An investigation of the relationship between microstructure and permeationproperties of ZSM-5 membranes

The influence of membrane microstructure on the transport properties of ZSM -5 membranes was investigated. Two zeolite membranes with (101)- and (002)- orientations were grown layer-by-layer onto seeded alumina support. The mem brane morphology was kept constant as well as the shape of the individual c rystal grains that made up the polycrystalline zeolite membrane layer. The membrane microstructure were characterized and quantified using six micro s tructural parameters that include membrane thickness (tau), grain size (d), grain morphology (M), zeolite population (N), crystal intergrowth (I-c) an d film orientation. Eight different gases including He, H-2, N-2, Ar, CH4, n-C4H10, i-C4H10 and SF6 were used as molecular probes to investigate the t ransport processes through the membrane of different thicknesses. By mainta ining a comparable non-zeolite flow, it was demonstrated that the (101)- an d (002)-oriented ZSM-5 membranes have comparable transport resistance. Also , the results of the multi-thickness comparison using the different sized m olecular probes indicate a strong similarity in the transport mechanism and diffusion pathway through these two membranes. The experiment suggests tha t the grain boundary is the main non-zeolite diffusion pathway in the membr ane and their elimination through grain growth can result in better membran e performance. (C) 2001 Elsevier Science B.V. All rights reserved.