CHARACTERIZATION AND PERMEATION PROPERTIES OF ZSM-5 TUBULAR MEMBRANES

ZSM-5 zeolite membranes with reproducible properties were prepared by in-situ synthesis on porous alpha- and gamma-alumina tubular supports and characterized by XRD, SEM and electron microprobe analysis. Single -gas permeances for H-2, CH4, N-2, CO2, n-butane, and i-butane increas e over some temperature range, but some gases exhibit maxima or minima . The highest ideal selectivities at room temperature are 299 for N-2/ SF6, 392 for H-2/n-butane, and 2,820 for H-2/i-butane. These membranes can separate n-butane/i-butane, H-2/n-butane and H-2/i-butane mixture s. All n-butane/i-butane separation selectivities have maxima as a fun ction of temperature and are higher than ideal selectivities because n -butane inhibits i-butane permeation. Thus, separation is not by size selectivity but is due to pore blocking. Temperature dependencies of s ingle-gas permeances and separation selectivities depend strongly on t he location of zeolite crystals and the location is determined by prep aration procedure. Ideal selectivities also depend strongly on the pre paration procedure. When the zeolite forms a continuous layer on the i nside surface of the support tubes, pure i-butane permeates faster tha n pure n-butane so that the single-gas permeances are not determined j ust by molecular size. The i-butane permeance also increases much more with temperature than the n-butane permeance. The permeation behavior may be the result of permeation through nonzeolitic pores in parallel with zeolite pores. When zeolite crystals are dispersed throughout th e pores of alpha-alumina supports, permeances are lower and gas permea tion and separation properties are quite different Ideal selectivities are lower, pure n-butane permeates faster than i-butane, and the perm eances increase much less with temperature. Separation selectivities a re lower but can be maintained to higher temperatures.