Microstructural engineering of zeolite membranes through composite seed layers

Abstract

MFI membranes were synthesized using composite seeds consisting of zeolite nanocrystals and amorphous silica nanoparticles. The weight compositions of zeolite and silica particles were tuned to 100/0, 75/25, 50/50, and 25/75, respectively, which were then deposited as seed layers on porous alumina support through rubbing. At the seeded growth condition at 135 °C for 24 h, the film thickness of zeolite membranes grown from 4 different seed layers was similar to 3 μm, along with preferential crystal orientation to (h 0 h) direction. The upper parts of the zeolite membrane shared comparable film characteristics independent of seed composition, according to FCOM images of the MFI membranes. On the other hand, the zeolite membrane prepared from a seed layer of 75 wt% zeolite and 25 wt% silica particles had the greatest N2/SF6 gas selectivity of >175 and N2 gas permeance of >3300 GPU. Porosimetry also showed that there are fewer non-zeolite pathways in the zeolite membrane. Such results were attributed to differences in silica loading in the seed layer, where the silica species effectively altered the microstructure of zeolite membranes. The grain size of zeolite membranes increases as silica particle loading increases, reducing the chance of non-zeolitic defects occurring at grain boundaries. The composite seed with a silica loading of 25% was particularly successful because it did not inhibit the intergrowth of zeolite seed crystals while sacrificing slight N2 gas permanence. Thus, adding silica nanoparticles to the seed layer can help reduce non-zeolite defects in MFI membranes, according to this research. Further research is needed to prove that this method can be applied to zeolite membranes with other zeolite frameworks.