Separations of C4 and C6 Isomers in ZSM-5 Tubular Membranes

Mixtures of linear and branched hydrocarbon isomers were separated in continuous flow experiments with ZSM-5 zeolite membranes. The method of membrane preparation has a dramatic effect on the selectivity. Changing the preparation procedure apparently changes the amount of the non-zeolitic pores that are in parallel with the zeolite pores and/or the length of the permeation pathways. The highest n-hexane/2,2-dimethylbutane separation selectivity is 2580. The n-hexane permeance and the n-hexane/2,2-dimethylbutane selectivity slowly decreased with time, but the membrane could be regenerated by calcination at 723 K. The highest n-butane/isobutane separation selectivity of 57 was obtained at 498 K, and the n-butane/isobutane selectivity correlated with the n-hexane/2,2-dimethylbutane selectivity but not with the N2/SF6 ideal selectivity measured at room temperature. Separations of organic vapors in our membranes are not due to molecular sieving. The separation selectivities are much higher than the ideal selectivities; the n-alkane effectively blocks the permeance of the branched-chain hydrocarbon.