Fig. 4: SEM images and XRD patterns of ZSM-5 zeolite membranes synthesis
by one-layer (Series SLS, M2), Dual-layer (Series DLS, M5 and M7) and
Triple-layer (Series TLS, M9 and M11) seeding methods, (♦) are from the
alumina substrate peaks
This is due to the fact that large seeds accumulate less in density at
the top layer, resulting to form larger gaps and lower space limitation
suppression. This allows large seeds to grow in different directions
with a slower rate than that of small seeds [11]. While, the small
seeds, due to their lower inter-crystalline gaps, mainly grow along the
direction perpendicular to the support surface with a fast growth rate
resulting in the thicker zeolite layer [14]. It can be concluded
that the seed size at the top layer determines the final thickness of
the membrane. The SEM images in Figures 4 and S2 indicate also that the
membranes in TLS series have more uniform and smooth surfaces than those
in DLS and SLS series. In fact, intergrown of the penetrated small seeds
from the top layer into the spaces and inter-crystalline gaps at the
bottom layer densifies the density of the zeolite layer resulting in the
more uniform and continuous membrane surface.