The Preparation of NaA Zeolite Membrane on the Inner Surface of Hollow Fiber Supports

Zeolite membranes can be used in the separation of gas and liquid mixtures due to their unique pore structures, adsorption properties, strong mechanical strength, chemical stability and thermal stability. At present, most membranes reported in literatures were synthesized on the outer surfaces of flat plates or large diameter tubular supports. Only a few studies have been reported in literature regarding the preparation of zeolite membranes on the inner surfaces of the tubular supports. Growing a zeolite layer on the inner surface of the tubular support is attractive from the viewpoint of industrial application because the membrane layer is better protected from mechanical damage. On the other hand, in order to increase the flux of zeolite membranes, multi-channel supports are greatly preferred for zeolite membrane preparation. The zeolite membrane layers can only be synthesized on the internal surfaces of multi-channel supports. In previous studies, most of the efforts have been focused on the preparation of inner side zeolite NaA membranes on tubular supports with inner diameter (i.d.) larger than6mm, no studies have reported the preparation of zeolite membranes on the inner surfaces of hollow fiber supports with inner diameter less than2mm, which is obviously more challenging. The use of supports with small diameter can greatly reduce the volume of membrane module, leading to the reduced manufacture price, which is preferred for industrial application.Here we study the synthesis of zeolite NaA membranes on the inner surface of hollow fibers with inner diameter of～1.0mm and their properties. Influence of adding polyvinyl pyrrolidine (PVP) intermediate layer on the membrane formation is investigated. The main experimental results and conclusions are summarized as follows:(1) Preparation of zeolite NaA membranes on the inner surface of hollow fibersComposite hollow fiber (CHF) supports are used first for the preparation of inner side zeolite NaA membranes since CHF supports provide uniform zeolite particles on their surfaces as seeds, leading to the omission of the seeding process. The effects of synthesis composition, synthesis time and synthesis method on the membrane formation are investigated. The properties of the as-synthesized membranes on them are characterized by scanning electron microscopy (SEM) and vacuum pressure measurement. The optimal synthesis conditions are found to be as follows:synthesis composition of7.5Na2O:2SiO2:Al2O3:600H2O, synthesis temperature of100℃, synthesis time of4-5h, using a rotating autoclave with rotation of30rpm. The membranes prepared under above mentioned conditions are continuous and good enough for pervaporation (PV) test according to our PV results of zeolite membranes on outer surfaces of CHF supports. The optimal synthesis conditions obtained are applied to α-Al2O3hollow fiber supports to find a facile seeding method. Clear solution generated from synthesis solution can effectively increase the adhesion of seed particles with the support surface and work as binder to anchor seed particles to the support surface, leading to the formation of well-distributed seed layer. The effects of synthesis composition, synthesis time and synthesis method on the membrane morphology and properties are also investigated. The properties of the as-synthesized membranes are characterized by scanning electron microscopy (SEM) and vacuum pervaporation for the separation of90wt.%ethanol/water mixture at75m. Zeolite membranes with high separation factor up to10000at fluxes of6.9±0.3kg/m2h can be obtained under the conditions:synthesis composition of7.5Na2O:2SiO2:Al2O3:600H2O, synthesis temperature of100℃, synthesis time of5h, using a rotating autoclave (30rpm).(2) Influence of organic intermediate layer on the preparation of zeolite NaA membranesRecently, organic intermediate layer is used to prepare high qualified membranes since it can work as covalent linker to anchor zeolite crystals to the support surface. In order to find a superior seeding method, the influence of adding polyvinyl pyrrolidine (PVP) intermediate layer on the membrane formation is investigated. It is found well-distributed seed layer with a suitable seed weight gain (SWG) can be obtained by adding PVP intermediate layer. The synthesized membrane has a separation factor higher than10000in the pervaporation of90wt.%ethanol/water mixtures at75℃. The membranes synthesized without PVP intermediate layer had a higher flux compared to those synthesized with PVP intermediate layer. With increasing the concentration of the PVP aqueous solution, the fluxes of the zeolite membranes correspondingly decreased.