Department of Chemical and Biological Engineering, University of Colorado, Boulder, Colorado 80309-0424, USA.
Acc Chem Res. 2011 Nov 15;44(11):1196-206. doi: 10.1021/ar200083e. Epub 2011 Aug 2.
Since their first synthesis in the 1940s, zeolites have found wide applications in catalysis, ion-exchange, and adsorption. Although the uniform, molecular-size pores of zeolites and their excellent thermal and chemical stability suggest that zeolites could be an ideal membrane material, continuous polycrystalline zeolite layers for separations were first prepared in the 1990s. Initial attempts to grow continuous zeolite layers on porous supports by in situ hydrothermal synthesis have resulted in membranes with the potential to separate molecules based on differences in molecular size and adsorption strength. Since then, further synthesis efforts have led to the preparation of many types of zeolite membranes and better quality membranes. However, the microstructure features of these membranes, such as defect size, number, and distribution as well as structure flexibility were poorly understood, and the fundamental mechanisms of permeation (adsorption and diffusion), especially for mixtures, were not clear. These gaps in understanding have hindered the design and control of separation processes using zeolite membranes. In this Account, we describe our efforts to characterize microstructures of zeolite membranes and to understand the fundamental adsorption and diffusion behavior of permeating solutes. This Account will focus on the MFI membranes which have been the most widely used but will also present results on other types of zeolite membranes. Using permeation, x-ray diffraction, and optical measurements, we found that the zeolite membrane structures are flexible. The size of defects changed due to adsorption and with variations in temperature. These changes in defect sizes can significantly affect the permeation properties of the membranes. We designed methods to measure mixture adsorption in zeolite crystals from the liquid phase, pure component adsorption in zeolite membranes, and diffusion through zeolite membranes. We hope that better understanding can lead to improved zeolite membranes and eventually facilitate the large-scale application of zeolite membranes to industrial separations.
自 20 世纪 40 年代首次合成以来，沸石在催化、离子交换和吸附等领域得到了广泛的应用。尽管沸石具有均匀的分子尺寸孔道和优异的热稳定性和化学稳定性，但它们作为理想的膜材料仍存在一些挑战。直到 20 世纪 90 年代，人们才首次制备出用于分离的连续多晶沸石层。最初，人们尝试通过原位水热合成在多孔载体上生长连续的沸石层，得到了具有基于分子尺寸和吸附强度差异进行分离的潜力的膜。此后，进一步的合成努力导致了许多类型的沸石膜和质量更好的膜的制备。然而，这些膜的微观结构特征，如缺陷尺寸、数量和分布以及结构灵活性，人们了解甚少，渗透（吸附和扩散）的基本机制，特别是对于混合物，也不清楚。这些理解上的差距阻碍了使用沸石膜设计和控制分离过程。在本报告中，我们描述了我们为表征沸石膜的微观结构和理解渗透溶质的基本吸附和扩散行为所做的努力。本报告将重点介绍应用最广泛的 MFI 膜，但也将介绍其他类型沸石膜的结果。通过渗透、X 射线衍射和光学测量，我们发现沸石膜结构具有一定的柔韧性。缺陷的大小会因吸附和温度变化而发生变化。这些缺陷尺寸的变化会显著影响膜的渗透性能。我们设计了从液相测量沸石晶体中混合物吸附、沸石膜中纯组分吸附以及沸石膜中扩散的方法。我们希望更好的理解可以导致改进的沸石膜，并最终促进沸石膜在工业分离中的大规模应用。
