4. Conclusion
High-quality ZSM-5 zeolite membranes can be synthesized on the α-alumina
supports by applying a novel hybrid secondary growth method consisting
of different-sized seeding in combination with the variable
temperature/time profile. Using this method, the selectivity and
permeance of the membrane are increased via minimizing the
defects/inter-crystalline gaps and reducing the effective thickness of
the membrane (reducing the mass transfer resistance), respectively. The
microstructure of membranes can be manipulated by the different-sized
seeding method resulting in a thin, fully inter-grown, and dense layer
on the top, and a porous layer at the bottom resulting in a significant
increase in selectivity without scarifying the permeance. It was found
also that changing the seed size at the bottom layer, mainly influence
membrane permeance, while changing seed size on top, mainly affects
selectivity. Using the variable temperature/time method further
eliminates the inter-crystalline gaps/defects via balancing the
nucleation and grow reaction rates during the secondary growth
synthesis.