Parallel pathways for transport in ZSM-5 zeolite membranes

Changes in ZSM-5 zeolite membranes during preparation were studied by characterizing the gas permeation properties as the zeolite synthesis template was removed by calcination for 4 h at progressively higher temperatures or for longer times at 573 K. Both zeolite and intercrystalline pores appear to be present; the nonzeolitic pores are larger and have lower ideal selectivities, but these pores are small enough and/or in low enough concentrations that high ideal and mixture selectivities are obtained for the final membranes. The tetrapropylammonium hydroxide template is removed from the larger intercrystalline pores first, and thus ideal selectivities changed significantly with calcination temperature or time. The N-2/SF6 ideal selectivities also depend on the measurement method; they were higher for a pressure drop across the membrane than when a sweep gas was used, apparently because of the change in driving force for SF6, which adsorbs more strongly than Ng. Permeances continue to increase as membranes are calcined up to 873 K, but the N-2/SF6 ideal selectivities are the same for calcination from 673 to 873 K.