Single gas permeation through compositionally different zeolite NaA membranes: Observations on the intercrystalline porosity in an unconventional, semicrystalline layer

The successful application of zeolite A membranes in the industrial market has thus far been restricted to the pervaporative dehydration of solvent streams in the pharmaceutical and engineering industries. Their application in gas separation processes remains elusive, largely due to the existence of uncontrollable, intercrystalline diffusion pathways in the boundary regions of neighbouring crystals.In this study the gas permeation characteristics of an unconventional, semicrystalline (70%) zeolite NaA layer, where the boundary phase is filled with amorphous aluminosilicate, was examined and compared to a traditional polycrystalline membrane. To this end, we used single gas permeation of H2, N2 and SF6 at two temperatures. In general, the permeances of all gases through the conventional layer increased in response to temperature, indicating a governing contribution of activated (gaseous) diffusion over the temperature interval tested. Typical permeances of H2 were in the order of 4.8 × 10−7 (23 °C) to 6.5 × 10−7 mol m−2 s−1 Pa−1 (107 °C). For the semicrystalline layer, the same trends were observed for N2 and SF6, while the permeance of H2 decreased from 8.7 × 10−7 (23 °C) to about 3.9 × 10−7 mol m−2 s−1 Pa−1 at 107 °C. This was attributed to the expansion of the crystal/amorphous interface at higher temperature.By comparing the permselectivity values with the respective Knudsen factors, it was shown that diffusion through the semicrystalline layer, at lower temperature, was predominantly based on molecular sieving (PS H2/SF6 = 63.8), which yielded higher selectivities than the traditional membrane under the same conditions (PS H2/SF6 = 11.4). However, at higher temperature the H2/SF6 permselectivity of the crystalline layer (5.7) was somewhat higher than that of the semicrystalline membrane (5.2).Based on theoretical considerations, it was concluded that the crystal/amorphous interface in the semicrystalline membrane constituted a denser closure of the boundary interface, which could be attributed to a lower charge barrier presented by the amorphous phase (Si/Al > 1), but this integrity was lost at higher temperature due to the thermal instability of the amorphous material. The results therefore suggested that future research should focus on lowering the intercrystalline charge barrier during synthesis, which could reduce the intercrystalline porosity and improve the gas separation properties of NaA membranes in general.