Fig. 7: SEM micrographs of surface and cross-section, and XRD patterns of ZSM-5 zeolite membranes prepared by variable temperature/time profile, (♦) are from the alumina substrate peaks
The surface and cross-sectional SEM images of synthesized membranes listed in Table 5 are shown in Fig. 7. Results in Table 5 and fig. 7 reveal that the membranes synthesized at lower initial temperatures have relatively thinner zeolite layer compared to those synthesized at constant temperature. It can be ascribed to the domination of nucleation reactions at the low initial temperature of 140˚C. The produced zeolite nuclei can penetrate into the spaces between fixed zeolite seeds in the top, intermediate and bottom layers. Their higher growth rate compared to that of larger seeds at the high temperature (180˚C) stage effectively fills the non-zeolitic pores and defects resulting in a dense and uniform zeolite layer [27].
The SEM images in Fig. 7 demonstrate the different microstructures of the M16 and M17 membranes, in which lower intergrown zeolite framework, higher roughnessو and some micron-sized gaps can be observed on the surface of membranes. It could be due to long-term synthesis at low temperatures and subsequent short-term synthesis at high temperatures resulting in the production of a larger number of small crystals. This confirms the fact that the longer residence time at higher temperatures results in larger crystals, thicker zeolite layers, and a more continuous and denser zeolite layer. It can be also observed in Fig. 7 that the membranes M14 and M15 exhibit more uniform and continuous zeolite films via their optimum synthesis temperature/time profiles. The membranes (M9 and M13) synthesized at high temperature (180˚C) for 10 h, show thicker zeolite layer and rougher surface due to the domination of the growth reactions over the nucleation reaction via longer exposure at high temperature at final stage.
N2 and SF6 permeances and N2/SF6 ideal selectivity of all the synthesized membranes are shown in Table 5 and Fig. 8. The results in Fig. 8 show that the ZSM-5 membranes synthesized at the prolonged low initial temperature (140˚C), if the duration of the final stage remains constant (8 h at 180˚C), have higher N2/SF6 ideal selectivities than those synthesized at a short-term low-temperature initial stage, confirming the influence of the nucleation process on improving the selectivity of the final zeolite membranes. The effectiveness of low-temperature initial stage can be approved also by comparing the results of M16 and M17, in which a 4 h increase in the duration of the initial stage, enhances the selectivity up to 50 %. Therefore, the high selectivity of the M14 membrane shows that an initial 4 h synthesis at 140˚C significantly improves the membrane quality via the formation of enough nuclei that grow faster during the final stage (8 h at 180˚C) resulting in a well-intergrown zeolite layer. However, prolonging the low-temperature initial stage up to 8 h simultaneously with shortening the final stage to 6 h (M15 membrane) drastically decreases the selectivity, showing that the 6 h synthesis at high temperature (180˚C) is not enough to form a complete and uniform zeolite layer. The low selectivities of M16 and M17 in Fig. 8 reveal also that the high-temperature synthesis time should be longer than 6 h to achieve selectivities higher than 200.