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.
