Characterization of the formation of NaA zeolite membrane under microwave radiation

Abstract

Zeolite membranes have gained much attention in recent years, due to their potential application in various fields such as catalysis, gas separation, pervaporation, etc. [1–4]. There are many reports on the synthesis, characterization and applications of zeolite membranes [1–4], but literatures about the formation mechanism of zeolite membrane are fewer [5–7]. To prepare zeolite membranes, many methods have been developed, including hydrothermal crystallization [8], vapor phase synthesis [9], and microwave heating [10, 11]. It was reported that microwave heating and crystal seeding could help the formation and improve the performance of zeolite membranes [10]. It was proposed that the formation of zeolite membranes included the following three stages: sol or gel adhesion on the substrate to form the gel layer, the nucleation and crystallization stage, and the dissolving stage [10, 12, 13]. However, there are few reports on the direct observation of these stages, especially under microwave radiation. In this report, by using scanning electron microscopy (SEM), the formation stages of zeolite NaA membrane in microwave radiation environment were observed and confirmed with the results from X-ray diffraction (XRD) and gas permeation measurements. The disk shaped substrates (30 mm in diameter, 3 mm in thickness) used were porous α-Al2O3 made by casting, with pore radius of 0.3–0.5 μm and ca. 50% porosity. The precursor of the gel was prepared by mixing sodium hydroxide, sodium aluminate, water glass and water, and aging for 24 h with vigorous stirring. The final molar composition of the gel was 3Na2O: Al2O3: 2SiO2: 200H2O. After one face of the substrates was rubbed with zeolite NaA crystals as seeds, the substrates were put vertically in Teflon vessels containing 120 ml gel. Then the vessels were transferred into a microwave oven, heated up to 90 ◦C within 1 min, and reacted for a range of specified times. After the reaction, the membranes were vigorously ultrasonically vibrated in water for 10 min to remove the species physically absorbed on the substrates, washed by de-ionized water to pH = 7.0 and dried at 150 ◦C for 3 h. The membranes were characterized by scanning electron microscopy (SEM) on a JEM-1200E scanning electron microscope and Xray diffraction (XRD) using Cu Kα (λ = 0.154 nm) radiation operating at 40 kV and 100 mA on a Rigaku D max /b powder diffractometer. The integrity of the membrane was evaluated with gas permeation tests.