(568b) One-Step Secondary Synthesis of Thin Zeolite Beta Membranes

Zeolite membranes possess extremely regular and stable microporous frameworks and are, therefore, highly promising materials for gas separation applications which require thermal and chemical stability of separation media. Gas separations employing membranes are energy efficient and can be easily integrated into existing processes, which has translated in a strong industrial interest in their development. However, the development of zeolite membranes is a complicated process due to many experimental parameters that have an impact on the quality of these thin-layer separation devices.

In this paper we report a study of BEA zeolite membranes (pore diameter = 7.5Å). Although initial reports of BEA membrane synthesis have appeared recently, these syntheses are complicated. In situ crystallization of zeolite BEA membranes requires multiple steps with intermediate calcination to remove the template after each in situ crystallization step, which lasts close to 1 month. On the other hand, although being faster, seeded secondary growth also requires 2 or more steps of seeded growth to grow uniform thin layers of zeolite BEA as evidenced by SEM. Moreover, no compelling evidence is presented in these earlier studies which suggested that these intial BEA membranes were essentially defect-free. Therefore, it is desirable to develop a new simplified synthesis procedure for nearly defect-free BEA membranes.

In this study, we report nearly defect-free BEA zeolite membranes produced in a single hydrothermal synthesis step following seeding the support with small BEA crystals. The membranes were synthesized on disc-shaped α -alumina supports with a mean pore size of 200 nm, a diameter of 20 mm and a thickness of 2 mm. The supports were dip-coated with BEA seeds with a mean size of 290 nm. The structure-directing agent and the silica source used in the secondary synthesis were, respectively, tetraethyl ammonium hydroxide (TEAOH) and fumed silica. The synthesis resulted in smooth thin layers of zeolite BEA. The scanning electron microscopy (SEM) cross-sectional images showed the BEA layer thickness in a range of 5 µm to 20 µm. The structural characteristics of BEA zeolite were investigated by X-ray diffraction (XRD), which confirmed the presence of the zeolite BEA layer. The elemental Si/Al ratio in the membrane layer was analyzed by horizontal and vertical cross-sectional EDS line scans, indicating Si/Al = 11, which is similar to the reported Si/Al ratio in the bulk BEA zeolite. The membrane permeation performance was investigated with nitrogen and oxygen by an unsteady-state method at 25-500°C and a pressure difference of 100-200 kPa. The gas permeance (@ 10 ^-7 mol/s m² Pa) increased for low pressure differences across the composite membrane which indicated a good membrane quality. Moreover, an ideal N₂/O₂ selectivity of 1.35 was observed in the unsteady-state permeation test at ambient temperatures which is significantly higher than the Knudsen factor (KF = 1.06). This higher than expected selectivity may be attributed to the differences in N₂ and O₂ adsorption in the BEA zeolite (Si/Al=11). This effect is likely to be important when the gas molecules diffuse mostly through essentially defect-free zeolite layer indicating superior quality of these zeolite BEA membranes.