Crystallization of microporous crystals

Abstract

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.