(716f) Development of Evaluation Method for the Ideal Separation Property of Microporous Membrane: A Case Study for Ag-X Membrane for Propylene/Propane Separation

Since zeolite membrane has polycrystalline structures, there are two kinds of pathways across a membrane, intracrystalline (micropore of zeolite) and intercrystalline (defects among crystals) pathways. Separation property of zeolite membrane is always influenced to varying degrees by defects present in a membrane. Generally, a few intercrystalline pathways readily spoil separation performance of zeolite membrane, and thus reducing the intercrystalline pathways would help zeolite membranes to improve permselectivity.

It is important to develop a method to evaluate the ideal selectivity when molecules permeat only through the intercrystalline pathways, excluding the influence of permeation through the intercrystalline voids. We recently reported Ag-X membrane for propylene/propane separation based on selective adsorption.¹ In this study, we developed a novel method to evaluate the ideal separation property of Ag-X membrane for propylene/propane.

Na-X membrane was synthesized by a secondary growth method on a porous a-alumina tubular support (outer diameter, 10 mm; inner diameter, 7 mm; length, 30 mm, Noritake Co. Ltd.). The mean pore diameter of support was ca. 150 nm. The support was seeded by means of a dip coating method with a USY seed slurry. A seeded support was immersed in a synthesis solution for hydrothermal treatment. Hydrothermal treatment was carried out at 343 K for 24 h. After the hydrothermal treatment, Na-X membrane obtained was rinsed with distilled water and dried at 343 K overnight.

Ag-X membranes were prepared by an ion exchange method for Na-X membranes obtained. Na-X membrane was immersed into the nitrate salt aqueous solution (10 mM) and kept while stirring for 1 h. Ion exchanged X membranes were washed with distilled water and dried at 343 K overnight prior to use.

Separation test to evaluate the ideal selectivity was carried out as follows. A propylene/propane equimolar mixture was fed to Ag-X membrane at 313 K and atmospheric pressure. The permeate side was swept with flowing helium. To eliminate the effect of permeation through the intercrystalline pathway, the permeation side was pressurized from 0.1 to 0.4 MPa by using a back pressure valve.

When the permeation side was pressurized, we observed that the flux of propane drastically decreased while that of propylene was not deceased so much, and thus the separation factor increased with increasing pressure. For example, the propane fluxes at 0.1 and 0.4 MPa of pressure in the permeation side were 3.19 x 10^-2 and 0.704 x 10^-2 mmol m^-2 s^-1, respectively, resulting in that separation factor improved from 29 to 102. These results of permeation test suggest that propane permeation through the intercrystalline voids would be inhibited by helium counter-flowed from the permeate side.

This evaluation method could reduce the influence of permeation through intercrystalline voids and be effective to estimate an ideal selectivity of porous crystalline membrane. In addition, we can conclude that there is still room for improvement of the separation property for propylene/propane by reducing defects existing in Ag-X membrane.

(1) Sakai, M.; Sasaki, Y.; Tomono, T.; Seshimo, M.; Matsukata, M. Olefin Selective Ag-Exchanged X-Type Zeolite Membrane for Propylene/Propane and Ethylene/Ethane Separation. ACS Appl. Mater. Interfaces 2019, 11, 4145-4151.

Acknowledgment

This work was partially supported by JST CREST (Japan Science and Technology agency, Create REvolutionary technological seeds for Science and Technology innovation program), Grant Number JPMJCR1324, Japan.