeds. During the induction period, the seeds underwent dissolution. The SiO2/Al2O3 ratio and crystal size, pretreatment (calcination), and seed addition time had a significant influence on seed dissolution behavior, crystallization pro-cess, and product. Morphological studies revealed that the seed residues produced by dissolution (except for pure silica) resulted in the formation of“immobilized”surface nuclei, which allowed for the dense growth of fresh small zeolite Beta crystals. The dissolved small seed fragments yielded dispersed nuclei, which formed relatively scattered small zeolite Beta crystals thought to be the main nuclei source of the pure silica seed. It is suggested that the use of an appropriately high SiO2/Al2O3 ratio, small size, and precalcined zeolite Beta seed is helpful for the synthesis of highly crystalline and pure zeolite Beta from the organic structure-directing agent-free gel.