Synthesis And Characterization Of NaA Zeolite Membranes

Zeolite membranes have excellent thermal stability, chemical stability and mechanical stability. NaA zeolites are three-dimensional open-framework and high hydrophilicity with the molar ratio Si/A1 nearly equal to 1. Due to their uniform molecular-sized pores, they are ideal materials for separation of small molecular gas mixture. NaA zeolite membranes have been widely applied in pervaporation in recent years.In this work, NaA zeolite were synthesized by hydrothermal method and microwave heating method. NaA zeolite membranes were also synthesized on the surface ofα-Al2O3 substrates by in-situ hydrothermal synthesis, secondary growth method and microwave synthesis. The influence of several operating parameters such as synthesis time, number of zeolite layers was investigated. The effect of aging time to microwave method is discussed in detail.The phase structure of the powers and the membrane were investigated by X-ray diffraction (XRD). The surface morphology of the raw support and the membranes synthesized by conventional method as well as microwave method were studied by using a scanning electron microscope (SEM). The multipoint Brunauer, Emmett and Teller (BET) method is used to measure the Surface area, pore size distribution. Aging of the synthesis mixture has an enormous influence to the final zeolites and zeolite membranes both in hydrothermal synthesis and microwave synthesis, the increase of aging time benefit the formation of pure NaA zeolites and continuous compact membranes.NaA zeolites with crystal size of 0.3μm were successfully synthesized by hydrothermal synthesis by using the 30h aged synthesis gel for 4 hours. In microwave synthesis, NaA zeolites were formed from the 25h aged mixture for only 5 minutes. The synthesis time is shorted and the crystals are smaller to ~0.2μm in size with a much narrower size distribution. NaA zeolite membrane was prepared after a 4-stage hydrothermal synthesis with the thickness of about 15μm. Swelling and cracks were observed on the membrane surface due to the different coefficient of thermal expansion of the support and the membrane during the heat treatment process. Membranes prepared by secondary grow method show better properties than those prepared by in situ hydrothermal synthesis. The membrane is dense and continuous with no swelling or cracks were observed. Its thickness is about 10μm. Bi-layered zeolite membrane was obtained from the 4-stage hydrothermal synthesis with a thickness of 15μm.The experimental results showed that continuous compact zeolite NaA membranes could successfully deposit on the surface ofα-Al2O3 by microwave heating. Microwave heating method could effectively shorten the synthesis time compared with hydrothermal method under the same synthesis condition. SEM and XRD results demonstrated that there existed continuous compact zeolite NaA membranes with the average thickness of 9μm after microwave heating with synthesis time for 10 min.