Abstract The synthesis of flat tablet-shaped ZSM-5 crystals from a gel using metakaolin as aluminosilicate source and n -butyl amine as structure directing agent was investigated. The evolution inside the solid phase was characterized by X-ray diffraction, scanning electron microscopy, energy dispersive spectroscopy, thermogravimetry and mass spectrometry. A kinetic study indicated that the nucleation of the majority crystals occurred concurrently with the formation of the gel upon heating the starting liquid suspension. Microstructural evidences undeniably showed that the gel precipitated on ZSM-5 crystals and mineral impurities originating from kaolin. As a result, crystal growth was retarded by gel entrapment, as indicated by the configuration and morphology of the embedded crystals. The results presented herein are harmonized with a solution-mediated nucleation and growth mechanism. Our observations differ from the autocatalytic model that suggests that the nuclei rest inside the gel until released when the gel is consumed. Our results show instead that it is crystals that formed in an early stage before entrapment inside the gel that rest inside the gel until exposed at the gel surface. These results illustrate the limitation of the classical method used in the field to determine nucleation profiles when the crystals become trapped inside the gel.

中文翻译：

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凝胶截留阻碍 NBA-ZSM-5 晶体的溶液介导生长

摘要 研究了以偏高岭土为硅铝酸盐源，正丁胺为结构导向剂，从凝胶中合成扁平片状ZSM-5晶体。固相内部的演化通过 X 射线衍射、扫描电子显微镜、能量色散光谱、热重分析和质谱分析来表征。动力学研究表明，在加热起始液体悬浮液时，大多数晶体的成核与凝胶的形成同时发生。微结构证据不可否认地表明凝胶沉淀在 ZSM-5 晶体和源自高岭土的矿物杂质上。结果，如嵌入晶体的构型和形态所示，凝胶截留阻碍了晶体生长。本文中呈现的结果与溶液介导的成核和生长机制相一致。我们的观察与自催化模型不同，自催化模型表明原子核停留在凝胶内，直到凝胶被消耗时释放。相反，我们的结果表明，在凝胶内部滞留直到暴露在凝胶表面之前的早期阶段形成的晶体在凝胶内部。这些结果说明了在该领域使用的经典方法在晶体被困在凝胶内时确定成核曲线的局限性。相反，我们的结果表明，在凝胶内部滞留直到暴露在凝胶表面之前的早期阶段形成的晶体在凝胶内部被截留。这些结果说明了在该领域使用的经典方法在晶体被困在凝胶内时确定成核曲线的局限性。相反，我们的结果表明，在凝胶内部滞留直到暴露在凝胶表面之前的早期阶段形成的晶体在凝胶内部被截留。这些结果说明了在该领域使用的经典方法在晶体被困在凝胶内时确定成核曲线的局限性。