Zeolite Membranes

无机膜具有高选择性和在高温和反应气氛中工作的能力。这使得这些膜适用于成熟的有机聚合物膜之外的应用。例如，一种应用是在膜反应器中，为了提高产率，在膜反应器中进行诸如烃类脱氢之类的平衡受限反应，同时进行氢分离。沸石分子筛膜对多种气体混合物具有高度选择性分离的潜力。它们可以在环境温度下工作，在稳定状态下进行分离，而目前的分离是通过非稳定压力或变温吸附进行的。或者，它们可以在高温下用于催化膜反应器，以提高反应产率或选择性。为了实现这一潜力，必须将沸石膜制备为支撑在多孔管或其他支撑元件上的机械稳定的薄层。在本项目中，将研究在多孔管壁内合成薄层沸石的技术。通常的方法是以这样的方式部署反应物，即有利于沸石结晶的条件仅在多孔载体内的薄区占优势。将研究三种特定的技术来划定结晶区。第一种是基于反应物在两个水溶液之间的反向扩散，第二种是将反应前沿固定在水和有机相之间的界面上，第三种是基于前体凝胶的局部沉淀。沸石的形成将通过X射线衍射仪、固体核磁共振、红外和物理吸附来确定，而扫描电子显微镜将用来确定层的位置和厚度。将测量几种气体在常温和高温下的渗透速率系数和分离系数。最后，探讨了分子筛膜作为膜反应器在异丁烷脱氢和异丙苯烷基化等几种催化反应中的应用。

Inorganic membranes have the potential for high selectivity and the ability to operate at elevated temperatures and reactive atmospheres. This makes these membranes suitable for applications outside the range of the well-established organic polymer membranes. One application, for example, is in membrane reactors where an equilibrium-limited reaction such as hydrocarbon dehydrogenation is carried out with simultaneous hydrogen separation in order to increase the yield. Zeolite membranes have the potential of highly selective separations for a variety of gas mixtures. They can operate at ambient temperatures to carry out, at steady state, separations which are currently performed by unsteady pressure or temperature swing adsorption. Or they can be used at elevated temperatures in catalytic membrane reactors to increase the reaction yield or selectivity. To realize this potential, the zeolite membranes must be prepared as thin but mechanically stable layers supported on porous tubes or other support elements. In this project, techniques for synthesizing thin zeolite layers within the walls of porous tubes will be investigated. The general approach is to deploy the reactants in such a way that conditions favorable to zeolite crystallization prevail only over a thin region inside the porous support. Three specific techniques will be investigated for delimiting the crystallization region. The first will be based on counterdiffusion of reactants between two aqueous solutions, the second on fixing the reaction front at the interface between an aqueous and an organic phase, and third on the localized precipitation of a precursor gel. The formation of the zeolite will be ascertained by X-ray diffraction (XRD), solid state nuclear magnetic resonance (NMR), infrared and physical adsorption, while scanning electron microscopy (SEM- EDX) will be used to define the location and thickness of the layer. Permeation rate coefficients and separation factors will be measured for several gases at ambient and elevated temperatures. Finally, the use of zeolite membranes as membrane reactors will be explored in a few catalytic reactions such as the dehydrogenation of isobutane and the alkylation of cumene.