Synthesis and gas permeation properties of highly b-oriented MFI silicalite-1 thin membranes with controlled microstructure

Herein we report continuous crack-free b-oriented silicalite-1 thin membranes with controlled microstructure on porous α-Al2O3 supports by using our micro-tiles-and-mortar method. The critical parameters identified for successful fabrication include: (1) polymer intermediate layers to get highly b-oriented seed crystal layers with high packing density and uniform surface coverage, (2) metal passivation layers to prevent out-of-plane nucleation and growth of seed crystals while allowing in-plane nucleation and growth of crystals, and (3) filling the gaps between seed crystals with nano-sized seed crystals. The membranes have three different types of microstructures: one with non-activated grain boundaries, another with activated grain boundaries, and the third with activated grain boundaries that are overgrown. To the best of our knowledge, membranes with the first two types of microstructures have never been reported previously. The performance of these membranes has been evaluated based on the binary separation of xylene isomers in the temperature range of 25–175 °C showing different permeation properties. The maximum p-/o-xylene separation factor was about 4 possibly due to grain boundary defects resulting from Au/Pd nanoparticles deposited on the sidewalls of seed crystals as well as on the supports.

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► Highly b-oriented MFI silicalite-1 thin membranes were prepared using secondary growth.
► Au/Pd passivation prevents out-of-plane nucleation and growth of oriented seed crystals while allowing in-plane nucleation and growth.
► Membranes with three different grain boundary structures were prepared.
► Performance of the membranes was evaluated based on the binary separation of xylene isomers in the temperature range of 25–175 °C.