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.