微孔晶体的晶化

Crystallization of microporous crystals

从宏观和微观角度简要总结了微孔晶体晶化过程及晶化机理的研究,包括微孔晶体的成核与晶体生长(宏观角度)、晶化机理的主要观点、结构导向效应、研究微孔晶体晶化的主要技术手段及最新的研究思路和策略(微观角度).借鉴于致密晶体晶化行为的研究,微孔晶体的晶化过程分为成核与晶体生长两个阶段.在成核阶段,提出了晶核可以从液相(液相成核)、固相(固相成核)或固液相(双相成核)中成核,后被统一为"通用"成核机制.在晶体生长阶段,研究主要集中于生长模式.受制于当前较低分辨率和灵敏度的表征手段以及合成体系的复杂度,人们对晶体的成核与生长机制的认识还存在着极大的争议.在微孔晶体的晶化过程及晶化机理研究中,一项重要的内容是通过特定的表征手段确认在晶化过程中生成的小结构单元,包括色谱、电喷雾质谱、原位和非原位核磁共振、紫外拉曼光谱等.结合实验数据和理论计算可以确定一些单靠实验数据不能确认的结构单元.在微孔晶体的晶化过程中,结构导向剂起到了极其关键的作用.本文简要总结了结构导向剂的种类和导向的典型结构.最后介绍了新近提出的"反向进化"法,该方法可用于研究在晶化的早期阶段生成的小结构单元及晶化起点的结构.

Zeolites and related microporous crystalline materials have been widely used as ion-exchangers in detergent industry, catalysts in petrochemical and chemical industry, and adsorbents in air separation by pressure swing adsorption. Its crystallization mechanism has been an important issue in zeolite field. Great efforts have been made to understand the events happened in the crystallization process and how the crystallization started. However, a well-accepted conclusion has far been achieved due to the inherent complexity of the synthesis systems and the low resolution of current characterization techniques. This paper briefly reviewed the studies on the crystallization mechanism and crystallization process of microporous crystals. The main points of view on the nucleation and crystal growth were introduced, including ＂solution-mediated transport mechanism＂, ＂solid hydrogel transformation mechanism＂, ＂dual-phase transition mechanism＂, and ＂generalized mechanism＂. In the study of the crystallization of microporous crystals, the identification of the fragments or small structural units is also very important. The typical characterization techniques that can provide such information are briefly introduced, including high resolution chromatography, in situ and ex situ nuclear magnetic resonance （NMR） with newly developed cross polarization （CP）, magic angle spinning （MAS）, dipolar diphasing, total side-band suppression, multiple-quantum magic-angle spinning （MQMAS）, and rotational-echo double resonance （REDOR） methods, electrospray ionization mass spectrography （ESI-MS）, and ultraviolet Raman spectra （UV-Raman）. Combination of the experimental data and the corresponding calculation can determine the small structural units that cannot be identified by the experimental data only. In the crystallization of microporous crytals, templates or structure-directing agents played an extremely imporant role. In this paper, the diversity of the templates or structure-directing agents was reviewed and the corresponding classification was applied. Finally, a recently developed ＂reverse temporal evolution＂ crystallization process was introduced, which might be used to get the structural information of the species finally included into the structure of the resulting open framework at the very early stage of the formation process as well as the structural information of the starting point of the crystallization that a single crystal grew from.