Synthesis of NaA zeolite membrane with high performance

Abstract

In recent years, zeolite membranes have been hotly investigated due to their potential to be combined in reaction/separation devices such as membrane reactors and chemical sensors, and to achieve gas separation under steady-state operation [1–4]. Molecules with different sizes and shapes can be discriminated or separated by zeolites through their channels. A-type zeolite, due to the pore size (0.3–0.5 nm) of its channel system, can be selective for small molecules, such as H2, O2, N2, CO2 and H2O from other molecules or from each other. Aoki et al. [5] formed zeolite A-type membranes on a porous support tube, and obtained good results for gas separation, of which the permeance of H2 permeance was about 10−7 mol · s−1 · m−2 · Pa−1. Recently, we [6–8] reported the preparation of zeolite NaA membranes by multistage synthesis, the results of pure gas separations were very good, while the permeance of H2 was also about 10−7 mol · s−1 · m−2 · Pa−1. As mentioned above, zeolite NaA membranes usually have a good separation factor, but the permeance is too low for practical applications [9, 10]. The relationship of the permeance and the permselectivity of the membrane is a trade-off, which means to get a high permeance, the permselectivity has to be sacrificed to a certain extent, and reverse also affirms we have to sacrifice the permeance to improve permselectivity. Thus one of the challenges in the field of zeolite NaA membrane is to prepare zeolite membranes with high permeance, while maintaining high separation selectivity. The zeolite membrane is composed of three parts: the zeolite crystal layer, the intermediate layer and the substrate [11]. The intermediate layer is composed of the substrate and the zeolite crystals in the pore of the substrate. The permeance of the membrane is mainly controlled by the thickness of the zeolite layer and the intermediate layer. In order to increase the permeance of the membrane, it’s necessary to reduce the thickness of these two layers. As reported, an active mesoporous layer can assist the formation of zeolite membrane [12], thus possibly forming a continuous but thin zeolite membrane in a shorter time of reaction. And during the formation of the zeolite membrane, the reaction bulk may penetrate into the substrate to form a thick intermediate layer. In this paper, combining these two factors together by adding a more active mesoporous layer on the macroporous substrate before synthesis, we successfully synthesized zeolite A-type membrane with high permeance and gas selectivity (Fig. 1). The initially porous α-Al2O3 substrate used was made by casting technology, with 30 mm in diameter, 3 mm in thickness, 0.3–0.5 μm in pore radius and ca. 50% porosity. One face of the substrate was dipped into a solution of 1.0 M of AlOOH with 1 wt% of PEG400 and 2 wt% of PVA-72000. The dipping time was about 9–12 s. Then the substrate was dried at room temperature for 24 h, heated to 700◦C at a rate of 0.3◦C/min, held at 700◦C for 3 h and cooled down at 0.5◦C/min. The dipping-calcining process was repeated to the same face of the substrate for 3 times to form a continuous thin mesoporous top layer. Then the modified side was coated with NaA zeolite crystals as nucleation seeds. The synthesis mixture was prepared by mixing sodium aluminate, water glass, sodium hydroxide, and water under vigorous stirring for 24 h to form a homogenous gel with molar composition of 3Na2O : 2SiO2 : Al2O3 : 200H2O. After the precursor was poured into a stainless steel autoclave, the substrate was put vertically in the synthesis mixture supported by a Teflon holder, with the unseeded face protected from the reaction bulk. The autoclave was transferred into the oven preheated at 90◦C and reacted for 12 h. After the reaction, the membrane was cooled down, washed in water, and dried at 150◦C for 3 h. Finally, the formation of the zeolite membrane was confirmed by X-ray diffraction (XRD) using a Ragaku D max/b powder diffractometer with Cu Kα (λ = 0.154 nm) radiation and operating at 40 kV and 100 mA. The surface morphologies of the membrane were observed with scanning electron microscopy (SEM) on a JEM-1200E scanning electron microscope. The integrity of the membrane was evaluated by gas permeation tests. The membrane was sealed in a permeation module with the zeolite membrane on the high-pressure side. The gas permeances of the membrane were measured by a soap-film flowmeter under