Separation of Hexane Isomers through Compact Na-ZSM-5 Zeolite Membrane

ZSM-5 type zeolite membranes have intensively being studied for their potential applications such as hexane isomers separation. However, many of ZSM-5 membranes poorly separate such isomers at higher temperatures above 423 K, possibly due to the presence of non-zeolitic permeation pathways.

In this communication, we report structural properties and separation properties of Na⁺-exchanged ZSM-5 (NaZSM-5) membranes crystallized for different periods. NaZSM-5 membranes were prepared on the outer surface of tubular porous a-alumina supports by a secondary growth method. After the seeding, hydrothermal treatment was carried out at 453 K for different synthesis periods in the synthesis mixture with the composition of 10 Na₂O:0.15 Al₂O₃:36 SiO₂:1200 H₂O.

By decreasing non-zeolitic permeation pathways, separation factor of the n-hexane (n-Hex)/2,2-dimethylbutane (DMB) binary equimolar mixture remarkably increased from unity to above 300 at 573 K. For a compact NaZSM-5 membrane (crystallized for 12 h) with Si/Al ratio of about 12, increasing temperature from 373 to 573 K enhanced the separation factors of n-Hex/DMB, n-Hex/2-methylpentane (MP), and MP/DMB mixtures from 14 to 470, 5 to 38, and 1.1 to 13, respectively. Detailed permeation and separation properties of hexane isomers through the NaZSM-5 membranes will be presented.