Fig. 3: SEM images and particle size distributions of the bare support
surface (a), seeded support surface by 1.5 μm (b), 300 nm (c), and 60 nm
(d) ZSM-5 zeolite seeds. The SEM cross-section of seeded support (e),
and the XRD patterns of three different sized ZSM-5 seeds (f) are also
available
The XRD patterns and SEM images of surface and cross-section of five
typical membranes of series SLS, DLS, and TLS are shown in Fig. 4. The
SEM micrographs and XRD’s of all other samples are also available in the
supplementary information (Figures S2 and S3). The membrane thickness
was estimated using SEM images and reported in Tables 1-3. It is clearly
observed that the tubular supported zeolite membranes synthesized with
different combination of seed layers all reflect the diffraction peaks
of the ZSM-5 zeolite. Indicating that they contain pure ZSM-5 phase and
the ratio of (1 0 1) and (0 2 0) peak show that the zeolite crystals in
the membrane are almost c-oriented [17]. Furthermore, the strong
peaks of alumina supports in the membranes confirm that the zeolite
layers are rather thin. The peak intensity ratio of the ZSM-5 zeolite
layer to that of alumina support indicates the crystallinity and
consequently the thickness of the zeolite layer.
The SEM images in Figures 4 and S2 show that the membranes synthesized
with 300 nm seeds at the primary layer, (samples M10, M11, and M12 in
TLS series) are c-oriented with column-shaped grain crystals. The
microstructures of these membranes are different from those synthesized
by large seeds (1.5 μm) at the primary layer. The XRD results in Figures
4 and S3 are also in good accordance with the SEM analysis in which the
higher the intensity ratio of the (1 0 1) peak to that of the (0 2 0)
peak (I101/I020), the higher c-oriented
degree of the membrane [18].
This special crystal orientation can be ascribed to the density of seeds
at the support surface so that higher seed density of small crystals
leads to higher space limitation suppression forcing for the growth of
seeds along the direction perpendicular to the support surface [19].
The same explanation can be relied on for comparing the membrane
thickness. The results in Tables 2 and 3 show that the thickness of the
membranes with small seeds (60 nm) at the top layer is higher than those
synthesized using mid-size seeds (300 nm) at the top layer.