Study of crystallization process of SAPO-11 molecular sieve

The crystallization process of SAPO-11 was studied using a combination of X-ray diffraction, scanning electron microscopy, X-ray fluorescence, nuclear magnetic resonance, and X-ray photoelectron spectroscopy. In the initial stage of crystallization, SAPO-11 was formed along with an unknown crystalline material composed of Si-P-Al. As crystallization evolved, the crystalline material dissolved. The SAPO-11 formation rate increased greatly, which is characteristic of fast crystallization. After 2.33 h, the relative crystallinity of SAPO-11 reached ∼100% and remained at a high level until crystallization was complete. Si was incorporated into the SAPO-11 framework from the initial stage of crystallization. The Si content of the solid samples increased with crystallization time. Most of the Si atoms existed as Si islands in the SAPO-11 framework, resulting in the presence of multiple coordination environments, i.e., Si(nAl, (4 − n)Si), n = 0–4. X-ray photoelectron spectroscopy analysis revealed Si enrichment on the external surfaces of the SAPO-11 crystals. Based on the experimental results, the distribution of Si in the crystals is not uniform, showing an increasing trend from the core to the surface.

摘要采用X射线衍射、X射线荧光光谱、扫描电镜和固体核磁等方法研究了SAPO-11分子筛的水热晶化过程. 结果表明, 晶化初期, SAPO-11和一种具有磷硅铝组成的未知晶相同时生成; 随着晶化的进行, 中间相溶解, SAPO-11的生成速率大大增加, 呈现快速晶化的特征; 至2.33 h后, SAPO-11的结晶度接近100%, 并保持至晶化结束. 硅从晶化初期即参与了SAPO-11的形成, 它在晶体中的含量随晶化时间的延长而逐渐增加. 硅原子主要以硅岛的形式分布于SAPO-11分子筛骨架中, 从而导致多种硅配位环境的存在. 分析显示, SAPO-11分子筛呈现外表面富硅的特点, 结合晶化过程的分析可推测, 硅在SAPO-11分子筛晶体中的分布不均匀, 其含量从内向外递增.

Graphical AbstractThe formation of SAPO-11 exhibited fast crystallization characteristics. The Si content of SAPO-11 increased with time, which led to a non-uniform distribution of Si in the crystals.Figure optionsDownload as PowerPoint slide