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.