Isomorphous substitution of Al, Fe, B, and Ge into MFI-zeolite membranes

ZSM-5 zeolite membranes with Al, Fe, B, or Ge substituted for Si in the fra mework structure were prepared by in situ crystallization onto porous stain less steel supports. Their single-gas and mixture separation properties wer e compared to a silicalite-l membrane. For all membranes, single-gas permea nces at 473 K showed a decreasing trend as the kinetic diameter of the diff using molecule increased. All membranes separated n-C4H10/i-C4H10, n-C4H10/ H-2, and H-2/i-C4H10 mixtures, and the selectivities depended on temperatur e and the substituted cation, but no trend with acidity or hydrophobicity w as observed. For most separations studied, the substituted membranes had hi gher selectivities than a silicalite-l membrane, and the B-ZSM-5 membrane h ad the highest selectivities. The n-C4H10/i-C4H10 separation was due to dif ferences in diffusion rates and adsorption coverages, and the highest selec tivity was 45. The n-C4H10/H-2 separation had a maximum value of 110 at 300 K, and separation was due to preferential adsorption of n-C4H10, which per meated faster than H-2 For H-2/i-C4H10 mixtures, however, H2 permeated fast er, but i-C4H10 adsorption reduced the H-2 permeation below the single-gas value so the mixture selectivity was below the ideal value. (C) 2000 Elsevi er Science B.V. All rights reserved.