Permeation properties of templated and template-free ZSM-5 membranes

Following the secondary growth procedure, ZSM-5 zeolite membranes were synthesized over porous alpha-alumina supports from clear template-free precursor solution and their permeation properties were compared with membranes prepared using templates. SEM analysis revealed that the templated membrane (M1) contained smaller crystals with more number of faces than their template-free counterpart (M2). Identical XRD patterns confirmed their ZSM-5 crystal structure. The expected size selectivity could not be achieved for M1 as this membrane possessed some micro defects, induced by the calcination procedure. M2 could achieve the required size selectivity at ambient temperatures and shape selectivity at 200 °C and gas permeation was mostly activated diffusion. Selectivity ratio of 44 was achieved for butane isomers through M2 with n-butane molecules experiencing maximum activation (43 kJ/mol). In general, permeation values measured for M2 membranes were an order lower than those of M1 membranes.