(514b) Rapid Defect-Healing of Zeolite Membrane By Alkaline-Treatment in the Presence of Surfactant

Chemical separations require about the half of industrial energy use and 15% of the total energy consumption in US. Membrane separation is fervently expected as an energy-efficient, economical, CO₂-free purification process to reduce the energy consumption in chemical production.

Zeolites have uniformly sized micropores defined by their crystal structures. The homogeneous distribution of pore size corresponds to superior size-exclusion ability called molecular sieving property.

Well-prepared zeolite membranes showed high separation performances based on the molecular sieving property. However, inter-crystalline defects such as pinholes and cracks in zeolite membrane often spoiled the separation performance. Improvement of productivity of membrane by a simple post-treatment could contribute to the reduction of membrane cost.

In this study, silicalite-1, siliceous MFI-type zeolite, membrane was alkaline-treated for defect-healing. The effect of treatment conditions on the permeation property was investigated.

Experimental

Silicalite-1 membrane was synthesized by a secondary growth technique. Seed crystals were loaded on α-alumina support by a dip-coating method. Alkaline-treatment was performed on the obtained silicalite-1 membrane. Silicalite-1 membranes were immersed in an aqueous solution containing 0.1 M NaOH and 0.05 M CTAB (Cetyltrimethylammonium bromide) at 358 K for given periods. After that, the membrane was washed with hot water and calcined at 673 K. n-Hexane/2,3-dimethylbutane permeation separation test was performed to evaluate the molecular sieving property of the membrane. Changes in the ratio of non-zeolitic pathway were evaluated by the nano-permporometry test.

Results and discussion

Fig. 1 shows the relationship between the permselectivity and the ratio of non-zeolitic pathway. The non-zeolitic pathways on silicalite-1 membranes were occluded in the early stage of the alkaline treatment, resulting in the enhancement of separation performance. The separation factor for n-hexane/2,3-dimethylbutane mixture increased from 86.5 to 559 after just 15-min treatment. In contrast, the permeance of 2,3-DMB increased with decreasing separation factor when the alkaline treatment was performed for >30 min. These results indicated that large defects formed in the later stage.

To study the role of CTAB, post-treatments with only NaOH or CTAB were carried out, respectively. The separation performance of silicalite-1 membranes treated with only NaOH or CTAB did not improve. These results suggested that the combination of NaOH and CTAB was essential for the improvement of separation performance.

To confirm the reproducibility of this method, alkaline-treatments were performed on other four silicalite-1 membranes in the same way. As a result, in all cases, the separation factors successfully increased in all membranes tested. In addition, the treatment was carried out for other types of zeolite membranes (Na-*BEA, Na-ZSM-5 and Na-MOR), and then the separation performances of these membranes increased as well. We believe that this simple alkaline-treatment with surfactant is promising technique of defect-healing for molecular sieving zeolite membranes.