The SEM images of surface and cross-section of the bare and the seeded tubular α-alumina supports are shown in Fig. 3. As can be seen in Fig. 3b, the surface of support was not fully covered by the 1.5 μm coffin shape zeolite seeds. However, the second layer of 300 nm sphere shape seeds (Fig. 3c) significantly covers the gaps and spaces between large seeds at the bottom layer. The third 60 nm sphere shape seed layer (Fig. 3d) ultimately prepares a smooth, uniform, and defect-free seed layer. The average seed particle size and the seed size distribution of each seed layer are also shown in Fig. 3. As can be seen, the seeds prepared in this study displayed average particle sizes of 1.5 μm, 300 nm, and 60 nm for the bottom, intermediate, and top layers, respectively. The narrow particle size distribution curves also prove that the adjustment of seed size by controlling synthesis time was performed successfully. The XRD patterns of different size synthesized seeds are presented in Fig. 3f. The patterns indicate that all the seeds used in this study were pure crystalline with the characteristic peaks of ZSM-5 zeolite. The intensity of characteristic peaks enhanced with the increase of seed size.