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.