SEPARATIONS OF C-4 AND C-6 ISOMERS IN ZSM-5 TUBULAR MEMBRANES

Mixtures of Linear and branched hydrocarbon isomers were separated in continuous now experiments with ZSM-5 zeolite membranes. The method of membrane preparation has a dramatic effect on the selectivity. Changi ng. 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-dimethylb utane separation selectivity is 2580. The n-hexane permeance and the n -bexane/2,2-dimethylbutane selectivity slowly decreased with time, but the membrane could be regenerated by calcination at 723 K. The highes t 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 N-2/SF6 ideal selecti vity measured at room temperature. Separations of organic vapors in ou r membranes are not due to molecular sieving. The separation selectivi ties are much higher than the ideal selectivities; the n-alkane effect ively blocks the permeance of the branched-chain hydrocarbon.