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 te mperatures or for longer times at 573 K. Both zeolite and intercrystal line pores appear to be present; the nonzeolitic pores are larger and have lower ideal selectivities, but these pores are small enough and/o r in low enough concentrations that high ideal and mixture selectiviti es are obtained for the final membranes. The tetrapropylammonium hydro xide template is removed from the larger intercrystalline pores first, and thus ideal selectivities changed significantly with calcination t emperature or time. The N-2/SF6 ideal selectivities also depend on the measurement method; they were higher for a pressure drop across the m embrane than when a sweep gas was used, apparently because of the chan ge in driving force for SF6, which adsorbs more strongly than Ng. Perm eances 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.