Synthesis and gas permeation properties of an NaA zeolite membrane

Abstract

A high quality NaA zeolite membrane, which shows a H 2 /n-C 4 H 10 permselectivity of 106, has been synthesized on a seeded a-Al 2 O 3 support by a multistage synthesis method.The synthesis of zeolite membranes and their application in separation and catalysis have attracted great interest in recent years. 1,2 For gas separation, most studies have concentrated on MFI type (silicalite-1 and ZSM-5) zeolite membranes. Although high separation factors were obtained for mixtures of organic gases, 3-6 MFI zeolite membranes do not show good separation performance for permanent gases or permanent gas/ organic gas mixtures because of the large pore size and hydrophobic nature of the zeolite. Thus the investigation of zeolite membranes with small pore size has attracted much attention. 7-9 NaA zeolite has a smaller pore size (0.41 nm) than that of MFI zeolite (ca. 0.55 nm). However, most studies on NaA zeolite membranes focused on the dehydration of liquid mixtures by pervaporation 10,11 and few gas permeation results were reported. 9,12 Furthermore, it was shown, from the gas permeation results, 9,12 that the quality of these NaA zeolite membranes was poor. Here, a high quality NaA zeolite membrane has been synthesized on a seeded a-Al 2 O 3 support by a multistage synthesis method.A self-made porous a-Al 2 O 3 disk (30 mm in diameter, 3 mm in thickness, 0.1-0.3 mm pore radius, ca. 50% porosity) was used as the support. Before synthesis, one side of the support was coated with NaA zeolite crystals as nucleation seeds by dipping the support in a 0.5 wt% NaA zeolite suspension in a single step for 30 s. The 0.5 wt% NaA zeolite suspension was prepared by peptizing 0.5 g NaA zeolite in 100 ml deionized water. In order to form a thin and uniform NaA zeolite membrane on the support surface and to inhibit the formation of zeolite in the pores of the support, a uniformly sized NaA zeolite of dimensions ca. 1 mm, larger than those of the pores of the support, was used as the seed. The synthesis mixture was prepared by mixing sodium aluminate, water glass, sodium hydroxide and water. The molar ratio of the resultant gel mixture was 3Na 2 O+2SiO 2 +Al 2 O 3 +200H 2 O. The synthesis was carried out at 90 °C for 24 h in a stainless steel autoclave with the seeded support held vertically in the synthesis mixture by a Teflon holder. After the synthesis, the composite membrane was washed several times with water until the pH of the washings was neutral, then dried at 150 °C for 3 h. In order to obtain a high quality NaA zeolite membrane, the synthesis was repeated twice more (in all, a three-stage synthesis). The assynthesized membrane was characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and gas permeation studies. The permeances of the gases were measured at different temperatures by a soap-film flowmeter under a pressure difference of 0.10 MPa. The permselectivity of A/B is defined as the permeance ratio of gas A to gas B.The XRD pattern of the as-synthesized membrane prepared by the three-stage synthesis i...

Synthesis of zeolite NaA membranes with high permeance under microwave radiation on mesoporous-layer-modified macroporous substrates for gas separation

“…By comparing the NaA membrane synthesized by microwave heating on macroporous substrate with that by conventional hydrothermal synthesis by Aoki et al [22], the permeance of our NaA membrane was at least 7 times larger, while the H 2 /C 3 H 8 permselectivity was similar. Compared to that of multiple syntheses in gel [14], the permeance of the NaA membrane here was at least 2 times larger; the H 2 /C 3 H 8 permselectivity was not so high as we previously obtained [14], partly due to the choose of the permselectivity H 2 /C 3 H 8 here instead of H 2 /n-C 4 H 10 as before.…”

“…Multiple growths may improve the permselectivity and permeance if the reaction parameters are carefully controlled. Previously, we reported a NaA membrane with high permselectivity and a good permeance of 2.86 × 10 −7 mol s −1 m −2 Pa −1 by using multiple growth in gel with the aid of seeding of NaA crystals [14].…”

Synthesis and perfection evaluation of NaA zeolite membrane

“…It will not only cut the equipment and operation cost off, but also be operated continuously if the zeolites can be configured into a membrane form. Since 1990s, much effort has been paid in the synthesis of zeolite membrane [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20]. Till now, different types of zeolite membranes, e.g.…”

“…Although high separation factors have been obtained for these mixtures, the MFI zeolite membrane has not shown good separation performance for permanent gases or permanent gas/organic gas because of the large pore size and the hydrophobic nature of the zeolite [12][13][14][15]. Therefore, the investigation of the zeolite membranes with small pore size has attracted much attention [19][20][21][22][23][24][25].…”

Synthesis of NaA zeolite membrane on a ceramic hollow fiber

“…The IR spectrum of the powders confirmed the XRD results. Our previous study also showed that only NaA zeolite membrane formed on a disk ceramic support by using the same synthesis condition [26]. From the SEM image (not shown), it can be seen that crystals formed on the ceramic hollow fiber support after one-stage synthesis.…”